114 ESTIMATION OF TOTAL SOLIDS AND ASH. 



The result must be corrected for the volume of the fat and 

 protein thrown down as directed under milk-sugar. 



Cowing - Scopes' Method. L. Gowing-Scopes has investi- 

 gated the method originated by Deniges, which consists in 

 oxidising citric acid to acetone dicarboxylic acid, and the con- 

 version of this into an insoluble basic mercury salt. He finds 

 that, to obtain exact results, strict attention must be paid to 

 details of manipulation. 



For the estimation of citric acid in milk, the clear filtrate 

 obtained by adding acid mercuric nitrate to milk as for the 

 estimation of milk-sugar is used ; 10 c.c. of this is neutralised 

 exactly with alkali, using phenol-phthalein as indicator ; a precipi- 

 tate will form, but this will be redissolved on the addition of 10 c.c. 

 of a reagent prepared by covering 51 grammes of mercuric nitrate 

 and 51 grammes of manganese nitrate with 68 c.c. of strong 

 nitric acid, and after the addition of 100 c.c. of water to dissolve 

 the salts, making up the volume to 250 c.c. and filtering. 



This solution, made up to 200 c.c., is boiled under reflux for 

 three hours, filtered through a weighed Gooch crucible, and the 

 precipitate washed well with cold water ; the deposit on the 

 sides of the flask may be removed by adding 1 or 2 c.c. of 1 per 

 cent, nitric acid, and rubbing with a rod ; after drying for five 

 hours the precipitate is weighed. The colour of the precipitate 

 should be nearly white ; if it is yellow, this is due to the presence 

 of basic salts, and the result will be high. 



The weight of the precipitate multiplied by 0-1667 will give 

 the amount of citric acid, and in calculating the percentage in 

 the milk, the corrections for the volume of the precipitated 

 proteins and fat must be made as in Vieth's method of milk- 

 sugar estimation. 



The method given above departs slightly from Gowing-Scopes' 

 original method, as, owing to the presence of mercury in the 

 filtrate, slightly more mercuric salts are present than he prescribes, 

 but his researches have shown that variations of the amount 

 of mercury used have far less influence on the results than vari- 

 ations in the other ingredients of his reagent. 



Stahre's method may also be used ; 50 c.c. of milk are 

 treated with 20 c.c. of 50 per cent, sulphuric acid, 2 c.c. of 40 per 

 cent, potassium bromide solution, and 20 c.c. of 10 per cent, 

 phospho-tungstic acid, and diluted to 200 c.c. and filtered. To 

 150 c.c. of the filtrate are added 25 c.c. of freshly prepared satu- 

 rated hydrobromic acid, the solution heated to 50 for five min- 

 utes, and then treated with 10 c.c. of 5 per cent, potassium 

 permanganate,being stirred the whole time. The precipitate which 

 consists of penta-bromacetone is collected, dried over sulphuric acid 

 and weighed, the weight multiplied by 0*236 giving citric acid. 



