With fairly pure ores, free from sulphate, the assay may be made more quickly as follows: Dissolve 1 gram of the finely powdered ore by boiling gently with 40 c.c. of dilute hydrochloric acid for 15 minutes; cool; add a few drops of permanganate; neutralise with ammonia, add acetic acid and a little sodium acetate. Titrate with standard chromate.
COLORIMETRIC PROCESS.
This is based upon the brown coloration produced in very dilute solutions of lead by the action of a solution of sulphuretted hydrogen. The quantity of lead in the 50 c.c. of the assay solution must not much exceed 0.5 milligram, nor be less than 0.01. The sulphuretted hydrogen is used in the form of a solution, and is not bubbled through the assay. The principle of working is the same as previously described.
Standard Lead Solution.—Each c.c. of this should contain 0.1 milligram of lead. It is made by diluting 10 c.c. of the solution of lead nitrate, described under the volumetric process, to 1 litre.
Sulphuretted hydrogen water is made by passing a current of the washed gas into water till the latter is saturated.
Five c.c. of the sulphuretted hydrogen water are put into a Nessler tube, the measured portion of the assay solution added, and the whole diluted with water to the 50 c.c. mark. Into the standard Nessler tube the same amount of the sulphuretted hydrogen water is put, and diluted to nearly 50 c.c. The standard lead solution is then run in till the tints are equal. The assay solution must not contain much free acid, and if the conditions will allow it, may with advantage be rendered alkaline with ammonia. The chief cause of disturbance is the precipitation of lead sulphide forming a black turbid solution instead of a brown clear one. This may be caused by using hot solutions or an excess of acid. Other metals precipitable by sulphuretted hydrogen must be absent as well as strong oxidising agents.
Effect of Varying Temperature.—The effect of increased temperature is to change the colour from brown to black, and to render the estimation difficult.
| 1 | c.c. | at 15° C. | showed the | colour of | 0.5 | c.c. | at 60° C. |
| 2 | " | " | " | " | 1.5 | " | at 60° C. |
| 3 | " | " | " | " | 5.0 | " | at 50° C. |
Effect of Varying Time.—The colour becomes lighter on standing: 2 c.c. on standing 10, 20, and 40 minutes became equal in colour to 1.7 c.c.
Effect of Acids and Ammonia.—Two c.c. of the solution with 2 c.c. of dilute hydrochloric acid became cloudy and equivalent to about 2.5 c.c.; and a similar result was got with 2 c.c. of dilute sulphuric acid. With 2 c.c. of dilute ammonia the solution became somewhat darker, or equal to 2.3 c.c.; but gave a very clear solution easy to compare.