Selective elctrodes. A little theory
Contents
1. Everyone knows what temperature does to measurements of potential The behaviour of selective electrodes with variations in temperature is not as well known as that of OH electrodes which is why we do not refer to compensa tion for temperature in measurements with ISE s Laboratory Catalogue 2004 1st Edition 55 The temperature must remain constant during analysis with selective electrodes both during calibration with standards as during sample measurements lonic strength The potential of selective electrodes responds to ion activity not concentration For ion activity and concentration to be similar an ionic strength adjuster ISA is added both to the standards and to the samples An lonic Strength Adjuster ISA is a solution with a high ionic strength which does not interfere with the sample and which equalizes the ionic strength of standards and samples In the user manual for each ion selective electrode the ionic strength adjuster solution required for each case is 1 S E given pH of the sample The pH of the sample should be on a certain scale in some cases because of H and OH interferences and in others because the membranes work properly in a certain pH area Electrode interferences If there are ions present which affect the selective electrode it will respond both to the ions in which the user Is interested and with those which interfere That is why the user must insure that there are no interfering2. ions prior to starting analysis In the manual for each selective electrode there is a table indicating the ions and level of interference they possess Interference with the method If the ion to be measured is not free i e is linked to other ions or adsorbed it cannot be measured In these cases the sample must be treated beforehand so that the ion to be measured is found in its free state Any CRISON ISE electrode will work with other brands of instruments if CRISON
3. lon selective electrodes A little theory These are electrodes with a sensitive membrane selective for a particular ion When the selective electrode is immersed in the sample a potential develops in the membrane caused by a selective and spontaneous reaction To measure this potential a reference electrode is required alongside the indicator selective electrode Variables to be considered when measuring with an ISE Measuring range This is the scale of concentration of the ion to be meas ured in which the electrode is sensitive to variations It can be divided into two areas Linear area Non linear area Working within the linear area is always preferred because the non linear area requires calibration using at least 5 standards and manual interpolation Typical curve for a selective electrode 0 01 0 1 1 10 100 1000 ppm Reference electrode The reference electrode provides a constant potential against which variations due to the indicator electrode can be measured It is very important to select an appropriate reference electrolyte for which the user should bear in mind The ionic strength of the electrolyte should be much higher than that of the sample It should be an equi transterrent solution The cation and anion speeds should match as far as possible It should not react with the sample It should not contaminate the sample It should never contain the ion to be measured Temperature
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