Determination of the Solubility Product of Various Iron Hydroxides and Jarosite by the Chelation Method

Abstract

A modification of the EDTA method of determining ion activities in soil developed by Norvell was used to measure the solubility of various ferric hydroxides. The solubility products of jarosite (KFe3(SO4)2(OH)6) and amorphous Fe(OH)3 were also determined on a natural jarosite nodule taken from an acid sulphate soil.For amorphous ferric hydroxide and lepidocrocite, the method gave pK° values of 39.02 ± .35 and 42.48 ± .23, respectively. Hematite and goethite were too inert to yield reproducible solubilities.For a mixture of jarosite and ferric hydroxide, the respective pK° values were 98.56 and 39.93. These values were used to develop an Eh‐pH stability diagram that helps to explain the occurrence of jarosite under acid oxidizing conditions in soils.

Selective Extraction of Amorphous Iron Oxide by Edta From a Mixture of Amorphous Iron Oxide, Goethite, and Hematite

“…measurable reduction could be observed in the peak ding to goethite plus hematite, which may On the extractable compound is X-ray amorphous. The DTA curve excludes, however, polymeric iron hydroxide, and its absence is verified to some extent b the solubility roduct calculated above, which is smaller than et al, 1963 s o h n , 1967; Vlek et al, 1974). This leads to the assumption that the iron extracted by EDTA is present in poorly crystalline iron oxide.…”

“…It may be goethite and to carry out a selective extraction of oxide mixed with fematite, with solubility roducts of 10-44 to I O -"~ (Vlek et al, 1974) are herefore, the selectivity must be due to an extreme inertness of goethite thermodynamically unsta g le in all the EDTA extractants given in Fig. 2 and hematite towards dissolution in EDTA solutions, resulting in dissolution rates of these compounds which are orders of magnitude smaller than the dissolution rate of amorphous iron oxide.…”

“…This result seems to indicate that the E g TA-extractable fraction DiSCUSk The present investigation shows that it by means of EDTA oethite and hematite. It may be goethite and to carry out a selective extraction of oxide mixed with fematite, with solubility roducts of 10-44 to I O -"~(Vlek et al, 1974) are herefore, the selectivity must be due to an extreme inertness of goethite thermodynamically unsta g le in all the EDTA extractants given inFig. 2.The reciprocal of undissolved EDTA-extractable iron plotted against extraction time.…”

Selective Extraction of Amorphous Iron Oxides by Edta From a Danish Sandy Loam

“…These iron oxides are considered to be amorphous, as the solubility product of amorphous iron oxides are - (Vlek et al, 1974;Borggaard, 1976;Christensen and Borggaard, 1977).…”

Copper Industrial Byproducts for Improving Iron Deficient Calcareous Soils1

“…The findings of the present experiment with "jarosite" are no exception to the general pattern. The laboratory data of Vlek et al (1974) suggest that pure "jarosite" was of low Fe solubility and in the same order of magnitude as other Fe compounds such as amorphous ferric hy-…”