Enhanced spectrophotometric methods for trace metal determination in waters: zinc as an example

Abstract

Although spectrophotometric methods seem to be outdated as analytical tools for the analysis of trace metals in waters, there is no doubt that they still present a series of advantages over other advanced techniques (simplicity, speed, low cost and maintenance, portable instrumentation, etc.). Even more, if experimental conditions are strictly controlled, it is possible to develop highly competitive methods. In this sense, method validation turns into a key feature to improve and demonstrate its applicability. Bearing this in mind, a simple and very sensitive spectrophotometric method for the direct determination of μg L⁻¹ levels of zinc in natural waters has been developed and in-house validated. It is based on the reaction of Zn(II) with di-2-pyridyl ketone benzoylhydrazone (dPKBH) under slightly acidic conditions (pH 6.4) and 15% (v/v) ethanol to produce a 1 : 2 (Zn : dPKBH) complex (λ_max 370 nm). Beer's law is obeyed in the range 6.3–3000 μg L⁻¹Zn(II) with a detection limit of 0.7 μg L⁻¹. Different parameters such as selectivity, recovery, linearity, limits of detection and quantification, precision, and uncertainty of the measurements were evaluated in order to validate the proposed method. Then, the new method was applied to the analysis of different water samples (e.g., tap and river water), demonstrating its applicability to the determination of Zn at environmental levels.