PASCO Specialty & Mfg. PASCO Scientific SODIUM ION SELECTIVE ELECTRODE CI-6734 Benutzerhandbuch
012–06615A
Sodium Ion Selective Electrode
1
Introduction
The PASCO scientific Sodium Ion Selective Electrode is used to quickly, simply, accurately, and economically
measure sodium ions in aqueous solutions.
Theory
The Sodium Ion Selective 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 an ISE Amplifier
and a ScienceWorkshop computer interface. The level of sodium ions, corresponding to the measured potential, is
described by the Nernst equation.
E = E0 + S log X
where:
E = measured electrode potential
E
0
= reference potential (a constant)
S = electrode slope (
≈
59
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, C
ƒ
, by the activity coefficient,
γ
by:
X =
γ
C
ƒ
Activity coefficients may vary, depending on the total ionic strength, I, determined as:
I = 1
2
Σ
C
X
Z
X2
where:
C
X
= concentration of ion X
Z
X
= 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.