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# Re: Hardness in ppm

```Steve P. wrote-
>
> I propose that we stop using units of hardness to refer to concentration
> of magnesium and calcium. The appropriate units for those things are in
> ppm or mg/L of Ca++ or Mg++. The usage of mg/L of CaCO3 is somewhat
> useful because it allows one to use direct weights of the most commonly
> used calcium salt however, test kits should give one and only one result
> and that is the concentration of calcium ions (one test) and the
> concentration of magnesium ions (another test).

Unfortunately, the tests we have don't work like that.  We actually
measure Ca++ and Mg++ with one test, getting a total molarity, and then
measure one of the two alone (Ca++, I think).  The other we then get by
difference.  The molarities can then be converted to ppm results
for the individual species, but in practice it's simpler to stop at
the "equivalent CaCO3" stage, so we tend to do so.

The tests measure the concentrations in terms of numbers of ions
per litre (that is what molarities are).  If you have (say) apples and grapes
and can count the two together, or apples alone, you can easily get the
number of grapes by difference.  If you want the total _weights_ of each,
and can only measure by counting (but know the weights of individual
apples and grapes), you must do the subtraction to get the numbers of
each _before_ you try to calculate weights.  The calculations are not
at all difficult, but they must be done.
>
> For measuring alkalinity, I propose that we should use units of
> equivalent CO3-- however there may be a more precise scientific measure.

The obvious one to use would be the one actually there: HCO3-.
You won't like the next bit though - the obvious _unit_ to use is
millimolarity.  This goes for Mg++ and Ca++ as well.  The good bit
about using GH, KH, and ppm CaCO3 equivalent is that they are _all_
directly related to molarities (numbers of ions in solution), and that
is what our tests measure.  ppm Ca++, ppm Mg++ are derived measures.
They are not difficult to get - simply multilply by the atomic ("ionic")
weight, but there is no avoiding doing the calculations, as long as
some of our tests measure two or more things together.

> The issue is NOT:
>
> "are we analytically-enlightened individuals able to apply conversion
> factors?"

If you want Mg++ and Ca++ in ppm, you will have to do _some_
calculations after using your test kits.

--
Paul Sears        Ottawa, Canada
```