[Charles Dalby]

How do I handle multiple acids and determine a combined pH value ?
This is engine exhaust and the condensate is assumed to be at equilibirum with the gas stream.

thanks!

[Borek]

That's not a trivial question. General solution gives 6^th degree polynomial (see http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-solution, equation 11.16 - you will have to ignore quotient on the right side and repeat large quotient on the left for all acids present). Plus you should take into account gases solubility. Really difficult task.

You may try to use PHREEQC - free program for aqautic equilibria/chemistry - it should be able to deal with the problem. But the program itself is  not easy to master.

Finally, you may assume that pH is result of only SO₂ presence - it is the strongest of all acids present and it is in the excess).

What kind of engine has more SO₂ than CO₂ in exhaust gases?

[AWK]

At the presence of air and water, NO2 forms mixture of HNO3 and HNO2, ans SO2 is oxidized to H2SO4 by NO2 even without oxygen.

[Charles Dalby]

I will take up your suggestions.

CO2 concentrations are somewhat greater than I have indicated,
but the emphasis is on the sulfuric and nitric portions.

Interestingly an equilibrium constant calculation for 370 PPM CO2 ambient levels
leads to a pH of 4.9, and 5.7pH for natural rainfall after . the original acid rain.

Addition of sulfuric and nitric acids should drop the pH down towards 2 or 3.

Charles