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posted Jun 16, 2017, 2:13 AM by John Doe   [ updated Jun 16, 2017, 2:13 AM ]

Methods of Detecting Gold in Sea-Water.

John R. Don

Sonstad's Methods.—Sonstadt gave three methods for the detection of gold in sea-water, two of which he recommended as easily applicable. In the first of these, 150 to 200 c.c. of sea water is acidulated with hydrochloric acid, ferrous sulphate is added, and the water is concentrated by boiling. The film of ferric oxide found in the bottom of the dish is treated with chlorine-water, and the solution obtained is tested for gold with tin chloride. In the second method, a small quantity of baric chloride is added to the sea-water, and both gold and silver are found to be precipitated.

The first method I tried upon water from the Pacific, at St Clair head, near Dunedin, and obtained with tin chloride a distinct coloration, doubtless due to gold. § But I found it impossible to obtain beads of gold by fusing with borax and pure lead, as Sonstadt directs. (This is not surprising, since my subsequent determinations would give, as the quantity of gold in 200 c.c. of this sea-water, less than 0.000012 grain.) The necessity of boiling makes this method unsuitable for the treatment of samples large enough to yield a weighable bead of gold.

I therefore tried the precipitation with baric chloride. Sonstadt makes the remarkable assertion that in order to precipitate the gold, the baric chloride does not need to be added in sufficient quantity to precipitate as baric sulphate all the soluble sulphates in the sea-water, but, on the contrary, that the amount added to a liter of sea-water need not exceed that required to form about 1 grain of precipitate. Baric chloride being one of the least soluble of salts, this statement seems inexplicable; nevertheless, I have been convinced, whatever be the explanation, that Sonstadt's method is as effective as if the whole of the soluble sulphates were precipitated as baric sulphate.

He explains the precipitation by baric chloride by supposing the gold to be present as an aurate. To test this question, artificial sea-water was prepared, and the aurate of potassium was added to it. Subsequent treatment with baric chloride precipitated no gold. The experiment was repeated, with the sanie result.

It seems unlikely, therefore, that the gold exists in sea-water as an aurate. I confess that I can form no conception of its state of combination. The subject would repay a more thorough investigation.

The Author's Method.—Having been led, by evidence which I will not here repeat, to doubt whether the precipitation with baric chloride was complete, I tried a different one, which, if successful at all, would certainly precipitate all the gold present. All ordinary salts of gold are reduced to the metallic state by moderate heating. Applying this principle first to an artificial solution, I added to 112 pounds of artificial sea-water* ten times as much calcic iodate as Sonstadt found, and a solution of auric chloride in sodium chloride, containing 0.005 grain of gold, so that the water would contain 0.1 grain of gold per ton. This solution was allowed to stand in a dark place for a week, that the gold might have time to form possible new combinations The whole was then evaporated; the mixed salts (over 4 pounds) were heated dull red and lixiviated with water; the insoluble residue (123 grains) was fused with borax and pure litharge, and 0.0043 grains of gold was obtained. The experiment was repeated, the gold being added as an iodate, dissolved in excess of potassic iodate; and in this case 0.0052 grain of gold was recovered. (The slight excess may have been due to a small particle of the cupel remaining in the button. It was not due to silver in the litharge.)

The method was then applied to actual sea-water; only the sample was doubled in weight, to allow for the smaller propertion of gold. The mixed salts (about 8 pounds) resulting from the evaporation of 0.1 ton of sea-water were heated dull red and lixiviated, and the residue (principally sand, with a little oxide of iron) was fused and cupelled as before. The weight of pure gold obtained from 0.1 ton of sea-water was, in the first experiment 0.0065, and in the second 0.0071 grain. In both cases, the prill contained absolutely no silver.

The following table summarizes the experiments above dsscribed:
Determinations of Gold in Sea-Water.

I have said nothing concerning the silver in sea-water. Precipitation by baric chloride certainly saves some silver, but on! about only fourth as much as was reported by Malaguti and Durocher. Probably the precipitation is not complete.

§ The experiment is more successful with the modification of the tin-chloride test prepared by T. K. Rose (Chem. News, vol. lxvi, p. 271).
* Prepared according to the analysis by Roscoe and Schorlemmer.