Pour on the assay in the flask 50 c.c. of a "ferric chloride mixture," made by dissolving 600 grams of calcium chloride and 300 grams of ferric chloride in 600 c.c. of hydrochloric acid, and making up to 1 litre with water.
Firmly cork up the apparatus, and boil over a small Bunsen-burner flame for fifteen or twenty minutes, but avoid evaporating to dryness. Disconnect the flask, and pour away its contents at once to prevent breakage of the flask by their solidification. The arsenic will be condensed in the U-tube, together with the greater part of the hydrochloric acid; transfer the distillate to a beaker washing out the tube two or three times with water; add a small piece of litmus paper; neutralise with ammonia; render faintly acid with dilute hydrochloric acid; add 2 grams of bicarbonate of soda in solution; and dilute to 250 c.c. The solution is now ready for titrating.
The arsenic comes over in the early part of the distillation, as will be seen from the following experiment, made on 1 gram of copper precipitate; in which experiment the distillate was collected in separate portions at equal intervals, and the arsenic in each portion determined:—
| Time Distilling. | Iodine Required. | Equivalent to Arsenic in the Distillate. |
| 5 minutes | 12.0 c.c. | 0.0450 gram |
| 5 " | 0.17 " | 0.0005 " |
| 5 " | 0.0 " | |
| 5 " | 0.0 " | |
| To dryness | 0.0 " |
The volume of each distillate was about 5 c.c.
In this operation the metals are converted into chlorides by the action of ferric chloride, which gives up a part of its chlorine, and becomes reduced to the ferrous salt. The calcium chloride does not enter into the chemical reaction, but raises the temperature at which the solution boils, and is essential for the completion of the distillation.[105] Two experiments with material containing 3.48 per cent. of arsenic gave—(1) with ferric chloride alone, 2.74 per cent.; and (2) with the addition of calcium chloride, 3.48 per cent.
It is always necessary to make a blank determination with 1 gram of electrotype copper, to find out the amount of arsenic in the ferric chloride mixture.[106] Unfortunately, a correction is always required. This amounts to about 0.15 per cent. of arsenic on each assay, even when the mixture has been purified; and this constitutes the weakness of the method, since, in some cases, the correction is as much as, or even greater than, the percentage to be determined.
The acid distillate containing the arsenious chloride may be left for an hour or so without much fear of oxidation; but it is safer to neutralise and then to add the bicarbonate of soda, as the following experiments show. Several portions of a solution, each having a bulk of 100 c.c., were exposed for varying lengths of time, and the arsenic in each determined.
| Time Exposed. | Acid Solutions. "Iodine" Required. Arsenic Found. | Neutralised Solutions. "Iodine"Required. Arsenic Found. |
| — | 18.2 c.c. = 0.0136 gram | 18.1 c.c. = 0.0136 gram |
| 1 hour | 18.2 " = 0.0136 " | 18.2 " = 0.0136 " |
| 2 hours | 17.7 " = 0.0133 " | 18.0 " = 0.0135 " |
| 4 " | 17.5 " = 0.0131 " | 18.4 " = 0.0138 " |
| 5 " | 17.0 " = 0.0127 " | 18.3 " = 0.0137 " |
The Titration.—Make a standard solution of iodine by weighing up in a beaker 16.933 grams of iodine and 30 grams of potassium iodide in crystals; add a few c.c. of water, and, when dissolved, dilute to 1 litre: 100 c.c. will equal 0.500 gram of arsenic.