Talk:Acid dissociation constant

message: 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)
replies: text: When K_\mathrm{a} = \frac{K^\ominus}{\Gamma}  and both K^\ominus and \Gamma  have the same dimension, K_\mathrm{a} has no dimension. See, for example, Rossotti and Rossotti (1961). Petergans (talk) 09:40, 19 September 2019 (UTC)
replies: text: What exactly is the dimension of Γ? Isn't it usually considered non-dimensional?--109.166.139.27 (talk) 14:38, 10 November 2019 (UTC)
replies: text: The answer is given above. The dimension of K^\ominus and \Gamma  will depend on the unit used in the measurement of concentration, commonly mol dm-3. LogK is possible when the value of \Gamma  is (implicitly) equal to 1, whatever the unit of measurement. Petergans (talk) 20:35, 10 November 2019 (UTC)
replies: text: More precisely: the answer is given above in the Talk page section Discussion of dimension, as well as in the article section Dimensionality. Dirac66 (talk) 01:32, 19 November 2019 (UTC)
thread_title:  pKa and the logarithmic expression 

message: I have found an interesting new source for this section:  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)
replies: text: Can you send me a reprint - peter.gans@hyperquad.co.uk? Another "simple" example is spermidine which has 3 non-equivalent amino groups, 3 micro-states each for LH+ and LH22+. Petergans (talk) 10:53, 5 March 2020 (UTC)
thread_title: New source

message: 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)
replies: text: Putting the quotient of activity coefficients Γ to be equal to 1 is an arbitrary assumption. It can be applied to any equilibrium constant since KT=KΓ. The assumption is needed because the thermodynamic constant, KT, must be dimensionless; otherwise Log K does not exist. It is equivalent to the assumption that the properties of solution are the same as properties of an ideal solution, so KT has the same numerical value as K. Values for activity coefficients can be calculated using the Debye-Hueckel equation, Pitzer equations and others.Petergans (talk) 08:35, 28 April 2020 (UTC)
replies: text: I think that values in tabular form for the dependence on ionic strength of Γ when a fixed reference state (as mentioned in a section above) is used should be included. The experimental values of activity coefficients for these acids, obtained by various methods, not those based on calculations (with the mentioned equations) which rely on various assumptions like Debye-Huckel, are more valuable for the article. The activity coefficients values for concentrated strong acids like those mentioned also remove the problem of negative pKa discussed above which arises when activity coefficients are ignored.--109.166.139.1 (talk) 01:48, 10 May 2020 (UTC)

text: Also the connection to the law of dilution including activity coefficients for concentrated strong acids should be emphasized. Usually in absence of lower than 1 or greater than 1 values of activity coefficients the law of dilution in its classic form has a non-constant constant Ka.--109.166.139.1 (talk) 02:03, 10 May 2020 (UTC)
replies: text:  For a salt ApBq, the quantities \gamma_\ce{A^+} and \gamma_\ce{B^-} cannot be determined individually from experimental data. The quantity that can be obtained is the  mean activity coefficient,  \gamma_\pm=\sqrt[p+q]{\gamma_\mathrm{A}^p\gamma_\mathrm{B}^q}. The value of this quantity varies with solute concentration. The effect of this variation is illustrated by the graphic taken from acid dissociation constant and repeated here. Petergans (talk) 10:15, 10 May 2020 (UTC)
thread_title:  Examples of values when the quotient of activity coefficients is different from 1 

message: As seen in the Wikipedia app running on Android 10, there are multiline blanks for … formatted expressions. I don't know the syntax, so am not trying to correct these. Nikevich 18:54, 21 December 2021 (UTC)
thread_title:  Inline text blanks 

message: 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)
replies: text: It is defined in the Introduction by the equation pKa = -log10Ka, but admittedly there is a problem for readers who don't know what log (logarithm) means. Perhaps we should add a simple example: For a weak acid with Ka = 10-5, log Ka is the exponent which is -5, so that pKa = 5. And for acetic acid with Ka = 1.8 x 10-5, pKa is close to 5. Dirac66 (talk) 14:29, 10 January 2023 (UTC)
replies: text: We also could explain the trend, because the whole idea of "equilibrium constant" is already technical. Something like "A higher Ka means the compound is more acidic because at equilibrium it is more dissociated. For convenience and due to the range of values encountered in nature, it is often converted to a logarithmic scale, pKa, where a lower value means a compound is more acidic." Compare how our article here seems only advanced level, with the lead-section of pH, which includes lay-reader-level details: "Acidic solutions (solutions with higher concentrations of H+ ions) are measured to have lower pH values". DMacks (talk) 15:28, 10 January 2023 (UTC)

text: I have now added the above explanations to the article, at the end of the introduction. Dirac66 (talk) 02:41, 29 March 2023 (UTC)
thread_title:  Definition of pKa