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pH Normally Distributed?

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  • This topic has 4 replies, 3 voices, and was last updated 12 years ago by Lee.
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  • #52668

    Lee
    Participant

    I, getting into a project, and am not sure of what to expect for the data distribution. 
    We mix chemicals (about 100+), and when done test the pH of the mixture.  Refresher: pH is a measure, on a log scale, of the hydrogen ion concentration.  The scale runs from 0 to 14 (i.e., is bounded).
    For the actual measurements, should I expect they would be normally distributed, some other distribution?  Bottom line is that the measurements are what they are, but is there any fondamental reason to expect a particular distribution?

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    #185542

    BTDT
    Participant

    Eugene:The shape of the pH titration curve for your final mixture will depend very much on the buffering properties of the mixture near its final composition. Ask the chemists.Cheers, Alastair

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    #185544

    Lee
    Participant

    Thanks, I’ll seek out a chemist.  I had thought that for a given mixture that the distribution would be more dependent on the pH meter response characteristics and the stability of the mixing process — and somehow recognize that the meter readings are based on a log scale but that the process variations are (presumably) something other than a log scale.

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    #185555

    Eric Maass
    Participant

    Eugene,BTDT gave you good advice, but let me expand on it
    to help you when you meet with the chemist.If your mixture contains a buffer,then the
    dependence of the pH on the amount of chemicals like
    acids or bases in the mixture will be very non-
    linear. Please visit the url to see the graph below.
    http://www.files.chem.vt.edu/chem-
    ed/titration/graphics/titration-strong-acid-35ml.gifYou might be able to model the distribution of pH
    using Monte Carlo simulation, if you get the
    titration graph as BTDT described from the chemist
    (perhaps similar to the graph in the url above), and
    if you make some assumptions about the distributions
    of the quantity and concentrations of the
    ingredients to the mixture. Best regards,
    Eric

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    #185575

    Lee
    Participant

    Thanks, that also explains why the measured pH for some mixtures have a standard deviation that is a lot wider than for other mixtures.

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