AN AUTOMATED METHOD FOR THE DETERMINATION OF STRONG AND WEAK ACIDS IN FRESHWATER
The Gran procedure described by Molv©¡rsmyr and Lund (1983) was modified and automated for the routine analysis of strong and weak acids in freshwater. The titrations are carried out by coulometric generation of hydroxide ions in the pH range 3.6-10.3. The method measures the amount of non-volatile strong and weak acids in freshwater with a precision of ¡¾ 5 ¥ìmol/1. The concentrations of strong and weak acids in 94 coloured lakes from two areas in Norway were measured by the method. The results showed that the free acidity in samples with pH below 5 was largely explained by strong acids. Measured negative strong acids of samples with pH above 5.3 correlated well with alkalinity values. Total organic carbon and non-labile aluminium accounted for more than 80 % of the weak acids. The automated method represents a reproducible procedure for measuring strong and weak acids on a routine basis.