RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROMATE. 173 



means of accomplishing the purification ; ten successive crystalHzations 

 having failed to yield a pure salt. Moreover, inconvenience arises from 

 the fact that it is stable under the solution only between 26° and G2.8°, 

 and because its solubility has only a comparatively small temperature 

 coefficient, the yields are small. 



Attempts to prepare the salt by recrystallizing chromic anhydride 

 were likewise abandoned as unprofitable. The best chromic anhydride 

 obtainable was found to contain much sulphate, which w^as very slow 

 to leave during the particularly wasteful process of its crystallization. 

 In one case twelve successive crystallizations of a kilogram of chromic 

 anhydride gave only a few grams of product which still contained 

 traces of sulphur. These unsuccessful methods are recounted only 

 to save other experimenters the trouble of attempting the same 

 processes. 



Another method which is distinctly better than the preceding is per- 

 haps worthy of more detailed discussion, although a still better and 

 much simpler procedure was afterwards devised. 



The sulphate can be quickly eliminated by relying upon the very 

 different solubilities of silver chromate and sulphate. If a chromate is 

 slowly precipitated by silver nitrate in dilute solution, using not quite 

 enough of the precipitate to remove the chromate wholly from the solu- 

 tion, the precipitate contains very little sulphate. The chromate of 

 silver is then thoroughly washed, and easily converted into sodium 

 chromate by treatment with a solution of pure sodium chloride. The 

 presence of an excess of precipitated silver chromate ensures the absence 

 of all but the merest traces of chloride in the solution, if time has been 

 allowed for the attainment of equilibrium. If greater purity is desired, 

 the chromate may be reprecipitated by silver nitrate, and again treated 

 with pure common salt. The very appreciable quantity of silver which 

 the solution of sodium chromate still contains is easily precipitated by 

 copper wire or gauze, and the cupric chromate, being easily soluble, is 

 quickly eliminated upon crystallizing the sodium chromate as dekahy- 

 drate. It is idle to attempt to get rid of the silver chromate by crystal- 

 lization, since its solubility is affected about as much proportionally by 

 temperature as that of sodium chromate is. After the silver has been 

 removed, five crystallizations were enough to remove all the copper and 

 other impurities, and a specimen of sodium chromate was obtained 

 which gave every evidence of being pure. 



Yet another available method, the most satisfactory one of all, de- 

 pends upon the fact that sodium bichromate may be easily and quickly 

 freed from sulphate by recrystallization. The bichromate is very solu- 

 ble, and possesses a moderately great change of solubility with the tern- 



