191 1] John L. Kantor and William J. Gies 269 



(2) the removal of much of the copper from Solution in the only 

 metallic reduction process thus far attempted. 



While the preparation of a colorless biuret reagent is theoretic- 

 ally desirable, it is quite probable that a colorless reagent would fail 

 to exceed the blue biuret reagent in effectiveness. The color con- 

 trast between the blue reagent and the red product with protein 

 appears to be helpful in forming a conclusion in difficult cases. 

 Thus far the original biuret reagent has met every expectation in 

 tests for the smallest proportions of protein that can be detected 

 with the aid of the biuret test as applied in the classical way and, 

 pending the successful preparationofasatisfactory colorless reagent, 

 if we ever succeed in devising it, we strongly recommend the blue 

 reagent. 



The biuret reagent may be prepared in the following propor- 

 tions: Into 1000 c.c. of 10 per cent. sodium hydroxid Solution pour 

 25 c.c. of 3 per cent. cupric sulfate Solution, a few cubic centimeters 

 at a time, with vigorous shaking after each addition. The beautiful 

 blue Solution is the reagent. Weaker or stronger copper Solutions 

 may be made, by changing the volume of added copper sulfate Solu- 

 tion, but our experience indicates that, for routine work, the propor- 

 tions indicated above are as satisfactory as any. These Solutions 

 keep indefinitely, but Silicate, ferric hydroxid and other materials 

 may form Sediments, if the chemicals are impure or the walls of 

 the Container are attacked. 



If filtration of the biuret reagent is desirable at any time, glass 

 wool may be used to accomplish it. Filter paper and cotton wool 

 cannot be employed for this purpose because of the attraction be- 

 tween cellulose and the copper of the reagent, a fact to which we 

 have already made public allusion and to which we intend to retum 

 with details at an early date.^ 



* Kantor and Gies : loc. cit. We have already published our Observation of 

 the fact that when filter-paper strips are suspended in the biuret reagent, the 

 paper " absorbs " the copper, is colored blue as a consequence and the reagent 

 itself is decolorized. We have lately observed the same blue coloration of filter 

 paper strips suspended in colorless ammoniacal biuret reagent. 



