Strontium dichromate

Strontium Dichromate, SrCrjO. SHjO, is formed by dissolving the normal chromate in chromic acid solution and allowing to crystallise, or by treating freshly precipitated strontium chromate with solid chromic anhydride. It yields large reddish-brown monoclinic crystals,... 

Self regulating chromium The self-regulating chromium solutions were introduced to eliminate the need for maintaining the correct catalyst concentration by periodic analysis they depend on the addition of a sparingly soluble sulphate to the bath which supplies the correct amount of SO 4 automatically. Initially strontium sulphate (solubility approx. l-75g/l at 30°C and 21 g/1 at 40°C) was employed for this purpose. The strontium sulphate forms a layer on the bottom of the bath, which must be stirred from, time to time. A bath with a CrO, concentration of 250 g/1 would have a catalyst content of l 52g/l SrS04 and 4-35 g/1 of KjSiF. Potassium dichromate and strontium chromate have also found application as additives for the control of the saturation solubility of the catalyst. 

Strontium Chromate. Strontium chromate is precipitated from solutions of sodium dichromate and strontium chloride, followed by filtration, washing, drying, and grinding. A primer composition based on strontium and calcium chromates is described in [5.70], [5.118]. 

Solubility The chromates of the alkali metals and of calcium and magnesium are soluble in water strontium chromate is sparingly soluble. Most other metallic chromates are insoluble in water. Sodium, potassium, and ammonium dichromates are soluble in water. 

Chromates are usually yellow or red in colour, and, except those of ammonium, the alkali metals, calcium, strontium, and magnesium, are practically insoluble in w ater. They are obtained by oxidation of chromites, by fusion of chromium sesquioxide with the appropriate base in presence of air or of an oxidising agent by oxidation of chromium salts in solution or by double decomposition. Normal, di-, and tri-clrromates, etc., are derived from one and the same acid oxide KaCrOj behaves like an alkali torvards CrOg, since it is quantitatively converted into dichromate. A large number of complex double chromates are known. 

Cobalt(III) Co -" Strontium Sr -" Dichromate Cr20 - Permanganate Mn04... 

Chromium occurs in three basic forms metallic chromium (Cr(0)), trivalent chromium (Cr(III)), and hexavalent chromium (Cr(VI)). Hexavalent chromium can exist as chromium hexavalent ion and as part of a number of compounds including calcium chromate, chromic acid, chromium trioxide, lead chromate, strontium chromate, potassium dichromate, and zinc chromate. 

Hexavalent chromium compounds are classified as substances known to be carcinogenic to humans. This is based upon sufficient evidence of carcinogenicity in humans exposed in chrome production facilities, chromium-alloy facilities, in the chrome plating industry as well as in chrome pigment industries. This exposure results in an increased incidence of lung cancer among these workers. The incidence of cancers at other sites may be increased in these occupational workers There is not sufficient evidence to show that barium chromate, calcium chromate, chromium trioxide, lead chromate, sodium dichromate and strontium chromate are carcinogenic in humans. 

Acid neutralisation Sodium dichromate 12.13 Strontium carbonate production... 

To prepare potassium dichromate from this strontium chro-See H. A. Doerner, Chem. and Met, Eng., 47 (1940), 688. 

If calcium chromate be treated with a soln. of potassium sulphate, the calcium chromate is converted into calcium sulphate, which is precipitated, and potassium chromate, which remains in soln. Instead of leaching the calcium chromate with a soln. of potassium sulphate, W. J. Chrystal showed that if ammonium sulphate is used, a soln. of ammonium chromate is produced, and J. J. Hood found that if the soln. of potassium salt be treated with sodium hydrosulphate, potassium sulphate crystallizes from the soln., while sodium dichromate remains in soln. According to F. M. and D. D. Spence and co-workers, if a mixtiu-e of ammonia and carbon dioxide be passed into the aq. extract of the calcium chromate. Calcium carbonate is precipitated while ammonium and alkali chromate remain in soln. If the liquid be boiled, ammonia is given off, and sodium dichromate remains in soln. S. Pontius used water and carbon dioxide under press, for the leaching ptocess. J. Brock and W. A. Rowell purified alkali chromite by treating the soln. with strontium hydroxide, and digesting the washed precipitate with a soln. of alkali sulphate or carbonate W. J. A. Donald used calcium hydroxide or barium chloride as precipitant. A mixture of chromite with calcium carbonate and potassium carbonate was formerly much employed. Modifications of the process were described by W. J. A. Donald, A. R. Lindblad, C. J. Head, 8. G. Thomas, W. Gow, J. Stevenson and T. Carlile, L. I. Popoff,G. Bessa, P.Weise, P. N. Lukianoff,... 

For the equilibrium between barium chromate and dichromate, vide supra, chromic acid. I. Meschtschersky found that a litre of water dissolved 4-3 mgrms. at 100°. C. R. Fresenius said that at 0°, a litre of water dissolves 1-19 mgrms. The salt is less soluble after it has been ignited. E. Schweizer said that in that state a litre of water dissolves 0-62 mgrm. L. M. Henderson and F. C. Kracek pointed out that the solubilities of the alkaline earth chromates decrease as the at. wt. of the alkaline earth metal. Thus, at 15°, the solubilities of the anhydrous calcium, strontium and barium chromates are of the order 25,1-2, and 0-0033 grms. per litre respectively. The solubility of ladiom chromate seems to follow the rifle, and a separation of radium and barium can be effected by fractional precipitation as chromates. The partition factor for acidic soln. is about 15-5. 

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