Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  


    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 74

The Relative Weights of Gold and Silver Dissolved by Potassium Cyanide Solutions from Alloys of these Metals

page break

The Relative Weights of Gold and Silver Dissolved by Potassium Cyanide Solutions from Alloys of these Metals.

page break

The relative weights of gold and silver dissolved by potassium cyanide solutions from alloys of these metals.

In the extraction, by means of potassium cyanide solutions, of gold and silver from ores containing them, it is found that the percentage of gold recovered is almost always larger than the percentage of silver. This fact must be due to one or both of two causes; 1st, in an alloy of gold and silver the gold is more readily dissolved than the silver, or, 2nd, gold generally exists in the metallic state, whilst silver is often combined with sulphur, tellurium, &c., forming compounds which are only slowly dissolved by the cyanide. In order to test the first of these hypotheses, alloys of gold and silver of varying composition were prepared, and, after being rolled into sheets, circular plates were stamped out of them. These plates were then exposed, fa Nessler test-glasses, to the action of a 0.5 per cent, solution of potassium cyanide for two hours. The cyanide solutions were evaporated to dryness, and the bullion and gold determined by ordinaryassay methods.

page 1277

The results are contained in the following table.

Percentage of gold in plate. Percentage of gold in bullion dissolved by KCN.
20 20.0
50 47.8
80 77.5

These results show that, practically, gold and silver are [unclear: dissolvel] from an alloy of these metals in the proportions (by weight) in [unclear: which] they exist in the alloy. At first sight this appears to be in direct opposition to my results on the rate of solution of the two [unclear: metals] when separate (Trans., 1895, 67,199), as it was then shown that "the ratio of the amount of gold dissolved by any given cyanide [unclear: solution] to that of the silver dissolved by the same solution is nearly in this ratio of their atomic weights," or, in other words, for every 197 part of gold dissolved only 108 of silver pass into solution. In an alloy [unclear: of] the two metals, let A represent the weight of gold, and B the [unclear: weight] of silver, then the relative areas of the metals exposed to the [unclear: cyanide] will be gold = A/sp. gr. of Au = A/19.3, silver = B/sp. gr. of [unclear: Ag]= B/10.45*

But, as already shown (when the metals are separate), the weight of gold dissolved : the weight of silver dissolved from equal surface; the atomic weight of gold : atomic weight of silver, or Au/Ag = 196.85/107.66, and, assuming that this relation holds good when metals are alloyed, we get
  • Weight of Au dissolved = A 196.85/19.3 = A 1.02.
  • Weight of Ag dissolved = B 107.66/10.45 = B 1.0206.
But 196.85/19.3 = atomic volume of gold, and 107.66/10.45 = atomic volume of silver, and, as these atomic volumes are [unclear: practical] equal, the relative weights of gold and silver dissolved are [unclear: proper] tional to A and B, that is, to the weights of the respective metalsi [unclear: in] the alloy. As the results given in the above table are in [unclear: accordanc] with this hypothesis, we may conclude that it is correct, and that from an alloy of gold and silver, the metals are dissolved in the of their atomic volumes.

Harribon and Sons. Printers in Ordinary to Hrr Majesty, St. Martin's [unclear: La]

* This becomes evident if we consider a cube of the alloy to be divided into [unclear: ti] layers, and the layers into infinitely small prisms of gold and silver, the number [unclear: of] prisms of the respective metals being in the proportion of their volumes.