Apparatus, Reagents, etc.—The same as in the last analysis, except that the chromate is not required. For analysis the student may take a solution prepared by the demonstrator, or a sample of ‘ gold ’ cyanide (KCN), which, if the potassium salt, will contain from 95% to 98% KCN. This salt now frequently contains NaCN, so that if estimating in percentages of KCN the results in these terms will run above 100%.
Method, Reactions. —If to a solution of KCN a solution of AgNO3 be added the following reaction takes place,
KCN + AgNO3 = AgCN + KNO3
but this AgCN is soluble in KCN, forming the soluble double cyanide KCN, AgCN, and as long as there remains in the solution some KCN not converted into this double salt no permanent precipitate forms, but immediately the last trace of KCN is absorbed to form the double salt then the slightest excess of AgNO3 will produce a permanent precipitate.
2KCN + AgNO3 = KCN, AgCN + KNO3
It is evident, then, that if the strength of the AgNO3 solution is known the quantity of CN present may readily be estimated, and, as is frequently required, calculated to KCN. In technical work, where the presence of ferrocyanides and other salts may obscure the end point, the addition of a few c.cs. N. NaHO and a few drops of E. KI is advisable, and even then, unless precautions be taken (see Sutton, pp. 218-226), the end point may still be indistinct, but with the materials mentioned for practice the student should have no difficulty, especially if he use an alkali and KI, in obtaining a distinct end point marked by a permanent opalescence.
The Standard Solution. —The balance of the N/10 AgNO3, used in the last estimation will serve again here.
1 c.c. N/10 AgNO3 = .0052 gm. CN= .013 gm. KCN
as by the second equation one molecule of AgNO3 is required for every two molecules of KCN to form the soluble double cyanide KCN,AgCN. In routine work ‘ empirical ’ solutions are generally used. For example, in the United States 6.535 gms. AgNO3 to the litre forms the standard solution. Then 10 c.cs. KCN solution are taken for the estimation, and each c.c. of AgNO3 solution used represents 1 lb. of KCN in the ton (2000 lbs.) of solution.
Or again, as is the practice in the British Colonies, the standard solution contains 17 gms. AgNO3 per litre. Then 13 c.cs. of the cyanide solution to be tested are taken, and the number of c.cs. of AgNO3 used divided by ten gives the percentage of KCN in the solution.
The strength of the solution has already been checked by precipitation.
The Analysis.—Weigh out quickly about 10 gms. of the roughly powdered cyanide and transfer to the 1000 c.c. test-mixer. Add about 250 c.c. distilled water. Stopper and shake. When dissolved make up to 1000 c.cs. With the pipette transfer 50 c.cs. into a 200 c.c. Erlenmeyer flask. Add a few drops E. KI, and titrate with the N/10 AgNO3, running in a little, and then rapidly swirling the contents of the flask. After a number of additions and agitation by swirling, the precipitate formed hangs much longer before dissolving. The solution is then run in drop by drop with agitation till a faint yet permanent opaline or milky tint is formed. This is best judged by placing a small square of black paper under the flask.
Note the number of c.cs. used, and repeat the estimation on another 50 c.cs. of the solution.
Calculation of Results.—Assume that 38.2 c.cs. AgNO3 are required for 50 c.cs. KCN solution, 38.2 c.cs. N/10 AgNO3 = .013 x 38.2 = .4966 gms. KCN. But 50 o.cs. contain .5 gm. of the salt, therefore 100 c.cs. will contain 1 gm. of the salt, of which .4966 x 2 = .9932 gms. are potassium cyanide, or in other words the salt contains 99.32% KCN (assuming no NaCN to be present).
Note on Testing Solutions containing KCN.—The student may test a solution if dilute (say below 5%) by measuring out 50 c.cs. and titrating as before. If a strong solution is to be tested it should be diluted to a given volume, and 50 c.cs. of the diluted solution taken and the results multiplied according to the degree of dilution. The methods used in technical work have been indicated above, and the student may adopt them when he returns to this work in Laboratory Ore Tests, Part III.