Why is Ph climbing?

[Rangeball]

If I recall correctly, when my alk was very high as PH rose, alk tended to rise as well. It may very well have been from the process you stated occuring simultaneously, but when my ALK is below 200 it isn't as noticeable.

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

[chem geek]

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

My point exactly. Simply aerating just increases the pH (ignoring the increase in alkalinity you saw at high alkalinity levels). It is the adding of acid that lowers the alkalinity (and also lowers the pH). The reason you add the acid first is that the outgassing of CO2 is more rapid at lower pH so by adding acid first you make the process more efficient (occur more rapidly). It would still work if you aerated first and then added acid -- it would just take a lot longer.

[waterbear]

When you inject CO2 for "pH control" you are implying that there is some sort of pH drift (demand) already that you are trying to counteract. Since injecting CO2 makes the pH drop, the implication is that you have something else in the aquarium that is trying to make the pH rise (i.e. is basic or alkaline). So the combination of adding CO2, which just makes the pH drop, plus this basic source, causes the pH to be stable (or to drop less) and for the alkalinity to increase.

CO2 injection is used in aquariums (usually freshwater live plant tanks to promote growth) and also used in commercial pools for pH control. The net effect in both cases is a rise in TA

Richard

just for clarification here are some links of commercial equipment
http://www.brockent.com/CategoryView.asp?CategoryId=377
http://www.poolequip.com/Merchant2/m...egory_Code=CO2
http://www.aquasol.com/co2_systems.htm

If you read the info on the links they blame the rise in TA on the size of the CO2 bubbles injected.

[chem geek]

The only explanation I can come up with that is consistent with what these guys are saying is that injecting CO2 into the water increases the Carbonate Alkalinity, which is a true statement. However, there is a difference between carbonate alkalinity and Total Alkalinity. The former does not include [OH-] while the latter does. For the pool alkalinity tests, you measure Total Alkalinity, not Carbonate Alkalinity. For practical purposes, the two are very close in a pool (except at high pH), unless there is CYA present (in which case there is the CYA adjustment that can be made).

What can I say? Perhaps you can do an experiment if you've got one of these CO2 injectors. Just be sure to do the experiment in water that doesn't have other things going on (i.e. living things doing photosynthesis and respiration, etc.).

Anyway, I'm off on vacation. Talk to y'all after next week!

Richard

[waterbear]

The only explanation I can come up with that is consistent with what these guys are saying is that injecting CO2 into the water increases the Carbonate Alkalinity, which is a true statement. However, there is a difference between carbonate alkalinity and Total Alkalinity. The former does not include [OH-] while the latter does. For the pool alkalinity tests, you measure Total Alkalinity, not Carbonate Alkalinity. For practical purposes, the two are very close in a pool (except at high pH), unless there is CYA present (in which case there is the CYA adjustment that can be made).

What can I say? Perhaps you can do an experiment if you've got one of these CO2 injectors. Just be sure to do the experiment in water that doesn't have other things going on (i.e. living things doing photosynthesis and respiration, etc.).

Anyway, I'm off on vacation. Talk to y'all after next week!

Richard

I have started a new thead in the China shop to continue this side discussion since it is getting a bit too technical for the general forum.
http://www.poolforum.com/pf2/showthr...9517#post29517