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Re: Conductivity & TDS

>Date: Tue, 05 Aug 97 07:45:36 -0700
>From: gomberg at wcf_com
>Subject: Re: Aquatic Plants Digest V2 #876
>Well to take the HCl example, I claim the Cl- ions do not carry a
>little tag with them to tell you they were last bonded with an H+
>vs. an Na+.  And CO3- ions do not have a marker to tell you whether
>they came from H2CO3 or K2CO3.

No, ions in solution do not run around wearing tags specifying their
origin, but the properties of the solute are still relevant.  This is
especially easy to show in a simple solution like HCl(aq).  In more complex
solutions things are a bit more muddled, as usual, but I think the HCl(aq)
example is quite clear cut.

>So yes, they do count as "solids"
>and an HCl solution does NOT have 0 TDS.

Hmm.  This is indeed the broadest interpretation of the word solid that I
have ever seen.  What about dissolved oxygen, for example.  Are you going
to call that a "solid" too, or is that special treatment reserved for gases
that ionize in water?  What about ionizing liquids?  Is anything that
ionizes in water a "solid"?  What, exactly, is your definition of "solid"
with respect to TDS?  IMO, the concept you have expressed is well beyond
any reasonable definition of TDS.

Here is, I think, a reasonable definition of Total Dissolved Solids:

	The total weight of all solids that are dissolved in a given
	volume of water, expressed in units of mg per unit volume of 
	water (mg/L).

There's nothing tricky involved here.  It means exactly what it says.  No
unconventional definition of "solid" is required.

To further illustrate my point, lets look at the standard method for
determining TDS[1].  Briefly, this method is:

	(1) filter the sample to remove suspended solids and other particulate

	(2) evaporate the sample to dryness at a specified temperature[1]

	(3) weigh the residue

	(4) TDS = mass of residue (mg) / volume of sample (L)

Now, the temperature used for the evaporation step is not just picked out
of a hat.  It is specifically chosen to maximize the removal of water,
dissolved gases, and volatile liquids, while minimizing the decomposition
of heat sensitive solids.

If one subjects our example HCl(aq) solution to this procedure you will
indeed find that it has zero TDS!.  All of the HCl(aq) will be lost to the
atmosphere as HCl(g) during the evaporation phase, just as it should be.
HCl is a gas, not a solid, so we don't *want* to measure it and we don't.
Like all analytical methods, this one is designed to minimize interference
from substances which are not part of the property which we are trying to
measure.  In this case that includes any solute that is not in fact a solid.

It is possible that trapped water and/or non-volatile miscible liquids will
remain as part of the residue, thus causing the TDS result to be
artificially high.  This does not mean the these substances are "solids"
and therefore components of TDS.  They are just interferences.  It is also
possible that some heat sensitive solids will decompose, causing the
measured TDS to be low.  This does not mean that these substances are not
solids.  It is just a limitation of the method.

To summarize, I think it's clear that dissolved gases or liquids, whether
ionic or not, should not generally be considered as contributing to TDS.



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