The deposition of the copper under the conditions given is satisfactory, but, as already stated, if the solution contain more than 10 per cent. of nitric acid it is not thrown down at all; or if a stronger current is used, say that from three Bunsen cells, it will be precipitated in an arborescent brittle form, ill adapted for weighing. It may be noted here that increasing the size of the cells does not necessarily increase the intensity of the current.
In two determinations on pure electrotype copper the following results were obtained:—
| Copper Taken. | Copper Found. |
| 0.8988 gram | 0.8985 gram |
| 0.8305 " | 0.8303 " |
The presence of salts of ammonia, &c., somewhat retards the deposition, but has no other ill effect.
The organic matter generally present in copper ores interferes more especially in the colorimetric determination of the residual copper. It can be detected on dissolving the ore as a light black residue insoluble in nitric acid. It is filtered off at once, or, if only present in small amount, it is carried on in the ordinary process of the assay and separated in the last filtration before electrolysis.
The following experiments were made to test the effect of the presence of salts of foreign metals in the solution during the precipitation of copper by electrolysis:—
| Copper Taken. | Other Metal Added. | Copper Found. |
| 0.1000 gram | 0.1000 gram of silver | 0.1800 |
| 0.1050 " | 0.1000 " " | 0.2000 |
| 0.1030 " | 0.1000 " mercury | 0.2010 |
| 0.1037 " | 0.1000 " " | 0.2015 |
| 0.1020 " | 0.1000 " lead | 0.1020 |
| 0.1030 " | 0.1000 " " | 0.1028 |
| 0.1010 " | 0.1000 " arsenic | 0.1010 |
| 0.1007 " | 0.1000 " " | 0.1022 |
| 0.1030 " | 0.1000 " antimony | 0.1050 |
| 0.1034 " | 0.1000 " " | 0.1057 |
| 0.0990 " | 0.1200 " tin | 0.0990 |
| 0.1014 " | 0.1000 " " | 0.1015 |
| 0.1000 " | 0.1000 " bismuth | 0.1662 |
| 0.1040 " | 0.1000 " of cadmium | 0.1052 |
| 0.1009 " | 0.1300 " zinc | 0.1017 |
| 0.1014 " | 0.1000 " nickel | 0.1007 |
| 0.1079 " | 0.1200 " iron | 0.1089 |
| 0.1054 " | 0.1000 " chromium (Cr2O3) | 0.1035 |
| 0.1034 " | 0.1000 " " (K2CrO4) | 0.1010 |
| 0.1075 " | 0.1000 " aluminium | 0.1078 |
| 0.1010 " | 0.1000 " manganese | 0.0980 |
It will be seen from these that mercury, silver, and bismuth are the only metals which are precipitable[52] along with the copper under the conditions of the assay. Mercury, which if present would interfere, is separated because of the insolubility of its sulphide in nitric acid.
Bismuth is precipitated only after the main portion of the copper is thrown down. It renders the copper obviously unsuitable for weighing. It darkens, or forms a greyish coating on, the copper; and this darkening is a delicate test for bismuth. In assaying ores containing about three and a half per cent. of copper, and known to contain bismuth in quantities scarcely detectable in ordinary analysis, the metal deposited was distinctly greyish in colour, and would not be mistaken for pure copper. Ten grams of this impure copper were collected and analysed, with the following results:—