CO2 to reduce pH

[waste]

Hello all you chem folk

The other day someone said to use CO2 instead of muriatic acid to reduce pH in a feeder mechanism. The argument over using the acid is that if the valve somehow got stuck open and no one checked the pH for a period of time the lowest the CO2 would take the pH is ~6.4 while the unchecked acid would eventually drop the pH to ~.5 It didn't quite sound right to me, so I put it up to you who are much more kowledgeable than I

Isn't it the outgassing of CO2 that raises pH? (as in lowering the alkalinity)

Thanks for any thoughts - Ted

[JasonLion]

CO2 out gassing removes CO2 from the water and the PH goes up. CO2 injection does the opposite, CO2 is added to the water and the PH goes down.

CO2 injection is a fairly common method of lowering PH in large pools. It is not commonly used for residential pools because the equipment is more expensive than for acid injection and gas tanks can be quite heavy and thus difficult for a home owner to handle.

[chem geek]

Ted,

When CO₂ either outgasses or is added via injection the pH moves but the TA does not. The reasons are technical but in simplest terms when CO₂ combines with water it produces H₂CO₃ which is carbonic acid that then dissociates to form mostly bicarbonate ion, HCO₃^-, and a little carbonate ion, CO₃^2-, at least at the normal pH in pools ( 7-8 ). TA is increased when there are more carbonates in the pool, but is decreased when there is more hydrogen ion (i.e. lower pH) and these exactly cancel each other out as they are in pairs -- technically CO₃^2- counts twice towards TA while each H⁺ counts once and there are two of them in H₂CO₃.

As for the argument that the lowest the pH would get to is 6.2 via injection of carbon dioxide into the water, that is patently false. The pH can go as low as you want, but you will be fighting outgassing and at the point when the injection rate equals the outgassing rate then that is where the pH will ultimately end up. Since outgassing is a function of aeration (among other things), there is no way that someone can predict 6.2 as the ultimate low point. In fact, carbonated beverages have a low pH that varies from 2 to 4.5, though this is done by injecting CO₂ under pressure.

Richard

In actual practice with CO2 injecton for pH control there IS an increase in TA over time that requires the addition of acid anyway to get the TA lowered.

[chem geek]

I can understand an increase in total carbonates (including carbonic acid and dissolved carbon dioxide), but I wonder why the TA goes up over time (at a constant pH for comparison)? I wonder what component of TA is increasing over time. Perhaps there is some source of rising pH other than CO₂ outgassing as that would cause the TA to rise as well. For newer plaster, that would be a source of pH rise so perhaps even older pools have some continued, albeit slow, curing going on. Dissolving of pool plaster or of calcium carbonate scale would also be a source of rising pH and TA; perhaps near the CO₂ injection point as that could be at low enough pH to cause some minor corrosion. In 10,000 gallons, corrosion of one pound of plaster would raise the TA by about 10 ppm (and the pH from 7.5 to 8.5). High quantities of CO₂ especially at lower pH can be corrosive via dissolving of the calcium oxide component of calcium silicate:

CaO(s) + CO₂ --> Ca²⁺ + CO₃^2-

Richard

Chemgeek,
CO2 injection has been around for a long time for pools and also in aquariums. In both cases it causes a rise in TA over time in actual use. In aquariums TA is often called carbonate hardness or dKH. In a closed system with only CO2 added and bicarbonate ions in the system it is true that the addition of CO2 would have no effect on TA but in real life there are other chemical systems occurring such as the addition and consumption of sanitation products like cal hypo, liquid chlorine etc. or the reaction of plaster surfaces in a pool.
You cannot look at a chemical reaction "in a glass bubble" and expect that it will occur exactly the same way under real life conditions.

I think a telling test would be to measure TA, use CO2 injection to control pH for a period of time, then stop CO2 injection and allow pH to stabilize and measure TA again to see if it has stayed the same.

[waste]

Thanks for answering guys!

If you hadn't guessed, it was the Goldline guy who said this last week when I went to the seminar (I thought it was only to help us troubleshoot the units, but came away as a certified warantee center [?]) I didn't get a chance to go 1:1 with the guy, but have his e-mail address so I can ask questions that aren't of immediate concern.

Goldline's latest thing is the 'Sense and Dispense' which has ORP and pH sensors and a pH chem feeder (CO2 or muriatic) - it should be available this spring (it's funny, Autopilot has had that for a few years )

If I had thought about it for another minute, I should have realized that if taking CO2 out of the water raises pH, then (obviously ) adding CO2 would lower the pH (It's like my golf swing... I always mess up on the follow through )

Anyway, thanks for the info - and I hope that this thread will continue - before I know it, I'm gonna have to deal with one of these systems.

[chem geek]

I recently learned about some additional reasons why the TA will tend to rise in pools that are using CO₂ injection for pH control. If one is using chlorinating liquid as the source of chlorine, then the "excess lye" slowly raises the pH and the TA, but the CO₂ only lowers the pH with no change in TA. So there is a net rise in TA over time unless acid is added. For an ideally perfect balance, one should add Muriatic Acid to compensate for the "excess lye" and should inject CO₂ to compensate for the outgassing of carbon dioxide, though the latter effect can be reduced by lowering the TA level since higher TA results in faster carbon dioxide outgassing.

Evaporation and refill will tend to increase TA (and CH) over time as well.

[waste]

Great memory Richard!

Thanks for clarifying the old thread