[CF-metadata] new standard name request for pH

From: Jonathan Gregory <j.m.gregory>
Date: Sat, 2 May 2009 13:53:34 +0100

Dear John

Thanks for your thoughtful email. I follow the argument but in the end I do
not (yet) agree with the conclusion, that sea_water_pH_total_scale should be a
standard name. I am sorry if you find that exasperating. As in previous such
discussions, it is not because I am suggesting that you or your pH expert are
mistaken about what terms are normally used and what they mean. My difficulty
is that "total scale" is not informative to a general scientific readership,
which CF standard names are designed to be.

I've had a look at Zeebe and Wolf-Gladrow. Thanks for that reference.
Wikipedia summarises similar ideas. I also looked at a IUPAC document on the
subject. It seems that problem of definition arises because pH is actually
impossible to measure, as defined! Hence the true value is somewhat unknowable.
These different scales represent different methods of measurement and
estimation. I think we could take two views about that:

* The different pH scales are actually trying to measure the same geophysical
quantity. In that case the standard name should just be sea_water_pH. Then the
scale is an aspect of the method of measurement, and should be recorded in
some other attribute. This is somewhat related to the other thread, about
quantities relating to the techniques of measurement.

* The different pH scales are different geophysical quantities. In that case
they need distinct standard names, and it ought to be possible to indicate
in some non-jargon way what the difference refers to, even though it couldn't
be fully described in the name, so we still need a definition. Following this
route, the total scale could be called sea_water_pH_including_bisulfate_ions,
for instance. (A side-issue is a question of whether to call them bisulfate
or the IUPAC name of hydrogen_sulfate.)

pH is a dimensionless quantity. It has no units. It's the logarithm of
a number which does have units, so the units of that quantity need to be
specified in the definition.

> Yet the second
> sentence of the following statement does not hold true for pH, given
> the depths at which these measurements are being made:
>
> On Apr 30, 2009, at 9:22 AM, Jonathan Gregory wrote:
> >Boussinesq models (most ocean climate models are Boussinesq) treat
> >density
> >as constant 1000 kg m-3 except in the computation of pressure
> >gradients, where
> >it matters to the dynamics. Therefore in dealing with concentrations
> >of
> >tracers, per kg and per litre are identical and to choose one or the
> >other
> >would be arbitrary and hence unhelpful for data exchange.

Actually this is a digression now, since all the quantities concerned are
logs of mol per kg. However, I maintain the above is a correct statement about
ocean *models*. I did not say it's a correct statement about nature! It is
not, as you point out. However, Boussinesq models are only approximate. In
such models, that distinction could not be made. No-one has yet requested
pH for an ocean model, but I expect it will come up sometime.

Best wishes

Jonathan
Received on Sat May 02 2009 - 06:53:34 BST