Thermodynamics of the dissociation of morpholinium ion in seawater from 5 to 40�C

Abstract

The dissociation constant of morpholinium ion has been determined in five synthetic seawaters at eight temperatures from 5 to 40°C. The seawater solvents contained NaCl, MgCl2, Na2SO4, CaCl2, and KCl in amounts corresponding to salinities of approximately 10, 20, 30, 35, and 40‰ (parts per thousand) and ionic strengths ranging from 0.2 to 0.8 mol-kg−1. Electromotive-force measurements of cells without liquid junction, with hydrogen gas electrodes and Ag/AgCl electrodes, were used to derive the dissociation constant and associated changes of enthalpy, entropy, and heat capacity for the dissociation equilibrium. The pK values over the range of salinity S and thermodynamic temperature T studied are given by the equation $$\begin{gathered} pK = (1396.58 + 1.822S)/T - 2.14236 InT + 16.0138 + 0.001912S \hfill \\ - 9.722 \times 10^{ - 5} S^2 + 1.025 \times 10^{ - 6} S^3 \hfill \\ \end{gathered}$$ At a salinity of 35‰, corresponding to ‘normal seawater’, pK is higher by 0.206 at 25°C than in the water solvent, while ΔHo is increased by 292 cal-mol−1 and both ΔSo and ΔC p o are substantially unchanged. The buffer composed of 0.04 molal morpholinium chloride and 0.04 molal morpholine is useful for pH control in synthetic seawater media.