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Re: Acids to lower pH (was: RE: Lime Deposits)
From: "Mike Wise"
Sent: Friday, May 05, 2000 11:46 AM
> Would I be correct to assume that phosphoric acid would be
> more effective at lowering pH because it releases 3 H+ ions
> for every PO4 -3 ion in solution?
The advantage to phosphoric acid is its buffering point - for practical
purposes laying between that of peat and carbonates. The downside is the
build-up of phosphates in the closed tank environment - how do you like your
algae?
As weak polyprotic acids step through the ionization process, the
corresponding ionization constants grow ever smaller. This usually produces
at least an order or two of *magnitude*'s difference in the ratios, so even
though releases three times as many hydrogen ions the additional gain is
statistically insignificant. In the case of H3PO4, the constants are K1=7.5
x 10-3, K2=6.3 x 10-8, and K3=3.6 x 10-13 - so you can see the overall gain
is puny, indeed. (As a plus, these large differences make for more stable
buffering compounds.)
Compare hydrochloric to sulfuric, a much stronger diprotic. As sulfuric
ionizes to sulfurous acid, the intermediate acid is almost as strong to
ionize as the original. For practical purposes again, having the two
ionization constants so closely "in synch" does double the amount of
hydrogen produced. And the added SO4 ions only affect the hardness without
feeding any unwanteds...
-Y-
David A. Youngker
nestor10@mindspring.com
...who apparently is far more sleepless than Bob D. tonight...
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