532 SMITH'S INTERMEDIATE CHEMISTRY 



The dichromates of potassium and sodium are made by adding 

 sulphuric acid to the crude solution of the chromate obtained from 

 chromite (p. 531). They crystallize when the liquid cools, and 

 the mother-liquor, containing the potassium sulphate and un- 

 deposited dichromate, is used for extracting a fresh portion of 

 cinder. As the dichromates are much less soluble than the chro- 

 mates, they crystallize from less concentrated solutions, and can 

 therefore be obtained in purer condition. For this reason the 

 extract is always treated for dichromate. 



Chemical Properties of the Dichromates. 1. When 

 concentrated sulphuric acid is added to a dichromate, chromic 

 anhydride OO 3 separates in red needles: 



H 2 SO 4 - Na 2 S0 4 + H 2 + 2CrO 3 j . 



2. Although a dichromate lacks the hydrogen, it is essentially 

 of the nature of an acid salt. Hence, when potassium hydroxide 

 is added to a solution of potassium dichromate, potassium chro- 

 mate is formed: 



K 2 Cr 2 7 + 2KOH -* 2K 2 Cr0 4 + H 2 O. 



The solution changes from red to yellow, and the chromate is 

 obtained by evaporation. In this way the pure alkali chromates 

 are made. 



3. With free acids the dichromates give powerful oxidizing 

 mixtures, in consequence of their tendency to form chromic salts 

 (see below). 



Uses of Dichromates. When paper is coated with gelatine 

 containing a soluble chromate or dichromate and, after being 

 dried, is exposed to light, chromic oxide is formed by reduction, 

 and combines with the gelatine. This product will not swell 

 up or dissolve in tepid water, as does pure gelatine. This action 

 is used in many ways for purposes of artistic reproduction. Thus, 



