Recently several ion-selective electrodes, such as those for NH4, NO3, Cl, F. K, Ca, etc, are becoming available. If I understand it correctly, these electrodes measure ion activity, meaning ionic strength of the solution and ion pairing or complexing would effect the reading. Are these electrodes really measuring free ions? and how can we keep the ionic strength of the solution constant (easily and quickly), yet avoiding ion pairing or complexing?
Dear Nguyen Hue,
the most common way to maintain the ionic strength of a solution constant, when using ISEs, is to use a TISAB solution (Total Ionic Strength Adjustment Buffer)(see also http://en.wikipedia.org/wiki/Total_ionic_strength_adjustment_buffer). Usually, every manufacturer constructing ISEs provides the corresponding TISAB solution.
Best regards
Dear Mr Hue, completing the best answers above, you can also use biological buffers for adjusting pH and ionic strength such as PIPES (http://www.sigmaaldrich.com/catalog/search?interface=All&term=PIPES&N=0&mode=match%20partialmax&focus=product&lang=pt®ion=BR) or TRIZMA (http://www.sigmaaldrich.com/catalog/search?interface=All&term=TRIZMA&N=0&mode=match%20partialmax&focus=product&lang=pt®ion=BR). These buffers are generally used when you wants to measure biological compounds using biosensors (example: urea, glucose, creatinine, etc). These ISEs has enzyme immobilized on surface of a primary ISE such as glass or ammonium electrodes. The enzymatic degradation produces an ion, hydrogen cation or ammonium for example, that are detected by the primary electrode. In this case TISAB cannot be adequate, because it has many ions in your composition that may be interferents. I hope that this answer can also help you.
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