Analytical aspects of the application of sodium pyrophosphate reagent in the specific extraction of the labile organic component of humus and soils

Abstract

The effectiveness of the 0.1 M Na 4P 2O 7 (sodium pyrophosphate) leach commonly used in soil science to extract humic and fulvic acid components was studied for application to humus samples collected in exploration geochemistry programs. This digestion is viewed as an alternative to the stronger aqua regia attack, to preferentially dissolve those elements bound to or scavenged by humic and fulvic complexes present in soils. Study of the contact time of 1 g of humus in 100 ml of 0.1 M Na 4P 2O 7 solution for 30 samples revealed that the conventional 16-h duration could be decreased to 1 h (with constant agitation) without significantly lessening the amount of Co, Cu, Fe, Ni. Pb or Zn extracted. In fact, the amount of Ni apparently extracted increased, reflecting readsorption of Ni onto the substrate in the longer procedure. About 10–20% more element, including organic carbon, was extracted by a second 1-h treatment with fresh Na 4P 2O 7 solution but Mn behaved differently in that a greater proportion was dissolved, about 30% more than by the first leach. This pattern was further shown by another group of 125 humus samples to which two 3-h Na 4P 2O 7 leaches were applied. The greater amount of Mn extracted in the second leach (30 vs. 10–20% for the other elements) is probably due to dispersion of Mn-containing colloidal particles rather than to organic dissolution. At 1 g subsampling, results for two in-house humus control samples and the international standard reference lake sediment, LKSD-4, demonstrated relative standard deviations in the range 2–7% when element concentrations were a decade above instrumental detection limits. Similar precision was shown by replicate samples of 37 humus samples sieved to < 80 mesh. Detection limits in the order of 1 ppm for these elements in the original sample by atomic absorption spectrometry are improved by one to two orders of magnitude by flow injection inductively coupled plasma mass spectrometry. The latter technique requires dilution of the leachate by about 20-fold to obviate severe drift due to salt build-up on the sampling orifice, even with the use of flow injection.