[orgo814]

I read in my inorganic textbook that CO2 interferes with measuring the pH of weakly acidic cations? How does this happen? My book didn't really give an explanation as to why/how it would interfere.

[Hunter2]

What happens if CO₂ dissolved in water?

[orgo814]

You get carbonic acid. Why is this only a big deal with weak acids? Is it because they are around the same acidity?

[Hunter2]

Carbon dioxide creates acidic solutions of carbonic acid. So the measuring of other weak acids will be disturbed.

[orgo814]

Ok. And i am guessing that it doesnt affect moderately/strong/very strong acidic cations because they are strong enough to resist it?

[Hunter2]

Check the pKa values of the acids.

[Borek]

Ok. And i am guessing that it doesnt affect moderately/strong/very strong acidic cations because they are strong enough to resist it?

It depends not only on the acid strength, but also on the concentration. If the acid is weak or diluted, amount of H⁺ produced by the dissociation of carbonic acid can substantially change the pH of the solution.

pH of pure water, saturated with atmospheric CO₂ is always below 6 (something like 5.6 if memory serves me well). 10^-6M solution of HCl (strong acid) left in contact with air will not have pH 6, but a little bit lower one. For weak acids situation is not much different - just the effect is more prominent.