Thermo Fisher Scientific Ion Selective Electrodes Silver - Manual (Page 21)

Instruction Manual Silver/Silver Sulfide Electrode
21
ELECTRODE THEORY
Electrode Operation
The silver/sulfide electrode is composed of a silver sulfide crystal membrane bonded into an epoxy
or glass body. When an electrode potential develops across the membrane, the membrane is in
contact with a solution containing sulfide or silver ions and is capable of measuring free sulfide or
silver ions. This electrode potential is measured against a constant reference potential, using a
pH/mV meter or an ion meter. The level of sulfide or silver ions, corresponding to the measured
potential, is described by the Nernstian equation:
E = Eo + S log X
where
E = measured electrode potential
Eo = reference potential (a constant)
S = electrode slope (-26 mV for sulfide; +57 mV for silver)
X = level of sulfide or silver in solution
The activity, X, represents the effective concentration of the ions in solution. 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 Cx Zx 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.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster (ISA)
is added to samples and standards. The recommended solution for sulfide is SAOB, used to prevent
oxidation and free sulfide ion from hydrogen ion, in addition to adjusting the ionic strength. The
recommended ISA for silver is NaNO
3
. Solutions other than these may be used as ionic strength
adjusters as long as ions that they contain do not interfere with the electrode's response to sulfide
ions or to silver ions. Samples with high ionic strength (greater than 0.1M) do not need ISA added
and standards for these solutions should be prepared with a composition similar to the samples.
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
ions diffuse at different rates. When making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing solution as well as in the sample
solution or the change in liquid junction potential will appear as an error in the measured electrode
potential.