Sensitive assay for oxygen solubility in molten alkali metal carbonates by indirect flame atomic absorption spectrometric Cr(VI) determination

A precise and accurate indirect analytical method for the assessment of O₂ solubility in molten (Li_0.62K_0.38)₂CO₃ and (Li_0.52Na_0.48)₂CO₃ is described. The method is based on the oxidation of Cr₂(SO₄)₃ (added in excess to the melt) by the oxygenate species, which are formed inside the melt when it is in contact with oxygen gas, and subsequent determination of trace amount of Cr(VI) in withdrawn frozen melts by flame atomic absorption spectrometry (FAAS). The samples (1.0-2.0 g) are dissolved in dilute hydrochloric acid at room temperature. The speciation of Cr(VI) is carried out by complexation with ammonium pyrrolidine dithiocarbamate (APDC), followed by extraction into methyl isobutyl ketone (MIBK), which is introduced directly into the flame. Optimisation of the flame composition provided maximum Cr signal in organic phase under lean acetylene-air flame. The separation and preconcentration parameters such as sample volume/extractant volume ratio, pH sample solution, chelating concentration and extraction time are evaluated. Under the optimised conditions Cr(VI) is efficiently separated from Cr(III), which exceeded 200-folds. The results of the analysis of synthetic samples using standards in MIBK medium give rise to recoveries of 98-99%. The Cr(VI) detection limit of 4×10⁻⁶ g L⁻¹ using 12.5-fold preconcentration and relative standard deviation of 1% at the 0.10 mg L⁻¹ level are obtained. The sampling-to-sampling reproducibility was typically 3-5% relative standard deviation. By changing the preconcentration factor and the dilution of the sample melt, it is possible to analyse oxygen concentrations in molten alkali metal carbonates as low as 1.5×10⁻⁹ mol O₂ per gram melt.