128 CHEMICAL STATICS 



solution of cupric chloride, the mixture is at first green, indicating 

 the presence of lower hydrates of cupric chloride, but on adding 

 more of the solution it becomes blue, and simultaneously with 

 the appearance of a pure blue color, but not before, precipitation 

 of cupric caseinate occurs. If, to 5 cc. of a 2 per cent solution in 

 formic acid, we add 1|, 2, or 2| cc. of a saturated solution of 

 cupric chloride, no precipitation of the caseinate occurs, but on 

 diluting this mixture with water a precipitate results, and the 

 appearance of this precipitate coincides with the attainment of a 

 clear blue color on the part of the mixture. 



About six cubic centimetres of water are required to produce 

 a permanent precipitate. This precipitate redissolves on heat- 

 ing and the mixture simultaneously becomes green; on cooling the 

 blue color reappears and with it the precipitate. If formic 

 acid be added to the mixture the precipitate redissolves as soon 

 as the mixture becomes green. If the precipitate be very slight 

 it will redissolve on adding alcohol. It cannot be urged that the 

 formation of cupric caseinate requires the presence of a sufficient 

 concentration of cupric ions, because green solutions of cupric 

 chloride contain an abundance of ions * and casein will react 

 with very small amounts of metal ions, for although it is itself 

 insoluble it will drive carbonic acid out of the sparingly soluble 

 calcium carbonate to form a freely soluble caseinate of calcium 

 (vide Chap. V). 



If instead of adding water to a mixture of 5 cc. of 2 per cent 

 casein in formic acid and 2 cc. of saturated CuCl2, we add alcohol, 

 no coagulation occurs until the mixture changes in color from 

 green to brown when a coagulum of cupric caseinate is produced 

 which redissolves on adding water. 



Similar results are obtained when a 2 m. solution of cobalt 

 chloride is employed instead of a saturated solution of cupric 

 chloride. If to 5 cc. of a 2 per cent solution of casein in formic 

 acid we add 2 to 3 cc. C0CI2 we obtain a blue-purple solution; 



* Green solutions containing, probably, a mixture of the anhydrous brown 

 salt and the fully hydrated blue salt. Even on the basis of the hypothesis 

 urged by Donnan (11), (12), therefore, a considerable number of cupric ions 

 must exist in green solutions. It is important to notice that the precipitation, 

 as stated above, does not occur until the solution is pure blue in color; mix- 

 tures so slightly green that they appear wholly blue until viewed alongside 

 pure blue mixtures produce no precipitate. 



