A REVISION OF THE ATOMIC WEIGHTS OF IODINE AND SILVER. 



125 



possible traces of chlorides, and was preserved in a platinum flask. Even when 

 made from the chloride, the doubly distilled product was absolutely free from 

 chlorides. The approximate concentration of the solution was determined 

 shortly before use by titration against a standard solution of iodic acid or 

 potassiimi permanganate. PreUminary experiments showed that there is no 

 danger of the reduction of the silver salts by a slight excess 

 of hydrazine in nitric acid solution. 



Special pains were taken in the reduction of the iodic acid 

 with hydrazine to avoid any possibiUty of loss of iodine by 

 volatiUzation. The solution of the iodic acid was trans- 

 ferred to an 8-liter bottle with a carefully ground stopper, 

 and, after the iodic acid had been neutralized with ammonia, 

 very slightly less than the theoretical quantity of hydrazine 

 hydroxide was added. Since in alkaline solution hydrazine 

 is without effect upon iodic acid, the solution was next 

 made acid by slow addition of nitric acid. 



As soon as the neutral point has been passed, iodine is 

 freed by interaction of the hydriodic acid with the iodic 

 acid. Lest any of this iodine be volatilized from the solu- 

 tion before it could be reduced by the hydrazine the nitric 

 acid was introduced at the bottom of the solution through a 

 long funnel. Thus iodine was sure to be completely re- 

 duced before reaching the surface of the solution. A specially 

 devised stopper, shown in the accompanying diagram, pro- 

 vided for the escape of the nitrogen through a long col- 

 imin of bulbs, which served to catch spatterings or soluble 

 vapors, if any reached this point. As a further precaution 

 against the escape of iodine a small quantity of sulphurous 

 acid was poured into the bottle through the train of bulbs. This sulphurous 

 acid served also to complete the reduction of the iodic acid. The amount of 

 sulphuric acid formed in this way was not sufficient, however, to produce 

 appreciable occlusion of silver sulphate. 



After the reduction and subsequent dilution of the solution to a concentra- 

 tion between thirtieth and fiftieth normal, the hydriodic acid was precipitated 

 with a dilute solution of a weighed amount of silver as before described. On 

 accovmt of the large bulk of the mother-liquor, 6 to 7 liters, the end-point as 

 determined in the nephelometer was somewhat uncertain, especially since it 

 seemed to be complicated by a minute trace of silver iodide held in suspension. 

 Hence, instead of attempting to use exactly the theoretical quantity of silver, an 

 amount was employed a few tenths of a milligram in excess of the required quan- 

 tity. Then this excess was determined by evaporating the supernatant liquid 

 to a very small bulk, precipitating the excess of silver as silver iodide and deter- 

 mining the silver iodide either gravimetrically upon a Gooch-Munroe-Neubauer 



Fig. 5. — Appara- 

 tus for reducing 

 iodic add. 



