so METHODS OP QUANTITATIVE ANALYSIS. 



Ten c. c. of milk are diluted with 50 c. c. of water 

 and treated with 10 c. c. of a 15 per cent, aqueous solu- 

 tion of trichloracetic acid. This causes a complete pre- 

 cipitation of the albuminoids. The reaction takes place 

 in the cold. The precipitate is allowed to stand a few 

 hours and then washed with dilute trichloracetic acid 

 and finally the nitrogen content of the residue deter- 

 mined by the Kjeldahl method. 



VII. DETERMINATION OF LACTOSE. 



For the estimation of lactose any one of the following 

 methods may be used. First — Titration with Fehling's 

 solution. Second — Gravimetric Analysis by the Soxhlet- 

 Allihn method. Third — Determination by circular po- 

 larization. 



A. TITRATION WITH FEHLING'S SOLUTION. 



Before titrating with Fehling's solution the milk sam- 

 ple must be freed of albuminoids. For this purpose the 

 filtrate obtained from the precipitation of the albumin- 

 oids by the Munk-Ritthausen method may be used (page 

 48). This filtrate, together with all of the wash water 

 is well mixed and made up to a definite volume (usually 

 200 c. c). An aliquot portion of this solution is then 

 titrated with Fehling's solution of known reducing pow- 

 er. This reagent is prepared in the following manner. It 

 consists of two solutions which should be kept separate. 



Solution I. 34.639 grams pure crystallized copper sul- 

 phate are dissolved in warm water and the solution after 

 cooling made up to exactly 500 c. c. 



Solution 2. 173 grams of pure crystallized Siegnette 

 salts (potassium sodium tartrate) and 50 grams chemic- 

 ally pure sodium hydrate are dissolved separately in dis- 

 tilled water and the cooled solutions then united and made 

 up to 500 c. c. 



