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Re: Aquatic Plants Digest V3 #367
> Date: Mon, 06 Jul 1998 06:53:25 -0400
> From: Neil Frank <nfrank at mindspring_com>
> Subject: Hardness in ppm
I generally agree with what you wrote, but some of the questions you
ask, have at least partly been answered.
> This is a good example why degrees of hardness is not a > desireable way for people to compare hardness values, unless we > are all using the same conversion factors.
In my opinion it is NOT the *conversion factors* that introduce the
confusion, but the different "degrees" i.e. German, American, British,
etc. You might assume that in a book published in England they will use
English degrees (Clark) to measure water hardness, but I would not bet a
nickel on it!
> Also, we all have to remember that when hardness is expressed in > ppm, we MUST KNOW if we are talking about ppm of calcium carbonate > or calcium oxide or calcium or calcium+magnesium or ???
That uncertainty HAS been settled, at least in the USA, Canada and most
of Europe: The convention (promulgated in the US by the EPA) is that
when one talks about water *hardness*, then one talks in terms of CaCO3.
Even if the hardness is caused 100% by magnesium compounds! There is a
very good practical reason why, but it is outside the scope of aquatic
On the other hand, one can talk in terms of calcium content. Then ppm in
*water solution* mean weight/volume ratio of that element. It is a
little bit off from the "absolute" wt/wt ratio, but in practice it is
Other units that are widely used in scientific publications are
"equivalents" or rather "milliequivalents". The are very useful for
certain specific usage, but less so in water treatment or aquaria.
> When a test measures total hardness and separately presents > measurements of calcium and magnesium, it seems that the units > for hardness are not presents as CaCO3 or CaO. If it is, does > anyone know what it means to present the magnesium compound > concentration in terms of equivalent Calcium or Calcium > carbonate?
There is only one test that I know of, that determines total hardness
and then hardness due to magnesium. There might be others. IF the test
presents water *hardness*, then the convention is to express it as CaCO3
If the test measures calcium, magnesium, or whatever other element, then
the convention "in the water business" is to express it in ppm.
Principally biologists and physicists, prefer other units, because they
simplify their calculations. But then the units are ALWAYS indicated.
So, the scene in scienceland is not as messy as it might appear. But
using "degrees" without their pedigrees are for the most part confusing.