Determination of alkalinities of estuarine waters by a two-point potentiometric titration

Gran plots of titrations of seawater with acid are straight lines after protonation of all weak acids when ion-pairing is taken into account. This property is used to calibrate the pH electrode and to determine the endpoint of what is essentially a two-point alkalinity titration of the sample. First the initial sample pH is measured; then a standard addition of acid is made giving a pH near 3.2 (pH1); a further acid addition is made giving a pH near 2 (pH2). The slope of the electrode response and the total alkalinity are calculated from pH2 and pH1. The advantages of this method are that no separate calibrations are necessary: no corrections for variations in activity coefficients are needed because pH values are obtained on the seawater pH scale; and the instruments used for the determinations are very simple. The standard deviation of the alkalinity determination of seawater by the proposed technique was − 0.10%.