RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROMATE. 175 



Some time was spent in determining the conditions under which 

 phenolphthalein gives satisfactory results in the presence of pure 

 sodium chromate. Schreinemakers * states that the chromate is neu- 

 tral to phenolphthalein and recommends the use of this indicator in 

 analyzing solutions of chromates for excess of either hydroxyl or 

 chromate ion. In our experiments it appeared that the solubility 

 of the indicator in very concentrated sodium chromate solutions is too 

 small under ordinary conditions to permit of a visible reaction with the 

 low concentration of ionized hydroxyl which must exist there. This is 

 especially true when alcoholic phenolphthalein is dropped into a satu- 

 rated solution of the chromate. In this case the indicator seems to be 

 precipitated by the salt in such form as to dissolve only very slowly on 

 subsequent dilution. This difficulty was avoided by diluting the indi- 

 cator with pure water in the first place. On the other hand, a dilute 

 solution of the salt is easily shown to be appreciably hydrolyzed, the 

 color of phenolphthalein being distinctly visible superposed on the yel- 

 low of the salt. Increasing temperature of course causes increasing 

 hydrolysis ; in boiling solutions of the purest very concentrated sodium 

 chromate the hydrolysis is sufficiently great to cause a distinctly 

 reddish tint. 



As an outcome of these experiments the neutralization was conducted 

 in concentrated solutions of sodium chromate at room temperature, the 

 indicator in dilute solution being added to the separate test portions. 

 To the main solution sodium hydroxide was added until the phenol- 

 phthalein just began to show its color in the separate test, and then 

 sodium dichromate until the color just disappeared. This solution 

 may have contained a constant and very slight excess of dichromate, 

 but this substance is easily removed by subsequent crystallization, 

 being heteromorphous as regards sodium chromate. 



The neutral salt prepared in this way may be further purified by 

 crystallizing either as the dekahydrate, hexahydrate, or tetrahydrate. 

 The first named phase, Na2Cr04 • IOH2O is to be preferred on account of 

 economy of material, except as a means of eliminating sodium sulphate. 



To crystallize the salt with four molecules of water the temperature 

 of the solution must be kept above 26° ; but at 20° 100 parts of water 

 dissolve 86 parts of sodium chromate, whereas at 0°C. 100 parts of 

 water dissolve only about .32 parts of sodium chromate in equilibrium 

 with dekahydrate ; hence arises one reason for the greater economy in 

 crystallizing the dekahydrate at the lower temperatures. Moreover, a 

 greater rate of purification from every common impurity except sodium 



* Zeitschr. f. Phys. Chem., 56, 72 (1906). 



