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Re: Hardness, Calcium, Magnesium

On Tue, 10 Apr 2001, Charles Kuehnl wrote:

> Following the calculation for Ca++ concentration is pretty straightforward
> but I do not follow the rational behind the Mg++ calculation.  Why is the
> atomic mass of CaCO3 used instead of the atomic mass of MgCO3 as the
> denominator of the Mg calculation?  I would guess that the source of my
> problem relates to the nature of the test and the resultant measurement
> being in terms of CaCO3.

I think you're probably right about the source of your problem.  Hardness
measurements aren't as straightforward as their simplicity seems to imply.
Hardness tests repond to the number of equivalents of calcium and
magnesium present in the water.  By standard reporting practice, the
equivalents are reported in terms of the amount of calcium carbonate that
would be dissolved in pure water to create the same hardness.

An equivalent is a mole of ionic charge; an equivalent weight is the
molecular weight of an ion divided by it's electrical charge; for calcium
40/2=20, for magnesium 24/2=12.  The equivalent weight of CaCO3 is it's
molecular weight divided by 2; 100/2=50.  To get picky, we're actually
using milliequivalents (abbreviated meq).

Mr. Fitch's test kit reported total hardness as CaCO3 and the calcium
component of the hardness, also as CaCO3.  The magnesium component
calculated by difference is also expressed as CaCO3.

To convert from calcium as CaCO3 to calcium ion you first figure out how
many equivalents are represented by the CaCO3, then you find out how much
calcium is required to make that many equivalents:

calcium hardness/50 = meq Ca++;
Ca++ concentration = (20 mg/meq)*(meq Ca++) or 0.4*calcium hardness;

similarly for magnesium

magnesium hardness/50 = meq Mg++;
Mg++ concentration = (12 mg/meq)*(meq Mg++) or 0.24*magnesium hardness

Mr. Fitch glossed over the "divide by two" step and went more directly to
the end result.

> >Umm.  Buy low, sell high.
> Although probably not one of the more eloquent offerings of wisdom on this
> forum, it is surely the most practical.  Have you perfected this too or are
> you just good with plants and fish?

Who says I'm good with fish?  It's just something to keep in mind when the
stock market gets bumpy.

Roger Miller