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Re: GEOSTATS: interpolating pH values

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  • Rane Curl
    ... I am responding to this as a chemist, and not as expert in kriging. I am also sure that others have dealt with pH values in this context in one way or
    Message 1 of 6 , Feb 5 11:34 PM
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      On Fri, 6 Feb 1998, Inakwu Ominyi Odeh wrote:

      > The transformation bias of pH values as negative log of H+ concentrations being
      > referred to here is important iff the pH range within the immediate domain used
      > for kriging is large. If the range is within a unity, then there is probably
      > not much to be gained by transforming back to H+ concentration before kriging.
      > If, however, the pH range is 2 or more, then theoretically, additivity bias may
      > be introduced as a result of any quantitative interpolation. One should judge
      > each case carefully before embarking on kriging of pH values or their H+ conc.
      > But then other problems may arise as the H+ concentration (in soil) is
      > generally very low and may vary (spatially) exponentially.

      I am responding to this as a chemist, and not as expert in kriging. I am
      also sure that others have dealt with pH values in this context in one way
      or another, but I would raise questions about the procedures being
      recommended and the theoretical foundation for them.

      While [H+] is "additive" as a strong acid with nothing else present, in an
      real environment there are many other solutes present as well as reactive
      minerals. Natural solutions are usually buffered, so mixing two solutions
      of different pH does *not* average the [H+]. I would suggest that the
      objective be looked at more deeply and a transformation of pH for the
      purposes of kriging be chosen on a more realistic basis than additivity of
      [H+].

      There may even be justification for using pH. After all, the *solutions*
      do not get mixed when kriging - just the numbers. pH does represent a real
      thermodynamic quantity, which is the Gibbs Free Energy of formation of
      [H+] in solution and, under some conditions, free energies are additive.
      Other reactions also respond in a multiplicative manner to [H+] (via
      mass-action equilibria), so the pA of (the activities) of those other
      species respond in a generally additive manner to changes in pH.

      I presume that someone, somewhere, has done a much more thorough
      investigation of the meaning of kriging in pH, but I would not dismiss it
      out of hand unless some more fundamental objections can be put forward
      that the "additivity of [H+]" (sometimes, but rarely).

      --Rane L. Curl
      Prof. of Chemical Engineering


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