Volumetric properties of MES, MOPS, MOPSO, and MOBS in water and in aqueous electrolyte solutions

Abstract

4-Morpholineethanesulfonic acid (MES), 4-morpholinepropanesulfonic acid (MOPS), 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO), and 4-( N-morpholino)butanesulfonic acid (MOBS), are useful for pH control as standard buffers in the physiological region of 5.5–6.7 for MES, 6.5–7.9 for MOPS, 6.2–7.6 for MOPSO, and 6.9–8.3 for MOBS, respectively. On the basis of density measurements at 298.15 K, the apparent molar volumes, V ϕ , of the above-mentioned buffers in water and in (0.05, 0.16, and 0.25) mol kg −1 aqueous solutions of NaCl, KCl, KBr, and CH 3COOK have been calculated. The partial molar volumes at infinite dilution, V ϕ o , obtained from V ϕ , have been used to calculate the volume of transfer, Δ t r V ϕ o , from water to aqueous electrolyte solutions. It was found that both V ϕ o and Δ t r V ϕ o vary linearly with increasing the number of carbon atoms in the alkyl group side chain of the zwitterionic buffers. These linear correlations have been utilized to estimate the contributions of the zwitterionic end group (morpholinium ion, –SO 3 −) and –CH 2– group to V ϕ o and Δ t r V ϕ o . The values of V ϕ o and Δ t r V ϕ o for some functional group contributions of the zwittierionic buffers with salts have also been reported.