ANALYTICAL ASPECTS OF THE APPLICATION OF SODIUM PYROPHOSPHATE REAGENTIN THE SPECIFIC EXTRACTION OF THE LABILE ORGANIC-COMPONENT OF HUMUS AND SOILS

The effectiveness of the 0.1 M Na4P2O7 (sodium pyrophosphate) leach co mmonly used in soil science to extract humic and fulvic acid component s was studied for application to humus samples collected in exploratio n geochemistry programs. This digestion is viewed as an alternative to the stronger aqua regia attack, to preferentially dissolve those elem ents bound to or scavenged by humic and fulvic complexes present in so ils. Study of the contact time of 1 g of humus in 100 ml of 0.1 M Na4P 2O7 solution for 30 samples revealed that the conventional 16-h durati on could be decreased to 1 h (with constant agitation) without signifi cantly lessening the amount of Co, Cu, Fe, Ni, Pb or Zn extracted. In fact, the amount of Ni apparently extracted increased, reflecting read sorption 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 Na4P2O7 solution but Mn behaved differently in that a greater proportion was dissolved, about 30% more than by the fi rst leach. This pattern was further shown by another group of 125 humu s samples to which two 3-h Na4P2O7 leaches were applied. The greater a mount of Mn extracted in the second leach (30 vs. 10-20% for the other elements) is probably due to dispersion of Mn-containing colloidal pa rticles rather than to organic dissolution. At 1 g subsampling, result s for two in-house humus control samples and the international standar d reference lake sediment, LKSD-4, demonstrated relative standard devi ations in the range 2-7% when element concentrations were a decade abo ve instrumental detection limits. Similar precision was shown by repli cate 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 at omic absorption spectrometry are improved by one to two orders of magn itude by flow injection inductively coupled plasma mass spectrometry. The latter technique requires dilution of the leachate by about 20-fol d to obviate severe drift due to salt build-up on the sampling orifice , even with the use of flow injection.