Thermo Fisher Scientific Ion Selective Electrodes Sodium - Manual (Page 12)

Sodium Electrode Instruction Manual
10
The edge of the shaded area (the straight line) shown in Figure 3 indicates a minimum pH at which
dilute sodium measurements can be made with less than 10% hydrogen ion interference. The pH
should be adjusted to a pH greater than 9 by the addition of ISA to all standards and samples for
optimal results over the entire concentration range of sodium. Additional ammonium hydroxide
may be necessary to adjust the pH to the desired level in some cases.
Electrode Life
A sodium ion electrode will last six months in normal laboratory use. On-line measurements might
shorten operational lifetime to several months. In time, the response time will increase and the
calibration slope will decrease to the point calibration is difficult and electrode replacement is
required.
Electrode Storage
The sodium ion electrode should be stored in the sodium electrode storage solution, never in air or
in distilled water. A more dilute sodium chloride solution (with pH adjusted through the use of ISA)
may be used for storage before low level measurements. For longer storage (longer than two
weeks), rinse and dry the sensing glass and cover the glass tip with any protective cap shipped with
the electrode. The reference portion of the combination electrode (or the outer chamber of the
reference electrode) should be drained of filling solution, if refillable, and the rubber insert placed
over the filling hole.
ELECTRODE THEORY
Electrode Operation
The sodium ion electrode is composed of a sodium-selective glass membrane bonded to a glass
body. When the membrane is in contact with a solution containing sodium ions, an electrode
potential develops across the membrane. This electrode potential is measured against a constant
reference potential, using a pH/mV meter or an ion meter. The level of sodium ions, corresponding
to the measured potential, is described by the Nernst equation.
E = Eo + S log X
where:
E = measured electrode potential
Eo = reference potential (a constant)
S = electrode slope (~56 mV/decade)
X = level of sodium ions in solution
The activity, X, represents the effective concentration of free sodium ions in the solution. The
activity is related to the free ion concentration, Cf, by the activity coefficient, γ , by:
X =
γ Cf
Activity coefficients may vary, depending on the total ionic strength, I, determined as:
I
= 1/2
Σ CxZx 2
where: Cx = concentration of ion X
Zx = charge of ion
Σ = sum of all of the types of ions in the solution