On May 12, 2009, at 1:10 AM, Jonathan Gregory wrote:
Dear Roy
I agree with what you say. But John and his colleague tell us that
it is not possible simply to describe the difference between pH
total scale and sea-water scale. Like you, I suggested that they
could be described as being just [H+], or the sum of [H+] and other
ions, but that's not right, apparently (and I am
certainly not an expert). John says
"There are several systems for measuring pH in sea water, and this
one [total scale] best takes into account the chemistry and other
interactions at the depths we make these observations."
Sorry, my terminology again was ambiguous. When I wrote "there are
several systems for measuring pH in sea water", a more precise
phrasing would have been "there are several systems of sea water
chemistry (or its analogs) within which pH is measured or computed."
I would say it is the chemical systems that vary, not merely the
measurement technique.
On May 11, 2009, at 10:29 AM, Jonathan Gregory wrote:
If the difference is so detailed and hard to summarise, it seems to
me that it really is a matter of
technique, and not of different geophysical quantities. I would
expect it to be possible to say, in a phrase, what the *essence* is
of the difference between geophysical quantities, whereas that
cannot necessarily be done for methods.
Can your "expectation of elegance" be justified by some analysis? I
suspect that in the case of complex systems, reduction of differences
to a simple essence is not always feasible -- that different
geophysical quantities may be different in complex ways.
In such cases, possibly the difference may be captured by a phrase
that appears to be a 'specialist term', if only because the system's
complexity is not yet broadly appreciated. In our particular case,
our offered phrases may be more specialist terms than reflective of
the essence of the difference, but the latter quality also seems
present to a degree.
On May 12, 2009, at 1:10 AM, Jonathan Gregory wrote:
Their opinion is that pH is always measuring the same geophysical
quantity, but that it is hard to measure. In turn, that might be
because it is hard to define just what pH really is; I have to admit
that it is not clear to me from what I have read. If you know others
who could help us with this, that would
be great.
My non-expert impression is that the notion of pH is fairly well
defined, but that different chemical systems will produce different pH
measurements or calculated values, through no failure of the core
definition of pH. It is the need to know which system was in play when
the measurement/calculation was made -- in order to properly interpret
the values that are given -- that drives the naming.
In short, I concur with Roy's opinion that the quantities are
different, and his analogies seem to the point. Apologies if my
previous wording led you astray.
John
--------------
John Graybeal <mailto:grayb...@mbari.org> -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Interoperability Project: http://marinemetadata.org
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