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The Bjerrum plot shows how the different species of dissolved inorganic carbon (DIC) are distributed in sea water as a function of pH. The higher the pH, the more carbonate ion, less dissolved $\ce{CO2}$ (i.e. $\ce{H2CO3}$), and vice versa. And according to Takahashi et al. (1993), this is relevant for the $\ce{pCO2}$. That is, the higher the pH, the lower the $\ce{pCO2}$.

Are there processes in nature that can change alkalinity, as defined by Dickson et al. (2007) (i.e. sum of negative charges of non-conservative ions), without changing pH?

Here is the definition:

4.2 Total alkalinity

The total alkalinity of a sample of sea water is a form of mass-conservation relationship for hydrogen ion. It is rigorously defined (Dickson, 1981) as “. . . the number of moles of hydrogen ion equivalent to the excess of proton acceptors (bases formed from weak acids with a dissociation constant K ≤ 10 –4.5 at 25°C and zero ionic strength) over proton donors (acids with K > 10 –4.5 ) in 1 kilogram of sample.” Thus

$A_T = [\ce{HCO3-}] + 2[\ce{CO3^2-}] + [\ce{B(OH)4^-}] + [\ce{OH-}] + [\ce{HPO4^2-}] + 2[\ce{PO4^3-}] + [\ce{SiO(OH)3^-}] + [\ce{NH3}] + [\ce{HS-}] + ... - [\ce{H+}]_F - [\ce{HSO4-}] - [\ce{HF}] - [\ce{H3PO4}] ...$ (13)

where the ellipses stand for additional minor acid or base species that are either unidentified or present in such small amounts that they can be safely neglected. $[\ce{H+}]_F$ is the free concentration of hydrogen ion

This last term is defined later in the text according to

$[\ce{H+}] = [\ce{H+}]_F\left(1+\frac{S_T}{K_S}\right)$ where $S_T = [\ce{SO4^2-}] + [\ce{HSO4-}]$ and $K_S$ is the $K_a$ for $\ce{HSO4-}$.

Zeebe & Wolf-Gradow (2001) show in their figure 1.1.3 that the air-sea exchange of $\ce{CO2}$ does not change alkalinity (the horizontal line). Although it does change pH (diagonal background lines). This is in fact a problem related to antrhopogenic climate change, as $\ce{CO2}$ is taken up by the ocean, causing acidification, and posing threats to marine ecosystems. I was wondering if there is a process that draws a line that is parallel to the pH isolines in the figure below.

enter image description here

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    $\begingroup$ What are non-conservative ions? $\endgroup$
    – Ivan Neretin
    Commented Nov 28, 2022 at 9:53
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    $\begingroup$ Note that this is a follow up question from Earth Science SE - Why does decreasing surface alkalinity increase surface pCO2? $\endgroup$
    – Poutnik
    Commented Nov 28, 2022 at 10:01
  • $\begingroup$ Some participants have there an interesting idea that changing [OH-] does not change [H+] and therefore nor the pH. $\endgroup$
    – Poutnik
    Commented Nov 28, 2022 at 10:08
  • $\begingroup$ I think conservative ions are those whose concentrations won't change due to changes in the environment (temperature, pH, salinity), such as Na+ or Mg++. Non-conservative ions, such as carbonate ions, do. Does it make sense? doi.org/10.1016/j.marchem.2007.01.006 $\endgroup$
    – ouranos
    Commented Nov 28, 2022 at 13:00
  • $\begingroup$ It does make sense, but I would rather expected conserved as a passive form. Conservative sounds to me like such ions are actively conserving, protecting something. Or at least proponents of conservation. But I am not an EN native. Perhaps conservative is closer to conserved than to conserving. $\endgroup$
    – Poutnik
    Commented Nov 28, 2022 at 13:07

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If there are 2 controlled processes, one increasing alkalinity, e.g. by dissolving $\ce{MgO(s)}$ or alkaline earth carbonates, and the other dissolving $\ce{CO2(g)}$, so the concentration ratios of $\ce{CO2(aq)}$, $\ce{HCO3-(aq)}$ and $\ce{CO3^2-(aq)}$ are constant, then yes.

It is theoretically possible, as the above constant ratios would form pH buffers with constant [$\ce{H+}$], [$\ce{OH-}$], but with variable concentrations and therefore alkalinity.

But in geological context, it is highly inprobable both processes occur in such a balanced way, so generally no, it is not possible and can occur just accidentally.

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  • $\begingroup$ I just learned about the example: $\ce{CO2}$ absorption in sea water changes pH, but doesn't change alkalinity, since it adds a carbonate ion at the same ratio it adds a H+ (Figure 1.1.3 in Zeebe & Wolf-Gradow: geosci.uchicago.edu/~kite/doc/Zeebe_CO2_In_Seawater_Ch_1.pdf ). Since it is possible in one way, I was wondering if something similar could exist in the other direction too. $\endgroup$
    – ouranos
    Commented Nov 29, 2022 at 8:35
  • $\begingroup$ @ouranos Well, as the procedure of alkalinity determination involves titration with methyl orange pH indicator and heating the sample to get rid of CO2, its dissolution alone is not really relevant, if not followed by dissolving of alkaline matter. $\endgroup$
    – Poutnik
    Commented Nov 29, 2022 at 14:20

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