# Re: hardness in ppm

```> (From Neil Frank)
>
> Sure.... but only if you know how to calculate molar concentrations! <g>
>
> Let us practice:
>  "atomic weight in g per Liter = 1 mole" (in mg/L = 1 millimole).

1 molar, 1 millimolar.  If you take the molecular weight of a
compound in grams, you have 1 _mole_ of the substance, and if you dissolve
that in 1 litre, you have a 1 _molar_ solution.
>
>  For example, 100 mg/L CaCO3 = 1 millimole CaCO3.

1 millimolar solution of CaCO3 (with the usual caveat that it isn't
actually CaCO3).  CaCO3 is an obvious one, because the molecular weight is 100.
Let's suppose we have 50 ppm of Ca++.  The atomic (and  "ionic") weight of
Ca++ is 40, so the concentration is 50 mg per litre, which is 50/40
millimoles per litre, or 1.25 millimolar.
>
> If I got this right I will say it 10 times. <g>

I would rather you tried 10 different calculations....    :)

> GH and KH are odd units to use in lots of ways, but at
> >least one is in no doubt what they are.
>
> I think this conversation has gone full circle... or maybe has been around
> the block a few extra times <g>. Several people have already said that
> degrees hardness have had many definitions in the aquarium literature and
> unless it is footnoted each time used, they may (or may not) mean the same
> thing!

In most of the stuff that I have read, the "10 mg CaO per litre" is
the jumping off point.  Unfortunately, in the aquarium textbooks, things
are frequently screwed up immediately by an assertion that 10 mg MgO will
do the same thing, which is _not_ the case.  10 mg CaO is equivalent to
10(40/56) mg MgO, or 7.1 mg.  MgO molecular weight is 40, CaO 56.

> Regarding this thread, "enough is too much"

I'm inclined to agree, but we definitely have an ongoing problem
with confusion over this subject.  When I first started keeping aquaria,
I read the "water chemistry" stuff with increasing suspicion, as several
things made little sense (e.g. CO2 not being available at high KH),
but fortunately I then came across an assertion that the same amounts
of MgO and CaO gave the same water hardness.  This was clearly wrong,
so I looked up the acid constants for "H2CO3" in a chemistry textbook,
and started from there.  I then worked backward from a pH/KH/CO2 table to
the definition of GH.  It wasn't then difficult to go through the books
I had, making corrections.  This should _not_ have been necessary!!!!!

--