Beginners Guide to ISE Measurement. Chapter 8.

CALIBRATION PRACTICE

A single standard solution is usually diluted sequentially to produce concentrations of 1000 ppm, 100 ppm, 10 ppm, 1 ppm and even 0.1 ppm for a complete calibration (or this can be done using decades of Molarity units). However, if the approximate concentration range of the samples is known then the calibration should be made using standards which closely bracket this range. Large errors may occur if samples are measured by extrapolation beyond the range of the calibration. If you are confident that you are working in the linear range of the electrode then only a two point calibration is necessary. But three or more are recommended in order to confirm the linearity and to detect any errors in diluting the standards, or to define the curve in the non-linear range. Since the slope of the calibration line is temperature dependent, the temperature of the calibrating solutions and the samples should be the same within a tolerance of ±2°C.

If more than three standards are being measured, it may help to minimise potential drift and hysteresis during measurement, if the solutions are measured twice, the second time in the reverse order (i.e. 1, 2, 3, 4, .., etc. then .., 4, 3, 2, 1 - all at regularly spaced intervals) and the average mV value taken to construct the calibration graph. In this case, having defined the calibration slope accurately, it may only be necessary to recalibrate using only one representative standard, nearest to the range of the sample concentrations, and simply normalise the rest of the points to this new value. For example, if a subsequent reading for the recalibration standard gives a millivolt reading of 2 mV below the initial (average) value, then you can either subtract 2 mV from all the other initial calibration data and re-plot the graph or, probably more easily, add 2 mV to any sample readings taken immediately after the recalibration and then read the concentrations directly from the original graph.

Alternatively (or possibly additionally) it may help (particularly for the lowest concentration standard and during sample measurement) to always immerse the electrodes in pure water for a short time, then drying, before immersing in the next solution. However, it must be noted that prolonged immersion in pure water may damage the electrodes and they should only be immersed for as long as it takes for the voltage to fall below that of the lowest standard (or rise above for negative ions).

Despite these possible improvements to the conventional calibration method, the most effective way of reducing potential drift is to use the Standard Addition method to measure two standards (which bracket the known range of the samples) to find the average slope and then measure the samples by Standard or Sample Addition methods - see Chapter 10.

For the most precise results it is best to measure samples soon after calibration. Ideally each sample measurement should be immediately preceded by a calibration (see Chapter 11 for more details of how to achieve the most precise results). This is relatively quick and easy if only a one-point recalibration is employed as described above. Ultimately the operator must decide what is the best compromise between the time taken to make frequent calibrations and the precision requirements, but for many applications it will be sufficient to make just one calibration graph before taking a series of sample measurements over several hours.