WET ASSAYING This employs the volumetric, gravimetric, and electrolytic methods. Volumetric methods are in particular favour, as less manipulation is required, and the sample may often be titrated without lengthy chemical separations. The weight of ore taken for assay is much smaller than in Fire Assaying, 0.25-2.0 grammes generally being sufficient. It is essential that the sample shall be finely crushed, especially if it is to be dissolved in acids, as particles of the mineral may be encased in the insoluble gangue. Minerals which are not decomposed by acids are fused with a flux which will render them soluble in water or acids.
The cupellation assay was used formerly, for the determination of silver in many of its alloys. This method has been replaced in the majority of mints and assay offices by volumetric methods. The chief of these is the "Gay Lussac" assay, which was introduced into the Paris mint in 183o, and has since been adopted in most offices. An exact weight of the bullion is dissolved in nitric acid, and very nearly all the silver is precipitated at once by the addition of a known volume of a standard solution of salt. When the precipitate has settled, the remaining silver is precipitated by the further addition of a small quantity of a more dilute solution of salt, the precipitate forming a white cloud in the supernatant liquid. The quantity of this silver is judged by the appearance of the white cloud.
The material is attacked with suitable acids, and the copper is carried into solution. There are three methods of assay available. (I) In the cyanide assay, the solution is rendered ammoniacal, which results in the formation of a deep blue liquid. This colour is discharged by titration with a standard solution of potassium cyanide, the volume of the latter being a measure of the copper present. (2) In the iodide assay, the copper is first separated from the other metals present, and is then redissolved in Nitric acid. The mineral acid is destroyed by adding an excess of zinc acetate. Potassium iodide is then added, which results in the reduction of the copper and the liberation of a quantity of iodine which is proportional to the amount of copper present. This iodine is measured by titration with a standard solution of sodium thiosulphate, using starch solution as an inside indicator (q.v.), towards the end of the titration. (3) In the electrolytic assay, any hydrochloric acid is expelled from the solution, and the acidity is carefully adjusted to the correct strength. The solution is then subjected to electrolysis, the copper being deposited on the cathode. The increase in weight of the latter gives the amount of copper present.
The ore is brought into solution by acid attack, the iron is reduced by one of the usual chemical processes, and the ferrous solution is oxidised by titration with a standard solution of potassium dichromate, or of potassium permanganate. In the former case a weak solution of potassium ferricyanide is used as an outside indicator (q.v.).
Minerals of this metal are dissolved in hydrochloric acid, sometimes assisted by nitric acid. The lead is separated by evaporation with sulphuric acid, and the solution is cooled, diluted, and filtered. The precipitated lead sulphate is redissolved in hot ammonium acetate solution, and titrated, just below the boiling point, with a standard solution of ammonium molybdate. A solu tion of tannic acid is used as an outside indicator. An alternative method is to reprecipitate the lead as chromate, filter, and dissolve the precipitate in a hydrochloric acid solution of salt. Potassium iodide is added, which results in the liberation of iodine. This is titrated with thiosuplhate as in the copper assay.
The ores of tin are decomposed by fusion with sodium peroxide in an iron or nickel crucible, or by ignition with zinc oxide and metallic zinc. In either case the residue is leached out with water and dissolved in hydrochloric acid. The tin is reduced by suspending a nickel coil in the solution and boiling vigorously for 4o-6o minutes. The solution is cooled in an atmosphere of carbon dioxide, and the tin is oxidised by titration with a stand ard solution of iodine, using starch as an inside indicator.
These ores are attacked with acids, and the solution is subjected to a process of separation which follows, in principle, the system adopted in Qualitative Analysis. (See CHEMISTRY : Analytical.) The clear zinc solution is rendered slightly acid, and is titrated, just below the boiling point, with a standard solution of potassium ferrocyanide. A solution of uranium nitrate is used as an outside indicator.
(Georgius), De Re Metallica (Basle 1556), Bibliography.-Agricola (Georgius), De Re Metallica (Basle 1556), Eng. trans. by Mr. and Mrs. H. C. Hoover (The Mining Magazine, 1912) ; E. A. Smith, The Sampling and Assay of the Precious Metals (new edition in preparation) ; E. A. Wraight, Assaying in Theory and Practice (1914) ; W. W. Scott, Technical Methods of Metallurgical Analysis (1924). (C. W. D.)
