Talk:Acid dissociation constant/Archive 2

Table of values for common substances
I am concerned that no information about ionic strength is given in this table. Petergans (talk) 10:13, 15 August 2008 (UTC)
 * 1) references at the head of the table
 * 2) removed aniline again!
 * 3) removed compounds listed in the thermodynamics table, but added link to that table.
 * 4) revised adenine and codeine which are bases, not acids.

Well, as we say in the introduction, equilibrium constants are usually determined at concentrations high enough that the activity ratio is 1. As such, I've never in my chemical career yet run into a table of ionic strengths. EagleFalconn (talk) 21:08, 15 August 2008 (UTC)
 * I suggest you read the IUPAC guidelines for publishing equilibrium constants. In the thermodynamics tables the values are stated to be for I=0. If the sources for the final table contain information on I, it would be sufficient to quote them in the header. The point is, a reader might find a different pK value somewhere else because it relates to a different ionic strength. In elementary texts ionic strength and activity considerations are usually completely ignored, but in WP we have to be more rigorous. Petergans (talk) 09:07, 16 August 2008 (UTC)
 * I agree, I was simply pointing out that they're not broadly accessable. The resources I've identified so far are the National Physical Laboratory and an article in the Iranian Journal of Science and Technology, which I don't have access to. Taghikhani, V.; Modarress, H.; Vera, J. H. Activity coefficients of individual ions.    Iranian Journal of Science and Technology  (2002),  26(B2),  299-306.EagleFalconn (talk) 18:46, 16 August 2008 (UTC)
 * That's not what I meant. When a pK value is given in a table, the temperature and ionic strength at which the value was obtained should accompany it. We have the temperature at the head of the table. The sources I meant were Lange's handbook and Knovel, where the values came from. Do they give the ionic strengths? If not, we might have to say "at various ionic strengths", but that would not be very satisfactory. 0.1M is the most commonly used, but we can't assume that this applies to all. My database does give ionic strength with the values, but only lists organic ligands if they form metal complexes, so I can't check all of them. Petergans (talk) 10:09, 17 August 2008 (UTC)
 * Aren't pK values for reference purposes extrapolated to I=0? They ought to be (but I wouldn't bet on it ;) Physchim62 (talk) 14:54, 19 August 2008 (UTC)
 * Ah. Lange's extracts to I=0. I can't find anything specific from Knovel, but if what Physchim62 says its true, it'd be a good guess to assume it does. EagleFalconn (talk) 05:39, 20 August 2008 (UTC)
 * In that case we should stick to values given by Lange and Goldberg. If Knovel does not state I=0 explicitly that is a fundamental flaw. Petergans (talk) 08:04, 20 August 2008 (UTC)

Minor revision
The article has been "refereed". Go here for comments. I have done some minor revision in the light of these comments, bearing in mind that the author is a specialist in pK values in non-aqueous solvents. He has offered to write short paragraphs on organic synthesis and liquid chromatography. Should we take him up on this offer? Petergans (talk) 14:05, 18 August 2008 (UTC)
 * It can't hurt. EagleFalconn (talk) 05:39, 20 August 2008 (UTC)

Go for it
The article appears to be stable. Why not go for FA NOW? Petergans (talk) 10:10, 7 October 2008 (UTC)


 * Sounds good. I'm still a bit afraid that some people will complain that there are some paragraphs with no references, but perhaps I'm being overly paranoid and the best way to find out is to try. --Itub (talk) 05:29, 8 October 2008 (UTC)

I don't think this article has a chance for Featured Article status as it is right now. Just two things that I can note right off the back. It does not have a history section (which of course the usage of the acid disassociation would), and it does not explaining how pKa=pH at the half-equivalence point outside of the Henderson-Hasselbach equation. From my experience in College Chemistry Lab, this is critical for experimental determination of pKa values (which is then used to find Ka) in monoprotic acids, so the fact that it gets one sentence will not cut it.--Jorfer (talk) 13:17, 8 October 2008 (UTC)

I tried to give the following so that students can better understand how pKa=pH relates directly to the definition of Ka and not just the Henderson-Hasselbach equation: The point at which 50% neutralization of the acid occurs is called the half-equivalence point. At this point since
 * $$K_a = \mathrm{\frac{[A^-][H^+]}{[HA]}}$$

and $$[A^-] = [HA]$$, the definition of the equilibrium constant can be rendered
 * $$K_a = \mathrm{\frac{[A^-][H^+]}{[A^-]}}$$

which cancels the [A-] out and leaves $$K_a = [H^+]$$. Since, pKa = -log10Ka and pH = -log[H+], pKa = pH at the half-equiavalence point.--Jorfer (talk) 13:19, 8 October 2008 (UTC)
 * User Jorfer has not apparently understood that this is the same derivation as in the article which states:

After rearranging the expression defining Ka, and putting pH = −log10[H+], one obtains pH = pKa – log ( [AH]/[A−] )
 * The only difference is that logarithms are taken at the beginning, not the end. This formulation is more useful because it allows an easy definition of the buffer range as well as the half neutralisation point.
 * Regarding history, it is there in the definitions section. Petergans (talk) 08:33, 9 October 2008 (UTC)
 * I realize that Henderson-Hasselbach is taken directly from the definition, but I feel that this needs to be clarified (if not simply stated). Also, my mistake, the definition does cover a brief history, just not in a historic style. This section could be buffed up though with a history of determining the Ka of substances experimentally (i.e. the techniques used, notable contributors, etc.).--Jorfer (talk) 16:41, 9 October 2008 (UTC)

Archive
All discussions prior to FAC have been archived. Petergans (talk) 09:18, 30 October 2008 (UTC)

Equilibrium symbol
There is some discussion at Wikipedia talk:Accessibility that you may find of interest. --Itub (talk) 15:53, 17 October 2008 (UTC)
 * Thanks for this info. In fact I already use a picture in the documentation for my programs (HTML, Dreamweaver). BUT, $$\rightleftharpoons$$ is not the only $$ ...$$ in the article, so if it's not rendered correctly then there may be problems elesewhere. Unless we get a FAC comment to the contrary I suggest things are left as they are. Petergans (talk) 08:19, 19 October 2008 (UTC)
 * I've changed my mind on this and have replaced $$\rightleftharpoons$$ with throughout. However, I think the alignment is not quite right. In
 * acid + base conjugate ...
 * I suggest the space between the horizontal lines should line up with the (lower case) text characters. Petergans (talk) 17:20, 29 October 2008 (UTC)

Comments brought over from FAC
I was in the middle of updating my comments at FAC when Sandy archived the nomination, meaning that it will not be promoted to FAC this time around. I thought it might be useful to leave my remaining comments here. I will only add comments here on issues that you have responded to but for which I still feel there is an issue.


 * The "struck O" notation is in error in a couple of places in that you have " O " instead of "O". I changed all the struck Os to thetas because of this, before I figured out that the problem was just the sequence of tags.
 * I still don't think the table of dielectric constants needs to be there.
 * With regard to the listy section; yes, I think separate paragraphs might work. The main templates do need to go, though.

If you would like an outside reviewer for any further changes, please just let me know. I will respond if I have time. Mike Christie (talk) 23:18, 25 October 2008 (UTC)

Notes Oct 30th
More notes.
 * In the section on monoprotic acids, this appears: "The extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated when the pKa and analytical concentration of the acid are known. See ICE table for details." I think it would flow more smoothly if we could avoid the "see" construction by integrating the reference to the ICE table into the text.  How about: "When the pKa and analytical concentration of the acid are known, the extent of dissociation and pH of a solution of a monoprotic acid can be easily calculated using an ICE table."?
 * Single sentence paragraphs are somewhat jerky to read. The suggested revision above would make that paragraph a single sentence.  I think it could be moved up and appended to the paragraph that currently ends "classified as strong bases."
 * Both agreed. BTW, ICE table is one of many articles linked from this page that I have revised. Gibbs free energy, however, was not one of them, which is why the notation is different there. Petergans (talk) 09:59, 31 October 2008 (UTC)
 * OK, that explains the notational discrepancy. Mike Christie (talk) 21:22, 1 November 2008 (UTC)


 * In the section on "Acidity in nonaqueous solutions" you mention methanol as an alternative solvent. The following discussion does not mention methanol again.  Is methanol of sufficient interest as a solvent in this discussion to warrant its inclusion in the table of dielectric constants, or to add a column for it in the tables of pKa values?
 * Methanol is important but I don't have any pK data to hand. The research paper could be cited but, as the title suggests, it is not specifically about pKa values; it is about the determination of stability constants of coordination complexes in which pKa values are needed as subsidiary information, an application which is mentioned in the Importance... section. I may be able to fish out some pKa values from my database. Petergans (talk) 09:59, 31 October 2008 (UTC) In the database I found a few horribly complicated molecules have been done in pure methanol, but more publications referred to mixed methanol/water systems. Unfortunately, my database does not cover molecules of pharmaceutical interest, which would be the main area of application. Petergans (talk) 10:55, 31 October 2008 (UTC)
 * The question is if another expert in this field were to read this section of the article, would they have a legitimate complaint that the article does not treat methanol as thoroughly as it should? I'll take your word for it either way on this; I have no way of knowing. Mike Christie (talk) 21:22, 1 November 2008 (UTC)
 * The sentence referring to methanol has been removed. Other solvents could have been mentioned too, but determination of pKa in those solvents is a minority sport. Petergans (talk) 12:14, 2 November 2008 (UTC)
 * OK. I've struck this comment on the assumption that it's OK not to mention methanol. Mike Christie (talk) 14:52, 2 November 2008 (UTC)


 * It would be nice to avoid this single-sentence paragraph: "In solvents of low dielectric constant ions tend to associate forming ion pairs and clusters, which complicates the interpretation of pKa values". Is there more that can be said here?  I'm not knowledgeable about chemistry, so these suggestions may not be suitable, but some additional questions that could be answered in this paragraph occurred to me: What would be a reason for using a solvent of low dielectric constant despite this problem?  What is a typical example of such a solvent in wide use?  Do the complications prevent the measurement of pKa completely?  If not, are there techniques that could be mentioned?
 * The sentence has been removed, as it is of marginal significance. Low dielectric constant is much more significant for metal ion coordination complexes, which is not the topic here. Petergans (talk) 10:55, 31 October 2008 (UTC)


 * Instead of "temperature /Kelvin" I would suggest either "temperature in degrees Kelvin" or "temperature in Kelvin", whichever would sound the most natural to someone with a chemistry background. The current version may be standard usage but looks odd to a lay reader.
 * OK Petergans (talk) 09:59, 31 October 2008 (UTC)

More later, or possibly over the weekend -- I am travelling tomorrow and may not be editing much till Saturday or Sunday, though I may get a short time online tomorrow night. Mike Christie (talk) 23:35, 30 October 2008 (UTC)

Notes Nov 1st

 * The caption to the image in the lead starts "The weak acid acetic acid donates a proton to water in an equilibrium reaction to give the acetate ion and the hydronium ion". Two adjacent links that run blue text together are discouraged, and I think "acetic acid" is more useful to state first here, so how about: "Acetic acid, a weak acid, donates a proton to water in an equilibrium reaction to give the acetate ion and the hydronium ion"?
 * Accepted.Petergans (talk) 12:14, 2 November 2008 (UTC)


 * I also think you can cut the word "Key:" from the caption; shorter is better for captions, and it is evident that the following text provides a key.
 * OK Petergans (talk) 12:14, 2 November 2008 (UTC)


 * The "See also" section links to several articles that are already linked in the body of this article. See WP:SEEALSO for the guideline on this; generally there's no need to have a link here if it appears in the body.  There is an exception for editorial discretion, if necessary; are any of these needed?  Personally I tend to think that anything important enough to link here should probably be linked somewhere in the article, so I usually try to eliminate these sections.  The ones that are repeated in the article are isoelectric point, Henderson–Hasselbalch equation, Determination of equilibrium constants and Hammett equation.  Dissociation constant appears in the navbox near the top (which I will comment on in a bit; I think it might be better as a template).  I'd suggest deleting all the above links from the "See also" section.  That leaves two: Hydrolysis of metal salts and QSAR.  Is there a natural place where these could be linked from the body of the article?
 * Removed all but one of the links. The remaining one is a related topic which is not suitable for linking from this article. Hammett equation and Taft equation are close relatives, see below. Petergans (talk) 12:14, 2 November 2008 (UTC)
 * OK. Dissociation constant is in fact linked from one of the navboxes, so it could be cut too, but I see the logic in leaving it there.  If that navbox were to end up at the bottom of the article as a template I'd suggest cutting it in that case, as the two links would be quite close to each other then. Mike Christie (talk) 14:56, 2 November 2008 (UTC)


 * The external links in the "External links" section should each have a short description so that a reader can understand why they might wish to go to that site. The layout guideline for links is here.
 * Done Petergans (talk) 12:14, 2 November 2008 (UTC)

-- Mike Christie (talk) 22:49, 1 November 2008 (UTC)
 * There are two navboxes: one in the lead, and one early in the first section. WP:ACCESS recommends that a navbox in the lead comes before the text, to make it easier to read.  Graham87, who is blind and uses a screen reader, commented on the accessibility of the text at this talk page section at Sandy's request.  You can see his comments there.  How about making one of the navboxes an end-of-page template, along the lines of the "Solutions" template already at the end?  More than one template at the end is OK.  I would also suggest making the navbox that is not already a template into a template, just to make it easier to edit.  Graham didn't say that the location of the navbox after the intro text is a problem, but it might be best to try it in both locations and move it to comply with the guideline unless it causes problems.
 * The first navbox has been removed. All the essential links contained in it either were in the article or have now been added, except for dissociation constant and proton affinity which are related articles, but which are not on topic here. Petergans (talk) 16:03, 2 November 2008 (UTC)
 * OK. Graham's remaining comments were that it would be good to be able to hide the navbox, and that the equilibrium symbol should use Template:Eqm.  On the first point I have left a note with another editor who is expert on templates to see if it is easy to make Template:Chemical equilibria hideable.  On the second point, he doesn't give a reason, but I would guess it's because the image you're using (Image:Equilibrium.svg) can't be read easily by speech synthesizers.  Is there any reason not to use Template:Eqm? Mike Christie (talk) 17:23, 2 November 2008 (UTC)
 * Geometry guy just made the template hideable, so that resolves that question. I think it should start visible, as it is now, but I will check with Graham and see if there is a strong preference for it to start hidden for any reason. Mike Christie (talk) 17:50, 2 November 2008 (UTC)
 * I see most of the harpoons have been replaced with Eqm. I have replaced the remaining two.  I don't have time to do more tonight, and may not have much time tomorrow, but will see what I can work in. Mike Christie (talk) 02:35, 4 November 2008 (UTC)


 * I've added a link to the Taft equation and have revised that article. It contained an elementary error of the kind that gives WP the reputation of being "patchy" in quality of articles. The error was to use k, not K, for an equilibrium constant. This might seem trivial but it is not. k is used to represent a kinetic rate constant. What made be blow my top was the fact that this error was copied over from the IUPAC gold book, which many WP editors believe is gospel! Petergans (talk) 12:14, 2 November 2008 (UTC)


 * The article on the Taft equation was actually correct; this equation is used for calculating rate constants. Perhaps, like other free energy relations, it is also used with equilibrium constants, but it is certainly used with rate constants and I think that was the original use (the rates of a certain hydrolysis reaction). I looked at several books and all most used the lowercase k. --Itub (talk) 13:55, 2 November 2008 (UTC)
 * Thanks for this correction. The article did not say that k is a rate constant, I just assumed it was an equilibrium constant. I'll revert and remove Taft from this article. The comment about Gold Book still stands as that was the source of the unexplained equation. Petergans (talk) 16:03, 2 November 2008 (UTC)

Notes Nov 2nd

 * The images are all copyright compliant; at least one is missing a description but that doesn't have to be fixed for this article. However, the graphs all need a source of some kind.  The source doesn't have to supply the identical graph, or the exact numbers used in the construction of this graph; it just has to give an interested reader a place to go to see that the graph presented matches a reliable source.  A reference to a handbook of data, or to a textbook that contains similar graphs, would suffice.  The image articles can contain the source information themselves -- it doesn't have to be added to this article.
 * I think the graphs you to which you refer are ones that I created myself in EXCEL or, for speciation diagrams, with my own program HySS. I have previously tried to to modify the source information for an image, but could not see how to do it. What is the procedure? Petergans (talk) 16:03, 2 November 2008 (UTC)
 * There's no requirement to do it a special way; I'd suggest you just hit "Edit this page" when looking at the image and add a "==References==" or "==Sources==" section, and add the sources in whatever way you like. Here is an image that I added references to recently; this one actually needed three or four references. Mike Christie (talk) 17:45, 2 November 2008 (UTC)
 * Done Petergans (talk) 09:46, 3 November 2008 (UTC)

-- Mike Christie (talk) 15:27, 2 November 2008 (UTC)
 * Some of the references don't have page numbers or chapter references so that the reader can locate the relevant information. In some cases this may be legitimate if the reference is to a discussion that is carried on through an entire book or article, but otherwise a more specific location is needed.  I looked at this version (specified in case you add or change references, since I want to refer to the footnotes by number).  The following references seem to me to need a page number or range, chapter number, or section number: 5, 7, 10, 13, 23, 25, 34, 35, 36, 37, 38, 40, 41, 42, 43 and 44.
 * Fixed all but 34 to 44 (current 36 to 46). These last are not references to any specific material cited in the article. Rather, they are a kind of additional reading in relation to the applications to which pKa values have been put. I think they come under the "legimitimate" category mentioned above. The book titles indicate that the whole book is concerned with a particular topic. Petergans (talk) 11:12, 3 November 2008 (UTC)
 * All appear to be fixed except reference 13, March & Smith's Advanced Organic Chemistry, which appears to be used just for the fact that acetonitrile is used to obtain pKa values for organic compounds. Mike Christie (talk) 11:49, 4 November 2008 (UTC)

Template to category?
As this article has evolved the templates have become less and less important. I now suggest that the remaining template:Chemical equilibria be replaced by a new category: Equilibrium chemistry. This category will contain all the topics now linked in template:Chemical equilibria and many more, as detailed in the provisional list below. Unfortunately I don't know how to create a category, so if this suggestion is acceptable, can some please help to create it.

name: Equilibrium chemistry

sub-category of : Chemistry

home chemical equilibria

categories: pH indicators, Acid-base chemistry, Thermodynamics, Separation processes, Solutions

acid-base extraction*acid-base homeostasis*acid–base reaction*acid dissociation constant*binding constant* Brønsted–Lowry acid-base theory*buffer solution*dissociation constant*determination of equilibrium constants*distribution coefficient*distribution ratio*equilibrium constant*equilibrium stage*equilibrium unfolding*Gibbs free energy*Haber Bosch process*haemoglobin*Henderson–Hasselbalch equation*hydrolysis*ICE table*law of mass action*Le Chatelier's principle*liquid-liquid extraction*Partition coefficient*phase diagram*phase rule*Protein pKa calculations*reversible reaction*reaction quotient*self-ionization of water*solubility equilibrium*relative volatility*sulfuric acid*stability constant*theoretical plate*thermodynamic equilibrium*vapor-liquid equilibrium*van 't Hoff equation

Petergans (talk) 10:22, 4 November 2008 (UTC)


 * I can help with this. If you haven't yet seen it, you may be interested in this, which gives a lot of background information and lists the pros and cons on three methods of organizing information: lists, categories and templates.  One point it makes is that it is often useful to have both categories and templates on the same topic; they are not mutually exclusive.  Mike Christie (talk) 11:56, 4 November 2008 (UTC)
 * I would like to go ahead with creating the category. We can decide later if the navigation template: chemical equilibria should also be retained. It was originally created by Happy Camper for chemical equilibria. I feel that is is not qute right for this article because it contains a number of links which are irrelevant to the topic. What is your opinion Ivan (Itub)? Any other opinions? Petergans (talk) 14:13, 4 November 2008 (UTC)
 * I'm generally not a big fan of these navigation templates, except for very well defined sets of topics (otherwise they end up getting bloated). I think a category should be enough in this case. The relevant articles are already linked when they appear in the text, and if someone wants a comprehensive list they can go to the category. --Itub (talk) 15:03, 4 November 2008 (UTC)
 * Agreed, nem. con. Have you the time to create the category? The provisional list above is already in alphabetical order. It would be better if a chemist did it, don't you think?.Petergans (talk)
 * I'm not sure about the template idea, because equilibrium chemistry is so large a field, pretty much all of chemistry. I'm not hostile to the idea of a category, but I'd like to see a little more discussion and a more concrete proposal as to where it fits with the other categories. Physchim62 (talk) 16:52, 5 November 2008 (UTC)
 * The main reason for suggesting this category is that equilibrium chemistry is under-represented in WP. It covers a very broad range of disciplines and so will be complementary to existing categories which tend to be restricted to one discipline. The articles I have collected together in the provisional list above all mention equilibrium or pK explicitly. But look at articles like the chelate effect and molecular recognition to see how badly quantitative equilibrium chemistry is sometimes treated. No article even exists dealing with the stability constants of metal complexes. These are examples of things on my to do list. The equilibrium chemistry category will draw attention to the fact that such diverse topics have a fundamental property in common. Petergans (talk) 08:45, 6 November 2008 (UTC)

(reset indent) Nav templates IMO are more user-accessible than categories, since they give a certain structure. Also, not all articles need be listed. Some nav templates I've done include:


 * Reaction mechanisms
 * Chromatography
 * Organometallics
 * Chiral_synthesis

Some relevant discussion here: Wikipedia_talk:WikiProject_Chemistry/Archive_16

On the other hand, categories will just give a hodge-podge of all articles listing that topic. Creating a cat is simple: simply add to all articles which you want. Go to Category:Acid dissociation constant, add a Acid dissociation constant and that's about it. --Rifleman 82 (talk) 17:30, 5 November 2008 (UTC)

Notes Nov 5th
I think the "Importance of pKa values" section could be improved by trying to make the sentences flow more from each other. I will work on this in a sandbox and post a draft here, if I can come up with anything better than what is there already.

Sorry I haven't had much time to review the article over the last couple of days; I may have a little more time tomorrow but I do owe another article a GA review. I should have more time over the weekend. I think it's now quite close to a renomination for FA but I'd like to give it a couple more reads through first. Mike Christie (talk) 03:44, 6 November 2008 (UTC)

In Section "Factors that determine the relative strengths of acids", the sentence
 * "Proton sponge, 1,8-Bis(dimethylamino)naphthalene, has a pK$a$ value of 12.1. It is one of the strongest amine bases known. The high basicity is attributed to the relief of strain upon protonation and strong internal hydrogen bonding."

seems wrong. At the very least, it needs to be referenced. Physchim62 (talk) 08:51, 6 November 2008 (UTC)


 * What part of it seems wrong? It sounds plausible enough to me, and it agrees with what the proton sponge article says, which references for all these claims. I haven't checked the references, though, but I'm sure one or two of them could be useful for this article. --Itub (talk) 09:51, 6 November 2008 (UTC)
 * Checking again, the incorrect part seems to be that the conjugate acid has a pK$a$ of 12.1, which makes it a stronger base than trimethylamine for example (and much stronger than aniline). The reference is in Proton Sponge, and I would transfer it except that I would like comment on the exact nomenclature that we're using here. This is a topic which is very "nomenclature-sensitive", so I shan't change without being sure that I'm not introducing a new style! Physchim62 (talk) 13:38, 6 November 2008 (UTC)
 * Also, I don't see where the strain is in unprotonated Proton Sponge: it has limited rotational freedom, yes, but that isn't strain (merely an entropy problem in its synthesis), and that is not relieved on protonation, quite the reverse! Physchim62 (talk) 13:47, 6 November 2008 (UTC)
 * The content here was essentially copied from proton sponge so any error here is also an error there. Given that the original literature is cited there I very much doubt if there is an error. Yes, proton sponge is a stronger base than trimethylamine. Petergans (talk) 14:36, 6 November 2008 (UTC)

Notes Nov 9th
Here are notes on all the remaining issues with the article that I can see. In these suggested rewrites I haven't preserved links because I wanted to be able to work with the text more easily; if we agree on the sequence of sentences I will be glad to make the parallel changes to the article text.

Importance section
Here’s an attempted rewrite of the "Importance" section. I’m posting it here rather than to the article because of the likelihood that in this rewrite I may have made subject-matter errors. What I’m trying to do is make the text flow more naturally. Comments?


 * A knowledge of pKa values is important for the quantitative treatment of systems involving acid–base equilibria in solution. Many applications exist in biochemistry; for example, the pKa values of proteins and amino acid side chains are of major importance for the activity of enzymes and the stability of proteins.   Protein pKa values cannot always be measured directly, but may be calculated using theoretical methods.   Buffer solutions are used extensively in biochemistry to provide solutions at or near the physiological pH for the study of biochemical reactions; the design of these solutions depends on a knowledge of the pKa values of their components.   Important buffer solutions include MOPS, which provides a solution with pH 7.2, and tricine, which is used in gel electrophoresis.  Buffering is an essential part of acid base physiology including acid-base homeostasis  and is key to understanding disorders such as acid-base imbalance.


 * Buffer solutions also play a key role in analytic chemistry. They are used whenever there is a need to fix the pH of a solution at a particular value. Compared with an aqueous solution, the pH of a buffer solution is relatively insensitive to the addition of a small amount of strong acid or strong base. The buffer capacity of a simple buffer solution is largest when pH = pKa. In acid-base extraction, the efficiency of extraction of a compound into an organic phase, such as ether, can be optimized by adjusting the pH of the aqueous phase using an appropriate buffer. At the optimum pH, the concentration of the electrically neutral species is maximized; such a species is more soluble in organic solvents having a low dielectric constant than it is in water. This technique is used for the purification of weak acids and bases.


 * In pharmacology, ionization of a compound alters its physical behavior and macro properties such as solubility and lipophilicity (log p). For example, ionization of any compound will increase the solubility in water, but decrease the lipophilicity. This is exploited in drug development to increase the concentration of a compound in the blood by adjusting the pKa of an ionizable group.


 * Knowledge of pKa values is important for the understanding of coordination complexes, which are formed by the interaction of a metal ion, Mm+, acting as a Lewis acid, with a ligand, L, acting as a Lewis base. The ligand may also undergo protonation reactions, so the formation of a complex in aqueous solution could be represented, symbolically by the reaction
 * [M(H2O)n]m+ +LH [M(H2O)n−1L](m−1)+ + H3O+
 * To determine the equilibrium constant for this reaction, in which the ligand loses a proton, the pKa of the protonated ligand must be known. In practice, the ligand may be polyprotic; for example EDTA4− can accept four protons; in that case, all pKa values must be known. In addition, the metal ion is subject to hydrolysis, that is, it behaves as a weak acid, so the pK values for the hydrolysis reactions must also be known.


 * Assessing the hazard associated with an acid or base may require a knowledge of pKa values. For example, hydrogen cyanide is a very toxic gas, because the cyanide ion inhibits the iron-containing enzyme cytochrome c oxidase. Hydrogen cyanide is a weak acid in aqueous solution with a pKa of about 9. In strongly alkaline solutions, above pH 11, say, it follows that sodium cyanide is "fully dissociated" so the hazard due to the hydrogen cyanide gas is much reduced. An acidic solution, on the other hand, is very hazardous because all the cyanide is in its acid form. Ingestion of cyanide by mouth is potentially fatal, independently of pH, because of the reaction with cytochrome c oxidase.


 * In environmental science acid–base equilibria are important for rivers and lakes. For example, humic acids are important components of natural waters. Another example occurs in chemical oceanography: free copper(II) is toxic to phytoplankton but when copper(II) is complexed to naturally-ocurring organic ligands it is not toxic . Petergans (talk) 11:37, 10 November 2008 (UTC)


 * The design of a universal pH indicator requires a mixture of indicators whose adjacent pKa values differ by about two. This is because the transition range of a pH indicator is about pKa ± 1: this is the range over which the color is intermediate between the colors of the acidic and basic forms of the indicator.

(end of rewrite)

I cut two sentences because I did not understand their relevance to this article or the adjacent material well enough to include them in the rewritten version:
 * The isoelectric point of a given molecule is a function of its pK values, so different molecules have different isoelectric points. "Isoelectric focussing is a technique used for separation of proteins by 2-D gel polyacrylamide gel electrophoresis." Does isoelectric focusing use pKa values, or just make use of buffer solutions?  If the latter I’m not clear why we need to mention it here.
 * Suggested addition in magenta. The application depends on the pK values of different proteins being different. Petergans (talk) 11:37, 10 November 2008 (UTC)


 * "The reaction that converts adenosine triphosphate to adenosine diphosphate is very pH sensitive." Same question: does this need to be mentioned?  If so, can we specifically say what it is about pKa values that is critical here?
 * This sentence was added by Eaglefalconn, who presumably had a reference for it. The statement is true but needs a citation which I cannot provide just now. Probably best to remove it. Petergans (talk) 11:37, 10 November 2008 (UTC)

In addition I would like to expand the "environmental science" paragraph, or cut it, or combine it with another paragraph. I don’t see an easy combination target. Is this something for which an example can be given of a specific situation where pKa values are used? Or can more be written about this?
 * I've suggested a couple of examples. (ref 46) has lots more, but I don't have a copy of it to hand. Likewise, with Millero's book - I know that this is an active research topic at the Universities of Miami and Messina, amongst others. The emphasis is usually on metal-ion complexation.
 * You've done a splendid job on the rewrite. Shall I incorporate it into the article? Petergans (talk) 11:37, 10 November 2008 (UTC)


 * Thank you; please go ahead! Mike Christie (talk) 11:43, 10 November 2008 (UTC)


 * I've gone ahead and redone this section based on the discussion above. Could you just check that the environmental science section is correctly cited?  I had to move the references to where I thought they should be, so it would be good if you could confirm.  I also don't see that we have a citation for the comments about isoelectric point and isoelectric focusing; or can that be covered by one of the two existing gel electrophoresis citations, further up? Mike Christie (talk) 00:07, 11 November 2008 (UTC)

Lead
The lead does not appear to discuss the "Factors that determine the relative strengths of acids" section, or the "Experimental determination of pKa values" section. (See WP:LEAD.) I’ve added some material to the draft below to try to correct this, and cut somewhat elsewhere to compensate. As above I have put the rewrite here for fear of technical/subject matter mistakes. The "factors" section includes a couple of short sentences but I felt unable to summarize accurately the information about Pauling’s second rule or the subsection on thermodynamics; perhaps another more knowledgeable editor could fill in the missing information.


 * An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution: the larger the value the stronger the acid and the more the acid is dissociated, at a given concentration, into its conjugate base and the hydrogen ion. The strength of a base is nowadays quantified by the Ka value of its conjugate acid.


 * Ka is an equilibrium constant. For an equilibrium between a generic acid, HA, and its conjugate base, A−, HA A− + H+, Ka is defined, subject to certain conditions, as
 * $$K_a = \mathrm{\frac{[A^-][H^+]}{[HA]}}$$
 * where [HA], [A−] and [H+] are equilibrium concentrations of the reactants. The term acid dissociation constant is also used for pKa, which is equal to −log10 Ka. pKa is proportional to the standard Gibbs free energy change for the reaction. While the standard enthalpy change for a weak acid dissociation reaction may be positive (endothermic reaction) or negative (exothermic reaction), the standard entropy change is always negative. pKa values for endothermic reactions increase with increasing temperature; the opposite is true for exothermic reactions. This is in accord with Le Chatelier's principle.
 * In aqueous solutions, acids that release a single proton are partially dissociated to an appreciable extent in the pH range pKa ± 2. The actual extent of the dissociation can be calculated, using the pKa value, if the analytical concentration of the acid and pH (or analytical concentration of added mineral acid) are known. Acidic behavior can also be characterized in non-aqueous solutions.  Factors that determine the magnitude of pKa values include Pauling's rules and, for organic acids and bases, inductive effects and mesomeric effects; these effects are summarized in the Hammett equation.  Structural effects, such as inter-molecular hydrogen bonding, can also be important.  pKa can be experimentally determined by potentiometric (pH) titration, but for values of pKa less than about 2 or more than about 11 spectrophotometric or NMR measurements may be required.


 * A knowledge of pKa values is essential for the understanding of the behaviour of acids and bases in solution. For example, many compounds used for medication are weak acids or bases, so a knowledge of the pKa and log p values is essential for an understanding of how the compound enters (or does not enter) the blood stream. There are many other applications, including aquatic chemistry, chemical oceanography, buffer solutions, acid-base homeostasis, protein chemistry and enzyme kinetics. A knowledge of pKa values is also a prerequisite for a quantitative understanding of the interaction between acids or bases and metal ions to form complexes in solution.

(end of rewrite)

I suggest cutting the following:
 * "The term pKb is used in relation to bases, though pKb has faded from modern use due to the easy relationship available between pKb and pKa, the strength of its conjugate acid." It doesn’t seem important enough relative to the other items which do warrant mention in the lead.
 * True, but something needs to be said about bases. I hope my sentence does it.


 * "such as Michaelis–Menten kinetics, which involve a pre-equilibrium step": I suggest cutting this just to shorten the last paragraph, which is longer, relative to the body section it relates to, than some of the other lead material.
 * Your changes are fine. I've added a summary of the thermodynamic aspect and edited some of the technical stuff. Petergans (talk) 12:48, 10 November 2008 (UTC)
 * Looks good. I have to head off to work now, so won't be editing again till this evening; I think your version is fine to go ahead and edit into the article. Mike Christie (talk) 12:53, 10 November 2008 (UTC)


 * I have made these changes, with some tweaks. I added a few links to the new material but it might be good to glance over the lead and see if anything else should be linked. Mike Christie (talk) 00:28, 11 November 2008 (UTC)

Other issues
The following list contains everything else I know of that needs to be addressed for FA.
 * The article is inconsistent in its use of American English versus British English. For example, it has both "-ize" and "-ise" endings;  "behaviour" is a British spelling but "gray" is the preferred American spelling.  It doesn’t matter which is chosen, since this is not a topic which has a regional connection, but it does need to be consistent through the article.  I’ll be happy to go through and make the necessary changes if you can tell me which you prefer – American or British English.
 * I'll go for British, of course! Petergans (talk) 15:40, 10 November 2008 (UTC)

Done Petergans (talk) 15:40, 10 November 2008 (UTC)
 * I have a couple of questions about the units style. The relevant style guide is WP:UNITS; I will fix a few things myself.  The guidelines I am referring to are these:
 * The first instance of each unit needs to be spelled out in full and linked; it can be abbreviated throughout the article thereafter.
 * Where a unit is not sufficiently familiar that the abbreviation will be quickly recognized by readers, the abbreviation should be shown after the first use – e.g. "decimeter (dm)". One possible exception: M stands for Molar concentration I presume?  If so, since the M is not an abbreviation, I think it would suffice just to link the first occurrence of the "M" to the molar concentration article.
 * A nonbreaking space goes between the number and the unit; the code is "&amp;nbsp;"
 * The product of units is indicated by a middle dot: "&amp;middot;".
 * There's an instance of "300 /M" but it is definitional and I think is OK.
 * This is a quotation so, MUST stay as it is. Petergans (talk) 15:40, 10 November 2008 (UTC)


 * There's a mix of exponential and division notation in this: "ΔHO /kJ mol−1"; it looks like a standard usage, but I'd like to ask if it would be as readable to the target audience if converted to be consistently exponential. If it should stay as it is for readability I think that's fine.
 * In this case I think WP should give a lead and use this notation which is precise, unambiguous and, once the convention is understood, clear. Otherwise, how is the general public ever to come to terms with standard scientific notation? In text the slash can be replaced by "in" which effectively has the same meaning as "divided by". Petergans (talk) 15:40, 10 November 2008 (UTC)
 * There appears to be a conflict between the WP and IUPAC recommendations for composite units. In this table, numerical values have been obtained by dividing (slash) the physical quantity, ΔHO, by the unit of measurement, kJ mol−1. "kJ per mole" is hardly ever used in chemistry texts. The slash convention is particularly valuable when a number has been multiplied or divided by a factor since the old usage of ,e.g. times 1000, can be ambiguous. In this case the quantity in Joules per mole has been divided by 1000. The unit kJ mol−1 is standard in thermodynamics.  Middle dot in units is obsolete in chemistry. Petergans (talk) 13:35, 10 November 2008 (UTC)

Done Petergans (talk) 15:40, 10 November 2008 (UTC) Done Petergans (talk) 15:40, 10 November 2008 (UTC)
 * Why is "Bis" capitalized in the "factors" section?
 * "The total volume of added strong base should be small compared to the initial volume of to keep the ionic strength nearly constant." A word is missing.

That’s everything I could see to fix. Mike Christie (talk) 21:11, 9 November 2008 (UTC)
 * A few comments on the comments!
 * I'll let Peter choose for the spelling, but remember that British spelling is not always consistent between -ize and -ise (people at WP:FAC will probably be aware of this).
 * M is a very common abbreviation for mol dm$–3$. It should probably be defined at its first usage and restricted to article text (not tables or formulae).
 * Use of a middle dot to create a product of units is very rare in scientific articles on Wikipedia, and almost unknown in other scientific contexts. We should use a thin space, but a nonbreaking space is quite normal given current browser limitations.
 * Division of a quantity by a unit, as in "ΔHo /kJ mol−1" is normal scientific practice for tables and graph axes, as per ISO 31-0: it avoids having to repeat the unit on each line.
 * Physchim62 (talk) 13:17, 10 November 2008 (UTC)
 * Comments on the comments on the comments: I agree with the first three points (of course, ize/ise should be consistent within the article even if British English is chosen). Regarding the fourth point: it is of course correct, but putting the units in parentheses is also very common even among scientists and I think is much more intuitive to non-scientists. I suggest putting the units in parentheses. --Itub (talk) 13:25, 10 November 2008 (UTC)

Remaining issues
I've been back through the talk page looking for outstanding issues. I thought it would be more convenient if I summarised the situation here, rather than making people read the long text above.

DonePetergans (talk) 13:17, 11 November 2008 (UTC) DonePetergans (talk) 13:17, 11 November 2008 (UTC) DonePetergans (talk) 13:17, 11 November 2008 (UTC) I stopped using the (unit) convention because it can be ambigous if a numerical factor is involved, though that is not an issue in this article. Petergans (talk) 13:17, 11 November 2008 (UTC)
 * Reference 13 still doesn't have a page number; it does look like it needs one.
 * The changes to the lead and to the importance section have been done but should be reviewed to make sure I didn't mess anything up. This includes checking for additional possible links in the new material, mostly in the lead, and checking that the references for environmental science are correct.
 * I moved some text around in the bit on isolectric focusing, and it has no reference as it stands; that should be fixed.
 * I've converted to British English throughout. However, I am unreliable on this issue as I've spent decades on both sides of the Atlantic, so it would be best for someone else to check for any remaining American English usages.
 * I don't have a strong opinion on the remaining units usage issues; I agree with the suggestion that using parentheses rather than a slash for dividing by units in tables and graphs might be easier for lay readers. I understood the usage once it was pointed out but the parens would be clear too.  Anyway, I think this is up to the regular editors here.  There might be some discussion of this at FAC.

There are also two points I've asked for external advice on. One is a stylistic issue: the formatting of "see also" and "external links" sections. See enzyme kinetics, for example, which uses em dashes; I've found that most featured articles just use bare links without explanation, which I think is suboptimal. I've left a query with a style expert but have not heard back. We could renominate at FAC without resolving this.

The other issue is the lack of citations for long sections of the article. For example, the section on equilibrium constants has essentially no citations for the majority of the text; the references are apparently to specific points. I suspect that this is because nothing in this section would be regarded as even faintly necessary to cite in a professional journal; it would be common knowledge. Here it's necessary to go further, but I am not sure what the current style is and have asked the FAC director for an opinion. For example, it might be the case that a given text that covers the entire area could simply be cited, once, at the end of the section. Or it might be more usual to cite at once at the end of each paragraph. I'll post a note here when I hear back.

I'll come back tomorrow evening and look through one more time. Does anyone else think there's anything else that needs working on before we go back to FAC? Mike Christie (talk) 02:15, 11 November 2008 (UTC)
 * I have commented out the navigation template as I feel that the article has evolved so much since it was introduced that it is now redundant. Is there a consensus for this?
 * Creation of an Equilibrium chemistry category. This is a separate issue from the one above. Looking through articles to which it might apply, the majority give Physical chemistry or Analytical chemistry as the category (with no consistency!). I suggest that an equilibrium chemistry category would be more appropriate. When the Thermodynamics category is present the new category should be added. See Portal:Chemistry/Categories for a list of categories currently in use in chemistry. Equilibrium chemistry would fit nicely into this list. Petergans (talk) 20:52, 11 November 2008 (UTC)
 * I do not understand why the template was removed from the section on Experimental determination ...  There is now no link to that article, which does in fact give a lot more detail. What should be done? Petergans (talk) 13:17, 11 November 2008 (UTC)

The points above are now mostly taken care of; I haven't heard back from the FAC director on the sources but will ask someone else if I don't hear tomorrow. For the main template issue: the reason I removed it is that it's intended for summary style. It indicates that the entire linked articles would be inserted at that point if it were not for length constraints. Hence it is used for articles that are of necessity too broad in scope to fully cover their material; it permits long sections to be summarized and removed to subsidiary articles. See chemistry for multiple examples of this usage. In this article it doesn't seem to be appropriate because the section on equilibrium constants wouldn't be expected to include the entire article on determination of those constants -- some of what's in that article doesn't specifically relate to the acid dissociation constant. If there is no natural place to link to the "determination" article from the existing text, I'd suggest either looking for a natural paraphrase which would leave us with text that could be used for the link, or else adding it to the "See also" section at the end.

If you're OK with that, then there's not much else to do. I think it's important to get another opinion on the references, since that could lead to quick opposes if we don't understand the usual citation style. Other than that the remaining topics of discussion, such as units and the navbar, seem to be the sort of thing that the editors here can agree on and which aren't likely to provoke disagreement at FAC. I will try to get back here in the next day or two with a response on the references. Mike Christie (talk) 03:26, 12 November 2008 (UTC)
 * I have put an unlinked category in my sandbox. If someone will set up the skeleton category for me, I will hopefully be able to fill in the details. This will make it easier to assess the idea. Petergans (talk) 10:16, 12 November 2008 (UTC)


 * I'm afraid I've never created a category or done anything with them, so I can't help; I took a look at WP:CAT but I'd have to do some experimenting to figure out the right way to do it. I'll leave a note with a helpful admin I know who I'm sure will know more than I do on this. Mike Christie (talk) 00:39, 13 November 2008 (UTC)
 * Category pages can be edited and created like any other page. However the list of articles in the category is generated automatically by the software from the articles which actually are in the category. Geometry guy 20:02, 13 November 2008 (UTC)

Are we ready?
Many thanks to Mike Christie for all the time and hard work he has put in, especially on such alien subject matter. The text now runs much more smoothly.

On the matter of references, I have now included the chapter titles in some of them. Hopefully this helps in regard to matters of "common knowledge". I've only done it for books that I have copies of, so it may be useful to add some more chapter titles. Petergans (talk) 11:08, 16 November 2008 (UTC)

I invite interested editors to do a last read-through/edit/comment before nominating again for FAC. Petergans (talk) 10:58, 14 November 2008 (UTC)


 * The category:Equilibrium chemistry has been created. In the end it turned out very easy to do.
 * Create the category page and put in the template:catmain with the name of the principal file. This is optional.
 * Add the link to each page (e.g. acid dissociation constant) that is to be included in the category. This automatically creates a link which will be seen when the category:equilibrium chemistry page is opened. Petergans (talk) 14:10, 14 November 2008 (UTC)

Small changes in section headings and ALT text for LaTeX equations and images
L'Aquatique and I added ALT text to the LaTeX-formatted equations and to the first image of the lead to make them accessible to people who use screen readers, per this discussion on the Talk page of the accessibility guidelines. Do they seem reasonable and accurate to everyone here? You'll find them buried in the &lt;math&gt; and Image: tags in editing mode. If they're OK, we should probably add ALT text to the remaining images.

We also changed a few of the section headings slightly to remove the formatted text pKa in the section headings and to avoid repetition of the article title in the section headings, per this discussion on the accessibility talk page and per MOS:HEAD. Hopefully, everyone will be fine with those as well. Proteins (talk) 21:35, 21 November 2008 (UTC)
 * I'm not happy about removing pKa from headings. pKa is is standard notation in chemistry for these quantities. The heading to section 7 does not mean the same as before. The subscript is important to differentiate pKa from pKb. pK by itself is used when the context makes it clear that that subscript a is implied, as in the phrase "the pK of the acid is ..". There are three alternatives for section headings: i) accept that standard chemical notation should be used, ii) use the text "pKa" or pK or iii) spell it out - "acid dissociation constant".


 * "acid dissociation constant" may be the article title, but it is also the name of the quantity being discussed, so some repetition is inevitable. Petergans (talk) 00:11, 22 November 2008 (UTC)

First, am I correct in understanding that you approve of the ALT text messages in the equations and lead image? Would you like me to add it for the remaining images, or perhaps you would like to do it yourself? The goal is to describe each image so that visually impaired readers can appreciate its content as well. As it is, blind readers hear the figure caption repeated twice, as you can tell by turning off the images on your browser. Some captions do describe their image, but many just give its significance, like delivering the "punchline" of a joke without its setup.

As for the section headings, I'm fine with using pKa myself, although it seems to run afoul of the consensus on accessibility to the blind and for style. You can find more information about those concerns by following the links to discussions and guidelines above.

Although they may seem unfamiliar at first, I think the section headings aren't made significantly worse by eliminating the article title from them. MOS:HEAD is only a guideline, but you know the old saw about customs having more force than law; people will ask you why it isn't being followed. For example, if this were Martin Luther, people would ask you to replace a section heading "Luther's education" with "Education". If you think about it, it's not that ambiguous whose education is being discussed, although the form may be unfamiliar. I think we agree that "acid dissociation constant", its abbreviation Ka and the equivalent form pKa are the same for the purposes of MOS:HEAD, but you could argue that point, I suppose. I modified section heading 7 so that I think you'll agree with it.

I see that the article is up for FAC, so I'll read through it carefully and give my review this weekend. A few suggestions from my brief scan:


 * a longer discussion of the sigma factors for electronic induction, with a short table illustrating the most outstanding/common functional groups? Given all the other tables, a short table of a few rows might not be out of place.  You mention the Hammett equation, but it's only one sentence, and I'm afraid that people who don't know it already won't get much out of it.  Perhaps you think the topic is too technical?  You might consider illustrating it with a homologous series, such as methanol to methanol substituted with halides to phenol to phenol substituted with other electron-withdrawing groups?  BTW, is there a discussion of resonance affecting pKa values?
 * I've expanded the bit about the Hammett equation. I started with a table of values, but decided in favour of a more general discussion. There is a table of original values in hammett equation. I've also added a more up-to-date reference. Petergans (talk) 16:24, 23 November 2008 (UTC)


 * Do you think an extra "Notes" column could be added to the pKa tables so that non-(bio)chemists don't have to follow hyperlinks to know what glycine is or why it has two titratable groups? Right now, the tables seem uncomfortably similar to those found in the CRC manual or a textbook, and people might criticize the article along the lines of WP:NOTTEXTBOOK and WP:IINFO.  You might consider adding a molecular diagram of each acid, perhaps with the titratable proton highlighted in red?  Visual aids might help you get your point across to non-chemists.


 * pKa shifts in proteins? The maleic/fumaric example of pH shifts due to hydrogen bonding is good – I use it in my own lectures – but it would be nice to mention their role in enzymatic catalysis and protein stability.  On the other hand, perhaps that's too parochial.

Good luck with the FAC! Proteins (talk) 13:47, 22 November 2008 (UTC)
 * You make a lot of good points. Firstly let me explain a misunderstanding. I thought that the changes you made were part of preparing the article for FAC, which is why I responded here, rather than editing the article. I need to have a definitive answer on the use of subscripts in section headings.


 * Unfortunately, I can't give you a definitive answer, especially given that I'm a relative newbie on Wikipedia. I believe that it's OK per WP:ACCESS; I personally checked that pKa in section headings is read correctly by JAWS and Fire Vox, and Graham87, who knows much more, found the same.  I think the greatest opposition will come from people interpreting MOS:HEAD strictly, in particular its discouragement of "special characters" and the repetition of the article title.  I posted a few questions at WT:MOS, so you should have your answer in a day or two. Proteins (talk) 16:26, 22 November 2008 (UTC)


 * The ALT text additions were by and large OK, though I have edited them for scientific content. I would prefer that you added the alt text elsewhere, too, as I think that the outsider's description is likely to be more informative than mine as I am too close to the subject matter.


 * Great! I'll be happy to add the ALT text. Proteins (talk) 16:26, 22 November 2008 (UTC)


 * I will consider the other points over the weekend. I did consider putting structural formulas alongside pK table entries for the reasons that you outline, but decided against this idea as it would have increased the row heights too much. I'll do some trials in my sandbox. Regarding the effect of hydrogen bonding on protein pK values, please feel free to add something yourself, with references. I am not familiar with this area. Petergans (talk) 14:43, 22 November 2008 (UTC)


 * I'll think it through and try to find some good examples that aren't too obscure. The earliest examples in protein science would probably be the pKa shifts found first spectrophotometrically and enzymatically in ribonuclease A in the 1950's, which are associated with Tyr-Asp hydrogen bonds and with the catalytic histidines.  That example might allow us to talk more about how NMR is used to determine the pKa's of individual sidechains, and maybe pD?  A more modern example might be more illuminating and more fun, however. Proteins (talk) 16:26, 22 November 2008 (UTC)