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Author: Subject: ascorbic acid and pH levels
kikoko39
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[*] posted on 8-4-2008 at 17:26
ascorbic acid and pH levels


I have a doubt, cause right now im in high school, IBO, so im doing a couple of experiments, and we did an experiment by taking the pH value of orange juice, and the oxidation of which, but i cant currently find any explanation for the fluctuations of the pH, when reaching 24 hours after the first pH test, the result was the highest and at 2 hours it is the lowest and the pH values just jump over the place.. So, i was wondering if anybody can help me with this explanation,
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ScienceGeek
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[*] posted on 9-4-2008 at 02:02


By saying the IBO, do you mean the International Baccalaureate?

It's kind of hard to understand your problem! Can you give a better description on what you mean by "fluctuations", and a more detailed procedure for the experiment?




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kikoko39
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[*] posted on 9-4-2008 at 15:33


yes, international bacculauerate organization, the fluctuation is that if i start with a sample with 3.5 pH, then every hour I check the pH, the values are 3.5,3.2,3.45,3.48,3.48,3.5,3.4,3.55,3.6,3.5,3.4,3.55,3.6,3.35,3.6,3.6,3.6,3.45,3.65,3.65,3.7,3.75,3.65,3.65
is it normal for it to jump around so much, and if there is any explanation for so much movement?

the experiment is using a pH meter, we take the pH every hour and the solution is orange juice, and it is at a constant 5°C.

[Edited on 9-4-2008 by kikoko39]
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Saerynide
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[*] posted on 9-4-2008 at 18:47


That seems like very little variation to me (~+/- 7% max). Highschool-grade pH meters are not the most accurate things, so I wouldnt expect them to read constant for 24 hours.

The pH reading will fluctuate when the fluid is moving because the pH meter relies on a difference in voltage. Moving fluid changes the conductivity and this will be reflected in the pH reading. Moving a fluid will not actually change the real pH.

Also, it's hard to really keep something at 5C, unless you have it in an electronic temperature controlled water bath (I wouldnt rely on the fridge to be constant 5C). So changes in temp will affect the real pH. It might be easier to do it at 0C, then you can use an icewater bath.

Why dont you do a control with plain distilled water or pH7 buffer soln (store bought) and see if the pH varies in 24 hours? Then you can know if its your pH meter.

[Edited on 4/9/2008 by Saerynide]




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[*] posted on 10-4-2008 at 00:55


I agree with Saerynide! pH meters at my school, at least, have a tendency of fluctuating at the 0,1 decimals.

Nice you attend the IB programme! I do to ;)




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[*] posted on 8-5-2008 at 14:04


I would not use distilled water as a control, due to the lack of any buffering capacity I would expect the fluctuations to be even greater.

It would probably be beneficial to standardize the pH meter every 3 hours or so, to avoid any potential problems associated with drift.

Over 24 hours at 5C I doubt there would be enough oxidation of ascorbic acid to dehydroascorbic acid that would yield an increase in pH. At room temperature perhaps.

Good Luck...

m_e




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