Before calibrating sensors

- What is ISAB and when is it used?

Ionic Strength Adjustment Buffer is added equally to samples and standards to minimize any errors due to differences in ionic strength between samples and standards which will cause differences in activity coefficients which can cause the concentration to be under estimated by up to 50 or 60% in the worst cases. In some cases ISABs can also include ingredients which minimize 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 stabilization time) and may not be necessary at higher IS, depending on precision requirements.

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 analyzed by the Standard Addition or Sample Addition method.

Note: Libelium does not provide ISA Buffers, but we recommend the use of them when more precision is required.

- 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 to be about 3.4 % per 10°C, ie: if the slope is about 55 mV/dec then a 10 °C rise will increase this to about 57 mV/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.

- How frequently should I re-calibrate the electrodes?

This depends on the precision requirements for the results and the rate of drift of the electrode system. Apart from drift, any large temperature changes (greater than 2 °C) will cause a change in the calibration. Ideally, the temperature of the calibrating solution and any sample solutions should not differ by more than about ±1 °C. For the most precise results, it may even be beneficial to calibrate between every sample.

- After immersing the electrodes, how long should I wait before taking a reading?

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 stabilization or not.

- What is the concentration range and detection limit for an ISE measurement?

The total measuring range for each ISE is given in the individual specification sheets where the lower figure is the detection limit - but note that these limits are necessarily only rough estimates of what is possible since the errors escalate dramatically in the non-linear range as the slope reduces and the detection limit is approached. The best precision can only be achieved in the linear range of the electrode.

The lower limit of linearity is also difficult to define precisely and will vary slightly depending on the individual ISE/Reference electrode combination and particular laboratory conditions. In some cases it can be an order of magnitude higher than the detection limit. If users wish to analyze samples near to this value then it is necessary to make measurements with their own set of electrodes to determine what is the lowest limit for acceptable results in their own particular application.

The upper concentration limit is often quoted as 1 Molar, but in practice it is difficult to obtain reliable results above about 0.1 Molar because of uncertainties in the effect of high ionic strength on the activity coefficient.