The colorimetric determination may be made in the manner described under that head, but where a number of assays are being carried out it is more convenient to have a series of standard phials containing known amounts of copper in ammoniacal solution. By comparing the measured volume of the assay solution with these, the amount of copper present is determined at a glance. These standard bottles, however, can only be economically used where a large number of assays are being made daily.
A convenient plan is to get a quantity of white glass four-ounce phials, like that in fig. 55, and to label them so that they shall contain 100 c.c. when filled up to the bottom of the labels. The labels should be rendered permanent by coating with wax, and be marked with numbers indicating the milligrams of copper present. The bottles are stopped with new clean corks, and contain, in addition to the specified quantity of copper, 6 c.c. of nitric acid and 10 c.c. of strong ammonia, with sufficient water to make up the bulk to 100 c.c. The copper is best added by running in the requisite amount of a standard solution of copper, each c.c. of which contains 0.001 gram of the metal.
The standard bottles should be refilled once every three or four months, since their colorimetric value becomes slowly less on keeping. The following determinations of a set which had been in use for three months will illustrate this. The figures indicate milligrams of copper in 100 c.c.: the first row gives the nominal and the second row the actual colorimetric value of the standards. The difference between the two shows the deterioration.
| 1 | 2 | 3 | 4 | 6 | 8 | 10 | 12 | 14 |
| 1 | 2 | 3 | 3.7 | 5.5 | 7.5 | 9 | 11 | 13 |
The amount of copper in the assay is got by increasing that found colorimetrically by one-half and adding to that found on the platinum cylinder. The percentage is calculated in the usual way. The following examples will illustrate this, as well as the method of recording the work in the laboratory book:—
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Cylinder I. + Cu 9.5410
Cylinder I. 9.5170
——————
0.0240
By colour 100 c.c. = 0.0015}
0.0007} 0.0022
—————— ——————
0.0022 0.0262
IX. Sample. Took 5 grams.
Copper = 0.52%
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Cylinder VI. + Cu 10.5705
Cylinder VI. 10.0437
——————
0.5268
By colour, 100 c.c. = 0.0070}
0.0035} 0.0105
—————— ——————
0.0105 0.5373
Matte, No. 1070. Took 1.5 gram.
Copper = 35.82%
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Cylinder XIII. + Cu 12.0352
Cylinder XIII. 11.0405
——————
0.9947
By colour 100 c.c. = 0.0005}
0.0002} 0.0007
—————— ——————
0.0007 0.9954
X. Sample, Cake copper. Took 1.0053 gram.
Copper = 99.00%
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In the electrolytic assay of metals, alloys, precipitates, and other bodies rich in copper, the preliminary separation of the copper by sulphuretted hydrogen is unnecessary. It is sufficient to dissolve the weighed sample in 10 c.c. of nitric acid, boil off nitrous fumes, dilute to 100 c.c. with water, and then electrolyse.
General Considerations.—In the preliminary work with the copper sulphide there is a small loss owing to its imperfect removal in washing the filter paper, and another small loss in dissolving in nitric acid owing to the retention of particles in the fused globules of sulphur. To determine its amount the filter-papers and sulphur were collected from forty assays, and the copper in them determined. The average amount of copper in each assay was 0.175 gram; that left on the filter paper was 0.00067 gram; and that retained by the sulphur 0.00003 gram; thus showing an average loss from both sources of 0.00070 gram. The determinations from another lot of forty-two similar assays gave on an average
| Copper left on filter paper | 0.00070 | gram |
| Copper retained by sulphur. | 0.00004 | " |
The loss from these sources is trifling, and need only be considered when great accuracy is required.