Solution for storing a glass electrode

Question

What kind of storage solution is required for a glass electrode?

Is 3 M $\ce{KCl}$ sufficient for this purpose? If so, what is the approximate pH of that solution (the solution I use presently has a pH $\approx$ 5)?

Can filling solutions be used as storage solutions for glass electrodes?

Answer 1

A glass electrode is usually stored in a 3 or 4M KCl solution, depending on whatever the manufacturer advices you to do.

But the pH that you measure from this solution is absolutely not the pH. It's the measured potential of this concentrated KCl solution that is (in this case wrongly) converted into the pH scale.

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The normal pH measurements with glass electrodes cannot be done without the calibration of your device through references solutions, which are common buffer solutions with acidic, neutral and basic pH values like, e.g., pH 4, 7, 9.

In order to calibrate the pH electrode, you or actually your device saves the measured potential for each of these known buffer solutions and calculates a regression function that should give you something near: $$E = 414.12 - 59.16~pH~~~~~(\text{at}~25~^\circ C)$$

(The time when I had to do this in practical courses gave us $E = 401.93 - 56.711~pH$ ... I have no idea what the temperature was :D )

For further informations on all this you can read Mettler-Toledo's pH guide.

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Now, as your KCl solution has a certain potential the device converts it through this regression equation into a pH value. But as I said, that is wrong - I guess that pH indicators like the universal indicator would show no big difference from pH 7 (I've never done this actually, so this is seriously a guess).

Your actual question should rather be about what the potential of a 3M or 4M KCl solution should be. And that, as can be found here, is:

Answer 2

Most pH electrode storage solutions are 4 M KCl with a pH about 4. The solution you have should be fine if you can change the internal solution on a monthly basis (more often if you use it continuously).

The internal solution is also saturated with silver chloride. I would only use it for the external storage solution if I couldn't make the external solution, but it should not be a problem for a short term.