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Re: CO2 Capacity.
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>
>From: STDIXON <stdixon at bechtel_com>
>Date: Mon, 03 Feb 1997 08:50:14 -0500 (EST)
>Subject: CO2 Capacity?
>
>Paul Krombholz wrote:
>
>>Having some carbonates in the tank increases the capacity of
>>the water to hold CO2, but there is little benefit in continuing to add
>>carbonates.
>
>Could you tell us a little more about that Paul? I noticed something
>similar a couple of weeks ago reading descriptions of CO2 reactors. My
>interest is along the following lines: Assume a steady CO2 bubble size and
>flow rate, and a steady flow of water through a given CO2 reactor. If I
>have say, about 7 mg/l CO2 at a pH of 7.0 with KH of 2.5, would I have a
>greater CO2 mg/l concentration if I raised the KH to say 4? In other
>words, if I would be willing to accept a higher base case pH, could I
>increase the CO2 levels for a given supply of CO2?
What I meant when I said there is little benefit in continuing to add
carbonates is that adding carbonates can only increase the capacity of the
water to hold CO2 at a given pH. Adding carbonate can't actually add any
CO2. Because of low solubility, there is a limit as to how much carbonate
you can get in the water. Some really hard water I have seen from the
Madison, Wisconsin area actually precipitates out calcium carbonate when
exposed to air. With water like that, the only way to keep all that
carbonate in solution is to keep the amount of dissolved carbon dioxide
much higher than the amount that would occur if the water were in
equilibrium with the atmosphere.
Most aquarium plants can only utilize free, dissolved CO2. Some, coming
from well-lit habitats crowded with plants where CO2 is at a premium, can
also extract CO2 from the bicarbonate ion, leaving behind the hydroxide
ion.
The following table is from a Finnish aquaria magazine
(Akvaariomaailma) and was posted by Pauli Hopea.
[I copied it from the Krib, to which it came by way of George Booth.]
The relationship of CO2 , pH and KH
-----------------------------------------------------------------------
\ pH | 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 8.0
KH\ |
-----------------------------------------------------------------------
0.5 | 15 9.3 5.9 3.7 2.4 1.5 0.9 0.6 0.2
1.0 | 30 19 12 7 5 3 1.9 1.2 0.3
1.5 | 44 28 18 11 7 4 2.8 1.8 0.4
2.0 | 59 37 24 15 9 6 4 2.4 0.6
2.5 | 73 46 30 19 12 7 5 3 0.7
3.0 | 87 56 35 22 14 9 6 4 0.9
3.5 | 103 65 41 26 16 10 7 4 1.0
4.0 | 118 75 47 30 19 12 6 5 1.2
5.0 | 147 93 59 37 23 15 9 6 1.5
6.0 | 177 112 71 45 28 18 11 7 1.8
8.0 | 240 149 94 59 37 24 15 9 2.4
10 | 300 186 118 74 47 30 19 12 3
15 | 440 280 176 111 70 44 28 18 4
-----------------------------------------------------------------------
| CO2 milligrams/liter
-----------------------------------------------------------------------
As you can see from the table, the free CO2 would go from 7 to 12 mg/liter
if you raised the KH from 2 to 4 and kept the pH the same.
Paul Krombholz Tougaloo College, Tougaloo, MS 39174
In warm, humid Jackson, Mississippi where the plants are beginning to think
that winter is over.