Yes, it’s true! (Hey, it’s in the wiki so it must be true! /sarc )
We have (bold added):
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution. In the natural environment carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and presence of carbon dioxide in the atmosphere. Other common natural components that can contribute to alkalinity include borate, hydroxide, phosphate, silicate, nitrate, dissolved ammonia, the conjugate bases of some organic acids and sulfide. Solutions produced in a laboratory may contain a virtually limitless number of bases that contribute to alkalinity. Alkalinity is usually given in the unit mEq/L (milliequivalent per liter). Commercially, as in the swimming pool industry, alkalinity might also be given in parts per million of equivalent calcium carbonate (ppm CaCO₃).
So most alkalinity is due to carbonate rocks and CO₂ in the atmosphere. But adding more CO₂ doesn’t make thing more alkaline:
Alkalinity is sometimes incorrectly used interchangeably with basicity. For example, the pH of a solution can be lowered by the addition of CO₂. This will reduce the basicity; however, the alkalinity will remain unchanged
So there you have it. CO₂ in the atmosphere causes alkalinity and adding CO₂ to a solution will leave the alkalinity unchanged.
“Paging (not a) Doctor Connolley to the wiki-Metaphor Enforcement Ward, STAT!”
Now, in reality, there’s a bit of a word game going on here. Or perhaps a “definition game”…
You see, in common use, “alkaline” is used to mean “basic” or “opposite of acidic”. That isn’t strictly correct; though often the two things, alkalinity and basicity, are ‘fellow travelers’ in chemical reactions, so folks commonly treat them as the same thing. They are not.
An acid is a proton donor (there are a couple of other definitions too, such as based on electron pairs or …) where a base is a proton acceptor. Alkaline does not mean that. It means a specific neutralization point related to carbonate. That difference makes all the difference in the world as it relates to CO₂.
Alkalinity can be measured by titrating a sample with a strong acid until all the buffering capacity of the aforementioned ions above the pH of bicarbonate or carbonate is consumed. This point is functionally set to pH 4.5. At this point, all the bases of interest have been protonated to the zero level species, hence they no longer cause alkalinity. For example, the following reactions take place during the addition of acid to a typical seawater solution:
The key point there being the pH 4.5 point. So adding CO₂ to sea water at present does NOT “increase acidity” as the ocean is not acidic, it is basic (being pH bigger than 7 toward the 8 basic side). More importantly, adding CO₂ also does not make the ocean “less alkaline” either! Yes, it’s true! (Hey, it IS in a wiki article…)
As a sidebar note: Freshwater clams and other shell formers are happy to make shells at pH up to 4.5 or so. Leads me to wonder if they care about pH or Alkalinity… https://chiefio.wordpress.com/2012/03/08/clams-do-fine-in-acid-water/
Addition of CO₂
The addition (or removal) of CO₂ to a solution does not change the alkalinity. This is because the net reaction produces the same number of equivalents of positively contributing species (H+) as negative contributing species (HCO₃- and/or CO₃²-).
At neutral pH values:
CO₂ + H₂O → HCO₃− + H+
At high pH values:
CO₂ + H₂O → HCO₃ ²- + 2H+
So there you have it. Next time a Warming “chicken little” screams at you about “ocean acidification” and CO2, just point out that the ocean is alkaline and adding more CO2 to the ocean does not change the alkalinity at all… and wait for it…