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Re: Carbonate Hardness
On Tue, 19 Sep 2000, George Slusarczuk wrote:
> There IS a relatively simple way to measure "true" Carbonate Hardness.
> It is based on the fact, that carbonate hardness equals to "temporary
> hardness" (that's where the term "carbonate hardness" came from).
> Measuring Total Hardness and then Permanent Hardness, and then
> subtracting the second from the first gives one temporary (carbonate)
> hardness.
"Carbonate hardness" as you describe it here measures that part of the
total hardness that is balanced by carbonates. If the water contains more
carbonate than general hardness then this method does not measure the
amount of bicarbonate in solution (which is, after all, what we want to
measure).
My tap water for instance, contains about 1 degree of general hardness
and 7 degrees of alkalinity--all apparently from bicarbonate. Going
through the procedure you describe, the water would have 1 dGH before
boiling (Total Hardness) and 0 dGH after boiling (Permanent Hardness) and
the method would determine 1 dGH of "carbonate hardness".
Uh... That's not a very useful result.
The boiling method works in some water, but not in others. The tip off
comes from the measure of Permanent Hardness. If that is 0 then the
method can't be used. As it happens this value often is 0.
As an aside, the USGS (and EPA, I believe) use the term "non-carbonate
hardness", but I have found no use in modern English-language
water-quality literature for the term "carbonate hardness".
> Of course, many waters might not have any Permanent Hardness or any
> Carbonate Hardness, but most will probably have a mixture of both.
> Waters in the South-Western US will probably have a relatively large
> "other alkalies" component that will add to the _alkalinity_
> measurement, but NOT to the water hardness value.
For what it's worth, its very rare for unpolluted natural water from the
southwest to contain a significant amount of anything that will contribute
to alkalinity except bicarbonate. I have seen analyses that contained
significant amount of HS- and I understand that acetate may be more common
than is widely recognized. Either of these could appear as part of the
measured alkalinity but these are rare in natural water; HS- might appear
in some oil field brines, but typically both of these flag heavily
polluted water such as septic tank or landfill leachate and will never
show up in a public water supply. These pollutants are probably more
common outside the southwest US.
Roger Miller