Sulphur.—The amount of sulphur in refined copper is very small, seldom exceeding 0.005 per cent. In coarse copper, as might be expected, it is found in larger quantities.
In determining it, it is first converted into sulphuric acid, and then precipitated and weighed as barium sulphate. The precipitation cannot be effected from a nitric acid solution. Ten grams of copper are dissolved in nitric acid, as for the other determinations, and then boiled with excess of hydrochloric acid till the nitric acid is completely removed. There is then added a few drops of a dilute solution of baric chloride, and the solution is allowed to stand for some hours. The baric sulphate is filtered off and weighed.
The necessity for precipitating from a hydrochloric acid solution is seen from the following determinations. In each experiment 10 grams of copper was used, and a known weight of sulphur, in the form of copper sulphate, added.
| Sulphur added. | Sulphur found in Hydrochloric Acid Solution. | Sulphur found in Nitric Acid Solution. |
| 5 milligrams | 8 milligrams | 0.03 milligrams |
| 10 " | 11 " | 0.03 " |
| 15 " | 17 " | 12.00 " |
Bismuth.—Nearly all samples of copper contain bismuth, but only in small quantities. It is best determined colorimetrically as described under Bismuth. The method of concentrating and preparing the solution for colorimetric assay is as follows. Dissolve 10 grams of copper in nitric acid, as in the other determinations; neutralise with soda; add 1 or 1.5 grams of bicarbonate of soda and boil for ten minutes; filter, dissolve the precipitate in hot dilute sulphuric acid; add sulphurous acid and potassium iodide in excess, and boil till free from iodine. Filter and dilute to 500 c.c. Take 50 c.c. of the yellow solution for the determination. A few c.c. of a dilute solution of sulphurous acid (1 in 100) will prevent the liberation of iodine. The following experiments test the method of separation. Ten grams of copper were treated as above and precipitated with 1.5 gram of "soda;" the precipitate contained 0.6 milligram of bismuth (= 0.006 per cent.). The filtrate treated with another 1.5 gram of "soda" gave a precipitate which was free from bismuth. To the filtrate from this was added 1.0 milligram of bismuth, and another fraction was precipitated with 1.5 gram of "soda." In this precipitate was found 1.0 milligram of bismuth. To the filtrate another milligram of bismuth was added and the separation with "soda" repeated. The bismuth was separated from this precipitate with ammonic carbonate before determination, and 0.9 milligram was found.
Arsenic.—The proportion of arsenic in copper varies from 0.01 to 0.75 per cent. whilst in coarse copper it may amount to 2 or even 3 per cent. To determine it, dissolve 5, 10, or 20 grams of the copper (according to the amount of arsenic present) in 18 c.c., 35 c.c., or 70 c.c. of nitric acid, and an equal volume of water. Boil off the nitrous fumes, dilute to 100 c.c. and neutralise with soda; add 1.5 or 2 grams of carbonate of soda dissolved in a little water, and boil. Filter (washing is unnecessary) and dissolve back into the flask with a little dilute hydrochloric acid; add 30 c.c. of dilute ammonia and 25 c.c. of "magnesia mixture," and allow to stand overnight. The whole of the arsenic is precipitated as ammonic-magnesic arsenate in one hour, but it is advisable to leave it longer. The precipitate may be dried and weighed, or, better, titrated with uranium acetate. (See Arsenic.) To test this method of separation 10 grams of pure copper were taken and 0.200 gram of arsenic dissolved with it. The arsenic was determined by titration with uranium acetate, and 0.200 gram was found. Two other similar experiments with 0.080 and 0.010 gram of arsenic added, gave 0.079 and 0.012 gram respectively.
Antimony or bismuth may be present without interfering with the titration. With 0.100 gram of antimony and 0.100 gram of arsenic, 0.100 gram of arsenic was found; and in another case, with 0.100 gram of bismuth and 0.060 gram of arsenic, 0.060 gram was found. In these experiments the antimony and bismuth were present in the assay solution when titrated. For a gravimetric determination they would require to be removed before precipitating with "magnesia mixture."
Phosphorus, if present, counts as arsenic in the proportion of 1 to 2.4; but, except in the case of coarse coppers, it is always absent.
Iron, if present, interferes by forming a white flocculent precipitate of ferric arsenate after the addition of the sodium acetate and preliminary to the titration. Each milligram of iron abstracts, in this way, 1.3 milligrams of arsenic.
Iron.—Refined coppers carry traces of iron, varying from 0.001 to 0.01 per cent. It is best determined during the arsenic estimation. The precipitate of the ammonic-magnesic arsenate will contain the whole of the iron as ferric hydrate. On dissolving in hydrochloric acid, neutralising with ammonia, adding 5 c.c. of sodic acetate, diluting, and boiling, it reappears as a white precipitate of ferric arsenate. It is filtered off (the arsenic being estimated in the filtrate), dissolved in warm hydrochloric acid, and determined colorimetrically as described under Iron. A series of experiments testing the separation is there given.