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Topic: pH of the sample increases continuously  (Read 2581 times)

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Offline stdacet

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pH of the sample increases continuously
« on: June 09, 2016, 11:34:52 AM »
I am facing a few problems when taking pH measurements.
When measuring pH of a sample like water samples with pH meter, the reading does not stabilize and goes on increasing. If the sensor probe or sample vial is shaken to homogenize the sample, the pH reading increases rapidly say from 7.5 to 7.8, and then continues to increase swiftly. What should I do to get an actual pH reading of the sample.

Secondly, if the pH meter with automatic temperature compensation is calibrated with the buffers at temperature 25 degree C, then even if the sample is at  any temperature like 20 degree C, the meter will actually display the pH of the sample at calibration temperature i.e. 25 degree C, is it right?

Any help in this regard will be highly appreciated.
thanks.

Offline Arkcon

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Re: pH of the sample increases continuously
« Reply #1 on: June 09, 2016, 09:05:46 PM »
Well the second part is easier to answer:

Quote
Secondly, if the pH meter with automatic temperature compensation is calibrated with the buffers at temperature 25 degree C, then even if the sample is at  any temperature like 20 degree C, the meter will actually display the pH of the sample at calibration temperature i.e. 25 degree C, is it right?

Yup.  That's exactly what the temperature compensation is for.


I am facing a few problems when taking pH measurements.
When measuring pH of a sample like water samples with pH meter, the reading does not stabilize and goes on increasing. If the sensor probe or sample vial is shaken to homogenize the sample, the pH reading increases rapidly say from 7.5 to 7.8, and then continues to increase swiftly. What should I do to get an actual pH reading of the sample.


Any help in this regard will be highly appreciated.
thanks.

Borek's webpage probably goes into much better detail, however, you have to realize that a pH electrode is only good for measuring the pH a buffer, or a dilute weak acid or base, or a very dilute strong acid or base (and the last one can get dubious rapidly.)  Plain water simply doesn't have the conductivity to make a useful reading.  You can check this yourself, your samples are likely not reproducible -- you won't get the same reading later in the day from the same sample.

The longer you have to leave the electrode in the water, the worse your reading, and the worse the next reading will be.  You're depleting the electrolyte through the junction.  This happens whenever you leave the electrode in solution too long.  You should probably change the filling solution and soak it in storage solution.

Also, your results are odd to me.  Reasonably pure water isn't typically pH 7.0, usually 6.6-6.8 because of dissolved CO2. When someone say water's pH is 7, I know they've only read about it, and have never done it.

The classical way to take a pH electrode reading of water is to a a neutral salt, pure NaCl is the cheapest best one, to give the water some conductivity.  IIRC, something like 0.1 M NaCl should do the trick.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline stdacet

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Re: pH of the sample increases continuously
« Reply #2 on: June 10, 2016, 03:25:08 AM »
The water sample is actually the river water, not simple potable water, that's why pH is more than 7.

Offline Arkcon

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Re: pH of the sample increases continuously
« Reply #3 on: June 10, 2016, 05:26:30 AM »
Ah.  Buffered nicely in the alkaline region.  Ok, the main problem is a lack of conductivity in the sample.  Add chemically pure neutral salt and see if reproduciblity improves.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline stdacet

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Re: pH of the sample increases continuously
« Reply #4 on: June 11, 2016, 02:04:01 PM »
Ok, So the pH does not stabilize due to lack of conductivity.

thanks for this suggestion.

With regard to automatic temperature compensation, there is still some confusion.

 A pH meter with automatic temperature compensation (ATC) is calibrated with buffers at 25 degree C. The sample temperature is 20 degree C. Which of the following two statements is correct.??

1. The pH meter will correct the temperature error in sample pH value, and displayed pH reading will be the pH of the sample at 20 degree C corrected with reference to the calibration temperature i.e. 25 degree C.

2. The pH meter will compensate the temperature difference and the displayed pH reading will be the corrected pH of the sample at 25 degree C.

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