68 Gooch and Browning — Method for the 



acid previous to titration, and general convenience in manipu- 

 lation, seemed to be best was 10 cm 3 of the mixture made by 

 diluting the acid with an equal volume of water. This we 

 fixed upon for use in future experiments and set the limit of 

 concentration at 40 cm 3 . 



It is manifest from the phenomena described that when 

 much hydriodic acid remains in the solution the last portions 

 of free iodine cannot be completely removed by heat without 

 volatilization of the arsenic. We experimented, therefore, 

 upon the effect of very dilute sulphurous upon the remnant of 

 iodine in liquids constituted as described and concentrated to 

 40 cm 3 , the point of minimum color, the solution of sulphurous 

 acid which we employed corresponding approximate^ to cen- 

 tinormal iodine. We found that upon adding the sulphurous 

 acid drop by drop to the hot concentrated solution the point 

 at which the color vanished could be determined without diffi- 

 culty, but that if the solution was permitted to stand a single 

 minute the color of iodine returned, doubtless developed by 

 the action of air upon the hot hydriodic acid. We adopted, 

 therefore, the plan of at once diluting the solution with cold 

 water as soon as the sulphurous acid had done its work and 

 immediately neutralizing with potassium carbonate. When 

 this mode of proceeding was followed we were unable to find 

 evidence of reversion of arsenious acid to arsenic acid, magne- 

 sia mixture producing in the solution no precipitate of the 

 ammonium magnesium arseniate. 



Following out the same general lines, therefore, we pro- 

 ceeded to the quantitative examination of the process. Portions 

 of a standard solution of the dihydrogen potassium arseniate 

 were measured from a burette into counterpoised Erlenmeyer 

 flasks of 250 cm 3 capacity, and the increase in weight was taken 

 as the measure of the actual amount of the solution employed. 

 Potassium iodide in solution, and 10 cm 3 of sulphuric acid 

 [1 : 1] were added, and the liquid was diluted with water to a 

 volume of about 100 cm 3 . A trap made, as described in our 

 paper upon the reverse of this process, by cutting down a 

 two-bulbed drying tube, was hung in the neck of the flask to 

 prevent mechanical loss, and the liquid was rapidly concentrated 

 by boiling until the volume of 40 cm 3 , the point at which the 

 color of iodine had faded to a pale yellow, was reached. At 

 this point the flask was removed from the flame, its sides and 

 the trap were quickly washed down, the weak sulphurous acid 

 was added drop by drop from a burette until the color of the 

 free iodine had just vanished, the liquid was immediately 

 diluted with cold water, the free acid was nearly neutralized 

 with potassium carbonate and the point of neutralization was 



