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A Cheap CO2 Meter


  1. CO2/KH/PH Ratio
    by (Uwe Behle) (Tue, 25 Jan 1994)

CO2/KH/PH Ratio

by (Uwe Behle)
Date: Tue, 25 Jan 1994

George Booth (booth-at-hplvec.LVLD.HP.COM) wrote:
: In rec.aquaria, (Eric Yun-Sung Fang) writes:

:     [my stuff about KH/pH/CO2 ratios deleted]

:     Now, are you sure that this is true? (I'm not, but I don't know the 
:     answer either.)

: Yes.

:     All three are simultaneously true; they are not independent. If you add
:     three equations  together, you get:

:       CO2 + H2O  <-->   2 H+  +   CO3-

:     Which implies that if you know the pH  (essentially H+ concentration)
:     the KH (which I have assumed is equivalent to CO3- concentration) you
:     know the CO2 concentration.

: Yes.

:     Therefore, even if you use weird-ass pH buffers and shit, this equation
:     should still hold, since it only depends on the pH. 

: Well, no (I think). 

: Buffers are strange animals, if I remember Karla's Lab Days correctly. 

: To counter your question, let me ask this:  if you have a bicarbonate 
: buffer set up, it will try to hold the pH at the pk1 pH, 6.37.  If you 
: have a phosphate buffer set up, it will try to hold the pH at the pk2 
: pH of 7.21.  What happens if you mix them?  

:     Check this with your wife; she has the PhD. 

: Her PhD is BIOchemistry; buffers were just a minor annoyance along the
: way :-).  She says she's not sure, but thinks that the relationship 
: won't hold.  We're going to try an experiment at home: set up a KH/pH/CO2
: mixture and verify the results then add some pH-UP or -DONW (left over
: from our less sohisticated days) and see what happens.  

:     I'm just a TA. (BA Chem).
:     That doesn't make me an expert, or even competent ;).

: Do you have a professor available?  Ask him or her.  (Or are you afraid
: the question might turn into an "exercise for the TA" :-).

: ------
: George

Krause describes a method in his book on aquarium water that is supposed to
work with any kind of water. Not absolute pH is the key, but the change of
pH by two units is used to determine correct CO2 concentration.

Take a sample of your water and aerate it for some time until all CO2 is
removed. In that case the concentration of CO2 is in equilibrium with 
the surrounding air (0.6ppm). Measure pH of the water (=X).

Next exhale through a pipe into the water sample. After a while the
of CO2 in the water will assume 60ppm. Measure  pH of the water (=Y).

The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of 
the difference between X and Y: pH,opt = X +.67*(Y-X).

This will work even with buffered water, although the change in pH might be
small and only detectable with an electronic pH meter.


NAME    Uwe Behle, HP Boeblingen Instruments Division
EMAIL (internet), (packet radio)

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