Thermo Fisher Scientific Ion Selective Electrodes Cyanide - Manual (Page 13)

Instruction Manual Cyanide Electrode
13
Electrode Operation
Eutech Cyanide Ion Electrodes consist of a solid membrane containing a mixture of inorganic silver
compounds bonded into the tip of a glass or epoxy electrode body. An electrode potential develops
across the membrane when the electrode is in contact with solution containing cyanide ions and is
capable of measuring free cyanide ions. This potential is measured against a constant reference
potential, using a standard pH/mV meter or an ion meter. The Nernst equation describes the level of
cyanide ions in solution corresponding to the measured potential:
E = Eo - S log X
where:
E = measured electrode potential
Eo = reference potential (a constant)
S = electrode slope (
∼57 mV/decade)
X = level of cyanide ions in solution
The activity, X, represents the effective concentration of free cyanide ions in the solution. Total
cyanide concentration, Ct, may include some bound as well as free cyanide ions. Since the electrode
only responds to free ions, the concentration of the free ions, Cf, is found by:
Cf = Ct - Cb
where Cb represents the concentration of all bound or complexed cyanide ions.
The activity is related to the free ion concentration, Cf, by the activity coefficient, γ , by:
X =
γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½
Σ CxZx 2
where:
Cx = concentration of ion X
Zx = charge of ion X
Σ = sum of all of the types of ions in the solution
In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient,
γ , is constant and the activity, X, is directly proportional to the concentration.
All samples and standards containing cyanide ions have ionic strength adjuster (ISA) added so that
the background ionic strength is high and constant relative to variable concentrations of cyanide.
The recommended ISA for the cyanide electrode is sodium hydroxide, NaOH, though other basic
solutions can be used as long as they do not contain ions that would interfere with the electrode's
response to cyanide.
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally
across the solution boundary resulting in a potential difference between the two solutions, since