CO2 calculation - My doubts

[Milan]

I'm trying to figure out the way the CO2 calculation works, and I have my serious doubts about it.

Here is the formula:
CO2=3*KH*10^(7.0-pH)

There is also a table based on this. It appears to me it has an empirical background, but that's OK. What bothers me is that it's based on a single point pH/KH measurements, which in my mind is fundamentally wrong. Let me illustrate this:

Case 1:
Tap water is KH=6, pH=7.5
After X amount of CO2 dissolved, KH remains 6, but pH drops to 6.8.

Case 2:
Tap water is KH=6, pH=8.0
After X amount of CO2 dissolved KH remains 6, but pH drops to 6.8.

Case 3:
Tap water is mixed with RO and the final readings are KH=6, pH=6.8
No CO2 used (X=none)

Is the X in all 3 cases the same? I doubt it. But the above formula suggests
they are (CO2=29ppm). In Case 3, it's even absurd. This really blows my mind.

I my mind, CO2 concentration calculations should reflect the work (X amount
injected) needed to move the pH from point A to point B for a given KH. So,
two points, start and end pH! Could it be that in the above formula part in
brackets "(7.0-pH)" considers 7.0 (neutral) being a start point?

Chemistry not being my field, I may be seriously missing something here, but
if someone could comment on this, I would appreciate.

Thanks

 

[malkore]

No, the equation assumes that only carbonate hardness is acting as a buffer. Some people have very funky water with other buffers...sometimes phosphate based, which completely invalidate the Kh/pH.

pH and Kh are inter-related...so the equation does not assume a pH of 7.0, but rather assumes that no other buffers are present, so that pH and Kh exist in their normal 'harmonious' state, and as such, any lowering of the pH from the 'normal' level in relation to Kh must be attributed to the carbonic acid created by CO2 injection.

In case 1, that pH coincides with the Kh level - so CO2 table is accurate within 2ppm

In case 2, pH is too high for the Kh level...another buffer is present. CO2 table is inaccurate...severity of inaccuracy depends on the % of buffer that is not carbonate.

In case 3, you don't indicate percentage RO used, but assuming the RO is perfect, 7.0pH, 0 Kh, and you used 50%, then your Kh =3 pH around 7.2-7.4


Does that clear up things?

 

[Milan]

Thanks Malkore,

I believe this is my missing part :

... assumes that no other buffers are present, so that pH and Kh exist in their normal 'harmonious' state, ...

Now that I'm starting to wrap my mind around it, I'll shoot some more questions if you don't mind ...
Could you be more specific as to what is the "harmonious state"? Are there any tables to go by, or formulas ...? For instance, the tap water I am using is is KH~6, and pH=7.8-8.0 ... Is it harmonious, hence my CO2 calcs accurate?

 

[czcz]

Your tap KH and pH are pretty much the same as mine, fwiw. As you can see tap yields about 3ppm CO2, which is the expected range. I feel confident about CO2 values from the standard formula, from observation of plant health and limited algae. HTH

 

[malkore]

Actually I kind of mis spoke for sake of simlicity. The chart assumes that MOST of your buffering is from carbonate.

In that sense, with a dKh of 9 and pH of 7.8, i'm 'normal' Your Kh of 6 and pH of 7.8 is reasonable too. I'd say you have a little something else buffering, but not enough to freak out over. I'd run your CO2 on the higher side, say 30ppm per the chart, to ensure you have enough.

A liquid CO2 kit won't give you any better results than the table, because it's using the same principle as the table, so you're results would be consistently 'skewed'.

I have not found a nice chart that shows proper pH for a given Kh...but czcz is on the right track. If your non-injected water gives a pH & Kh that works out to about 3ppm of CO2, then your water is 'harmonious'.


I have seen some very strange water in my days. dKh of 3, with a pH of 8.0...not normal. Kh of 7, adn a pH of 6.9....some strong acid is present, or likely pH Down was used.