# [Wet Thumb Forum]-A different look at pH-KH-CO2



## Roger Miller (Jun 19, 2004)

I constructed two diagrams that can be used to determine the CO2/KH ratio from pH. Both diagrams are based on the standard method published by the US Geological Survey for determining CO2 concentration (Skougstad et al, 1979)

This diagram is to be used if your KH kit gives you KH in degrees









This diagram is to be used if your KH kit gives you KH in ppm or mg/l









Both diagrams work the same way. You measure the pH and KH. Find the pH reading on the bottom axis of the graph and trace up graph to the corresponding point on the green line then left to the left axis to read the CO2/KH ratio. You get the CO2 concentration by multiplying your measured KH value by the ratio you got off the graph.

For example, if you measure a pH of 6.8 and KH of 4 degrees then the CO2/KH ratio from the graph is about 5.5. The CO2 content is 4*5.5, of 22 ppm.

If you don't like the png files posted above then you might try the pdf versions of the same graphs:

http://members.spinn.net/~roger/co2khdegrees.pdf
or
http://members.spinn.net/~roger/co2khppm.pdf

Roger Miller

------------
_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


----------



## jart (Mar 13, 2005)

Roger, as always, thanks for sharing and taking the time to post this.


----------



## anonapersona (Mar 11, 2004)

Nice, can we get that added to the library here?


----------



## Robert Hudson (Feb 5, 2004)

Sure, if Roger doesn't mind, I can add it to the Library.

I am always looking for new and original material to add to the library. Anyone who feels inclined to write an article is welcome to send it to me to go in the Library. Diana Walstad has already offered. Anything well written is welcome!

Robert
King admin
www.aquabotanic.com


----------



## Faruk Gençöz (Nov 4, 2005)

Roger,

Would it be possible to use pH-kH-CO2 tables in every aquarium condition to read actual CO2 levels?

Suppose CO2 is not injected and there is a torf layer in the filter media. As a result while pH and gH are declining little by little, kH remains nearly the same. In such a condition, according to the table we must read a higher CO2 level although we did not inject it at all. Can there be such situations and if they do happen, can we still use the tables for an accurate estimate of CO2?

Faruk Gençöz


----------



## Roger Miller (Jun 19, 2004)

There are cases where the pH-KH-CO2 method does not work. The method assumes that the measured KH is all due to bicarbonate. Other weak acids can also be measured as part of the KH. If the KH in your tank includes weak acids other than bicarbonate then the method does not give accurate results.

Weak acids can be introduced to your tank as commercial buffers used to control pH, from peat or sometimes from decomposing wood. I'm not sure what torf is. If it's similar to peat then it may cause problems.

The effect from other acids causes the pH-KH-CO2 method to produce CO2 results that are too high. The first sign that the method isn't working is usually unrealistically high CO2 readings or readings that don't vary in proportion to CO2 inputs.

Roger Miller

------------
_"The indispensible first step to getting the things you want out of life is this: Decide what you want" -- Ben Stein_


----------



## Faruk Gençöz (Nov 4, 2005)

Sorry about the Turkish word, "torf". I think it is in fact German. Torf means peat.

Faruk Gençöz

[This message was edited by fgencoz on Sat February 28 2004 at 08:57 PM.]


----------



## anonapersona (Mar 11, 2004)

Wait, if my KH = 8 and the pH is 7.6 .... my CO2 = 0.05x 8 = 0.4ppm???

Um, I don't think that is right!


----------



## Caught29 (Apr 1, 2004)

Anona, it seems you were calculating using the wrong chart. You should have been using the top chart, meant for Kh measured in degrees. The claculation would be 0.85 x 8 = 6.8ppm. Your Co2 concentrations are still a bit low by most standards, but much higher than .4ppm.


----------



## bigpow (Oct 3, 2004)

Guys, please help me out here...
I've been scratching my head since last week

Tank #1
pH = 6.8
KH = 120ppm

Tank #2
pH = 6.4
KH = 120ppm

Using calculation, my CO2 level for both tanks are way high... doesn't sound right since this is from DIY Yeast (2liter bottles)


----------



## Roger Miller (Jun 19, 2004)

tank 1 would have about 28 ppm CO2 and tank 2 would have about 90 ppm CO2. 28 ppm is at least in the realm of possibility. 90 ppm probably isn't.

Are you confident in your pH and KH numbers? If they are correct then there must be something in your aquarium that is reading as part of the KH. Have you added a pH buffer of any kind?


Roger Miller


----------



## feefeefish (Apr 5, 2004)

What is the normal CO2 range, and how much CO2 does Flourish Excel add to a tank please?

Thank you!


----------



## Roger Miller (Jun 19, 2004)

Normal CO2 in well-aerated clean water is about 0.5 ppm.

Normal CO2 in an aquarium without a lot of plant growth is around 3-5 ppm.

The normal target range for CO2 enrichment is 15 to 30 ppm.

Flourish Excel adds no CO2 to a tank. It is a substitute for CO2.


Roger Miller


----------



## feefeefish (Apr 5, 2004)

My pH is 6.6 and my KH is 40ppm; am I correct in calculating the CO2 at 20ppm? If so, do I need to add any to the planted tank? I intend to add a little pinch of baking soda to the water to prevent a pH crash, as my water has a naturally low KH, so that may change the ratio later. The GH is 150ppm.


----------



## Roger Miller (Jun 19, 2004)

Your calculation is right. If you aren't already adding CO2 to your tank then that result is probably too high. The KH value you have measured may be effected by a non-carbonate buffer.

Unless your tank is very small "a little pinch" of baking soda won't have much effect.


Roger Miller


----------



## feefeefish (Apr 5, 2004)

Surely a buffer would raise the pH rather than keep it low? Or am I mis-understanding the roles of buffers? Unfortunately, the water from my faucet has these results. The pinch was about 1/4 tsp in 29 gallons. I'll check and recalculate tomorrow.

Thank you for your help!


----------



## Roger Miller (Jun 19, 2004)

Non-carbonate buffers usually cause the pH to be lower than with the carbonate buffer only.

Are you using well water?


Roger Miller


----------



## imported_Creature (Feb 6, 2003)

> quote:
> 
> Originally posted by feefeefish:
> Surely a buffer would raise the pH rather than keep it low? Or am I mis-understanding the roles of buffers? Unfortunately, the water from my faucet has these results. The pinch was about 1/4 tsp in 29 gallons. I'll check and recalculate tomorrow.
> ...


I'm sure you've answered your question about buffers, but just in case, all a buffer does is resist changes in pH. As Roger stated, carbonate buffers keep the solution buffered at a higher pH while others, at a lower pH.

So by raising your KH, buffer capacity, while maintaining your original pH setpoint, more CO2 must be added to get to your pH setpoint because the carbonate buffer is resisting the change in pH. Result? More dissolved CO2 in solution for plants to use. Want to know how they use it? I got it here somewhere....


----------



## imported_Creature (Feb 6, 2003)

> quote:
> 
> Flourish Excel adds no CO2 to a tank. It is a substitute for CO2.


Roger, I wonder what source of carbon is. I know during photosynthesis, there is an exchange of electrons caused by light causing CO2 to split thereby enabling the carbon to be used in the manufacture of glucose, the Calvin cycle.

The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2.

I don't see how Flourish can claim to provide carbon to plants along a different pathway or reaction. Perhaps I just haven't read enough.


----------



## Roger Miller (Jun 19, 2004)

The simple formula for photosynthesis is a gross oversimplification. The actual process involves addition of a carbon atom to a larger organic molecule, then division of the molecule into smaller fragments. Most of the smaller fragments are used to recreate a supply of the original organic molecule. About one fragment in six is used by the plant to create other organic molecules. Glucose may or may not be a final product in the reaction.

My understanding is that Excel provides a simple organic molecule similar to some of those smaller fragments that plants use to make sugars and other compounds. Plants can adsorb the short-chain organic compound provided by Excel and incorporate it directly into their metabolic processes in the same way that they would use the small fragments that they produce during photosynthesis. It's also possible that the compound added with Excel might be incorporated directly into molecule-building reactions that have nothing to do with photosynthesis.

I don't have trouble with that, but there are two things I don't understand about Excel.

The compounds in Excel are very reactive and biologically available. They're great bacteria food. I think a very large proportion of the carbon added with Excel may actually be consumed by bacteria rather than plants. I don't know how much might actually go to plants.

SeaChem also (at least informally) claims that Excel is not available to algae and that it may actually kill algae. I don't understand that at all.


Roger Miller


----------

