Iron ions, aqueous stability constants

In a review on the design of ligands for selective complexation of metal ions in aqueous media and a book on the principles underlying stability constants and on the design of metal complexes for various medical applications iron complexes and their solution chemistry take their appropriate place. 

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

A study of sucrose interactions with the same raagnesivun and calcium ions showed that both form mono- and di- sucrose adducts, and the isolation of several complexes formed between these metal ions and D-glucurono-1,4-lactone in ethanolic and aqueous solutions has been reported. The interaction in aqueous solution of j8-D-fructose with hydrated salts of Zn(II), Cd(Il), and Hg(ll) has given rise to solid adducts of the type M(D-fructose)Xa.nHaO, X = Br, Cl which were characterised by F.T.-I.R. spectroscopy. X-ray powder diffraction and molar conductivity measurements. Potentiometric pH titration was used to measure the stability constants of the 1 1 complexes formed between a number of cations and tubercidin 5 -monophosphate. The complexation ofCopper(II) ions with sucrose has been studied and a number of complexes were identified. A molecular model has been used to describe the formation of iron(III)-glucosamine complexes in aqueous KOH at various mole ratios. ... 

Hexa.cya.no Complexes. Ferrocyanide [13408-634] (hexakiscyanoferrate-(4—)), (Fe(CN)6)4", is formed by reaction of iron(II) salts with excess aqueous cyanide. The reaction results in the release of 360 kJ/mol (86 kcal/mol) of heat. The thermodynamic stability of the anion accounts for the success of the original method of synthesis, fusing nitrogenous animal residues (blood, hom, hides, etc) with iron and potassium carbonate. Chemical or electrolytic oxidation of the complex ion affords ferricyanide [13408-62-3] (hexakiscyanoferrate(3—)), [Fe(CN)6]3-, which has a formation constant that is larger by a factor of 107. However, hexakiscyanoferrate(3—) cannot be prepared by direct reaction of iron(III) and cyanide because significant amounts of iron(III) hydroxide also form. Hexacyanoferrate(4—) is quite inert and is nontoxic. In contrast, hexacyanoferrate(3—) is toxic because it is more labile and cyanide dissociates readily. Both complexes liberate HCN upon addition of acids. 

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