CALCULATION OF THE PH OF RAINWATER Rainwater is normally low in all mineral constituents. Addition of dissolved CO2 will result in an increase in the H+concentration, with the result that the CO32 concentration will be negligible. The pH of rainwater is estimated at 25C as follows: CO2 concentration in air is 390ppmv P1=VV1P Electroneutrality principle states that the sum of positive and negative charges within the water must balance to zero