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 @[email protected] potential differences or results in a significant activity @[email protected] decrease, or incipient @[email protected] 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 @[email protected] may be problematic.
- Substances interacting with the @[email protected] itself, blocking the surface or changing its chemical composition [i.e. organic solvents for the liquid or poly(vinyl chloride) (PVC) @[email protected] electrodes] are grouped as interferences or electrode poisons.