Apparatus, Reagents.—Apparatus as before. For the standard solution, pure HgCl2 is required. For analysis the student may take further portions of the materials used in the last estimation.

Method, Reactions.—If to an ammoniaeal solution of KCN a solution of HgCl2, be slowly added, Hg(CN)2, is formed and is soluble in water. When this reaction is complete any further addition of HgCl2 results in the formation of a white precipitate,

HgCl2 + 2KCN = Hg(CN)2 + 2KCl
HgCl2 + 2NH4HO = NH2HgCl + NH4Cl + 2H2O

Knowing the strength and volume of the HgCl2 solution used, the quantity of CN or KCN may be calculated.

Standard Solution.—An HgCl2 will contain = 27.054 gms. of the salt per litre, and according to the equation 1 c.c. is equivalent to .0052 gm. CN or .013 gm. KCN.

It will be sufficient if the student prepare 250 c.cs. of this solution. Therefore dissolve 6.7635 gms. of the salt in distilled water and make up to 250 c.cs.

The student need not at present check the accuracy of this standard, but may compare the results obtained with those from the Silver Nitrate method.

The Analysis.—Proceed exactly as in the Silver Nitrate method, weighing or measuring the same quantities of material operated on, and titrating after adding excess of NH4HO(no KI) till a faint but permanent opalescence is obtained.

Run duplicates, calculate the percentage of KCN, and compare the results with those previously obtained. With pure salts this method is very accurate, but with impure salts it is not so reliable as the previous method.

Note.—When instructed to estimate the percentage of KCN present, it is assumed that all the CN radical is united to K. This may or may not be so, but in Cyanide work it has become the custom to speak of the strength of a solution in percentage of KCN.