L. O. Jones — Estimation of Boric Acid. 151 



run in to a faint indication, which if permanent on the addi- 

 tion of more mannite, may be taken as the reading point. 

 About 1-2 grm of mannite are necessary for a determination. 

 The boro-mannite compound is sufficiently acidic to liberate 

 iodine abundantly, but it appears to. be a time reaction, and at 

 the end of six hours only about 95 per cent of the theoretical 

 amount (considering B 2 3 as a bivalent acid) has been thrown 

 out. The combination of boric acid and mannite liberates in 

 the presence of iodide and iodate immediately only about one- 

 half the iodine required on the theory that B 2 3 under these 

 conditions acts as a bivalent acid, of with the neutralizing 

 power of metaboric acid, HOBO. When no mannite is present 

 phenolphthaline gives an alkaline indication when only about 

 one-half the amount of alkali theoretically necessary to form 

 the metaborate, NaOBO, has been added. Obviously, then, the 

 starch iodide coloration will not appear at all on the addition 

 of mannite, if about one-half the free boric acid is first neu- 

 tralized by the solution of alkali, and the remainder of the 

 alkali immediately added to complete neutralization. The 

 point at which the danger of the appearance of the iodide 

 coloration on the addition of mannite has been passed, is 

 roughly indicated before the mannite has been added by the 

 appearance of the strong alkaline indication of phenolphthalein. 

 This indicator would not need to be added at all, if the boro- 

 mannite compound quickly and regularly liberated iodine from 

 the iodide and iodate. The fact, however, that this compound 

 of boric acid and mannite — as -has been ascertained by experi- 

 ment — liberates, on standing twelve hours, about 99 per cent 

 of the theoretical amount of iodine, places the strength of this 

 acid above that of citric or tartaric acid as investigated by 

 Furry.* With phenolphthalein, however, the end reaction is 

 sharp and the small amount of carbonate present in the stand- 

 ard solution of alkali is precipitated by the barium chloride 

 already in the solution. The calculation must therefore be 

 based on the amount of free hydroxide in the standard solution 

 of alkali used, according to the following representation : 



B 2 3 + 2NaOH=2NaOBO + H 2 0. 



The best results and the most definite indications are 

 obtained in cold solution of a volume not greater that 

 50 cm3 . This fact accords with the observations of Magnaninif 

 that the relative electrical conductivity of the boro-mannite 

 solution is decreased by dilution and elevation of the 

 temperature. When silicates are present in solution, the 



* Am. Chem. Jour., vi, 341. 



f Gaz. Chim., xx, 428, and xxi, 134. 



