Thermo Scientific Orion 2117HL User Manual page 8

General Information
Thermo Scientific Orion 2117HL High Level Chloride Analyzer User Guide
The sensing electrode responds logarithmically to changes in the chloride
ion concentration. This response is described by the Nernst equation:
E = E + 2.3 (RT/nF) log (C/C
o
Where:
E = measured electrode potential, mV
E = potential, when C equals C
o
R = ideal gas constant
T = temperature of sample, degrees K
n = valence of ionic species (-1 for chloride ion)
F = Faraday's constant
C = effective chloride ion concentration (activity)
C = concentration (activity) of chloride ion where potential E is
iso
temperature independent (isopotential point)
The above equation indicates that the measured potential varies with both
temperature and the concentration of the ion of the interest. In order to
eliminate error caused by fluctuations in sample temperature, the 2117HL
microprocessor constantly updates temperature corrections from data
supplied by the ATC probe.
From the Nernst equation, the theoretical response of a chloride ion
selective electrode to a ten-fold change in concentration at 25 °C is
-59.16 mV. This is referred to as the electrode slope (S). Most electrodes,
however, do not exhibit a theoretical slope. Therefore, the analyzer is
calibrated to determine its actual value. Two standards are used to provide
information necessary for the microprocessor to compute the actual slope
and E for use during sample analysis.
0
)
iso
, mV
iso
I-4