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-bounces at cgd.ucar.edu [mailto:cf-metadata-bounces at 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 descrX-MailScanner-NERC-SpamCheck: not
> spam, SpamAssassin (not cached, score=-3.5,
> required 5, autolibed above) as to be inaccurate to
> the scientists that use this data. Indeed, they would not use these
> terms. Therefore we re-propose the original terms (B), (C), and (D),
> or alternatively simply (C), as these represent the common and most
> descriptive usage.
>
> John
>
>
> [1] Zeebe & Wolf-Gladrow (2001). CO2 in Seawater: Equilibrium,
> Kinetics,
> Isotopes. Published by Elsevier as part of their oceanographic series.
>
> [2] Ibid, p 54 ff. The exact quote was helpful to me, so I provide it
> here: "In high school we all learned that pH is the negative common
> logarithm of the concentration of hydrogen ions. ... Things are a bit
> more complicated for several reasons... The symbol H+ represents
> hydrate complexes rather than the concentration of free hydrogen
> ions.... In a refined theory one should use activity (an 'effective'
> concentration...) instead of concentration...." It goes on to explain
> the derivation of the various scales, in case that is of interest.
>
> [3] Ibid, p 55
>
> [4] Ibid, p 59
>
> [5] Section 45 in http://aurora.regenstrief.org/~ucum/ucum.html#section-Other-Legacy-Units
> (which unfortunately does not agree with the cited reference [1]
> about the meaning of this term.
>
> [6] Proposal chain for reference:
> a1) pH_per_unit_volume_in_sea_water =>
> a2) pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion =>
> a3)
> pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion_per_unit_volume
> =>
> a4) pH_of_sea_water_defined_by_moles_of_hydrogen_ion_per_unit_mass
> b1) pH_per_unit_mass_in_sea_water =>
> b2) pH_of_sea_water_defined_by_mass_concentration_of_hydrogen_ion =>
> b3)
> pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion_per_unit_mass
> =>
> b4) pH_of_sea_water_defined_by_mole_concentration_of_hydrogen_ion
> c1)
> mole_concentration_of_H_and_HSO4_per_unit_mass_in_sea_water_expressed_as_pH
> d1) mole_concentration_of_H_and_HSO4_and_HF_per_unit_mass
> in_sea_water_expressed_as_pH
> e1) pH_of_sea_water
>
> [7] If I understand CF canonical units correctly, it is OK to have
> one's standard-named data in units that are physically equivalent to
> the canonical unit -- for example, moles per gram is OK if the
> canonical unit is moles/kg -- but it is not OK if the conversion is to
> a fundamentally different unit -- like moles / liter in the example.
>
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John
--------------
John Graybeal <mailto:graybeal at mbari.org> -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Interoperability Project:
http://marinemetadata.org
Received on Thu May 07 2009 - 11:38:29 BST
