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3.1
Basic Theory
pH is the unit of measurement of the acidity or alkalinity of a
solution, and is expressed as the negative logarithm of the
hydrogen ion concentration:
pH = –log[H
+
]
pH 0 is very acidic, pH 14 is very alkaline and pH 7 is neutral.
For routine pH measurements a sensing electrode and a
reference electrode or a combination electrode (both electrodes in
one body) are used along with a meter capable of displaying the
measurements.
The pH sensing electrode has an internal solution with a constant
pH value, and develops a potential when placed in a solution. This
is caused by the activity of the H
+
ions in the solution. The reference
electrode has a defined, stable potential irrespective of the H
+
activity in the sample. The Model 440 measures and converts the
resulting minute electrode voltages into a pH reading.
The response of a pH electrode (or its slope) is defined by the
Nernst equation:
Electrode response = E
0
– 2.3RT
.
pH
nF
where: E
0
= a constant factor
T = the temperature (Kelvin)
R = the gas constant
n = the ionic charge
F = the Faraday constant
The theoretical slope for an electrode is 59.16 mV where the H
+
ionic charge (n) = 1 at 25°C (298K). This means that for a one
unit change in pH the system will sense a change of 59.16 mV.
The measurement of electrode slope is a good indication of
electrode condition.
Temperature is also an important consideration when measuring
pH. It affects the electrode slope. ATC probes are recommended
so that the slope can be corrected for temperature. There are
many other factors that affect pH.
Further information on pH theory and the factors that affect the
ability to do accurate pH readings is included in ‘Guide to pH
Measurement’.
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