High precision measurements of alkalinity and total carbon dioxide in seawater by potentiometric titration — 1. Presence of unknown protolyte(s)?

Abstract

An automated system for measuring alkalinity and total carbon dioxide in seawater by potentiometric titration is described. Single observation standard errors of ∼1 μequiv. kg −1 and ∼2 μmol kg −1 for alkalinity and total carbon dioxide are obtained using this system. The titration total carbon dioxide results are ∼21 μmol kg −1 higher than those obtained by gas extraction methods which, unlike the titration method, do not require detailed knowledge of the chemistry of the seawater samples. Possible explanations of these differences include: an error in the physico-chemical model; a persistent contamination problem; or the presence of an unknown protolyte (HX) in natural seawater. We find this latter hypothesis most intriguing. We show that a species with a p K between 6.0 (carbonic acid) and 8.7 (boric acid) would not affect the alkalinity, but would be recorded putatively as ‘total CO 2’ in a titration procedure, and suggest that such a species may occur at significant concentration in seawater.