Optimal Parameters of Silver Metal Based Air-Gap Cyanide Sensors

Analytical characteristics of air-gap cyanide sensor incorporating metallic silver as indicator electrode have been investigated as function of chemical composition, pH, buffer capacity, concentration and volume of the absorbent electrolyte film covering the indicator electrode. Simple silver nitrate electrolyte provides results equivalent to those obtained with a dicyanoargentate(l) electrolyte, but its preparation is easier. The pH of absorbing solution should be kept at pKHCN=9.3. Convenient buffer capacity can be assured by a medium concentrated (10-1M) appropriate buffer system (e.g. borax-borate). Optimal electrolyte concentration is about 10"* mol/l, while the volume of electrolyte must be of several microlitres for assuring good reproducibility. For interpretation of the results an equation correlating equilibrium activity of the acidified sample solution with that of the absorbing film has been determined, and the relationship between the e.m.f. and the pH of the film has been derived. Thus, the entire 4-phase system (sample solution, air-gap, absorbing film, potentiometric sensor) can be described as a closed equilibrium system.