Theory of  Operation Example Cyanide Electrode The cyanide electrode consists of a solid sensing clement containing a mixture of inorganic silver con bonded into the tip of an epoxy electrode body. When the sensing element is in contact with a cyanide solution, silver ions dissolve from the membrane surface. Silver ions within the sensing element move to the surface to replace the dissolved ions, setting up a potential difference that depends on the cyanide level in the solution.. This potential is measured against a constant reference potential with a my meter or ISE meter. The measured potential corresponding to the level of cyanide in solution, is described by the Nernst equation: E = E0 + S log (A) where: E = measured electrode potential E0 = reference potential (a constant) A = level of cyanide ion in solution S = electrode slope (about -57 mV) per decade for cyanide) S = 2.3RT/nF where:   R & F are constants T =  temperature degrees K n = ionic charge The ionic level, A, is the activity or “effective concentration The cyanide ion activity is related to free- ion concentration. Cf by the activity coefficient, y: A = y*Cf Ionic activity coefficients are variable a largely depend on total ionic strength. The ionic strength of a solution is determined by all of the ions present. It is calculated by multiplying the concentration of each individual ion by the square of its charge. adding all these values up, and then dividing by two Mathematically, ionic strength is defined as: 1/2 ∑(CiZiSquared) where: Ci = concentration of ion i Zi = charge of ion i ∑ = symbolizes the sum of all the types of ions in solution. If the background ionic strength is high and constant relative to the ion concentration, the activity coefficient is constant and activity is directly proportional to concentration. Ionic strength adjustor (ISA) is added to all standards and samples so that the background ionic strength is high and constant relative to variable concentrations of cyanide. For cyanide the recommended ISA is 10 M NaOH. Other solutions can be used as long as they do not contain ions that would interfere with the electrode’s response to cyanide. If samples have a high ionic strength (above 0.1 M), standards should be prepared with a composition similar to the samples. Back to Services