Page 88 - PBL71 OI/AXM630-EN AZTEC 600 SS (WIRO)

Aztec 600 ISE ammonia and fluoride
Single-stream ion-selective analyzers
D.2 Theory of Operation
Ion-selective electrodes are useful tools in analytical chemistry. Rapid measurements of a great variety of species over wide concentration ranges can be made with the appropriate ion-selective electrode with parts-per-billion sensitivity.
The ion-selective electrode used for fluoride measurement is a solid-state electrode. This type of electrode comprises a plastic body containing an internal reference and Lanthanum Fluoride half-cells. At equilibrium, the membrane potential is mainly dependent on the activity of the target ion outside the membrane and is described by the following Nernst equation:
E = E0 + (2.303RT/ nF) x Log (A) Where:
Appendix D – Principle of Operation – Fluoride Analyzers
E = E0 = 2.303 =
R = T = n = F = Log (A) =
total potential (in mV) developed between the sensing and reference electrodes
constant characteristic of the particular ISE / reference pair
conversion factor from natural to base10 logarithm
Gas Constant (8.314 joules/degree/mole) Absolute Temperature
charge on the ion (with sign)
Faraday Constant (96,500 coulombs) logarithm of the activity of the measured ion
The measured voltage (Eq.1) is proportional to the Logarithm of the concentration and the sensitivity of the electrode is expressed as the electrode Slope (in millivolts per decade of concentration). Therefore the electrodes can be calibrated by measuring the voltage in solutions containing, for example, 10 ppm and 100 ppm of the target ion. The Slope is the slope of the (straight) calibration line drawn on a graph of mV versus Log concentration.
for example:
mVHigh STD (100 ppm) – mVLow STD (10 ppm) Slope = -----------------------------------------------------------------------------------------------------------
Log100 – Log10
Therefore, the slope equals the difference in the voltages since
Log100 – Log10 = 1.
Unknown samples can be determined by measuring the voltage and plotting the results versus the concentration values on a calibration graph. Essentially, the only variables in the above expression are E – E0 (E) and concentration which, when plotted against each other, produces a linear relationship assuming a constant sample temperature.
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The exact value of the slope can be used as indication of the efficiency of the ion-selective electrode in use. The slope value for a monovalent cation determined at 25 °C = 59 mV (–59 mV for a monovalent anion). This indicates that for every decade change in concentration there will be a 59 mV change in the potential output from the electrode.
To maintain a high degree of measurement accuracy, periodic calibration is required. This is usually performed automatically in online analyzers at which time the slope is recalculated. However, most of the changes in the output from the electrode pair are a result of zero drift that over a period of time can become quite large (for example, ±50 mV) before it begins to adversely affect performance.