This is seldom called for, but it happens occasionally that an appreciable amount of sulphide is developed in the cyanidation of concentrate, and some sulphide is sometimes apparent in the press effluent from aluminium precipitation which it may be desirable to estimate. For such small amounts Clennell recommends the colorimetric method.
For this purpose a weak standard solution of sodium sulphide is prepared, as described below.
Two glass cylinders marked at 100 cc are taken, one of which is filled to the mark with the solution to be tested and a few drops of concentrated solution of sodium plumbite or alkaline lead tartrate added. If sulphides are present a brown coloration will become apparent. To the other cylinder which is filled nearly to the mark with water a similar amount of lead solution is added and the standard Na2S solution run in from a burette until the brown coloration produced matches that of the test. The amount of sodium sulphide in the test cylinder is then calculated from the quantity of Na2S used to match it.
It sometimes happens that the tone of color produced in the cyanide solution does not compare well with that in the water cylinder, and if the solution to be tested contains only traces of silver as would be the case with barren precipitate press effluent some of this solution may be shaken up with a little lead carbonate or litharge and filtered and the filtrate used instead of water for making up the second cylinder.
Standardizing the Sodium Sulphide
This is best done with standard zinc solution as described by Sutton (Vol. Anal.). The sulphide solution may be made by taking a measured amount of solution of caustic soda and saturating half of it with H2S gas and then adding the remainder of the caustic solution, or the fresh c.p. crystals may be used. It is best to make a fairly strong solution for the standardization and afterward dilute down to a suitable strength by adding the correct amount of water, since the method of standardizing is not satisfactory with very dilute solution.
The standard zinc solution is made by dissolving 44 grams of pure zinc sulphate in distilled water and making up to 1 litre. Or 10 grams of pure metallic zinc may be dissolved in hydrochloric acid and made up to 1 litre with distilled water. 1 cc of this solution will then equal 0.01 gram of metallic zinc. The indicator may be made by dissolving a little lead tartrate in a solution of caustic soda.
50 cc of the standard zinc solution is now taken and treated with ammonia to which a little ammonium carbonate solution has been added. A precipitate of zinc hydroxide will first be formed and addition of the ammonia must be continued until this re-dissolves to a clear solution. The sodium sulphide solution to be standardized is then run in from a burette and at intervals a drop of the liquid is withdrawn on a glass rod and placed on a piece of white filter paper: as soon as ever the filter paper has absorbed it a drop of the lead indicator is placed on the filter paper sufficiently near that the edges of the two circles of moisture will overlap one another. If no coloration appears at the point of contact the titration is not complete and addition of the sulphide is continued, a drop being withdrawn at intervals and tested in contact with the lead indicator on the filter paper, until a black or brown line appears at the point of contact of the two circles of moisture, indicating the end of the titration.
1 cc standard zinc solution =
0.01194 gm. Na2S
0. 01687 gm. K2S
When placing the drop of liquor from the test on the filter paper the precipitate of zinc sulphide will be seen to remain in the center portion of the circle while the clear liquor will expand beyond it: if the lead indicator spreads across this clear liquor and touches the inner circle of zinc sulphide it will react to give a black coloration regardless of whether the titration is finished or not so care should be taken to place the indicator sufficiently far away that it may only come in contact with the clear liquor without touching the precipitate.
The first trial will probably be overdone but a careful repetition will usually enable the exact end point to be reached. When the strength of the sodium sulphide solution has been determined a given amount may be measured out and diluted to a suitable strength for the purpose desired. This very weak solution deteriorates rapidly and a fresh batch should be diluted every few days. Even the strong stock solution decomposes steadily and should be frequently re-standardized.