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Ph Kh and CO2 relation

Jaap

Jaap

Member
Joined
30 Sep 2011
Messages
1,068
Location
Nicosia
Hi

I had a talk with Tom Barr last night and he suggested that a pH drop of 1 unit adds approximately 30ppm of CO2 regardles of kH.

Is this something we should consider?

What I knew up to now is that if I had a pH drop of something less than a unit but I had a high kH eg 9kH then the CO2 was at disarable levels.

Thanks
 
Behold, and do yourself and conclusion:

pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
Difference: ~11 mg/L CO2

pH 7.7, alkalinity = 5°dKH, temperature 25°C => 3.0 mg/L CO2
pH 6.7, alkalinity = 5°dKH, temperature 25°C => 30.6 mg/L CO2
Difference: ~28 mg/L CO2

pH 7.7, alkalinity = 10°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.7, alkalinity = 10°dKH, temperature 25°C => 61.3 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.7, alkalinity = 15°dKH, temperature 25°C => 9.1 mg/L CO2
pH 6.7, alkalinity = 15°dKH, temperature 25°C => 91.9 mg/L CO2
Difference: ~83 mg/L CO2

xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

pH 7.4, alkalinity = 2°dKH, temperature 25°C => 2.4 mg/L CO2
pH 6.4, alkalinity = 2°dKH, temperature 25°C => 24.5 mg/L CO2
Difference: ~22 mg/L CO2

pH 7.4, alkalinity = 5°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.4, alkalinity = 5°dKH, temperature 25°C => 61.2 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.4, alkalinity = 10°dKH, temperature 25°C => 12.2 mg/L CO2
pH 6.4, alkalinity = 10°dKH, temperature 25°C => 122.3 mg/L CO2
Difference: ~110 mg/L CO2

... or use my CO2 calculator: www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2
 
ardjuna said:
Behold, and do yourself and conclusion:

pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
Difference: ~11 mg/L CO2

pH 7.7, alkalinity = 5°dKH, temperature 25°C => 3.0 mg/L CO2
pH 6.7, alkalinity = 5°dKH, temperature 25°C => 30.6 mg/L CO2
Difference: ~28 mg/L CO2

pH 7.7, alkalinity = 10°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.7, alkalinity = 10°dKH, temperature 25°C => 61.3 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.7, alkalinity = 15°dKH, temperature 25°C => 9.1 mg/L CO2
pH 6.7, alkalinity = 15°dKH, temperature 25°C => 91.9 mg/L CO2
Difference: ~83 mg/L CO2

xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

pH 7.4, alkalinity = 2°dKH, temperature 25°C => 2.4 mg/L CO2
pH 6.4, alkalinity = 2°dKH, temperature 25°C => 24.5 mg/L CO2
Difference: ~22 mg/L CO2

pH 7.4, alkalinity = 5°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.4, alkalinity = 5°dKH, temperature 25°C => 61.2 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.4, alkalinity = 10°dKH, temperature 25°C => 12.2 mg/L CO2
pH 6.4, alkalinity = 10°dKH, temperature 25°C => 122.3 mg/L CO2
Difference: ~110 mg/L CO2

... or use my CO2 calculator: www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2
Click to expand...
How do you measure the co2? I mean you measure the alkalinity with a kh meter and the ph with a ph meter but do you have a co2 meter?

Thanks
 
But i have a pertinent and important question:

What are the consequences of this abrupt drop of PH and a suddenly raise of the CO2 in the fish and even in the plants?

Is not more reliable to inject Co2 in a stable or minor leverl so that as it is being injected is being consumed from the plants?

Best regards
 
Jaap said:
How do you measure the co2? I mean you measure the alkalinity with a kh meter and the ph with a ph meter but do you have a co2 meter?
Click to expand...
No, the above values are based just on KH and pH measurements and relationships. Still I believe that in most cases it should be quite close to the reality (but of course, in some cases it could be wrong).
 
So how do we really know kh and ph are related if we cannot independantly measure co2? Maybe Tom Barr ir right and maybe not...how can we know?
 
KH doesn't influence the amount of CO2 in the water, it influences the associated pH drop ( buffers it). So little KH, means little buffering -> larger pH drop, lots of KH, large buffering capacity, -> small pH drop.
 
Tom is right yes. Doesnt matter your KH, if you lower your ph by one unit you will have around 30 ppm of CO2 in your tank. This is very simple and no need to overthink it. Look at the ph-kh table and you should be able to see that at different KHs your ph will vary the same as to add 30 ppm CO2.

By the way you also dont need more CO2 added in a tank with higher KH compared to a lower KH one.
Basically you can forget about KH with regards to CO2.
 
I think Mr Barr is just offering simple guidelines to try and make our hobby easier ;)

If you have a high KH then the high buffering capacity means you don't need a 1 pH unit decrease to get 30ppm CO2. You're guaranteed at least 30ppm if you get a 1pH drop, but your fish might not appreciate it!

ardjuna, it's a pity your table doesn't work for Prague water!


P
 
pepedopolous said:
If you have a high KH then the high buffering capacity means you don't need a 1 pH unit decrease to get 30ppm CO2.
Click to expand...

This is just not true. But everyone seems to think it is no matter what you have in your water (eg phosphate buffers etc).
 
Hi Jose, I read the link you provided and still not sure. They seem to be talking about whether CO2 addition changes KH, and conclude that it doesn't.

I understand that KH/PH tables always underestimate CO2 concentration as pH is affected by other compounds in aquarium water. The only case where the tables apply accurately is inside a drop checker because we control the content of the drop checker solution and keep the KH at 4. The CO2 crosses the air gap to the drop checker solution and the pH decreases giving a colour change.

However in an aquarium, no matter the KH of the water and the pH before CO2 is added, when we add CO2 the pH decrease we measure during that time is simply a result of the added CO2. Other compounds in the water are highly unlikely to have a significant effect during this time.

Where you and I seem to disagree is that I think that the KH is relevant because if you add X amount of CO2 to low KH water you will see a bigger decrease in pH than in high KH water (buffering).

So I think you can add CO2 to aquarium water, making the concentration 30ppm and yet for high KH water the pH may not drop by a full unit.

Cheers,

P
 
Sorry about that. Ill look for the threads on the matter when I have some time but have a read around the barr report its all there.

"However in an aquarium, no matter the KH of the water and the pH before CO2 is added, when we add CO2 the pH decrease we measure during that time is simply a result of the added CO2. Other compounds in the water are highly unlikely to have a significant effect during this time."

This is totally true.

"So I think you can add CO2 to aquarium water, making the concentration 30ppm and yet for high KH water the pH may not drop by a full unit."

This last bit is where I disagree.

Lets do something. Look at the ph-kh table. Take two tanks as examples; One with KH 1 and the other one with KH 10. Suppose you have 8 ppm of CO2 at the starting equilibrium for both when you are not adding CO2 and write down their equilibrium ph. Then we start adding CO2 until we get to 30 ppm. At what ph are we now? write them down. Now do the first ph for each tank minus the second one after we add CO2. How much is the jump in ph?.
 
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ardjuna said:
Behold, and do yourself and conclusion:

pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
Difference: ~11 mg/L CO2

pH 7.7, alkalinity = 5°dKH, temperature 25°C => 3.0 mg/L CO2
pH 6.7, alkalinity = 5°dKH, temperature 25°C => 30.6 mg/L CO2
Difference: ~28 mg/L CO2

pH 7.7, alkalinity = 10°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.7, alkalinity = 10°dKH, temperature 25°C => 61.3 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.7, alkalinity = 15°dKH, temperature 25°C => 9.1 mg/L CO2
pH 6.7, alkalinity = 15°dKH, temperature 25°C => 91.9 mg/L CO2
Difference: ~83 mg/L CO2

xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

pH 7.4, alkalinity = 2°dKH, temperature 25°C => 2.4 mg/L CO2
pH 6.4, alkalinity = 2°dKH, temperature 25°C => 24.5 mg/L CO2
Difference: ~22 mg/L CO2

pH 7.4, alkalinity = 5°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.4, alkalinity = 5°dKH, temperature 25°C => 61.2 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.4, alkalinity = 10°dKH, temperature 25°C => 12.2 mg/L CO2
pH 6.4, alkalinity = 10°dKH, temperature 25°C => 122.3 mg/L CO2
Difference: ~110 mg/L CO2

... or use my CO2 calculator: www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2
Click to expand...

Wow never came across this before! Bit confused over the salinity part, how do you determine this?
 
Jose said:
http://www.barrreport.com/showthread.php/11066-Different-CO2-forms-under-low-vs-high-pH
Click to expand...
Jose, first I want to tell that "kwisatz" from barrreport.com, and "ardjuna" from ukaps.org is one and the same person ... me. So it was my thread on barrreport.com.

Secondly, it seems to me that you do a wrong assumption if you say that you can have 8 mg/L of CO2 before you start adding CO2 into your tank. If you don´t have some really bad tank with a lot of organics, then you can have around 3-5 mg/L CO2 at equilibrium with air + a few fish waste. Still, I don´t understand your logic. Please, can you give me some example of your counts? If I have pH 6.7 at KH 1, then I´ll have (theoretically) 6 mg/L of CO2. If I have pH 6.7 at KH 10, then I have (theoretically) 61 mg/L of CO2 ... but in reality I wont have 61 mg/L CO2, but I´ll have around 3-5 mg/L ... because that´s the equilibrium level before you add any artificial CO2 into your tank. The 61 mg/L CO2 is just theoretical level (in case you add artificial CO2). The same apply for pH 7.7 at KH 1 = 0.6 mg/L CO2, vs. pH 7.7 at KH 10 = 6 mg/L CO2.
 
You can estimate the salinity level based on conductivity: each 100 µS/cm (conductivity) ≈ 0.05 ‰ salinity.
Click to expand...
Ah right, and can this be determined using a TDS meter? I've came across a few conversion sites but no exactly sure which to convert.

Came Across this....
Electrical Conductivity units are expressed in micro-siemens/cm at 25°C (µS/cm).
1000 EC = 1000 µS/cm = 640 ppm (parts per million)

So 0.64 ppm per 1µS/cm?

Probably got that all wrong :D
 
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