FIA of fog- and rain-water acidity

Both pH 4.8 and 8.3 acidities were determined by FIA. The carrier solution was a carbonate-free 1X10-3 mol/l sodium hydroxide solution. For the pH 8.3 acidity measurement, the indicator was Bromothymol Blue; for pH 4.8, however, Bromophenol Blue was used. The half-width of the signal peak was used for the calibration. The values obtained by the FIA method agree with those obtained by the corresponding titration method, except for one case, in which the carrier sodium hydroxide concentration was much lower than those of the solutes in the sample solution. For all fog- and rainwater samples, both the pH 8.3 acidity obtained by FIA and the total acidity obtained by Gran's plots in the titration method were much larger than the hydrogen-ion concentrations because of the presence not only of ammonium ions, but also organic acids.