How important is the constancy of ionic strength in measuring potassium by an ion selective electrode? In what cases that the ionic strength can be allowed to fluctuate? What is the chemical formula of common ion strength buffer solutions?
ISEs respond to activity of the ion. Activity of the ion is moderated by its concentration and the ionic strength of the solution. For the same concentration the activity will decrease with an increase in the ionic strength. So it depends on what you are measuring K in. If you can control the ionic strength of your samples then you can calibrate the ISEs for concentration at the same ionic strength. If not then calibrate the ISEs against activity and measure K activity. Activity may be more important in biological systems than concentration anyway.
Rory gave you a great answer. I will try to simplify it a little bit or rephrase it. You are basically asking "when I do my electrochemical measurements does my electrolyte concentration matters ?" The answer is yes. I am assuming the K concentration is lower then your electrolyte (about 0.1 M usually) so by changing the [electrolyte] you effect not only your diffusion ability but your non faradic currents and the entire measurement.
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