Addition of Ferrous Sulphate to Precipitate Gold
The addition of ferrous sulphate to precipitate gold is quite simple and the operator must be sure the solution is fresh made from the pure salt and few drops of the solution added to a very dilute acid solution containing the simplest trace of gold, gives a beautiful light blue color to the liquid; but in a stronger acid solution with the smallest increase in the proportion of gold and corresponding increase in the proportion of the testing drops, a dark blue will be imparted to the solution, and when there is a fraction more gold in the solution, the depth of color increase to a brownish black and to a higher degree the richer is the solution. After a few hours rest for the gold to settle down, the liquid above it will change back to a pale green color or transparent clearness, which admits the rays of light passing through it.
A few drops of stannous chloride on the very dilute acid solution containing the gold particles, gives a brownish red color to the liquid, but in a stronger acid solution with the slightest increase in the proportion of gold, a brownish will at once be given to the liquid and when there is more gold in the solution the depth of color increase to a black. But unlike the ferrous sulphate, the stannous chloride does not clear the solution so as to admit the light passing through it, but remains the same color into which it is changed for a considerable time afterwards.
It is common to note that the more gold there happens to be in the sample liquids selected for trial the darker will be in the resulting colors from the action of both testing mixtures. Ferrous sulphate is the best and quickest precipitant for bulky solutions, seldom requiring more than a few hours to throw down the whole of the gold and clear the solution sufficiently for testing purposes. Stannous chloride is the most delicate test and by the far the best one to be employed in ascertaining if the ferrous sulphate precipitated the gold from the waste liquid. The color rinsing contains a considerable portion of gold. It has been noted that after coloring a batch of work, the pot was swilled out with water and this added to the rinsing through which the work had passed. The addition of stannous chloride indicated the presence of gold.
If the procedure is not practiced properly, a considerable quantity of gold is left in the rinsing water used in the coloring process. The stannous chloride is a fine test to assess the presence of gold when it is in solution. Oxalic acid is another test for gold, but no fine as the ferrous sulphate. This salt when dissolved in water and some of it added to a solution of gold produced a brown precipitated in the same manner as the ferrous sulphate, but the precipitation does not occur so rapidly. To assure a good precipitation is usually to wait 48 hours. This test when added to a gold solution acts very slowly in a cold solution, but its reaction is quickly in a hot solution. The great advantage is that precipitates gold to a metallic state without any other metal that may be present.