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CO2, Ph And Alkalinity from Carbonates



Here is the second part.
Sorry about the long posts.

Larry

CO2, Ph And Alkalinity from Carbonates

Co2 , Ph and carbonates are all related by the following three equations:

1. CO2 + H20 <------> H2CO2 (Carbonic Acid)

2.       H2CO2 <------> H+ + HCO3- (Bicarbonate)

3.        HCO3- <------> H+ + CO3-2(Carbonate)

Example of how increasing carbonates will either raise the Ph or need
counterbalancing CO2 added to maintain the ph level.

If NaHCO2 is added to aquarium water the additional carbonate ions will
cause a shift to the left side of equation (2). This will form more
carbonic acid extracting a H+ ion and thus raising the Ph. The additional
carbonic acid will drive equation (1) to the left creating CO2 which will
dissapate out of solution bringing equalibrium at a higher ph. In order to
maintain equalibrium at the original Ph equations (1) and (2) must be
shifted back t the right. This can be accomplished with the addition of
more CO2 into the aquarium. (Equation (3) becomes important as Ph approches
10.25)

These relationships are expressed in the familliar Chart with Kh/Ph and the
amount of CO2 needed in solution to maintain a specific Ph. The higher the
amount of carbonates/bicarbonates in the aquarium, the more CO2 is needed
to maintain a specific Ph.

    ph   6.0   6.2   6.4   6.6    6.8   7.0   7.2   7.4   7.6   7.8   8.0
 .........___________________________________________________
Kh
0.5 |      15   9.3   5.9   3.7   2.4   1.5   .93   .59   .37   .24   .15
1.0 |      30  18.6  11.8   7.4   4.7   3.0   1.7   1.2   .74   .47   .30
1.5 |      44   28   17.6  11.1   7.0   4.4   2.8   1.8   1.11  .70   .44
2.0 |      59   37    24   14.8   9.4   5.9   3.7   2.4   1.48  .94   .59
2.5 |      73   46    30   18.5   11.8  7.3   4.6   3.0   1.85 1.18   .73
3.0 |      87   56    35    22    14.0  8.7   5.6   3.5   2.2  1.40   .87
3.5 |     103   65    41    26    16.4 10.3   6.5   4.1   2.6  1.64  1.03
4.0 |     118   75    47    30    18.7 11.8   7.5   4.7   3.0  1.87  1.18
5.0 |     147   93    59    37     23   14.7  9.3   5.9   3.7   2.3  1.47
6.0 |     177  112    71    45     28   17.7  11.2  7.1   4.5   2.8  1.77
8.0 |     240  149    94    59     37    24   14.9  9.4   5.9   3.7   2.4
10. |     300  186    118   74     47    30   18.6  11.8  7.4   4.7   3.0
15. |     440  280    176  111     70    44    28   17.6  11.1  7.0   4.4
20. |     590  370    240  148     94    59    37    24   14.8  9.4   5.9
                                CO2 mg/liter




CO2 in excess of 40mg/liter is harmful to fish. Using this chart the
alkalinity of the water from bicarbonates is balanced against a desired ph
using CO2 to control the Ph, making sure a unhealthy amount of CO2 is not
called for.