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 ΔH^o is increased by 292 cal-mol⁻¹ and both ΔS^o 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.