Free Cyanide vs Total Cyanide Determination

It is obviously of the highest importance in controlling the action of a solution to find out what strength of cyanide and alkali it contains at any given time. Under this head are the tests for free cyanide, “total” cyanide, hydrocyanic acid, and free or “protective” alkali.

Besides these cyanide constituents, which have to be determined frequently each day, it is useful at times to ascertain the presence, (and if present the amount), of the following substances, zinc, copper, ferrocyanide, and sulphocyanate.

It is customary to express cyanide strength in terms of potassium cyanide (KCN) of 100% strength, even though sodium cyanide (NaCN), carrying a higher percentage of cyanogen (CN) per unit of weight, may be actually in use.

This seems somewhat illogical and the only thing to be said in its defence is that the theory and practice of cyanidation having been born and having grown up under the formula KCN a change of notation would mean readjustment of all the old standards and, for a time at least, a mental translation of the new notation to the old before it would convey any meaning. In view of the varying cyanogen content per unit of weight in the commercial cyanide now put on the market the obvious standard to use would be the percentage of CN but if in the future the pure sodium cyanide (98 to 99% NaCN) is to be universally supplied it would probably be more convenient to think and speak in terms of that salt and the titration of the solution could be reported in pounds NaCN per ton of solution. If a change is to be made, however, it would seem that it would only come about by concerted action between the various technical societies throughout the world.

Cyanide Titration with Silver Nitrate

The tests for free cyanide, total cyanide and hydrocyanic acid are made volumetrically, by slowly running silver nitrate solution from a burette into a known quantity of the cyanide solution. For this purpose a standard solution of silver nitrate is usually made up, of such a strength that the result may be read off without calculation.

Such a standard solution of silver nitrate may be made by dissolving 3.262 grams of silver nitrate (AgNO3) in distilled water and making up to 1 litre and this is the standard adopted and referred to in the following pages. (Should the least milkiness develop while dissolving it is an indication that the water is not sufficiently pure). The reaction taking place between the silver nitrate and potassium cyanide may be represented by the following equation:

AgNO3 + 2KCN = AgK (CN)2 + KNO3
Thus 169.9 gm. AgNO3 saturates 130.2 gram KCN or 3.262 gm. AgNO3 saturates 2.5 gm. KCN

Therefore 1 cc of above solution (=0.003262 gm. AgNO3) saturates 0.0025 gm. KCN. If 25 cc of mill solution is taken for titration, then 1 cc of nitrate solution = 0.0025 gm. KCN = 0.01% or 0.1 kilo KCN per metric ton or 0.2 lb. KCN per 2000 lb. ton of solution. Similarly, a solution containing 6.525 grams of silver nitrate per litre may be made up, 1 cc of which contains 0.006525 grams of AgNO3 equal to 0.005 gm. of KCN. Then if 10 cc of mill solution be taken for titration, 1 cc of nitrate solution = 0.005 gm. of KCN = 0.05% = 0.5 kilo per metric ton or 1 pound per 2000 lb. ton of solution; while if 50 cc of mill solution be taken, 1 cc of nitrate solution = 0.01% = 0.1 kilo per metric ton or 0.2 lb. per 2000 lb. ton of solution. Probably the most common strength for the standard silver nitrate solution is 13.05 grams per litre, and is such that if 10 cc of mill solution be taken for titration 1 cc of nitrate solution equals 0.1% KCN or 2 lb. per ton. This strength, however, is not recommended for reasons which will be stated later.

Free Cyanide Determination

Two methods are in use for this purpose, and there is a third that may be used on occasion, but it should be understood that there is no direct method yet devised which will give better than comparatively correct results.

If the determination, however, can be relied on to be relatively uniform and consistent, that is all that is required for practical working.

Determination of Free Cyanide Method No. 1

Take 25 cc of the solution to be tested and place in an Erlenmeyer flask, add a few drops of 5% solution of potassium iodide free from alkali, and slowly rim in the silver nitrate from the burette, shaking the flask meanwhile, until a distinct yellow opalescence appears. Then read off the number of cc of silver nitrate used and multiply by 0.01, and the result will be the amount of free cyanide in the solution in terms of percentage. To get the cyanide in kilos per metric ton of solution multiply the number of cc of silver nitrate by 0.1 or if the cyanide in terms of lb. per 2000-lb. ton of solution is desired multiply the number of cc of silver nitrate by 0.2. If the cyanide strength is over 0.2% KCN it is better to take only 10 cc of the cyanide solution for the test. In this case, when using as the standard solution 3.262 grams AgNO3 per litre, after the burette reading, has been noted move the decimal point one place to the left and divide by 4. For instance if B = burette reading in cc then B x 0.1/4 = % KCN or B/4 = kilos per metric ton or B x 2/4 = B/2 = lb. KCN per 2000-lb. ton of solution.

Method No. 1 is not to be recommended when the solution contains zinc (as it usually does), because the results are not uniform. When there is no free alkali in the cyanide solution the value obtained for free cyanide may approximate the correct one, but if any free alkali is present (and it almost always is, in a working solution), the cyanide reading will be in excess of the correct figure, and this excess error will be greater in proportion as the amount of free alkali is greater, until, if the latter happens to be about 0.08% (in terms of NaOH) or over, the reading supposed to represent free cyanide will coincide exactly with the amount of “total” cyanide, as shown in the method to be described later.

Free Cyanide Determination Method No. 2

In place of the foregoing the following is recommended, as giving the most correct and consistent reading for free cyanide under ordinary conditions. It needs a certain amount of practice to stop at the exact end-point of the reaction, but precision is easily and rapidly acquired.

Take 25 cc of the solution to be tested. (If not absolutely clear and colorless to transmitted light, filter and re-filter until it is.) Pour into a bulb parting-flask with the neck cut short, Fig. 1, or into a small conical parting-flask, Fig. 2, first having seen that the flask is as clean and transparent as the solution, for which purpose wash with some weak hydrochloric acid solution and rinse well. Do not add water to the solution to be tested, because it will tend to dissociate some of the zinc-potassium cyanide and make the free cyanide reading too high. Place the burette opposite a window with a good light (but not in the direct rays of the sun), and fix a black card vertically about 4 inches behind the spot where the flask is to be held, so that the light will strike the surface of the solution over the top of the card leaving the solution itself in shadow. Then, keeping the eye on a level with the flask, watch the behavior of every addition of silver nitrate as it touches the solution, not keeping it in continuous agitation but allowing the bluish- white cloud formed by each addition of silver to hang for a second or two before shaking and dissolving. The finish is indicated by the first appearance of a bluish haze dulling the original brilliancy of the solution. With a very little practice the operator will find he can check himself and others within a tenth of a cc on the burette. To ensure the success and delicacy of this reaction it is important that the silver nitrate solution should be dilute, and for this reason the standard already recommended (3.262 grm. AgNO3 per litre) is to be preferred to the usual standard of 13.05 grm. per litre. With a strong silver nitrate solution the tendency is to precipitate flocks instead of the dispersed opalescence necessary for a clearly defined end point.

Number of cc of standard AgN03 X 0.01 = % free KCN or X 0.1 = kilos KCN per metric ton of solution or X 0.2 = pounds KCN per ton (2000 lb.) of solution.

(NOTE).—It sometimes happens (that is, in the case of certain solutions) that the addition of a few drops of potassium iodide solution before titration will render this first haziness more easily distinguished and may give a slightly lower reading than without. In such cases, however, there is no harm in making use of it so long as it is clearly understood that it is the first faint blue-white opalescence that is to be looked for and not the yellow tint.

Walter Virgoe has pointed out that cyanide solutions containing copper, when titrated with silver nitrate without potassium iodide indicator, yield a result higher than the true value for free cyanide, owing to part of the copper-potassium cyanide being recorded as free cyanide. Thus, while in presence of zinc and free alkali the addition of neutral potassium iodide as indicator gives a higher and less correct reading than is recorded without the indicator, when copper is present without zinc the opposite is the case and the addition of neutral iodide indicator gives a lower reading than without, this lower reading being the correct one. So, when zinc is absent and copper present, method No. 1 should be used.

Analytical Determination Method of Free Cyanide No. 3

When copper and zinc are present together both tendencies are displayed in proportion to the relative amounts of each metal, and the result is thereby complicated. In this case, it is advisable to make titrations both with and without KI indicator and to base solution control on the lower of the two readings; or a method devised by Clennell for determining free cyanide may be tried, though the writer has not always found it to give concordant results.

First determine protective alkali, by Green’s method, carefully running in the acid drop by drop and recording the number of drops needed. (For this purpose and for the subsequent neutralization a mineral acid should be used, say decinormal nitric acid, because oxalic would precipitate the lime in the cyanide solution as oxalate, which would mask the end-point in the test for cyanide.) Then measure out another 25 cc of solution to be tested and run into it the same number of drops of decinormal acid as was previously recorded. The whole of the protective alkali in the cyanide solution is thus exactly neutralized so that it does not militate against the use of the iodide indicator. To the test solution so neutralized add at least 2 cc of neutral 5% solution of potassium iodide, and titrate with standard silver nitrate to first white flocculent precipitate. Calculate free cyanide as already explained.

The writer has found this white end point rather indeterminate, but the yellow end point is well marked and the readings uniform though the latter gives a higher and probably less correct result than the white end point.

In the use of this method the quantity of KI added has an important bearing on the result of the titration. The writer has found that smaller quantities than that stated give proportionally higher cyanide readings, though the amount of iodide may be materially increased without much apparent effect on the result.

Total Cyanide Determination

The term “total” cyanide is generally limited to mean zinc-potassium cyanide + free potassium cyanide + hydrocyanic acid. To determine total cyanide, make up an indicator consisting of 4 grams caustic soda and 1 gram potassium iodide in 100 cc of distilled water. Take 25 cc of the solution to be tested, add 3 to 5 cc of the indicator, and titrate with standard silver nitrate until a distinct lemon yellow opalescence appears.

Number of cc AgNO3 used X 0.01 = % “ total ” cyanide. Keep stock bottle of indicator well stoppered to avoid absorption of carbonic acid, which will tend to make the test cloudy with carbonate of lime. Should a whitish precipitate of zinc dense enough to mask the color come down before the yellow tint appears Clennell recommends the addition of a little ammonia before beginning the titration, but this is usually unnecessary.

Hydrocyanic Acid Determination

This of course need only be looked for in solutions containing no protective alkalinity. First determine free cyanide. Then take another portion, add a little solution of potassium or sodium bicarbonate free from monocarbonate, and titrate again for free cyanide. (Free carbonic acid must be absent.) The difference in the reading between the two titrations gives the equivalent in KCN of the hydrocyanic acid present (Bettel).

Estimation of Other cyanide Compounds

In trying to account for all the sources of cyanide loss during treatment of an ore it is sometimes useful to test the solution for cyanates, ammonia and ammonia compounds, formates and acetates.