Phosphorus.—Refined coppers do not carry phosphorus, although it may be present in "coarse copper" up to 1 per cent. or more. In such samples the following method is adopted for the estimation of both phosphorus and arsenic. Dissolve 10 grams of copper and 0.1, 0.2, or 0.3 gram of iron wire (according to the amount of arsenic and phosphorus present) in 35 c.c. of nitric acid and an equal volume of water. Add soda till the free acid is nearly neutralised. Next add a strong solution of sodium acetate, until the solution ceases to darken on further addition, then dilute with water to half a litre. The solution is best contained in a large beaker; it is next heated to the boiling point, and at once removed and allowed to settle. If the precipitate is light coloured it is evidence that sufficient iron has not been added, or, if it is green, from basic copper salts, it shows that the solution was not sufficiently acid. In either case start afresh. Filter off the precipitate and wash with hot water containing a little sodium acetate, dissolve it off the filter with hot dilute hydrochloric acid, add ammonia in excess, and pass sulphuretted hydrogen for five minutes. Warm at about 70° C. for a quarter of an hour. Filter. The clear yellow filtrate contains the arsenic and phosphorus. Add dilute sulphuric acid in excess; filter off the yellow precipitate of sulphide of arsenic, dissolve it in nitric acid, and titrate with uranium acetate, as described under Arsenic.
The filtrate from the sulphide of arsenic is rendered alkaline with ammonia and "magnesia mixture" added. The solution is stirred, and allowed to stand overnight. The precipitate of ammonic-magnesic phosphate is filtered off, dissolved, and titrated with uranium acetate, using the same standard solution as is used in the arsenic assay: 0.5 gram of arsenic equals 0.207 gram of phosphorus.
Copper.—The method of determining this has been described under Electrolytic Assay.
In the method of concentration by fractional precipitation with sodic carbonate (which is adopted in most of these determinations) the precipitate will contain all the bismuth, iron, and alumina; the arsenic and phosphorus as cupric arsenate and phosphate; and the greater part of the lead, antimony, and silver. The nickel and cobalt, and the sulphur as sulphuric acid, will remain in solution with the greater part of the copper.
PRACTICAL EXERCISES.
1. According to a wet assay 2 grams of a certain ore contained 0.3650 gram of copper. What would you expect the dry assay produce to be?
2. A standard solution is made by dissolving 25 grams of potassic cyanide and diluting to a litre. Assuming the salt to be 98 per cent. real cyanide, what would 100 c.c. of the solution be equivalent to in grams of copper?
3. How would you make a solution of "hypo" of such strength that 100 c.c. shall equal 0.633 gram of copper?
4. What weight of ore, containing 17.0 per cent. of copper, would you take in order to get about 0.5 gram of copper in solution for electrolysis?
5. The solution of copper in nitric acid is effected by the following reaction:—