Free Cyanide Determination

Free Cyanide Determination

You will perform this titration to obtain your Free Cyanide Determination.

  1. Take a 10ml aliquot of pregnant liquor.
  2. Make aliquot to 60ml with distilled water.
  3. Add 3 ml KI (10% solution)
  4. Add 5 ml of 1.5% NH4OH
  5. Titrate with 0.10 N Ag NO3 then; titre/100 = %CN


The endpoint is indicated by the first sign of a change from clear to slightly turbid conditions. A black background helps to perceive this transformation, and a comparison with a sample at equivalence point is also useful. If solution is already slightly turbid, a greed tinge will be noted at equivalence point.

Reagent Preparation:

Distilled water used at all times

Ag NO3        –                 originally N/50
–    dilute 1:1 to achieve N/100

10% KI                –    10 grams KI per 100 ml H20

1.5% NH4OH        –    1.5 mls Ammonia per 100 ml         H20

Ag NO3 should be kept in a dark colored bottle, away from light.

Standard silver nitrate solution is made by dissolving 4.33 grams silver nitrate, AgNO3, in distilled water and making up to 1 liter. The reaction between silver nitrate and sodium cyanide is represented by the following equation:

AgNO3 + 2NaCN = AgNa(CN)2 + NaNO3
Thus, 169.9 grams AgNO3 saturates 98 grams NaCN, or 4.33 grams AgNO3 saturates 2.5 grams NaCN.

For a detailed treatment of this subject the reader is referred to J. E. Clennell, The Chemistry of Cyanide Solutions, McGraw-Hill, 1910.
It should be pointed out that the presence of certain dissolved impurities in cyanide solution, particularly soluble sulphides, thiosulphates, and compounds of copper and zinc tends to make this free-cyanide determination unreliable unless special precautions are taken. The interference due to soluble sulphides may be overcome by adding 0.2 to 0.5 grams of litharge or lead carbonate to a 25 ml. solution sample, shaking for a few minutes, and then filtering before titration. In the presence of the cyanide complexes of copper and zinc, varying amounts of the combined cyanide report as free cyanide, depending upon whether or not KI is used and the degree of alkalinity of the solution. For copper some authorities recommend using up to 1 gram of KI for each titration, whereas in the presence of zinc, titration to an opalescent end point without KI is the more reliable. Where copper and zinc are both present Hamilton advises that titrations be made both with and without KI and the lower of the two results taken for solution control.

One cubic centimeter of the foregoing solution (= 0.00433 gram AgNO3) saturates 0.0025 gram NaCN. Therefore, if 25 cc mill solution be taken for titration, each cubic centimeter of silver nitrate solution required will equal 0.0025 gram NaCN or 0.01 per cent NaCN. For example, a 25-cc sample of cyanide solution is titrated, and it is found that 4.8 cc silver nitrate solution is used; the strength of the cyanide solution then is 0.048 per cent NaCN. Sometimes it is more convenient to use a 10-cc sample for titration when strong solutions are used. In such cases the same silver nitrate solution is used, and the burette reading is multiplied by 2.5.

The solution to be tested should be clear (filter if necessary). Twenty five cubic centimeters is put into a clean, transparent, 125-cc Erlenmeyer flask. The solution should not be diluted. The silver nitrate solution is added until the end of the reaction is indicated by the first appearance of a bluish haze, dulling the original brilliancy of the solution. This point is best determined against a black background, and the operator should experiment with various conditions of light until he is able to obtain consistent readings and check himself and others within at least 0.1 cc on the burette. A good setup is to place the burette against a window with a good light (but not in the direct rays of the sun) so that the flask will be about level with the eye against a black background.

The addition of a few drops of a 5 per cent neutral solution of KI imparts a yellowish tinge to the precipitate, which makes the exact finishing point more distinct. The effect is due to the precipitation of silver iodide in preference to silver cyanide when the solution contains no more free cyanide.