Sodium’s chemistry means that you cannot measure its concentration with a simple test kit or reagent and photometer combination like you would with many other water quality parameters. There are a number of available options (read more here), but using an ISE is the most commonly used within the water testing industry.
ISE measurements can be a tricky method to master and relies careful calibration and use to ensure your results are accurate.
When measuring sodium using an ISE, there is the added complication of sodium error in the electrode. The glass sodium ISE is basically like a pH electrode with a high sodium error but where the Na+ ions are replaced by H+ ions in the electrode. To combat this effect when measuring low levels of sodium, you will need to increase the pH of your sample to at least pH 12 or even pH 13. This works on the principle that if you eliminate the H+ then the electrode can detect the Na+. However, even if you are able to reduce your H+ to this level, calibration and staving off contamination can be a challenge. Sodium contamination is very common in lab chemicals such as deionised water and can also be present on glassware. Whilst ISE’s may claim to measure down to values as low as 0.002mg/L, even with good technique, around 0.01mg/L is as good as can be realistically achieved.
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