86 ANALYSIS OF MILK. 



Each c.c. of — caustic soda solution is equal to 0-0124 gramme 



crystallised boric acid or 0-0070 gramme boric anhydride. 



Phosphoric acid can be separated from boric acid by precipi- 

 tation as calcium phosphate, if not more than 0'2 per cent, of 

 crystallised boric acid be present. 



As excessive heating is apt to drive off boric acid, it is necessary 

 to carry the charring so far only as will give a colourless solution. 



This method tends to give rather low results, as a portion of 

 the boric acid remains in the calcium precipitate, while, on the 

 other hand, all the phosphoric acid may not be removed. Shrews- 

 bury recommends that after charring the milk, and dissolving 

 the ash in acid, a little phenol-phthalein solution be added, 

 and then dilute caustic soda solution and calcium chloride 

 solution alternately till a permanent pink is jJi'oduced. The 

 filtrate is then titrated with acid using methyl orange as indi- 

 cator, and then using phenol-phthalein ^^"ith alkali in the pre- 



sence of glycerol. If more than I'T c.c. — alkali is used the 



precipitate should be dissolved up and reprecipitated, the second 

 filtrate titrated, and the results added to the first titration ; if 

 necessary, the process should be repeated. 



Allen and Tankard have devised a method for the estimation 

 of boric acid, which consists in evaporating the licjuid to be 

 tested to dryness with a few cubic centimetres of 10 per cent, 

 calcium chloride solution ; in the case of milk or cream it is 

 advisable to add just sufficient alkali solution to neutralise it 

 to phenol-phthalein. 



To 10 to 25 c.c, add i of its bulk of 10 per cent, calcium chloride 

 solution, and just sufficient alkali to neutralise to phenol-phthalein; 

 evaporate to dryness ; ignite at a gentle heat till thoroughly 

 charred, boil the residue with 150 c.c. of distilled water, and 

 filter the liquid. The filter is returned to the dish, and the residue 

 ignited till white at a moderate temperature, and boiled with 

 a further 150 c.c. of water. The liquid is allowed to stand over- 

 night, and is filtered cold; the mixed liquids are evaporated 

 to a volume of 25 to 30 c.c. ; and after cooling neutralised with 

 N 

 j^ acid, usnig methyl alcohol as indicator. An equal volume of 



glycerol is added, and a little phenol-phthalein, and the solution 



titrated with — caustic soda (free from carbonate). The volume 



of j^ caustic soda required by an equal volume of glycerol is 



subtracted from the amount used, and the remainder multiplied 

 by 0-0062 will give the weight of H.BO.;. 



