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Re: KH = 1
> From: Wright Huntley <jwwiii at pacbell_net>
> Subject: Re: KH=1
> > Date: Mon, 24 Dec 2001 13:40:01 EST
> > From: Paulio629 at aol_com
> > If Im not wrong, raising the kh will in turn raise the pH. Am I right about
> > this?
> It would only change if a weak acid (like injected CO2, forming carbonic acid)
> was pushing the pH below the natural buffered pH level. Buffers, like KH, try
> to hold pH very constant, until something overcomes them.
A buffer system is the _combination_ of the acid and the anion,
in this case H2CO3 and HCO3-. The pH depends on the ratio of the two.
> The salt,
> (carbonate/bicarbonate) holds pH in the high 7s or low 8s (I forget the actual
> value), unless there is enough of the acid of that salt (carbonic) to lower
> the pH.
The H2CO3/HCO3- combination is pretty good in the range (roughly)
pH 6.5 to 8.5 or so. Above that, you are getting more into HCO3-/CO3--.
> It is an equilibrium balance, with pH telling you the ratio of the salt/acid,
> effectively. If you know how much salt (KH), the charts show how much acid
> (CO2). pH is just the indicator.
> That's why the KH-CO2 chart is such a great tool for measuring CO2
> concentration, in the absence of other acids or buffers. Higher KH tends to
> "buffer the pH high." It thus takes more CO2 to lower the pH by the same
> amount. The pH has thus become more "stable."
No. Whatever the KH, doubling the CO2 reduces the pH by 0.3.
The point about higher KH is that it provides better protection against
big pH reductions caused by the addition of _strong_ acids, e.g., nitric
from oxidation of fish waste ammonia (not too great a problem in many
planted tanks). KH 1 is fine for stability with H2CO3, but KH 0.001 is
not, and it takes less HNO3 to get there from KH 1 than from KH 3.
Adding strong acids destroys KH.
Doubling the KH increases the pH by 0.3 (for the same CO2 concentration),
but does not make it more stable if the CO2 concentration changes.
> If you already have significant CO2 dissolved, then raising the lowered pH by
> increasing KH is quite likely, especially when you start down around KH=1
> (much too little buffering for good stability, IMHO).
It _will_ raise the pH.
Paul Sears Ottawa, Canada