Voltammetric Determination of Free and Total Manganese in Tea Infusions

Tea infusions are a source of mineral nutrients, including manganese. Although it is classified as an essential element, the excess of manganese is toxic for humans and its toxicity depends on its bioavailability. In this context, we propose a simple procedure to determine free and total manganese in black tea infusions by square-wave voltammetry (SWV). Free manganese was directly quantified in the samples after simple sample dilution in 0.1 mol L−1 acetic acid/acetate buffer (pH = 4.5) while total manganese determination was carried out after sample digestion with oxidant mixture (HNO3 + H2O2). For comparison, cloud point extraction was performed to remove organic-bound manganese and the content of free metal (not extracted) measured by flame atomic absorption spectrometry (F AAS) agrees with the values obtained by the proposed SWV procedure. The limit of detection and quantification were estimated for manganese determination as 0.06 and 0.20 µg L−1, respectively, with a linear range observed between 20 and 250 µg L−1. The coefficient of variation was 9.7% (50 µg L−1, n = 10). The procedure was applied to determination of free and total manganese in black tea bags and the results were compared with literature procedures employing F AAS.