Electrochemical study of proton ionisation, copper(II) complexation and surfactant properties of piperazine-N-N′-bis[2-hydroxypropanesulfonic acid] pH buffer: Comparison with other N-substituted aminosulfonic acids pH buffers Original Research Article

The surface active behaviour of the piperazine-N-N′-bis[2-hydroxypropanesulfonic acid] (POPSO) and its capability to bind copper(II) were investigated at 25°C and 0.5 mol l−1 ionic strength. For acid dissociation and copper(II)–buffer complexation determinations, ion selective electrode potentiometric titration data (pH or pH plus pCu measurements) were evaluated with the help of SUPERQUAD computational program. The buffer concentration influenced markedly the results. At 10 mmol l−1 buffer, pKa1=7.69 and pKa2=8.20 were obtained. At 2.0 mmol l−1, pKa2=7.92 and insolubility of the buffer at pH≤7.0 prevented the determination of pKa1. Too low buffer power of the solution prevented the determination of the pKa2 value at 0.8 mmol l−1. At that POPSO concentration, the following Cu(II)–POPSO stability constants (log βab, corresponding to the equilibrium: aCu2++bL2−↔[MaLb]2a−2b; H2L=POPSO) were obtained: β11=4.1, β12=8.1, β13=12, β14=16, β15=19, β16=23. The values of βab (and the statistical quality of the fit) decreased when the buffer concentration increased (e.g., log β13=9.6 at 10 mmol l−1 POPSO). Aggregation of molecules of POPSO may be the cause of this behaviour. Demonstration of a marked surface active behaviour of 10 mmol l−1 POPSO solution was obtained by alternating current polarography. A conditional stability constant of Cu(II)–buffer at 10 mmol l−1 POPSO concentration and pH=8.0, was 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 us: N-[2-hydroxyethyl]piperazine-N′-[2-ethanesulfonic acid] (HEPES), N-[2-hydroxyethyl]piperazine-N′-[2-hydroxypropanesulfonic acid] (HEPPSO), 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and piperazine-N,N′-bis[2-ethanesulfonic acid] (PIPES).