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Re: KH, pH, buffering and baking soda

> From: "Birdy" <birdy at aa_net>
> Subject: Kh, Ph, Buffering, and Baking Soda.
> In order learn something about the relationship between Ph, Kh, and
> buffering, I conducted the following experiment.


> Using a 5-gallon bucket of water that I removed from my aquarium in the
> course of a normal water change, I began adding Baking Soda in 1/4
> tablespoon doses, resulting in the following results.
> Dose    Ph      Kh (dH)
> 0           6.5      4
> 1           6.6      5
> 2           6.7      6
> 3           6.8      9
> 4           7.0      11

	The KH increased from 4 to 11, a factor of 2.75, and the CO2
concentration wouldn't change significantly in a short time, so one
would expect the pH to rise by log(2.75), or 0.44.  It did, to within
the accuracy of the tests.

> In doing the Ph tests, I actually have 5 test tubes, so I can lay them side
> by side on white paper, and see straight progression of color from 6.5 to
> 7.0.

	As one would predict.  In fact one would predict (from the KH numbers)
6.5, 6.6, 6.7, 6.85, 6.94.  That is pretty good agreement.

> So, I want to ask anyone who knows... Previous to doing this test, I had
> understood that the term Ph Buffering refers to using a substance that adds
> alkalinity, (the ability to absorb acids), without actually changing Ph.

	There is more rubbish written on this than on just about anything
else in aquarium chemistry.  A buffering system slows the change in pH
when another acid is added - it doesn't stop it.  The pH is controlled by
a buffer in this way:

The equilibrium:
		H2CO3   <->   H+   +   HCO3- 
has an equilibrium constant -    [H+][HCO3-]/[H2CO3]   =   K (say)

so:		[H+]  =  K[H2CO3]/[HCO3-]
or:		pH  =  -log(K)  -  log[H2CO3]  +  log[HCO3-]

which is the basis of the tables on the Krib and elsewhere.  Double the
CO2 (H2CO3) concentration, the pH will fall by log(2), double the KH,
it will rise by log(2), or 0.3.

> But in this test, the Ph changed along with the Kh.

	and exactly as one would predict!
> So, in adding baking soda, I increased both the Ph, and the Kh.  Is this
> what Ph Buffering looks like?

> I am following the advise of those who say, "don't start adding CO2 to your
> aquarium until you understand Ph buffering and the relationship between Kh,
> and Ph."

	Probably not bad advice.
> So, if I added CO2 to the water described above, would there be a
> precipitous, unmanageable, Ph drop?

	No.  You had KH 4 to start with.  Look at the tables on the Krib.

Paul Sears        Ottawa, Canada