Passive CO2 diffuser - Page 8 - DIY Aquarium Projects

mistergreen · 07-30-2018, 08:50 AM

The CO2 in carbonic acid will eventually break loose, in addition to the free CO2, as we all know the pH in CO2 filled water will eventually rise left alone. It's the carbonic acid that affect the pH.

Gerald · 07-30-2018, 11:26 AM

Quote:

Originally Posted by tiger15

I don’t know if your undertanding is correct. I think Hernry coefficient is derived experimentally by comparing air concentration of co2 to total aqueous concentration of co2 that is sum of ionized (carbonic acid) and free co2. This is similar to total ammonia determination which is the sum of free and ionized NH3.

OK thanks tiger - so you're saying the K value for CO2 includes all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.

tiger15 · 07-30-2018, 04:25 PM

Quote:

Originally Posted by Gerald

OK thanks tiger - so you're saying the K value for CO2 includes all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.

Actually, I am not certain, but can only assume that K is derived from total dissolved CO2 that by definition includes free and ionized CO2. The two exist simultaneously and interchangeably. Free dissolved CO2 in equilibrium is not the same as CO2 mist created by artificially injection.

Ionized CO2 is carbonic acid. Carbonate and bicarbonate are dissolved salt of cations, not CO2. The only way to ascertain what is included in K is to review the lab procedure that derived K.

The reason CO2 has much lower K than other atmospheric gases is that it is a polar molecule that has great affinity for polar H2O, and can ionize. O2 and N2 are non-polar and dissolve in free phase only, no comparison.