Re: Calcium, Magnesium, TDS

> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Subject: Calcium, Magnesium, TDS
> I, Mr. AnalRetentive, have recently purchased yet more Lamotte test
> kits, this time a Hardness test kit (Total, Ca, Mg) and an electronic
> Total Dissolved Solids (TDS) tester.  The results of testing our
> current tanks are thus (all values mg/l):
>                       Total    Ca     Mg      TDS
> 100g Discus tank:       48     40      8      120
> 100g 'Bow tank:        104     88     16      140
> 120g 'Bow tank:        100     80     20      160
> (BTW, the total hardness values agree almost perfectly with a Tetra GH
> test kit using 10 ml of water (0.5 dGH per drop, 1 dGH = 17.8 ppm)).
	The problem with these numbers is that you have to know
what material the "mg/L" refers to.  The hardness numbers usually
refer to mg of calcium carbonate per litre.  The numbers are obtained
by measuring the molar concentration of the divalent ion (Ca++ or Mg++ or
both), and then working out how much CaCO3 contains that many moles.
This is almost certainly the case even when you are measuring magnesium,
but could you please check the fine print on the package to make sure. 
I say "almost certainly" because that is the only way things will add
up properly, which they do.  I assume that the method was a difference
method, with one measurement for Total Hardness, and another for Ca or
Mg.  I forget which is the easier to measure alone.
	The TDS from a conductivity measurement will depend on some
assumptions made about what is in the water.  If these assumptions are
wrong, then the result will be meaningless.  Is there anything that came
with the "TDS tester" that might give some clue about this?  The mobilities
of different ions are quite different, so the calibration will be different
for different materials in the water.  The Ca and Mg measurements and the
Total Hardness measurements are _only_ concerned with the Ca++ and Mg++
ions, the conductivity will depent on _all_ ions, positive and negative,
in solution.  Do you know what material the "mg/L" for the TDS tester
refers to?  If it is also CaCO3 then it is a fair bet that your tanks
contain significant amounts of other ions - Na+, K+, SO4--, Cl-?
	The nature of the counterion will affect the TDS measurement
even if there are no other positive ions present.  If the KH and GH
are equal, then bicarbonate will be the negative ion.  If the KH is
lower, the there is, in part, some other negative ion.  This will throw
off the calibration evin if Ca++ and Mg++ are the only positive ions 
present.  Also, the Ca++ and Mg++ ions have different mobilities, so
if the ratio is different from that assumed for the tester, the result
will be inaccurate.

> I've heard that TDS is a useful measurement (I believe Oleg Kiselov
> made that statement many years ago).  It has one or two (or maybe
> more) meanings to aquarists (correct me if I'm wrong here): the simple
> meaning is a quick way to determine hardness.  Higher TDS readings
> indicate more "total" hardness.  Since most hardness is Ca/Mg
> hardness, TDS hardness is a quick way to electronically measure "GH". 

	Quick, but hardly accurate.  It could be way off - look at your
own results.

> Could TDS in conjunction with Ca, Mg, and Fe tests give us a better
> picture of the nutrients available to plants?

	It won't distinguish between the different ions.  If the calibration
is for calcium bicarbonate (a reasonable assumption), then you will know
if there is a lot of material that doesn't show up in the hardness tests,
but not what it is.

Paul Sears     Ottawa, Canada