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13
P
H
MEASUREMENT
The instrument works with pH measurement electrodes, redox potential measurement electrodes
(ORP), and specific ion electrodes. The probes with 4 wire Pt100, 2 wire Pt1000 or Ni1000 sensors
may be used for measuring temperature or for the automatic compensation of the Nernst coefficient
with the pH electrode.
Some probes are fitted with SICRAM module that acts as an interface between the sensor on the
probe and the instrument. There is a electronic circuit with a permanent memory inside the module
that enables the datalogger to recognize the type of probe connected and to read its calibration in-
formation.
The Delta Ohm Pt1000 and Ni1000 probes complete with TP47 module , are automatically detected
while the direct 4 wire Pt100 temperature probe is not automatically detected by the instrument and
must be set up in the
Probe type
menu item (please see the description of the menu on page 8).
The pH or mV indication is displayed in the main line; the secondary line shows the temperature.
The electrode for pH measurement
The electrode for pH measurement, generally in glass, generates an electrical signal proportional to
the pH according to Nernst law. The following aspects of this signal are considered:
Zero point
: The pH where the electrode generates a potential of 0 mV. In most electrodes, this value
is found at about 7pH.
Offset or Asymmetry Potential
: mV generated by an electrode when immersed in a buffer solution at
7pH. Generally oscillates between ± 20 mV.
Slope
: response of the electrode expressed in mV per pH units. The theoretical electrode slope at
25°C is 59.16 mV/pH. When the electrode is new the slope is close to the theoretical value.
Sensitivity
: it is the electrode's slope expression in relative terms. It is obtained by dividing the ac-
tual value of the slope by the theoretical value, and is expressed as a %. The asymmetry potential
and the slope vary in time with the use of the electrode, which necessitates regular calibration.
The pH electrodes must be calibrated using the standard solutions (see the pH calibration chapter
below). The ORP and specific ion electrodes do not need calibration as their absolute voltage is
measured.
The redox standard solutions are only used to check the quality of a redox elec-
trode
.
No calibration of the temperature sensor is required by the user: the sensor is calibrated in
the factory and the Callendar Van Dusen parameters are recorded in the SICRAM module.
The probes are detected during turn on, and this cannot be performed when the instrument is
already on, therefore if a probe is connected and the instrument is on, it is necessary to turn it
off and on
.
Automatic or manual pH compensation
In a correct measurement of pH, the results need to be expressed together with the temperature
value at which the reading is performed.
The electrode slope varies according to the temperature in a known mode according to Nernst law:
e.g., a 1pH variation, that at 25°C means 59.16mV, at 100°C means 74.04mV.
