Estimation of boric acid. So 



ilistiJled to diyuess into about 10 c.c. of strong ammonia ; eight 

 successive portions of 10 c.c. each of methyl alcohol are added 

 and distilled off. 



.\bout 1 gramme of lime is ignited in a capacious pktiimm 

 basin in a muffle at the highest temperature attainable, and the 

 basin and lime weighed. The ammoniacal distillate is now added 

 and the liquid evaporated on the water-bath ; the basin is again 

 ignited in a muffle and weighed. The increase of weight repre- 

 sents the boric anhydride. 



Hehner prefers the use of a measured quantity of sodium 

 phosphate solution of known strength for fixing the boric acid 

 instead of ammonia and lime. lie distils directly into the 

 sodium phosphate solution, evaporates and cautiously ignites. 

 The weight of the residue of pyro-phosphate obtained from an 

 equal measure of sodium phosphate solution is subtracted fi'om 

 the weight of the residue ; the difference repiesents boric anhy- 

 dride. It is necessary, however, to ignite \cvy cautiouslv, as 

 sodium phosphate is liable to spurt. 



Thompson has shown that boric acid may be titrated with 

 caustic alkali, using pheiiol-phthalcin as indicator, provided at 

 least .'50 per cent, of glycerol l)c [)resent. His directions aiv ; 

 1 or 2 grammes of caustic soda arc addcil to UK) c.c. of milk 

 and the whole evaporated to drviicss in a platinum dish. The 

 residue is thoroughly ciiarred, heated with "JO c.c. of water an<l 

 liydrochloiic acid added drop by drop till all but carbon is dis- 

 solved. The whole is transferred ti> a 100 c.c. Mask, the bulk 

 not being allowed to get above 50 to (iO c.c, and half a jiramme 

 of dry calcium chloride added. To this mixture a few drops of 

 phenol-phthalein solution are added, then a lo per cent, solution 

 of caustic soda, till a permaneut pink colour is perceptible, and, 

 finally, 25 c.c. of lime water. In this way all the phosphoric 

 acid is precipitated as calcium phosphate. The mixture is made 

 u]) to 100 c.c, mixed, and filtered through a dry filter. To 5o c.c. 

 of the filtrate {= 50 c.c milk) normal sulphuric acid is added 

 till the pink colour is gone, then a few drops of methyl orani;e, 

 and the addition of acid continued until the yellow is just changed 



N . 



to pink. . caustic soda solution is added till the liquid assumes 



a vcllow tiiiuc, excess of soda being avoided. At this stage all 

 acids likely to be present exist as salts neutral to phenol-phthalein, 

 except boric acid and a little carbonic acid, which last is expelled 

 b)- a few minutes boiling. The solution is cooled, a little more 

 phenol-phthalein added, and as much ulycerol as will give at least 

 3(> ]>er cent, of that substance in the final solution, and titrated 



with ". cau-tic soda till a permanent pink colour is produced. 



