Re: [Killietalk] TDS vs Conductivity

[Barry Cooper <bjc3 at centurytel_net>]

Lee,

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. The reason that the relationship between conductivity 
and TDS is not 1:2 because things like ionized Ca compounds contribute 
three anions for each molecule (as an example). There must be any number 
of things that contribute charged particles, even organic compounds. 
Proteins and peptides can be charged and therefore even they could 
contribute. The failure of a TDS meter to match known analyzed ionized 
compounds as the chemical equivalent of "the silent majority", no? In 
this case, I suppose, it's the silent minority.

TDS meters are only the roughest of estimators, anyway. The term "total 
dissolved solids" is really misapplied, it seems to me. Sugar in pure 
water, for example, would have a high TDS but would register 0 on a TDS 
meter. Thus, TDS (i.e. conductivity) meters mostly measure ions and 
these only provide a very rough estimate of hardness, based on the 
assumption that water with high conductivity is mostly accounted for by 
Ca and Mg. This can be very misleading. In my own water, for example, 
the TDS is about 80-90, but the water has almost zero hardness, measured 
with a kit. The pH is also high. Thus, in my case, the conductivity 
seems to be accounted for mostly by Na, carbonate and bicarbonate ions.

In short, I don't think there is any substitute for a proper water 
analysis, particularly if you are having problems. If your water company 
can't provide it or, if you are on well water like me, you can get it 
done at reasonable cost. I think it cost me about $30 for a prettly full 
analysis a few years ago. If you are going to get a full water analysis, 
make sure you talk to the lab about how to "pack" the water (air-tight, 
non-reactive containers, etc.).

Barry

Barry J. Cooper
Sweet Home, OR 97386



LeeH920226 at aol_com wrote:

>In a message dated 2/2/06 1:14:25 AM, ron_schulz at gmail.com writes:
>
><< What is the relationship between conductivity and TDS?. I have a Hach
>conductivity meter which consistently measures my well water at 270uS/cm.
>The hardness of my water is consistently around 130ppm as CaCO3  measured
>with a Lamotte test kit.  >>
>
>With my water which contains (according to the water company) sodium 14 ppm, 
>potassium 3.7 ppm and calcium 24 ppm I calibrated my TDS meter (Hanna) against 
>my conductivity meter (Pinpoint) and the relationship is consistently 0.67 
>TDS/conductivity or 1.5 Conductivity/TDS. It will vary somewhat depending on 
>what cations are present. Potassium chloride is what they use for calibration and 
>that ratio is 0.5 or 2.0 depending on which fraction you use. Now, you might 
>wonder, if my TDS is about 200 ppm what are the other ions? I have no idea. I 
>also think the divalent ions like calcium contribute twice as much to 
>conductivity. Also they do not test for iron (trivalent) but I don't have any evidence 
>for iron in my water supply. The anions add up to more than the cations but 
>still not 200 ppm. There are other anions possible, like ammonium which they do 
>not test for. I don't find any either. As Wright says, it should not be a 
>worry, just pay attention to differences between your own operations -- like 
>adding water to tanks or wadding fish to tanks and also to whether it is high, 
>medium or low. I classify my own water supply as medium and doctor it for those 
>SAA that need low TDS and for those Tanganyikan fish that need higher.
>
>Lee Harper
>Media, PA
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