Chromium, nickel And molybdenum are the most important alloying elements steels. They are used in various combinations and different categories of alloy steels are obtained: chromium, chromium-nickel, chromium-nickel-molybdenum and similar alloy steels.
The influence of chromium on the properties of steels
The tendency of chromium to form carbides is average among otherscarbide-forming alloying elements. At a low Cr/C ratio of chromium content relative to iron, only cementite of the (Fe,Cr) type is formed. 3 C. With an increase in the ratio of chromium to carbon content in Cr/C steel, chromium carbides of the form (Cr,Fe) appear 7 C 3 or (Cr,Fe) 2 3C 6 or both. Chromium increases the ability of steels to be thermally hardened, their resistance to corrosion and oxidation, provides increased strength at elevated temperatures, and also increases the abrasive wear resistance of high-carbon steels.
Chromium carbides are also wear-resistant. They are the ones who provide durability to steel blades - it’s not for nothing that knife blades are made from chrome steels. Complex chromium-iron carbides enter the solid solution of austenite very slowly - therefore, when heating such steels for hardening, a longer exposure at the heating temperature is required. Chromium is rightfully considered the most important alloying element in steels. The addition of chromium to steels causes impurities such as phosphorus, tin, antimony and arsenic to segregate to the grain boundaries, which can cause temper brittleness in steels.
The influence of nickel on the properties of steels
Nickel does not form carbides in steels. In steels it is an element that contributes to the formation and preservation austenite . Nickel increases the hardening of steels. In combination with chromium and molybdenum, nickel further increases the thermal hardening ability of steels and helps to increase the toughness and fatigue strength of steels. Dissolving into ferrite Nickel increases its viscosity. Nickel increases the corrosion resistance of chromium-nickel austenitic steels in non-oxidizing acid solutions.
The influence of molybdenum on the properties of steels
Molybdenum readily forms carbides in steels. It dissolves only slightly in cementite. Molybdenum forms molybdenum carbides once the carbon content of the steel becomes high enough. Molybdenum is capable of providing additional thermal hardening during tempering of hardened steels. It increases the creep resistance of low-alloy steels at high temperatures.
Molybdenum additives help refine the grain of steels, increase the hardening of steels by heat treatment, and increase the fatigue strength of steels. Alloy steels containing 0.20-0.40% molybdenum or the same amount of vanadium slow down the occurrence of temper brittleness, but do not completely eliminate it. Molybdenum improves the corrosion resistance of steels and is therefore widely used in high-alloy ferritic stainless steels and in chromium-nickel austenitic stainless steels. High molybdenum content reduces the susceptibility of stainless steel to pitting corrosion. Molybdenum has a very strong solid solution strengthening effect on austenitic steels that are used at elevated temperatures.
The sixth group of elements of the periodic table includes chromium 24 Cr, molybdenum 42 Mo, tungsten 74 W and the radioactive metal seaborgium 106 Sg. Chromium occurs in nature in the form of four stable isotopes, of which 52 Cr predominates (83.8%). Natural molybdenum and tungsten are a complex mixture of seven and five isotopes, respectively, most of which occur in comparable quantities in the earth's crust. Thus, the dominant nuclide molybdenum-98 makes up only 24% of the total number of molybdenum atoms.
In 1778, the Swedish chemist K. Scheele obtained the oxide MoO 3 from the molybdenite mineral MoS 2, during the reduction of which with coal four years later R. Hjelm isolated a new element - molybdenum. Its name comes from the Greek “molybdos” - lead. The confusion stems from the fact that soft materials such as graphite, lead and molybdenite MoS 2 were previously used as writing leads. This is associated with the name of graphite “black lead” - black lead.
In 1781, K. Scheele and T. Wergmann isolated the oxide of a new element from the mineral CaWO 4 (scheelite). Two years later, Spanish chemists - brothers J. and F. d'Eloire - showed that the same element is an integral part of the mineral (Fe, Mn)WO 4 - wolframite. Its name comes from the German Wolf Rahm - wolf foam. When smelting tin, a large amount of metal was lost, turning into slag. This was caused by the fact that wolframite, accompanying cassiterite, interfered with the reduction of tin. Medieval metallurgists said that wolframite devours tin like a wolf eats a sheep. By reducing wolframite with coal, they obtained a new metal called tungsten.
In 1797, the French chemist L. Vauquelin studied the properties of the orange-red mineral crocoite PbCrO 4, sent to him from Siberia by the Russian geologist M. Pallas. When the mineral was boiled with potash, it obtained an orange-red solution.
3PbCrO 4 +3K 2 CO 3 + H 2 O = Pb 3 (CO 3) 2 (OH) 2 ¯ + 3K 2 CrO 4, + CO 2,
from which he isolated potassium chromate, then chromic anhydride and, finally, by reducing CrO 3 with coal - the new metal chromium. The name of this element comes from the Greek “chroma” - color and is associated with the variety of colors of its compounds. The mineral chromite, the most important modern raw material for chromium production, was found in the Urals in 1798.
Seaborgium was first obtained in 1974 by American scientists under the leadership of Albert Ghiorso in Berkeley (USA). The synthesis of an element in the amount of several atoms was carried out according to the reactions:
18 O + 249 Cf 263 106 Sg + 4 1 n,
248 Cf + 22 Ne 266 106 Sg + 4 1 n
The half-life of the longest-lived isotope 266 Sg is 27.3 s. The element is named after the American physicist and chemist Glenn Seaborg.
Following the general tendencies of filling the d-sublevel when moving through the period for elements of the sixth group, it would be necessary to assume the configuration of the valence electrons in the ground state (n-1)d 4 ns 2, which, however, is realized only in the case of tungsten. In chromium and molybdenum atoms, the energy gain caused by the stabilization of a half-filled sublevel and the complete absence of the destabilizing contribution of pairing energy turns out to be higher than the energy that must be spent on the transition of one of the s-electrons to the d-sublevel. This leads to a “jump” of the electron (see section 1.1) and the electron configuration (n-1)d 5 ns 1 for chromium and molybdenum atoms. The radii of atoms and ions (Table 5.1) increase during the transition from chromium to molybdenum and practically do not change upon further transition to tungsten; their close values for molybdenum and tungsten are a consequence of lanthanide compression. At the same time, despite this, the difference in properties between these two elements turns out to be much more noticeable than between the 4d and 5d elements of the fourth and fifth groups (zirconium and hafnium, niobium and tantalum): as you move away from the third group of influence lanthanide compression on the properties of atoms weakens. The values of the first ionization energies during the transition from chromium to tungsten increase, as for elements of the 5th group.
Table 5.1. Some properties of elements of the 6th group
| Properties | 24Cr | 42Mo | 74W |
| Number of stable isotopes | |||
| Atomic mass | 51.9961 | 95.94 | 183.84 |
| Electronic configuration | 3d 5 4s 1 | 4d 5 5s 1 | 4f 14 5d 4 6s 2 |
| Atomic radius *, (nm) | 0.128 | 0.139 | 0.139 |
| Ionization energies, kJ/mol: | |||
| First (I 1) | 653,20 | 684,08 | 769,95 |
| Second (I 2) | 1592,0 | 1563,1 | 1707,8 |
| Third (I 3) | 2991,0 | 2614,7 | |
| Fourth (I 4) | 4737,4 | 4476,9 | |
| Fifth (I 5) | 6705,7 | 5258,4 | |
| Sixth (I 6) | 8741,5 | 6638,2 | |
| Ionic radii**, nm: | |||
| E(VI) | 0.044 | 0.059 | 0.060 |
| E (V) | 0.049 | 0.061 | 0.062 |
| E (IV) | 0.055 | 0.065 | 0.066 |
| E (III) | 0.061 | 0.069 | – |
| E (II) *** | 0.073 (ns), 0.080 (s) | – | – |
| Electronegativity according to Pauling | 1.66 | 2.16 | 2.36 |
| Electronegativity according to Allred-Rochow | 1.56 | 1.30 | 1.40 |
| Oxidation states **** | (–4), (–2), (–1), (+2), +3, (+4), (+5), +6 | (–2), (–1), (+2), +3, (+4), (+5), +6 | (–2), (–1), (+2), (+3), (+4), +5, +6 |
* For coordination number CN = 12.
** For coordination number CN = 6.
*** The radius is indicated for low- (ns) and high-spin (hs) states.
**** Unstable oxidation states are indicated in parentheses.
In various compounds, the elements chromium, molybdenum and tungsten exhibit oxidation states from –4 to +6 (Table 5.1). As in other groups of transition metals, the stability of compounds with the highest oxidation state, as well as coordination numbers, increase from chromium to tungsten. Chromium, like other d-metals, in lower oxidation states has a coordination number of 6, for example, 3+, –. As the degree of oxidation increases, the ionic radius of the metal inevitably decreases, which leads to a decrease in its coordination number. That is why, in higher oxidation states in oxygen compounds, chromium has a tetrahedral environment, realized, for example, in chromates and dichromates, regardless of the acidity of the medium. The process of polycondensation of chromate ions, successively leading to dichromates, trichromates, tetrachromates and, finally, to hydrated chromic anhydride, is only a sequential increase in the chain of CrO 4 tetrahedra connected by common vertices. For molybdenum and tungsten, tetrahedral anions, on the contrary, are stable only in an alkaline medium, and upon acidification they increase the coordination number to six. The resulting metal-oxygen octahedra MO 6 condense through common edges into complex isopolyanions that have no analogues in chromium chemistry. As the degree of oxidation increases, the acidic and oxidizing properties increase. Thus, Cr(OH)2 hydroxide exhibits only basic properties, Cr(OH)3 exhibits amphoteric properties, and H2CrO4 exhibits acidic properties.
Chromium(II) compounds are strong reducing agents that are instantly oxidized by atmospheric oxygen (Fig. 5.1. Frost diagram for chromium, molybdenum and tungsten). Their reducing activity (E o (Cr 3+ /Cr 2+) = –0.41 V) is comparable to similar vanadium compounds.
Table 5.2. Stereochemistry of some Cr, Mo and W compounds
| Oxidation state | Coordination numbers | Stereometry | Cr | Mo, W |
| -4 (d 10) | Tetrahedron | Na 4 | ||
| -2 (d8) | Trigonal bipyramid | Na 2 | Na 2 | |
| -1 (d7) | Octahedron | Na 2 | Na 2 | |
| 0 (d6) | Octahedron | [Cr(CO) 6 ] | ||
| +2 (d 4) | flat square | - | ||
| square pyramid | - | 4 - | ||
| Octahedron | K 4 CrF 2 , CrS | Me 2 W(PMe 3) 4 | ||
| +3(d 3) | Tetrahedron | - | 2– | |
| Octahedron | 3+ | 3 - | ||
| +4(d 7) | Octahedron | K2 | 2 - | |
| Dodecahedron | - | 4 - | ||
| +5(d 1) | Octahedron | K2 | - | |
| +6(d o) | Tetrahedron | CrO 4 2 - | MO 4 2 - | |
| Octahedron | CrF 6 | in isopoly compounds | ||
| ? | - | 2 - |
The most characteristic oxidation state for chromium is +3 (Fig. 5.1). The high stability of Cr(III) compounds is associated with both thermodynamic factors - the symmetric d 3 configuration, which provides high strength of the Cr(III) - ligand bond due to the high energy of stabilization by the crystal field (ESF) in the octahedral field () of the ligands, and with the kinetic inertness of octahedral chromium(III) cations. Unlike molybdenum and tungsten compounds in higher oxidation states, chromium(VI) compounds are strong oxidizing agents E 0 ( /Cr 3+) = 1.33 V. Chromate ions can be reduced by hydrogen at the time of separation in hydrochloric acid solution to Cr 2+ ions , molybdates - to molybdenum(III) compounds, and tungstates - to tungsten(V) compounds.
Compounds of molybdenum and tungsten in lower oxidation states contain metal-metal bonds, that is, they are clusters. The best known are octahedral clusters. For example, molybdenum dichloride contains Mo 6 Cl 8: Cl 4 groups. The ligands that make up the cluster ion are bound much more tightly than the external ones, therefore, when exposed to an alcohol solution of silver nitrate, it is possible to precipitate only one third of all chlorine atoms. Metal-to-metal bonds are also found in some chromium(II) compounds, such as carboxylates.
Despite the close stoichiometry of the compounds of the elements of the sixth group of chromium and the sulfur group, the atoms of which contain the same number of valence electrons, only a distant similarity is observed between them. For example, the sulfate ion has the same dimensions as the chromate and can isomorphically replace it in some salts. Chromium(VI) oxochloride is similar in its ability to hydrolyze to sulfuryl chloride. At the same time, sulfate ions in aqueous solutions practically do not exhibit oxidizing properties, and selenates and tellurates do not have the ability to form isopolycompounds, although individual atoms of these elements may be included in their composition.
Compared to d-elements of the fourth and fifth groups, chromium, molybdenum and tungsten cations are characterized by a much higher Pearson “softness”, which increases down the group. The consequence of this is the rich chemistry of sulfide compounds, especially developed in molybdenum and tungsten. Even chromium, which has the greatest rigidity compared to other elements of the group, is capable of replacing the oxygen environment with sulfur atoms: for example, by fusing chromium(III) oxide with potassium thiocyanate, KCrS 2 sulfide can be obtained.
5.2. Prevalence in nature. Preparation and use of simple substances.
The elements of the sixth group are even and therefore more common than the odd elements of the 5th and 7th groups. Their natural galaxy consists of a large number of isotopes (Table 5.1). Chromium is the most common in nature. Its content in the earth's crust is 0.012% wt and is comparable to the abundance of vanadium (0.014% wt) and chlorine (0.013% wt). Molybdenum (3×10 -4% mass) and tungsten (1×10 -4% mass) are rare and trace metals. The most important industrial chromium mineral is chromium iron ore FeCr 2 O 4 . Other minerals are less common - crocoite PbCrO 4, chrome ocher Cr 2 O 3. The main form of occurrence of molybdenum and tungsten in nature is feldspars and pyroxenes. Of the molybdenum minerals, molybdenite MoS 2 is the most important, mainly due to the fact that it does not contain significant quantities of other metals, which greatly facilitates the processing of ore. The products of its oxidation under natural conditions are wulfenite PbMoO 4 and powellite CaMoO 4 . The most important tungsten minerals are scheelite CaWO 4 and wolframite (Fe,Mn)WO 4 , but the average tungsten content in the ores is extremely low - no more than 0.5%. Due to the similar properties of molybdenum and tungsten, complete solid solutions of CaMoO4-CaWO4 and PbMoO4-PbWO4 exist.
For many technical purposes, there is no need to separate the iron and chromium contained in chromium iron ore. An alloy formed when it is reduced with coal in electric furnaces
FeCr 2 O 4 + 4C Fe + 2Cr + 4CO,
Ferrochrome is widely used in the production of stainless steels. If silicon is used as a reducing agent, ferrochrome with a low carbon content is obtained, which is used for the production of strong chrome steels.
Pure chromium is synthesized by reduction of Cr 2 O 3 oxide with aluminum
Сr 2 O 3 + 2Al = 2Cr + Al 2 O 3
or silicon
2Cr 2 O 3 + 3Si = 4Cr + 3SiO 2.
In the aluminothermic method, a preheated mixture of chromium(III) oxide and aluminum powder with oxidizing agent additives (Footnote: the heat released during the reduction of chromium oxide with aluminum is not enough for the process to occur spontaneously. Potassium dichromate, barium peroxide, chromic anhydride are used as an oxidizing agent) is loaded into the crucible. The reaction is initiated by igniting a mixture of aluminum and sodium peroxide. The purity of the resulting metal is determined by the content of impurities in the original chromium oxide, as well as in reducing agents. It is usually possible to obtain metal of 97-99% purity, containing small amounts of silicon, aluminum and iron.
To obtain the oxide, chromium iron ore is subjected to oxidative melting in an alkaline environment
4FeCr 2 O 4 + 8Na 2 CO 3 + 7O 2 8Na 2 CrO 4 + 2Fe 2 O 3 + 8CO 2,
and the resulting Na 2 CrO 4 chromate is treated with sulfuric acid.
2Na 2 CrO 4 + 2H 2 SO 4 = Na 2 Cr 2 O 7 + 2NaHSO 4 + H 2 O
In some industrial plants, carbon dioxide is used instead of sulfuric acid, carrying out the process in autoclaves under a pressure of 7 - 15 atm.
2Na 2 CrO 4 + H 2 O + 2CO 2 = Na 2 Cr 2 O 7 + 2NaHCO 3.
At normal pressure the equilibrium of the reaction is shifted to the left.
Then the crystallized sodium bichromate Na 2 Cr 2 O 7 × 2H 2 O is dehydrated and reduced with sulfur or coal
Na 2 Cr 2 O 7 + 2C Cr 2 O 3 + Na 2 CO 3 + CO.
The purest chromium in industry is obtained either by electrolysis of a concentrated aqueous solution of chromic anhydride in sulfuric acid, a solution of chromium(III) sulfate Cr 2 (SO 4) 3 or chromium-ammonium alum. Chromium with a purity greater than 99% is released on a cathode made of aluminum or stainless steel. Complete purification of the metal from nitrogen or oxygen impurities is achieved by keeping the metal in a hydrogen atmosphere at 1500 °C or by distillation in a high vacuum. The electrolytic method allows one to obtain thin films of chromium, which is why it is used in electroplating.
To obtain molybdenum, ore enriched by flotation is roasted
900 – 1000 ºС
2MoS 2 + 7O 2 = 2MoO 3 + 4SO 2.
The resulting oxide is distilled off at the reaction temperature. Then it is further purified by sublimation or dissolved in an aqueous solution of ammonia
3MoO 3 + 6NH 3 + 3H 2 O = (NH 4) 6 Mo 7 O 24,
recrystallize and decompose again in air to the oxide. Metal powder is obtained by reducing the oxide with hydrogen:
MoO 3 + 3H 2 = Mo + 3H 2 O,
pressed and melted in an arc furnace in an atmosphere of inert gas or converted into an ingot using powder metallurgy. Its essence lies in the production of products from fine powders by cold pressing molding and subsequent high-temperature treatment. The technological process of manufacturing products from metal powders includes preparing the mixture, molding blanks or products, and sintering them. Molding is carried out by cold pressing under high pressure (30–1000 MPa) in metal molds. Sintering of products from homogeneous metal powders is carried out at temperatures reaching 70–90% of the melting temperature of the metal. To avoid oxidation, sintering is carried out in an inert, reducing atmosphere or in a vacuum. Thus, molybdenum powder is first pressed in steel molds . After preliminary sintering (at 1000-1200 °C) in a hydrogen atmosphere, the workpieces (stubs) are heated to 2200-2400 °C. In this case, individual crystallites melt from the surface and stick together, forming a single ingot, which is subjected to forging.
The starting material for the production of tungsten is its oxide WO 3 . To obtain it, ore (scheelite CaWO 4 or wolframite FeWO 4), previously enriched by flotation in solutions of surfactants, is subjected to alkaline or acid opening. Alkaline dissection is carried out by decomposing the concentrate in autoclaves with a soda solution at 200 °C
CaWO 4 + Na 2 CO 3 = Na 2 WO 4 + CaCO 3 ¯ .
The equilibrium shifts to the right due to the use of a threefold excess of soda and the precipitation of calcium carbonate. According to another method, wolframite concentrates are decomposed by heating with a strong solution of caustic soda or sintering with soda at 800-900 °C
CaWO 4 + Na 2 CO 3 = Na 2 WO 4 + CO 2 + CaO.
In all cases, the final decomposition product is sodium tungstate, which is leached with water. The resulting solution is acidified and tungstic acid is precipitated
Na 2 WO 4 + 2HCl = H 2 WO 4 ¯ + 2NaCl.
Acidic dissection of scheelite also produces tungstic acid:
CaWO 4 + 2HCl = H 2 WO 4 ¯ + CaCl 2.
The released tungstic acid precipitate is dehydrated
H 2 WO 4 = WO 3 + H 2 O.
The resulting oxide is reduced with hydrogen
WO 3 + 3H 2 = W + 3H 2 O.
The oxide used for the production of high-purity tungsten is pre-purified by dissolution in ammonia, crystallization of ammonium paratungstate and its subsequent decomposition.
When the oxide is reduced, tungsten metal is also obtained in the form of a powder, which is pressed and sintered at 1400 ºС, and then the rod is heated to 3000 ºС, passing an electric current through it in a hydrogen atmosphere. Tungsten rods prepared in this way acquire plasticity; from them, for example, tungsten filaments are drawn for incandescent electric lamps. Large-crystalline tungsten and molybdenum ingots are produced by electron beam melting in a vacuum at 3000-3500 o C.
Chromium is used in metallurgy in the production of stainless steels, which have unique corrosion resistance. Adding just a few percent chromium to iron makes the metal more susceptible to heat treatment. Chromium is used to alloy steels used in the manufacture of springs, springs, tools, and bearings. A further increase in the chromium content in steel leads to a sharp change in its mechanical characteristics - a decrease in wear resistance, the appearance of brittleness. This is due to the fact that when the content of chromium in steel is more than 10%, all the carbon contained in it passes into the form of carbides. At the same time, such steel is practically not subject to corrosion. The most common grade of stainless steel contains 18% chromium and 8% nickel. The carbon content in it is very low - up to 0.1%. Turbine blades, submarine hulls, as well as pipes, metal tiles, and cutlery are made of stainless steel. A significant amount of chromium is used for decorative corrosion-resistant coatings, which not only give products a beautiful appearance and increase their service life, but also enhance the wear resistance of machine parts and tools. Chrome plating with underlayer of copper and nickel well protects steel from corrosion, giving products a beautiful appearance. Protective and decorative chromium plating is applied to parts of cars, bicycles, devices, in which the thickness of the applied film usually does not exceed 5 microns. In terms of reflectivity, chrome coatings are second only to silver and aluminum, so they are widely used in the production of mirrors and spotlights. Nickel alloys containing up to 20% chromium (nichrome) are used for the manufacture of heating elements - they have high resistance and become very hot when current is passed. The addition of molybdenum and cobalt to such alloys greatly increases their heat resistance - gas turbine blades are made from such alloys. Along with nickel and molybdenum, chromium is a component of metal-ceramics, a material used in dental prosthetics. Chromium compounds are used as green (Cr 2 O 3 , CrOOH), yellow (PbCrO 4 , CdCrO 4 ) and orange pigments. Many chromates and dichromates are used as corrosion inhibitors (CaCr 2 O 7 , Li 2 CrO 4 , MgCrO 4), wood preservatives (CuCr 2 O 7), fungicides (Cu 4 CrO 7 ×xH 2 O), catalysts (NiCrO 4, ZnCr 2 O 4). World chromium production currently exceeds 700 thousand tons per year.
Molybdenum is also used in metallurgy to create hard and wear-resistant, chemically resistant and heat-resistant structural alloys, as an alloying additive to armor steels. The thermal expansion coefficients of molybdenum and some types of glass (they are called “molybdenum glass”) are close, therefore, inputs to glass electric vacuum devices and bulbs of powerful light sources are made from molybdenum. Due to its relatively small thermal neutron capture cross section (2.6 barn), molybdenum is used as a structural material in nuclear reactors . Molybdenum wire, tapes and rods serve as heating elements and heat shields in vacuum installations. Molybdenum, alloyed with titanium, zirconium, niobium, and tungsten, is used in aviation and rocketry for the manufacture of gas turbines and engine parts.
Tungsten is the best material for filaments and spirals in incandescent lamps, radio tube cathodes and X-ray tubes. The high operating temperature (2200-2500 o C) provides greater light output, and the low evaporation rate and the ability to hold shape (do not sag when heated to 2900 o C) ensure a long service life of the filaments. Tungsten is also used to create hard, wear-resistant and heat-resistant alloys in mechanical engineering and rocketry. Steels containing 20% tungsten have the ability to self-harden - blades of cutting tools are made from them. Tungsten alloys advantageously combine heat resistance and heat resistance not only in humid air, but also in many aggressive environments. For example, when 10% tungsten is added to nickel, its corrosion resistance increases 12 times. Tungsten-rhenium thermocouples allow measuring temperatures up to 3000 °C.
This article will look at chromium and its subgroup: molybdenum and tungsten. In terms of content in the earth's crust, chromium (6∙10 -3%), molybdenum (3∙10 -4%) and tungsten (6∙10 -4%) are fairly common elements. They are found exclusively in the form of compounds. The main ore of chromium is natural chromium iron ore (FeO∙Cr 2 O 3). Of molybdenum ores, the most important mineral is molybdenite (MoS 2), of tungsten ores - the minerals wolframite (xFeWO 4 ∙zMnWO 4) and scheelite (CaWO 4). Natural chromium consists of isotopes with mass numbers 50 (4.3%), 52 (83.8%), 53 (9.5%), 54 (2.4%), molybdenum - from isotopes 92 (15.9%) , 94 (9.1%), 95 (15.7%), 96 (16.5%), 97 (9.5%), 98 (23.7%), 100 (9.6%), and tungsten - from isotopes 180 (0.1%), 182 (26.4%), 183 (14.4%), 184 (30.7%), 186 (28.4%).
Physical properties:
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Density, g/cm 3 |
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Melting point, °С |
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Boiling point, °С |
When compacted, the elements are grayish-white shiny metals. Very pure metals lend themselves well to machining, but traces of impurities give them hardness and brittleness.
Receipt:
To obtain elemental chromium, it is convenient to start from a mixture of its oxide (Cr 2 O 3) with aluminum powder. The reaction that begins upon heating proceeds according to the equation (aluminothermy):
Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr + 129 kcal
When producing aluminothermic chromium, a little CrO 3 is usually added to the initial Cr 2 O 3 (to make the process more vigorous). As a result of the reaction, two layers are formed, of which the upper one contains red (from traces of chromium oxide) aluminum oxide, and the lower one contains approximately 99.5% chromium. The reduction of MoO 3 and WO 3 with hydrogen to metals easily occurs above 500 °C.
Molybdenum and tungsten can be obtained by reducing their oxides at high temperatures with coal or hydrogen. Chromium can be obtained in a similar way:
Cr 2 O 3 + 3H 2 → 2Cr + 3H 2 O
WO 3 +3H 2 →W+3H 2 O
MoO 3 +3H 2 →Mo+3H 2 O
Molybdenite is converted to MoO 3 by firing in air: 2MoS 2 + 70 2 = 4S0 2 +2MoO 3
Also, one of the ways to obtain chromium is the reduction of chromium iron ore with coal:
Fe(Cr0 2) 2 +2С→2С0 2 +Fe+2Cr (an alloy of iron and chromium is obtained - ferrochrome).
To obtain especially pure chromium from chromium iron ore, chromate is first obtained, then it is converted into dichromate (in an acidic medium), then the dichromate is reduced with coal (to form chromium oxide III), and then aluminothermy:
4Fe(Cr0 2) 2 +8Na 2 CO 3 +70 2 →8Na 2 CrO 4 +2Fe 2 O 3 +8С0 2
Na 2 Cr 2 O 7 + 2C → Cr 2 O 3 + Na 2 CO 3 + C0
Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr + 129 kka l
In the laboratory, a different reaction is often carried out:
(NH 4) 2 Cr 2 O 7 →N 2 +Cr 2 O 3 +4H 2 O, and then reduced to chromium as described above.
This is interesting:
Very pure chromium can be obtained, for example, by distilling the electrolytically deposited metal under high vacuum. It is plastic, however, even when stored in air, it absorbs traces of gases (0 2, N 2, H 2) and loses plasticity. From ores Cr, Mo and W are usually smelted not from pure metals, but from their high-percentage alloys with iron. The starting material for the preparation of ferrochrome (at least 60% Cr) is directly chromium iron ore. Molybdenite is first converted toMoO 3 , from which ferromolybdenum is then prepared (at least 55% Mo). Manganese-poor wolframites can be used to obtain ferrotungsten (65-80% W). .
Chemical properties:
In relation to air and water, Cr, Mo and W are quite stable under normal conditions. Under normal conditions, all three metals react noticeably only with fluorine, but with sufficient heating they combine more or less vigorously with other typical metalloids. What they have in common is the absence of chemical interaction with hydrogen. When moving in the subgroup from top to bottom (Cr-Mo-W), the chemical activity of metals decreases. This is especially evident in their attitude towards acids. Chromium is soluble in dilute HCI and H2SO4. They have no effect on molybdenum, but this metal dissolves in hot, strong H2SO4. Tungsten is resistant to all common acids and their mixtures (except for a mixture of hydrofluoric and nitric acids). The conversion of molybdenum and tungsten into a soluble compound is most easily accomplished by alloying with nitrate and soda according to the following scheme:
E+ 3NaNO 3 +Na 2 CO 3 =Na 2 EO 4 +3NaNO 2 +C0 2
Sodium tungstate, obtained from wolframite by similar fusion with soda, is decomposed with hydrochloric acid and the released H 2 WO 4 is calcined until it transforms into WO 3.
All metals form amphoteric oxides:
4Cr+30 2 →2Cr 2 O 3
This is interesting :
Cr 2 O 3 is a very refractory dark green substance, insoluble not only in water, but also in acids (it reacts with alkalis only in melts, with acids only with strong ones (for exampleHCl andH 2 SO 4) and only in a finely dispersed state), examples are below. Due to its intense color and great resistance to atmospheric influences, chromium oxide is an excellent material for the production of oil paints (“chrome green”).
2W+30 2 →2W0 3
2Mo+30 2 →2Mo0 3
4CrO 3 →2Cr 2 O 3 +30 2
All elements form the corresponding halides, by direct interaction, where they exhibit a +3 oxidation state:
2E+3Hal 2 →2EHal 3
The solubility of Mo0 3 and W0 3 in water is very low, but in alkalis they dissolve to form salts of molybdic and tungstic acids. The latter in the free state are almost insoluble powders of white (H 2 Mo0 4) or yellow (H 2 W0 4) color. When heated, both acids easily split off water and transform into the corresponding oxides.
Mo0 3 +2NaOH→Na 2 MoO 4 +H 2 O
W0 3 +2NaOH→Na 2 WO 4 +H 2 O
Similar salts can also be obtained by fusing metals with alkalis in the presence of oxidizing agents:
2W+4NaOH+30 2 →2Na 2 WO 4 +2H 2 O
W+2NaOH+3NaNO 3 →Na 2 WO 4 +3NaNO 2 +H 2 O
Likewise for molybdenum
2Mo+4NaOH+30 2 →2Na 2 MoO 4 +2H 2 O
Mo+2NaOH+3NaNO 3 →Na 2 MoO 4 +3NaNO 2 +H 2 O
According to the Cr-Mo-W series, the strength of acids H 2 EO 4 decreases. Most of their salts are slightly soluble in water. Of the derivatives of the most common metals, those that are highly soluble are: chromates - only Na +, K +, Mg 2+ and Ca 2+, molybdates and tungstates - only Na + and K +. Chromate salts are usually colored light yellow, CrO 4 2- ion, Cr 2 O 7 2- - orange; Molybdic acid and tungstic acid are colorless.
Tungsten dissolves only in a mixture of concentrated nitric and hydrofluoric acids :
W+10HF+4HNO 3 →WF 6 +WOF 4 +4NO+7H 2 O
Concentrated sulfuric acid also acts on molybdenum:
2Mo+6H 2 SO 4 (conc.) → Mo 2 (SO 4) 3 +3SO 2 +6H 2 O
Chromium is affected by both HCl, H 2 SO 4 (diluted), and H 2 SO 4 (concentrated), but concentrated - only when heated, since chromium is passivated by concentrated sulfuric acid:
27H 2 SO 4 (conc.) +16Cr=8Cr 2 (SO 4) 3 +24H 2 O+3H 2 S
2Cr+6HCl→2CrCl 3 +3H 2
3H 2 SO 4 +2Cr→Cr 2 (SO 4) 3 +3H 2
Being a typical acid anhydride, CrO 3 dissolves in water to form chromic acid characterized by medium strength - H 2 CrO 4 (with a lack of CrO 3) (or dichromic acid, with an excess of CrO 3 -H 2 Cr 2 O 7). Chromic anhydride is poisonous and very strong oxidizing agent.
H 2 O+2СrO 3(g) →H 2 Cr 2 O 7
H 2 O + CrO 3 (week) → H 2 CrO 4
2CrO 3 +12HCl→2CrCl 3 +3Cl 2 +6H 2 O
In addition to acids such as H 2 CrO 4 (chromate salts), for chromium and its analogues there are also those corresponding to the general formula H 2 Cr 2 O 7 (bichromate salts).
Solutions of dichromates show an acidic reaction due to the fact that the Cr 2 O 7 2- ion reacts with water according to the scheme
H 2 O+Cr 2 O 7 2- →2НCrO 4 → 2Н + +2CrO 4 2-
As can be seen from the equation, the addition of acids (H + ions) to the solution should shift the equilibrium to the left, and the addition of alkalis (OH - ions) to the right. In accordance with this, it is easy to obtain chromates from dichromates, and vice versa, for example, by the reactions:
Na 2 Cr 2 O 7 + 2NaOH = 2Na 2 CrO 4 +H 2 O
2K 2 CrO 4 +H 2 SO 4 =K 2 SO 4 +K 2 Cr 2 O 7 +H 2 O
Salts of chromic acids in an acidic environment are strong oxidizing agents. For example, they oxidize HI already in the cold, and when heated, HBr and HCl, the reaction equation in general form:
Na 2 CrO 4 +14НHal = 2NaHal + 2СrHal 3 +3Hal 2 +7H 2
This is interesting:
A mixture of equal volumes of a cold-saturated solution with a very strong oxidizing effectK 2 Cr 2 O 7 and concentratedH2SO4 ("chrome mixture") used in laboratories for washing chemical glassware.
When CrO 3 interacts with hydrogen chloride gas, chloride is formed lame(CrO 2 Cl 2), which is a red-brown liquid. Compounds of this composition are also known for Mo and W. All of them interact with water according to the scheme
EO 2 Cl 2 +2H 2 O→H 2 EO 4 +2HCl
This means chromyl chloride is the acid chloride of chromic acid. Chromyl chloride is a strong oxidizing agent.
CrO 2 Cl 2 +H 2 O+KCl→KCrO 3 Cl+2HC
Chromium exhibits several oxidation states (+2, +3, +4, +6). Molybdenum and tungsten derivatives will be partially considered, only those where these metals exhibit the main oxidation state: +6.
This is interesting :
Compounds where chromium and its analogs exhibit oxidation states of +2 and +4 are quite exotic.The oxidation state +2 corresponds to basic CrO oxide (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic environment (“with hydrogen at the time of release”).
Chromium analogue dioxides - brown Mo0 2 AndW0 2 - are formed as intermediate products during the interaction of the corresponding metals with oxygen and can also be obtained by reducing their higher oxides with gaseous ammonia (they are insoluble in water and, when heated in air, easily passVthree-axle):
Mo0 3 +H 2 →MoO 2 +H 2 O
3W0 3 +2NH 3 →N 2 +3H 2 O+3W0 2
2W0 3 +C→CO 2 +2W0 2
Also, to obtain tetravalent chromium oxide, the following reaction can be used:
2СrO 3 →2CrO 2 +0 2
The main functions of dioxides are met by the halides of tetravalent molybdenum and tungsten. Formed as a result of the interaction of Mo0 2 with chlorine when heated in the presence of coal brown MoCl 4 easily sublimes as yellow vapor:
Mo0 2 +2Cl 2 +2C→MoCl 4 +2CO
As mentioned above, compounds are more characteristic of chromium, where it exhibits an oxidation state of +: 6 or +3.
Dichrome trioxide is prepared by the reaction:
4Cr+30 2 →2Cr 2 O 3
But, more often, Cr 2 O 3 and salts corresponding to chromic acid are usually obtained not from a metal, but by reducing derivatives of hexavalent chromium, for example, according to the reaction:
K 2 Cr 2 O 7 +3S0 2 +H 2 SO 4 =K 2 SO 4 +Cr 2 SO 4) 3 +H 2 O
The action of a small amount of alkali on a solution of Cr 2 (SO 4) 3 can give a dark blue precipitate of slightly water-soluble chromium oxide hydrate Cr(OH) 3 . The latter has a clearly defined amphoteric character. With acids, it gives salts of chromium oxide, and under the action of an excess of alkalis, it forms a complex, with the anion [Cr (OH) 6 ] 3-, or, chromite salts are formed. For example:
Cr(OH) 3 +3HCl=CrCl3 +3H2O
Cr(OH) 3 + KOH=K 3 [Cr(OH) 6 ] + 2H 2 O
Cr(OH) 3 + KOH = KCrO 2 + 2H 2 O
2NaCrO 2 +3Br 2 +8NaOH=6NaBr+2Na 2 CrO 4 +4H 2 O
Cr 2 (SO 4) 3 +ЗH 2 0 2 +10NaOH=3Na 2 SO 4 +2Na 2 CrO 4 +8H 2 O
5Cr 2 O 3 +6NaBrO 3 +2H 2 O=3Na 2 Cr 2 O 7 +2H 2 Cr 2 O 7 +3Br 2
The oxidation state of chromium +6 corresponds to chromium oxide: CrO 3. It can be obtained by the reaction:
K 2 Cr 2 O 7 +H 2 SO 4 → 2CrO 3 +K 2 SO 4 +H 2 O
This oxide, as described above, corresponds to 2 acids: chromic and dichromic. The main derivatives of these acids, which necessary know -K 2 Cr 2 O 7 and Na 2 CrO 4 or Na 2 Cr 2 O 7 and K 2 CrO 4. Both of these salts are very good oxidizing agents:
2K 2 CrO 4 +3(NH 4) 2 S+8H 2 O=2Cr(OH) 3 +3S+4KOH+ 6NH 4 OH
K 2 Cr 2 O 7 +7H 2 SO 4 +6NaI→K 2 SO 4 +(Cr 2 SO 4) 3 +3Na 2 SO 4 +7H 2 O+3I 2
4H 2 0 2 +K 2 Cr 2 O 7 +H 2 SO 4 →CrO 5 +K 2 SO 4 +5H 2 O
The CrO 5 molecule has a structure. This is a salt of hydrogen peroxide.
Na 2 CrO 4 + BaCl 2 →BaCrO 4 ↓ + 2NaCl (qualitative reaction for barium cation 2+, yellow precipitate)
K 2 Cr 2 O 7 +3Na 2 SO 3 +4H 2 SO 4 →Cr 2 (SO 4) 3 +K 2 SO 4 +3Na 2 SO 4 +4H 2 O
K 2 Cr 2 O 7 +7H 2 SO 4 +3Na 2 S→3S +Cr 2 (SO 4) 3 +K 2 SO 4 +3Na 2 SO 4 +7H 2 O
K 2 Cr 2 O 7 +4 H 2 SO 4 +3C 2 H 5 OH→ Cr 2 (SO 4) 3 + K 2 SO 4 +3CH3COH+7 H 2 O
3H 2 C=CH-CH 2 -CH 3 +5 K 2 Cr 2 O 7 +20 H 2 SO 4 =
3H 3 C-CH 2 -COOH+3C 0 2 +5 Cr 2 (SO 4) 3 +5 K 2 SO 4 + 23 H 2 O
All derivatives of hexavalent chromium are highly toxic. When in contact with the skin or mucous membranes, they cause local irritation (sometimes with the formation of ulcers), and when inhaled in a sprayed state, they contribute to lung cancer. Their maximum permissible content in the air of industrial premises is 0.0001 mg/l.
Application:
The introduction of Cr, Mo and W into the composition of steels greatly increases their hardness. Such steels are used mainly in the manufacture of rifle and gun barrels, armor plates, springs and cutting tools. Typically, these steels are also very resistant to various chemical influences.
This is interesting:
Molybdenum was found in ancient Japanese swords, and tungsten was found in Damascus daggers. Even a small addition of molybdenum (about 0.25%) greatly improves the mechanical properties of cast iron.
Steel containing 15-18% W, 2-5% Cu and 0.6-0.8% C can be highly heated without loss of hardness. With a content of more than 10% Cr, steel almost does not rust. Therefore, in particular, turbine blades and submarine hulls are made from it. The alloy of 35% Fe, 60% Cr and 5% Mo is distinguished by its acid resistance. This applies to an even greater extent to alloys of Mo and W, which can in many cases serve as a replacement for platinum. The alloy W with Al (“partinium”) is used in the manufacture of automobile and aircraft engines. Molybdenum-based alloys retain mechanical strength at very high temperatures (but require an oxidation-protective coating). In addition to its introduction into special steels, chromium is used to coat metal products whose surface must provide great wear resistance (calibers, etc.). Such chrome plating is carried out electrolytically, and the thickness of the applied chromium films, as a rule, does not exceed 0.005 mm. Molybdenum metal is mainly used in the electric vacuum industry. It is usually used to make pendants for electric lamp filaments. Since tungsten is the most refractory of all metals, it is especially suitable for making light bulb filaments, certain types of alternating current rectifiers (called kenotrons), and the anticathodes of high-power X-ray tubes. Tungsten is also of great importance for the production of various superhard alloys used as tips for cutters, drills, etc.
Chromium oxide salts are used mainly as mordants for dyeing fabrics and for chrome tanning leather. Most of them are highly soluble in water. From the chemical side, these salts are interesting in that the color of their solutions changes depending on the conditions (temperature of the solution, its concentration, acidity, etc.) from green to purple.
Editor: Galina Nikolaevna Kharlamova
Program
Chemical activity of metals from the chromium subgroup. Basic valence states. Chromium complex compounds, structure and significance. hydration isomerism. Acid-base and redox properties of chromium (II), (III) and (VI) compounds. Polycompounds. Chromium peroxo compounds. Analytical reactions of elements of the chromium subgroup. Comparison of stability, acid-base and redox properties of higher oxygen compounds of elements of the chromium subgroup.
The chromium subgroup is formed by metals of the secondary subgroup of the sixth group - chromium, molybdenum and tungsten. The outer electronic layer of atoms of elements of the chromium subgroup contains one or two electrons, which determines the metallic nature of these elements and their difference from the elements of the main subgroup. In binary compounds Cr, Mo and W, all oxidation states from 0 to +6 are exhibited, since, in addition to the outer electrons, a corresponding number of electrons from the unfinished penultimate layer can also participate in the formation of bonds. The most stable oxidation states for Cr are +3 and +6, Mo and W +6. Compounds in higher oxidation states are usually covalent and acidic in nature, much like the corresponding sulfur compounds. As the oxidation state decreases, the acidic character of the compounds weakens.
In the series Cr - Mo - W, the ionization energy increases, i.e. the electron shells of atoms become denser, especially strongly during the transition from Mo to W. Tungsten, due to lanthanide compression, has atomic and ionic radii close to those of Mo. Therefore, Mo and W are closer in properties to each other than to Cr.
Cr, Mo and W are white shiny metals. They are very hard (scratch glass) and refractory. Modifications of Cr, Mo and W, which are stable under normal conditions, have the structure of a body-centered cube. Tungsten is the most refractory of metals. In the Cr – Mo – W series, an increase in the melting temperature and heat of atomization (sublimation) is observed, which is explained by the strengthening of the covalent bond in the metal crystal arising due to d-electrons.
Although Cr, Mo and W are in the stress series before hydrogen, they are little susceptible to corrosion due to the formation of an oxide film on the surface. At room temperature these metals are slightly reactive.
Cr, Mo and W do not form stoichiometric compounds with hydrogen, but when heated they absorb it in significant quantities to form solid solutions. However, upon cooling, the absorbed hydrogen (especially in Mo and W) is partially released. As in other subgroups d-elements, with an increase in the ordinal number of an element in the Cr-Mo-W series, the chemical activity decreases. Thus, chromium displaces hydrogen from dilute HCl and H2SO4, while tungsten dissolves only in a hot mixture of hydrofluoric and nitric acids:
E o + 2HNO 3 + 8HF = H 2 [E +6 F 8 ] + 2NO + 4H 2 O
Due to the formation of anionic complexes EO 4 2-molybdenum and tungsten also interact when alloyed with alkalis in the presence of an oxidizing agent:
E o + 3NaN +5 O 3 + 2NaOH = Na 2 E +6 O 4 + 3NaN +3 O 2 + H 2 O
In concentrated HNO 3 and H 2 SO 4 chromium is passivated.
Cr, Mo and W form numerous compounds with S, Se, N, P, As, C, Si, B and other non-metals. The most interesting are the carbides: Cr 3 C 2, MoC, W 2 C, WC, which are second only to diamond in hardness and have high melting points, are used for the manufacture of especially hard alloys.
In direct interaction with halogens, chromium forms only di-, tri- and tetrahalides, and molybdenum and tungsten - and higher - penta- and hexahalides. Most element halides in lower oxidation states are strong reducing agents and easily form complex compounds. Mo and W diamides are cluster-type compounds with MeMe bonds. Halides of elements in higher oxidation states are, as a rule, volatile compounds with covalent bonds that easily hydrolyze in water, usually with the formation of oxohalides:
MoCl 5 + H 2 O MoOCl 3 + 2HCl
Elements of the chromium subgroup form numerous oxide compounds corresponding to the main oxidation states. All oxides under normal conditions are solids. For chromium, the most stable is Cr 2 O 3, and for Mo and W – MoO 3 and WO 3. In the Cr - W series, the thermodynamic stability of acidic oxides EO 3 increases. Lower oxides are strong reducing agents and exhibit a basic character. An increase in the degree of oxidation is accompanied by an increase in acidic properties. Thus, Cr 2 O 3 is an amphoteric oxide, and CrO 3 (EO 3) is a typical acidic oxide with the properties of a strong oxidizing agent. The only highly soluble oxide - CrO 3 - when dissolved in water, forms chromic acid:
CrO 3 + H 2 O H 2 CrO 4.
MoO 3 and WO 3 are poorly soluble in water and their acidic nature manifests itself when dissolved in alkalis:
2KOH + EO 3 K 2 EO 4 + H 2 O.
Of the hydroxides of the E(OH) 2 type, only the poorly soluble base Cr(OH) 2 is known, which is formed when solutions of Cr 2+ salts are treated with alkalis. Cr(OH) 2 and Cr 2+ salts are strong reducing agents that are easily oxidized by atmospheric oxygen and even water to Cr 3+ compounds. Mo 2+ and W 2+ hydroxides are not released due to their instantaneous oxidation with water.
The gray-blue hydroxide Cr(OH) 3 precipitated from solutions of Cr 3+ salts has a variable composition Cr 2 O 3 n H 2 O. This is a layered multinuclear polymer in which the role of ligands is played by OH - and OH 2, and the role of bridges is played by OH - groups.
Its composition and structure depend on the conditions of preparation. Freshly obtained Cr(OH) 3 is highly soluble in acids and alkalis, which cause the rupture of bonds in the layered polymer:
3+ Cr(OH) 3 3-
Mo(OH) 3, which is poorly soluble in water and acids, is obtained by treating Mo 3+ compounds with alkalis or ammonia. It is a strong reducing agent (decomposes water with the release of hydrogen). The best known hydroxide derivatives are Cr +6 , Mo +6 and W +6 . These are, first of all, acids of the type H 2 EO 4 and H 2 E 2 O 7 and their corresponding salts. Chromic H 2 CrO 4 and dichromic H 2 Cr 2 O 7 acids are of medium strength and exist only in aqueous solutions, but the salts corresponding to them are yellow chromates (CrO 4 2- anion) and orange dichromates (Cr 2 O 7 2- anion) , are stable and can be isolated from solutions.
Mutual transitions of chromate and dichromate can be expressed by the equation:
2CrO 4 2- + 2H + 2HСrO 4 - Cr 2 O 7 2- + H 2 O
Chromates and dichromates are strong oxidizing agents. Molybdic and tungstic acids are sparingly soluble in water. When alkalis act on H 2 MoO 4 (H 2 WO 4), or when MoO 3 (WO 3) melts with alkalis, depending on the ratio of the amounts of reagents, molybdates (tungstates) or isopolymolybdates (isopolytungstates) are formed:
MoO 3 + 2NaOH Na 2 MoO 4 + H 2 O
3MoO 3 + NaOH Na 2 Mo 3 O 10 + H 2 O
Isopolycompounds Mo +6 have different compositions: M 2 + Mo n O 3 n +1 (n=2, 3, 4); M 6 + Mon O 3 n +3 (n = 6, 7); M 4 + Mo 8 O 26. The tendency to polymerize from chromium to tungsten increases. Mo and W are characterized by the formation of heteropolyacids, i.e. polyacids containing in the anion, in addition to oxygen and molybdenum (tungsten), another element: P, Si, B, Te, etc. Heteropolycompounds are formed by acidifying a mixture of salts and mixing the corresponding acids, for example:
12Na 2 EO 4 + Na 2 SiO 3 + 22HNO 3 Na 4 + 22NaNO 3 + 11H 2 O.
Cr +6, Mo +6, and W +6 are characterized by the formation of peroxo compounds. Peroxide CrO 5 is known, having the structure CrO(O 2) 2. This unstable dark blue compound, existing in solutions, is obtained by treating solutions of chromates or dichromates with diethyl ether and a mixture of H 2 O 2 and H 2 SO 4. This reaction detects chromium (Cr +6) even in small quantities. Peroxochromates K[(Cr(O 2) 2 O)OH)] H 2 O, M 3 , M= Na, K, NH 4 + were obtained.
