PKa vs. PKb

[Kostas Petritis]

JI2002,

You got it all wrong, from the beginning to the end and keep doing it . If you can not see what I (or others) meant, I am not going to bother to explain...

PS: Although you admit that the analogy is poor, you still presumed that I aliked you like that professor...

[Uwe Neue]

I have to admit that I am a pKa-man, but I also have to say that JT's question has some merit. If I have a compound with a pKa of 5, and pKb's do not exist. I have no clue if the compound will be more ionized or less ionized at pH 4. This is important for the chromatographer, since he would like to know if the retention increases or decreases if he goes from pH 5 to pH 3. If I have a pKb scale and a pKa scale, I know that the thing with a pKa of 5 is an acid, and I will get more retention at pH 4. I also will know that if the thing has a pKb of 9 (instead of a pKa of 5) it will be more ionized and thus have less retention at pH 4.

On the other hand, I think that one can confuse the hell out of oneself if one has a compound with a phenol function and an aniline function...

[HW Mueller]

Wow, now I am confused, it´s these simple things that do this, not really worthwhile to discuss, but: what confuses me again and again is this NEGATIVE log stuff, a smaller number is more acid rather than a larger one.....confusing. One can also start a flaming (is that what JI2002 did??) discussion on that convention. Anyway when confusion strikes I go to the K and the corresponding equilibrium equation to get me straight. Nevertheless, the PKa scale is nice as you can immediatly see at what pH a substance may be a buffer. If someone introduces a pOH, we might have to take pKb more serious?

[Victor]

It is technically wrong to refer to state that the pKa of a base e.g.pyridine is 5.2.

Rather, the pKa of the conjugate acid of pyridine is 5.2. If everyone made this distinction (as JI clearly and correctly has made in the posts) then there would be less of a confusion; although there could be confusion in the term "conjugate acid" itself.

I suppose this distinction is usually omitted because the correct termninology is more of a mouthful. There might be other problems with the use of pKb for compounds with acidic and basic groups as others have already discussed. There might even be problems with people referring to the pKb of acids (instead of referring to the pKb of their conjugate bases)..

[Bill Tindall]

Uwe, I can't recall a time, other than this discussion or a quizz I have long since forgotten, where I knew the pKa of something and didn't know its structure! So I have never in practice had the occasion to be confused about whether something with a pKa of XX would get more or less ionized with a change in pH. But would this not make a great trick prelim. question for an evil minded professor?

And for the others, it is not an arbitrary convention to discuss the acid/base characteristics of R3NH+ in terms of a pKa. If this compound has a pKa of 5 it is an acid as surely as benzoic acid is an acid. To discuss its acid/base properties in terms of a pKb makes no sense. I read the post that started this all and the only reason there was any confusion is because the original poster didn't provide the name or structure of what they were talking about-an unfortunate habit of posters.

[Uwe Neue]

Good point, Bill! I should have tried to stick with my lousy jokes instead of attempting to say something intelligent. The Germans say: Schuster, bleib bei deinen Leisten! (Free translation: stick to the stuff about which you know something...)