Without any addition, 15.0 c.c. were required; and in another experiment, in which 30 grams of ammonic salts were present, 15.6 c.c. of uranium solution were required. Such variations in the amount of ammonic salts as occur in ordinary working are unimportant.
Phosphates, of course, interfere. In fact, the uranium acetate solution can be standardised by titrating with a known weight of phosphate, and calculating its equivalent of arsenic. Thus, in an experiment with 0.6 gram of hydric sodic phosphate (Na2HPO4.12H2O), equivalent to 0.05195 gram of phosphorus, or 0.1256 gram of arsenic, 23.25 c.c. of a solution of uranium acetate were required. The same solution standardised with white arsenic gave a standard of which 100 c.c. = 0.5333 gram arsenic. On this standard the 0.6 gram of sodic phosphate should have required 23.5 c.c.
Experiments in which 0.1 gram of bismuth and 0.1 gram of antimony were present with 0.1 gram of arsenic, showed no interference on the titration. Ferric or aluminic salts would remove their equivalent of arsenic, and, consequently, must be removed before titrating.
Effect of Varying Arsenic.—Varying amounts of metallic arsenic were weighed up and dissolved in nitric acid, &c., and titrated:—
| Arsenic taken | 0.010 gram | 0.050 gram | 0.100 gram | 0.200 gram |
| Arsenic found | 0.010 " | 0.050 " | 0.100 " | 0.197 " |
These experiments show that the method yields good results within these limits.
Determination of Arsenic in Mispickel.—Weigh up 1 gram of the dried and powdered ore, and evaporate to near dryness with 20 c.c. of dilute nitric acid. Make up to 100 c.c. with water, and pass sulphuretted hydrogen to reduce the ferric iron to the ferrous state, then add 20 c.c. of dilute ammonia, and again pass sulphuretted hydrogen. Warm, filter, and evaporate the filtrate to drive off the excess of ammonia; then add 10 c.c. of nitric acid, and boil down till the sulphide of arsenic at first precipitated is dissolved; neutralise; add 5 c.c. of sodium acetate and acetic acid solution; transfer to a pint flask, boil, and titrate.
For example, an impure sample of ore required, in duplicate assay of half a gram each, when treated in the above-mentioned way, 39.6 and 39.5 c.c. of the uranium acetate solution (100 c.c. = 0.537 gram of arsenic), equivalent to 0.2114 gram of arsenic, or 42.3 per cent.
An alternative method is as follows. Powder the ore very finely and weigh up .5 gram. Place in a 2-3/4 inch berlin dish and add strong nitric acid, one drop at a time until the action ceases; with care there need be no very violent reaction. Dry over a water bath. Cover with 2 grams of nitre and over this spread 5 grams of a mixture of equal parts of nitre and carbonate of soda. Fuse in a muffle or over a large gentle blow-pipe flame for 4 or 5 minutes. This will spoil the dish. Allow to cool and boil out in a larger dish with 100 c.c. of water. Filter and wash into an 8 oz. flask. Acidify the liquor with nitric and boil down to about 100 c.c. The acid should not be in too large excess, but an excess is needed to destroy nitrites. Neutralise with soda or ammonia. Add 5 c.c. of the mixture of sodium acetate and acetic acid. Titrate with uranium acetate.
Determination of Arsenic (As) in Crude Arsenic.—The method given under the iodine titration simply determines that portion of the arsenic which is present in the substance as arsenious oxide or white arsenic. The following method will give the total arsenic in the sample. It would be incorrect to report this as so much per cent. of arsenious oxide, although it may be reported as so much per cent. of arsenic equivalent to so much per cent. of white arsenic, thus:—