| Autor: |
J Rouiller, M. Cromer, J.-P. Scharff, Y. Bizri, B Guillet |
| Rok vydání: |
1984 |
| Předmět: |
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| Zdroj: |
Geochimica et Cosmochimica Acta. 48:227-234 |
| ISSN: |
0016-7037 |
| DOI: |
10.1016/0016-7037(84)90246-1 |
| Popis: |
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 NaClO4 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 α 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 pKA values for each of the two weak acidities (pKA = 4.80 (COOH) and pKA = 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 PbA2 with stability constants (protometric determinations): logK1 = 4.2 and logK2 = 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. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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