[eml-dev] [Fwd: Re: EML units - pH]

[Matt Jones]

Uh, true, sort of.  In addition, I think the mass of a mole of particles 
can be calculated as the # moles x molecular weight, which for hydrogen 
is 1, so one mole of hydrogen = 1 gram of hydrogen.  So there is a 
direct relationship between moles and mass that varies as a function of 
molecular weight.  So a mole is a count that is directly porportional to 
mass.

In addition, "mole" is the fundamental SI unit for the NIST fundamental 
dimension "amount", so mole itself can be used in derived unit types. 
So the only problem I really encounter in the definition of the units 
for pH is how to deal with the log(), not the molar concentration issue.

Matt

Dan Higgins wrote:
> Hi All,
>    As I learned it, a mole (of anything) is not a mass (grams). It is a 
> _count _in units of Avagado's number (6.023x10^23) Or, as defined by NIST
> 
> The mole is the amount of substance of a system which contains as many 
> elementary entities as there are atoms in 0.012 kilogram of carbon 12
> 
> It thus really has no units.
> 
> Dan
> 
> ----
> 
> Matt Jones wrote:
> 
>>
>> ------------------------------------------------------------------------
>>
>> Subject:
>> Re: EML units - pH
>> From:
>> Matt Jones <jones at nceas.ucsb.edu>
>> Date:
>> Mon, 04 Apr 2005 10:46:18 -0800
>> To:
>> David Balsiger <dbalsiger at wisc.edu>
>>
>> To:
>> David Balsiger <dbalsiger at wisc.edu>
>> CC:
>> servilla at lternet.edu, sheldon at uga.edu, bjbenson at wisc.edu, 
>> pmkapadia at wisc.edu
>>
>>
>> This is a tricky one that we discussed a lot in the eml group but were 
>> ultimately stymied.  Here's the basics as I recall it:
>>
>> pH = -log(moles H/liter)
>>
>> moles H =~ grams because the molecular weight of H is 1
>>
>> thus,
>>
>> pH =~ -log(grams H/liter)
>>
>> For example, for a solution with hydrogen ion concentration of 10^−4, 
>> or 0.0001, moles per liter, the pH is 4.
>>
>> The problem we run into is how to deal with the -log, which doesn't 
>> really change the units.  STMML doesn't really deal with 
>> transformations like this, so we weren't really sure how to either.  
>> The unit seems like it should be -log(moles/l) [or for hydrogen maybe 
>> -log(grams/liter)], but we didn't really know what to make of it 
>> because STMML has no way to express functions such as logarithms as 
>> far as we could tell.  Its possible that transformaitons like this 
>> make the value dimensionless, but I'm not sure.  This problem extends 
>> beyond pH to other similar transformations such as sin, cos, etc., but 
>> pH seems a very common case. STMML probably would need to be extended 
>> with a general purpose expression language that supports common 
>> transformation functions to really define pH.  Suggestions are welcome 
>> on how to handle this in the EML units description.
>>
>> Matt
>>
>> David Balsiger wrote:
>>
>>> Matt, Mark, Wade,
>>>
>>> Surely someone has determined the correct unit and unit/type 
>>> descriptions for standard pH measurements. Can you save us some time 
>>> and effort in this matter?
>>>
>>> Thanks
>>> Dave
>>>
>>>
>>> Dave Balsiger   Center for Limnology  265-7944
>>
>>
>>
>> ------------------------------------------------------------------------
>>
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>>  
>>
> 
> 

-- 
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Matt Jones                                     jones at nceas.ucsb.edu
http://www.nceas.ucsb.edu/    Fax: 425-920-2439    Ph: 907-789-0496
National Center for Ecological Analysis and Synthesis (NCEAS)
University of California Santa Barbara
Interested in ecological informatics? http://www.ecoinformatics.org
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