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Re: CO2/KH/pH Questions

> Date: Mon, 31 Mar 1997 08:44:46 -0500 (EST)
> From: STDIXON <stdixon at bechtel_com>
> I was looking through Amano's books last night and thought of the
> KH/pH/CO2 relationship.  Amano often lists all three parameters for
> each tank.  Typical values are pH 6.8 and 2 dKH.  CO2 levels for such
> tanks are listed at 15 mg./l or greater.
> Yet, when I go to the KH/pH/CO2 chart I find lower CO2 values,
> sometimes quite a bit lower.  In the example above 9.4 mg./l. is what
> the chart shows (TOA pg. 182).  Has Amano missed the mark, or is the
> chart not "absolute," so to speak?

The chart depends on the water having only bicarbonate as a buffer.
It is "absolute" only when that condition is true. 

The chart also assumes that the numbers you use are accurate.  Typical
test kits have a rather wide margin of error.  For the example above,
if the pH was actually 6.6 instead of 6.8, the CO2 reading would
match.  Or if the KH was actually 3 instead of 2, the CO2 would match.
Or measured pH and/or KH and/or CO2 could all be a little off.  

If a CO2 test kit was used to determine CO2 and there were strong
mineral acids in the water, CO2 would read higher than it should.
Likewise, at lower KH values, CO2 test kits are more adversely
affected by other acids in the tank.

> Second related question:
> In the section on carbon dioxide, The Optimum Aquarium (English
> translation) states: "The greater the amount of bicarbonate dissolved
> in water (or higher the carbonate hardness) the more free carbon
> dioxide is necessary (see table)."  Section, pg. 79.  At 15
> dKH the table shows 50 mg./l of CO2.  Necessary for what?

I believe the implication is "necessary to achieve a neutral pH (7.0)."

> The next sentence talks about calcium precipitating out of the water
> if free CO2 falls.  Will calcium precipitate at 15 dKH if free CO2
> falls below 50 mg./l?  How does carbonic acid fit into the equation?

I think there are some generalizations here.  If a lot of the KH is
due to calcium carbonate (instead of sodium bicarbonate, for example),
the water may be saturated with calcium carbonate and that saturation
point is partly determined by the amount of dissolved CO2 (more CO2,
higher saturation point).  If CO2 drops, the saturation point drops
and calcium carbonate comes out of solution.  I think.  Perhaps one of
our chemistry gurus can confirm or refute that.

Carbonic acid forms as CO2 is dissolved into the water.  Three to five
percent of the dissolved CO2 is in the form of carbonic acid (I think,
see above).  Carbonic acid is not usually something we concern
ourselves with -- it is a consequence of the amount of CO2 in the
water and can't be controlled independently.

> Finally, I noticed that the optimum CO2 levels in TOA (green portions,
> pg 182) rise as the KH rises.  At 1 dKH 5-12 mg./l. CO2 is
> recommended, while at 10 dKH 12-30 mg./l CO2 is recommended?  Does
> anyone know why?

They are trying to maintain an "optimum" pH level of around 7.0 while
maintaining an "optimum" CO2 level of around 10-20 ppm.  Too bad they
don't come right out and say that.  

Proper use of the table is to first pick the pH you want then pick the
CO2 level you want.  Use the table to find the KH you should have to
meet the first two parameters.  Add sodium bicarb and/or calcium
carbonate to achieve that KH (and whatever GH you want) then inject
CO2 until the pH is right.  Voila.