150 Z. C. Jones — Estimation of Boric Acid. 



in this solution was estimated by the process of Winkler recom- 

 mended by Kiisler. The acid used to make this estimation 

 was hydrochloric, standardised by silver nitrate. 



The full method for the estimation of boric acid as finally 

 elaborated is as follows: The solution is made clearly acid to 

 litmus by hydrochloric acid and 5 cm3 of a solution (10^) of barium 

 chloride added. An amount of iodate and iodide of potassium 

 sufficient to liberate an amount of iodine at least equivalent to- 

 the excess of hydrochloric acid in the acidified solution is 

 mixed with starch in a separate beaker, and the iodine, which is 

 usually thrown out by this mixture, is just bleached by a dilute 

 solution of thiosulphate. 



To the now neutral solution of iodide and iodate a single 

 drop of the solution to be analyzed is transferred by a glass 

 rod. If a blue coloration is developed, the solution is acidic 

 with hydrochloric acid, and all the boric acid is in free condi- 

 tion. The amount of iodide and iodate used depends upon 

 the acidity of the solution containing boric acid. Usually 

 10 ctn3 of a 25 per cent solution of iodide and the same amount 

 of a saturated solution of iodate is sufficient. Any larger ex- 

 cess of hydrochloric acid should be neutralized by sodium 

 hydroxide before the iodide and iodate mixture is added. 

 After the addition of the iodide and iodate solution, containing 

 starch, to the boric acid solution, the liberated iodine should 

 be carefully bleached by thiosulphate. Any excess of thiosul- 

 phate in reasonable amount does not seem to be detrimental, 

 but in practice the starch iodide color is clearly bleached, and 

 no more then added. Carbonates prevent a definite indication 

 of the neutral point by thiosulphate and starch iodide, there- 

 fore the barium chloride (about 5 cm3 ) should be added before 

 this point in the process. The mixture of iodide and iodate 

 is not added to the solution to be analyzed until after it is 

 made acidic, for the reason that when the neutral point is 

 approached by the addition of hydrochloric acid, the starch 

 iodide is thrown out locally by the acid, and the small amount 

 of sodium borate remaining undecomposed does not again 

 bleach the coloration produced, thus obscuring the neutral 

 point which must be obtained before titrating for boric acid. 



The solution after the bleaching of the iodine by thiosul- 

 phate is colorless and contains only starch, neutral chloride, 

 potassium tetrathionate, iodide and iodate, and all the boric 

 acid present in uncombined condition. The carbonate lies out 

 of the sphere of action in insoluble form as barium carbonate. 

 A few drops of the indicator, phenolphthalein, are now added r 

 and the alkaline solution run in until a strong red coloration is 

 produced. A pinch of mannite is then added, which bleaches 

 the phenolphthalein coloration, and the alkali solution again 



