Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/179

 sideways to and fro. The mouth of the tube should not be closed with the linger, as in this manner some impurities may easily enter the mixture. Very often the dialysates become turbid upon the addition of the caustic soda solution, but this does not interfere with the reaction. In order to test for diffused albumen we have different methods at our disposal, of which the biuret reaction has been found to be the best. One could also make use of the precipitin formation that appears when prepared serum is employed, but such serum is not always at hand. Further, we may use ninhydrin, but it is not so sensitive to albumen.

Ninhydrin reacts, amongst others, with compounds which carry an amino group in a position to the carboxyl group; when it produces a bluish-violet colour, if the concentration of the reacting compounds is sufficiently strong. The albumen molecule contains a few free amino and carboxyl groups, and as soon as it is decomposed, these groups are set free. The ninhydrin reaction becomes stronger the more the albumen is decomposed, provided the various stages of decomposition are not withdrawn. At each stage an amino and carboxyl group are set free. The biuret reaction manifests itself quite differently. The greater the fractional decomposition of the albumen, the weaker is the biuret reaction. As soon as we pass a certain limit of decomposition the reaction ceases.