Ion-selective electrodes

An Ion-selective electrode (ISE) is a transducer (sensor) which converts the activity of a specific ion dissolved in a solution into an electrical potential which can be measured by a voltmeter or pH meter, and compared to the potential of a non-specific reference electrode. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. Ion-selective electrodes are used in biochemical and biophysical research, where measurements of ionic concentration in an aqueous solution are required, usually on a real time basis.

Types of ion-selective membrane
The sensing part of the electrode is usually made as an ion-specific membrane. There are four main types of ion-specific membranes used in ion-selective electrodes:

Glass membranes
Glass membranes are made from an ion-exchange type of glass (silicate of chalcogenide). This type of electrode has good selectivity, but only for a few types of single-charged cation; mainly H+, Na+, and Ag+. Chalcogenide glass also has selectivity for double-charged metal ions, such as Pb2+, and Cd2+. The glass membrane has excellent chemical durability and can work in very aggressive media. A very common example of this type of electrode is the pH glass electrode found in almost every chemical laboratory.

Crystalline membranes
Crystalline membranes are made from mono- or polycrystallites of a single substance. They have good selectivity, because only ions which can introduce themselves into the crystal structure can interfere with the electrode response. Selectivity of crystalline membranes can be for both cation and anion of the membrane-forming substance. An example is the fluoride selective electrode based on LaF3 crystals.

Ion exchange resin membranes
Ion-exchange resins are based on special organic polymer membranes which contain a specific ion channel or carrier (resin), often a drug or other biologically-derived substance. This is the most widespread type of ion-specific electrode, since the wide variety of ion-specific resins allows preparation of a wide variety of electrodes selective for different ions, single-atom or multi-atom. They are also the most widespread electrodes with anionic selectivity. However, such electrodes have low chemical and physical durability as well as "survival time". An example is the potassium selective electrode, based on the drug valinomycin as an ion-exchange agent.

Construction
These electrodes are prepared from glass capillary tubing approximately 2 millimeters in diameter, a large batch at a time. Polyvinyl chloride is dissolved in a solvent and plasticizers (typically phthalates) added, as is standard when making a vinyl polymer. Water and other ions will not be able to pass through the vinyl once polymerized; in order to provide the ionic specificity desired, the appropriate specific ion channel or carrier is added to the vinyl solution.

One end of a piece of capillary tubing about an inch or two long is dipped into this vinyl solution containing the specific ion carrier, and removed to let the vinyl solidify into a plug at that end of the tube. Using a syringe and needle, the tube is filled with salt solution from the other end, and may be stored in a bath of the salt solution for an indeterminate period. For convenience in use, the open end of the tubing is fitted through a tight o-ring into a somewhat larger diameter tubing containing the same salt solution, with a silver or platinum electrode wire inserted. New electrode tips can thus be changed very quickly by simply removing the older capillary electrode and replacing it with a new one.

Enzyme electrodes
Enzyme electrodes definitely are not true ion-selective electrodes but usually are considered within the ion-specific electrode topic. Such an electrode has a "double reaction" mechanism - an enzyme reacts with a specific substance, and the product of this reaction (usually H+ or OH-) is detected by a true ion-selective electrode, such as a pH-selective electrodes. All these reactions occur inside a special membrane which covers the true ion-selective electrode, which is why enzyme electrodes sometimes are considered as ion-selective. An example is glucose selective electrodes.

Use
In use, the electrode wire is connected to one terminal of a galvanometer or pH meter, the other terminal of which is connected to a reference electrode, and both electrodes are immersed in the solution to be tested. The passage of the ion through the membrane creates an electrical current, which registers on the galvanometer; by calibrating against standard solutions of varying concentration, the ionic concentration in the tested solution can be estimated from the galvanometer reading.

Sources of inaccuracy
In practice there are several issues which affect this measurement, and different electrodes from the same batch will differ in their properties. Leakage in the electrode, thereby allowing passage of any ions, will cause the meter reading to show little or no change between the various calibration solutions, and requires that that electrode be discarded. Similarly, with use the ion-sensitive channels may gradually become blocked or otherwise inactivated, causing the electrode to lose sensitivity. The response of the electrode and galvanometer is temperature sensitive, and also 'drifts' over time, requiring recalibration frequently during a series of measurements, ideally at least one calibration sample before and after each test sample. On the other hand, after immersion in the solution there is a transient 'settling time' which can be five minutes or even longer, before the electrode and galvanometer equilibrate to a new reading; so that timing of the reading is critical in order to find the most accurate 'window' after the response has settled, but before it has drifted appreciably.

Interference
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.

Effect of differences in total ionic strength
Ion-selective electrodes do not actually measure the concentration of an ion, but rather its activity, which depends on the product of its concentration and the activity coefficient in that solution, which in turn depends on the total ionic strength; when the total ionic strength is low, increases and decreases in the concentration of the measured ion itself also significantly increase and decrease the total ionic strength, thus the activity coefficient, thus exerting a greater effect on the potential measured by the electrode than the same change would in a solution with high total ionic strength. This has the result that, if an ion-selective electrode is calibrated using dilute solutions in distilled water, measurements in high ionic strength solutions, such as biological fluids or sea water, will be wrong.

In order to correct for this effect, a small amount of very high ionic strength buffer, known as Total Ionic Strength Adjustment Buffer, is added to the calibration standards, so that the change in total ionic strength, and therefore activity coefficient, between concentrations of the calibration solutions is minimal, and the calibration curve reflects only the change in concentration. Using the Total Ionic Strength Adjustment Buffer to raise the total ionic strength of the solutions being measured to the same level as the calibration standards allows direct comparison of the measurements without need for a correction factor to account for differences in total ionic strength.