Aztec 600 ISE ammonia and fluoride
Single-stream ion-selective analyzers
Appendix D – Principle of Operation – Fluoride Analyzers
84
OI/AXM630–EN Rev. I
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 = E
0
+ (2.303RT/ nF) x Log (A)
Where:
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:
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 – E
0
(
E) and concentration which, when
plotted against each other, produces a linear relationship
assuming a constant sample temperature.
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.
E
= total potential (in mV) developed between the
sensing and reference electrodes
E
0
= constant characteristic of the particular
ISE / reference pair
2.303
= conversion factor from natural to base10
logarithm
R
= Gas Constant (8.314 joules/degree/mole)
T
= Absolute Temperature
n
= charge on the ion (with sign)
F
= Faraday Constant (96,500 coulombs)
Log (A) = logarithm of the activity of the measured ion
Slope
mV
High STD
(100 ppm)
mV
Low STD
(10 ppm)
–
Log100
Log10
–
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