RESPONSE TIMES
No electrode response, when moving from one solution to another of different strength, is instantaneous.
The time taken to achieve a final steady value will vary depending upon such factors as electrode type,
degree of concentration change and temperature. Gentle stirring of the solution will improve response times
as will agitation of the electrode.
It is normal, under most conditions, for an electrode to reach 90% of its steady state value in less than 30
seconds.
Some instruments have an inbuilt end-point detector which automatically freezes the displayed reading when
the electrode signal reaches a pre-determined level of stability.
SALT BRIDGE
The salt bridge is that part of a reference electrode which contains the filling solutions to establish the
electrolytic connection between reference internal cell and the test solution.
SELECTIVITY
The ability of an ISE to determine the activity of a given ion in the presence of other ionic species.
SELECTIVITY COEFFICIENT
Defined as the ratio between the activities of the selected ion and an interferent ion which would give an
identical electrode response when present in solution.
SINGLE JUNCTION REFERENCE ELECTRODE
Contains a single electrolyte which forms a low-junction-potential liquid junction with the test solution.
SLOPE
An indication of electrode sensitivity. Theoretically an electrode will develop a potential of 59.16mV at 25°C
(see Nernst Equation). In practice electrode output might be less than this due to contamination of the
sensing membrane. Cleaning will help to restore a Nernstian response. Most pH meters are equipped with
a control to enable electrodes to be calibrated when developed potentials are non-Nernstian. (This is often
called the ‘slope’ control).
SOLID STATE ION SELECTIVE ELECTRODE
Membrane is a crystalline solid material; either a single crystal, a homogeneous or a heterogeneous mixture
incorporating an inert polymer matrix and the internal electrolyte is replaced with a metal contact pad. No
free-flowing liquids are used in the electrode construction which enables the electrodes to be small and
insensitive to electrode orientation.
TEMPERATURE EFFECTS
pH measurements are influenced in several ways by temperature. The change in electrode slope described
by the Nernst Equation is predictable and is usually corrected for with an ATC probe. Other changes are less
predictable and are, therefore, more difficult to correct for. Changes in internal reference potential occur and
the pH of samples can vary with temperature.
The impedance of the glass membrane can also cause problems at low temperatures. As a rule of thumb the
membrane impedance will double for each 8°C drop below 25°C.
TISAB (TOTAL IONIC STRENGTH ADJUSTMENT BUFFER), (see ISAB also)
The exact formulation which will vary with individual electrode and manufacturer, contains a decomplexing
agent, a pH buffer and a species to increase ionic strength.
TITRATION
An analytical technique to measure the concentration of a species by incremental additions of a reagent
which complexes or decomplexes the test species. Typically an S-shaped curve of electrode potential against
titrant volume is obtained; the equivalent point is at the steepest point of the curve. The technique may be
used to directly measure a sample species or indirectly determine an ion not directly sensed by the electrode.
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