# Re: Hardness, Calcium, Magnesium

```Charles Kuehnl <ckuehnl at mmcable_com> wrote:
> Subject: Re:  Hardness, Calcium, Magnesium

> First of all, the atomic weights of the elements involved
> C - 12.011, O - 15.999, Mg - 24.035, Ca - 40.08

One little mistake: Mg (MW) = 24.305

> Therefore
> MgCO3 - 24.035 + 12.011 + 3 * 15.999 = 84.043
> MgCO3 is 24.035/84.043 or 28.6% Mg by weight

Not quite. There is no such compound as Magnesium carbonate
(MgCO3). The closest thing I could find is magnesium carbonate
hydroxide (MgCO3)4 - Mg(OH)2 - 5H20, which has a MW of ~485.
If I'm not mistaken, it's the main ingredient in Milk of Magnesia.

> CaCO3 - 40.08 + 12.011 + 3 * 15.9999 = 100.088
> CaCO3 is 40.08/100.088 or 40.0% Ca by weight

Sounds good.

> 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.

Yes. Initially, many years ago, hardness was a measure of the
capacity of water to precipitate soap. Soap is precipitated chiefly
by Ca and Mg ions. Other cations, often complex or organically
bound, are used to calculate hardness, but their role in contributing
to hardness is often difficult to define. For this reason, total
hardnesses are defined as the sum of Ca and Mg ions in mg/L,
both expressed as calcium carbonate.

Hardness = 2.497 (Ca) + 4.118 (Mg), where 2.497 is 100.088/40.08
and 4.118 is 100.088/24.305.
Jamie    <"\\\>< Aquatic plants, water chemistry, and cichlids
Greenwood, SC     http://www.ais-gwd.com/~jjirons

```