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Topic: Why store pH electrode in KCL?  (Read 38053 times)

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Offline poobear

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Why store pH electrode in KCL?
« on: July 25, 2011, 12:39:32 PM »
Hi everyone,

I hope this is the correct place for this question.

Everyone seems to suggest to store pH electrodes in a KCL solution.

Why?

I can't figure out anything special with KCL, so I guess KCL is just a cheap way to get a pH that the electrode likes. But why does it like that pH, why do I see notes in capitals to never store it in pH 7 (water) ?

Thanks!

Offline fledarmus

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Re: Why store pH electrode in KCL?
« Reply #1 on: July 25, 2011, 02:43:49 PM »
This might help answer your question:

http://en.wikipedia.org/wiki/Glass_electrode#Construction

Note that the reference internal solution (6) is 0.1 M KCl, and that it is in contact through a porous plug (7) with the solution that you are measuring. The best way to prevent any possible change in that internal solution on storage by leakage through the porous plug is to ensure that the external solution is as close as possible in nature to the inside solution - i.e., 0.1 M KCl.

There are other types of electrodes with different internal standards which require different storage solutions, but this is standard for the usual pH glass electrode.

Offline zaphraud

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Re: Why store pH electrode in KCL?
« Reply #2 on: July 30, 2011, 01:45:35 PM »
yeah but the more obvious question is "what the hell is wrong with salt", and I'm pretty sure its the fact that iodized salt is ubiquitous and insidious and that the iodine might slowly damage the membrane.

Offline fledarmus

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Re: Why store pH electrode in KCL?
« Reply #3 on: August 01, 2011, 07:53:52 AM »
yeah but the more obvious question is "what the hell is wrong with salt", and I'm pretty sure its the fact that iodized salt is ubiquitous and insidious and that the iodine might slowly damage the membrane.

In a world where pH electrodes are being used, I doubt seriously that the salt is being bought at the grocery store. Sodium chloride and potassium chloride are available in similar purities at similar prices from most chemical supply houses, and enough 0.1 M KCl to store the electrode in is a trivial expense. No, the difference has to do with the difference in electrical properties between Na+ and K+ - if sodium ions are equilibrated with the potassium ions over a long period of time through your membrane, then the electrical properties of your electrode, and therefore the pH measurements that you will get when you use the electrode, will also drift over time. To preserve the consistency of your measurements, you want to make sure that the measurement conditions at different times are as close to identical as possible.

Offline poobear

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Re: Why store pH electrode in KCL?
« Reply #4 on: August 01, 2011, 08:16:29 AM »
This might help answer your question:

http://en.wikipedia.org/wiki/Glass_electrode#Construction

Note that the reference internal solution (6) is 0.1 M KCl, and that it is in contact through a porous plug (7) with the solution that you are measuring. The best way to prevent any possible change in that internal solution on storage by leakage through the porous plug is to ensure that the external solution is as close as possible in nature to the inside solution - i.e., 0.1 M KCl.

There are other types of electrodes with different internal standards which require different storage solutions, but this is standard for the usual pH glass electrode.

I looked at our solution and it is 3 M KCl. Does that mean that our internal solution is also 3 M KCl?

If not, how does that work? Over time the 0.1 M would become more and more increased.

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