On the true and the perceived minor interactions of dissolved phosphate and dissolved sulphate and some other constituents with the Alkalinity of oceanic seawater

Abstract

Abstract. We have become aware that there is some confusion in the literature on the interactions of dissolved phosphate and dissolved sulphate with the alkalinity of seawater. This paper aims to invite comments and corrections of colleagues towards resolving this confusion and eventually obtaining full agreement among ocean scientists. In the world oceans, the Alkalinity of seawater or Oceanic Alkalinity is defined as the small difference of electric charge between fully dissociated strong cations and the fully dissociated strong anions. As seawater must have an overall neutral electric charge, this small difference of the strong cations and anions must be compensated by a small difference between not fully dissociated weak cations and the weak anions. Oceanic Alkalinity can be determined by acid titration of a seawater sample, this leading to an ensuing value of Titration Alkalinity. In the titration procedure for Titration Alkalinity, both dissolved phosphate and dissolved sulphate play minor roles that can be assessed accurately from measured values of dissolved phosphate and the value of dissolved sulphate on the basis of measured salinity, respectively. In the past, a perceived role of biological uptake or release of dissolved phosphate has been suggested in the value of Oceanic Alkalinity. To the best of our knowledge, this perceived role is mistaken. Moreover, it has also been reported a perceived role of biological uptake/release of dissolved sulphate from seawater in the value of Oceanic Alkalinity. Latter perceived role in the value of Oceanic Alkalinity is not necessarily wrong, in theoretical principle. However, it deviates from how Alkalinity has traditionally been defined in the literature and how it is used. Moreover, the role of sulphate is not verifiable, because the small amount of biological assimilation of sulphate cannot be discerned from measurement of the very large background concentration value of dissolved sulphate. Consideration of nitrite in Alkalinity has also been suggested and is theoretically not incorrect, but insignificant versus the accuracy of the measurements of Titration Alkalinity. Therefore, it is reasonable to omit and ignore nitrite when considering Alkalinity. Finally, in the classical (1981) expanded equation for Titration Alkalinity, the negative sign of [H3PO4] is mistaken and yet another reason that this [H3PO4] term best would have been, or from now on should be, omitted from the expanded equation for Titration Alkalinity.