MILK 



93 



barium chloride. The flask is then changed, and the precipi- 

 tate dissolved in 10 c.c. of pure concentrated hydrochloric acid, 

 5 c.c. being used at a time, and the filter washed with 50 c.c. of 

 water after each addition. When the precipitate has been 

 dissolved, the filter flask is again attached to the pump. 



The solution, which is cloudy, is now poured into an 

 Erlenmeyer flask, heated to boiling, and again filtered through 

 the asbestos filter under slightly reduced pressure. Operating 

 in this way a perfectly clear liquid ought to be obtained. 

 After washing the filter with distilled water, the solution is 

 again brought into an Erlenmeyer flask and 20 c.c. of ammonia, 

 10 c.c. of which are to neutralise the acid, are added; also 

 10 c.c. of potassium cyanide (13 g. per litre), which is equivalent 

 to JV/10 silver nitrate. Ten drops of a 10 per cent, solution of 

 potassium iodide are used as indicator. The titration is made 

 with N/10 silver nitrate, which is slowly run in until the 

 cloudiness which forms does not disappear on shaking. 



Beau has prepared the table given below, from which the 

 amount of citric acid in the milk under examination is directly 

 shown, the quantity of citric acid being expressed in centi- 

 grams per litre for each cubic centimetre of JV/10 silver nitrate 

 from to 10. The table is only applicable where the method 

 of operation has been exactly as described above, and where 

 the filtrate = 100 c.c. obtained from 25 c.c. of milk. 



Citric Acid in Milk Corresponding to N/10 Silver Nitrate. 



