proved unavailing in solutions that contain a strong reducing agent the reducing agent leads to the formation of a metal here a silver deposit on the membrane according to the process of the invention the formation of such a metal deposit is prevented by providing a solution sample for use in the determination of the cyanide ion concentration which contains a complexing agent for the metal the complexing agent must be so chosen that it does not interfere with the test reaction for example triethylenetetramine ethylenediaminetetraacetic acid n 39 n 39 n n tetrakis 2 2 hydroxypropyl ethylenediamine citric acid tartaric acid 1 3 propanediamine diethylenetriaminepentaacetic acid rochelle salt mono di and trisodium salts of n hydroxy ethylene diaminetetraacetic acid nitrilotriacetic acid and its salts triethanolamine and cyclohexanediaminetetraacetic acid have proved to be suitable if metal ions which form complexes with the complexing agent are present in the bath solution specified for the concentration measurement care must be taken in accordance with the present invention that the complexing agent is added in an amount that leads to an excess of that required to complex the metal ions if bath solutions are to be measured for example baths for autocatalytic metal deposition on sensitized surfaces already contain a complexing agent for the metal ions along with a metal compound care must be taken in accordance.