Any substance, other than the ion being measured, whose presence in the sample solution
affects the measured emf of a cell. Interfering substances fall into two classes:
' interferences and 'chemical
' interferences. Examples of the first class include:
- Those substances which give a similar response to the ion being measured and whose
presence generally results in an apparent increase in the activity (or concentration)
of the ion to be determined (e.g. Na+
for the Ca2+
- Electrolytes present at a high concentration that give rise to appreciable liquid junction potential differences or results in a significant activity coefficient decrease, or incipient Donnan exclusion failure. The second class of substances that should be recognized as chemical interferences
Species that interact with the ion being measured so as to decrease its activity or
apparent concentration. The electrode continues to report the true activity (e.g.
present in the measurement of Ag+),
but a considerable gap will occur between the activity and concentration of the ions
even in very dilute solutions. Under these circumstances the determination of ionic concentration may be problematic.
Substances interacting with the membrane itself, blocking the surface or changing its chemical composition [i.e. organic solvents
for the liquid or poly(vinyl chloride) (PVC) membrane electrodes] are grouped as interferences or electrode poisons.
PAC, 1994, 66, 2527
(Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations
on page 2530
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook