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Re: Water hardness in ppm
In your answer to the Neil Frank/Roger Miller corerespondence (Aquatic
Plant Digest, V3 #370), I agree with the results of your calculations,
but not necessarily with your line of arguments and/or conclusions.
> > Roger, thanks for answering my question. After hearing all the > > interesting discussion about hardness, I decided to change my > > opinion about units: ppm is really the preferred way to present > > hardness.
> ppm of _what_? The usual problem is that people don't say, and one > is left wondering.
Well, if one presents *water hardness* in ppm, then the convention is
that it is expressed as mg/L of CaCO3 or ppm (weight/volume) -- no
wondering. This is true in the US, Canada and most of Europe. I would
think that the rest of the world follows suit.
> GH and KH are odd units to use in lots of ways, but at least one is > in no doubt what they are.
Well, not exactly. Because there exist different scales of "degrees" of
water hardness, offhand one DOES NOT know *which* "degrees" are being
used -- and that creates substantial uncertainty! I have seen aquarium
books published in England that use the "English degree" and others that
use the "German degree", but talk of "degrees". (Sometimes one can find
out which degrees are used from the conversion factors -- if they are
listed and IF they are correct.)
1 German degree = 17.9 ppm (as CaCO3)
1 American degree = 17.1 ppm
1 English degree = 14.3 ppm
1 French degree = 10.0 ppm
the possibility of serious error (10.0 vs. 17.9 = 79% difference!) is
not so farfetched.
Furthermore, if one follows the postings on this (or any other) list,
them you can find the units expressed as GH, TH, dGH, etc. NOBODY
specifies *which* degrees they are talking about, except, sometimes, the
kit manufacturer is mentioned. But does that give a non-owner of that
particular kit sufficient information? In my opinion NO.
If THAT were not confusing enough, some authors MAKE UP their own
"units" of hardness: In the June issue of FAMA, p. 204, the author
presents a hardness unit of "m/s" and the Editor LET IT PASS! Not
reassuring at all.
Now, let me NOT start on "KH"!!!
> > Wardleys has a nice hardness test kit (they sent it to me as a
> > freebee) which reports total hardness and calcium hardness. > > Mg++ hardness is calculated by difference. All are reported in > > ppm, which is nice. The only concern is that they say that to > > convert ppm to DH you multiply by 0.056.
(By the way, that 0.056 factor is 1/17.9 = 0.056, just as 1 degree dGH =
17.9 ppm CaCO3 hardness)
> This means that the concentrations are all in units of ppm of CaCO3; > _all_ of them, including the Mg. In order to do the subtraction in > a valid fashion, the numbers must be in molar units. GH is O.K. for > that; ppm Mg++ and ppm Ca++ are not, because the atomic weights are > different.
Atomic weights DO NOT enter into this particular calculation, if BOTH Ca
and Mg are expressed as *mg/L CaCO3*. A simple subtraction of "calcium
hardness" from "total hardness" will give one "magnesium hardness"
expressed as CaCO3, i.e. that part of total hardness caused by
magnesium! Pure and simple.
Admittedly, this CAN be considered to be in "molar units" in the sense
that 100 ppm of *calcium water hardness* equals 0.001 mole/L (=
millimole = mmole) of CaCO3 and 100 ppm *magnesium hardness*, *EXPRESSED
AS CaCO3* is also 0.001 mole, but of MgCO3, i.e. they are equimolar.
(Which is also your conclusion!)
Now, if one wants to calculate the respective *elemental* concentrations
from these water hardness data, the calculations are also quite simple,
but involve atomic weights:
Concentration of Ca (in ppm) = Ca-water hardness (in ppm) x 0.40.
Concentration of Mg (in ppm) = Mg-water hardness (in ppm) x 0.24.
The conversion factors are NOT a mystery. They are the atomic weight of
the element, calcium = 40, or magnesium = 24, divided by the molecular
weight of CaCO3, which happens to be 100!
In my opinion, using perts-per-million of "calcium carbonate hardness"
is just as easy, or easier than starting with "degrees". Both
calculations will gives an identical result if, AND ONLY IF, you choose
your "degrees" right. The results expressed in ppm are unambiguous!
> > So we are getting hardness as they say, but not Ca and Mg > > concentrations. If this is true, does anyone know if taking 40% of > > Wardley's ppm result will be an OK way get the approximate concentrations of the Ca and Mg cations.
> O.K. for Ca, but you want 24% for Mg. The atomic weight of Mg > is 24, that of Ca is 40, and that of CaCO3 is 100. You are > getting molar concentrations, pretending they are CaCO3, which > neither is, and stating the answers in ppm.
> 1 GH is 0.179 millimolar Ca++ or Mg ++.
> This is derived from the definition: 10 mg CaO per litre of water.
> The M.Wt. of CaO is 56, so 10/56 millimolar.
> 1 GH is 0.179 x 40 ppm Ca++ = 7.2 ppm Ca++
> 1 GH is 0.179 x 24 ppm Mg++ = 4.3 ppm Mg++
> I hope this helps. I have seen too much confusion lately!
> Paul Sears Ottawa, Canada