ELECTROCHEMICAL STUDY OF PROTON IONIZATION, COPPER(II) COMPLEXATION AND SURFACTANT PROPERTIES OF PIPERAZINE-N-N'-BIS[2-HYDROXYPROPANESULFONIC ACID] PH BUFFER - COMPARISON WITH OTHER N-SUBSTITUTED AMINOSULFONICACIDS PH BUFFERS

The surface active behaviour of the piperazine-N-N'-bis[2-hydroxypropa nesulfonic acid] (POPSO) and its capability to bind copper(II) were in vestigated at 25 degrees C and 0.5 mol l(-1) ionic strength. For acid dissociation and copper(II)-buffer complexation determinations, ion se lective electrode potentiometric titration data (pH or pH plus pCu mea surements) were evaluated with the help of SUPERQUAD computational pro gram. The buffer concentration influenced markedly the results. At 10 mmol l(-1) buffer, pK(a1) = 7.69 and pK(a2) = 8.20 were obtained. At 2 .0 mmol l(-1), pK(a2) = 7.92 and insolubility of the buffer at pH less than or equal to 7.0 prevented the determination of pK(a1). Too low b uffer power of the solution prevented the determination of the pK(a2) value at 0.8 mmol l(-1). At that POPSO concentration, the following Cu (II)-POPSO stability constants (log beta(ab), corresponding to the equ ilibrium: aCU(2+) + bL(2-) <-> [MaLb](2a-2b); H2L = POPSO) were obtain ed: beta(11) = 4.1, beta(12) = 8.1, beta(13) = 12 beta(14) = 16, beta( 15) = 19, beta(16) = 23 The values of beta(ab) (and the statistical qu ality of the fit) decreased when the buffer concentration increased (e .g., log beta(13) = 9.6 at 10 mmol l(-1) POPSO). Aggregation of molecu les of POPSO may be the cause of this behaviour. Demonstration of a ma rked surface active behaviour of 10 mmol 1-' POPSO solution was obtain ed by alternating current polarography. A conditional stability consta nt of Cu(II)-buffer at 10 mmol 1-' POPSO concentration and pH = 8.0, w as also determined by the Scatchard method: log K = 5.7 (/Cu(II)/bound / /buffer/(total): 0.088-0.19). The POPSO properties are compared with those of other similar zwitterionic compounds previously studied by u s: N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES), yd roxyethyl]piperazine-N'-[2-hydroxpropanesulfonic acid] (HEPPSO), bis(2 -hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and piperaz ine-N,N'-bis[2-ethanesulfonic acid] (PIPES). (C) 1998 Elsevier Science B.V. All rights reserved.