36 Browning — On the Interaction of 



and completely, as it seemed, in the cold. The solution was 

 then treated with acid potassium or sodium carbonate (about 

 5 grm.) in excess. At this point a precipitate formed unless 

 Rochelle salts had been added, as was sometimes the case, to 

 prevent it. To the alkaline solution iodine was added until a 

 yellow tinge showed it to be in excess, and the solution was 

 allowed to stand, with frequent shaking, about one-half hour, 

 at the end of which time the excess of the arsenious acid was 

 thoroughly acted upon. In order to test the permanency of 

 the iodine color in alkaline solution, two experiments were 

 made, in the first of which 5 grm. of acid sodium carbonate 

 were dissolved in about 100 cm 3 of water, and in the second 

 the amount of acid generally used (10 cm 3 ) was neutralized 

 with the acid carbonate and an excess added, the amount of 

 water present being about the same as in the first experiment. 

 Starch and a drop of iodine solution were then added. No 

 bleaching effect was apparent during two hours. Several 

 experiments, made without allowing the solution to stand, with 

 the excess of iodine seemed to show that the excess of the 

 arsenious acid had not been completely oxidized by the iodine. 

 As this took place mainly in those experiments in which the 

 precipitate had not been held up by Rochelle salts, I am 

 inclined to attribute it to the holding of the arsenious acid by 

 the precipitate. After this point in the process had been 

 reached, the excess of iodine was destroyed by the addition of 

 arsenious acid, starch was added and the blue color obtained 

 with iodine. Knowing the amount of arsenious acid originally 

 added together with the second amount, used to bleach the 

 excess of iodine, also the total amount of iodine employed and 

 its value in terms of arsenious acid, we are in position to deter- 

 mine the amount of arsenious acid used for the reduction ; and, 

 by referring to the reaction given above we can calculate the 

 amount of chromic acid originally present. The use of 

 Hochelle salts had a disadvantage, however, which should be 

 mentioned. If the precipitate is held up, the solution takes 

 on a dark green color which makes the starch iodide reaction 

 difficult to detect. The presence of the precipitate, except for 

 the apparent holding of some arsenious acid — a source of error 

 which seems to be largely obviated by the standing with the 

 excess of iodine — causes no inconvenience, being itself of a 

 very light green color and leaving the solution almost color- 

 less. I have tried filtering with no gain in convenience, since 

 the precipitate is of a gelatinous character and filters very 

 slowly, so that the filtration and thorough washing lengthen 

 the process very materially. 



The results follow in the table : 



