Propriedades eletroquimicas de hexacianoferratos absorvidos pela silica gel quimicamente modificada com oxido de Zr(IV)

AUTOR(ES)
DATA DE PUBLICAÇÃO

1994

RESUMO

Silica gel with a specific surface area of 424 m.g and an average pore diameter of 6 nm, was coated with Zr(IV) oxide by reacting ZrCl4 or ZrOCl2 with SiO2. The amount of Zr(IV) grafted on SiO2, hereafter denoted as SZrOH, was 470 mmol.g and 250 mol.g using respectively, ZrCl4 and ZrOCI2. The hexacyanoferrate(ll) complex (HCF) was adsorbed by SZrOH in acid solution, resulting in SZrHCF. The reaction of this material with Cu(II), resulted in the very stable film of copper hexacyanoferrate (II) complex (HCFCu) with atomic ratio Cu/Fe 0.25, hereafter denoted as SZrHCFCu. A similar complex was obtained from reaction of SZrOH with Cu(ll) at pH 6, followed by reaction with HCF(II), which showed atomic ratio Cu/Fe 4.0, hereafter denoted as SZrCuHCF. The infrared spectra of SZrHCF, SZrHCFCu and SZrCuHCF showed the CN stretching band (uCN) shifted to higher energy region in relation to the bulk phase HCF and HCFCu. The uCN bands of the samples of SZrHCF, SZrHCFCu and SZrCuHCF were observed at ~2080, 2090 and 2100 cm , respectively. The adsorbed HCF ( SZrHCF) was transformed into a Prussian Blue (PB) complex species with time. The electronic spectra showed a band at 730 nm, which is assigned to a charge transfer in the PB species, Fe+[Fe(CN)6] Fe [Fe(CN)6]. This absorption band was algo observed in SZrHCFCu due to excess of hexacyanoferrate on the surface (where atomic ratio Cu/Fe 0.25), however with lower intensity. This complex showed one characteristic absorption band at 480 nm, which is assigned to the charge transfer band FeCu. For SZrCuHCF (where atomic ratio Cu/Fe 4.0) only the absorption band at 480 nm was observed. Cyclic voltammetry measurements were carried out using a carbon paste electrode of the material as working electrode. The newly prepared samples of SZrHCF showed the mid-point potential. E1/2 , of the redox couple Fe at ca. 0.23 V vs SCE. After aging, one additional redox couple was observed at ca. 0.87 V vs SCE in KCI 1 M electrolyte solution. These same redox couples were observed in the aged samples of e SZrHCFCu due to the formation of PB species, beyond the characteristic redox couple with E1/2 at ca. 0.69 V vs SCE due to the formation of HCFCu complex. For SZrCuHCF samples only E1/2 at ca. 0.69 V vs SCE was observed. The E1/2 values of the redox couple Fe of the samples of SZrHCFCu and SZrCuHCF were recorded with higher reversibility in KCl and NH4CI electrolyte solutions. In 1 M KCI solution, the E1/2 was observed at ca. 0.69 V vs SCE for both samples, while in NH4CI, the E1/2 was observed at 0.78 and 0.74 V vs SCE respectively. showing that electroative species have higher afinity to NH4 ion. The Ip. u product for SZrCuHCF was constant at higher range of scan rate than was to the SZrHCFCu one, probably for a more effective "electron-hopping" mechanism. The newly prepared samples of SZrHCF were able to mediate electron transfer between the electrode and the solution containing ascorbic acid. The magnitude of the anodic peak currente showed a linear relationship against the ascorbic acid concentration in the range of 0.5 and 5 mM. This property was used to determine the amount of ascorbic acid in tablets of vitamin C and in fresh and processed juices.

ASSUNTO(S)

voltametria oxidos metalicos silica gel

ACESSO AO ARTIGO

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