1913] Alfred P. Lo f Itrop 557 



The complex colorless Cyanide o£ copper, which is readily soluble 

 in 10 per cent. sodium hydroxid soution, was tried. The Solution 

 was colorless and it gave a strong characteristic pink to purple reac- 

 tion when added to a dilute Solution of Witte peptone. Unfor- 

 tunately, however, the Solution gradually turned blue, It was 

 thought that this blue oxidation product might be reduced with an 

 excess of potassium Cyanide to drive the cupric ion into the colorless 

 complex ion. Experiment proved such to be the case, but this color- 

 less Solution would not give the biuret reaction when added to pep- 

 tone Solution. This is an extremely interesting fact from which 

 the inference may be drawn that the biuret reaction takes place only 

 with the positive copper ion. But if this inference is correct, how 

 is it that one obtains a reaction with the colorless Solution made by 

 adding K2Cu(CN)3 to 10 per cent. sodium hydroxide Solution? 

 This fact is easily understood when we consider the dissocia- 

 tion of this salt. The instability constant of the complex ion, 



(Cu"") X (CN")^ 



r rr\j\ = — ~' is 0.5 X lO"^'^, and the concentration of cuprous 



ion is approximately 3.7 X io~^ in a tenth molar Solution. But, 

 this very small concentration of cuprous ion is sufficient to Start 

 the reaction. In the strong alkaline Solution there is immediate oxi- 

 dation of Cu"^ to Cu"^"^, which reacts with the protein giving some 

 pink color; as soon as this happens more of the KoCu(CN)3 dis- 

 sociates, forming more Cu"^->Cu'^^ which results in more of the 

 pink product with the protein. If an excess of KCN is added, 

 there is a driving back of the ionization of K2Cu(CN)3, positive 

 copper ions are removed and hence no reaction occurs when protein 

 Solution is treated with the reagent. 



In Order to determine whether the reagent could be prepared 

 for use during a laboratory day without oxidation, 0.2 gm. of the 

 white double Cyanide was added to 50 c.c. of sodium hydroxid Solu- 

 tion, and observed for appearance of any blue color. From time to 

 time the flask containing the Solution was opened and portions were 

 withdrawn for the purpose of making tests upon proteins. After 

 five hours of such treatment the reagent began to acquire a light 

 blue color, due to the oxidizing effect of the air, the color deepening 

 to that of the regulär reagent. If air is excluded from the bottle, 

 the development of color proceeds more slowly. 



