RADIOMETER ABL800 FLEX, Reference Manual

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1. Potentiometric measuring principles ABL800 FLEX Reference Manual
General information,
Continued
The electrodes are active elements and must be calibrated regularly. Signals from
the electrodes change because of, e.g., protein build-up, worn-out membranes,
aging electrodes, etc.
Calibration
(continued)
The responses from the electrodes when measuring on the calibrating solutions are
checked to ensure that the amplified signals from the electrodes are converted to
accurate values for an unknown sample. The relationship between the electrode
amplifiers’ output and the pH/
p
CO
2
/electrolyte electrodes are simple mathematical
functions. Calibration data can therefore be determined by relating the electrode
signals during the calibration process to the values of the calibrating solutions.
Calibration line The calibration line expresses the relationship between the potential measured at
an electrode, and the concentration of the species specific to the electrode. The
calibration line forms the basis of the scale used by the analyzer to convert
electrode chain potentials to concentrations. Each electrode has a different
calibration line.
The pH electrode is used as an example to illustrate how the calibration line is
derived from two calibration solutions with known pH.
The calibration solutions give the following two points:
−
64 mV at pH 6.802 (Cal
2) and
−
100 mV at pH 7.398 (Cal 1)
Within the coverage range 6.300 to 8.000 the pH electrode is linear, and the
relationship between potential and pH is linear, so a line can be drawn between the
two points, as shown below:
Calibration line
pH
Measured
potential
(mV)
−
100
−
64
6.802
pH of Cal 2 sol.
7.398
pH of Cal 1 sol.
−
97
7.346
pH of sample
This is a two-point
calibration. In one-point
calibration, only the position
of the calibration line is
determined. The slope of the
calibration line is maintained
from the last 2-point
calibration.
The calibration line is stored in the computer and is used during measurement to
convert the potential measured at the pH electrode during sample analysis to an
actual pH value.
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