Pedro J. Morando

The reductive dissolution of iron oxides by ascorbate: The role of carboxylate anions in accelerating reductive dissolution

There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law R = k[HA−]12[H+] suggests the involvement of surface ≡FeIII A− complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant Ks = (9.57 × 108 M−1). Slow dissolution follows with an empirical rate law R = kobs(≡FeIIIA). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates. Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡FeII surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved FeII-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and FeII-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.

The reaction of cysteine with the pentacyanonitrosylferrate(2–) ion

The title reaction has been studied and it has been found that in the presence of oxygen a catalytic path for the oxidation of cysteine to cystine is established. The rate law is presented, and the mechanism is discussed on the basis of observed effects of the reagent concentrations on the extrapolated initial absorbance of the solution at 522 nm, and on the experimental pseudo-first-order rate of disappearance of colour. Cyanide ion has a remarkable influence on the course of the reaction.

Cleaning of stainless steel surfaces and oxide dissolution by malonic and oxalic acids

Abstract The effectiveness and materials compatibility of malonic acid as a decontamination and chemical cleaning reagent has been explored. Comparison with its homologous oxalic acid demonstrates that malonic acid is a milder reagent, that dissolves the oxide scales more slowly but at acceptable rates, and provides better conditions for base metal protection. The mechanism of attack of malonic acid onto magnetite involves an autocatalytic process, mediated by ferrous ions. The rate is extremely sensitive to the concentration of this ion, and therefore the solution redox potential indirectly determines the rate of attack.

The thermal decomposition of copper(II) nicotinate and isonicotinate

Abstract The thermal decomposition of copper(II) nicotinate and isonicotinate yields, in a sharp transition, metallic copper as the final solid residue along with the release of carbon dioxide and pyridine. In the case of the nicotinate, small amounts of nicotinic acid are also detected. The decomposition is started by homolytic RC(O)O-Cu bond scission, which is equivalent to electron transfer from RC(O)O− to Cu(II).

The thermal decomposition of iron(III) formate

Abstract The thermal decomposition of iron(III) formate takes place through competitive formate anion decomposition (to yield CO+H 2 O) and electron transfer from formate to Fe(III) (to yield Fe(II) formate(s) and CO, CO 2 , H 2 and H 2 O as gaseous products). The processes are overlapped by reactions that yield HCOH.

Dissolution of cobalt ferrites by thioglycolic acid

The dissolution of cobalt ferrites CoxFe3–xO4 by thioglycolic acid involves the chemisorption of thioglycolate anion onto FeIII ions of the solid, followed by an electron transfer from the ligand to the metal ion and subsequent release of FeII. Kinetic data suggest that two adjacent FeIII–L sites evolve to two FeII+L2. Substitution of CoII for FeII does not bring about any noticeable change in the kinetics for x < 0.6. For larger values of x, the early mechanism of dissolution changes, suggesting that electron hopping within the octahedral sites may produce a chain dissolution of FeII for each single original electron transfer from thioglycolate. Rate data in the presence of exogenous FeII are also discussed.

Reduction of vanadium(V) by oxalic acid in aqueous acid solutions

The reduction of vanadium(V) to vanadium(IV) by oxalate in acidic media proceeds at 50 °C via two parallel pathways that involve activated states of compositions {VO2+; 2H+; 2C2O42−}≠ and {VO2+; 2H+; 3C2O42−}≠, resulting in an apparent change in the partial order on oxalate as its concentration increases and a maximum in the rate/pH profiles. The best explanation for these results assumes outer sphere electron transfer as the rate determining step in both pathways, without formation of vanadium(III). The implications for the bioinorganic chemistry of vanadium are discussed.

Comparison of the thermal behaviour of the metal salts of simple and polymeric carboxylates

Abstract The thermal decomposition of several metal polyacrylates and poly(meta)acrylates is compared with the TG of simple carboxylates. The applicability of a general scheme of decomposition is discussed.

Preparation and spectral properties of the sodium salts of pentacyano(ligand)ferrate(II) complexes

SummaryPreparation of solids of general formula Nan{Fe(CN)5L} ·xH2O (where L is a pyridine or pyrazine derivative) is described. The i.r. spectra of the solids together with electronic and1H n.m.r. spectra of the aqueous solutions are presented, and the relationship between π-back donation and spectral properties is discussed.

The different pathways of the thermal decomposition of metal nicotinates and isonicotinates

Abstract The thermal decomposition (TD) of basic iron nicotinate and isonicotinate proceeds in several stages: (a) dehydration to yield a hydroxocompound; (b) dehydration of hydroxide bridges, overlapping with water attack on the anion and release of H(nic) and H(isonic); (c) homolytic RC(O)O-Fe(III) bond breaking; (d) formation of metallic iron. Sodium salts decompose yielding Na 2 CO 3 + C, that later evolves CO. The different types of chemical reactions involved in the TD of metal nicotinates and isonicotinates are compared.