ANNALS 



OF 



PHILOSOPHY. 



NOVEMBER, 1820. 



Article I. 



Description of Chromate and Bichromate of Potash. 

 By Thomas Thomson, M.D. F.R.S. 



oINCE the original discovery of chromic acid by Vauquelin 

 scarcely any addition has been made to our knowledge of the 

 salts which it forms, except a few very short notices respecting 

 them by John, and the analysis of a single chromate by Berze- 

 lius. Indeed the reserve with which they have been mentioned 

 by systematic writers will not surprise practical chemists, who 

 are aware of the great difficulty of preparing the chromates in a 

 state of purity, in consequence of the disposition which they 

 have to enter into triple compounds. I have repeatedly turned 

 my attention to these salts, and have been at considerable 

 pains to obtain them pure, and to determine their properties and 

 their composition. I propose at present to confine myself to an 

 account of the salts which chromic acid forms with potash ; 

 because these salts constituted the materials out of which all the 

 other chromates were formed. 



To obtain chromate of potash, I employed the method recom- 

 mended by Vauquelin many years ago. Chromate of iron was 

 reduced to powder, and heated for several hours in a crucible 

 mixed with half its weight of saltpetre. The mass was then 

 digested in water, and the same process was repeated with the 

 undissolved residuum. The yellow-coloured liquids thus ob- 

 tained were slightly alkaline. They were saturated with nitric 

 acid, and concentrated, till no more crystals of saltpetre could be 

 obtained from them. The yellow liquid was now set aside for 



Vol. XVI. N" V. X 



