Re: [CF-metadata] new standard name request for pH
Hi John,
Your suggestion that we disallow storage of NBS scale pH in CF by not creating
a Standard Name for it worries me. We have significant quantities of NBS-scale
pH in BODC (from estuarine systems and the days before ocean chemists
discovered the kilogram) that we may wish to put into CF in the future. Whilst
conversion based on approximation to free_scale is possible in some (but not
all) cases (need additional data), it's something I'd rather not do. I would
much rather label precisely and leave any manipulations up to the user who is
aware of the proposed usage and what is fit for that purpose.
My attraction to Jonathan's simple 'pH_in sea_water' is that it gives us a way
of handling data in CF where 0.1 pH units don't matter. We have one dataset
monitoring waters off a chrome plating plant where pH varies between 2 and 6.
At the same time I am acutely aware of the effects errors of that magnitude
have on deep ocean carbon budget calculations.
Cheers, Roy.
-Original Message-
From: cf-metadata-boun...@cgd.ucar.edu
[mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf Of John Graybeal
Sent: 02 May 2009 01:28
To: CF Metadata List; Lowry, Roy K
Subject: Re: [CF-metadata] new standard name request for pH
Hi everyone,
Thanks very much for considering and responding to the pH terms
proposal. To respond, I spent considerable time being educated by our
in-house pH expert, even reading pages myself from the referenced book
[1], and reviewed this email with him. So I claim this email
represents a high level of expertise in this scientific field. I
apologize for the length, but it seemed necessary to present
justifying details.
To recap, the original proposal as revised was for 4 names related to
pH:
A) sea_water_pH_NBS_scale (moles/liter)
B) sea_water_pH_free_scale (moles/kg)
C) sea_water_pH_total_scale (moles/kg)-- the one we care about
D) sea_water_pH_seawater_scale (moles/kg)
Through this thread we have had several suggestions for replacements
[5], most recently arriving at
a) pH_of_sea_water_defined_by_moles_of_hydrogen_ion_per_unit_mass
b) pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion
c)
mole_concentration_of_H_and_HSO4_per_unit_mass_in_sea_water_expressed_as_pH
d) mole_concentration_of_H_and_HSO4_and_HF_per_unit_mass
in_sea_water_expressed_as_pH
e) (added) pH_of_sea_water
I believe the focus of these responses was on the concentration of the
solution, but this mis-states the actual chemical definition of this
particular concept. (Unfortunately we incorrectly presented this in
the original definitions for B, C, and D.) In the reference[1] it is
made clear that:
(a) pH is strongly affected by abundance of hydrogen (actually
hydrate) ions, but is not defined by their quantity; it is determined
according to the *activity* of those ions, and this is not a linear
relationship (though it is close to 1 in some regimes)[2]. We will
change the proposed definitions to replace 'concentration' with
'activity'; my apologies for this error.
(b) The use of the NBS scale is not recommended for sea water[3], as
errors will be induced (up to at least 0.1 units in situ); in this
light we propose to simply withdraw [A] from consideration, especially
as we don't need it.
(c) There are significant differences in the values obtained for these
different quantitiies, up to .12 units; thus to say just
'pH_of_sea_water' is to have a very loose concern about the accurate
meaning of the so-labelled pH values[4]. For comparison, measurements
are often made, and needed, at the level of 0.0005 units.
(d) All 3 of {B, C, D} above are in regular use, but (C) is the most
promising for the common reference[4].
Additionally, the units are expressed as noted above by common
convention (see for example UCUM units section on pH [5]), in order to
make certain conversions handy for the enlightened. 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.
We therefore do not believe a substitution of moles/liter or moles/
m**3 is appropriate for the canonical moles/kg, as this would force
most practitioners to convert their data before naming it with this
name. [7] Obviously if there is a set of practitioners that are
working with seawater pH using a different canonical unit, that would
be another matter to consider; we think it is unlikely that any
observationalists are doing so.
So, in summary, the longer, explicit terms being proposed are so
approximate (for the reasons
Re: [CF-metadata] new standard name request for pH
I didn't mean to suggest we not create a standard name for it, I just
suggested it not be a part of this proposal. What I didn't like about
the original name is that it implies NBS scale is appropriate for sea
water, whereas the documentation I had suggested that was not the case
(and my impression was that the errors could be bigger than 0.1, but
I'm talking out of my hat there). So another name might be more
suited -- or this might be your preference. If you want that name I
won't stand in your way!
You know, an analogous name for some of this is
'sea_surface_temperature'. I know that's a favorite of yours Roy!
The argument (I think you've made this point very effectively in
presentations?) is What do we do with this? We don't know what the
values mean! So a number of more precise terms were defined (though
I don't know that the original is precluded, or even discouraged?). I
think 'sea_water_pH' is an excellent analog.
John
On May 7, 2009, at 5:14 AM, Lowry, Roy K wrote:
Hi John,
Your suggestion that we disallow storage of NBS scale pH in CF by
not creating a Standard Name for it worries me. We have significant
quantities of NBS-scale pH in BODC (from estuarine systems and the
days before ocean chemists discovered the kilogram) that we may wish
to put into CF in the future. Whilst conversion based on
approximation to free_scale is possible in some (but not all) cases
(need additional data), it's something I'd rather not do. I would
much rather label precisely and leave any manipulations up to the
user who is aware of the proposed usage and what is fit for that
purpose.
My attraction to Jonathan's simple 'pH_in sea_water' is that it
gives us a way of handling data in CF where 0.1 pH units don't
matter. We have one dataset monitoring waters off a chrome plating
plant where pH varies between 2 and 6. At the same time I am acutely
aware of the effects errors of that magnitude have on deep ocean
carbon budget calculations.
Cheers, Roy.
-Original Message-
From: cf-metadata-boun...@cgd.ucar.edu [mailto:cf-metadata-boun...@cgd.ucar.edu
] On Behalf Of John Graybeal
Sent: 02 May 2009 01:28
To: CF Metadata List; Lowry, Roy K
Subject: Re: [CF-metadata] new standard name request for pH
Hi everyone,
Thanks very much for considering and responding to the pH terms
proposal. To respond, I spent considerable time being educated by our
in-house pH expert, even reading pages myself from the referenced book
[1], and reviewed this email with him. So I claim this email
represents a high level of expertise in this scientific field. I
apologize for the length, but it seemed necessary to present
justifying details.
To recap, the original proposal as revised was for 4 names related to
pH:
A) sea_water_pH_NBS_scale (moles/liter)
B) sea_water_pH_free_scale (moles/kg)
C) sea_water_pH_total_scale (moles/kg)-- the one we care about
D) sea_water_pH_seawater_scale (moles/kg)
Through this thread we have had several suggestions for replacements
[5], most recently arriving at
a) pH_of_sea_water_defined_by_moles_of_hydrogen_ion_per_unit_mass
b) pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion
c)
mole_concentration_of_H_and_HSO4_per_unit_mass_in_sea_water_expressed_as_pH
d) mole_concentration_of_H_and_HSO4_and_HF_per_unit_mass
in_sea_water_expressed_as_pH
e) (added) pH_of_sea_water
I believe the focus of these responses was on the concentration of the
solution, but this mis-states the actual chemical definition of this
particular concept. (Unfortunately we incorrectly presented this in
the original definitions for B, C, and D.) In the reference[1] it is
made clear that:
(a) pH is strongly affected by abundance of hydrogen (actually
hydrate) ions, but is not defined by their quantity; it is determined
according to the *activity* of those ions, and this is not a linear
relationship (though it is close to 1 in some regimes)[2]. We will
change the proposed definitions to replace 'concentration' with
'activity'; my apologies for this error.
(b) The use of the NBS scale is not recommended for sea water[3], as
errors will be induced (up to at least 0.1 units in situ); in this
light we propose to simply withdraw [A] from consideration, especially
as we don't need it.
(c) There are significant differences in the values obtained for these
different quantitiies, up to .12 units; thus to say just
'pH_of_sea_water' is to have a very loose concern about the accurate
meaning of the so-labelled pH values[4]. For comparison, measurements
are often made, and needed, at the level of 0.0005 units.
(d) All 3 of {B, C, D} above are in regular use, but (C) is the most
promising for the common reference[4].
Additionally, the units are expressed as noted above by common
convention (see for example UCUM units section on pH [5]), in order to
make certain conversions handy for the enlightened. Yet the second
sentence of the following statement
