Re: [APD] CO2 Charts and High pH

["Roger Miller" <roger at spinn_net>]

Scott wrote:

> In the case of high pH values, no table is needed or useful
> because, Roger has told us, the CO2 all gets converted to
> carbonates in that situation, which I take from Roger's
> post to mean that the CO2 level won't rise if you add CO2
> so there's no point to try.

Just to be perfectly clear...

I put together a table to show pH, CO2 and bicarbonate at pH near 8.4 and to
illustrate what happens.  I know this table won't look right on everyone's screen.

.................HCO3- concentration
.....CO2/HCO3-...needed to get 5 ppm.....Corresponding 
pH...ratio.......of CO2...................alkalinity

8.1..0.0127......393.ppm...............322.ppm...18.1.degrees
8.2..0.0101......495.................. 406.......22.7
8.3..0.0080......624...................511.......28.6
8.4..0.0064......785...................643.......36.0
8.5..0.0051......988...................810.......45.4
8.6..0.0040......1244..................1020......57.1
8.7..0.0032......1566..................1284......71.9

When the bicarbonate concentration alone reaches 1000 ppm the water reaches a
point where few people would call it "fresh."  That happens around a pH of
8.5.  These numbers are based on one set of equilibrium constants.  A
different set of constants would provide slightly different results.  Higher
or lower temperatures would also produce slightly different results.

If you're interested in CO2 concentrations lower than 5 ppm then you can get
those with lower alkalinity values.  At low CO2 concentrations there may be
another factor that sets in.  The table above and every other chart, graph or
table you have seen for the pH-KH-CO2 system reflects equilibrium conditions.
 Disequilibrium conditions can allow the actual CO2 concentration to be
different from the table.  This may be a significant factor if you're adding
large amounts of CO2 to alkaline water; the actual CO2 concentration may be
higher than the value in the charts because the system is not in equilibrium.

To get back to the reason I posted this, you can add CO2 to water with a high
pH and get the CO2 to rise, but when the CO2 rises the pH will drop and the
water will no longer be at such a high pH.

And a brief bit about the buffer.  Josh was going to add NaOH to increase the
pH.  The OH- part of the NaOH reacts with CO2 to make HCO3- or with HCO3- to
make CO3-- plus water.  As a result, the buffer that stays in the water is
still the carbonate buffer.

The presence of another buffer in the water would not effect the results
above.  If the pH is 8.5 then the CO2/HCO3- ratio is 0.0051 regardless of what
causes the pH to be 8.5.  The presence of other buffers in a system cause
problems for the pH-KH-CO2 method only because it makes the KH test an
inaccurate meausure of HCO3- concentration.


Roger Miller
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