Evidence has been presented to show that seawater behaves as a "constant ionic medium", stabilizing the activity coefficients of solutes present in small concentrations and reducing or eliminating the residual liquid-junction potential encountered in measuring the pH of seawater systems. Under these conditions, an experimental scale of hydrogen ion concentration becomes a practical possibility. The standard emf of the H2/Ag;AgCI cell, needed to establish standard reference buffer solutions in seawater, has been determined In synthetic seawater at salinities from 20 to 45% and temperatures from 5 to 40 OC. Supplementary measurements in seawater to which HCI has been added lead to values of pHso.,, the formation constant of HS04-, in this range of salinity and temperature.In earlier contributions (1-3), we have examined the possibility of determining experimentally, by electrometric methods, the hydrogen ion concentration of natural seawater.It was concluded that seawater of 35%0 salinity possesses the favorable characteristics of a "constant ionic medium". Thus, the liquid-junction potential between seawater and the customary concentrated potassium chloride solution of the reference electrode is nearly constant when small changes in the composition of the seawater solution occur. Furthermore, the activity coefficients of solutes present in relatively small amounts in the seawater medium are stabilized, changing but little as the concentrations of these substances are altered. As a consequence of these favorable conditions, it appears feasible to standardize a scale of pH, defined as -log CH, for seawater that can be experimentally realized through measurements with the electrometric assembly of the usual glass-calomel type. The practical advantage of such a scale derives both from the utility of the quantity C H and from the greater simplicity On leave 1975-76 from the University of Malaya, Kuala Lum-On leave 1975-76 from Carleton College, Northfield, Minn. pur. I of the concept of hydrogen ion concentration as compared with that of the hydrogen ion activity. In practice, the pH of an "unknown" solution (X) is derived from the emf ( E x ) of the cell Glass electrode 1 Soln X (1 KCl, Reference electrode (A) and from the corresponding emf (Es) for the same cell in which a standard solution (S) of known or assigned pH [pH(S)] replaces Solution X. Use is made of the operational definition When the ionic strengths and compositions with respect to the preponderant electrolytes are the same in the solutions X and S, Equation 1 yields an experimental measure of p r n~( X )if p m~( S ) is known ( m = molality). In order to establish hydrogen ion concentrations or molalities for buffer solutions in synthetic seawater (sw) in the pH range close to that of natural seawater, we have chosen to study the emf of cells without liquid junction, in a manner similar to that used to derive pH(S) for the NBS standard reference solutions ( 4 ) . Hydrogen electrodes and silver-silver chloride electrodes are used in this method. The stand...

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