On May 6, 2009, at 5:37 AM, Jonathan Gregory wrote:
Dear John
He believes other terminology will be opaque
and ambiguous, and therefore essentially useless.
Perhaps you could ask him, not what he calls
the quantity normally, but how would he answer, in one sentence, the
question,
"What does that mean?", if he was giving a seminar in which he
mentioned the
concept of pH total scale to a scientifically literature audience
who were not
ocean chemists. Such people could be expected to know that pH measures
hydrogen ion concencentration in some way, but not any more details.
Surely it
must be possible to say, in a phrase, what "total scale" indicates
sufficiently
to get across the point of it, if not the details?
Jonathan, all,
Yes, I have asked that question. Short answer: No, I don't think it
is possible to say what this means in the type of phrase you are
looking for. Hence my conclusion that we may be stuck.
Having spent the last two weeks learning 'what does that mean' --
well, as much as high school chemistry allows -- I will give you his
answer, then mine. It may help the list to know that most folks don't
measure it directly (what we do at MBARI), especially in the deep
ocean. Instead, pH is more commonly determined as the solution to an
equation with several other measured components, and it appears to be
the entire interaction of that chemical system that is referenced by
the term, not simply a characterization of the single measurement.
(The other chemical parts of the system determine key equilibrium
constants that help specify the resulting interactions and the
conversions between scales.) That may be a reason it resists summary.
His answer: "There are several systems for measuring pH in sea water,
and this one best takes into account the chemistry and other
interactions at the depths we make these observations." (Yes, I know
that is not sufficiently descriptive for your purposes.) Any attempt
to get a more concrete summary immediately devolves into detailed
explanations of chemistry; our previous attempts at summary concepts
appeared to be unsatisfactory on their face. We have since asked 2 of
the leading experts in the field, with nearly identical results. (They
offered a second reference[1] in support, and another set of
explanatory detail, below -- note its final summary.) If there were
any readily acceptable answer I think we would have found it.
My answer, attempting to be more concrete: "Ocean chemists have
developed several regimes in which they perform their observations and
state their results; this regime best derives the activity of hydrate
ions taking into account temperature, pressure, high salinity
(especially), and a wide range of pressures in situ." Any more precise
summary of the context would require at least a few more sentences. To
synthesize all those nouns into a meaningful and sufficiently specific
phrase really would give you a Standard Paragraph.
So. There we are. Sorry I can not be more obliging, I really have
tried but this one defies easy answers.
The good news is, I will be off the list for the next week while on
vacation. So you have plenty of time to consider or ignore these
matters. :->
Thanks for all your help considering this matter.
John
[1] Pure Appl. Chem., Vol. 74, No. 11, pp. 2169–2200, 2002.
INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY -- MEASUREMENT OF
pH. DEFINITION, STANDARDS, AND PROCEDURES (IUPAC Recommendations 2002)
================== Expert Recommendation on Naming ================
The definitions are:
(A) pH(total) = - log ([H+]total / k0) = - log ( [H+]free (1 +
ST/K(HSO4-)) / k0)
(B) pH(sws) = - log ([H+]sws / k0) = - log ( [H+]free (1 + ST /
K(HSO4-) + FT/K(HF)) / k0)
(C) pH(IUPAC) = operational pH calibrated using IUPAC primary
standard buffers
NOTES:
1. Any measured pH value needs to also identify the temperature (&
pressure) at which it was measured.
2. Although pH is a dimensionless quantity, the concentrations in (A)
and (B) are specified in moles per kilogram of solution. That is k0
has the value of 1 mol/kg-soln.
For (C), conventionally 10^-pH is thought of as a single ion activity
for hydrogen ion with associated concentration units of mol/kg-H2O,
though this is essentially meaningless for sea water.
3. ST = total concentration of sulfate ion in seawater - f(salinity)
FT = total concentration of fluoride ion in seawater - f(salinity)
K(HSO4) = dissociation constant for HSO4 in seawater - free H+
scale; f(salinity, temperature)
K(HF) = dissociation constant for HF in seawater - free H+ scale;
f(salinity, temperature)
4. Values of pH(SWS) or pH (total) ultimately are not calculated from
the definitions (A) and (B) but are calibrated relative to hydrogen
ion concentration values assigned to Tris/Tris-H+ buffers in synthetic
seawater using a Harned cell. These buffers are then either used
directly to calibrate a pH probe, or are used to assign a dissociation
constant to a colorimetric indicator.
5. The reference for pH(IUPAC) is Buck et al. (2002) (attached).
Older data may have been calibrated using the same primary buffers
(originally developed at NBS in the late 1950s).
6. pH(IUPAC) implicitly includes a term (often of unknown magnitude)
that is dependent on the liquid junction design for the pH cell, and
is thus not recommended for precise work.
I am not sure you can convey all this in 8 words or less. My
preference is for definitions (A), (B) and (C), but if you are
bringing separate data sets together, the devil could be in the
details alluded to in the footnotes.
=============================
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