RICHARDS. — TRANSITION TEMPERATURES OF SODIUM CHROMATE. 177 



and like it, the chromate is efflorescent in the dry air of the steam- 

 heated ^ laboratory. In order to make sure of the composition of the 

 salt, a specimen which had been dried as much as possible without visi- 

 ble efflorescence was analyzed. 0.8316 gram of the salt lost on dehydra- 

 tion 0.4358 gram of water, or 52.43 per cent. The theoretical per- 

 centage is 52.70, hence there can be no doubt as to the formula 

 NaAOi-lOHzO.e 



^^he hexahydrate, Na2Cr04 • 6H2O has only a very small range of 

 stability, as will be seen from an examination of the diagram on page 

 1 78. The preparation of this hydrate was first described by Salkowski."^ 

 At temperatures below 19.53°, especially in contact with its solution, it 

 tends to pass into the dekahydrate modification, and above 26°, into 

 tetrahydrate. Inasmuch as the temperatures within which it is stable 

 are those most frequently found in the laboratory, it is the phase most 

 frequently met with. When the dekahydrate warms to 19.53° or the 

 tetrahydrate cools to 26° particles of hexahydrate in the atmosphere 

 induce the crystallization of this form so readily that it is often difficult 

 to prevent the appearance of this phase. In two analyses, 1.4152 

 grams and 1.7786 grams of the salt lost respectively 0.5705 gram and 

 0.7122 gram of water on drying at 180° to constant weight. These 

 results correspond to 40.3 and 39.4 per cent of water respectively, and 

 since the first sample was somewhat moist and thfe second was slightly 

 effloresced, there can be no doubt that the substance was really 

 Na2Cr()4 • eHzO, a salt possessing theoretically 40.0 per cent of water. 

 The material gives a transition point at about 25.9° (hydrogen scale), 

 being converted into tetrahydrate at that temperature. 



The tetrahydrate, Na2Cr04 • 4H2O, is stable in contact with its solution 

 only at temperatures above 26° and below 62.8° ±. Even in the dry 

 air of a steam-heated room it shows no evidence of efflorescence ; there- 

 fore its aqueous vapor tension must be very low. This hydrate is formed 

 when a hot concentrated solution of sodium chromate is allowed to cool 

 to a temperature above 26°, and the needle-shaped crystals melt at 

 about 62.8°. Occasionally other crystals of a hydrate or anhydride sep- 

 arate from the solution above 62.8". Because the hydration of the latter 

 crystals at low temperatures is slow, enough water should be added to 

 a very hot solution to keep the salt dissolved until the temperature 62.8° 



* Kopp's ascribing deliquescence to the salt must have been due to his 

 working in a very moist atmosphere. 



• These analyses, as well as those of the hexa- and tetrahydrate given further 

 on, were kindly made by J. B. Churchill under the direction of one of us. 



' Salkowski, Ber., 34, 1947 (1901). The salt had been made and analyzed 

 previously by Richards and Churchill, but no description had been published. 



