Re: GEOSTATS: interpolating pH values
- On Fri, 6 Feb 1998, Inakwu Ominyi Odeh wrote:
> The transformation bias of pH values as negative log of H+ concentrations beingI am responding to this as a chemist, and not as expert in kriging. I am
> 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.
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
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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