Determination of Thiocyanate by Colorimetric
To a 100- cc Nessler tube add 50 cc water and 5 cc (more if necessary) of the cyanide solution to be tested, then add 2 cc HCl and 10 cc of 5 per cent solution of ferric chloride, FeCl3 Mix, and dilute to the 100-cc mark with water.
If Prussian blue should form on the addition of FeCl3, the solution should be filtered and the precipitate washed with water until all the red color is washed from the paper. Return the filtrate to the tube, and dilute to the 100-cc mark.
To a second Nessler tube add 50 cc water, 2 cc HCl and 10 cc 5 per cent FeCl3 solution. Then dilute with water to almost the 100-cc mark. From a burette run in 0.01 N potassium thiocyanate, KCNS, solution a few drops at a time until the color in tube 2 matches that in tube 1. When matched, read the burette, and calculate the amount of KCNS added to effect the match.
0.01N KCNS SOLUTION. Dissolve 9.72 grams chemically, pure KCNS in water, and dilute to 1000 cc. This is 0.10N solution. Mix well, then take 100 cc of this solution, and dilute to 1000 cc. This is .010N solution.
1 cc = 0.000972 grams KCNS
The burette reading x 0.000972 = weight in grams of KCNS in the original sample.
Five Per Cent FeCl3 Solution. To about 100 cc water add 50 grams FeCl3 and 25 cc HCl. Warm to dissolve the FeCl3. Cool, and dilute to 1000 cc.
This method is sufficiently accurate for most purposes and may be done in a few minutes.
Determination of Thiocyanate by Permanganate Method
To 100 cc of cyanide solution add 10 cc H2SO4 and 10 cc 20 per cent solution of ferric sulphate, Fe2(SO4)3. Stir well, and filter into a 100-cc graduate cylinder until the 60-cc mark is reached. The filtrate represents 50 cc of the original solution. Wash into a 250-cc beaker, dilute to about 100 cc, and titrate with 0.10N potassium permanganate, KMnO4, solution until the red color is dispelled.
1 cc 0.10N KMnO4 = 0.00162 gram potassium thiocyanate, KCNS On 60 cc aliquot — 0.0324 gram per 1000 cc or 0.0648 lb. per ton of solution.
0.10N KMnO4 Solution. (See under “Determination of Reducing Power.”) Silver interferes, but it is unlikely to be found in sufficient quantities in the cyanidation of gold ores to cause an appreciable error.
Determination of Reducing Power
To 5 to 25 cc of solution, depending upon the amount of reducing agents present, add sufficient water to bring the volume to 200 cc. Then add 25 cc of 25 per cent H2SO4 solution and titrate with 0.10N potassium permanganate (KMnO4) solution to the first faint pink coloration, which remains permanent for 2 min.
The result is reported in cubic centimeters of 0.10N KMnO4 solution per 1000 cc of cyanide solution.
The amount of cyanide solution taken for this determination should be so adjusted as to require 5 to 10 cc 0.10N KMnO4 solution. Larger titrations generally result in fading end points.
0. 10N KMnO4 Solution. Dissolve 3.16 grams of potassium permanganate in water, and dilute to 1000 cc. This solution should be kept in a dark bottle.