Constantes de stabilité de complexes organo-minéraux. Interactions des ions plombeux avec les composés organiques hydrosolubles des eaux gravitaires de podzol

Details Constantes de stabilité de complexes organo-minéraux. Interactions des ions plombeux avec les composés organiques hydrosolubles des eaux gravitaires de podzol Constantes de stabilité de complexes organo-minéraux. Interactions des ions plombeux avec les composés organiques hydrosolubles des eaux gravitaires de podzol

Feb 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48..227b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 48, Issue 2, pp.227-234

Computer Science

Scientific paper

The complexation of lead(II) ions by water-soluble soil organic matter as ligand (recovered by percolating water through a A2 horizon of a podzolic soil) has been studied by potentiometric methods at 25°C in 0.1 M NaClO 4 medium. The total acidity of these solutions was resolved into strong, weak and very weak acidity. During the alkali titration, the dissociation of humic samples was characterized by an coefficient that may be determined according to a procedure derived from Irving-Rossotti's method. The so-called Henderson-Hasselbalch equation was used to obtain pK A values for each of the two weak acidities ( pK A = 4.80 (COOH) and pK A = 8.85 (OH)). For the determination of stability constants of metal-humate complexes (HA) in the acidic range, three methods have been investigated: --an approach using only pH measurements. --direct estimation of the free metal ion concentrations from ISE determinations. --application of Marinsky's method accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A]. In these experiments typical values used for total lead(II) ion concentrations and total ligand concentrations are respectively in the range 4.10 -5 M to 4.10 -4 M and 0,33 to 0,57 meq·1 -1 . All results agree with the existence of mono and bis-complexes PbA and PbA 2 with stability constants (protometric determinations): log K 1 = 4.2 and log K 2 = 3.7. The values obtained by direct estimation of free lead(II) concentration are slightly lower. An increase in stability constants was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH). Ligand conformational properties are expected to be of great importance in the complexation phenomena.

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