Delta OHM HD2205.2, Manual

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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. Of this signal the following aspects are considered:
Zero point : The pH where the electrode generates a potential of 0 mV. In most electrodes, this value
is found at about 7 pH.
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 calibration chapter be-
low). The ORP and specific ion electrodes do not need calibration.
The standard redox solutions
are only used to check the quality of a redox electrode.
User calibration of the temperature sensor is not required: The sensor is calibrated in the fac-
tory 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
The pH measurement is influenced by the measured solution temperature.
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.
6pH 7pH
pH
8pH
74.04mV
-74.04mV
59.16mV
-59.16mV
54.20mV
-54.20mV
0mV
mV
T=100°C, 74.04mV/pH
T=25°C, 59.16mV/pH
T=0°C, 54.20mV/pH
When a temperature probe is present (temperature only probe, combined pH/temperature probe or
conductivity/temperature probe), the instrument automatically applies the
ATC (Automatic Tem-
perature Compensation) function.