174 PROCEEDINGS OF THE AMERICAN ACADEMY. 



perature ; 100 parts of water dissolve 162 parts of the dry salt at 100° 

 and 109 parts at 15°. ^ Since, however, the salt crystallizes with two 

 molecules of water, the actual loss on crystallization is less than would 

 appear from a superficial inspection of these figures. The weight of 

 the crystals separated fi-om a saturated solution between these two tem- 

 peratures from 162 grams of anhydrous salt dissolved in 100 grams of 

 water is easily calculated fi-om the equation : 



Na^CrA ^ ^109 



when a? = 87 grams. Thus more than half the salt is easily recovered 

 from each crystallization, and if use is made of a wider range of tem- 

 perature, permitted by the high saturated boiling point (139° with 

 solubility 210 parts to 100 of water) a still larger yield may be obtained. 

 Since the solubility diminishes but slightly below 15°, further cooling 

 is not profitable. Thus the crystallization of this salt is convenient and 

 efficient ; and since sodium bichromate is not isomorphous with the cor- 

 responding salts either of potassium or of sulphuric acid (the most prob- 

 able and insidious impurities) crystallization forms an admirable means 

 of purification. The elimination of the sulphuric acid requires only 

 three or four crystallizations if the crystals are separated centrifugally. 

 A large excess of chromic acid causes unnecessary loss, because of the very 

 great solubility of the trichromate,^ but a small excess is convenient, 

 because the elimination of the dark brownish tint of the trichromate 

 from the mother liquor is a convenient guide as to the purity of the 

 salt. 



The crystallized, pure bichromate was treated with somewhat less than 

 the calculated amount of pure sodium carbonate, and with the help of 

 the excess of bichromate the carbon dioxide was easily removed by 

 boiling the solution during an hour. From a neutral solution all the 

 carbon dioxide cannot be expelled. The sodium carbonate for this 

 purpose should be prepared by repeatedly crystallizing the purest soda 

 of commerce in porcelain, or better in platinum dishes, until sulphates 

 and chlorides are certainly absent. 



The sodium chromate containing a small amount of sodium bichro- 

 mate, prepared as described above, was neutralized with the help of 

 phenolphthalein by means of a solution of pure sodium hydroxide, 

 made from sodium by allowing this metal to stand in a platinum dish 

 in a desiccator over a dilute solution of the hydroxide. 



« Stanley, Chem. News, 54, 194 (1886). 3 Ibid. 



