Here is a Qualitative Test Method to look for Cyanide Traces. To 500 to 1000 cc of the solution to be tested add 1 to 2 cc ammonium sulphide, (NH4)2S, and evaporate just to dryness. The final stages of evaporation should be done slowly. Cool, add 10 cc water, stir well, let settle, and filter. To the filtrate add 2 drops of saturated ferric chloride, FeCl3, solution. A red coloration indicates the presence of cyanide.

How to Detect Traces of Cyanide in Water

A simple test to determine if there is cyanide present in water is:

Evaporate 500 cc of the solution with 3 or 4 drops of ammonium sulphide. Bring to dryness on the water bath and take up with a small quantity of water or water and alcohol. Filter, and add a drop of ferric chloride solution. A red coloration indicates the presence of cyanide. By careful evaporation 1 part in 100,000 can be detected. The author states that by evaporating solutions containing known quantities of cyanide in this manner he has found it possible to obtain a rough colorimetric estimation of the amount of cyanide present.