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Author Topic: Standard solution for titration via ph meter?  (Read 831 times)

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pcm81

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Standard solution for titration via ph meter?
« on: June 12, 2018, 02:03:12 PM »

I realize that oxalic acid is used to create standard solutions for titrations, because is does not form hydrides and so it can be reliably weighed out unlike substances like NaOH. My question is if i can use a reasonably expansive 0.01 resolution pH meter ($130 Extech PH220-C, well calibrated against real standard solutions ) to achieve the same goal?
I used a 0.01g scale to create a 0.1M solution of NaOH. When diluted 99:1 with di water (so its now a 0.001M solution) i get exactly 11 on pH meter as expected.
If i want to use strong base and at some point strong acid for volumetric titrations,  can I "standardize" them based on pH alone or do i have to titrate vs oxalic acid? I realize this wont work with weak acids/bases, so the question is limited to strong acids and bases.

Thanks
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Borek

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Re: Standard solution for titration via ph meter?
« Reply #1 on: June 12, 2018, 08:52:30 PM »

If you mean "measuring pH to directly determine concentration of the substance" the answer is "no". Even assuming ideal behavior of the solution and no errors in measurements pH of 2.00 means anything between 1.995 and 2.00499, check how that translates to accuracy of the concentration and compare that to the accuracy you can get from the titration.

pH meter can be great in helping determining the equivalence point of the titration.
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pcm81

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Re: Standard solution for titration via ph meter?
« Reply #2 on: June 13, 2018, 11:31:19 AM »

If you mean "measuring pH to directly determine concentration of the substance" the answer is "no". Even assuming ideal behavior of the solution and no errors in measurements pH of 2.00 means anything between 1.995 and 2.00499, check how that translates to accuracy of the concentration and compare that to the accuracy you can get from the titration.

pH meter can be great in helping determining the equivalence point of the titration.

Thank you for the feedback.
It seems I have 3 options:
1. Buy a 99.99% pure, analytical grade, oxalic acid and titrate the NaOH solution to standardize it. This is the most expansive option.
2. Buy a 99.6% pure oxalic acid crystals and try  to recrystallize it to improve the purity; then weigh in some concentration of it and and titrate NaOH.
3. Recrystallize and fully hydrate the NaOH and weigh in the concentration i want directly, accounting for weight of the water of hydrate.

I am kind of leaning towards option #2. what do you think?

Goal of all this is to get a 0.1M NaOH to measure total acid and free acid of home made parkerizing solution. While also learning the "legit", lab grade, ways of doing it.
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Borek

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Re: Standard solution for titration via ph meter?
« Reply #3 on: June 13, 2018, 11:50:56 AM »

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pcm81

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Re: Standard solution for titration via ph meter?
« Reply #4 on: June 15, 2018, 01:08:48 PM »

Thanks. Titrated my 3 day old 0.1M NaOH  mixture with KHP (shipped in today) and determined that it was 0.061M... Ph also dropped as expected...
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