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Re: Water hardness, CO2
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To: Aquatic-Plants at actwin_com
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Subject: Re: Water hardness, CO2
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From: psears at emr_ca (Paul Sears)
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Date: Tue, 3 Oct 1995 09:25:55 -0400 (EDT)
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In-Reply-To: <199510021624.MAA01332 at looney_actwin.com> from "Aquatic-Plants-Owner at actwin_com" at Oct 2, 95 12:24:27 pm
>
> Total hardness dH (which I think means general hardness)
> measures the milligrams of calcium or magnesium oxide in a liter
> of water.
>
Yes, one degree of hardness corresponds to 10 mg/L
of calcium oxide in the water. This is a 1.79 x 10^-4 molar
solution of Ca++ ions. They are not there as the oxide!
This corresponds to giving fertilizer potassium concentrations
as K2O, which is never actually there and would be quite
difficult to make.
One degree of general hardness would also be 1.79 x 10^-4
molar magnesium ion concentration, which would be a different
mass of magnesium oxide, about 7.14 mg/L. You won't find
magnesium oxide around in the solution either.
> Carbonate hardness dCH, is a measure of carbonates: hydrogen
> carbonate and the calcium and magnesium carbonate salts.
>
Yes, but there won't be much carbonate in solution.
> Noncarbonate hardness refers to permanent hardness from compounds
> such as calcium sulphate, magnesium sulphate and others.
>
Yes - in fact any Ca or Mg salt of a strong acid. A strong acid
is one that is essentially completely ionized at the lowest pH of interest
to us (say 5). Remember, though, that the ions do not "belong"
to each other in solution. You cannot say that you have sodium chloride
and calcium bicarbonate in a solution. You have all four types of ion
wandering around separately. The total positive and negative charges
must balance, though.
> To confuse matters slightly more, aquarists add sodium bicarbonate
> (baking soda) and calcium carbonate to alter the carbonate
> concentration.......
Yes, though the CaCO3 would have to react with CO2 and water
to dissolve.
CaCO3 + CO2 + H20 -> Ca++ + 2(HCO3-)
> which is also affected by CO2 concentration.
No it isn't.
> Still confused? Me too.
>
Then why post this stuff?
To get the carbonate hardness, it is sufficient to look at the
concentration of HC03- (bicarbonate ion) in the solution. At pH 5 to 9
you can ignore CO3-- and OH-. 10^-3 molar bicarbonate is equivalent to
about 3 degrees of hardness.
Paul Sears, Ottawa, Canada.