PKa vs. PKb

[JI2002]

Hi, there,

Instead of using PKb for the bases to describe their relative basicity, I like to know why PKa are commonly used(for their protonated form, of course)? It's confusing sometimes. When people say the PKa=5 for compound XYZ, I don't know if XYZ is acid or base. It will be much clearer by using PKb for bases and Pka for acids. Thanks in advance.

[Bill Tindall]

Be thankful that this is your most serious worry because it is about to be eliminated by the Bronstead definition of acids and bases.

You say you don't know if something is an acid or a base. Well, if the structure or name is provided then you know what the material is. If the material is capable of providing a hydrogen ion in any reaction then it can be an acid. So, HCl, NH4+ , H2O and CH3OH are all acids, at least in some reactions.

Skeptical?

CH3OH + OH- == CH3O- + H2O

Similarly, if the stuff is capable of accepting a hydrogen ion in any reaction then the stuff can be a base. So, OH-, NH3, H2O and CH3OH are all bases.

Paired with the structure, the pKa or pKb will tell you how strong the acid or base species is. Small numbers mean stronger. If a pKa is provided then the reaction described by the pKa is an hydrogen ion dissociation.

Sign up for my class at Pittcon and learn more.....

[JI2002]

Bill,

Thank you for the reply, but you didn't answer my question: Instead of using PKb for a base, why is PKa used for its conjugate acid?

For example:

B + H2O = BH+ + OH-

In which B is a base and BH+ is its conjugated acid. Usually PKa value is provided for BH+, why not provise PKb value for B instead? Using PKa for both acids and bases can cause confusion.

Please see EARLY ELUTING PEAK thread

So it's not my most serious worry, it just makes me think about it. Because PKa of BH+ + PKb of B = 14 in water at 25C, it's easy to figure out the other one if one is known. So why not use PKa for acids and PKb for bases?

It's a simple question, but it could be more complicated than what you think. I'm sorry if I gave you the impression that I'm a high school or undergraduated student. MOre comments please.

[tom jupille]

The obvious comment is that pKa + pKb > 14 in mixed aqueous/organic solvents, but that't probably not relevant to your original point.

We use pKa for everything because "that's the way we do it". That may sound like I'm joking (well, OK, I am, to a acertain extent!), but I think any terminological convention involves a certain amount of arbitrariness. Once a convention has become established, it tends to stay. Think of any of the standard chromatographic definitions and ask yourself why they are defined as they are:
k' : why not just use the ratio of retention time to dead time (i.e, how many column volumes are required for elution)?
N : why not just use a ratio of peak width to retention time (or vice versa)? And why not use sigma instead of baseline width (that would eliminate some of the tailing-related variation)?
Rs: why not use a ratio of retention time difference to sigma?

[Bill Tindall]

I will try to be more clear on this attempt....

Your premise (I think) is that we can tidely divide all the chemicals into bases, for which we will list pKb and acids for which we will tabulate pKa, and neutrals. In making this division we can all just look at the chemical as tell which table it will go into and conversely everyone will know which table to use to look something up.

Which table, pKa or pKb, will you go to to look up the base or acid strength of HPO4-- ? While tabulating chemicals, which table will contain H2O and CH3OH? How about glycine?

The point of these examples is that whether something is an acid or a base depends on the chemistry it participates in. Hence, the world of chemicals can not be tidely divided into acids and bases and tabulated accordingly. So, in describing/tabulating a chemical's acidic or basic properties we have to also say what the chemistry is for which we are providing an equilibrium constant. That in turn leads to a choice of what table to put some chemicals in.

In the case of ammonia/ammonium I readily found its basic properties tabulated in a table of pKb's and its acid property tabulated in a table of pKa's.

However, the book that tabulated everything as an acid (eg. NH4+) probably found it cheaper to make one table and the reader searching for acid/ base information on a chemical need only look in one place to find what is needed. I think this is the answer to your question.

Perhaps most would find the "confusion" of having everything in one table less than the confusion of trying to decide what table contained a value for HPO4--, or glycine. The "one pKa Table fits all" proponents need only remember that big number mean things are weak acids. If something is a very weak acid I don't have a problem going foreward to realize that it's conjugate must be a strong base.

[Bill Tindall]

I thought of another example of this sort of consideration. The oxidation potential of oxidants could be tabulated in one table and reduction potential of reductants listed in another table. Of course, then one might have to look in both tables to fine hydrogen peroxide, which can act either way. I'm sure someone decided that it was easier to put everything in one table and hope people remember that positive values mean one thing and negative values mean the opposit, not unlike thae case of acids and bases listed in one table of pKa.

[JI2002]

I didn't say to list PKa for acids and PKb for bases in the books. What I was trying to say was that, in HPLC analysis, if you have seven analytes: two acids, three neutrals and two bases, why not provide PKa values for the acids and PKb values for the bases? After reading EARLY ELUTING PEAK thread, you've got to be kidding me if you tell me it didn't make somebody confused!

I'll take the "that's the way we do it" statement. It's a good answer, but it reminds me the article "Grandma's Ham" written by Dr. Dolan in LC.GC.

If the use of PKa for everything is established in a terminological convention, I'll take that too if it's true.

Tom, you went too far by stating PKa + PKb > 14 in mixed aquous/organic solvent. Pka is only used to describe relative acidity. According to your analysis, Pka will be unknown in a gradient HPLC run. Consider temp effect too! Remember PKa + PKb = 12 in water at 100C.

PKb came into use probably at the same time as PKa. I just wanted to know why it's not used in the field of HPLC. It's legitamate question and I didn't expect to be offerred some class at Pittcon. It's just laughable.

[Kostas Petritis]

JI2002,

About the "that's the way we do it" statement, it is pretty true and legitible, especially when it comes down to terminology.

Your post, reminded me of an experience I had during my defence of my Master degree. Although, my master thesis was in analytical chemistry, the master itself was very organic syntesis oriented (32 out of 35 people were doing organic syntesis for their thesis). As a result, during my thesis presentation I had 3 pure, hard core organic synthesis proffesors that didn't know too much about analytical chemistry.

In general, two of them were asking legitible questions, and one of them was asking tricky, destabilizing questions.

My collegue that was doing cyclodextine analysis by using LC, she got the question: What will happen to me if I eat the cyclodextrines that you are analyzing? Am I going to die, am I going to digest them or what?

When I finished with my presentation, the guy knowing that I am orgininally from Greece, he started by asking the definition of chromatography (too easy to be true). Then he said: "You know sir, we are now able to analyze compounds that do not have any colors. Is there any reason for calling it still chromatography? Shouldn't we change this term and if yes what would it be a good term?"

Of course any logical reply, was not good enough for him and he was always coming back that we need to change this term.

Knowing what the guy was trying to do, and conlcuding that no answer will be good enough for him, I said: "Sir, you have a point... if I will be famous enough in the future, I am going to change it". Everybody laughed in the room, and the guy with a smile, gave up.

Furthermore, take for example terms like "normal phase" and "reverse phase" chromatography. What shoud be normal and what reverse? Ion pairing chromatography had about 10 different names suggested for it...

Terms sometimes change but this is quite rare, such as HPLC (from high pressure to high performance...).

Finally, I would suggest not to take personally Bill's comment about his class in Pittcon. Remember that the participation in this forum is solely volonteer and along with some good advice/answer/comment some people might try to promote their product/services. For some, these are chromatographic supplies, for others chromatography classes etc...

Mentioning his class in PITTCON, it was more of an announcement for Bill, than a personal "you need to go back to school" type of attack...

[tom jupille]

I didn't say to list PKa for acids and PKb for bases in the books. What I was trying to say was that, in HPLC analysis, if you have seven analytes: two acids, three neutrals and two bases, why not provide PKa values for the acids and PKb values for the bases? After reading EARLY ELUTING PEAK thread, you've got to be kidding me if you tell me it didn't make somebody confused!

I'm the one who jumped to the wrong conclusion. Had the pKb been cited, I would probably have not made the mistake. But saving me embarassment is not likely to persuade the rest of the world to change its terminology .

I'll take the "that's the way we do it" statement. It's a good answer, but it reminds me the article "Grandma's Ham" written by Dr. Dolan in LC.GC.

The "Grandma's Ham" story strikes close to home because that's the way most humans operate (I'll stick a copy of the joke on the "Around the Water Cooler" board in case anyone's interested). Unless a different procedure or terminology has really dramatic examples over the "way we've always done it", things won't change. You (JI2002) have made some good points in favor of changing. Bill has made some points in favor of not changing. My guess is that most people will continue to simply use pKa.

Tom, you went too far by stating PKa + PKb > 14 in mixed aquous/organic solvent. Pka is only used to describe relative acidity. According to your analysis, Pka will be unknown in a gradient HPLC run.

I don't know that I'd agree with "unknown", but I would concur with "inexactly known" -- which has been cited as source of inaccuracy when making computer predictions of retention.

Consider temp effect too! Remember PKa + PKb = 12 in water at 100C.

Which reinforces the point I was trying to make: that you can't assume pKa + pKb = 14

PKb came into use probably at the same time as PKa. I just wanted to know why it's not used in the field of HPLC. It's legitamate question and I didn't expect to be offerred some class at Pittcon. It's just laughable.

I agree with Kostas; I think you read more into that sentence than Bill intended.

Kostas: re "chromatography". I'll have to remember (i.e., steal) that answer! (Of course the other possible response would have been "it's named in honor of its inventor".)

[Kostas Petritis]

Tom,

I used that one and much more in favour of not changing the name. The guy though would always say that in his point of view, as in reality we can analyze compounds that do not absorb in Vis, there is no point of calling it like this and the name had to change... Maybe he didn't believe it himself, and just wanted to make a point (which I didn't get at that point and I haven't get it up to date...).

As I said though, the guy was there trying to intimidate and was always the one asking the weird questions so proceeding with the logical answers was not the way to go...

[tom jupille]

Kostas: you're quite right. My last comment was a joke, but a joke actually created by Mikhail Tsvett: I believe the Russian word "tsvett" (Цвет) loosely translates as "color" (Hopefully, some of our Russian-speaking members can confirm or deny this). He named the technique after himself!

[Uwe Neue]

Tom, what do you expect from the poor Tswett guy. If he would have called it jupillography, nobody would have understood what it means...

[tom jupille]

That's easy for you to say; there are plenty of things named after you. For example, I just did a Google search on "Neue Trennsäule" and got 26 hits:
http://tinyurl.com/a9jhe

[Kostas Petritis]

Uwe,

You can instead play around with Tom's name though :

How about "atomography" ?

Hey! this is a good alternative name for chromatography!

[JI2002]

Tom, I was NOT trying to change the terminology, I was just trying to understand why. Like I said, I accept "that's the way we do it", I'm a little disppointed in the answer though. When I said PKa + PKb =14, I specified it's in water and 25C. Make no mistake there.

Kostas, Thank you for sharing the story. I was lucky that I didn't have that kind of headache in my defense.

First of all, I want to say you got me wrong. I didn't know what's wrong with your committee member"the Guy", but I want to make it clear that I was certainly looking for answers, not asking tricky questions.

Second, from the way you told your story, you probably wanted to tell "the guy" to shut up when he asked those questions. Just because he's your committee member, you didn't dare do so.

Third, I didn't expected a response like yours, but I started to realize that the question made some people upset. However, you are in NO position to tell me to shut up. Hey, what's wrong with trying to find an answer?

Like exactly what you said, participation in the discussion is voluntary. If you don't want to answer the question, you had the choice to ignore it( This is another difference from your defense in which you had to respond to the questions, poor analogy!)

Tom and Kostas, no matter what's Bill's intention, don't you think he can find some other ways to make his annoncement? Don't bother to cover it.