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 Na4P2O7 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 µg 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.     

Comparibility of Results Obtained by the Use of Different Selective Extraction Schemer for the Determination of Element Forms in Soils

Two selective extraction schemes, that commonly known as Tessier's and that used by the Geological Survey of Canada (GSC), are compared. The nominal phases identified include: adsorbed/exchangeable/carbonate, ‘AEC’; amorphous Fe oxyhydroxides (including Mn oxides), ‘amFeox’; organics and sulphides, ‘org/sul’; and residual (silicates and more resistant minerals). Three standard reference soil samples, SRMs 2709, 2710 and 2711, were analysed in triplicate for the elements Ca, Cd, Cu, Fe, Mn, Ni, P, Pb, Ti, V and Zn. Results by both methods agree extremely well for Ca, Mn and Cd, and for Ni and V in SRMs 2709 and 2711. For other elements, there is a strong tendency for a higher percentage to be dissolved earlier in the GSC scheme (in the AEC, amFeox and org/sul phases). Consistently more Fe reports to the amFeox and org/sul phases by the GSC method, otherwise present in the residual phase by the Tessier scheme. Results for P do not compare well, with a much greater proportion of the element being extracted in the amFeox phase by the GSC procedure. Very little P appears to be labile by the Tessier protocol. This paper suggests that more research is needed into the nature of phases purportedly extracted by selective leaches.     

Development of a Mercury Vapour Air Analyser for Measurement of Hg in Solution

The Tekran 2537A mercury vapour analyser, designed to measure Hg in air by cold vapour atomic fluorescence spectrometry, has been modified to determine Hg in solution. The new ‘front-end’, required to generate Hg° vapour from acidified waters or acid leachates, is described. Using 1% NaBH4 as reducing agent, a 12 mL water sample can be analysed, at a rate of 1 every 6 min, for Hg to a detection limit of 0.8 ppt (ng L-1). Instrumental precision is typically 1% relative standard deviation (RSD) at levels of Hg from 10 to 200 ppt. Results for 10 analyses of the international water standard, NIST 1642b, are 1530±20 ppt Hg, agreeing well with the certified value of 1480±130 ppt. Nineteen geological standard reference materials (soils, sediments and tills) were used to assess accuracy. Results for these samples, digested in aqua regia in triplicate, showed good agreement with recommended values for all but two, SO-3 and TILL-1. However, results by this method for these two standards were confirmed by an independent method, direct atomic absorption spectrometry. Average method precision was shown to be 5% RSD over the range 10 ng g- 1 to 35 μg g-1 Hg.