Titrimetric analysis is a method for determining the amount of a substance by accurately measuring the volume of solutions of substances that react with each other.
Titer- the amount of the substance contained in 1 ml. solution or equivalent to the analyte. For example, if the titer of H 2 SO 4 is 0.0049 g / ml, this means that each ml of the solution contains 0.0049 g of sulfuric acid.
A solution whose titer is known is called titrated. Titration- the process of adding to the test solution or an aliquot of an equivalent amount of a titrated solution. In this case, standard solutions are used - fixed channels- solutions with the exact concentration of the substance (Na 2 CO 3, HCl).
The titration reaction must meet the following requirements:
high reaction rate;
the reaction must proceed to completion;
the reaction must be highly stoichiometric;
have a convenient method of fixing the end of the reaction.
HCl + NaOH → NaCl + H 2 O
The main task of titrimetric analysis is not only to use a solution of exactly known concentration (fixanal), but also to correctly determine the equivalence point.
There are several ways to fix an equivalence point:
According to the intrinsic color of the ions of the element being determined, for example, manganese in the form of an anionMNO 4 -
By witness substance
Example: Ag + + Cl - "AgCl $
Ag + + CrO 4 "Ag 2 CrO $ 4 (bright orange color)
A small amount of salt K 2 CrO 4 is added to the flask where it is required to determine the chlorine ion (witness). Then, the test substance is gradually added from the burette, while chloride ions are the first to react and a white precipitate (AgCl) is formed, i.e. PR AgCl<< ПР Ag2Cr O4.
Thus, an extra drop of silver nitrate will give a bright orange color, since all the chlorine has already reacted.
III. Using indicators: for example, acid-base indicators are used in the neutralization reaction: litmus, phenolphthalein, methyl orange - organic compounds that change color when moving from an acidic to an alkaline environment.
Indicators- organic dyes that change their color when the acidity of the medium changes.
Schematically (omitting intermediate forms), the indicator equilibrium can be represented as an acid-base reaction
HIn + H 2 O In - + H 3 O +
H2O 
H++OH-
H + + H 2 O 
H3O+
The area of color transition of the indicator (position and interval) is affected by all the factors that determine the equilibrium constant (ionic strength, temperature, foreign substances, solvent), as well as the indicator.
Classification of methods of titrimetric analysis.
acid-base titration (neutralization): this method determines the amount of acid or alkali in the analyzed solution;
precipitation and complexation (argentometry)
Ag + + Cl - "AgCl $
redox titration (redoximetry):
a) permanganatometry (KMnO 4);
b) iodometry (Y 2);
c) bromatometry (KBrO 3);
d) dichromatometry (K 2 Cr 2 O 7);
e) cerimetry (Ce(SO 4) 2);
f) vanadometry (NH 4 VO 3);
g) titanometry (TiCl 3), etc.
The titrimetric method of analysis (titration) allows for volumetric quantitative analysis and is widely used in chemistry. Its main advantage is the variety of ways and methods, due to which it can be used to solve various analytical problems.
Principle of Analysis
The titrimetric method of analysis is based on measuring the volume of a solution of known concentration (titrant) that has reacted with the test substance.
For analysis, you will need special equipment, namely, a burette - a thin glass tube with applied graduations. The upper end of this tube is open, and at the lower end there is a stopcock. The calibrated burette is filled with the titrant to the zero mark using a funnel. The analysis is carried out to the end point of the titration (CTT) by adding a small amount of solution from the buret to the substance under study. The end point of the titration is identified by a change in the color of the indicator or some physical-chemical property.
The final result is calculated from the amount of titrant used and is expressed in titer (T) - the mass of the substance per 1 ml of solution (g / ml).
Process Justification
The titrimetric method of quantitative analysis gives accurate results because the substances react with each other in equivalent amounts. This means that the product of their volume and quantity are identical to each other: C 1 V 1 = C 2 V 2 . From this equation, it is easy to find the unknown value of C 2 if the remaining parameters are set independently (C 1 , V 2) and are established during the analysis (V 1).
Endpoint titration detection
Since the timely fixation of the end of the titration is the most important part of the analysis, it is necessary to choose its methods correctly. The most convenient is the use of colored or fluorescent indicators, but instrumental methods can also be used - potentiometry, amperometry, photometry.
The final choice of the LTT detection method depends on the required accuracy and selectivity of the determination, as well as its speed and the possibility of automation. This is especially true for cloudy and colored solutions, as well as aggressive environments.
Requirements for the titration reaction
In order for the titrimetric method of analysis to give the correct result, it is necessary to choose the right reaction that will underlie it. Its requirements are as follows:
- stoichiometry;
- high flow rate;
- high equilibrium constant;
- the presence of a reliable method of fixing the experimental end of the titration.
Suitable reactions may be of any type.
Types of analysis
The classification of titrimetric analysis methods is based on the type of reaction. On this basis, the following titration methods are distinguished:
- acid-base;
- redox;
- complexometric;
- precipitation.
Each type is based on its own type of reaction, specific titrants are selected, depending on which subgroups of methods are distinguished in the analysis.
Acid-base titration
The titrimetric method of analysis using the reaction of interaction of hydroxonium with hydroxide ion (H 3 O + + OH - \u003d H 2 O) is called acid-base. If a known substance in solution forms a proton, which is typical for acids, the method belongs to the acidimetry subgroup. Here, stable hydrochloric acid HCl is usually used as the titrant.
If the titrant forms a hydroxide ion, the method is called alkalimetry. The substances used are alkalis such as NaOH, or salts obtained by reacting a strong base with a weak acid such as Na 2 CO 3 .
In this case, color indicators are used. They are weak organic compounds - acids and bases, which differ in the structure and color of protonated and non-protonated forms. The most common indicators used in acid-base titrations are phenolphthalein, a single color indicator (a clear solution turns crimson in an alkaline environment) and a two-color methyl orange indicator (a red substance becomes yellow in an acidic environment).
Their widespread use is associated with high light absorption, due to which their color is clearly visible to the naked eye, and contrast and a narrow color transition region.
Redox Titration
Redox titrimetric analysis is a quantitative analysis method based on changing the ratio of the concentrations of the oxidized and reduced forms: aOx 1 + bRed 2 = aRed 1 + bOx 2.
The method is divided into the following subgroups:
- permanganatometry (titrant - KMnO 4);
- iodometry (I 2);
- dichromatometry (K 2 Cr 2 O 7);
- bromatometry (KBrO 3);
- iodatometry (KIO 3);
- cerimetry (Ce(SO 4) 2);
- vanadatometry (NH 4 VO 3);
- titanometry (TiCl 3);
- chromometry (CrCl 2);
- ascorbinometry (C 6 H 8 OH).
In some cases, the role of an indicator can be played by a reagent participating in the reaction and changing its color with the acquisition of an oxidized or reduced form. But they also use specific indicators, for example:
- when determining iodine, starch is used, which forms a dark blue compound with I 3 - ions;
- in the titration of ferric iron, thiocyanate ions are used, which form bright red complexes with the metal.
In addition, there are special redox indicators - organic compounds that have different colors of oxidized and reduced forms.
Complexometric titration
In short, the titrimetric method of analysis, called complexometric, is based on the interaction of two substances with the formation of a complex: M + L = ML. If mercury salts are used, for example, Hg(NO 3) 2, the method is called mercurymetry, if ethylenediaminetetraacetic acid (EDTA) - complexometry. In particular, with the help of the latter method, a titrimetric method for analyzing water is carried out, namely, its hardness.
In complexometry, transparent metal indicators are used, which acquire color when complexes are formed with metal ions. For example, when titrating ferric salts with EDTA, transparent sulfosalicylic acid is used as an indicator. It turns the solution red when complexed with iron.
However, more often metal indicators have their own color, which changes depending on the concentration of the metal ion. As such indicators, polybasic acids are used, which form fairly stable complexes with metals, which are rapidly destroyed when exposed to EDTA with a contrasting color change.
Precipitation titration
The titrimetric method of analysis, which is based on the reaction of the interaction of two substances with the formation of a solid compound that precipitates (M + X = MX ↓), is precipitation. It is of limited value, since usually the deposition processes proceed non-quantitatively and non-stoichiometrically. But sometimes it is still used and has two subgroups. If the method uses silver salts, for example, AgNO 3, it is called argentometry, if mercury salts, Hg 2 (NO 3) 2, then mercurymetry.
The following methods are used to detect the end point of the titration:
- Mohr's method, in which the indicator is a chromate ion, which forms a red-brick precipitate with silver;
- the Folhard method, based on the titration of a solution of silver ions with potassium thiocyanate in the presence of ferric iron, which forms a red complex with the titrant in an acidic medium;
- the Faience method, which involves titration with adsorption indicators;
- the Gay-Lussac method, in which the CTT is determined by the enlightenment or turbidity of the solution.
The latter method has not been practically used recently.
Titration methods
Titrations are classified not only by the underlying reaction, but also by the way they are performed. On this basis, the following types are distinguished:
- direct;
- reverse;
- substituent titration.
The first case is used only under ideal reaction conditions. The titrant is added directly to the analyte. So with the help of EDTA, magnesium, calcium, copper, iron and about 25 other metals are determined. But in other cases, more complex methods are more often used.
Back titration
It is not always possible to find the ideal response. Most often, it proceeds slowly, or it is difficult to find a way to fix the end point of the titration for it, or volatile compounds are formed among the products, due to which the analyte is partially lost. These shortcomings can be overcome by using the back titration method. To do this, a large amount of titrant is added to the substance to be determined so that the reaction goes to completion, and then it is determined how much of the solution remains unreacted. For this, the titrant residues from the first reaction (T 1) are titrated with another solution (T 2), and its amount is determined by the difference in the products of volumes and concentrations in two reactions: C T1 V T 1 -C T 2 V T 2.
The use of the reverse titrimetric method of analysis underlies the determination of manganese dioxide. Its interaction with ferrous sulfate proceeds very slowly, so the salt is taken in excess and the reaction is accelerated by heating. The unreacted amount of iron ion is titrated with potassium dichromate.
Substituent titration
Substituent titration is used in case of non-stoichiometric or slow reactions. Its essence is that for the substance to be determined, a stoichiometric reaction with an auxiliary compound is selected, after which the interaction product is subjected to titration.
This is exactly what is done when determining dichromate. Potassium iodide is added to it, as a result of which an amount of iodine equivalent to the analyte is released, which is then titrated with sodium thiosulfate.
Thus, titrimetric analysis makes it possible to determine the quantitative content of a wide range of substances. Knowing their properties and features of the course of reactions, it is possible to choose the optimal method and method of titration, which will give a result with a high degree of accuracy.
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Plan
1. Essence of precipitation titration
2. Argentometric titration
3. Thiocyanatometric titration
4. Application of precipitation titration
4.1 Preparation of standardized silver nitrate solution
4.2 Preparation of standardized ammonium thiocyanate solution
4.3 Determination of the chlorine content of a sample according to Volhard
4.4 Determination of the content of sodium trichloroacetate in a technical product
1. The essence of precipitationtitration
The method combines titrimetric determinations based on the reactions of precipitate formation of poorly soluble compounds. For these purposes, only certain reactions that satisfy certain conditions are suitable. The reaction must proceed strictly according to the equation and without side processes. The resulting precipitate should be practically insoluble and precipitate fairly quickly, without the formation of supersaturated solutions. In addition, it is necessary to be able to determine the end point of the titration using an indicator. Finally, the phenomena of adsorption (co-precipitation) must be expressed so weakly during titration that the result of the determination is not distorted.
The names of the individual precipitation methods are derived from the names of the solutions used. The method using a solution of silver nitrate is called argentometry. This method determines the content of C1~ and Br~ ions in neutral or slightly alkaline media. Thiocyanatometry is based on the use of a solution of ammonium thiocyanate NH 4 SCN (or potassium KSCN) and serves to determine traces of C1- and Br ~, but already in strongly alkaline and acidic solutions. It is also used to determine the silver content in ores or alloys.
The expensive argentometric method for determining halogens is gradually being replaced by the mercurometric method. In the latter, a solution of mercury nitrate (I) Hg 2 (NO 3) 2 is used.
Let us consider in more detail argentometric and thiocyanatometric titration.
2. Argentometric titration
The method is based on the reaction of precipitation of C1~ and Br~ ions by silver cations with the formation of sparingly soluble halides:
Cl-+Ag+=AgClb Br^- + Ag+= AgBr
In this case, a solution of silver nitrate is used. If the substance is analyzed for silver content, then a solution of sodium (or potassium) chloride is used. titration solution drug
Titration curves are of great importance for understanding the method of argentometry. As an example, consider the case of titration of 10.00 ml of 0.1 N. sodium chloride solution 0.1 N. a solution of silver nitrite (without taking into account the change in the volume of the solution).
Before the start of titration, the concentration of chloride ions in the solution is equal to the total concentration of sodium chloride, i.e. 0.1 mol / l or \u003d -lg lO-i \u003d 1.
When 9.00 ml of silver nitrate solution is added to the sodium chloride solution being titrated and 90% of the chloride ions are precipitated, their concentration in the solution will decrease by a factor of 10 and become equal to N0 ~ 2 mol/l, and pC1 will be equal to 2. Since value nPAgci= IQ- 10 , the concentration of silver ions in this case will be:
10th / [C1-] \u003d 10-10 / 10-2 \u003d 10-8 M ol / l, OR pAg \u003d - lg \u003d - IglO-s \u003d 8.
Similarly, all other points are calculated to plot the titration curve. At the equivalence point pCl=pAg= = 5 (see table).
Table Change in pC\ and pAg during titration of 10.00 ml of 0.1 N. sodium chloride solution 0.1 N. silver nitrate solution
|
AgNO 3 solution added, |
|||||
|
9.99 10.00 (equiv. point) 10.01 |
yu-4 yu-5 yu-6. |
yu- 6 yu- 5 yu-* |
|||
The jump interval in argentometric titration depends on the concentration of the solutions and on the value of the solubility product of the precipitate. The smaller the PR value of the compound resulting from the titration, the wider the jump interval on the titration curve and the easier it is to fix the end point of the titration using an indicator.
The most common is the argentometric determination of chlorine by the Mohr method. Its essence consists in the direct titration of a liquid with a solution of silver nitrate with an indicator of potassium chromate until a white precipitate turns brown.
Mohr's method indicator - a solution of K2CrO 4 gives a red precipitate of silver chromate Ag 2 CrO 4 with silver nitrate, but the solubility of the precipitate (0.65-10 ~ 4 E / l) is much greater than the solubility of silver chloride (1.25X _X10 ~ 5 E / l ). Therefore, when titrating with a solution of silver nitrate in the presence of potassium chromate, a red precipitate of silver chromate appears only after adding an excess of Ag + ions, when all chloride ions have already precipitated. In this case, a solution of silver nitrate is always added to the analyzed liquid, and not vice versa.
The possibilities of using argentometry are rather limited. It is used only when titrating neutral or slightly alkaline solutions (pH 7 to 10). In an acidic environment, the silver chromate precipitate dissolves.
In highly alkaline solutions, silver nitrate decomposes with the release of insoluble oxide Ag 2 O. The method is also unsuitable for the analysis of solutions containing the NH ^ ion, since in this case an ammonia complex is formed with the Ag + cation + - The analyzed solution should not contain Ba 2 +, Sr 2+ , Pb 2+ , Bi 2+ and other ions that precipitate with potassium chromate.Nevertheless, argentometry is convenient in the analysis of colorless solutions containing C1~ and Br_ ions.
3. Thiocyanatometric titration
Thiocyanatometric titration is based on the precipitation of Ag+ (or Hgl+) ions with thiocyanates:
Ag+ + SCN- = AgSCN|
The determination requires a solution of NH 4 SCN (or KSCN). Determine Ag+ or Hgi + by direct titration with a solution of thiocyanate.
Thiocyanatometric determination of halogens is carried out according to the so-called Volhard method. Its essence can be expressed in diagrams:
CI- + Ag+ (excess) -* AgCI + Ag+ (residue), Ag+ (residue) + SCN~-> AgSCN
In other words, an excess of a titrated solution of silver nitrate is added to a liquid containing C1~. The AgNO 3 residue is then back titrated with a thiocyanate solution and the result is calculated.
Volhard's method indicator is a saturated solution of NH 4 Fe (SO 4) 2 - 12H 2 O. As long as there are Ag + ions in the titrated liquid, the added SCN ~ anions bind to the precipitation of AgSCN, but do not interact with Fe 3 + ions. However, after the equivalence point, the slightest excess of NH 4 SCN (or KSCN) causes the formation of blood-red ions 2 + and +. Thanks to this, it is possible to determine the equivalent point.
Thiocyanatometric definitions are used more often than argentometric ones. The presence of acids does not interfere with the Volhard titration and even contributes to obtaining more accurate results, since the acidic medium inhibits the hydrolysis of the Fe** salt. The method makes it possible to determine the C1~ ion not only in alkalis, but also in acids. The determination does not interfere with the presence of Ba 2 +, Pb 2 +, Bi 3 + and some other ions. However, if the analyzed solution contains oxidizing agents or mercury salts, then the application of the Volhard method becomes impossible: oxidizing agents destroy the SCN- ion, and the mercury cation precipitates it.
The alkaline test solution is neutralized before titration with nitric acid, otherwise the Fe 3 + ions, which are part of the indicator, will precipitate iron (III) hydroxide.
4. Application of precipitation titration
4.1 Preparation of a standardized solution of silver nitrate
The primary standards for standardizing silver nitrate solution are sodium or potassium chlorides. Prepare a standard solution of sodium chloride and approximately 0.02 N. silver nitrate solution, standardize the second solution according to the first.
Preparation of standard sodium chloride solution. A solution of sodium chloride (or potassium chloride) is prepared from chemically pure salt. The equivalent mass of sodium chloride is equal to its molar mass (58.45 g/mol). Theoretically, for the preparation of 0.1 l 0.02 N. solution requires 58.45-0.02-0.1 \u003d 0.1169 g of NaCl.
Take a sample of approximately 0.12 g of sodium chloride on an analytical balance, transfer it to a 100 ml volumetric flask, dissolve, bring the volume to the mark with water, mix well. Calculate the titer and normal concentration of the stock sodium chloride solution.
Preparation of 100 ml of approximately 0.02 N. silver nitrate solution. Silver nitrate is a scarce reagent, and usually its solutions have a concentration not higher than 0.05 N. For this work, 0.02 n is quite suitable. solution.
In argentometric titration, the equivalent mass of AgN0 3 is equal to the molar mass, i.e., 169.9 g / mol. Therefore, 0.1 l 0.02 n. the solution should contain 169.9-0.02-0.1 \u003d 0.3398 g AgNO 3. However, it does not make sense to take exactly such a sample, since commercial silver nitrate always contains impurities. Weigh on technochemical scales approximately 0.34 - 0.35 g of silver nitrate; weigh the solution in a volumetric flask with a capacity of 100 ml, a solution in a small amount of water and bring the volume with water; store the solution in the flask, wrapping it in black paper and pour it into a dark glass bottle. Standardization of the sulfur nitrate solution by sodium chloride. silver and prepare it for titration. Rinse the pipette with sodium chloride solution and transfer 10.00 ml of the solution into a conical flask. Add 2 drops of saturated potassium chromate solution and carefully titrate with silver nitrate solution drop by drop while stirring. Ensure that the mixture turns from yellow to reddish with one excess drop of silver nitrate. After repeating the titration 2-3 times, take the average of the convergent readings and calculate the normal concentration of the silver nitrate solution.
Let us assume that for titration 10.00 ml of 0.02097 N. sodium chloride solution went on average 10.26 ml of silver nitrate solution. Then
A^ AgNOj . 10.26 = 0.02097. 10.00, AT AgNOs = 0.02097-10.00/10.26 = 0.02043
If it is supposed to determine the content of C1 ~ in the sample, then, in addition, the titer of the silver nitrate solution in chlorine is calculated: T, - \u003d 35.46-0. ml of silver nitrate solution corresponds to 0.0007244 g of titrated chlorine.
4.2 Preparation of standardized ammonium thiocyanate solutionI
A solution of NH 4 SCN or KSCN with a precisely known titer cannot be prepared by dissolving a sample, since these salts are very hygroscopic. Therefore, prepare a solution with an approximate normal concentration and set it to a standardized solution of silver nitrate. The indicator is a saturated solution of NH 4 Fe (SO 4) 2 - 12H 2 O. To prevent the hydrolysis of the Fe salt, 6 N is added to the indicator itself and to the analyzed solution before titration. nitric acid.
Preparation of 100 ml of approximately 0.05 N. ammonium thiocyanate solution. The equivalent mass of NH4SCN is equal to its molar mass, i.e. 76.12 g/mol. Therefore, 0.1 l 0.05 n. the solution should contain 76.12.0.05-0.1=0.3806 g of NH 4 SCN.
Take a sample of about 0.3-0.4 g on an analytical balance, transfer it to a 100 ml flask, dissolve, dilute the volume of the solution with water to the mark and mix.
Standardization of ammonium thiocyanate solution by silver nitrate. Prepare a burette for titration with the NH 4 SCN solution. Rinse the pipette with silver nitrate solution and measure 10.00 ml of it into a conical flask. Add 1 ml of NH 4 Fe(SO 4) 2 solution (indicator) and 3 ml. 6 n. nitric acid. Slowly, with continuous agitation, pour the NH 4 SCN solution from the burette. Stop the titration when a brown-pink 2+ color appears, which does not disappear with vigorous shaking.
Repeat the titration 2-3 times, take the average from the convergent readings and calculate the normal concentration of NH 4 SCN.
Let us assume that for titration 10.00 ml of 0.02043 N. silver nitrate solution went on average 4.10 ml of NH 4 SCN solution.
4.3 Definitioncontentchlorine in the sample according to Folgard
Volhard halogens are determined by back titration of the silver nitrate residue with a solution of NH 4 SCN. However, accurate titration is possible here only on the condition that measures are taken to prevent (or slow down) the reaction between silver chloride and an excess of iron thiocyanate:
3AgCI + Fe(SCN) 3 = SAgSCNJ + FeCl 3
in which the color that appears at first gradually disappears. It is best to filter the AgCl precipitate before titrating the excess silver nitrate with NH 4 SCN solution. But sometimes, instead, some organic liquid is added to the solution, it is not mixed with water and, as it were, isolating the ApCl precipitate from excess nitrate.
Definition method. Take a test tube with a solution of the analyte containing sodium chloride. A weighed portion of the substance is dissolved in a volumetric flask with a capacity of 100 ml and the volume of the solution is brought to the mark with water (the concentration of chloride in the solution should be no more than 0.05 N).
Pipette 10.00 ml of the analyzed solution into a conical flask, add 3 ml of 6N. nitric acid and add a known excess of AgNO 3 solution from the burette, for example 18.00 ml. Then filter the precipitate of silver chloride. Titrate the silver nitrate residue with NH 4 SCN as described in the previous paragraph. After repeating the definition 2-3 times, take the average. If the precipitate of silver chloride is filtered, then it should be washed and the washings added to the filtrate.
Let us assume that the sample weight was 0.2254 g. To 10.00 ml of the analyzed solution was added 18.00 ml of 0.02043 N. silver nitrate solution. For the titration of its excess, 5.78 ml * 0.04982 n. NH 4 SCN solution.
First of all, we calculate what volume 0.02043 n. silver nitrate solution corresponds to 5.78 ml of 0.04982 N spent on titration. NH 4 SCN solution:
consequently, 18.00 - 14.09 = 3.91 ml of 0.2043 n went to the precipitation of the C1 ~ ion. silver nitrate solution. From here it is easy to find the normal concentration of sodium chloride solution.
Since the equivalent mass of chlorine is 35.46 g/mol*, the total mass of chlorine in the sample is:
772 \u003d 0.007988-35.46-0.1 \u003d 0.02832 g.
0.2254 g C1 - 100%
x \u003d 0.02832-100 / 0.2254 \u003d 12.56%.:
0.02832 > C1 -- x%
According to the Folgard method, the content of Br~ and I- ions is also determined. At the same time, it is not required to filter out precipitates of silver bromide or iodide. But it must be taken into account that the Fe 3 + ion oxidizes iodides to free iodine. Therefore, the indicator is added after precipitation of all ions of I-silver nitrate.
4.4 Determination of trichl contentOsodium acetate | in a technical preparation (for chlorine)
Technical sodium trichloroacetate (TXA) is a herbicide for controlling grass weeds. It is a white or light brown crystalline substance, highly soluble in water. According to Folgard, the mass fraction of organochloride compounds is first determined, and then after the destruction of chlorine. By difference, find the mass fraction (%) of sodium chlorine trichloroacetate.
Determination of the mass fraction (%) of chlorine inorganic compounds. Accurately weighed 2–2.5 g of the drug is placed in a volumetric flask with a capacity of 250 ml, dissolve, dilute the solution with water to the mark, mix. Pipette 10 ml of the solution into a conical flask and add 5-10 ml of concentrated nitric acid.
Add from the burette 5 or 10 ml of 0.05 N. silver nitrate solution and its excess, titrate with 0.05 N. NH 4 SCN solution in the presence of NH 4 Fe(SO 4) 2 (indicator).
Calculate the mass fraction (%) of chlorine (x) of inorganic compounds using the formula
(V - l / i) 0.001773-250x100
where V is the volume exactly 0.05 n. AgNO 3 solution taken for analysis; Vi -- the volume is exactly 0.05 N. NH 4 SCN solution used for titration of excess AgNO 3 ; t is a sample of sodium trichloroacetate; 0.001773 is the mass of chlorine corresponding to 1 ml of 0.05 N. AgNO solution. Determination of the mass fraction (%) of total chlorine. Take 10 ml of the previously prepared solution into a conical flask, add 10 ml of a solution with a mass fraction of NaOH 30% and 50 ml of water. Connect the flask to a reflux bead condenser and boil the contents for 2 hours. Let the liquid cool, rinse the condenser with water, collecting the wash water in the same flask. Add 20 ml of dilute (1:1) nitric acid to the solution and pour 30 ml of 0.05 N. silver nitrate solution. Titrate excess silver nitrate with 0.05 N. NH 4 SCN solution in the presence of NH 4 Fe(SO 4) 2. Calculate the mass fraction (%) of total chlorine (xi) using the above formula. Find the mass fraction (%) of sodium trichloroacetate in the preparation (х^) using the formula
x2 \u003d (x1 - x) (185.5 / 106.5),
where 185.5 is the molar mass of sodium trichloroacetate; 106.5 is the mass of chlorine contained in the molar mass of sodium trichloroacetate.
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Introduction
Titration is the gradual addition of a titrated solution of a reagent (titrant) to the analyzed solution to determine the equivalence point. The titrimetric method of analysis is based on measuring the volume of a reagent of exactly known concentration, spent on the reaction of interaction with the analyte. The equivalence point is the moment of titration when an equivalent ratio of reactants is reached.
The following requirements apply to the reactions used in quantitative volumetric analysis:
1. The reaction must proceed in accordance with the stoichiometric reaction equation and must be practically irreversible. The result of the reaction should reflect the amount of analyte. The equilibrium constant of the reaction must be sufficiently large.
2. The reaction must proceed without side reactions, otherwise the law of equivalents cannot be applied.
3. The reaction must proceed at a sufficiently high rate, i.e. in 1-3 seconds. This is the main advantage of titrimetric analysis.
4. There must be a way to fix the equivalence point. The end of the reaction should be determined fairly easily and simply.
If a reaction does not satisfy at least one of these requirements, it cannot be used in titrimetric analysis.
1. systems
A distinctive feature of redox reactions is the transfer of electrons between the reacting particles - ions, atoms, molecules and complexes, as a result of which the oxidation state of these particles changes, for example
Since electrons cannot accumulate in a solution, two processes must take place simultaneously - losses and gains, i.e., the process of oxidation of some and reduction of other particles. Thus, any redox reaction can always be represented as two half-reactions:
aOx1 + bRed2 = aRed1 + bOx2
The starting particle and the product of each half-reaction constitute a redox pair or system. In the above half-reactions, Red1 is conjugated to Ox1 and Ox2 is conjugated to Red1.
The potential of any redox system, measured under standard conditions against a hydrogen electrode, is called the standard potential (E0) of this system. The standard potential is considered to be positive if the system acts as an oxidizing agent and an oxidation half-reaction occurs on the hydrogen electrode:
or negative if the system plays the role of a reducing agent, and a reduction half-reaction occurs on the hydrogen electrode:
The absolute value of the standard potential characterizes the "strength" of the oxidizing agent or reducing agent.
The standard potential - a thermodynamic standardized value - is a very important physicochemical and analytical parameter that makes it possible to evaluate the direction of the corresponding reaction and calculate the activities of the reacting particles under equilibrium conditions.
To characterize the redox system under specific conditions, the concept of the real (formal) potential E0 "is used, which corresponds to the potential established at the electrode in this particular solution when the initial concentrations of the oxidized and reduced forms of potential-determining ions are equal to 1 mol / l and the fixed concentration of all other components solution.
From an analytical point of view, real potentials are more valuable than standard potentials, since the true behavior of the system is determined not by the standard, but by the real potential, and it is the latter that makes it possible to predict the occurrence of a redox reaction under specific conditions. The real potential of the system depends on the acidity, the presence of foreign ions in the solution, and can vary over a wide range.
2. Curvestitration
In titrimetric methods, the calculation and plotting of a titration curve makes it possible to assess how successful a titration will be and allow the choice of an indicator. When constructing a redox titration curve, the potential of the system is plotted along the ordinate axis, and the volume of the titrant or the percentage of titration is plotted along the abscissa axis.
2.1 Influenceconditionstitrationonmovecurves
The titration curve is built based on the values of redox potentials, therefore, all factors affecting the potential will affect the shape of the titration curve and the jump on it. These factors include the values of the standard potential of the analyte and titrant systems, the number of electrons involved in half-reactions, the pH of the solution, the presence of complexing reagents or precipitants, and the nature of the acid. The greater the number of electrons involved in the redox reaction, the flatter the curve characterizes this titration. The jump in titration is the greater, the greater the difference in the redox potentials of the oxidizing agent and reducing agent. With a very small difference in their redox potentials, titration is impossible. So the titration of Cl- ions (E = 1.36V) with permanganate (E = 1.51) is practically impossible. It is often necessary to expand the potential interval in which the jump is located if it is small. In such cases, jump control is resorted to.
A decrease in the concentration of one of the components of the redox pair significantly affects the size of the jump (for example, with the help of a complexing reagent). Let us assume that phosphoric acid, fluorides, or oxalates, which form complexes with iron (III) and do not interact with iron (II), are introduced into the solution, while the potential of the Fe3+/Fe2+ pair decreases. If, for example, due to the reaction of competing complexation, the concentration of Fe3+ ions in the solution decreases by a factor of 10,000, the potential jump on the titration curve will begin not at E = 0.95V, but at E = 0.71V. It will end, as before, at E = 1.48V. Thus, the region of the jump on the titration curve will be significantly expanded.
Increasing the temperature, respectively, increases the potential of the system of the titrant and analyte.
So, when choosing the optimal conditions for redox titration, one should first of all take into account their effect on the state of the redox system, and, consequently, on the real redox potential.
2.2 Definitionpointsequivalence
In redox titration methods, as well as in acid-base interaction methods, various ways of indicating the equivalence point are possible.
1. Non-indicator methods are applicable when using colored titrants (solutions of KMnO4, I2), a slight excess of which gives the solution a visually fixable color.
2. Indicator methods can be chemical if chemical compounds are used as indicators that sharply change their color near the equivalence point (within the jump on the titration curve).
Sometimes acid-base indicators are used in redox titration methods: methyl orange, methyl red, Congo red, etc. These indicators at the end point of the titration are irreversibly oxidized by an excess of the oxidizing agent and at the same time change their color.
It is possible to use fluorescent and chemiluminescent indicators when titrating reducing agents with strong oxidizing agents. Fluorescent indicators include many substances (acridine, euchrysine, etc.) that emit in the visible region at certain pH values of the solution after irradiation with ultraviolet radiation. Chemiluminescent indicators are substances (luminol, lucigenin, siloxene, etc.) that emit in the visible region of the spectrum at the end point of the titration due to exothermic chemical processes. Chemiluminescence is observed mainly during oxidation reactions with hydrogen peroxide, hypochlorites and some other oxidizing agents. The advantage of fluorescent and chemiluminescent indicators is that they can be used to titrate not only transparent and colorless, but also cloudy or colored solutions, for which conventional redox indicators are unsuitable for titration.
Indicator methods can also be physicochemical: potentiometric, amperometric, conductometric, etc.
2.3 Redoxindicators
To determine the equivalence point in redoximetry, various indicators are used:
1. Redox indicators (redox indicators) that change color when the redox potential of the system changes.
2. Specific indicators that change their color when an excess of titrant appears or the analyte disappears. Specific indicators are used in some cases. So starch is an indicator for the presence of free iodine, or rather triiodide ions. In the presence of starch, it turns blue at room temperature. The appearance of the blue color of starch is associated with adsorption on amylase, which is part of the starch.
Sometimes ammonium thiocyanate is used as an indicator when titrating iron(III) salts; cations with ions form a red compound. At the equivalence point, all ions are reduced to and the titrated solution turns from red to colorless.
When titrated with a solution of potassium permanganate, the titrant itself plays the role of an indicator. At the slightest excess of KMnO4, the solution turns pink.
Redox indicators are divided into: reversible and irreversible.
Reversible indicators - reversibly change their color when the system potential changes. Irreversible indicators - undergo irreversible oxidation or reduction, as a result of which the color of the indicator changes irreversibly.
Redox indicators exist in two forms, oxidized and reduced, with the color of one form different from the color of the other.
The transition of the indicator from one form to another and the change in its color occurs at a certain potential of the system (transition potential). The indicator potential is determined by the Nernst equation:
When the concentrations of the oxidized and reduced forms of the indicator are equal. At the same time, half of the indicator molecules exist in the oxidized form, and half in the reduced form. The transition interval of the indicator (IT) lies within the concentration ratios of both forms of the indicator from 1/10 to 10/1.
When carrying out a redox titration, it is necessary to select the indicator in such a way that the indicator potential is within the potential jump on the titration curve. Many indicators of redox titration are acidic or basic and can change their behavior depending on the pH of the medium.
One of the most famous and commonly used redox indicators is diphenylamine:
The restored form of the indicator is colorless. Under the action of oxidizing agents, diphenylamine is first irreversibly converted to colorless diphenylbenzidine, which is then reversibly oxidized to blue-violet diphenylbenzidine violet.
The two-color indicator is ferroin, which is a Fe2+ complex with o-phenanthroline
Titration by the indicator method is possible if for a given reaction EMF? 0.4V. At EMF = 0.4-0.2V, instrumental indicators are used.
3. Classificationmethodsredoxtitration
If the redox reaction proceeds non-stoichiometrically or not fast enough, indirect titration methods are used: back titration and substitution titration. For example, in the cerimetric determination of Fe3+, the substitution titration method is used:
Fe3+ +Ti3+ = TiIV + Fe2+ + + CeIV = Fe3+ + Ce3+.3+ does not interfere with titration.
A redox titration is possible if one suitable oxidation state of the analyte is present in the solution. Otherwise, before starting the titration, it is necessary to carry out a preliminary reduction (oxidation) to a suitable oxidation state, as is done, for example, when analyzing a mixture of Fe2+ and Fe3+ by permanganatometry. Preliminary reduction (oxidation) should provide a quantitative transfer of the element being determined to the desired oxidation state.
The reagent introduced for this purpose should be such a compound, from the excess of which, before the start of titration, it is easy to get rid (by boiling, filtering, etc.). In some cases, redoximetry is used to determine compounds that do not change their oxidation state.
So, by substitution titration, calcium, zinc, nickel, cobalt and lead ions are determined in permanganatometry, strong acids in iodometry.
Table 1
Redox Titration Methods
|
Method name |
Standard solution (titrant) |
Equations for half-reactions of the titrant system |
Method features |
||
|
Standard solution - oxidant |
|||||
|
Permanganatometry |
MnO4?+ 8H+ + 5e? = Mn2++ 4H2O MnO4?+ 4H+ + 3e? = MnO2 + 2H2O MnO4? + 2H2O + 3e? = MnO2 + 4OH? |
Non-indicator method, used in a wide pH range |
|||
|
Bromatometry |
BrO3?+ 6H+ + 6e? = Br?+ 3H2O |
The indicator is methyl orange. Wednesday - highly acidic |
|||
|
Cerimetry |
Ce4+ + e? = Ce3+ |
The indicator is ferroin. Wednesday - highly acidic |
|||
|
Chromatometry |
Cr2O72?+ 14H+ + 6e? = 2Cr3++2H2O |
The indicator is diphenylamine. Wednesday? strongly acidic |
|||
|
Nitritometry |
NO2- + 2H+ + e? = NO + H2O |
The external indicator is iodide starch paper. Wednesday? subacid |
|||
|
Iodimetry |
Indicator - starch |
||||
|
Standard solution - reducing agent |
|||||
|
Ascorbino-metry |
С6H6O6 +2H+ +2 e? = С6H8O6 |
Indicators - variamine blue or potassium thiocyanate for the determination of Fe3 + ions. Wednesday - sour |
|||
|
Titanometry |
TiO2+ + 2H+ + e? =Ti3+ + H2O |
The indicator is methylene blue. Wednesday - sour |
|||
|
Iodometry |
S4O62?+ 2e? = 2S2O32? |
The indicator is starch-small. Auxiliary reagent - KI. Medium - slightly acidic or neutral |
4. permanganatometry
Permanganatometry is one of the most commonly used redox titration methods. As a titrant, a solution of potassium permanganate is used, the oxidizing properties of which can be controlled depending on the acidity of the solution.
4.1 Peculiaritiesmethod
The most widely used in analytical practice is the permanganometric method of determination in acidic media: the reduction of MnO4- to Mn2+ is fast and stoichiometric:
A feature of the method is the strong influence of the concentration of hydrogen ions on the standard potential of the MnO4-/Mn2+ system. Sulfuric acid is most often used in titration in strongly acidic media. Hydrochloric and nitric acids should not be used, since competing redox reactions can occur in their presence. The reduction of the permanganate ion in an alkaline medium proceeds sequentially: first to the manganate ion MnO42-, and then to manganese dioxide MnO2:
Quantitatively, the reduction of permanganate in an alkaline medium to manganate proceeds in the presence of a barium salt. Ba(MnO4)2 is soluble in water, while BaMnO4 is insoluble; therefore, further reduction of MnVI from the precipitate does not occur.
Permanganometrically in an alkaline medium, as a rule, organic compounds are determined: formate, formaldehyde, formic, cinnamic, tartaric, citric acids, hydrazine, acetone, etc.
The indicator of the end of the titration is the pale pink color of excess KMnO4 titrant (one drop of 0.004 M titrant solution gives a noticeable color to 100 ml of solution). Therefore, if the titrated solution is colorless, the achievement of the equivalence point can be judged by the appearance of a pale pink color in excess of the KMnO4 titrant in the direct titration or by the disappearance of the color in the reverse titration. When analyzing colored solutions, it is recommended to use the ferroin indicator.
The advantages of the permanganometric method include:
1. Possibility of titration with KMnO4 solution in any medium (acidic, neutral, alkaline).
2. The applicability of a solution of potassium permanganate in an acidic medium for the determination of many substances that do not interact with weaker oxidizing agents.
Along with the listed advantages, the permanganatometry method has a number of disadvantages:
1. KMnO4 titrant is prepared as a secondary standard, since the initial reagent, potassium permanganate, is difficult to obtain in a chemically pure state.
2. Reactions involving MnO4- are possible under strictly defined conditions (pH, temperature, etc.).
4.2 Applicationmethod
1. Definition of reducing agents. If the redox reaction between the determined reducing agent and MnO4- proceeds quickly, then the titration is carried out in a direct way. This is how oxalates, nitrites, hydrogen peroxide, iron (II), ferrocyanides, arsenic acid, etc. are determined:
Н2О2 + 2MnO4- + 6Н+ = 5О2 + 2Мn2+ + 8Н2О
54- + MnO4- + 8H+ = 53- + 2Mn2+ + 4H2O
AsIII + 2MnO4- + 16H+ = 5AsV + 2Mn2+ + 8H2O
5Fe2+ + MnO4- +8H+ = 5Fe3+ + 2Mn2+ + 4H2O
2. Determination of oxidizing agents. Add an excess of the reducing standard solution and then titrate its residue with KMnO4 solution (back titration method). For example, chromates, persulfates, chlorites, chlorates and other oxidizing agents can be determined by the permanganometric method, first acting with an excess of a Fe2+ standard solution, and then titrating the unreacted amount of Fe2+ with a KMnO4 solution:
Cr2O72- + 6Fe2+ + 14H+ = 2Cr3+ + 6Fe3+ + 7H2O + (Fe2+) - excess-
Fe2+ + MnO4- + 8H+ = 5Fe3+ + Mn2+ + 4H2O - residue
3. The determination of substances that do not have redox properties is carried out indirectly, for example, by substitution titration. To do this, the determined component is converted into the form of a compound with reducing or oxidizing properties, and then titration is carried out. For example, calcium, zinc, cadmium, nickel, cobalt ions precipitate in the form of sparingly soluble oxalates:
M2+ + C2O4- = vMC2O4
The precipitate is separated from the solution, washed and dissolved in H2SO4:
MC2O4 + H2SO4 = H2C2O4 + MSO4
Then H2C2O4 (substituent) is titrated with KMnO4 solution:
2MnO4- + 5C2O42- + 16H+ = 2Mn2+ + 10CO2 + 8H2O
4. Determination of organic compounds. A distinctive feature of the reactions of organic compounds with MnO4- is their low rate. The determination is possible if an indirect method is used: the analyzed compound is pre-treated with an excess of a strongly alkaline permangant solution and the reaction is allowed to proceed for the required period of time. The permanganate residue is titrated with sodium oxalate solution:
C3H5(OH)3 + 14MnO4- + 20OH- = 3CO32- + 14MnO42- + 14H2O +
(MnO4-), excess residue
2MnO4- + 5C2O42- + 16H+ = 2Mn2+ + 10CO2 + 8H2O residue
redox titrimetric
5. essenceAndclassificationprecipitationmethods
Precipitation titration methods are titrimetric analysis methods that use titrants that form precipitates with analytes.
Requirements for reactions and analytes:
1. The substance to be determined must be highly soluble in water and must form ions that would be active in precipitation reactions.
2. The precipitate obtained in the reaction should be practically insoluble (PR< 10 -8 ? - 10 , S < 10 -5).
3. The titration results should not be distorted by adsorption phenomena (co-precipitation).
4. Precipitation should occur quickly enough (i.e. no supersaturated solutions should form).
5. It should be possible to fix the equivalence point.
Classification of precipitation titration methods depending on the titrants used:
Argentometry (titrant AgNO 3);
Mercurometry (titrant Hg 2 (NO 3) 2);
Thiocyanatometry (NH 4 SCN titrant);
Sulfatometry (titrants H 2 SO 4, BaCl 2);
Chromatometry (titrant K 2 CrO 4);
Hexacyanoferratometry (titrant K 4 ).
6. CurvestitrationAndtheiranalysis
The construction of titration curves is carried out on the basis of calculations according to the rule of the product of solubility and, respectively.
The titration curve is built in coordinates that show the change in the concentration of the ion being determined depending on the volume of the added titrant.
The larger the jump in the titration on the curve, the more opportunities for choosing the appropriate indicator.
Factors that affect the magnitude of the jump on the precipitation titration curves:
1. Concentration of titrant and target ion solutions The higher the concentration, the greater the jump on the titration curve.
2. The solubility of the precipitate that is formed during the titration (the lower the solubility, the greater the titration jump).
Dependence of the titration jump on the solubility of a sparingly soluble electrolyte.
3. Temperature
The higher the temperature, the greater the solubility of the precipitate and the smaller the jump in the titration curve. The titration is carried out at room temperature.
4. Ionic strength of the solution
The influence is relatively minor, since the ionic strength of the solution, compared to other factors, does not change the solubility of the precipitate so much; however, the higher the ionic strength of the solution, the higher the solubility and the smaller the titration jump.
7. Argentometry
Argentometry is a method of precipitation titration, which is based on the formation of hardly soluble salts of Argentum:
X - + Ag + \u003d AgX,
where X - = Cl - , Br - , I - , CN - , SCN - etc.
Titrant: AgNO 3 - secondary standard solution.
Standardization: for the primary standard solution of sodium chloride NaCl:
The indicator for standardization is 5% potassium chromate K 2 CrO 4 . The titration is carried out until a brown-red precipitate of argentum chromate appears:
Depending on the method of titration and the indicator used, argentometry methods are classified into:
Non-indicator: - Gay-Lussac method (equal haze method)
Method to the point of enlightenment
Indicator: - Mohr's method
Faience-Fischer-Khodakov method
Folgard method
More method
Titrant: AgNO 3 - sec. std. solution.
AgNO 3 + NaCl \u003d AgCl? + NaNO 3
The indicator is 5% potassium chromate K 2 CrO 4 (until brown-red argentum chromate appears):
2AgNO 3 + K 2 CrО 4 = Ag 2 CrО 4 ?+ 2KNO 3
Determined substances: chlorides Cl - , bromides Br - .
Medium: pH ~ 6.5-10.3.
Application: quantitative determination of sodium chloride, potassium chloride, sodium bromide, potassium bromide in the substance of medicinal substances.
Application restrictions:
1. Do not titrate acidic solutions:
2CrО 4 2- + 2H + = Cr 2 O 7 2- + H 2 O
2. It is impossible to titrate in the presence of ammonia and other ions, molecules that can act as ligands with respect to Argentum ions in complex formation reactions.
3. It is impossible to titrate in the presence of many cations (Ba 2+ , Pb 2+ , etc.), which form colored precipitates with chromate ions CrO 4 2- .
4. Do not titrate in the presence of reducing agents that react with CrO 4 2- chromate ions, converting them into Cr 3+ ions.
5. It is impossible to titrate in the presence of many anions (PO 4 3-, AsO 4 3-, AsO 3 3-, S 2-, etc.), which form colored precipitates of argentum with argentum ions.
Faience-Fischer-Khodakov method
Titrant: AgNO 3 - sec. std. solution
Standardization for the first. std. with a solution of sodium chloride NaCl by pipetting:
AgNO 3 + NaCl \u003d AgCl? + NaNO 3
The indicator for standardization is a 5% solution of potassium chromate K 2 CrO 4 (until a brown-red precipitate of argentum chromate appears):
2AgNO 3 + K 2 CrО 4 = Ag 2 CrО 4 ?+ 2KNO 3
Medium: pH ~ 6.5-10.3 when determining chlorides and pH ~ 2.0-10.3 when determining bromides and iodides.
Method indicators:
Fluorescein in the determination of chlorides;
Eosin in the determination of bromides and iodides.
Mechanism of action of indicators: adsorption. Adsorption indicators are indicators whose adsorption or desorption by a precipitate is accompanied by a color change in the T.E. or near it.
AgNO 3 + NaCl \u003d AgCl? + NaNO 3
HInd x H + + Ind - .
Titration conditions:
1. Acidity of solutions
2. Concentration of reacting solutions
3. Accounting for the adsorption capacity of indicators and ions present in the solution.
4. Titration near te should be done slowly
5. Titration with adsorption indicators is carried out in scattered light.
Application: quantitative determination of chlorides, bromides, iodides, thiocyanates, cyanides.
Folgard method
Titrants: AgNO 3 , ammonium or potassium thiocyanate NH 4 SCN, KSCN - secondary standard solutions.
Standardization of AgNO 3 for the first. std. NaCl solution by pipetting:
AgNO 3 + NaCl \u003d AgCl? + NaNO 3
The indicator for the standardization of AgNO 3 is a 5% solution of potassium chromate K 2 CrO 4 (until a brown-red precipitate of argentum chromate appears):
2AgNO 3 + K 2 CrО 4 = Ag 2 CrО 4 + 2KNO 3
Standardization of NH 4 SCN, KSCN for AgNO 3 standard solution:
AgNO 3 + NH 4 SCN = AgSCN + NH 4 NO 3
The indicator for the standardization of ammonium or potassium thiocyanate are ferum salts (ІІІ) (for example, NH 4 Fe (SO 4) 2 12H 2 O in the presence of nitrate acid):
Fe 3+ + SCN - \u003d 2+
Titrate until a faint pink color appears.
Wednesday: nitrate.
Method indicators: ferum salts (ІІІ) NH 4 Fe(SO 4) 2 ?12H 2 O in the presence of nitrate acid.
Determined substances: halide ions, cyanides, thiocyanates, sulfides, carbonates, chromates, oxalates, arsenates, etc.
Hal - + Ag + (excess) = AgHal
Ag + (residue) + SCN - = AgSCN,
and after the equivalence point:
Fe 3+ + SCN - \u003d 2+
(pink-red coloration)
When determining iodides, the indicator is added at the end of the titration to avoid a parallel reaction:
2Fe 3+ + 2I - = 2Fe 2+ + I 2
Benefits of the Volhard method - titration capability:
In very acidic solutions;
In the presence of many cations that interfered with the determination by the Mohr method (barium, plumbum, etc. cations, which formed colored precipitates of chromates).
8. Mercurometry
Mercurometry is a method of precipitation titration, which is based on the use of reactions of formation of salts of mercury (I) Hg 2 2+, hardly soluble by precipitation:
2Cl - + Hg 2 2+ \u003d Hg 2 Cl 2 Ї PR \u003d 1.3 × 10 -18
2I - + Hg 2 2+ \u003d Hg 2 I 2 Ї PR \u003d 4.5 H10 -29
Titrant: sec. std. Hg 2 (NO 3) 2 solution.
Standardization: for standard NaCl solution:
Hg 2 (NO 3) 2 + 2NaCl \u003d Hg 2 Cl 2 Ї + 2NaNO 3
Indicators: 1) solution of ferum (ІІІ) thiocyanate (from red to discoloration)
2Fe(SCN) 2+ + Hg 2 2+ = Hg 2 (SCN) 2 Ї + 2Fe 3+ ;
1-2% alcohol solution of diphenylcarbazone (until a blue color appears).
To account for the volume of titrant that was used to titrate the indicator, titrate a “blind sample”:
2) The indicator is added before the end of the titration, since if it is added first, it may be long before the t.e. diphenylcarbazide of mercury (II) is formed and gives a blue color sooner than the halide is titrated.
Determined substances: chlorides and iodides.
Environment: very acidic (can be up to 5 mol/l H + ions).
Disadvantage: Mercury (I) salts are very toxic.
9. Sulfametry
Sulfatometry is a method of precipitation titration, which is based on the use of reactions of the formation of sparingly soluble salts - sulfates.
Sometimes barymetry is distinguished - a method of precipitation titration, which is based on the use of reactions for the formation of insoluble barium salts.
The method is based on the reaction of barium sulfate precipitation formation:
Ba 2+ + SO 4 2- \u003d BaSO 4 Ї
def. titrant substance
Titrants: sec. std. solutions H 2 SO 4 , Ba(NO 3) 2 , BaCl 2 .
Standardization: H 2 SO 4 solution with Na 2 B 4 O 7 or Na 2 CO 3 with methyl orange; Ba (NO 3) 2 and BaCl 2 for H 2 SO 4 with nitrochromazo or orthonyl A.
Indicators: metallochromic indicators are used (they change their color in the presence of metal ions) - nitrchromazo (orthanilic C), ortanyl A. These indicators are pink in solution, and purple in the presence of barium cations.
Determined substances in direct titration:
sulfate acid - the content of barium;
barium chloride or barium nitrate - sulfate content.
Conclusion
Of the titrimetric methods of analysis, redox titration is widely used, the limits of application of this method are wider than those of acid-base or complexometric methods. Due to the wide variety of redox reactions, this method makes it possible to determine a large number of a wide variety of substances, including those that do not directly exhibit redox properties.
Permanganatometry is used to determine the overall oxidizability of water and soil. At the same time, all organic components (including humic acids of soils and natural waters) react with MnO4 - ion in an acidic medium. The number of millimol equivalents of KMnO4 used for titration is a characteristic of oxidizability (by permanganate).
Permanganatometry is also used for the analysis of easily oxidized organic compounds (aldehydes, ketones, alcohols, carboxylic acids: oxalic, tartaric, citric, malic, and also hydrazo groups). In the food industry, permanganatometry can be used to determine the sugar content in food products and raw materials, the nitrite content in sausages.
In the metallurgical industry, the iron content in salts, alloys, metals, ores and silicates is determined by permanganatometry.
Listliterature
1. Analytical chemistry. Chemical methods of analysis / ed. O.M. Petrukhin. Moscow: Chemistry, 1992, 400 p.
2. Vasiliev V.P. Analytical chemistry. At 2 pm Part 1. Gravimetric and titrimetric methods of analysis. M.: Higher school, 1989, 320 p.
3. Fundamentals of analytical chemistry. In 2 books. Book. 2. Methods of chemical analysis / ed. Yu.A. Zolotova. Moscow: Higher school, 2000, 494 p.
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Methods of titrimetric analysis are subdivided according to the titration option and according to those chemical reactions that are selected to determine the substance (component). In modern chemistry, quantitative and
Types of classification
Methods of titrimetric analysis are selected for a specific chemical reaction. Depending on the type of interaction, there is a division of titrimetric determination into separate types.
Analysis methods:
- Redox titration; the method is based on a change in the oxidation state of the elements in the substance.
- Complexation is a complex chemical reaction.
- Acid-base titration involves the complete neutralization of interacting substances.
Neutralization
Acid-base titration allows you to determine the amount of inorganic acids (alkalimetry), as well as calculate the bases (acidimetry) in the desired solution. This method is used to determine substances that react with salts. When using organic solvents (acetone, alcohol), it became possible to determine a larger number of substances.
complex formation
What is the essence of the method of titrimetric analysis? It is supposed to determine substances by precipitation of the desired ion as a poorly soluble compound or its binding into a poorly dissociated complex.
redoximetry
Redox titration is based on reduction and oxidation reactions. Depending on the titrated reagent solution used in analytical chemistry, there are:
- permanganatometry, which is based on the use of potassium permanganate;
- iodometry, which is based on oxidation with iodine, as well as reduction with iodide ions;
- bichromatometry, which uses oxidation with potassium dichromate;
- bromatometry based on oxidation with potassium bromate.
Redox methods of titrimetric analysis include such processes as cerimetry, titanometry, vanadometry. They involve the oxidation or reduction of ions of the corresponding metal.
According to the method of titration
There is a classification of titrimetric analysis methods depending on the titration method. In the direct variant, the ion to be determined is titrated with the selected reagent solution. The titration process in the substitution method is based on the determination of the equivalence point in the presence of unstable chemical compounds. Residue titration (reverse method) is used when it is difficult to find an indicator, as well as when the chemical interaction is slow. For example, when determining calcium carbonate, a sample of a substance is treated with an excess amount of titrated
The value of analysis
All methods of titrimetric analysis involve:
- accurate determination of the volume of one or each of the reacting chemicals;
- the presence of a titrated solution, due to which the titration procedure is performed;
- revealing the results of the analysis.
Titration of solutions is the basis of analytical chemistry, so it is important to consider the main operations performed during the experiment. This section is closely related to daily practice. Having no idea about the presence of main components and impurities in a raw material or product, it is difficult to plan a technological chain in the pharmaceutical, chemical, and metallurgical industries. Fundamentals of analytical chemistry are applied to solve complex economic issues.
Research methods in analytical chemistry
This branch of chemistry is the science of determining a component or substance. Fundamentals of titrimetric analysis - methods used to conduct an experiment. With their help, the researcher draws a conclusion about the composition of the substance, the quantitative content of individual parts in it. It is also possible in the course of analytical analysis to identify the degree of oxidation in which the constituent part of the substance under study is located. When classifying chemistry, it is taken into account what kind of action is supposed to be performed. To measure the mass of the resulting sediment, a gravimetric research method is used. When analyzing the intensity of a solution, photometric analysis is necessary. The magnitude of the EMF by potentiometry determines the constituent components of the study drug. The titration curves clearly demonstrate the experiment being carried out.
Division of Analytical Methods
If necessary, in analytical chemistry, physicochemical, classical (chemical), as well as physical methods are used. Under chemical methods, it is customary to understand titrimetric and gravimetric analysis. Both methods are classical, proven, and widely used in analytical chemistry. involves determining the mass of the desired substance or its constituent components, which are isolated in a pure state, as well as in the form of insoluble compounds. The volumetric (titrimetric) method of analysis is based on determining the volume of the reagent used in a chemical reaction, taken in a known concentration. There is a division of chemical and physical methods into separate groups:
- optical (spectral);
- electrochemical;
- radiometric;
- chromatographic;
- mass spectrometric.
Specifics of titrimetric research
This branch of analytical chemistry involves measuring the amount of a reagent that is required to carry out a complete chemical reaction with a known amount of the desired substance. The essence of the technique is that a reagent with a known concentration is added dropwise to a solution of the test substance. Its addition continues until its amount is equivalent to the amount of the analyte reacting with it. This method allows high-speed quantitative calculations in analytical chemistry.
The French scientist Gay-Lusac is considered as the founder of the technique. The substance or element determined in a given sample is called the substance being determined. Among them may be ions, atoms, functional groups, associated free radicals. Reagents are called gaseous, liquid, which react with a certain chemical substance. The process of titration consists in adding one solution to another while constantly mixing. A prerequisite for the successful implementation of the titration process is the use of a solution with a specified concentration (titrant). For calculations, that is, the number of gram equivalents of a substance that is contained in 1 liter of solution is used. Titration curves are built after the calculations.
Chemical compounds or elements interact with each other in well-defined weight amounts corresponding to their gram equivalents.
Options for preparing a titrated solution by weighing the initial substance
As the first method of preparing a solution with a given concentration (a certain titer), one can consider dissolving a sample of the exact mass in water or another solvent, as well as diluting the prepared solution to the required volume. The titer of the resulting reagent can be determined from the known mass of the pure compound and from the volume of the prepared solution. This technique is used to prepare titrated solutions of those chemicals that can be obtained in pure form, the composition of which does not change during long-term storage. For weighing the substances used, bottles with closed lids are used. This method of preparing solutions is not suitable for substances with increased hygroscopicity, as well as for compounds that enter into chemical interaction with carbon monoxide (4).
The second technology for the preparation of titrated solutions is used at specialized chemical enterprises, in special laboratories. It is based on the use of solid pure compounds weighed in exact quantities, as well as on the use of solutions with a certain normality. Substances are placed in glass ampoules, then they are sealed. Those substances that are inside the glass ampoules are called fixanals. During the direct experiment, the ampoule with the reagent is broken over a funnel, which has a punching device. Next, the entire component is transferred to a volumetric flask, then by adding water, the required volume of the working solution is obtained.
For titration, a certain algorithm of actions is also used. The burette is filled with ready-made working solution to the zero mark so that there are no air bubbles in its lower part. Next, the analyzed solution is measured with a pipette, then it is placed in a conical flask. Add a few drops of indicator to it. Gradually, the working solution is added dropwise to the finished solution from the burette, and the color change is monitored. When a stable color appears, which does not disappear after 5-10 seconds, the completion of the titration process is judged. Then proceed to the calculations, the calculation of the volume of the spent solution with a given concentration, draw conclusions from the experiment.
Conclusion
Titrimetric analysis allows you to determine the quantitative and qualitative composition of the analyte. This method of analytical chemistry is necessary for various industries, it is used in medicine, pharmaceuticals. When choosing a working solution, its chemical properties must be taken into account, as well as the ability to form insoluble compounds with the substance under study.
