[urtula]

Hello all,

I am aware that adding CO₂ to water will decrease the pH and the CO₂ + H₂O <--> H+ + HCO₃- <--> 2H+ + CO₃^2-equilibrium.
However, I do not understand why CO₂ would have a higher solubility at higher pH.
Would it be too simply to state this is due to Henry's law? If the pH of the water is high, limited amounts of CO₂ are in the water and if I then inject air rich in CO₂ in the water, a higher solubility of CO₂ in the water will be reached because of a lot more CO₂ is present in the air, shifting the equilibrium more towards dissolving (higher solubility) CO₂ in the water?

[Hunter2]

Higher pH means solution is more alcaline. CO2 is in water an acid, which neutralize the higher pH to lower one, what means more CO2 will be dissolved and forming Carbonate.

[urtula]

Higher pH means solution is more alcaline. CO2 is in water an acid, which neutralize the higher pH to lower one, what means more CO2 will be dissolved and forming Carbonate.

Yeah, that I understand, but is that the same as having a higher solubility?
I get that at a higher pH more CO2 will be able to dissolve in the water (and thus reduce the pH and ultimately increase the concentration of cO2 in the water), however is this the same as having a higher solubility at higher pH?
If this is the case, is it then because the higher the pH, the less CO2 is present so the concentration of CO2 in the water is lower (at high pH) than at low pH (high CO2 concentration) ?

Perhaps I am just struggling with the wording (I am not a native speaker).

[Hunter2]

The soloubility is higher at higher pH, because of chemical reaction. Not Opposit.

[Borek]

Yeah, that I understand, but is that the same as having a higher solubility?

Define "solubility" and we will start from there.

[urtula]

Define "solubility" and we will start from there.

I was thinking about how good a substance will dissolve ( at what rate, so higher solubility means a faster rate of dissolving).
So I am thinking about how a substance (eg salt) will dissolve better (faster) in water without any salt in it yet (lower concentration of salt in water, thus better dissolving of the salt you put it) or when the temperature is higher, it will dissolve beter.
So here: for the pH, I am not sure how this works. I figured it was because at higher pH there is less CO2 (like the salt example I gave) so CO2 can dissolve more compared to at lower pH?

The soloubility is higher at higher pH, because of chemical reaction. Not Opposit.

I am still having a hard time grasping why the higher pH would cause a higher solubility. The higher pH would increase (promote) the chemical reaction?

[Hunter2]

The dissolving of a salt  like NaCl is a physical process. The dissolving and its behaviour is more or less described what you wrote. The pH is almost neutral.
The dissolving of  a gas CO2 is also in one  part a physical process but it also  reacts with water chemically to carbonic acid.  The pH will decrease. If the pH of the water is already high then this acid will neutralized and more CO2 can be dissolved in comparison to water with neutral or even lower pH.

[Borek]

I was thinking about how good a substance will dissolve ( at what rate, so higher solubility means a faster rate of dissolving).

Dissolving rate depends mostly on mass transport (mixing/diffusion/convection), properties of the solvent and solute are much less important and in many cases completely negligible.

[urtula]

I was thinking about how good a substance will dissolve ( at what rate, so higher solubility means a faster rate of dissolving).

Dissolving rate depends mostly on mass transport (mixing/diffusion/convection), properties of the solvent and solute are much less important and in many cases completely negligible.

I am confused now, so you are telling me now that the pH does not really influence the dissolving of the CO2?

[Borek]

No. I feel like you are confusing solubility (which is about equilibrium) with kinetics (which is about how fast we get to the equilibrium). Solubility is a function of pH. time required to saturate the solution depends more on other factors than on pH.