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There is a problem re pKa as logarithm of a quantity Ka which is non-dimensionless. How does the dimension and its units affect the numerical values of the logarithmic pKa? This aspect must be clarified in the attached note!-- 109.166.129.52 ( talk) 12:53, 18 September 2019 (UTC)
(moved from larger_than_about_12_is_more_than_99%_dissociated_in_solution , above) I have now verified some of the textbook references in the section Strong acids and bases, and will make some changes. The book by Dasent does not mention solvent leveling, so I will replace it as a reference by Porterfield and also Shriver and Atkins, with some revision of content. I have been unable to find a source saying that the limiting pKa values are –2 for strong acids and 12 for strong bases. The article now attributes the latter value to Shriver and Atkins, who actually say that pKa is approximately 0 for strong acids and pKb is approximately 0 (or pKa approximately 14) for strong bases. Note that these values are symmetric about pH 7 as they should be. The buffer capacity curve above is also symmetric about pH 7, so it cannot lead to an asymmetric prediction such as –2 and 12, which is in fact unsourced if the value attributed to Shriver and Atkins is corrected. Dirac66 ( talk) 02:53, 16 November 2019 (UTC)
Two comments on this section:
First, since pKa = 14 - pKb for a base is more complicated than pKa for an acid, it would be better to start with an example having two nonequivalent sets of acid groups, such as citric acid. Then spermine with two nonequivalent sets of basic groups could be mentioned as a second example starting with the word "Similarly".
Also the most recently added point about the additivity of microreaction constants is of interest, but the equation cannot be correct. Surely it should be Ka (or Kb) which should be additive and not the logarithmic pKa. And as always it would be best to have a source for the statement, which would serve here to confirm the correct additivity relation. Dirac66 ( talk) 15:24, 17 February 2020 (UTC)
A macro-constant value is always equal to the sum of the micro-constant values. With spermine,
Similar relationships do not apply to log K or pK values. Obviously, the total concentration is the sum of the concentrations of the micro-species.
Also, any dissociation constant value is the reciprocal of the corresponding association constant value.
Aha! Here you have an expression for (macro understood) with two terms in the denominator, which I suspect is correct. However your four numbered equations above include Dissociation: (2), with two terms in the numerator. It would seem that to obtain the simple sums (3) and (4), we must define to be additive rather than itself. Like combining electrical resistances in parallel. Since you are more familiar than me with the details of equilibrium constants, I will ask you if the literature agrees. Dirac66 ( talk) 19:01, 22 February 2020 (UTC)
I have found the origin of the problem. Using association constants, adding one proton to spermine, L
So, the macro- association constant value is equal to the sum of the micro- association-constant values.
But, for removing one proton from LH and using dissociation constants
The macro- dissociation-constant value is not equal to the sum of the micro- dissociation-constant values. Petergans ( talk) 13:23, 24 February 2020 (UTC)
Petergans ( talk) 19:15, 28 February 2020 (UTC)
I have found an interesting new source for this section: Splittgerber, A. G.; Chinander, L.L. (1 February 1988). "The spectrum of a dissociation intermediate of cysteine: a biophysical chemistry experiment". Journal of Chemical Education. 65 (2): 167. It presents an undergraduate experiment for the determination of microconstants for the deprotonation of cysteine at the nitrogen and at the sulphur, using simple UV spectroscopy. As a Journal of Chemical Education article it is somewhat more pedagogical than the spermine paper, and starts by explicitly presenting equations relating micro and macro constants, for subsets of reactions corresponding to three cases of interest here.
For parallel deprotonation of the neutral cysteine zwitterion at two sites (N and S), so that one reactant leads to two products, the JCE paper states that the macroconstant K2 = KA + KB (using the authors' notation). This case is equivalent to the deprotonation of citric acid which I have mentioned above. And incidentally, citric acid does not have C3 symmetry, since the central carbon is bound to 2 -CH2COOH groups and 1 -COOH group, not 3 equivalent groups.
For convergent deprotonation of the cysteine (-1) ions produced in the above paragraph to a common (-2) ion, 1/K3 = 1/KC + 1/KD, so 1/K is additive here. Of course if we consider the protonations (associations) in the reverse direction, then K for association is additive. So we have a source for saying that if K is additive for reactions in one direction (here protonation), then 1/K is additive in the other direction (here deprotonation). Another example of this case is the spermine example in this Wikipedia article, for which parallel protonation of one neutral base leads to two (-1) anions, and again the macroconstant K for association (protonation) should be additive. At the moment however pK is incorrectly marked as additive, implying that K is multiplicative.
The third macroconstant considered is for the double deprotonation of the zwitterion at both N and S, which can proceed by two parallel paths through either (-1) ion to the final unique (-2) ion product. For this case the dissociation constants K are really multiplicative so that the pK are really additive.
I believe this article can be used as a source for a more complete Microconstants section presenting the various cases. Dirac66 ( talk) 02:25, 5 March 2020 (UTC)
I think the article should present examples of values for this constant when the quotient of activity coefficients Γ is different from 1 for acids like sulfuric acid, nitric acid, phosphoric acid, carbonic acid, etc.-- 109.166.136.166 ( talk) 14:39, 26 April 2020 (UTC)
What sources could are known to include such data for these acids?-- 109.166.136.166 ( talk) 14:48, 26 April 2020 (UTC)
I have just noticed that the section "Association and dissociation constants" uses an unsourced and nonstandard notation which conflicts with rest of the article. The usual textbook notation as in the rest of the article uses Ka for an acid DISsociation constant, where the subscript a is for acid, and Kb is the association constant for a base. However this section now claims that Ka is an "ASsociation constant" for an acid, a definition which is not the usual Ka, and that Kd (a symbol not usually used in this topic) is the acid dissociation constant normally denoted as Ka. This is extremely confusing and conflicts with the rest of the article and also with every textbook I know.
The two recent incorrect edits by 94.63.152.174 seem to have been an attempt to clarify this confused notation. I think it would be better to either delete this section entirely, or else to find sourced equations which presumably would use Ka for the acid dissociation constant as in the rest of the article, and another notation (Ka-1?) for the acid association constant. Dirac66 ( talk) 02:55, 9 August 2020 (UTC)
We could adopt some slightly modified notation for the association constant, instead of Ka which is already used for acid dissociation constant in the rest of the article. Perhaps KA or Ka or Kassoc. Any of these would be slightly different from Ka and so preserve the convention that one symbol should have a unique meaning in a given article. But at the same time any of these is close enough to Ka so that it would be reasonable to add that it is called Ka in computer programs for equilibrium constants. Could I ask Petergans to suggest which of these variations seems most appropriate? Dirac66 ( talk) 20:52, 13 August 2020 (UTC)
Many articles list the basicity constant of a substance, so the table should be extended in such a way to simplify comparing basicity and to common bases.
As seen in the Wikipedia app running on Android 10, there are multiline blanks for <chem>…</chem> formatted expressions. I don't know the syntax, so am not trying to correct these. Nikevich 18:54, 21 December 2021 (UTC)
The term is used a lot (more than 150 times in the article), but I am not finding an explanation for what it actually means, unless it is hidden somewhere deep in the text. Please define technical terms and symbols at the first appropriate opportunity, preferably at first use. Cheers,· · · Peter Southwood (talk): 05:16, 10 January 2023 (UTC)
This is the
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Acid dissociation constant article. This is not a forum for general discussion of the article's subject. |
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Acid dissociation constant is a former featured article candidate. Please view the links under Article milestones below to see why the nomination failed. For older candidates, please check the archive. | ||||||||||||||||
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There is a problem re pKa as logarithm of a quantity Ka which is non-dimensionless. How does the dimension and its units affect the numerical values of the logarithmic pKa? This aspect must be clarified in the attached note!-- 109.166.129.52 ( talk) 12:53, 18 September 2019 (UTC)
(moved from larger_than_about_12_is_more_than_99%_dissociated_in_solution , above) I have now verified some of the textbook references in the section Strong acids and bases, and will make some changes. The book by Dasent does not mention solvent leveling, so I will replace it as a reference by Porterfield and also Shriver and Atkins, with some revision of content. I have been unable to find a source saying that the limiting pKa values are –2 for strong acids and 12 for strong bases. The article now attributes the latter value to Shriver and Atkins, who actually say that pKa is approximately 0 for strong acids and pKb is approximately 0 (or pKa approximately 14) for strong bases. Note that these values are symmetric about pH 7 as they should be. The buffer capacity curve above is also symmetric about pH 7, so it cannot lead to an asymmetric prediction such as –2 and 12, which is in fact unsourced if the value attributed to Shriver and Atkins is corrected. Dirac66 ( talk) 02:53, 16 November 2019 (UTC)
Two comments on this section:
First, since pKa = 14 - pKb for a base is more complicated than pKa for an acid, it would be better to start with an example having two nonequivalent sets of acid groups, such as citric acid. Then spermine with two nonequivalent sets of basic groups could be mentioned as a second example starting with the word "Similarly".
Also the most recently added point about the additivity of microreaction constants is of interest, but the equation cannot be correct. Surely it should be Ka (or Kb) which should be additive and not the logarithmic pKa. And as always it would be best to have a source for the statement, which would serve here to confirm the correct additivity relation. Dirac66 ( talk) 15:24, 17 February 2020 (UTC)
A macro-constant value is always equal to the sum of the micro-constant values. With spermine,
Similar relationships do not apply to log K or pK values. Obviously, the total concentration is the sum of the concentrations of the micro-species.
Also, any dissociation constant value is the reciprocal of the corresponding association constant value.
Aha! Here you have an expression for (macro understood) with two terms in the denominator, which I suspect is correct. However your four numbered equations above include Dissociation: (2), with two terms in the numerator. It would seem that to obtain the simple sums (3) and (4), we must define to be additive rather than itself. Like combining electrical resistances in parallel. Since you are more familiar than me with the details of equilibrium constants, I will ask you if the literature agrees. Dirac66 ( talk) 19:01, 22 February 2020 (UTC)
I have found the origin of the problem. Using association constants, adding one proton to spermine, L
So, the macro- association constant value is equal to the sum of the micro- association-constant values.
But, for removing one proton from LH and using dissociation constants
The macro- dissociation-constant value is not equal to the sum of the micro- dissociation-constant values. Petergans ( talk) 13:23, 24 February 2020 (UTC)
Petergans ( talk) 19:15, 28 February 2020 (UTC)
I have found an interesting new source for this section: Splittgerber, A. G.; Chinander, L.L. (1 February 1988). "The spectrum of a dissociation intermediate of cysteine: a biophysical chemistry experiment". Journal of Chemical Education. 65 (2): 167. It presents an undergraduate experiment for the determination of microconstants for the deprotonation of cysteine at the nitrogen and at the sulphur, using simple UV spectroscopy. As a Journal of Chemical Education article it is somewhat more pedagogical than the spermine paper, and starts by explicitly presenting equations relating micro and macro constants, for subsets of reactions corresponding to three cases of interest here.
For parallel deprotonation of the neutral cysteine zwitterion at two sites (N and S), so that one reactant leads to two products, the JCE paper states that the macroconstant K2 = KA + KB (using the authors' notation). This case is equivalent to the deprotonation of citric acid which I have mentioned above. And incidentally, citric acid does not have C3 symmetry, since the central carbon is bound to 2 -CH2COOH groups and 1 -COOH group, not 3 equivalent groups.
For convergent deprotonation of the cysteine (-1) ions produced in the above paragraph to a common (-2) ion, 1/K3 = 1/KC + 1/KD, so 1/K is additive here. Of course if we consider the protonations (associations) in the reverse direction, then K for association is additive. So we have a source for saying that if K is additive for reactions in one direction (here protonation), then 1/K is additive in the other direction (here deprotonation). Another example of this case is the spermine example in this Wikipedia article, for which parallel protonation of one neutral base leads to two (-1) anions, and again the macroconstant K for association (protonation) should be additive. At the moment however pK is incorrectly marked as additive, implying that K is multiplicative.
The third macroconstant considered is for the double deprotonation of the zwitterion at both N and S, which can proceed by two parallel paths through either (-1) ion to the final unique (-2) ion product. For this case the dissociation constants K are really multiplicative so that the pK are really additive.
I believe this article can be used as a source for a more complete Microconstants section presenting the various cases. Dirac66 ( talk) 02:25, 5 March 2020 (UTC)
I think the article should present examples of values for this constant when the quotient of activity coefficients Γ is different from 1 for acids like sulfuric acid, nitric acid, phosphoric acid, carbonic acid, etc.-- 109.166.136.166 ( talk) 14:39, 26 April 2020 (UTC)
What sources could are known to include such data for these acids?-- 109.166.136.166 ( talk) 14:48, 26 April 2020 (UTC)
I have just noticed that the section "Association and dissociation constants" uses an unsourced and nonstandard notation which conflicts with rest of the article. The usual textbook notation as in the rest of the article uses Ka for an acid DISsociation constant, where the subscript a is for acid, and Kb is the association constant for a base. However this section now claims that Ka is an "ASsociation constant" for an acid, a definition which is not the usual Ka, and that Kd (a symbol not usually used in this topic) is the acid dissociation constant normally denoted as Ka. This is extremely confusing and conflicts with the rest of the article and also with every textbook I know.
The two recent incorrect edits by 94.63.152.174 seem to have been an attempt to clarify this confused notation. I think it would be better to either delete this section entirely, or else to find sourced equations which presumably would use Ka for the acid dissociation constant as in the rest of the article, and another notation (Ka-1?) for the acid association constant. Dirac66 ( talk) 02:55, 9 August 2020 (UTC)
We could adopt some slightly modified notation for the association constant, instead of Ka which is already used for acid dissociation constant in the rest of the article. Perhaps KA or Ka or Kassoc. Any of these would be slightly different from Ka and so preserve the convention that one symbol should have a unique meaning in a given article. But at the same time any of these is close enough to Ka so that it would be reasonable to add that it is called Ka in computer programs for equilibrium constants. Could I ask Petergans to suggest which of these variations seems most appropriate? Dirac66 ( talk) 20:52, 13 August 2020 (UTC)
Many articles list the basicity constant of a substance, so the table should be extended in such a way to simplify comparing basicity and to common bases.
As seen in the Wikipedia app running on Android 10, there are multiline blanks for <chem>…</chem> formatted expressions. I don't know the syntax, so am not trying to correct these. Nikevich 18:54, 21 December 2021 (UTC)
The term is used a lot (more than 150 times in the article), but I am not finding an explanation for what it actually means, unless it is hidden somewhere deep in the text. Please define technical terms and symbols at the first appropriate opportunity, preferably at first use. Cheers,· · · Peter Southwood (talk): 05:16, 10 January 2023 (UTC)