Influence of the Cation Na/Ca/Ag Ratio on the Ion Exchange Rate in Zeolite A-Modified Carbon Paste Electrodes

Abstract

In contrast to previous electrochemical studies of silver-ion-exchanged zeolite-modified electrodes (Ag + sZMEs), in which only two different compensating monocations in the zeolite have been used, in this work is presented a more complicated case of zeolite A containing three cation types, one electroactive soft cation and two nonelectroactive hard cocations: Ag + , Na + , and Ca 2+ , respectively. Na 2 Ca 4 Ag 2 sA, Na 2 Ca 3 Ag 4 sA, and Na 2 Ag 10 sA were obtained by ion exchange from the commercial sodium form of Lynde type-A (Na 12 sA). It was checked by 27 Al and 29 Si MAS NMR and X-ray diffraction (XRD) that no dealumination or loss of crystallinity occurred during the ion-exchange procedure. The exchanged zeolites were used to prepare zeolite-modified carbon paste electrodes (ZMCPEs). An alternative access to diffusion phenomena was achieved by a combination of open-circuit potential (OCP) and cyclic voltammetry (CV) measurements. Both techniques were performed in aqueous electrolytic medium 0.1 M NaNO 3 , both quiescent and disturbed. This novel experimental route was applied to investigate the influence of the nature and content of cations in the ion-exchange rate of AgCaNasA ZMCPEs. At open circuit, the potential shifts are associated, for the first time, with the flux of cations from the zeolite framework to the electrode/solution interface. Voltammetric behavior of ZMCPEs are mainly ascribed to interactions and stabilization of extra-framework cations, which is determined by the nature of the exchange sites inside the cages and cavities. A mechanism of extrazeolitic charge transfer in CV is clear in scan potentials performed subsequently to the first one. However, at short time periods of immersion electrode, in the first cycle of electrical perturbation, the intrazeolitic character of the electrochemical processes is not discarded. In this way, the ion exchange rate decreases as the amount of divalent cation, Ca 2+ , increases. This is due to the strong interaction of the divalent cation with water molecules and with the framework. Besides, cation diffusion is modified by the presence of silicon oil binder in the carbon paste.

Electrochemical Aspects of Silver Sulfosalts Dissolution in Acid Thiourea Solution

“…Silicon oil binder could modify the adsorption of thiourea and/or their oxidation products, which could produce the oxidation pre-peak for the CPE in thiourea. Moreover, the reduction of silver occurs mainly onto the carbon paste and for this reason is observed the characteristic feature of the deposition on foreign substrates; whereas silver dissolution can occur at the interface CPE-Ag 3 SbS 3 /Tu and probably within the paste (11) leading to the formation of a new silver-thiourea complex, which explains the second peak at A 1 .…”

Electrochemical characterization of carbon paste electrodes modified with MgZnGa and ZnGaAl hydrotalcite-like compounds

“…It has been proven as a sensitive and effective measure for monitoring spontaneous phenomena at zero current occurring at the electrode/solution interface, e.g., metal corrosion [24], adsorption [25], and absorption [26,27] of neutral species and ions onto metal surfaces, and it could be a sensitive means for a better understanding of the behavior of electrochemical measurements with regard to ionic exchange dynamics [28].…”

An Approach to the Reactivity of Isomorphous Proustite (Ag3AsS3) and Pyrargyrite (Ag3SbS3) in Cyanide Solutions

“…Carbon paste electroactive electrodes (CPEEs) were prepared in the same manner as reported elsewhere (7). The finely powdered solid compound (electroactive species) was mixed with spectroscopically pure graphite powder in a ratio of 3 × 10 -4 mol to 1 g carbon paste.…”