Siemens Rapidlab 1200 Operator's Manual page 50

1-30
In this equation, the reference electrode potential and the potential of ISE inner reference
element are constant; the liquid junction potential can be controlled. Therefore, the
potential remaining is the potential generated at the membrane. The membrane potential
corresponds to the ion activity and is related directly to the concentration of the ion in
solution. The cell potential is expressed quantitatively by the Nernst equation.
E = K + (2.3 RT/ZF) log a
cell
where
E = electrochemical cell potential
cell
K = a constant from various sources such as the liquid junction
R = gas constant
T = absolute temperature
Z = ionic charge
F = Faraday's constant
a = activity of the ion in the sample
i
This equation states that the cell potential is logarithmically related to the activity of the
analyte in the sample.
The potential that the sensor actually measures is the activity of the analyte in solution. In
clinical chemistry, it is typical that the results be expressed in the concentration of total
substance rather that the activity of the substance. For this reason, the measured results
must be expressed in units of concentration.
The activity equals the numerical value of the concentration of the ion (mol/L) times the
activity coefficient. The activity coefficient is a measure of the degree with which the ion
interacts with other ions in solution. The activity coefficient is dimensionless and depends
on the ionic strength of the solution:
I = 1/2 ∑ m * z
where
I = ionic strength of the solution
m = concentration of the ion (mol/L)
z = the charge number of the ions in solution
The activity coefficient generally decreases with increasing ionic strength.
02087462 Rev. V
Rapidlab 1200 Operator's Guide: System Overview and Intended Use
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