Is the proton with the highest or lowest pka removed first when titrated with a base?

Answer 1

It has been a while but from memory it is the lowest.

Answer 2

I THINK, I'm not totally sure because I'm going on memories from long ago, but I THINK the proton comes off when the pH=pKa. In that case, as you're titrating some acid with a base (meaning you're slowly raising the pH) of the solution, the protons with the lowest pKa will come off first. Hopefully that rings a bell?

Answer 3

I agree, lowest pKa reacts first, take for example phosphoric acid, pK1 = 2.12, pK2 =7.21, and pK3=12.67 Its third proton is quite difficult to make it react, you will need very basic conditions. So you can easily get sodium phospate monobasic and dibasic, but tri-basic, requires very high pH. If you use a proper indicator, such as methyl orange to titrate phosphoric acid using say, sodium hydroxide solution, you will get the color change at pH around 4, when all the first proton is gone but not the second or the third. But if you use phenolphtalein, you will get the color change at around 11, when all the first and second proton is reacted, but not the third. So in such cases you must make the calculations accordingly. Finally, I believe it would be impractical trying to titrate the third proton, since there isn't an indicator or pH meter that is dependable at such high pH as 12.67