110 ESTIMATION OF TOTAL SOLIDS AND ASH. 



equal measure of sodium phosphate solution is subtracted from 

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

 dride. It is necessary, however, to ignite very cautiously, as 

 sodium phosphate is liable to spurt. 



Thompson has shown that boric acid may be titrated with 

 caustic alkali, using phenol-phthalein as indicator, provided at 

 least 30 per cent, of glycerol be present. His directions are : 

 1 or 2 grammes of caustic soda are added to 100 c.c. of milk 

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

 residue is charred thoroughly, heated with 20 c.c. of water and 

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

 solved. The whole is transferred to a 100 c.c. flask, the bulk 

 not being allowed to get above 50 to 60 c.c., and half a gramme 

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

 phenol-phthalein solution are added, then a 10 per cent, solution 

 of caustic soda, till a permanent 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 

 up to 100 c.c., mixed, and filtered through a dry filter. To 50 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 orange, 

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



N 



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

 o 



a yellow tinge, 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 

 by a few minutes boiling. The solution is cooled, a little more 

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

 30 per cent, of that substance in the final solution, and titrated 



N 

 with caustic soda till a permanent pink colour is produced. 



N 



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

 o 



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 ofi 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, 



