<p>The answer is right there on the figure, also known as <a href="https://en.wikipedia.org/wiki/Bjerrum_plot" rel="nofollow noreferrer">Bjerrum plot</a>. If you decrease alkalinity (i.e. increase pH) you shift the relative proportions of the plot to the right, that is, decreasing H2CO3 (or dissolved CO2) concentrations, and increasing bicarbonate and carbonate ions concentrations. And vice versa. Increasing alkalinity decreases pH, increasing pCO2.</p>

CC BY-SA 4.0
2022-11-27T13:15:41.430Z
One can change alkalinity without changing pH. This is why I mentioned "at constant pH" in the question. For example, if one adds OH- ion to sea water, it doesn't change pH. Why does it decrease pCO2?
earthyguy
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2022-11-27T13:24:24.587Z
Sorry, this doesn't answer the question. "At constant pH" is important here.
earthyguy
CC BY-SA 4.0
2022-11-27T13:27:21.197Z
Decreasing alkalinity is not a synonym for increasing pH. Alkalinity is a sum of non-conservative ions, including H+ but not restricted to it.
earthyguy
CC BY-SA 4.0
2022-11-28T06:49:37.620Z
@earthyguy It is possible to change alkalinity without changing pH, but it is very unnatural process, that has to be controlled by supervising human.
Poutnik
CC BY-SA 4.0
2022-11-28T08:42:44.627Z
@Poutnik thanks for the comment. But the weathering of basic minerals, like MgOH or MgO, increase ocean alkalinity (by adding OH- and Mg++) without changing pH (no H+ added or removed). Not that uncommon in nature, I think.
ouranos
CC BY-SA 4.0
2022-11-28T08:48:41.657Z
@ouranos - Changing concentration of OH- (e.g.due dissolving natural carbonates and their hydrolysis)  directly impacts concentration of H+ via $\ce{H+ + OH-  <=> H2O}$  with Kw=[H+][OH-].
Poutnik
CC BY-SA 4.0
2022-11-28T09:09:15.243Z
Ah ok @Poutnik. But how about adding MgO? It would increase [Mg++], thus make water more alkaline?
ouranos
CC BY-SA 4.0
2022-11-28T09:16:23.960Z
Yes, it does, by increasing the OH- concentration // $\ce{MgO(s) + H2O(l) ->  Mg(OH)2(s) <=>[H2O]Mg^2+(aq) + 2 OH-(aq)}$ // $\ce{OH-(aq) + H+(aq) <=>>H2O(l)}$ // pH increases as OH- ions are scavenging H+ ions until the equilibrium is reached.
Poutnik
CC BY-SA 4.0
2022-11-27T13:11:31.613Z
2022-11-27T13:11:31.613Z
ouranos
<p>According to Takahashi et al. (1993), pCO2 (partial pressure of CO2) in the surface ocean changes due to 4 surface variables: temperature, salinity, total carbon (dissolved inorganic carbon, DIC) and total alkalinity (ALK). The latter relationship, in specific, is inversely proportional. Increasing alkalinity decreases surface pCO2 and vice versa. Why?</p>
<p>If anything, I would expect that decreasing alkalinity would decrease pCO2. I would believe this because removing carbonate and bicarbonate ions (which compose alkalinity), at constant pH, would force some CO2 to dissolve into these ions again to maintain the relative distribution at a constant pH, thus decreasing pCO2. This relative balance is illustrated for example in the figure below from chapter 8 in Sarmiento (2013). Why is the opposite true?</p>
<p><a href="https://i.sstatic.net/DGTMU.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DGTMU.png" alt="enter image description here" /></a></p>
<p>Taro Takahashi; Jon Olafsson; John G. Goddard; David W. Chipman; S. C. Sutherland (1993). Seasonal variation of CO<sub>2</sub> and nutrients in the high-latitude surface oceans: A comparative study. , 7(4), 843–0. doi:10.1029/93gb02263</p>
<p>Sarmiento, Jorge L. &quot;Ocean biogeochemical dynamics.&quot; Ocean Biogeochemical Dynamics. Princeton University Press, 2013.</p>

CC BY-SA 4.0
2023-01-02T22:18:05.760Z
Is this about The Revelle Factor? Revelle Factor/buffer factor is a measure of the resistance of the ocean surface layer to absorption of atmospheric CO2. https://en.wikipedia.org/wiki/Revelle_factor
Ken Fabian
<oceanography><geochemistry><carbon-cycle><biogeochemistry>
Why does decreasing surface alkalinity increase surface pCO2?
earthyguy
# Why does decreasing surface alkalinity increase surface pCO2?

(Asked by: username_0 on 2022-11-25)

According to Takahashi et al. (1993), pCO2 (partial pressure of CO2) in the surface ocean changes due to 4 surface variables: temperature, salinity, total carbon (dissolved inorganic carbon, DIC) and total alkalinity (ALK). The latter relationship, in specific, is inversely proportional. Increasing alkalinity decreases surface pCO2 and vice versa. Why?

If anything, I would expect that decreasing alkalinity would decrease pCO2. I would believe this because removing carbonate and bicarbonate ions (which compose alkalinity), at constant pH, would force some CO2 to dissolve into these ions again to maintain the relative distribution at a constant pH, thus decreasing pCO2. This relative balance is illustrated for example in the figure below from chapter 8 in Sarmiento (2013). Why is the opposite true?

[![enter image description here](https://i.sstatic.net/DGTMU.png)](https://i.sstatic.net/DGTMU.png)

Taro Takahashi; Jon Olafsson; John G. Goddard; David W. Chipman; S. C. Sutherland (1993). Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: A comparative study. , 7(4), 843–0. doi:10.1029/93gb02263

Sarmiento, Jorge L. "Ocean biogeochemical dynamics." Ocean Biogeochemical Dynamics. Princeton University Press, 2013.

**Tags:** oceanography, geochemistry, carbon-cycle, biogeochemistry

> **Comments:**
> **username_1**: Is this about The Revelle Factor? Revelle Factor/buffer factor is a measure of the resistance of the ocean surface layer to absorption of atmospheric CO2. https://en.wikipedia.org/wiki/Revelle_factor  

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## 1 answers

### Answer by username_2 on 2022-11-27. Score: 1

The answer is right there on the figure, also known as [Bjerrum plot](https://en.wikipedia.org/wiki/Bjerrum_plot). If you decrease alkalinity (i.e. increase pH) you shift the relative proportions of the plot to the right, that is, decreasing H2CO3 (or dissolved CO2) concentrations, and increasing bicarbonate and carbonate ions concentrations. And vice versa. Increasing alkalinity decreases pH, increasing pCO2.

> **Comments:**
> **username_0**: One can change alkalinity without changing pH. This is why I mentioned "at constant pH" in the question. For example, if one adds OH- ion to sea water, it doesn't change pH. Why does it decrease pCO2?  
> **username_0**: Sorry, this doesn't answer the question. "At constant pH" is important here.  
> **username_0**: Decreasing alkalinity is not a synonym for increasing pH. Alkalinity is a sum of non-conservative ions, including H+ but not restricted to it.  
> **username_3**: @username_0 It is possible to change alkalinity without changing pH, but it is very unnatural process, that has to be controlled by supervising human.  
> **username_2**: @username_3 thanks for the comment. But the weathering of basic minerals, like MgOH or MgO, increase ocean alkalinity (by adding OH- and Mg++) without changing pH (no H+ added or removed). Not that uncommon in nature, I think.  
> **username_3**: @username_2 - Changing concentration of OH- (e.g.due dissolving natural carbonates and their hydrolysis)  directly impacts concentration of H+ via $\ce{H+ + OH-  <=> H2O}$  with Kw=[H+][OH-].  
> **username_2**: Ah ok @username_3. But how about adding MgO? It would increase [Mg++], thus make water more alkaline?  
> **username_3**: Yes, it does, by increasing the OH- concentration // $\ce{MgO(s) + H2O(l) ->  Mg(OH)2(s) <=>[H2O]Mg^2+(aq) + 2 OH-(aq)}$ // $\ce{OH-(aq) + H+(aq) <=>>H2O(l)}$ // pH increases as OH- ions are scavenging H+ ions until the equilibrium is reached.  

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