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

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, Vϕ, 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 CH3COOK have been calculated. The partial molar volumes at infinite dilution, Vϕo, obtained from Vϕ, have been used to calculate the volume of transfer, ΔtrVϕo, from water to aqueous electrolyte solutions. It was found that both Vϕo and ΔtrVϕo 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, –SO3−) and –CH2– group to Vϕo and ΔtrVϕo. The values of Vϕo and ΔtrVϕo for some functional group contributions of the zwittierionic buffers with salts have also been reported.