84 ANALYSIS OF MILK. 



may be gravimetrically estimated, and, if the amount of milk be 

 sufficient, the phosphoric acid may be separated from another 

 portion by the molybdic acid method. Such modifications will 

 be found in works on inorganic analysis, and need not be 

 described in detail. 



If boric acid be present, it will be found to interfere with 

 the results of the analysis, as a portion of this remains in the 

 insoluble ash ; this may be removed by evaporating the acid 

 solution to dryness and repeatedly evaporating with small por- 

 tions of methyl alcohol. It will also interfere with the estimation 

 of alkalinity in the soluble ash, as the alkali shown by methyl 

 orange will be due to borax ; the chlorine is best estimated in 

 this case gravimetrically as silver chloride. 



Boric acid is detected by slightly acidifying the ash with 

 hydrochloric acid and dipping a piece of turmeric paper into 

 the solution ; on drying, this will assume a pinkish-brown color- 

 ation, turned a very dark green — almost black — on moistening 

 with a solution of sodium bicarbonate. Cribb and Arnaud pre- 

 pare turmeric paper by boiling 2 grammes of turmeric and 

 2 grammes of tartaric acid with 80 per cent, alcohol till the latter 

 is dissolved, and soaking strips of filter paper in this solution. It 

 is very delicate, and should be kept in the dark. Another test is 

 to moisten the ash with dilute sulphuric acid and add strong 

 alcohol ; if boric acid be present, the alcohol will burn with a 

 greenish flame on applying a match. 



Boric acid may also be detected in the milk direct, by acidi- 

 fying with hydrochloric acid, and dipping the turmeric paper 

 in the serum. 



Another simple test for the presence of boric acid consists in 

 putting about ,| oz. of milk in a glass, adding half its bulk of 

 phenol-phthalein, and dilute caustic soda solution drop by drop, 

 with constant stirring, till a faint permanent pink colour is 

 produced. Some of the pink-coloured milk is poured into 

 two test tubes. To one tube is added an equal bulk of 

 water, and to the other an equal bulk of a neutral mixture of 

 1 part pure glycerol and 1 part water. In genuine milk both 

 tubes lemain pinlc, and the colours are practically identical, 

 but in the presence of boric acid the water tube becomes 

 darker in tint, and the gl3'cerol tube much lighter — usually 

 quite white. 



If boric acid be present, 25 to 50 grammes of milk should be 

 taken for estimation. After addition of about 0-2 gramme caustic 

 soda, the milk is evaporated and thoroughly charred by ignition ; 

 the residue is extracted ))y dilute acetic acid, and well washed 

 with as small a quantity of water as possible ; the solution i.s 

 filtered into a small flask, to which a condenser is fitted, and 



