I have found out concentrations of organic ions such as formic acid, acetic acid and also measured inorganic ions which contribute acidity to the rain water using Ion chromatograph.
Could you titrate a sample? Say, 20ml of your sample titrated with 1.00 M NaOH.
M(acid) x Volume(acid) = M(NaOH) x Volume(NaOH) This will (I think) give you the molarity of the hydrogen ions.
Rainwater is completely unbuffered so the pH will change all the time depending on the partial pressure of carbon dioxide, the atmospheric concentrations of sujlphur and nitrogen oxides the temperature and other parameters such as the material of your collecting vessel and whether you collect the sample at the start, the middle or the end of a rainfall event. I suggest you record the pH continuously while collecting and then analyse samples for anions and see if they match the pH values. A single measurement of a dynamic system is only useful in the context of other measurements. pH measurements made in unbuffered water must be subject to rigorous QA procedures if the results are to be used for anything but entertainment..
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