Answer / rajesh

pH is a measure of the acidity or basicity of a solution. pH
is a potentiometric measurement where the electrode used
produces an electrical signal which the pH meter converts to
a pH reading. The signal produced and measured is a voltage
and since voltage is a potential difference, two potentials
are needed to make a measurement:

a) The sensing electrode provides a potential proportional
to the logarithm of the hydrogen ion activity in the sample.
b) The reference electrode ideally provides a stable and
consistent potential independent of the activity of the sample.

The potential difference between the reference and sensing
electrode is measured by the meter and converted into a pH
value.

All pH sensors consist of a measuring glass electrode and
reference electrode. A combination electrode is an electrode
which contains both the reference and measuring electrode in
the one body. The reference electrode provides a constant
potential regardless of the composition of the solution it
is placed in. The voltage difference between the reference
and measuring electrodes is a function of the pH value of a
solution.

Calibration is done using pH Buffers. Most modern pH meters
are supplied with the facility for automatic buffer
recognition. The manufacturer will have programmed in a pH
versus temperature profile into memory for a specific type
of buffer. If a different buffer to that specified by the
manufacturer is used for calibration this will result in
calibration errors. For proper calibration, buffer solutions
of carefully selected accuracy and pH range are required.
The calibration should be performed using two buffers whose
values bracket the expected value of the samples being
measured. In practice many pH meters are equipped with auto
buffer recognition - this requires the use of the buffer
values specified by the meter manufacturer.

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