"Charles" wrote in message
...

with a bit of variation.

bicarbonate hardness, also called temporary hardness, can be reduced
by boiling. The calcium and magnesium bicarbonates are converted to
calcium and magnesium carbonates, much less soluble, which settle out
on the boiling container as lime scale. if the boiled water is then
moved into a separate container, the calcium and magnesium carbonates
are left behind and the total hardness has been reduced.

so called permanent hardness is from calcium and magnesium sulfates
which are unaffected by boiling.

Thanks for the clarification, Charles. You are right of course -- for the
previous balance to be reestablished after adding CO2, you would have
to boil the water in the tank (which I doubt all that many people will
do ;-)

I guess boiling water isn't all that useful for fishkeeping though -- usually,
people are interested in dropping KH and GH together, but boiling
produces water that has quite a large difference between the two, with
a low KH and a (comparatively) high GH.

BTW, normally GH is always greater than KH, because

GH - KH = NKH

or

Total hardness - carbonate hardness = non-carbonate hardness

(You can find this formula in a lot of older books on fish keeping.)

However, some waters measure a KH greater than GH, which appears
impossible. The reason is that a GH test measures alkaline earth metal
ions, that is, Mg++, Ca++, and Sr++. On the other hand, a
KH test measures carbonate ions and hydrogen carbonate ions
(CO3-- and HCO3-).

So, if KH is greater than GH (there are more carbonate and hydrogen
carbonate ions in the water than alkaline earth metal ions), the excess
CO3-- and HCO3- ions are bound by alkali metal ions (Na+, and
K+). So, if KH measures greater than GH, that is simply an artifact
of the measuring technique that is used. You could also say that
GH (which is the German abbreviation for "total hardness") is a
misnomer, because it doesn't measure all of the hardness, just
some of it.

Historically, all this confusion is caused by the fact that, prior to modern
tests, people added calibrated soap solution to a water sample and
kept shaking until a steady head of foam was formed. The amount
of soap used was taken as measure of the hardness of the water because
Mg and Ca ions bind with the soap and precipitate out so, the
more soap was used, the "harder" the water was. (This method was
in use by aquarists until about the mid-fifties.) Then, later, people
found out that some of the hardness disappears when when you
boil water. This removable component of the "total" hardness
was called "temporary hardness" or "carbonate hardness" because
it is caused by bicarbonates. Then people came up with the
idea that GH = KH + NKH which, strictly speaking, is wrong
and has caused endless confusion for aquarists who found water
with a KH GH.

Modern chemistry avoids the entire issue by not talking about
hardness at all and instead simply quoting the ion concentrations.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com