Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 17)

Instruction Manual Fluoride Electrode
- 14 -
Electrode Storage
The fluoride electrode may be stored for short periods of time in 1.0X10
-2
M fluoride solution with
TISAB added. For longer storage (longer than two weeks), rinse and dry the sensing pellet and
cover the membrane 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. The fluoride electrode
should never be stored in distilled water.
ELECTRODE THEORY
Electrode Operation
The fluoride electrode consists of a single crystal of lanthanum fluoride as the membrane, bonded
into a glass or an epoxy body. Only fluoride ions are mobile in the ionic conductor crystal. When
the membrane comes in contact with a solution containing fluoride ions, a potential develops across
the membrane. This potential is measured against an external (or internal) constant reference
potential with a standard pH/mV meter or an ion meter and depends on the level of free fluoride
ions in the solution. The Nernstian equation describes the level of fluoride 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 fluoride ions in solution
The activity, X, represents the effective concentration of free fluoride ions in the solution. Total
fluoride concentration, Ct, may include some bound as well as free fluoride 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 fluoride 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