Beginners Guide to ISE Measurement, Chapter 9.

MEASURING PROCEDURES

As noted above (Chap. 6) ISAB (Ionic Strength Adjustment Buffer) is normally added to samples and standards in order to ensure that all measured solutions have the same Ionic Strength and avoid errors due to differences between the measured activity and the actual concentration. Indeed, many ISE manufacturers advocate always adding ISAB to samples and standards, irrespective of whether the samples have high ionic strength or not, because this can help to stabilise the liquid junction potential of the reference electrode and hence reduce errors in measurement, and reduce the time taken to reach a stable reading - but this may not be necessary if the reference electrode has an "equi-transferrent" filling solution (i.e. both ions have the same, or nearly the same, mobility - e.g. Lithium Acetate, or Potassium Nitrate).
Nevertheless, it must be noted that most electrode systems will give faster stabilisation with ISAB when measuring low concentration samples in the non-linear range.

For many applications, however, it may be unnecessary (for samples with IS below 0.01M for monovalent ions and 0.001M for divalent ions), or ineffective (for samples with IS greater than about 0.1M), or inconvenient, to add ISAB to all the samples and standards.

One simple way to avoid adding ISAB is to dilute the samples to a level where the activity effect is insignificant. But this requires a knowledge of the Ionic Strength of the samples, and care must be taken to avoid diluting so much that the measurements would fall within the non-linear range of the electrode. In some applications, where only the approximate concentration of the samples are required, or the differences between samples are more important than the actual concentrations, the effect of the ionic strength can often be ignored. Alternatively, if the highest possible precision and accuracy is required then using the Sample Addition or Standard Addition methods may be a better solution than adding ISAB.

If it is decided that ISAB should be added then the most important factor is that it should be added equally to standards and samples. Thus if the ISE instructions say, for example, that ISAB should be added "2%v/v" it must be recognised that this is only an approximate recommendation, and it will be more convenient to add 2ml of ISAB to 100ml of sample and standard (or 1ml to 50ml) rather than adding 2ml to 98ml.

There are several schools of thought as to the best way to take measurements. Some authorities suggest that the solutions should be stirred slowly with a magnetic stirrer at 50 - 100 rpm during immersion of the electrodes - but care must be taken to ensure that there is no heat exchange between the stirrer and the solution and that the stirrer is always at the same speed. Big differences in mV readings can occur if the speed is varied. Moreover, the magnitude of this effect is related to concentration (most variation in dilute solutions) and hence will affect the slope of the calibration graph. Some prefer to take a reading whilst stirring, others suggest that it is better to switch off the stirrer and take a reading in a still solution - but this can greatly increase the time for the measurement because it may take several minutes for the mV reading to stabilise whilst stirring, and several more to stabilise after the stirrer is switched off. Alternatively, and more easily, the solution can simply be swirled manually after immersion of the electrodes (to ensure good, homogeneous contact between solution and membrane- i.e. no air bubbles) and then left to stand. This avoids the problems of heat transfer and the inconvenience of adding and removing the magnetic stirrer every time. It must be noted however that some electrode systems stabilise more quickly and give more reproducible measurements when stirred whilst others give better results in still solutions - this is sometimes mentioned in the electrode operating instructions.

The time at which the measurements are taken can also vary depending on the characteristics of the particular type of electrode system being used and the balance between time constraints and precision requirements. In some cases it is best to wait for a stable mV reading (this can take several minutes). In others it is better to take all readings after a pre-specified time after immersion. Generally, the mV reading changes rapidly in the first 10 or 20 seconds as the ISE membrane equilibrates with the solution, then more slowly and exponentially as the reference electrode liquid junction poential stabilises. Always taking a reading after say 1 or 2 minutes (depending on which electrode system is being used) should ensure that all are taken in the shallow part of the stabilisation curve where only small and insignificant changes are occuring. A third alternative is to observe the drift in reading as the electrodes equilibrate after immersion and then take a reading at the point where the direction of drift is definitely reversed - i.e. a different electrochemical process begins to dominate - but this last effect is not common.

If the highest possible precision is required then it is suggested that each operator should make his own simple experiments by comparing the reproducibility of repeated measurements on the same sample using each method discussed above, in order to determine which is most suitable for his particular application. But note that whichever method is preferred, the same procedure must be used for all standards and samples in order to ensure that the electrode response will be as uniform as possible.

The electrode tips must be rinsed by spraying with a jet of deionised water and gently dabbed dry with a low-lint laboratory tissue between measurements. For the most precise results, it may help to minimise hysteresis effects if the electrodes are soaked in deionised water for 20 or 30 seconds after rinsing, before every measurement, so that each new reading is approached from the same direction, (i.e. always from low concentration to high - as recommended for calibration measurements, to also minimise cross contamination) rather than just being dependent on the last sample measured.