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Pristine Rainwater

What is the chemical composition of pristine rainwater?

Preliminary Consideration

‘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)

It is an example par excellence of the open carbonate system. The partial pressure of CO₂(g) is 0.00039 atm. In hydrochemistry, it is quite common to express the partial pressure by the pCO2 value:¹

(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.

pH	=   5.61
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