I needed to calculate mg/l CaCO3 values from partial CO2 pressures, but I'd like to double check with others to see if I did it correctly since the books aren't always clear on that.
I used the following process
%pCO2/100 --> atmospheres CO2
(atm CO2)*10^(-1.48)*1000 --> mM of H2CO3
(mM H2CO3)*61 --> mg/L H2CO3
(mg/l H2CO3)*1.22 --> mg/l CaCO3
There are no other components other than pCO2
Are you adding the CO2 to fresh or to sea water, and what temperature is the water at?
Alkalinity is independent of the pCO2 according to its definition so I cannot understand the basis for your calculation.Sea surface water with a high alkalinity and a mountain lake on granite with typical low alkalinity will both be in equilibrium with the CO2 of the atmosphere and thus have the same [H2CO3*]. Where did you get the conversion factor (1.22) from?
I agree with Henriks answer. If you wish to calculate alkalinity or CaCO3 concentrations for aqueous solutions you require a second CO2 system parameter apart from pCO2. There is the CO2sys software available for this kind of calculations (link below). Playing around with it might give you a better feeling for the constraints of the CO2 equilibrium reactions.
http://cdiac.ess-dive.lbl.gov/ftp/co2sys/?C=S;O=A
I have just read the last line of the question which states "There are no other components other than pCO2." so obviously the question is about adding CO2 to distilled water, with some calcium ions added? I assume the conditions are 1 bar and 25C.
To clarify: the system has 200 mg/l of Ca2+ added as CaCl2 and was stabilized at pH 7.5 with an organic buffer. What I am asking for (I guess) is the total IC present in the system. The people I report this to also want the IC presented as mg/l CaCO3 for... reasons.
In the system you describe the alkalinity and IC would both be different from that you calculation take into consideration.
Inorganic carbon (IC) is different from alkalinity and it is defined in absence of metal complexes: Inorganic carbon = IC = [H2CO3 ] + [HCO3- ] + [CO32-].
The definition of carbonate alkalinity for the system would for your system be: AlkC = 2 [CO32-] + [HCO3-] + [OH-] - [H+]
You mentioned there is an organic buffer added to the system. The base component of this buffer would contribute to the alkalinity and thus your system's total alkalinity would be: AlkT = 2 [CO32-] + [HCO3-] + [OH-] - [H+] + [Base in buffer]
If you goal is to calculate the inorganic carbon (IC) you also need to consider [HCO3-] (Besides H2CO3) which you calculate using the fixed pH of 7.5 and the [H2CO3*] you already have calculated from pCO2 using Henry's law.
There is a description here: https://sciencing.com/calculate-alkalinity-caco-5328969.html of "How to Calculate Alkalinity As Concentration of CaCO3". I have formatted it to make it more readable and attached my file.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Atmosphere