The assay without iodide.—The standard solution of silver nitrate is placed in a small burette divided into tenths of a c.c. Ten c.c. of the cyanide solution to be assayed is transferred to a small flask and diluted with water to about 70 c.c. The silver solution is then run in from the burette (with constant shaking of the flask), a little at a time but somewhat rapidly, until a permanent turbidity appears. Since 1 c.c. of the silver nitrate solution corresponds to .01 gram of potassium cyanide, it also corresponds to .1 per cent. of this salt counted on the 10 c.c. of cyanide solution taken. The titration should be performed in a fairly good uniform light. The learner should practice on a fairly pure solution of potassium cyanide at first, and this may conveniently have a strength of about 1 per cent. For practice with solutions containing zinc make a solution containing 1.1 gram of crystallised zinc sulphate in 100 c.c. and slowly add measured quantities of from 1 to 5 c.c. of this to the 10 c.c. of cyanide liquor before diluting for the titration.
If a cyanide solution blackens on the addition of the silver nitrate it contains sulphide. In this case, shake up a considerable bulk of the liquor with a few grams of lead carbonate, allow to settle and make the assay on 10 c.c. of the clear liquor.
If the cyanide liquor be suspected to contain free prussic acid, take 10 c.c. for the assay as usual; but, before titrating, add .1 or .2 gram of sodium carbonate. On no condition must caustic soda or ammonia be added. The difference between the results, with and without the addition of carbonate of soda, is supposed to measure the quantity of free prussic acid. If this has to be reported it is best done as "prussic acid equivalent to ... per cent. of potassium cyanide." Suppose, for example, the difference in the two titrations equals 1 c.c. of standard silver nitrate; the prussic acid found would be equivalent to .1 per cent. of potassium cyanide.
The assay with iodide.—The standard solution of silver nitrate is placed in a burette divided into tenths of a c.c. Take 10 c.c. of the cyanide liquor, which should previously have been treated with white lead for the removal of sulphides if these happened to be present. Transfer to a small flask, add 3 or 4 drops of a solution of potassium iodide and 2 or 3 c.c. of a solution of sodium hydrate; dilute to 60 or 70 c.c. with water. If much zinc is present the soda may be increased to 20 or 30 c.c. with advantage. The standard solution should be run in somewhat rapidly, but a little at a time, so that the precipitate at first formed shall be small and have only a momentary existence. The titration is continued until there is a permanent yellowish turbidity. The most satisfactory and exact finish is got by ignoring any faint suspicion of a turbidity and accepting the unmistakable turbidity which the next drop of silver nitrate is sure to produce. This finishing point gives results which are exactly proportional to the quantity of cyanide present; and it can be recognised with more than ordinary precision even in solutions which are not otherwise perfectly clear.
Each c.c. of the standard silver nitrate solution corresponds to .01 gram of potassium cyanide; and if 10 c.c. of the liquor are taken for assay this corresponds to .1 per cent. or 2 lbs. to the short ton or 2.24 lbs. to the long ton. As already explained the result should be reported as "cyanide equivalent to so much per cent. of potassium cyanide."
The following experimental results were obtained with a solution of potassium cyanide made up to contain about 1.2 per cent. of the salt.
Effect of varying cyanide.—The bulk before titration was in each case 60 c.c.; 2 c.c. of soda and 3 drops of potassium iodide were used in each case.
| Cyanide added | 40 c.c. | 30 c.c. | 20 c.c. | 10 c.c. | 5 c.c. | 1 c.c. |
| Silver required | 47.0 c.c. | 35.25 c.c. | 23.5 c.c. | 11.7 c.c. | 5.8 c.c. | 1.15 c.c. |
Accepting the result for 40 c.c. as correct, the others are in very satisfactory agreement.
Effect of varying dilution.—The conditions were those of the 40 c.c. experiment in the last series; but varying amounts of water were used in diluting.