2110XP Ammonia Analyzer User Guide (1574 Kb) - Thermo Scientific

General Information
The sensing electrode responds logarithmically to changes in the ammonia
ion concentration. This response is described by the Nernst equation:
Where:
E = E o + 2.3 (RT/nF) log (C/C iso )
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
F
= valence of ionic species (+1 for ammonia ion)
= Faraday’s constant
C = effective ammonia ion concentration (activity)
C iso = concentration (activity) of ammonia 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 2110XP
microprocessor constantly updates temperature corrections from data
supplied by the ATC probe.
From the Nernst equation, the theoretical response of a ammonia 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 instrument 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 ammonia, the 2110XP adjusts the
sample pH. This pH adjustment is accomplished by the patented passivediffusion
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.
Thermo Scientific Orion 2110XP Ammonia Monitor User Guide
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