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CO2 Chart

Contents:

  1. (F) CO2-KH-pH Table
    by booth-at-hplvec.LVLD.HP.COM ("GEORGE L""BOOTH") (Wed, 13 May 1992)
  2. CO2 / KH / pH relations table.
    by hopea-at-cs.Helsinki.FI (Pauli Hopea) (11 Jan 92)
  3. CO2 tests vs. looking it up in a table
    by "Roger S. Miller" <rgrmill/rt66.com> (Sun, 6 Feb 2000)
  4. testing for CO2 in water
    by "Roger S. Miller" <rgrmill/rt66.com> (Sat, 5 Feb 2000)


Alternate axes chart derived by George Booth, rendered by Jeff Dietsch

(F) CO2-KH-pH Table

by booth-at-hplvec.LVLD.HP.COM ("GEORGE L""BOOTH")
Date: Wed, 13 May 1992

Here is a rough table to help you determine how much CO2 is present in
your aquarium water based on accurate measurements of pH and KH.  I
would recommend a LaMotte pH test kit and a Tetra KH test kit to
determine the values.  The CO2-KH-pH equilibrium should be correct
unless you are taking extreme measures to adjust pH, such as adding
lots of phosphate buffers like pH-UP or pH-DOWN or using sulfuric
acid.

There's a trick to calculating these numbers that I have forgotten.  I
will try to convince Karla to refresh my memory and post that
information also.

KH values are in German degrees, CO2 values in mg/l (ppm).  CO2
concentrations in the range of 15-25 mg/l seem to work the best for
superior plant growth.  Concentrations above 30 mg/l may cause
respiratory problems in your fish.

The CO2 values for KH=1 and KH=10 are exact values in case you want to
draw a chart.  The other values are rounded to the nearest whole
value.  The lines of constant pH are straight and should pass through
CO2=0, KH=0. 

ph=6.6
KH       1     3     4     5     6     7     8    10
CO2    7.56   23    30    38    45    53    60  75.55

ph=6.8
KH       1     3     4     5     6     7     8    10
CO2    4.78   14    19    24    29    33    38  47.75

ph=6.9
KH       1     3     4     5     6     7     8    10
CO2    3.79   11    15    19    23    27    30  37.89

ph=7.0
KH       1     3     4     5     6     7     8    10
CO2    2.98    9    12    15    18    21    24  30.05

ph=7.1
KH       1     3     4     5     6     7     8    10
CO2    2.39    7    10    12    14    17    19  23.92

ph=7.2
KH       1     3     4     5     6     7     8    10
CO2    1.90    6     8     9    11    13    15  19.00

ph=7.4
KH       1     3     4     5     6     7     8    10
CO2    1.20    3     5     6     7     8     9  11.98

------
George

CO2 / KH / pH relations table.

by hopea-at-cs.Helsinki.FI (Pauli Hopea)
Date: 11 Jan 92
Newsgroup: alt.aquaria


The following table is from a finnish aquaria magazine (Akvaariomaailma)
When I was a beginner I did'n quite figure this up until I saw this
table, so maybe it will be helpful to somebody else too. Something like
this might be a good addition to the FAQ-file too?
---

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
---------------------------------------------------------------------------


The article suggested that 5-20 mg/l CO2 would be good for a normal
community tank due to the fact that most fish don't like CO2 to be
over 20mg/l and plants need at least 5mg/l for photosynthesis.

---

Then the question:

 Is this the ultimate-chemical-truth or is it possible to have
pH and KH like above but the amount of dissolved CO2 different than
in the table.

=================

CO2 tests vs. looking it up in a table

by "Roger S. Miller" <rgrmill/rt66.com>
Date: Sun, 6 Feb 2000

I an earlier letter I cautioned about the need for precise pH values when
determining CO2 by some methods.  In reply, James wrote:
 
> Roger, doesn't this same caution apply to the use of the CO2 - pH - KH
> table, which a lot of people here seem to depend upon? In looking at the
> table (posted by George Booth and archived at
> http://www.thekrib.com/Plants/CO2/co2-booth-faq.html), the pH measurement is
> incremented in units of 0.2 and the KH in units of 0.5.

It does, but there is a difference in degree.  The first time I used the
CO2 charts I also checked to see what the range of possible answers was
from my test.  I measured a KH of 7 degrees; KH could have been anything
from 6 to 7, and I measured pH at 7.4, but I used a cheap kit with 0.4 pH
units between color references, so I took my possible pH range to be 7.2
to 7.6.  My CO2 concentration could have been anywhere from less than 5
ppm to about 13 ppm.  I could have used better test kits to get a smaller
range of possible results.

If I used the log(CO2) method from my reply to Sylvia with a better pH kit
(one with readings at 0.2 pH units), and measured the tank pH at 7.4 to
7.6 and the aerated pH at 8.8 to 9.0, then the possible range of CO2
concentrations would be 8 to 20 ppm.

The method that Sylvia cited anticipated the possibility that the pH would
be strongly controlled by something other than the CO2-bicarbonate buffer,
so that a big change in CO2 could cause only a small change in pH.  In
that case the entire range from 0.6 to 60 ppm CO2 could fall in a pH range
between color patches on an inexpensive pH test kit, making the method
useless without a pH meter.

If you can use the KH-pH-CO2 charts or tables then they will give you the
estimate with the smallest problems caused by precision in the
measurement.  the log(CO2) method is always very sensitive to precision,
and the method that Sylvia cited may or may not be very sensitive,
depending on the degree of buffering.


Roger Miller


testing for CO2 in water

by "Roger S. Miller" <rgrmill/rt66.com>
Date: Sat, 5 Feb 2000

On Sat, 5 Feb 2000, Sylvia wrote: 
 
> I would appreciate if others already successfully injecting CO2 could 
> evaluate the following formula and see if it is, indeed, accurate with 
> respect to pH levels in their own tank water. This could be immensely helpful 
> to those reading and having difficulty determining bubble rate/CO2 level/pH 
> and finding results from charts and CO2 tests questionable.
> 
> This is posted at the Krib. Thanks for your help.
> 
> Sylvia
> 
>  <A HREF="http://www.thekrib.com/Plants/CO2/co2-meter.html">A Cheap CO2 Meter<
> /A> 

The method is not generally accurate.  It assumes that there's a straight
line relationship between CO2 content and pH; there isn't.  Things might
not be too bad if there is only a small pH difference between the high pH
you get on the aerated water and the low pH you get from the breathed on
water.  If the difference is more than a pH unit or so, then the error can
get pretty big.  I think that for common conditions this method could
easily overestimate CO2 levels by a factor of two.  And that doesn't even
consider the possible problems that come up if the CO2 content of the
aerated water isn't 0.6 ppm or the CO2 content of the breathed on water
isn't 60 ppm.

If you do have the case where there is a small difference between the two
extremes (implying a large buffer capacity), then you will need to use a
pH meter to get numbers that are accurate enough to use.

If you're willing to depend on the pH and CO2 content of aerated water,
then you might try something else.  Take a sample of your tank water and
measure its pH (call it pH(t)), then aerate the sample so it's pH rises
and remains constant.  At that pH (call it pH(a)) it should contain about
0.5 ppm CO2 - in equilibrium with air.  Now put those values into this
equation:

log(CO2) = pH(a) - pH(t) - 0.3

This is based on the assumption that the bicarbonate content of the water
doesn't change significantly when the sample is aerated.  It would be
possible to make a simple graph of this formula comparable to the
pH/KH/CO2 charts.

I tried this in one of my tanks but it didn't work because the pH of the
aerated sample rose above the range of my test kit.  So I made up this
example.

A sample of the tank water has a pH of 7.5.  That's pH(t).  After the
sample was aerated its pH rose to 8.9.  That's pH(a).  Putting those
values into the equation above, I get:

log(CO2) = 8.9 - 0.3  - 7.5 = 1.1

using a calculator, CO2 is 10^(1.1), or about 13 ppm.

The biggest problem with using this equation is that your pH numbers need
to be precise; if any pH measurement is off by 0.1 unit up or down, the
resulting CO2 estimate is off by 20% to 25%. The estimate that the aerated
water will contain 0.5 ppm CO2 is also a source of problems.  I don't
think there's a problem with my assumption that the bicarbonate content
doesn't change significantly when the sample is aerated, but heck, maybe
there is.

You can't use this method to split hairs, but you should be able to use it
in the presence of other buffers.


Roger Miller


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This page was last updated 30 July 2000