pH and C02 relationship - The Planted Tank Forum
 
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post #1 of 4 (permalink) Old 05-04-2005, 07:10 AM Thread Starter
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pH and C02 relationship

Hi,

While on my search on finding a good DIY C02 system, I've been reading a lot about how everything is supposed to work the motives behind C02 injection.

Basically, when C02 levels are high, pH goes down, therefore what is the actual equation between carbon dioxide and carbonic acid?

(I should know this, I have an AP chemistry test new Tuesday...)

Is the equation:

HC03 -> H20 + C02 (threfore, a high concentration of C02 will push the reaction to the left, producing carbonic acid -> lowers pH)

Thanks for the help!
Bronson

P.S. - I think this water chemistry should help me with my AP chemistry studying, hopefully... Spending a lot of time on these boards rather than with my books...
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post #2 of 4 (permalink) Old 05-04-2005, 01:28 PM
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Quote:
Originally Posted by bc_hawaii
Basically, when C02 levels are high, pH goes down, therefore what is the actual equation between carbon dioxide and carbonic acid?

(I should know this, I have an AP chemistry test new Tuesday...)

Is the equation:

HC03 -> H20 + C02 (threfore, a high concentration of C02 will push the reaction to the left, producing carbonic acid -> lowers pH)

Thanks for the help!
Bronson

P.S. - I think this water chemistry should help me with my AP chemistry studying, hopefully... Spending a lot of time on these boards rather than with my books...
First, good luck on the AP test. One of my co-workers was chair of the chemistry test committee last year (I am a chemistry professor). There should be plenty of acid-base equilibrium questions on it since we teach so much of that in freshman chemistry!

The above reaction needs to be corrected slightly:

H2CO3(aq) <----> H2O(l) + CO2(aq or g)

The CO2 on the right can be considered aqueous, though it does react quickly to produce the carbonic acid on the left. You are right on the addition of the CO2 producing more carbonic acid. There is more to it though:

H2CO3(aq) <----> HCO3-(aq) + H+(aq)

The carbonic acid is a weak acid which dissociates to bicarbonate (KH buffer!) and hydronium ions (written as the proton above). This is why the pH drops. Also, increasing bicarbonate ions (KH) will then resist the dissociation of carbonic acid, keeping the pH higher (for the same level of dissolved CO2).

Keep studying!

Kevin

Kevin

72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias

2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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post #3 of 4 (permalink) Old 05-04-2005, 11:38 PM Thread Starter
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Thanks for the informative reply, I think I understand the process now...

So, there are two equilibrium equations which involve carbonic acid. One with the C02 and the other an acid/base reaction to form bicarbonate.

Just a side question, are these basic equations the reasoning behind controlling KH in a planted aquarium?

Is this how the thought process works:

Need to inject CO2 -> inject C02
CO2 increased -> Carbonic Acid Increase -> pH Lower
pH lower -> add bicarbonate to raise kH -> buffer (control) pH

Therefore, is there a three way relationship going on here? A relationship between C02, pH, and kH?

Thanks for all the help,
BC
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post #4 of 4 (permalink) Old 05-05-2005, 04:27 PM
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Yes, the relationship is expressed as follows;

CO2 (ppm) = 3.0 * KH * 10^(7.00 - pH)

A sample chart can be found in many places, here is one; http://aquaticconcepts.thekrib.com/

Cheers,
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