Ion Meters

Q: What is an ion-selective electrode (ISE)?

A: An Ion Selective Electrode measures the potential of a specific ion in solution. (The pH electrode is an ISE for the Hydrogen ion). This potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of that specific ion.

Q: How do you measure concentration using an ion-selective electrode?

A: Standard solutions of known concentrations must be accurately prepared. These solutions are then measured with the pH/mV meter. The mV reading of each solution is noted and a graph of concentration vs. mV reading must be plotted. The unknown solution can now be measured. The mV value of the unknown solution is then located on the graph and the corresponding solution concentration is determined. Dedicated ion meters can construct a calibration curve automatically.

Q: What are the important characteristics of an ion-selective electrode?

A: There are five important characteristics of an ISE.
1. Selectivity
2. Slope of the linear response
3. Range of linear response
4. Detection limit
5. Response time

Q: What is the selectivity coefficient (K) of an ion-selective electrode?

A: ISEs are not ion-specific. All are sensitive to some other ions to some extent. For many applications these interferences are insignificant (unless there is a high ratio of interfering ion to primary ion) and can often be ignored. In some extreme cases, however, the electrode is far more sensitive to the interfering ion than to the primary ion and can only be used if the interfering ion is only present in trace quantities or even completely absent. The ability of an ion-selective electrode to distinguish between different ions in the same solution is expressed as the Selectivity Coefficient. If the primary ion for which the electrode is sensitive is A and the interfering ion is B then a selectivity coefficient of 0.1 would mean that the electrode is ten times more sensitive to A than to B. If the coefficient is 1 then the electrode is equally sensitive to both.

Q: What is the relationship between electrode response and concentration?

A: The slope of the linear part of the measured calibration curve of an ion-selective electrode has a theoretical value according to the Nernst equation of: 59.16 [mV per decade of concentration] for a single charged ion at 25°C or 59.16/2 = 29.58 [mV per decade of concentration] for a double charged ion at 25°C. A useful slope can be regarded as 50-60 mV [per decade of concentration] for a single charged ion or 25-30 mV [per decade of concentration] for a double charged ion.

Q: What is the typical linear response range of an ion-selective electrode?

A: At high and very low target ion concentrations there are deviations of the electrodes response from linearity. Typically an ion-selective electrode calibration curve exhibits a linear response range between 10-1M and 10-5M.

Q: What is the typical detection limit of an ion-selective electrode?

A: In practice, detection limits of 10-5 to 10-6M are measured for most of ion-selective electrodes. The observed detection limit is often governed by the presence of other interfering ions or impurities.

Q: What is the typical response time of an ion-selective electrode?

A: ISE's often require a long time for the readings to stabilize. Approximately fifteen minutes should be allowed for the reading to settle when measuring standard or sample solutions.