Electrochemical Evidence of Surfactant Activity of the Hepes pH Buffer Which May Have Implications on Trace Metal Availability to Culturesin Vitro

Hydrogen ion buffers are required for many different types ofin vitrobiological and chemical studies, but they may not be inert enough, thus interfering with the system under study.N-Hydroxyethylpiperazine-N′-ethanesulfonic acid (Hepes), a zwitterionic buffer in the 7.0–8.0 pH range, has been used in different biological studies. In this work, surfactant activity of Hepes and the buffer's capability to bind copper(II) were investigated through different electrochemical techniques: ion-selective electrode potentiometry, potentiometric stripping analysis, differential pulse anodic stripping voltammetry, normal pulse anodic stripping voltammetry, and alternating current polarography. Hepes concentrations between 0.1 (0.24 g liter−1) and 25 mmol liter−1(6.0 g liter−1) were studied at pH 8.0 (and at pH 6.5 for comparison). At pH 8.0 Hepes displayed surfactant activity, particularly when |Hepes| > 10 mmol liter−1(2.4 g liter−1). The ability of Hepes to bind copper(II) was detected when Hepes was in large excess (|Hepes|/|Cu| between 10,000 and 2000). Eventual implications in the bioavailability of copper(II) are also discussed.