Section 3
Operation
3.1 Theory of pH Measurement
pH is a unit of measurement which defines the degree of acidity or alkalinity of a solution. It is
usually measured on a scale of 0 to 14. The pH value quantifies the degree of hydrogen ion activity
of an acid or a base.
The internationally accepted symbol, pH, is derived from “p”, the mathematical symbol of the
negative logarithm and “H”, the chemical symbol for Hydrogen. The pH value is the negative
logarithm of Hydrogen ion activity as shown in the mathematical relationship pH=-log[H
+
].
The pH value of a solution is directly related to the ratio of the hydrogen ion [H
+
] and the hydroxyl ion
[OH
-
] concentrations. If the concentration of H
+
is greater than OH
-
, the material is acidic and has a
pH value of less than 7. Conversely, if the concentration of OH
-
is greater than H
+
the material is
basic, with a pH value greater than 7. If the concentrations of H
+
and OH
-
are equal the material is
neutral with a pH value of 7.
It can, therefore, be seen that pH is a measurement of both acidity and alkalinity, even though by
definition it is a selective measurement of hydrogen ion activity. The logarithmic relationship
between hydrogen ion concentration and the pH unit means that a change of one pH unit
represents a ten-fold change in hydrogen ion concentration.
3.2 pH Measurement
pH can be measured by using either pH papers/indicators or a pH meter, dependent on the level of
accuracy required. pH papers or indicators change colour as the pH level varies. These can be
used as a guide to the pH colour level, but can be limited in accuracy and difficult to interpret
correctly in murky or coloured samples.
For greater accuracy the use of a high impedance pH meter is recommended, together with a pH
measuring electrode and reference electrode.
Each component part of the measurement system can be described as follows:
a)
the pH meter - is a high impedance amplifier used to accurately measure the minute
electrode voltages produced. The pH meter will display the results directly in pH units on either an
analogue or digital display. Voltages can also be read for special applications, ORP (Oxidation-
Reduction Potential) measurements or with Ion Selective Electrodes.
b)
the pH electrode - is a hydrogen ion sensitive glass bulb, with a millivolt output that varies with
the changes in the relative hydrogen ion concentration inside and outside of the bulb. The pH
electrode has very high internal resistance, making the voltage change with pH difficult to measure.
The input impedance of the pH meter and leakage resistances are therefore important factors.
c)
the reference electrode - these cells consist of an internal element, usually a silver/silver
chloride wire, electrolyte (KCl) and a liquid junction. The liquid junction provides a leak path for the
internal electrolyte to “weep” into the sample chamber and provide an electrical contact with the
liquid to be measured. If the liquid junction is inefficient then measurement will be inaccurate. It is
common for the reference electrode to be incorporated into the pH electrode. It is then called a
combination electrode. The Model 3505 is supplied with a combination electrode.
350 550/REV A/03-03
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