A simple complexation titration

A simple example of the application of a complexation reaction to a titration procedure is the titration of cyanides with silver nitrate solution. When a solution of silver nitrate is added to a solution containing cyanide ions (e.g. an alkali cyanide) a white precipitate is formed when the two liquids first come into contact with one another, but on stirring it re-dissolves owing to the formation of a stable complex cyanide, the alkali salt of which is soluble  

When the above reaction is complete, further addition of silver nitrate solution yields the insoluble silver cyanoargentate (sometimes termed insoluble silver cyanide) the end point of the reaction is therefore indicated by the formation of a permanent precipitate or turbidity. 

The only difficulty in obtaining a sharp end point lies in the fact that silver cyanide, precipitated by local excess concentration of silver ion somewhat prior to the equivalence point, is very slow to re-dissolve and the titration is time-consuming. In the Deniges modification, iodide ion (usually as KI, ca 0.01 M) is used as the indicator and aqueous ammonia (ca 0.2M) is introduced to dissolve the silver cyanide. 

The iodide ion and ammonia solution are added before the titration is commenced the formation of silver iodide (as a turbidity) will indicate the 

During the titration any silver iodide which would tend to form will be kept in solution by the excess of cyanide ion always present until the equivalence point is reached  

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