These results show that large variation in the quantity of bicarbonate has no effect.
Effect of Free Acid.—In these experiments, the arsenic taken, the starch, and the bulk were as before, but no bicarbonate was added. In one case the solution was rendered acid with 5 c.c. of acetic acid, and in the other with 5 c.c. of hydrochloric acid; in both cases the interference was strongly marked, and no satisfactory finishing point could be obtained. This was much more marked with the hydrochloric acid.
Effect of Foreign Salts.—The process for getting the arsenic into solution will exclude all metals except tin, but the solution will be charged with sodium or ammonium salts in the process of neutralising, so that it is only necessary to see if these cause any interference. The alkaline hydrates, including ammonia, are plainly inadmissible, since no free iodine can exist in their presence. Monocarbonates similarly interfere, but to a much less extent; hence the necessity for rendering the assay distinctly acid before adding the bicarbonate of soda.
With 20 c.c. of arsenic solution; and with bulk, soda, and starch as before, the results obtained were:—
| "Iodine" required. | |
| With 20 grams of ammonic chloride | 20.0 c.c. |
| " 20 grams of sodium chloride | 20.0 " |
| " 20 grams of sodium acetate | 20.0 " |
| " 0.050 gram of tin, as stannic chloride | 19.6 " |
| Without any addition | 20.0 " |
The interference of the stannic salt is probably mechanical, the precipitate carrying down some arsenious acid.
Effect of Varying Arsenic.—With bulk, starch, and soda as before, but with varying arsenic, the results were:—
| Arsenic added | 1.0 c.c. | 10.0 c.c. | 20.0 c.c. | 50.0 c.c. | 100.0 c.c. |
| "Iodine" required | 1.1 " | 9.9 " | 20.0 " | 50.0 " | 100.0 " |
Determination of Arsenic in Metallic Copper.—Put 1 gram of the copper filings, freed from particles of the file with a magnet, into a 16-oz.-flask; and distil with the ferric chloride mixture, as above described. Neutralise the distillate; acidify; add bicarbonate of soda and starch; dilute; and titrate with the standard solution of iodine.[107] Make a blank determination with 1 gram of electrotype copper, proceeding exactly as with the assay; and deduct the amount of arsenic found in this experiment from that previously obtained.
Working in this way on a copper containing 0.38 per cent. of arsenic and 0.80 per cent. of antimony, 0.38 per cent. of arsenic was found.