614 CHROMATES. 



Terchromate of potash, KO, 3Cr O 3 , is obtained crystallized 

 when a solution of the bichromate is mixed with nitric acid, and 

 evaporated. Bichromates of soda and of silver exist, which are 

 anhydrous, like the bichromate of potash, (Warington.) 



Chromate of soda, Na O, Cr O 3 + lOHO. By the evaporation 

 of a concentrated solution of this salt, it is obtained in large 

 fine crystals, having the form of glauber salt. 



Chromate of lead, PbO, Cr O 3 ; 2046.3 or 163.97- This 

 compound, so well known as chrome yellow, is obtained by mix- 

 ing the nitrate, or acetate of lead, with the chromate or bi- 

 chromate of potash. The precipitate is of a lighter shade from 

 dilute than from concentrated solutions. It is entirely soluble 

 in potash or soda, but not in dilute acids. 



Subchr ornate of lead, 2Pb O, Cr O 3 , is of a red colour. It is 

 formed when a solution of neutral chromate of potash, with as 

 much free alkali added to it as it already contains, is added to a 

 solution of nitrate of lead. But the finest vermilion-red sub- 

 chromate is formed when one part of the neutral chromate of 

 lead is thrown into five parts of nitre, in a state bf fusion by 

 heat. Water dissolves the chromate and nitrate of potash of 

 the fused mass, and leaves the subchromate of lead, as a crys- 

 talline powder, (Liebig and Wb'hler.) An orange pigment may 

 be obtained very economically by boiling the sulphate of lead, 

 which is a waste product in making acetate of alumina from alum 

 by means of acetate of lead, with a solution of chromate of potash. 

 The subchromate of lead forms a beautiful orange upon cloth, 

 which is even more stable than the yellow chromate, not being 

 acted upon by either alkalies or acids. One method of dyeing 

 chrome orange, is to fix the yellow chromate of lead first in the 

 calico, by dipping it successively in acetate of lead and bichro- 

 mate of potash, and then washing it. This should be repeated, 

 in order to precipitate a considerable quantity of the chromate 

 in the calico. A milk of lime is then heated in an open pan, 

 and when at the point of ebullition, the yellow calico is im- 

 mersed in it, and instantly becomes orange, being deprived of a 

 portion of its chromic acid by the lime, which forms a soluble 

 chromate of lime. At a lower temperature, lime-water dissolves 

 the chromate of lead entirely, and leaves the cloth white. 



Chromate of silver falls as a reddish brown precipitate when 

 nitrate of silver is added to neutral chromate of potash. Dis- 

 solved in a hot and concentrated solution of ammonia, it gives, 



