Copper(II) Complexation Properties and Surfactant Activity of 3-[N,N-Bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic Acid andN-(2-Hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic Acid pH Buffers Which May Affect Trace Metal Speciation inin VitroStudies

Abstract

Disadvantages of the some zwitterionic pH buffers are (i) that they can interact with metal ions as well as protons, and (ii) that they may have a surfactant effect in chemical orin vitrobiological or biochemical studies. This has to be taken into account when a buffer is selected. Here, the copper-complexing capacity and the surfactant activity of three compounds, 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO),N-(2-hydroxyethyl)piperazine-N′-(2-hydroxypropanesulfonic acid) (HEPPSO), and piperazine-N,N′-bis(2-ethanesulfonic acid) (Pipes), were investigated. Global stability constants (log βabc) of copper(II)–buffer complexes were determined, at 25°C, 0.5 M ionic strength, and at 0.8 mM buffer concentration, by pH and pCu ion-selective electrode measurements. Here, βabccorresponds to the equilibrium: aCu2++ bL−+ cH+↔ (CuaLbHc)(2a−b+c); HL = DIPSO or HEPPSO; c: +1 = proton; −1 = hydroxide. Using SUPERQUAD constants were calculated, giving: DIPSO: log β110= 5.02, log β120= 8.99, log β130= 13.0, log β140= 16.3, log β14−1= 9.26, log β14−2= 0.645, log β150= 20.5, log β160= 24.3, log β16−1= 16.1, log β16−2= 8.98; HEPPSO: log β110= 4.29, log β120= 8.35, log β130= 12.1, log β140= 15.9, log β150= 19.6, log β160= 23.4, log β16−1= 14.9. The pKavalues were determined at higher buffer concentrations, giving a value 7.33 for DIPSO and 7.84 for HEPPSO at 2.0 mM buffer concentration. Effects of buffer concentration on stability and acidity constants were investigated and compared with measurements using voltammetric and potentiometric stripping analysis, confirming no copper(II) complexation by Pipes. Surfactant activities were determined using alternating current polarography, confirming marked surface activity of 10 mM of DIPSO or HEPPSO.