GRAN PLOT
A mathematical transformation of the titration curve in order to improve a method’s precision or limit of
detection.
HYSTERESIS
Electrodes sometimes display a memory effect which is exhibited when moving between solutions of signifi-
cantly different pH. Large changes in other ambient influences can also induce this effect, i.e. temperature
or light levels.
INCREMENTAL METHODS
A variety of spiking techniques whereby known volumes of standard or sample solutions are added. They
can be separated into known addition/subtraction and sample addition/subtraction.
INPUT IMPEDANCE
pH meters used in conjunction with glass pH electrodes must have a high input impedance to match the
impedance of the glass membrane. This is typically 10
13
ohms.
The signal developed by a pH electrode has very little power and the device, therefore, has a very limited
capability to drive current into a measuring circuit.
With modern electronic devices the necessary input impedance is not difficult to achieve but can be degraded
by a damp or dirty operating environment.
INTERFERENCE
Ion Selective Electrodes are highly selective but seldom respond specifically to a given ion. Different ions will
respond to an electrode to variable amounts and this is expressed as it’s selectivity coefficient. A value of 10
-
3
means that a concentration of interferent ion 1000 times greater than the measured ion is required before
an equal electrode response is seen.
INTERNAL REFERENCE ELEMENT
A reference electrode situated within the glass pH sensing electrode.
IONIC STRENGTH (I; units: mol/m
-3
, mol/g)
The total effect of all of the ions present in a solution is expressed as the ionic strength and is given by:
ionic strength = 1/2 C
i
Z
i
2
, where
C = concentration of ion i
Z = the charge of ion i
n = the number of species present in the solution
The ionic strength determines the activity coefficient of each ion in solution.
ION SELECTIVE ELECTRODE
Electroanalytical sensors which respond selectively to specific ions, the signal depends upon the activities of
the ions in solution. A potential develops across the membrane, an electrically conducting material which
separates the solution from an internal solution or contact. These electrodes are classed as glass, liquid or
solid-state.
ISAB (IONIC STRENGTH ADJUSTMENT BUFFER)
A solution of high ionic strength added to sample solutions before measurement to minimise differences in
ionic strength between standard and test solutions, thus eliminating the effect of ion activity coefficient and
permitting direct measurement of concentration. A non-interfering salt, pH buffer and decomplexing agent
are used, although the formulation can vary.
ISOPOTENTIAL POINT
A point at which the potential developed by an electrode is not influenced by temperature. For a normal glass
pH electrode this potential will be close to 7pH. Temperature effects are greater the further the level of the
sample is from the isopotential point (see ATC and Temperature Effects).
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