[Killietalk] TDS vs. Conductivity

[Charles & Sue Harrison <csharrison at inkmaker_net>]

>Date: Thu, 2 Feb 2006 13:53:51 EST
>From: LeeH920226 at aol_com
>Subject: Re: [Killietalk] TDS vs Conductivity
>To: killietalk at aka_org
>In a message dated 2/2/06 1:27:06 PM, bjc3 at centurytel_net writes:
>
><< Am I not correct in saying that the conductivity, which is what all
>these meters read, is dependent on the number of charged species in the
>water, mostly simple ions like Na+, Ca2+, Mg2+, Cl-, carbonate,
>bicarbonate, etc >>
>
>Exactly right. TDS does not mean what it says. What is important are those
>colligative materials that affect the osmotic pressure. Conductivity is a very
>simplified way  of measuring it, although very useful. Expressing it as TDS is
>useful, if misleading. Probably should be called apparent ionic dissolve
>solids. However that creates an acronym, already in use.
>
>Lee Harper


I would like to add some to this: The organic acids and other organic 
compounds will add to conductivity in an unpredictable way. There is 
their ionization coefficient to be concerned with. The same is true 
with Bicarbonates, and Phosphates. Plant people use pH to judge their 
CO2 concentration along with the Carbonate hardness.

Some of these guys go rattling off the CO2 conc.  and pH and Hardness 
as if they have control over it all. Then they add in Ferrous 
glycosides and/or some Phosphates and try to show a relationship with 
the TDS meters.

Just once some of these pseudo chemists should take a weighed 
container of their water and evaporate away the water just to see 
what they really have as Total Dissolved Solids.

So, TDS  is what is left when one evaporates away all the water from 
the solution. That is very clear and should be explained that way.

Conductivity is a very different thing. It is a measure of the 
conduction of electrical current through the medium or as we have 
come to learn it: the resistance of the medium to conduction.
-- 
Charles & Sue Harrison
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