Electrochemical Behavior of Methylene Blue and Its Leucoform at the Mercury Electrode

Abstract

The electrochemical behavior of methylene blue (MB) and its leucoform (LMB) in aqueous solutions at pH 7.9 was investigated with a mercury electrode. The polarographic results showed that at very low concentrations (4 • 10 .6 M/liter) and E1/z of "Brdi~ka's prewave" is the same as the standard potential of the MB/LMB couple and moves from about --0.290 to --0.180V (NCE) as the concentration increases. The electrocapillary curves indicate the following: (i) at low concentrations the LMB is less adsorbed than the MB; (ii) ~ decidedly decreases when the surface concentration of LMB reaches a value high enough to form a solid deposit on the mercury surface. We suggest that: (a) "Brdi~ka's prewave" which occurs at the lowest concentrations is actually the normal wave of the MB/LMB couple; (b) "Brdicka's normal wave" is a step that develops at the standard potential, but on an electrode covered by an insoluble film of LMB.The electroreduction of methylene blue (MB) to leucomethylene blue (LMB) on a mercury electrode takes place with an anomalous behavior. This system was studied polarographically by Brdi~ka (1, 2) and it is the classical example of reduction product adsorption cited in every textbook of polarography (3-5).Brdigka shows that at pH = 7.96 at concentrations of MB below 6 • 10-~M a single step ("prewave") is observed, while at higher concentrations a second step ("normal wave") appears at more negative potentials.In the interpretation of Brdi~ka the "prewave" corresponds to reduction of unadsorbed MB to adsorbed LMB; the height of this step is limited, not by the rate of diffusion of MB, but by the surface available for adsorption of the reduced molecules. He states that the product of MB reduction is adsorbed at the electrode surface as soon as it is formed and this facilitates, from an energy point of view, the reduction itself by allowing it to take place at a potential less negative than the standard potential of the MB/LMB couple. According to Brdi~ka, the "normal wave" is observed when the surface is fully occupied by LMB and requires a more negative potential since the reduced form finds itself in a higher energy state (unadsorbed as opposed to adsorbed). Under these conditions there is no process aiding the reduction and therefore the normal wave should develop in the neighborhood of the standard potential.After Brdi~ka much work was done (6-14) using different techniques in order to provide the MB/LMB redox couple with new information. Although the majority of these investigations have retained substantially unchanged Brdi~ka's point of view, experimental evidence exists and points out that the MB/LMB system is actually more complicated than Brdicka's scheme assumes it to be.Recently Baumgartner et aL (15) have shown by using multiple specular reflection spectroscopy applied to a thin layer electrochemical cell, that, in {he first 0.5 sec of the (I X 10-5M) MB reduction process, the LMB formed precipitates on the gold electrode surface.We are presenting here experimental evidence pertaining to th...

Surface tension of solutions of pure sodium dodecyl sulfate

“…After thorough drying it was extracted with reagent grade acetone (the reverse order gives much inferior results). Gas chromatography of the alcohol obtained by acid hydrolysis showed the presence of 0.39% (12) Kloubek, J. J. Colloid Interface Sci. 1972, 41, 1, 7, 17.…”