Acid-Base Properties of 2-Morpholinoethanesulfonic Acid (MES), Complexation Reaction of CuII-MES, and Interaction of Hydrous Manganese Oxide Surface with CuII in MES Buffer

Abstract

The acidity constants of 2-morpholinoethanesulfonic acid (MES) have been determined at various ionic strengths; pK_a1 = 1.99 and pK_a2 = 6.21 at ionic strength I = 0.1 M. The conditional stability constant of Cu^II-MES complex has been measured by means of a Cu^II-ion selective electrode: lg ^cK_Cu-MES = 1.39 ± 0.07 at pH = 5.57. Furthermore the interaction between Cu^II and hydrous manganese oxide surface in MES buffer was studied by differential pulse polarography. Conditional stability constants for Cu^II-MnO_x complex (SCu) at three pH values were calculated by two methods: van den Berg-Ružić method as well as the Scatchard plot yield corresponding values for lg ^cK_SCu. Both methods indicated the presence of one class of equivalent binding sites. The results of this investigation corroborate that MES is a suitable buffer for studying metal-MnO_x interactions.