Comparison of Two Reagents, Sodium Pyrophosphate and Sodium Hydroxide, in the Extraction of Labile Metal Organic Complexes

The international reference lake sediment, LKSD-4, was used to compare Hg, organic C and Zn extracted from its’ soluble organic’ phase by two commonly used reagents: 0.1 M Na₄P₂0₇ solution at pH 10 and 0.5 M NaOH solution at pH 12. While recoveries of Hg and Zn by 0.1 M Na₄P₂O₇ are not affected by changes in sample weight to reagent volume ratio (W/V) or contact time, those by NaOH show a marked dependency. In general, the NaOH leach extracts more organic C and Hg from LKSD-4 but less Zn. Over the range of conditions studied, the NaOH-based method extracted 4.7–9.8% C, 27-103 ng g−1 Hg and 19–69 υg g−1 Zn from LKSD-4, compared to 2.3–2.8% C, 17–24 ng g−1 Hg and 64–72 μg g−1 Zn by the Na₄P₂O₇ leach. Clearly, different groups of organic substances are being dissolved by these two reagents and therefore a comparison of data from different laboratories becomes meaningless. This paper suggests that more research is needed into the exact nature of metal-organic associations extracted by selective leaches and into associated artifacts of extraction such as readsorption phenomena.