Biuret reagent, preparation

K17. Kibrick, G. R., On the determination of protein in serum and in fractions obtained from serum with a biuret reagent prepared with sodium hydroxide. J. Lab. Clin. Med. 34, 1171-1174 (1949). 

Mix the contents thoroughly and transfer them to a plastic bottle. 2-6. The biuret reagent just prepared should be a deep royal blue. This solution may be stored indefinitely. If a black precipitate is observed in the storage container, however, discard the solution and prepare a fresh solution. 

Guanidine gives a positive biuret reaction (p. 271), the reaction with sodium pentacyanoammoferroate (p. 361) (13) similar to urea, the reaction with thymol and hypochlorite (14), and the reaction with sodium 1,2-naphthoqui-none-4-sulfonate (p. 320) (15). It is also precipitated with Nessler s reagent (p. 211). Guanidine gives a weakly soluble picrate, which can be prepared by precipitation of a neutral aqueous guanidine solution with an aqueous 1% ammonium picrate solution. 

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