The standard silver nitrate solution is made by dissolving 5 grams of fine silver in 50 c.c. of dilute nitric acid, boiling off nitrous fumes, and diluting to 1 litre.
The indicator is a saturated solution of iron alum, or a solution of ferric sulphate of equivalent strength made by titrating acid ferrous sulphate with potassium permanganate. Use 2 c.c. for each assay.
The sulphocyanate solution is standardised by placing 50 c.c. of the silver nitrate solution in a flask with 20 c.c. of dilute nitric acid, diluting to 100 c.c. with water, and running in the sulphocyanate until the greater part of the silver is precipitated; then adding 2 c.c. of the ferric indicator, and continuing the titration until a reddish-brown colour is developed, and remains permanent after shaking continuously. The assay is similarly performed, the silver being used in the state of a nitric acid solution.
The effect of variations in the conditions of the assay may be seen from the following experiments, in which 20 c.c. of standard silver nitrate were used:—
Effect of Varying Temperature:—
| Temperature | 10° C. | 30° C. | 70° C. | 100° C. |
| Sulphocyanate reqd. | 19.6 c.c. | 19.3 c.c. | 19.0 c.c. | 18.6 c.c. |
Effect of Varying Nitric Acid:—Varying nitric acid has no effect, except that with a fairly acid solution the finishing point is somewhat sharper.
| Nitric acid added | 5 c.c. | 10 c.c. | 20 c.c. | 50 c.c. |
| Sulphocyanate reqd. | 19.6 c.c. | 19.5 c.c. | 19.6 c.c. | 19.6 c.c. |
Effect of Varying Bulk:—
| Bulk | 50 c.c. | 100 c.c. | 200 c.c. | 300 c.c. |
| Sulphocyanate reqd. | 19.5 c.c. | 19.6 c.c. | 19.6 c.c. | 19.7 c.c. |