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:
'
electrode/electrochemical' 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+
electrode).
- 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
includes:
-
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.
CN−
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.
Source:
PAC, 1994, 66, 2527
(Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations
1994))
on page 2530
Cite as:
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.
