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27
E.
pH Measurements
The pH meter is an instrument used for
measuring the pH of solutions. It consists of a probe
which acts as an electrochemical cell
whose potential difference (voltage) depends upon the
hydrogen ion concentration of a solution
measured against a standard silver, silver chloride solution
contained within the probe.
This potential difference is measured
by a voltmeter whose scale is calibrated in pH units instead
of volts. The meter must be standardized
regularly in order to obtain accurate pH readings.
1.
General Notes:
a)
pH meters are very expensive and require
special care, therefore, handle them carefully.
Because of the high resistance of the
glass membrane, only very low currents can be drawn,
in the order of 10
-13
A. This requires a special voltmeter.
b)
The glass electrodes are very fragile
(and expensive) and can be cracked with just a sharp
tap. Therefore, NEVER use the electrode
as a stirring rod.
c)
The glass membrane and the wick providing
a connection to the reference electrode must
not be allowed to dry out. When
the pH meter is not being used, keep the probe in the
storage buffer solution provided.
d)
In very basic solutions ( pH above 12),
the glass electrode is unreliable and is gradually
dissolved by the high concentration of
hydroxide ions; therefore,
DO NOT allow the electrode to sit in strongly
basic solutions. When you have completed a
titration, immediately wash off the electrode
with distilled water and replace it in the storage
buffer.
e)
To prevent contamination of solutions,
always wash the electrode with distilled water and
gently dry it with a kimwipe when moving
the electrode to a different solution (including the
storage buffer).
f)
The pH meter works best if it is kept
plugged in continuously.
2.
Calibration Procedure
