Air Pumps & PH

[JamesFish]

For those reading this don't consider me completely bonkers after my PH settled I've rarely tested any tanks for a long time.

Nothing is ill or showing signs of stress I wanted to see effect a different substrate is having. Meter read 7.9 little odd as the tap is 7.2 so what caused it to increase the tank has no gravel or anything else that would normally push it up. Has a soil style substrate.

Tested tank 2 - 7.7 again odd as no gravel its quartz so wouldn't buffer it normally few pebbles but nothing I would expect to raise it.

Checked tank 3 - 7.7 same on tank 4.

Tested water in the water butt 8.2. Tested tap yup reads 7.2. Looked in water butt with a light little sand at the bottom rinsed and cleaned out. refilled left 24 hours come back home tested it again 8.2. Now slightly puzzled as nothing should be pushing up the PH.

Looked at all 4 tanks the 2 high reading and the water butt all run off air pumps or have one backing up the main filter.

Did a little reading and appears the rate I'm driving in air might actually be pushing it up is this possible?

The tanks where its dropping fastest I only ripple the surface the slow drop tanks have APS 50's or a APS 150 on them.

Anyone seen this happen before?

[racoll]

Aerating water drives out dissolved C02, increasing the pH.

What method are you using to measure the pH, by the way?

[JamesFish]

Hi,

Thanks for that was guessing I was removing something along with the chlorine out the water just couldn't think what.

I'm using a electronic PH meter its same as you find on ebay for a few £ its not been calibrated for a while so no promises its 100% accurate but does let me see what's happening.

[.Plecomania.]

tap water usually gives a lower pH rating right out of tap (has some CO2 dissolved) than after you let it sit (ie. in aquarium). once the CO2 leaves the water (air will off-gass the CO2 by dissolving more O2 and other gasses) the pH will normally rise. More buffered/harder water will not see drastic changes.

[dw1305]

Hi all,

Aerating water drives out dissolved C02, increasing the pH.

More buffered/harder water will not see drastic changes.

Racoll and Plecomania are right, it is a CO2 effect. Lower CO2 levels will mean that the (very small) proportion of CO2 that dissolves as H2CO3 is even smaller. H2CO3 is an acid ("H+ ion donor"), depressing pH. Harder water (dKH) has more bicarbonate HCO3- ions (a base ~ "H+ ion acceptor") than soft water, which means that the added acid won't have as greater effect on pH (pH is a ratio of acids:bases).

air will off-gass the CO2 by dissolving more O2 and other gasses

I don't think oxygen replaces CO2, I think their relative concentrations are independent, also I don't think a lot more oxygen is dissolved (although some will be, and oxygen is a base, as in O-H) it is more that the air bubbles increase the gas exchange surface area and this causes the O2/CO2 levels to equilibrate with atmospheric levels more quickly.

If you saturated the whole water column with oxygen (usually via the oxygen evolved during photosynthesis), you would get a higher pH, because oxygen is a base. If you had very soft water, a lot of submerged plants and low surface area:volume ratio you end up with very large diurnal pH swings, as the CO2:O2 ratio changes dependent upon whether the plants are net oxygen producers or consumers.


from <http://www.fondriest.com/environmental- ... ity/ph/#p3>.

cheers Darrel

[2wheelsx2]

I don't think oxygen replaces CO2, I think their relative concentrations are independent

I don't think that's quite accurate since the partial pressures of all the dissolved gases in a fluid are interrelated. So having higher O2 and other gases will affect how much CO2 may be dissolved in a fluid. I may be wrong as it's been a few decades since my inorganic chemistry classes and in geology I normally deal with magmatic and conic waters.

[jvision]

I don't think oxygen replaces CO2, I think their relative concentrations are independent

I don't think that's quite accurate since the partial pressures of all the dissolved gases in a fluid are interrelated. So having higher O2 and other gases will affect how much CO2 may be dissolved in a fluid. I may be wrong as it's been a few decades since my inorganic chemistry classes and in geology I normally deal with magmatic and conic waters.

While it's true that all gases in a fluid are interrelated, pO2 and pCO2 are not directly inverted. If CO2 is driven off, O2 is not the only gas that may increase in partial-pressure. I think this is what dw1305 is refering to.

[2wheelsx2]

Thanks for clarifying Jason. I didn't mean inverted. I meant that they are not independent. The partial pressures are dependent on the other partial pressures of all gases in solution, that's all.

[.Plecomania.]

(air will off-gass the CO2 by dissolving more O2 and other gasses)

I meant Nitrogen and other atmospheric gasses too, not just O2. the air isn't mostly O2 anyway.

[dw1305]

Hi all,

While it's true that all gases in a fluid are interrelated, pO2 and pCO2 are not directly inverted. If CO2 is driven off, O2 is not the only gas that may increase in partial-pressure. I think this is what dw1305 is refering to.

Yes I am, sorry it wasn't very clear what I meant. Even though CO2 is much more soluble than oxygen etc., you only have ~400ppm CO2 in the atmosphere, against ~ 21% Oxygen. The outcome of all of this is that you can have high CO2 and oxygen levels simultaneously.

You have a good example of this in high tech planted tanks (where CO2 is injected) to give you levels of ~30ppm CO2 (usually estimated using a drop checker containing the narrow pH range indicator bromothymol blue in a 4dKH "water", and using the known relationship between pH, CO2 and dKH <http://www.aquascapingworld.com/magazin ... nship.html>).

During the photo-period you often get "pearling" from the photosynthesizing plants, where oxygen bubbles form on the plant leaves. This is because the tank water is now 100% saturated with oxygen and the "excess" oxygen can't go into solution.

cheers Darrel