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relationships b/w hydroxides and KH
OK, when I brought this up, my main concern was with whether the
advice I found was misleading people when it suggested Ca(OH)2
addition for increasing GH and not KH.
The point I intended to stress was that the hydroxide will react with
H2CO3 to form HCO3, increasing the KH. If people are trying to
supplement calcium without increasing KH, they are better off using
CaCl2 (which was also suggested in the archives), CaSO4, CaNO3, or
any other calcium salt of the conjugate base of a strong acid.
The observation that any hydroxide can raise KH was intended as a
side-note. Yes, NaHCO3 also accomplishes this, and is simple and
effective. But it's not the only solution; using hydroxides allows
you to add cations other than sodium that might not have stable
bicarbonates, and temporary lowers the CO2 by about 7.5 ppm per unit
dKH increase. And adding a strong base isn't as dangerous for the
pH as it sounds as long as the CO2 levels are high.
Consider a tank with KH 1.5, CO2 44 ppm, pH 6.0. Raising KH to 4.5
by adding KOH requires addition of 115 mg/gal KOH. This drops CO2
levels (and increases potassium levels) by 23 ppm and raises pH to
about 6.8. Sufficient NaHCO3 to raise the KH to 4.5 would make the
pH about 6.5. And to break the buffer and really crank the pH up,
you would have to have added 490 mg/gal.
The moral of the story: don't use Ca(OH)2 if your goal is to raise
GH but not KH. But if you want to raise KH while keeping GH the
same, hydroxides allow you to add cations other than sodium, if
you so desire.