ISEs measure the activity of free ions in aqueous solutions.

Activity is the effective concentration - i.e. that portion of the ions which are free to take part in a given reaction - in this case coming into contact with the membrane surface. Activity is always less than concentration due to inter-ionic interactions in the solution, which inhibit the movement of ions and prevent some of them from reaching the membrane. It becomes proportionately less as the concentration increases. In practice this effect is negligible (within the error limits of the measurement) below about 0.01M for monovalent ions and 0.001M for divalent ions. The difference between activity and concentration is expressed as the Activity Coefficient.

Ionic Strength Adjustment Buffer is added equally to samples and standards to minimise any errors due to differences in ionic strength between samples and standards which will cause differences in activity coefficents which can cause the concentration to be understimated by up to 50 or 60% in the worst cases. In some cases ISABs can also include ingredients which minimise interference effects, and ensure that the pH is optimum for the ISE measurement. Furthermore, for some ions, the addition of ISAB can help to reduce the time required to reach a stable reading after immersing the electrodes in a new solution.

ISAB is not normally necessary if the total ionic strength of the samples is less than 0.01 Molar for monovalent ions or 0.001 M for divalent ions (unless required for control of pH or interference or stabilisation time) and may not be necessary at higher IS, depending on precision requirements. For more details see Electrode Operating Instructions

The recommended ISAB for each ISE can be found in the electrode specification sheet. However, it must be noted that most ISABs only increase the IS to about 0.1 Molar so will not be effective for samples which already approach this level. In this case there is no point adding more strength to the samples and it is necessary to bring the standards up to the same level by making them with a matrix similar to the samples but not containing the target ion or any that would interfere with it. Alternatively, high Ionic Strength samples can be analysed by the Standard Addition or Sample Addition method.

Ion-selective electrodes are not completely ion-specific. All are sensitive to some other ions to some extent. The degree of sensitivity to another ion is given by the Selectivity Coefficient, where a value of 0.1 indicates that the electrode is ten times more sensitive to the primary ion than to the interfering ion. A value of 1 indicates equal sensitivity to both ions.

The reference electrode outer filling solution, which is in contact with the external test solution, must not contain any ions that will interfere with the ion being detected.

The recommended reference electrode can be found in the ISE specification.

All ELIT ISEs are all-solid-state and contain no liquid or gel solutions - so there is nothing to replace.
ELIT reference electrodes contain only long-lasting gel electrolytes. The units are completely sealed and the electrolyte(s) cannot be replaced. The only need for reference electrode filling solutions is in order to keep a small drop in the protective cap to prevent the electrolyte and ceramic frit from drying out during storage.

If the signal is very erratic and jumps by tens or even hundreds of millivolts then this is probably due to minute bubbles in the reference electrode electrolyte. These can develop during transport or prolonged storage. This can normally be cured by holding the electrode firmly with the active tip pointing downwards and shaking down several times with a flick of the wrist, as with old medical mercury thermometers ( i.e. down vigorously but up gently so that the gel is propelled towards the ceramic frit and any bubbles away from it).

Large deviations may also be due to poor connections in the wiring or moisture on the contacts and in this case all connections should be checked and cleaned. Random deviations of a few millivolts may be due to contamination of the ISE membrane.
(see "How do I clean contaminated membranes" ) or to external electrostatic fields. In the latter case, if the operator or passers by are wearing static-producing clothing, it may help to ensure that every one remains still for a few seconds whilst the reading is taken.

If the slope is only a few millivolts outside specification but is stable and reproducible then the ISE can still be used satisfactorily, although the lower the slope the higher the errors on the measurement of activity (or concentration).
See "What precision can I expect from an ISE measurement".

If the slope gradually becomes lower each time it is measured then the membrane is probably contaminated and should be rejuvenated by cleaning.
See "How do I clean contaminated membranes".

Crystal membranes can be cleaned by washing with alcohol to remove any organic deposits and gently polishing the crystal surface with fine emery paper to remove any obvious markings or discoloration. Then wash with de-ionised water and wipe dry with a low lint tissue, ensuring that no particles are left on the membrane. The crystal surface should be smooth and shiny. PVC membranes must not be abraded, or even touched. These can also be washed in alcohol and then regenerated by soaking in the appropriate 1000 ppm standard solution overnight, or even for several days. They must only be gently dabbed dry after washing (not wiped) to avoid scratching the surface. The ceramic frits at the tips of reference electrodes can also be cleaned with alcohol to remove organic contamination, and soaked in 0.1Molar HCl to remove other deposits.

Most electrode systems require about three or four minutes to reach a completely stable reading. Nevertheless, most electrode combinations get to within one or two millivolts of the final value in less than thirty seconds, so it depends on your precision requirements as to whether you wait for complete stabilisation or not.

Assuming that there are no systematic errors from interference or activity coefficient effects, the precision depends on the error in the measurement of the electrode potential (mV) and the slope of the calibration line:- a 1mV error represents a ~4% error in the concentration for monovalent ions (slope~54) and an ~8% error for divalent ions (slope~27), when working in the normal linear range of the electrodes.
Nevertheless, it has been shown that, under optimum conditions, a precision of better than ± 2% can be achieved by making frequent recalibrations and taking multiple readings of the electrode signals.

Drift is the gradual change in electrode response over a period of time. The rate and extent of drift can vary depending on which particular ISE and reference electrodes are being used and the age and degree of contamination of the electrodes. If a series of samples are measured repeatedly it will be observed that each successive reading of the same solution will be slightly different from the previous one. This problem can be overcome by frequently measuring one calibration solution in between sample measurements, then re-calibrating when the mV value has drifted beyond an acceptable level - depending on precision requirements.

13.) What is the effect of temperature change on ISE measurements ?

Unfortunately this is a complicated relationship which cannot be simply quantified in terms of mV change per °C since the effect is different at different concentrations and actual value of the mV. Moreover, the electrode slope, the liquid junction potential, and solubility of the salts in the reference system all vary with temperature. However, the magnitude of the effect of temperature change on the slope can be calculated from a modified form of the Nernst equation (Slope = 2.303RT/nF = T x Constant) to be about 3.4% per 10°C - ie: if the slope is about 55mV/dec then a 10°C rise will increase this to about 57mV/dec. Thus, in order to avoid any errors due to temperature change it is advisable to recalibrate with standards at the same temperature as the sample solutions if the sample temperature deviates by more than about 2°C from the original calibration temperature.

This depends on the precision requirements for the results and the rate of drift of the electrode system. If only an ‘order of magnitude’ measurement is required then it may only be necessary to calibrate once a day or even less frequently. Apart from drift, any large temperature changes (greater than 2°C) will cause a change in the calibration. For the most precise results, it may even be beneficial to calibrate between every sample.

These figures are given in the specifications for each ISE.

In general it can be noted that most ISEs cannot be used for concentrations below about 0.1 ppm and many will only give a linear response and reasonable precision above 1 or 2 ppm. The upper concentration limit is usually quoted as 1Molar, but in practice it is difficult to obtain reliable results above about 0.5Molar because of uncertainties in the effect of high ionic strength on the activity coefficient.

In general, for overnight or longer storage, ISEs should always be rinsed with de-ionised water and gently dabbed dry with a low-lint tissue, and the black plastic cap should be replaced to protect the membrane from atmospheric oxidation/corrosion.

Reference electrodes must be prevented from desiccation whenever they are not in use by replacing the protective cap containing a small quantity of the appropriate outer filling solution.

However, if the electrodes are being used on a daily basis, with several analytical sessions each day then it is not necessary to replace the protective caps as long as they are left hanging in the electrode holder and protected from physical damage. In this situation it is probably most convenient to keep both the electrodes immersed in a standard solution which lies within the concentration range of the samples being analysed when not in use.

BUT NOTE that no electrode should be left soaking in pure water for more than a few minutes at a time.

ELIT Ion Selective Electrodes are exceedingly robust and durable. The shelf life of a new unused solid-state ISE is many years - providing that the protective cap is kept in place to limit any atmospheric corrosion. They are guaranteed for six months during normal operation, but in practice have been found to last much longer than this. In the Nico2000 test laboratory some electrodes have been used intermittently for several years with no deterioration in their performance.

The gel-filled reference electrodes, however, cannot be expected to last as long. But with careful use and attention to keeping the frit moistened with filling solution when not in use, some of these have also given useful service for several years.

ISE operation is based on ion-exchange / charge-transfer principles. Thus there is hardly any consumption of materials or wear and tear on components when samples are analysed. Assuming that the electrode is not exposed to damaging chemicals, the only deterioration is due to the gradual corrosion (oxidation, hydration, leaching) of the membrane, the plastic body, and the adhesive used to attach the membrane to the body (or to physical damage to the membrane surface or to the pin connecting to the electrode head). The extent of the corrosion is affected by the length of time the electrode is immersed or exposed to the atmosphere rather than the number of samples analysed.

No, this is not necessarily a problem. The measured potential difference is the difference between the membrane potential and sum of the reference electrode potential plus any other potential differences that may be generated at any or all of the metal-liquid or liquid-liquid junctions in the circuit. It is this value which is seen when the electrodes are immersed in pure water or any other solution which does not contain the target ion. This explains why the measured voltage is not expected to be zero when no target ion is present and also why it is not necessarily always positive when the target ion is present - it all depends on the difference between the ISE membrane voltage and the sum of all the other voltages in the circuit. For example, for a monovalent positive ion, the voltage could be -25 mV in 10ppm and +30mV in 100ppm (or even -60 mV in 10ppm and -5mV in 100ppm) but this still gives a slope of +55mV per decade of concentration and indicates that the ISE is functioning correctly. Reversing the charges above would describe the situation for a monovalent negative ion.

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CCR/07/08/03 - Last Update 22 Sept. 2008