Thermo Scientific Orion 2110XP Ammonia Analyzer

General InformationThe sensing electrode responds logarithmically to changes in the ammoniaion 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, mVE o = potential, when C equals C iso , mVR = ideal gas constantT = temperature of sample, degrees KnF= valence of ionic species (+1 for ammonia ion)= Faraday’s constantC = effective ammonia ion concentration (activity)C iso = concentration (activity) of ammonia ion where potential E istemperature independent (isopotential point)The above equation indicates that the measured potential varies with bothtemperature and the concentration of the ion of the interest. In order toeliminate error caused by fluctuations in sample temperature, the 2110XPmicroprocessor constantly updates temperature corrections from datasupplied by the ATC probe.From the Nernst equation, the theoretical response of a ammonia ionselective electrode to a ten-fold change in concentration at 25 °C is59.16 mV. This is referred to as the electrode slope (S). Most electrodes,however, do not exhibit a theoretical slope. Therefore, the analyzer iscalibrated to determine its actual value. Two standards are used to provideinformation necessary for the microprocessor to compute the actual slopeand E 0 for use during sample analysis.In order to eliminate interference from hydrogen ions, which can becomesignificant when measuring low levels of ammonia, the 2110XP analyzeradjusts the sample pH. This pH adjustment is accomplished by thepatented passive-diffusion process wherein the sample passes througha length of tubing contained in the reagent bottle. The reagent diffusesthrough the tube wall and mixes with the sample, which adjusts the samplepH to below 4.Thermo Scientific Orion 2110XP Ammonia Analyzer User GuideI-4