Titration of cyanides with silver nitrate is based on the complexation reaction:
Ag+ + CN- = [Ag(CN)2]-
The precipitation of silver cyanoargentate Ag[Ag(CN)2]at the end point can be avoided by the addition of ammonia solution, in which it is readily soluble, and if a little potassium iodide solution is added before the titration is commenced, sparingly soluble silver iodide, which is insoluble in ammonia solution, will be precipitated at the end point. Viewing against a black background best sees the precipitation.
Weigh out accurately about 3.5 g of potassium cyanide from a glass-stoppered weighing bottle, dissolve it in water and make up to 250 mL in a graduated flask. Shake well. Transfer 25.0 mL of this solution to a 250 mL conical flask, add 75 mL water, 5-6 mL 6M ammonia solution, and 2mL 10 per cent potassium iodide solution. Place the flask on a sheet of black paper, and titrate with standard 0.1 M silver nitrate. Add the silver nitrate solution dropwise as soon as the yellow colour of silver iodide shows any signs of persisting. When one drop produces a permanent turbidity, the end-point has been reached.