Potassium Chromate and Dichromate

In Group VIII, each position instead of being filled by a single element is occupied by a group of three elements. Thus there appear in triads iron, cobalt, and nickel ruthenium, rhodium, and palladium and osmium, iridium, and platinum. In this group there is no subdivision into families, but all the members are heavy metals. 

Of the heavy metals discussed above, the ones that are of most frequent occurrence and that are to receive detailed treatment in this chapter are chromium, manganese, and iron. 

On account of the difficulty of carrying out the industrial process on a laboratory scale, the following less economical procedure is recommended The raw material, chromic oxide, Cr203, is first treated with fused KOH. The oxidizing agent, KN03, is then added and it works more effectively than if it had been added at the outset. The aqueous extract of this fusion is allowed to crystallize to obtain a preparation of potassium chromate. The mother liquor from these crystals is acidified With acetic acid to obtain a preparation of potassium dichromate. Potassium compounds are prepared because they are less soluble and crystallize better than the corresponding sodium compounds. 

A saturated solution contains for each 100 grains of water the given number of grams of the anhydrous salt. 

Apparatus 4-inch sheet iron crucible with cover. 

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Pour potassium chromate and dichromate solutions into separate test tubes and add a solution of a barium salt to each of them. What is the composition of the precipitates See how the obtained salts react with dilute acids. Perform similar experiments with a lead salt. Write the equations of the reactions. 

Dry the potassium dichromate on a white paper towel at room temperature. Put the potassium chromate and dichromate into separate 2-ounce cork-stoppered bottles. Calculate the theoretical yield of potassium chromate from 50 grams of chromic oxide. Deduct your yield of chromate from this result, and calculate the theoretical yield of potassium dichromate. 

Potassium chromates.—The modes of preparation and an account of the properties of potassium chromate and dichromate are given in Volume VII. 

Experiment 182.— (a) Examine crystals of potassium chromate and dichromate, and state their most obvious physical properties. From previously determined facts, predict their solubility or insolubility in water. Verify the prediction. 

Green JH, Maddock AG (1949) (n, y) Recoil effects in potassium chromate and dichromate. Nature 164 788... 

An alloy of sodium and potassium (NaK) is used as a heat-transfer medium. Many potassium salts are of utmost importance, including the hydroxide, nitrate, carbonate, chloride, chlorate, bromide, iodide, cyanide, sulfate, chromate, and dichromate. 

Manufacture The primary iadustrial compounds of chromium made directly from chromite ore are sodium chromate, sodium dichromate, and chromic acid. Secondary chromium compounds produced ia quantity include potassium dichromate, potassium chromate, and ammonium dichromate. 

The NIOSH recommended exposure limit for carcinogenic hexavalent chromium is 1 lg/m Cr(VI) as a 10-h TWA, and for noncarcinogenic Cr(VI) the 10-h TWA is 25 lg/m Cr(VI), including a 15-min maximum exposure of 50 lg/m Cr(VI). According to NIOSH, the noncarcinogenic Cr(VI) compounds are chromic acid and the chromates and dichromates of sodium, potassium, lithium, mbidium, cesium, and ammonia. NIOSH considers any hexavalent chromium compound that does not appear on the preceding Hst carcinogenic (145). 

Zirconium reduces almost all oxygen-containing salts. This is the case for alkali hydroxides (accidents with the lithium, sodium and potassium compounds) and zirconium hydroxide, lithium, sodium and potassium carbonates, alkaline sulphates sodium tetraborate and copper (II) oxide. This is true especially for oxidising salts such as alkaline chromates and dichromates, chlorates (accident with potassium salt) and nitrates (accident with potassium salt). 

Munjal, N. L. et al., Amer. Inst. Aero. Astronaut. J., 1972, 10, 1722-1723 The effects of dichromates in promoting ignition of non-hypergolic mixtures of red fuming nitric acid with cyclohexanol, 2-cresol or 3-cresol, and furfural were studied. Ammonium dichromate was most effective in all cases, and the only effective catalyst for cyclohexanol. Potassium chromate and potassium dichromate were also examined. 

Pour an ammonium sulphide solution into a potassium chromate or dichromate solution and heat the mixture. How does the solution s colour change What is the composition of the precipitate Write the equation of the reaction. What properties does chromium (VI) exhibit in this reaction How does an acidified potassium chromate or dichromate solution react with hydrogen sulphide, sulphur dioxide, and an iron(II) salt Write the equations of the reactions. 

Some chromium(VI) compounds, such as, chromium trioxide (chromic acid), potassium dichromate, potassium chromate, sodium dichromate, and sodium chromate, are very caustic and can cause bums upon dermal contact. These burns can facilitate the absorption of the compound and lead to systemic toxicity. 

Chromate and Dichromate. To a yellow solution of potassium chromate add 6 N H2SO4, and note that the color changes to orange red. To an orange red solution of potassium dichromate add 6 N NaOH and note that the color changes to yellow. 

The chromates and dichromates are made by a method which has general usefulness for preparing salts of an acidic oxide—the method of fusion with an alkaline hydroxide or carbonate. The carbonate functions as a basic oxide by losing carbon dioxide when heated strongly. Potassium carbonate is preferred to sodium carbonate because potassium chromate and potassium dichromate crystallize well from aqueous solution, and can be easily purified by recrystallization, whereas the corresponding sodium salts are deliquescent and are difficult to purify. 

Chromyl Chloride, CrOjClj, sometimes termed cldorocliromic acid, is readily prepared by distilling a mixture of fused potassium chromate or dichromate and sodium chloride with concentrated sulphuric acid ... 

In dilute aqueous solution the chief constituent is undoubtedly dichromic acid, HjCr O, and a study of the molecular solution volumes and molecular refraetivities of chromic acid, potassium chromate, and potassium diehromate affords further evidence in support of tliis formula for the acid. ... 

An unstable monoclinic form, isomorphous with rubidium and ammonium dichromates, has been obtained adnhxed with the triclinic variety, by concentrating, after filtration from aluminium hydroxide, a mixture of potassium chromate and aluminium sulphate, and allowing the solution to crystallise. It may also be obtained in yellowish brown, doubly refracting monoelinic plates by crystallisation of a mixture of solutions of potassium diehromate and potassium thiocyanate. ... 

A method, which is considered sufficiently accurate for technical purposes, for the estimation of alkali chromates and dichromates in presence of each other, depends upon titration mth acid or alkali. Potassium dichromate is titrated with potassium hydroxide in presence of phenolphthalein ... 

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