OPEN CIRCUIT
The symptoms of an open circuit are either large jumps in displayed reading or a continual drift leading to an
out of range reading.
The measurement circuitry relies on good contact of both the inner element and screen of the sensor to the
instrument input.
pH ELECTRODE
An electrode which is specifically sensitive to hydrogen ion activity. Electrodes will normally perform satis-
factorily over the entire 0 - 14pH range, but under certain circumstances anomalies can exist in strongly
acidic solutions (acid error) and, more commonly, in alkali solutions by response to sodium or other cations
(sodium error) in basic solutions.
Special electrode configurations are available to suit most analytical situations.
pH UNIT
A logarithmic index for the hydrogen ion concentration in an aqueous solution. Given by:
pH = log10 1
H+
i.e. a solution of pH7 is 10-7M in hydrogen ion activity and a pH8 solution is 10-8M.
POTENTIAL
When two phases containing electrically charged particles come into contact, an electrical potential differ-
ence develops. The potential of an ISE is measured against a suitable constant potential (reference elec-
trode).
POTENTIOMETRY
A method of directly measuring wither concentration or activity of a sample solution by comparison with
results from two or more known standards. The temperature, ionic strength, pH and composition of the
standard and sample solutions must be kept constant.
PRIMARY STANDARDS
A range of aqueous buffer solutions defined by the National Bureau of Standards (N.B.S) in the U.S and by
British Standards Institute (BS1647) which provide stable liquid junction potential and uniform electrode
sensitivity.
REDOX POTENTIAL
The potential developed by a metallic electrode when placed in a solution containing a species in two differ-
ent oxidation states.
REDOX ELECTRODE
These units have a metallic sensing element, usually platinum but sometimes gold, which monitors oxidation
reduction reactions.
REFERENCE ELECTRODE
The reference electrode is the half of the electrode pair which has to maintain a constant potential regardless
of solution composition. It should not be easily polarised, possess no temperature hysteresis and a small
temperature coefficient.
RELATIVE MILLIVOLTS
A mode in which the actual sensor potential can be displayed. Can be modified to enable a reading to be
taken which is relative to some specific point in a measurement cycle.
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