PRACTICAL NOTES ON THE IODIDE PROCESS OF COPPER ASSAYING.
For the following remarks and experiments we are indebted to Mr. J.W. Westmoreland, who has had considerable experience with the process. Having dissolved the ore he converts the metals into sulphates by evaporating with sulphuric acid. The copper is then separated as subsulphide by means of hyposulphite of soda, and the precipitate is washed, dried, and calcined. The resulting oxide of copper is then dissolved in nitric acid; and to the concentrated solution, a saturated solution of carbonate of soda is added in sufficient quantity to throw down a considerable proportion of the copper. Acetic acid is added to dissolve the precipitate, and when this is effected more of the acid is poured on so as to render the solution strongly acid. To this potassium iodide crystals are added in the proportion of ten parts of iodide to each one part of copper supposed to be present. The solution is then titrated with "hypo" as usual.
For the examination of technical products experiments made in sulphuric acid solutions have no value, since arsenic acid, which is generally present to a greater or less extent, affects the end reaction. In such solutions bismuth may also interfere.
The solution best suited for the assay is one containing acetate of soda and free acetic acid. The presence of acetate of soda counteracts the interference of arsenic and of bismuth.
The return of the blue colour after titration is due to the excessive dilution of the assay, or to an insufficiency of potassium iodide, or to the presence of nitrous fumes. The interference of an excess of sodium acetate is avoided by adding more iodide crystals to the extent of doubling the usual amount.
The interference of lead can be avoided by the addition of sulphuric acid or of phosphate of soda to the acid solution containing the copper, and before neutralising with carbonate of soda. The end reaction is, however, with care distinguishable without this addition. The following experiments, each containing .0648 gram of lead, were made by him in illustration:
| Copper taken. | Reagent added. | Copper found. | End reaction. |
| .2092 gram | — | .2077 gram | fairly satisfactory |
| .2101 " | — | .2092 " | " |
| .2167 " | sulphuric acid | .2152 " | " |
| .2117 " | " | .2108 " | " |
| .2109 " | phosphate of soda | .2092 " | good, colourless |
| .2205 " | " | .2174 " | rather yellow |
Effect of Sodium Acetate.—Each solution contained .3343 gram of copper.
| a.b.c. | d. | e. | f. | g. | |
| grams. | grams. | grams. | grams. | grams. | |
| "Acetate" added | — | 16.2 | 16.2 | 16.2 | 16.2 |
| "Iodide" added | 3.5 | 3.5 | 7.0 | 3.5 | 7.0 |
| Copper found | .3343 | .3324 | .3351 | .3269 | .3356 |
In these experiments, except with the excessive quantities of acetate of soda and the insufficiency of potassium iodide in the cases of d and f, there was no difficulty with the after-blueing.