Stability constant of Ag CN

The solubility of precipitated silver iodide in solutions of potassium cyanide at 25 °C was determined by Randall and Halford (J. Amer. Chem. Soc. 1930, 52, 187). The solutions were made up to 0.02 M in sodium hydroxide to prevent the formation of hydrogen cyanide. After several days, samples were acidified with nitric add, heated to remove hydrogen cyanide and to predpitate silver iodide which was collected on a filter, dried, and wdghed. Tj-pical results are given in table 1 where a denotes the stoichdometric concentration of potassium cyanide and b denotes that of dissolved silver iodide. 

The (thermodynamic) solubility product of silver iodide is s = 1.0 X 10 mole 1  

We diall first determine the equilibrium constant for the reaction AgI(c) + 2CN- Ag(CN)7 + I- 

Since it may be assumed that effectively all the silver in solution is present as Ag(CN)7 we have 

Since there are two singly charged ions both in the numerator and in the denominator of this equilibrium concentration quotient and thi ionic concentration never exceeds 0.1 mole 1 , we assume that th. value of is equal, within about 10%, to that of the (thermodynamic equilibrium constant for the reaction. Dmding this constant by thf solubility product of silver iodide, we obtain the equilibrium constant K - Q s  

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