172 PROCEEDENGS OF THE AAIERICAN ACADEMY. 



what less easy to purify than the sulphate, and when it is impure, 

 a given percentage of impurity causes a larger error in the transition 

 temperature than in the case of the sulphate. This is largely because 

 the transition is from the dekahydrate to tetra- or hexahydrate, instead 

 of to anhydrous salt, so that the amount of solution formed to dissolve 

 the impurities is less, and therefore the concentration of the impurity 

 in the solution is greater. Another difficulty arises from the fact that 

 chromic acid is a weak acid, and hence its salts are easily capable of 

 absorbing acid impurities. This was shown experimentally by passing 

 pure carbon dioxide into the solution. Large volumes of the gas were 

 absorbed, and the solution became perceptibly orange because of the 

 formation of the dichromate. Boiling expels the gas but slowly, 

 because the hydrolysis increases with rise of temperature. Indeed, 

 a boiling solution of the purest salt is so much hydrolyzed as to be red 

 in color and to react strongly with phenolphthalein. Probably the 

 passing of pure air, or boiling at low temperature for a long time in 

 a partial vacuum, would be needed to displace all of the carbon dioxide, 

 and even this might not serve. Of course the solution of the salt ab- 

 sorbs hydrochloric acid and other strong acid vapors yet more easily ; 

 and even the dry salt, which is unaffected by carbon dioxide, is attacked 

 by stronger substances. 



Theoretically the tendency of the solution to hydrolyze offers no ob- 

 jection to the use of the salt as a means of fixing a definite point on 

 the thermometer scale, because the Phase Rule, which determines the 

 definiteness of the points, does not consider the state in which the sub- 

 stance exists in any of the phases. The feeble nature of chromic acid 

 is annoying and objectionable only for the practical reason that it makes 

 the preparation and preservation of the pure salt more difficult. When 

 the salt has once been prepared in the pure state, it must be carefully 

 guarded in order to preserve its purity. 



These difficulties, however, all disappear when the experimenter is 

 forewarned against them ; for easily effected precautions are capable 

 of surmounting them all. 



Preparation of Pure Sodium Chromate. 



Several methods of further purification of sodium chromate were 

 tried. Crystallization as the dekahydrate is of no avail as a means of 

 ridding the crystals of their most usual impurity, the sulphate, because 

 this is isomorphous with the chromate. Indeed the sulphate tends into 

 the crystals, leaving the mother liquor somewhat purer than before the 

 crystallization. The tetrahydrate is but little more satisfactory as a 



