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:

1. 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).
2. Electrolytes present at a high concentration that give rise to appreciable @L03584@ potential differences or results in a significant activity @C01124@ decrease, or incipient @DT06889@ failure. The second class of substances that should be recognized as chemical interferences includes:
3. 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 @I03174@ may be problematic.
4. Substances interacting with the @MT06878@ itself, blocking the surface or changing its chemical composition [i.e. organic solvents for the liquid or poly(vinyl chloride) (PVC) @MT06878@ electrodes] are grouped as interferences or electrode poisons.