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  1. #1
    Senior Member Snicklefritz's Avatar
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    any Soil science people here?

    Hey is anyone out there into soil science or environmental chemistry? I'm helping someone
    with some soil science homework but some of the topics are outside my area of expertise.
    It has to do with calculating and deriving Eh as a function of PH. Send me a PM if you've done this stuff.

  2. #2
    Senior Member randya's Avatar
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    you generally plot Eh vs. pH to determine the stability fields for various ionic compounds of elements like calcium, iron, etc. in the natural environment. I'm not sure you can actually calculate Eh from pH though, they are usually both field-measured parameters, at least when collecting groundwater quality samples.

  3. #3
    Senior Member Snicklefritz's Avatar
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    yeah, that makes sense. I'm actually helping my brother-in-law with a graduate level course in soil science and most of the stuff I have some idea about even though soil science is not my area, but the Eh and pH thing was kind of weird as you said.

    the problem goes something like this:

    Fe(OH)3(s) = Fe3+ + 3OH- log K = -39.1

    The teacher then asks them to reexpress the equation for Eh so that it is explicitly defined in terms of pH

    Eh = Eho - (0.059/n)log(Fe2+/Fe3+)

    I never asked any of my students to do a calculation like this and I don't see it in any of my textbooks either. The prof then wants people to calculate Eh for the half-cell in the first equation as a function of pH. Half cell calculations, no problem, but I just wondered about the pH part.

  4. #4
    Senior Member randya's Avatar
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    well, I suppose there is a mathematical relationship between Eh and pH for any point on an Eh-Ph plot, but I couldn't tell you much about how that would be defined or expressed mathematically. I do know that pH is the negative log of the H+ ion concentration, which explains the log part of your equation...


  5. #5
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    Hmmm, let's see howmuch I remember (it's been a loooong time):

    So,the problem is that your equation

    Eh = Eho - (0.059/n)log(Fe2+/Fe3+)

    Need to be a function of pH? Well, isn't the ratio between Fe2+ and Fe3+ dependent on pH?
    By looking at how the ratio Fe2+/Fe3+ behaves under different pH's, you could subsitute that part with a function of pH.

    But then again, I do much larger scale soil science (well, geo-hydrology these days) and the chemistry is nothing but a painfull memory.......(I hated it with a passion)

  6. #6
    Still can't climb
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    i thought foo was just for silly posts. This thread hurts my brain, but that doesn't take much these days.

  7. #7
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    Hey, did you check out this wiki page?

    http://en.wikipedia.org/wiki/Eh_(chemistry)

    It talks about this, and includes an equation very similar to yours called the Nernst equation.

    PS.If I making yself look like an idiot....I am a 'fallen' soil scientist, I do mostly statistics these days, and am loving it

  8. #8
    Senior Member randya's Avatar
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    There's a bunch more info if you google 'Nernst Equation'

    http://www.google.com/search?client=...UTF-8&oe=UTF-8

  9. #9
    Senior Member Snicklefritz's Avatar
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    yeah Tinuz, that's it. I wasn't sure how to incorporate H+ into the equation.

    the "Foo" forum is awesome!!!



    thanks guys!!!!
    Last edited by Snicklefritz; 02-08-09 at 02:14 PM.

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