It will be seen that in working under these conditions the results are proportional and the method satisfactory.
Examination of Tin Phosphide.—(Phosphor Tin.)—This substance is used in the manufacture of "phosphor bronze" and similar alloys. It is a crystalline, imperfectly-malleable, metallic substance. It is soluble in hydrochloric acid with effervescence; phosphoretted hydrogen, which inflames on the addition of a drop or two of nitric acid, being evolved. It is attacked by nitric acid, yielding a white powder of stannic phosphate; this is not easily decomposed by ammonium sulphide or readily soluble in hydrochloric acid.
"Phosphor-tin" is made up only of tin and phosphorus. For the estimation weigh up 1 gram. Place in a weighed Berlin dish; and cover with 10 c.c. of nitric acid and 3 or 4 c.c. of water. Let the reaction proceed (under a clock-glass) on the water-bath till complete. Remove the glass; evaporate to dryness, and ignite, at first gently over a Bunsen burner, and afterwards in the muffle at a red heat. Cool in the desiccator, and weigh as quickly as possible when cold. The substance contains the tin as stannic oxide, SnO2, and the phosphorus as phosphoric oxide, P2O5. The increase in weight on the gram of substance taken gives the weight of the oxygen taken up by the phosphorus and tin, and since 1 gram of tin takes up only 0.271 gram of oxygen, and 1 gram of phosphorus takes up 1.29 gram, the proportion of tin to phosphorus can be calculated from the increase in weight. For example, 1 gram of a sample gave 1.3410 gram of mixed oxides, which is 0.070 gram in excess of that which would be got with pure tin. If the substance was all phosphorus the excess would be 1.0190 gram; consequently the proportion of phosphorus in the substance is 0.070 / 1.019, or 6.87 per cent. The tin is calculated by difference, 93.13 per cent.
Another method of separating and determining the phosphorus is as follows:—Take 1 gram of the substance and add to it 15 c.c. of hot aqua regia. Boil till dissolved, dilute, and precipitate the tin with sulphuretted hydrogen. To the filtrate add ammonia and "magnesia mixture." Filter; wash the precipitate with dilute ammonia; dry, ignite, and weigh as magnesic pyrophosphate. Calculate the phosphorus, and take the tin by difference.
A sample of phosphor tin gave—
| Tin | 93.1 | per cent. (by difference) |
| Phosphorus | 6.9 | " |
| ——- | ||
| 100.0 |
Tin Arsenide.—This is met with in tin-smelting; it closely resembles the phosphide, but the crystals have a duller grey appearance. It contains simply tin and arsenic. The determination is made by treating 1 gram of the substance with nitric acid and weighing the mixed oxides of tin and arsenic in the same manner as in the case of the phosphide. One gram of arsenic will give 1.533 gram of arsenic oxide, As2O5; consequently the excess of weight of the mixed oxides over 1.271 gram must be divided by 0.262; the result multiplied by 100 gives the percentage of arsenic. In consequence of the higher atomic weight of arsenic the results by this method are not so close as with the phosphide. Each milligram of excess weight (over 1.271) represents 0.38 per cent. of arsenic, As. Both in this and in the corresponding phosphide determination care must be taken to avoid absorption of moisture, by allowing the oxides to cool in a desiccator and weighing quickly.
The percentage of arsenic is better determined as follows:—Weigh up 1 gram of the substance, dissolve in aqua regia, dilute, and pass sulphuretted hydrogen. Render alkaline with ammonia, and add ammonium sulphide till the precipitate is dissolved. Add "magnesia mixture." Filter off the precipitate, wash with dilute ammonia, ignite with a few drops of nitric acid, and weigh as magnesic pyrarsenate. Calculate the arsenic and take the tin by difference. A sample treated in this way gave—
| Tin | 96.8 | per cent. by difference |
| Arsenic | 3.2 | " |
| ——- | ||
| 100.0 |
Examination of Black Tin.—Dry the ore, and reduce it to a fine powder. Weigh up 2 grams, and boil with 20 c.c. of hydrochloric acid and 2 c.c. of nitric for ten or fifteen minutes. Filter, and reserve the filtrate.