Ion Selective Electrode - Interferences

Interferences

The most serious problem limiting use of ion-selective electrodes is interference from other, undesired, ions. No ion-selective electrodes are completely ion-specific; all are sensitive to other ions having similar physical properties, to an extent which depends on the degree of similarity. Most of these interferences are weak enough to be ignored, but in some cases the electrode may actually be much more sensitive to the interfering ion than to the desired ion, requiring that the interfering ion be present only in relatively very low concentrations, or entirely absent. In practice, the relative sensitivities of each type of ion-specific electrode to various interfering ions is generally known and should be checked for each case; however the precise degree of interference depends on many factors, preventing precise correction of readings. Instead, the calculation of relative degree of interference from the concentration of interfering ions can only be used as a guide to determine whether the approximate extent of the interference will allow reliable measurements, or whether the experiment will need to be redesigned so as to reduce the effect of interfering ions. The nitrate electrode has various ionic interferences, i.e. perchlorate, iodide, chloride, and sulfate. These interferences vary markedly in the extent to which they interfere. Thus, perchlorate gives a response which is about 50,000x as great as an equal amount of nitrate, while 1000x as much sulfate produces about a 10% error in the reading. Chloride causes a 10% error when present at about 30x the nitrate level, but can be removed by the addition of silver sulfate. Alternatively, nitrate can be determined by using an ammonia gas sensing electrode. This technique allows the user to determine both ammonium and nitrate ions sequentially. The procedure makes use of the reducing ability of titanium chloride. Trivalent titanium reduces any nitrate ion, up to 20 ppm, to ammonium ion (i.e., reverse nitrification). At pH 12-13, any ammonium ion in the sample is converted to ammonia gas and is ultimately detected by the electrode.