# Reasons we can't accurately calculate CO2 from alkalinity and pH...



## oviparous (30 Mar 2015)

Hi all,

I always read that we can't calculate our CO2 concentration from alkalinity and pH because there is other "stuff" that interferes with our readings. 
Because i'm always curious i wanted to know what that other "stuff" is, how much influence it has and in what circumstances it is likely to occur.
After reading almost every article about it on (mostly) aquarium websites (i know, never get info from an aquarium website), i was a bit disappointed because I never  found a satisfying answer.  We know what contributes to alkalinity, but not what influence it has and in which circumstances.
So i tried searching for  scientific data... Aha... Some real answers. 
The most interesting where these 2 papers:
http://www.biogeosciences-discuss.net/11/11701/2014/bgd-11-11701-2014.pdf
And
http://www.biogeosciences.net/8/3069/2011/bg-8-3069-2011.pdf

The first is really interesting, because they sampled water from different  types of water/biotopes. This way you also can get an idea how much influence it has on your own tank.
I'll post the abstract, but you really should read the whole paper it's really interesting!


> Inland waters have been recognized as a significant source of carbon dioxide
> (CO2) to the atmosphere at the global scale. Fluxes of CO2 between
> aquatic systems and the atmosphere are calculated from the gas transfer
> velocity and the water–air gradient of the partial pressure of CO2
> ...



The second one is easier to read, but not as thorough.


> Total alkalinity (TAlk) has long been used to evaluate the buffering capacity of aquatic systems. TAlk has also been used, together with measurements of either pH or dissolved inorganic carbon (DIC), to indirectly estimate the partial pressure of carbon dioxide (_p_CO2) in inland waters, estuaries, and marine systems. These estimates typically assume that carbonate and bicarbonate ions comprise nearly all the species contributing to TAlk; however, other inorganic and organic acids have the potential to contribute significant non-carbonate alkalinity. To evaluate the potential for error in using TAlk to estimate _p_CO2, we measured pH, TAlk, and DIC in samples of river water. Estimates of _p_CO2 derived from TAlk and pH measurements were higher than _p_CO2 estimates derived from DIC and pH by 13–66%. We infer that this overestimate is due to the presence of significant non-carbonate alkalinity (NC-Alk). This study also describes the relative proportions of carbonate- and non-carbonate alkalinity measured in 15 river systems located in northern New England (USA) and New Brunswick (Canada). NC-Alk represents a significant buffering component in these river systems (21–∼100% of TAlk), and failure to account for NC-Alk (which cannot directly contribute to _p_CO2) leads to the overestimation of carbon dioxide release to the atmosphere.



Have fun reading!


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## Jose (8 Sep 2015)

I think those other substances that affect ph kh equilibrium are basic substances like silicates or phosphates. Anything that affects ph without changing kh. This is a good question that also buggs me. Acids affect ph but also kh so they should fall into the equilibrium, but in reality acids also mess up the ph kh table. Too Many questions.


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