Talk:Carbonic acid/Archives/2021

Sparkling waters people drink could be mentioned at a "uses" entry

Some sparkling waters claim "pinpoint carbonation", and there does seem to be a different in carbonation sizes in sparkling waters. What else is carbonic acid used for? I heard it might have been a disinfectant late 19th century. — Preceding unsigned comment added by 50.38.106.1 (talk) 07:23, 2 January 2021 (UTC)

pK value (urgent)

There are contradictory statements concerning the first pK value of carbonic acid.

1. "cologarithm of the first dissociation constant has a value of ca. 3.6 at 25 C"
2. "cologarithm of the first dissociation constant, pKa1, has a value of ca. 6.35 at 25 C"

The value of 3.5 is for the equilibrium

H₂CO₃ ⇌ HCO₃^- +H⁺

Does the value of 6.35 refers to the equilibrium

CO₂ + H₂O ⇌ HCO₃^- +H⁺?

The issue needs to be resolved because the value of 3.6 is repeatedly being replaced in the text by 6.35, and then it is being reverted back. The plot appears to refer to the second definition.

The disrepancy seems to arise because in nature, that is when pH ≈ 7, the concentration of carbonic acid in solution is negligible. Is the second definition specified in Stumm and Morgan? If so, what is the page number where it appears? Please reply here so that I can modify the text. Petergans (talk) 11:51, 12 February 2021 (UTC)

In all chemical textbooks I have referenced, the pKa value is specified as 6.3. This is also the value used in the chemical speciation modelling software Minteq. Where does your value of 3.6 come from? Can please give the reference? Furthermore, this issue should be exlained more clearly in the article. I also suggest that both values and the roots of their difference be specified in the table, this will prevent that the values will be changed constantly. — Preceding unsigned comment added by 143.169.155.111 (talk) 15:52, 12 February 2021 (UTC)

A 2016 publication with the pK_a value of 3.49 is freely available on PubMed "here".. It points out that the compound decomposes reversibly in aqueous solution (to CO₂ + H₂O). This paper and the further references it quotes would all be worth incorporating. Note that this account also mentions the pK_a 6.35 value and explains the difference between them. Mike Turnbull (talk) 16:54, 15 February 2021 (UTC)

Many thanks. That publication gives defining chemical expressions for all the equilibrium constants, which is what is needed. I will revise the WP article when I have time to do it. Petergans (talk) 09:33, 16 February 2021 (UTC)

The text has now been modified. The difficulty arose because Pines et al. defined pK_a differently from the definition used in this article. Petergans (talk) 21:30, 17 February 2021 (UTC)

Equilibrium constants: Note 2

Is there a more detailed explanation for Note 2, which seems to indicate that the same reaction

HCO3 = CO3 + H

has an association constant of log beta2, but a disassociation constant of log beta1 - log beta2

I would have expected a simple -log beta2

Was the reaction meant to have been

H2CO3 = CO3 + 2 H?

Bendel boy (talk) 16:28, 23 February 2021 (UTC)

β₁ and β₂ are overall association constants whereas pK refers to the log₁₀ value of a stepwise dissociation constant. Numbering for dissociation constants is the reverse of the numbering of association constants, so log β₁ = pK₂ etc. Petergans (talk) 12:09, 1 March 2021 (UTC)

Overly aggressive automatic archiving

On a side note, can anyone here explain why "ClueBot III" is so darn aggressive in archiving this talk page? It appears from the history that there has been some discussion on the subject of the equilibrium constants every few months, but "ClueBot III" has reduced it to disjoint snippets spread over several archives. I don't find this very conductive to getting this problem solved; even if I were to have some time for it, I'd be bereft of the opinions of my fellow editors unless I sink even more time in a crawl through the archives of the discussion page. OneAhead (talk) 12:41, 14 August 2021 (UTC)

Update: I might have figured out the "ClueBot III" issue at least. The age at which to automatically archive a thread was set to 720, but for "ClueBot III", that's 720 hours (not days, as with "lowercase sigmabot III"), which amounts to 30 days. I don't know how people here feel about that, but to me, automatically getting rid of discussions after 1 month is ridiculously short. I'm tempted to speculate that whoever put that into was going for "about 2 years" (which would have been effective if the unit of "age" was days). Accordingly, I took the liberty to actually change the parameter to 2 years. Note that this is for automatic archiving; as I understand it, people retain the freedom to manually archive a thread before that time if they feel it is no longer relevant. OneAhead (talk) 13:12, 14 August 2021 (UTC)

Hydration constant and acid dissociation constants missing

Over the last year and a half, I've visited this page several times for its clear and elaborate discussion of the hydration constant and acid dissociation constants, only to find that part missing. Sure, the pre-2020 "Acidity of carbonic acid" section was growing ridiculously out of control and something had to be done about it, but the subsequent efforts to cure it have amounted to throwing the baby out with the bathwater. In the present state, there's no mention of the difference between the "real" and apparent pKa₁, even though I find that a pretty important nuance. Furthermore, the acid dissociation constants, which I would assume to be familiar to readers with basic chemistry background, have been replaced with the "overall protonation constants β₁ and β₂". Even I (as an actively practicing chemist with a PhD) find that more difficult to follow. While the original effort in overhauling the page focused on making it more accessible to a general public, there's no way one can claim that the current version is actually more accessible than the pre-2020 one. - a disappointed OneAhead (talk) 12:41, 14 August 2021 (UTC)

I may have been the person most responsible for some of the changes that you mention. The point that you make about stepwise and cumulative constants is a good one. I will address it.

Regarding hydration, do you mean CO₂ + H₂O = H₂CO₃? If so, quantification is difficult because there is no change in pH for this reaction, and there is also a gas/solution equilibrium to take into account. Nevertheless, it's an important aspect of respiration which merits a qualitative discussion.

Further constructive comments will be welcome. Petergans (talk) 09:44, 15 August 2021 (UTC)

I too was very surprised and disappointed to see that the table of species concentrations vs. pH had been removed in the 19:12 22 October 2020 Peterglans edit. I have been using that table in my fourth year Bioinorganic chemistry lecture when discussing the CO2 <--> carbonate + H+ concentrations vs. CO2 pressure and how both species are in much higher concentration in soda pop under pressure, then after opening the pop the CO2 is released and concomitant decrease in carbonate also. The "pH and Composition of Carbonic Acid Solutions" was a great section and table and I want it back!! Kcsunshine999 (talk) 02:36, 23 September 2021 (UTC)

My concern with reinstating the table to which you refer (copied below), is that the range of p(CO₂) values in column 1 is unrealistic, as the magnitude of the range of actual and predicted changes is very much smaller. Also the pH values do not go high enough for sea-water, which is alkaline.

The pH and species' concentrations can be obtained, as follows. First calculate the concentration of CO₂ in water, given the pressure in the atmosphere and the known solubility of CO₂ in water. Then, solve the mass-balance equations. Writing CO₂ as X,

T_X = [X] + β_1[X][H] + β_2[X][H]²

T_H = [H] + β_1[X][H] + 2β_2[X][H]² -K_W[H]^-1

T_X is the given value of the total concentration of dissolved CO₂. T_H has a value of zero in the absence of added mineral acid. These equations can be reduced to a single equation of degree 3 in the value of [H] (=10^-pH) for a given value of T_X. That equation can be solved, in an EXCEL spreadsheet, with the help of SOLVER. Petergans (talk) 10:10, 23 September 2021 (UTC)

$p_{{\text{CO}}_{2}}$ (atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3−] (mol/L)	[CO32−] (mol/L)
1.0 × 10−8	7.00	3.36 × 10−10	5.71 × 10−13	1.42 × 10−09	7.90 × 10−13
1.0 × 10−7	6.94	3.36 × 10−09	5.71 × 10−12	5.90 × 10−09	1.90 × 10−12
1.0 × 10−6	6.81	3.36 × 10−08	5.71 × 10−11	9.16 × 10−08	3.30 × 10−11
1.0 × 10−5	6.42	3.36 × 10−07	5.71 × 10−10	3.78 × 10−07	4.53 × 10−11
1.0 × 10−4	5.92	3.36 × 10−06	5.71 × 10−09	1.19 × 10−06	5.57 × 10−11
3.5 × 10−4	5.65	1.18 × 10−05	2.00 × 10−08	2.23 × 10−06	5.60 × 10−11
1.0 × 10−3	5.42	3.36 × 10−05	5.71 × 10−08	3.78 × 10−06	5.61 × 10−11
1.0 × 10−2	4.92	3.36 × 10−04	5.71 × 10−07	1.19 × 10−05	5.61 × 10−11
1.0 × 10−1	4.42	3.36 × 10−03	5.71 × 10−06	3.78 × 10−05	5.61 × 10−11
1.0 × 10+0	3.92	3.36 × 10−02	5.71 × 10−05	1.20 × 10−04	5.61 × 10−11
2.5 × 10+0	3.72	8.40 × 10−02	1.43 × 10−04	1.89 × 10−04	5.61 × 10−11
1.0 × 10+1	3.42	3.36 × 10−01	5.71 × 10−04	3.78 × 10−04	5.61 × 10−11