52 METHODS OF QUANTITATIVE ANALYSIS. 



out and the filtrate placed in a small test tube and held 

 over a white paper. In this way any blue coloration may 

 be readily detected. In case the solution be still blue this 

 filtered portion may be returned to the evaporator and 

 titrated further with the sugar solution. If the filtrate 

 shows a yellowish tint it is probable that too much of the 

 sugar-bearing filtrate had been used. 



The amount of filtrate required to reduce 20 c. c. of 

 Fehling solution is equivalent to 0.134 gram of lactose. 

 Upon this fact, the calculsCtions are based. For example, 

 if one uses 10 c. c. of milk and separates out the fats and 

 the albuminoids and makes up the filtrate to 200 c. c. 

 and finds that 50 c. c. of it are required to reduce 20 c. c. 

 of Fehling's solution, then the percentage of milk sugar 

 contained in the milk will be calculated by the formula 

 4 X 0.134 X 10, which is equivalent 5.36 per cent. 



Where great accuracy is not required this method of 

 titration is useful because results may be quickly ob- 

 tained. It is especially well adapted for technical analy- 

 ses, for analyses of food stuffs, and for clinical tests. In 

 cases where a greater degree of accuracy is demanded 

 gravimetric methods of analysis should be used. For 

 standardizing and controlling the Fehling's solution a 

 solution of chemically pure milk sugar or dextrose is 

 used. In the latter case one-tenth of a gram will reduce 

 20 c. c. of Fehling's solution. 



B. SOXHELT AND ALLIHN'S GRAVIMETRIC 

 METHODS. 



In estimating the lactose by these methods also, the al- 

 buminoids and fats of the milk must be eliminated. For 

 this purpose the modified Ritthausen method is employed. 

 The filtrate is collected in a 200 c. c. flask and the volume 

 completed with water. By using 10 c. c. of milk and 

 making to 200 c. c. the most favorable concentration is 



