The process of estimating free cyanide depends upon the fact that when a solution of iodine in potassium iodide is added to a solution of a simple cyanide, the reddish-brown color of the iodine solution disappears so long as the cyanide is in excess, since the reaction results in the formation of an iodide of an alkali metal and cyanogen iodide, both of which are colorless:
KCy + I2 = KI + ICy.
The finish of the reaction is sharply marked by the permanence of the yellowish tint in the solution under examination, after agitation. The end-reaction may be made even more distinct by using a drop of the starch indicator prepared as follows:
One part of pure, fresh starch is rubbed into a thin paste with a little cold water, then gradually poured into 150 to 200 times its weight of boiling water, the boiling continued for a few minutes, and the liquid then allowed to stand and settle. Only the clear solution is used for the indicator. The finishing point is now marked by the permanence of an intense blue or bluish-violet tint. The starch solution does not retain its sensitiveness for long, and must generally be freshly prepared. The substances sometimes added to preserve it, e.g., caustic alkalis, zinc chloride and mercuric iodide, would be inadmissible for testing cyanide solutions.
An iodine solution, suitable for cyanide titrations, may be prepared by dissolving 3.899 grams of chemically pure iodine and about G grams of potassium iodide in a small volume of water, and diluting with distilled water to 1,000 c.c. With this solution, 1 c.c. iodine = 0.001 gram KCy.
It is not necessary, in practice, to prepare chemically pure iodine, as the solution may be very conveniently standardized by means of a solution of pure potassium cyanide, the strength of which has been accurately ascertained by silver nitrate method. The value of the iodine solution is thus made to depend on the accuracy of the standard silver solution. For cyanide testing, this method of standardizing is more convenient than the usual method with standard thiosulphate, and probably quite as accurate, if not more so, since nitrate of silver is easily obtained in a condition of great purity.
The method of titration with iodine is described by Fordos and Gelis (Journal de Chim. et de Pharm., 23, 48), and generally ascribed to them, but the reaction on which it depends appears to have been originally mentioned by Serullas and Wohler.
Limitations of the Method of Estimation of Free Cyanide using Solution of Iodine and Potassium is, of course, not applicable where other substances are present which are also capable of reacting on iodine, but in some cases such interfering substances may be removed, as described below.
The iodine method is useful in certain cases where the soluti is free from zinc, but turbid with suspended matter which can conveniently be removed by filtration. With such a solution, ti tion with silver nitrate would be difficult, if not impossible, as end-point could not be accurately observed. When the starch ind cator is used, the end-point with iodine is quite distinct, even turbid solutions.
In presence of the zinc double cyanide the finishing point is un certain and indefinite. A white precipitate occurs gradually at i certain stage of the titration, probably consisting of zinc cyanide as follows:
K2ZnCy4 + 2I2 = ZnCy2 + 2KI -f- 2ICy.
The substances commonly occurring in cyanide solutions whi interfere with the iodine process are caustic alkalis, monocarbona ammonia, sulphides, thiosulphates, and probably most organic reducing agents. Ferrocyanides, fcrricyanides and thiocyanates (sul-phocyanides) do not interfere. According to W. J. Sharwood, small quantities of sulphides do not interfere, since the sulphur liberated combines with an equivalent of cyanide. (See above.)
Estimation of Free Cyanide by Iodine in Presence of Alkalis.
In the presence of alkalis the finishing point with iodine becomes very indefinite, and, moreover, represents more than the amount of free cyanide present. It was originally recommended to add carbonic acid water (ordinary soda water), in order to convert both hydrates and monocarbonates into bicarbonates, for example:
KOH + CO2 = KHCO3.
K2CO3 + CO2 + H2O = 2KHCO3
Since bicarbonates have no action on iodine, the solution may now be titrated by standard iodine, and the amount of cyanide correctly determined.
Researchers recommend adding 50 c.c. of ordinary soda water to the solution to be tested, and titrating at once with decinormal or centinormal iodine.
It is evident, however, that any excess of carbonic acid will decompose cyanide with liberation of hydrocyanic acid, thus:
KCy + CO2 + H2O = KHCO, + HCy, though probably with dilute solutions, if titrated at once, the effect of a small excess of CO2 would not be very noticeable.
However, the following method gives accurate results, and avoids the danger of loss from the use of excess of acid:
To a measured volume of the solution silver nitrate is first ad< until a permanent turbidity is produced, the exact amount ad< being of no consequence. A drop of phenol phthalein is now add to this somewhat turbid solution, and standard acid (for example N/10 H2SO4, HCl or HNO3) is added until the pink color just disappears. The amount required represents, as will be shown later the quantity of acid necessary to convert hydrates into neutral sail and monocarbonates into bicarbonates, without any decomposith of cyanide taking place. If, now, we take a fresh measured volume of the original solution and add to it, with agitation, the quantity of dilute acid shown to be necessary by the previous experiment, we obtain a solution which may at once be titrated with iodine.
The chemistry of cyanide solutions resulting from the treatment of ores by Clennell, J. E. (John Edward) P.29-30