BORIC ACID. Ill 



and then dilute caustic soda solution and calcium chloride 

 solution alternately till a permanent pink is produced. The 

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

 cator, and then using phenol-phthalein with alkali in the pre- 



U 

 sence of glycerol. If more than 1-7 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 liquid 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 charred 

 thoroughly, 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 



acid, using methyl orange as indicator. An equal volume of 



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



N 

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



N 

 of 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 3 B0 3 . 



The author and Miller have found that it is quite unnecessary 

 either to evaporate the milk, ignite it, or to use any indicator 

 other than phenol-phthalein ; the method is to a measured 

 or weighed quantity of milk (10 c.c. suffices) add half its bulk 

 of a 0-5 per cent, solution of phenol-phthalein, and run in alkali 

 till a pink colour appears, boil, and titrate back while still boiling 



N 

 with acid solution till white, and finally with alkali till faintly 



pink. The colour, though faint, is quite distinct, and no attempt 

 should be made to obtain a pronounced pink colour. Add 30 per 



N 

 cent, of glycerol, and continue the titration with alkali 



