General Information
I-4
Thermo Scientific Orion 2120XP Calcium Hardness Analyzer User Guide
The sensing electrode responds logarithmically to changes in the calcium
ion concentration. This response is described by the Nernst equation:
E = E
o
+ 2.3 (RT/nF) log (C/C
iso
)
Where:
E = measured electrode potential, mV
E
o
= potential, when C equals C
iso
, mV
R = ideal gas constant
T = temperature of sample, degrees K
n = valence of ionic species (+2 for calcium ion)
F = Faraday’s constant
C = effective calcium ion concentration (activity)
C
iso
= concentration (activity) of calcium ion where potential E is
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 2120XP
analyzer microprocessor constantly updates temperature corrections from
data supplied by the ATC probe.
From the Nernst equation, the theoretical response of a calcium ion
selective electrode to a ten-fold change in concentration at 25 °C is
+29.58 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
0
for use during sample analysis.
In order to eliminate interference from hydrogen ions, which can become
significant when measuring low levels of calcium hardness, the 2120XP
analyzer adjusts the sample pH. This pH adjustment is accomplished by
the patented passive-diffusion process wherein the sample passes through
a length of tubing contained in the reagent bottle. The reagent diffuses
through the tube wall and mixes with the sample, which adjusts the sample
pH to below 4.
