154 THE DIALYSATION PROCESS 



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 albu- 

 men 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 com- 

 pounds 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 of decomposition are not withdrawn.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. 



