Top tips for ion selective electrode (ISE) calibrations and measurements

An ISE calibration requires elimination of any contamination and good laboratory practice. See below our top tips on ISE calibration:

• Keep fingers off the inside of your glassware! It is very important that the glassware you use in analytically clean, even if they have been through an autoclave, if any finger prints are on the inside the glass this will contaminate your readings – each finger could add 5ppm of sodium for example.
• Always use deionised water to make up your dilutions. A common mistake is when people use standard tap water to make up their standard dilutions – this has all sorts of contaminants which can significantly affect your measurements.
• Rinse your measuring cylinder out with the standard you’re using next – if you have just measured out your 1000 ppm standard and are about to measure out your 100ppm standard, use your 100ppm standard to rinse out any drips of your 1000ppm standard remaining in your cylinder – don’t cross contaminate
• Label up your beakers. Simple laboratory practice but don’t get confused or mix up your dilutions – clearly label your 2/3 dilutions with their concentrations and your sample. 
• Add the same amount of ISAB to your sample as you did to your standard dilutions. Ionic strength adjustment buffers (ISABs) are used to ensure there is an equal chance of the ion your are measuring touching your electrode regardless of the ionic strength of the standards/sample. 
• The volume of ISAB added is different for specific ions. Different ions require different amounts of ISAB added, for example for fluoride, 1ml of ISAB to 50ml sample would be used, for other ions you may need 2ml or 5ml.