CHROMATS AND DICHROMATS. Chromates are salts of chromic acid H2CrO4, which does not exist in a free state, obtained only in the form of aqueous solutions with a concentration of no more than 75%. They are obtained by reacting chromium(VI) oxide or a solution of chromic acid H 2 CrO 4 with metal oxides, hydroxides, carbonates or by an exchange reaction involving soluble chromate salts. Chromates of alkali metals and magnesium dissolve well in water, and for alkaline earth metals the solubility decreases so sharply in the series CaCrO 4 – SrCrO 4 – BaCrO 4 – RaCrO 4 that it was possible to construct a laboratory method for separating calcium, strontium and barium: calcium chromate does not precipitate into a precipitate from millimolar (0.001 mol/l) solutions, and the precipitate of strontium chromate, unlike barium chromate, dissolves under the action of acetic acid.
Crystalline chromates and their solutions in water are yellow. When they are acidified, for example, by adding a small amount of dilute sulfuric acid, the CrO 4 2- ions are converted into dichromate ions Cr 2 O 7 2-, eliminating water and acquiring an orange color:
2CrO 4 2- + 2 H 3 O + Cr 2 O 7 2- + 3H 2 O
Unlike chromates, almost all dichromate salts are highly soluble in water. Dichromates are strong oxidizing agents. In an acidic environment, they are characterized by a half-reaction:
Cr 2 O 7 2- + 14H + + 6 e- = 2Cr 3+ + 7H 2 O,
where each chromium(+VI) atom in the anion receives three electrons and turns into a Cr 3+ cation, which immediately attaches water molecules to itself, turning into a hexaaquacation. The solution turns green.
Ammonium dichromate has special properties: it decomposes easily. When heated weakly, ammonium dichromate (NH 4) 2 Cr 2 O 7 spontaneously ignites with the emission of sparks (this experiment is called a chemical volcano) - hot particles of chromium(III) oxide Cr 2 O 3, nitrogen gas N 2 and water vapor. Potassium chromate K 2 CrO 4 melts without decomposition at 968 ° C, and potassium dichromate K 2 Cr 2 O 7 decomposes in accordance with the reaction equation:
The most widely used are potassium chromate and dichromate and ammonium dichromate.
Potassium chromate K 2 CrO 4 – yellow crystals that turn red when heated. Does not form crystalline hydrates. Density 2.732 g/cm 3 , melting point 968.3° C, solubility at 20° C is 63.0 g/100 g of water.
Potassium dichromate K 2 Cr 2 O 7 - orange-red crystals, does not form crystalline hydrates, density 2.684 g/cm 3, melts at 397 ° C (further heating of the melt leads to decomposition), solubility at 20 ° C is equal to 12.48 g of salt per 100 g of water. The technical name of potassium dichromate K 2 Cr 2 O 7 is chromium. Widely used as an oxidizing agent in the match industry, pyrotechnics and as a reagent in chromatometry.
Chromatometry (more precisely, dichromatometry) is a volumetric (titrimetric) method of chemical analysis ( cm. TITRATION), based on the use of a K 2 Cr 2 O 7 solution of a certain concentration and allowing to determine the content of reducing agents and some metal cations.
A mixture of equal volumes of a cold-saturated solution of K 2 Cr 2 O 7 and concentrated sulfuric acid (chromic mixture) has a very strong oxidizing effect and is used in laboratories for washing chemical glassware.
Ammonium dichromate also does not form crystalline hydrates, but decomposes when heated. These are orange crystals with a density of 2.15 g/cm 3 and a solubility of 35.6 g/100 g of water (at 20 ° C). A convenient starting reagent for obtaining high-purity chromium(III) oxide (a stable green pigment and an integral part of some catalysts and a well-known mixture for polishing optics - GOI paste). In addition, ammonium dichromate is used as a component of the photosensitive layer of photographic materials and a number of pyrotechnic compositions, a wood preservative, an oxidizing agent in organic synthesis and a bleaching agent for fats, wax and paraffin.
Chromates are also used as a mordant for dyeing, tanning agents in tanning, for seed mordant (K 2 CrO 4), corrosion inhibitors (CaCrO 4, BaCrO 4, ZnCrO 4), pigments (barite yellow BaCrO 4, crown yellow PbCrO 4, red crown Pb 2 CrO 5) and also act as components of materials for thermochromic sensors.
Chromates are found in nature, primarily, for example, the mineral crocoite PbCrO 4. It was this mineral, brought to France from the Urals (“Siberian red lead”), that in 1797 served as the raw material for the production of chromium oxide, from which Nicolas Vauquelin first isolated a new element - chromium. Other chromate minerals - lopecite (potassium dichromate, potassium chromium) and tarapacaite (potassium chromate) are much less common.
Lyubov Alikberova
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Salts of chromic acids in an acidic environment are strong oxidizing agents. Substances such as H2S, H2SO3, HJ, etc., in the cold, and HBr and HC1, when heated, are easily oxidized by chromium salts. A mixture of equal volumes of a saturated solution of K2CrO7 with concentrated H2SO4 (the so-called chromium mixture) is often used in laboratory practice as a strong oxidizing agent for cleaning chemical glassware from grease deposits.
Chromic acid salts PbCl4 and BaClO4 are used as durable mineral paints. Chromium alum K2Cgg (5O4) 4 - 24NaO is used in tanning leather, making waterproof fabrics, and is used in dyeing as a mordant.
Salts of chromic acid in solutions and in solid form are yellow in color, salts of dichromic acid are orange-red. Solutions of dichromate salts have an acidic reaction.
Chromic acid salts corresponding to this oxidation state play an important role in chemical technology.
Salts of chromic acid are called chromates, and salts of dichromic acid are called dichromates. The Cl2O4 ions are colored yellow, and the Cr2Og ions are orange.
Salts of chromic acid are called chromates, and salts of dichromic acid are called dichromates.
Salts of chromic acid are called chromates, and salts of dichromic acid are called dichromates. Chromates and bichromates hydrolyze in an aqueous solution. Solutions of chromates have an alkaline reaction, while solutions of bichromates have an acidic reaction.
Salts of chromic acids are colored: chromates are usually yellow, and dichromates are orange of varying intensities. Therefore, they are used as paints. For example, lead chromate PBCrO4 (insoluble in water) - yellow crown - is used to prepare yellow oil paint; included in primers.
Salts of chromic acid are yellow in color, while salts of dichromic acid are orange-red.
Salts of chromic acid are called chromates, and salts of dichromic acid are called dichromates. Chromates and dichromates in aqueous solution undergo hydrolysis. Solutions of chromates have an alkaline reaction, while solutions of bichromates have an acidic reaction.
Salts of chromic acid are called xpoMaTaMn (VI); often the oxidation state of chromium is not indicated), for example K2CrO4 - potassium chromate.
Salts of chromic acids are strong oxidizing agents. Oxidation is carried out in an acidic solution. Cr2O ions serve as oxidizing agents; containing hexavalent chromium. By adding electrons, they become green trivalent chromium ions.
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Chromic acid acts only with difficulty on a solution of oxyzolepiden in acetic acid, even at boiling temperature. The reaction is never violent and I have only on rare occasions found traces of benzyl in the resulting products.
Chromic acid, 70 g of chromic anhydride are dissolved in 100 ml of water.
Chromic acid and chromyl chloride (Etar's reagent) also interact with paraffin hydrocarbons, and in terms of the degree of selectivity, their reactivity is basically similar to the reactivity of chlorine atoms. Susceptible to this action are the a-positions of the side chain of aromatic compounds, the tertiary C - H - groups of paraffins and cycloparaffins and the C - H - groups of ethers adjacent to oxygen; but no steric hindrance was observed and the differences in reactivity between the various compounds of these types are relatively small. C-D bonds are attacked more slowly than C-H bonds. In addition, such oxidation processes occur more slowly in the complete absence of oxygen, which is absorbed by the reaction mixtures to a weak extent. A rather curious oxidation with chromic acid, catalyzed by acids, proceeds at a rate directly proportional to the total content of chromium (VI) in the oxidized solution, and is not a heterolytic oxidation, as in the case of alcohols, aldehydes and ketones (Chapter
Chromic acid, caustic alkalis, concentrated and diluted acids have a destructive effect on human skin, clothing, fabrics and wood. Acid splashes on clothing can burn through them and land on the body if they are not immediately washed off with water and the fabric is washed with a diluted soda solution. If chromic acid gets on your body, it is recommended to immediately wash it off with a solution prepared from one part alcohol, one part hydrochloric acid and two parts water, and then wash your hands well with soap.
Chromic acid HaCgO4 is dibasic and is one of the strong acids.
Chromic acid tends to produce polychromic acids, the complex anions of which contain several atoms of chromium. The formation of polychromic acids occurs only in an acidic environment. Therefore, yellow-colored chromate ions are stable only in an alkaline environment or in the form of medium salts.
Chromic acid does not affect side chains that lack a-hydrogen.
Chromic acid in a sulfuric acid environment oxidizes allyl alcohol to acrolein, which has a characteristic odor and has a strong tear effect.
Chromic acids and their salts are strong oxidizing agents. Acting as an oxidizing agent, chromium, which is part of chromic acids and their salts, is reduced to the trivalent state. In this case, the solution turns from orange-red to green-violet.
Chromic acids exist only in aqueous solution. However, their salts are very stable.
Chromic acids form two series of salts: chromates - the so-called salts of chromic acid, and dichromates - the so-called salts of dichromic acid.
Chromic acid dissolves magnesium hydroxide and many salts well, but does not dissolve magnesium alloy, as a result of which it is widely used to clean the surface of parts from non-greasy contaminants, to dissolve corrosion products and flux inclusions when it is necessary to maintain the dimensions of parts. Chromic acid is also a good passivator for magnesium alloys.
Chromic acid is widely used in the oxidation of the alkene double bond, either as a solution of O3 in concentrated acetic acid in the presence or absence of sulfuric acid, or as an aqueous solution of sodium or potassium dichromate with sulfuric acid.
Chromic acid dissolves magnesium hydroxide and many salts well, but does not dissolve magnesium alloy, as a result of which it is widely used to clean the surface of parts from non-greasy contaminants, to dissolve corrosion products and flux inclusions when it is necessary to maintain the dimensions of parts. Chromic acid is also a good passivator for magnesium alloys.
Chromic acid is a very useful substance for any chemical laboratory or large factory. It does an excellent job of cleaning many surfaces and is very inexpensive.
Physical and chemical properties of chromic acid
Chromic acid is a crystalline substance with a red tint. This is a medium strength electrolyte. By itself, it is not capable of conducting electric current, but its melts and solutions are excellent conductors. The chromic acid solution has a red tint. Chromium in this acid has an oxidation state of +6. This is the maximum value for chromium, so in all chemical reactions it will oxidize to lower oxidation states (donate electrons). Almost all salts of chromic acid are poisonous and, like the acid itself, are strong oxidizing agents. They are widely used in paint and varnish production and leather tanning. Chromic acid is capable of reacting with oxides and hydroxides of potassium and sodium. As a result of this reaction, water and the chromate of the corresponding metal are obtained. This acid dissolves almost all metals. Iron and aluminum are exceptions; as a result of the reaction on them, an oxide film appears (acid passivates).Applications of chromic acid
Chromic acid is an indispensable substance in any good chemistry laboratory. With its help, you can easily clean dishes from organic residues, which usually do not react with many substances. The glassware does not have to be changed frequently, which significantly saves the laboratory budget. Using solutions of chromic acid, you can modify the surface of many metals (except iron and aluminum) and some other materials. The result is an intricate relief that looks very beautiful and is expensive. Materials treated with chromic acid retain their acquired shape for a long time and become immune to harmful environmental influences. Such treatment is not cheap, but the result is worth it. Catalysts are made based on chromic acid, which significantly reduce the concentration of harmful substances in exhaust gases. This interesting property has allowed engineers to create environmentally friendly engines that have almost no harmful impact on the environment. Chromic acid is used to create special rod mixtures. They are highly durable and can easily change shape when heated. With the help of this acid, they also give strength to parts that may be damaged during processing.Introduction
Chromic acid- red crystalline substance; isolated in a free state upon cooling saturated aqueous solutions of chromate. Chemical formula H 2 CrO 4 .
1. Properties
1.1. Chemical
Chromic acid is a medium strength electrolyte. Isopolychromic acids exist in aqueous solutions, colored red. Chromic acid salts are strong oxidizing agents and poisonous. In chromic acid, the oxidation state of chromium is +6 (or VI). Chromium, like most transition metals, can exist in several oxidation states. The sixth oxidation state is the highest for chromium; the third degree is the most stable, the second degree as well. Reacts with basic oxides and hydroxides of sodium and potassium, for example, amphoteric oxides and zinc hydroxide with aluminum hydroxide in its presence or in reaction with chromic acid will behave as weak bases. The reaction products are water and metal chromates.
Chromic acid corresponds to salts - chromates, isopolychromic acids - isopolychromates (also dichromates, isopoly compounds).
2. Receipt
3. Application
Solutions of chromic acid are used in electrolytic chrome plating and the production of chromium by electrolysis. Chromic acid is an intermediate in chrome plating and is also used in ceramic mortar and colored glass compositions. Because the compound of chromic acid in sulfuric acid (also known as "sulfochromic acid") is a powerful oxidizing agent, this mixture can be used to clean laboratory glassware, or when washing away insoluble organic residues.
Chromic acid is used to produce:
- high purity metal chromium,
- catalysts,
- electrolytic chromium,
- superhard materials,
- in the production of castings as part of molding and core mixtures,
- for chrome plating, chromating and passivation processes,
- for etching processes.
This abstract is based on an article from Russian Wikipedia. Synchronization completed 07/13/11 23:36:51
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