236 ESTIMATION [CH. X 



Amides can be recrystallised from dilute (50 p. c.) 

 alcohol. 



Estimation of Froteids. 



The methods based on weighing the precipitated pro- 

 teids are seldom satisfactory, as the precipitates are gener- 

 ally impure. The quantity of proteid in the precipitate 

 can most conveniently be estimated by determining the 

 nitrogen present and multiplying by the appropriate factor 

 for the method employed. 



The estimation of nitrogen may be made by any of the 

 methods used in organic analysis, but Kjeldahl's method 

 and Wanklyn's albuminoid ammonia process possess the 

 great advantage that the precipitate or residue does not 

 require to be powdered or intimately mixed with a solid, a 

 process always difficult in such cases and frequently almost 

 impossible. 



The soda-lime method^ of Will and Varentrap much 

 used at one time for the determination of nitrogen in 

 organic residues, has been largely replaced by Kjeldahl's 

 process. 



For the estimation of peptones and albumoses, the 

 nitrogen in the dried sodium phosphotungstate precipitate 

 may be determined by Kjeldahl's process ; or the precipi- 

 tate may be decomposed by alkali and the peptones etc. 

 in solution estimated by "Wanklyn's method ; or by com- 

 parison of the colour obtained in biuret reaction, with 

 the colour given by standard peptone solutions under 



1 A full aceonnt of the soda-lime process is given in Fresenius, 

 Quant. Analysis, 7th ed. vol. ii. pt. i. 



