PPM vs. Degrees
- Hardness in ppm
by Paul Sears <psears/nrn1.NRCan.gc.ca> (Tue, 7 Jul 1998)
by Paul Sears <psears/nrn1.NRCan.gc.ca>
Date: Tue, 7 Jul 1998
> From: Neil Frank <nfrank-at-mindspring.com>
> Subject: Re: Hardness in ppm /Red Rotala
> >From: "Roger S. Miller" <rgrmill-at-rt66.com>
> >I think this is always done on an equal equivalents basis. In words, when
> >magnesium is expressed as its calcium equivalent the number is the weight
> >of calcium that would carry the same electrical charge as the weight of
> >magnesium that's actually present. It amuses me that someone, sometime
> >thought this system actually made things easier!
The point is that _both_ are derived from _molar_ concentrations,
which _are_ the easiest to use.
> 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. GH and KH are odd units to use in lots of ways, but at
least one is in no doubt what they are.
> Also important is to start presenting Ca++ and Mg++ concentrations instead
> of Hardness.
Yes, but you will have to do some conversions, I suspect...
> Some manufacturers are starting to move in that direction. Maybe. <g>.
> 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.
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.
> So it appears that they present the Ca hardness concentrations as ppm of
> CaCO3 and Mg as the equivalent of MgCO3.
.....Mg as the equivalent amount of CaCO3.
> 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