100 RESEARCHES UPON ATOMIC WEIGHTS. 



acid which might have had its source in either copper or sulphuric acid must 

 have been completely eliminated, for it must have been almost wholly in the 

 ionized condition. 



A considerable quantity of this sulphurous acid was poured into one of the 

 precipitating flasks, and the weighing tube, containing the iodine, was intro- 

 duced, after the lower stopper had been allowed to drop out of the tube into the 

 flask. The sulphurous acid immediately sealed the open end of the tube so that 

 no iodine vapor could escape. The other stopper was then removed by means of 

 a platinum wire, the wire was rinsed into the flask, and the flask was quickly 

 closed by means of its glass stopper. Solution of the iodine in the sulphurous 

 acid was hastened by gently agitating the flask. Any iodine vapor which es- 

 caped from the tube must have been instantly converted into hydriodic acid. 

 After sufficient time had been allowed for every trace of this hydriodic acid to be 

 absorbed by the solution, the flask was opened and the solution was transferred 

 to another precipitating flask, and a slight excess of the purest ammonia was 

 added. When iodine is dissolved in a large excess of sulphurous acid the solu- 

 tion becomes colored yellow, owing to the formation of an iodide of sulphur,* 

 and upon standing, this solution may deposit sulphur. But since the solution 

 was made alkaline with ammonia as soon as the iodine was dissolved, with 

 the complete disappearance of the color and without the formation of the 

 slightest trace of a precipitate, no danger was to be feared from this 

 source. 



From the weight of the iodine, the weight of silver necessary exactly to com- 

 bine with it was calculated. This silver was weighed out and dissolved in nitric 

 acid as described previously. The solution was diluted until not stronger than 

 I per cent and was added slowly to the already dilute solution of ammonic iodide 

 in the precipitating flask. The flask was shaken for some time, and was then 

 made acid with a considerable excess of nitric acid. Long-continued shaking, 

 followed by several days' standing, yielded a clear supernatant solution. 25 c.c. 

 portions of this solution were pipetted into nephelometer tubes and were 

 tested with cubic centimeter portions of hundredth normal silver nitrate and 

 hydriodic acid solutions in a nephelometer. If an excess of either iodine or 

 silver was present, the deficiency of the other was made up in the remaining 

 solution by addition of standard silver nitrate or hydriodic acid until the exact 

 end-point was reached. It was never necessary to add more than o.i mg. of 

 either iodine or silver, so that the liquid removed from the flask for the tests 

 could be neglected. This end-point is very sharp in the case of silver and pure 

 iodine; for so little silver iodide is dissolved that the two nephelometer tubes 

 remain almost absolutely clear. An excess of o.i mg. of silver in a liter of solu- 

 tion is easily detected. This almost complete lack of opalescence in the nephelo- 

 meter tubes is strong evidence of the absence of even a trace of either chlorine or 



' Dammer : Handb. d. anorg. Cketn., i, 557. 



