A detailed analysis of the polarographic behaviour of methylene blue in phosphate buffer on mercury |
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Authors: | Giovanni Piccardi Francesco Pergola Maria Luisa Foresti Rolando Guidelli |
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Institution: | Institute of Analytical Chemistry, University of Florence, Florence (Italy) |
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Abstract: | Polarographic current-potential characteristics and current-time curves for the reduction of methylene blue (MB) to methylene blue leucoform (MBL) in a pH 7.9 aqueous phosphate buffer have been examined in detail over a wide concentration range. It has thus been shown that the so called “normal” or “main” reduction wave of MB actually consists of two separate steps, the former with a half-wave potential practically coinciding with the formal potential E0=?0.250 V/SCE of the MB/MBL couple and the latter with a half-wave potential of about ?0.310 V/SCE. As soon as the well-known MB adsorption prewave has attained its maximum height (which occurs at a MB bulk concentration c0*?5×10?5M), a further slight increase in c0* causes the appearance of the wave with E1/2=?0.310 V. The height of the linear potential-sweep voltammetric peak corresponding to the latter wave increases proportionally to the sweep rate, thus revealing the “adsorption” nature of this wave. A comparison with a previous chronocoulometric investigation of the MB/MBL system has permitted us to conclude that the wave with E1/2=?0.310 V is due to reduction of the MB molecules which, after having reached the surface of the dropping electrode by diffusion, are adsorbed at the top of the adsorbed monolayer of MBL in direct contact with the electrode and remain in this adsorbed state after reduction. For c0*>7×10?5M the wave with E1/2?E0=?0.250 V starts to develop. This wave is due to the electroformation of MBL molecules which diffuse back into the solution. The shape of polarographic current-potential characteristics and current-time curves has been accounted for semiquantitatively through an approximate solution of the corresponding diffusional problem. |
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