KARL FISCHER TITRATION
A method for determining the water content of samples. A polarising current is applied to a pair of platinum
electrodes immersed in a solution containing iodine, pyridine, methanol and sulphur dioxide. A polarising
current, typically 10 microamps, is applied to the electrodes and the resulting potential is monitored.
LIQUID JUNCTION
The point of contact between the sample and the reference electrode filling solution.
LEAK RATE
The reference electrode fill solution will inevitably leak into the sample solution. An irregular leak rate may
cause the liquid junction potential to vary causing unstable or drifting readings. Samples of small volume that
need to be retained can also suffer an unacceptable level of contamination.
MEMBRANE
In a pH electrode the membrane is a thin pH sensitive glass. A potential difference is developed across the
membrane due to the formation of double layers with ion-exchange properties.
MILLIVOLT
Based on the SI unit of potential difference, electrical potential or e.m.f. such that the potential difference
across a conductor is 1 millivolt when a current of 1 milliamp flows through a resistance of 1 ohm.
MOLALITY
The concentration of a solution expressed as a number of moles of dissolved substance per kilogram of
solvent.
MOLARITY
The concentration of a solution expressed as the number of moles of dissolved substance per dm
3
of solution.
MONOVALENT ION
An ion with a single positive or negative charge (Na+, H+, Cl-, etc).
NERNST EQUATION
A mathematical relationship which describes the theoretical behaviour of pH electrodes
E = Ex + 2.3 RT logai
nF
E is the potential, in millivolts, developed between the sensing and reference electrodes.
Ex varies with the choice of reference electrodes and is a constant.
2.3 RT
is the Nernst Factor. R is the gas constant, i.e. 8.31441J.K-1 mol-1.
nF
T is the thermodynamic temperature (in K)
n is an integer with sign and magnitude corresponding to the charge on the ion.
F is the Faraday Constant and is equal to 9.648456 x 104 C.mol-1.
Ai is the activity of the ion to which the electrode is responding.
At T=25°C, E is equal to 59.16mV when n=1.
NOISE
Random and rapid changes in displayed electrode potential. This is often due to the pick-up of strong static
charges or electromagnetic interference from adjacent equipment.
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