ESTIMATION OF ACID DISSOCIATION-CONSTANTS IN SEAWATER MEDIA FROM POTENTIOMETRIC TITRATIONS WITH STRONG BASE .1. IONIC PRODUCT OF WATER - KW

The various assumptions implicit in the calculation of acid dissociation constants (based on ionic medium standard states) from potentiometric titrations using a cell with liquid junction (i.e. a pH measuring cell) have been examined. It was concluded that results can be obtained having an accuracy commensurate with the experimental precision. It has been shown that although the precise composition of the medium is a function of the hydrogen ion concentration (because of the protolytic nature of some of the ions in the media, e.g., sulphate and fluoride), the effect of such variations in the medium composition can be compensated for when defining the activity of hydrogen ion on an ionic medium standard state by defining the concentration of hydrogen ion as: [H]SWS=h(1 + βHSOST + βHFET) where βHSO and βHF are the relevant association constants and ST and FT are the total concentrations of sulphate and fluoride, respectively. This approach was used to obtain values for the ionic product of water (KW) in artificial seawater media at various temperatures and ionic strengths. These were fitted to give the equation (molal concentration units): pKw= 3441.0 T+2.256-0.7091 1 2 (rms deviation 0.01)where I is the formal ionic strength of the artificial seawater medium and T is the absolute temperature. The values obtained are in reasonable agreement with those found by previous workers. © 1979.