Instead of igniting the precipitate with nitric acid, it may be collected on a weighed filter-paper, dried at 100° C., and weighed as ammonic-magnesic arsenate (2AmMgAsO4.H2O), which contains 39.5 per cent. of arsenic. The results in this case are likely to be a little higher. The drying is very tedious, and is likely to leave behind more water than is allowed for in the formula. In a series of determinations in which the arsenic was weighed in both forms, the results were:—
| Ammonic-magnesic Arsenate in grams. | Arsenic in grams. | Magnesium Pyrarsenate in grams. | Arsenic in grams. |
| 0.0080 | 0.0032 | 0.0065 | 0.0031 |
| 0.0400 | 0.0158 | 0.0330 | 0.0160 |
| 0.0799 | 0.0316 | 0.0633 | 0.0306 |
| 0.1600 | 0.0632 | 0.1287 | 0.0623 |
| 0.4000 | 0.1580 | 0.3205 | 0.1551 |
| 0.7990 | 0.3156 | 0.6435 | 0.3114 |
VOLUMETRIC METHODS.
There are two methods: one for determining the arsenic in the lower, and the other in the higher state of oxidation. In the first-mentioned method this is done by titrating with a standard solution of iodine; and in the latter with a solution of uranium acetate. Where the arsenic already exists as arsenious oxide, or where it is most conveniently separated by distillation as arsenious chloride, the iodine method should be used; but when the arsenic is separated as ammonic-magnesic arsenate or as sulphide, the uranium acetate titration should be adopted.
IODINE PROCESS.
This is based on the fact that sodium arsenite in a solution containing an excess of bicarbonate of soda is indirectly oxidised by iodine to sodium arsenate,[103] and that an excess of iodine may be recognised by the blue colour it strikes with starch. The process is divided into two parts—(1) the preparation of the solution, and (2) the titration.
Preparation of the Solution.—For substances like crude arsenic, in which the arsenic is present as arsenious oxide, the method is as follows:—Take a portion which shall contain from 0.25 to 0.5 gram of the oxide, place in a beaker, and cover with 10 c.c. of sodic hydrate solution; warm till dissolved, put a small piece of litmus paper in the solution, and render acid with dilute hydrochloric acid. Add 2 grams of bicarbonate of soda in solution, filter (if necessary), and dilute to 100 c.c. The solution is now ready for titrating.
Where the arsenic has to be separated as arsenious chloride, the process is as follows:[104]—Weigh up 1 gram of the finely-powdered ore (metals should be hammered out into a thin foil or be used as filings), and place in a 16-ounce flask provided with a well-fitting cork, and connected with a U-tube, as shown in the drawing (fig. 68). The U-tube should contain 2 or 3 c.c. of water, and is cooled by being placed in a jar or large beaker of cold water. The water used for cooling should be renewed for each assay.