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

[oviparous]

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
(pCO2). Currently, direct measurements of water pCO2 remain
scarce in freshwaters, and most published pCO2 data are calculated
from temperature, pH and total alkalinity (TA). Here, we compare
calculated (pH and TA) and measured (equilibrator and headspace)
water pCO2 in a large array of temperate and tropical freshwaters. The
761 data points cover a wide range of values for TA (0 to
14 200 µmol L-1), pH (3.94 to 9.17), measured pCO2
(36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to
3970 µmol L-1). Calculated pCO2 were > 10 %
higher than measured pCO2 in 60 % of the samples (with a median
overestimation of calculated pCO2 compared to measured pCO2 of
2560 ppmv) and were > 100 % higher in the 25 % most organic-rich
and acidic samples (with a median overestimation of 9080 ppmv). We suggest
these large overestimations of calculated pCO2 with respect to
measured pCO2 are due to the combination of two cumulative effects:
(1) a more significant contribution of organic acids anions to TA
in waters with low carbonate alkalinity and high DOC concentrations; (2) a
lower buffering capacity of the carbonate system at low pH, which increases
the sensitivity of calculated pCO2 to TA in acidic and
organic-rich waters. No empirical relationship could be derived from our
data set in order to correct calculated pCO2 for this bias. Owing to
the widespread distribution of acidic, organic-rich freshwaters, we conclude
that regional and global estimates of CO2 outgassing from freshwaters
based on pH and TA data only are most likely overestimated,
although the magnitude of the overestimation needs further quantitative
analysis. Direct measurements of pCO2 are recommended in inland waters
in general, and in particular in acidic, poorly buffered freshwaters.

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 (pCO2) 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 pCO2, we measured pH, TAlk, and DIC in samples of river water. Estimates of pCO2 derived from TAlk and pH measurements were higher than pCO2 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 pCO2) leads to the overestimation of carbon dioxide release to the atmosphere.

Have fun reading!

 

[Jose]

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.