What is the chemical composition of pristine rainwater?
‘Pristine rainwater’ is pure water in chemical equilibrium with the CO2 gas of the atmosphere (no other acids and bases):
|(1)||pristine rainwater = H2O + CO2(g)|
|(2)||pCO2 = – log P = – log 0.00039 = 3.408|
Calculation and Results
We start with pure water (button H2O) and select “Open CO2 System” – as shown in the right screenshot. It displays the default value pCO2 = 3.408, which represents the partial pressure of CO2 in the atmosphere. [It is easy to check it: Select in the blue drop-down list “atm”, then 0.0039 atm appears.]
Click on Start to run the calculations. The first information you get is that pH changes from 7 to 5.61. The complete results are shown in the output tables (including table Ions) for 25.
|DIC||= 0.0157 mM||(= 0.189 mg/L)|
|CO2||= 0.0133 mM|
|HCO3-||= 0.0024 mM|
|CO3-2||= 4.7 · 10-8 mM|
Here, CO2 represents the composite carbonic acid H2CO3*. The sum of CO2 and HCO3- yields the DIC. In this slightly acidic water there is virtually no carbonate ion, CO3-2.
Remarks & Footnotes
Many textbooks use the (rounded) pCO2 value of 3.5. So, one can rerun the above calculations with pCO2 = 3.5, if desired. What we then obtain is: pH 5.66 and DIC = 0.0130 mM. ↩