Methylene Blue adsorption current

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

The methods described above for the study of TCE are representative of the methods we are currently using to study the adsorption of a variety of organic molecules by clay mineral surfaces. We shall briefly summarize the results obtained so far in two other cases, one dealing with the adsorption of methylene blue to clays and the other dealing with a recalibration of the alkyl ammonium ion method for surface charge determination. In both of these studies, essentially the same computational techniques as those described above were used. 

Adsorption of the methylene blue cation on a positively charged mercury surface has been investigated by Los and Tompkins [8, 9], The measurements were performed in the absence of faradaic current by two methods 1) direct determination of the adsorption from the change in concentration of methylene blue during desorption from the surface of mercury, transferred from the methylene... 

Adsorption of the leuco form of methylene blue on mercury was investigated by Laitinen and Hosier [19], who measured the differential capacity of the double electric layer in the absence of faradaic current. They found that at the maximum adsorption potential (—0.6 V, s.c.e.) the adsorption of the leuco form satisfied the Langmuir equation with a B value of 1.67-10 liter/mole. This is almost three orders of magnitude smaller than the value obtained from the Brdicka theory. 

An argument in favour of this suggestion is the existence of this peak in acidic solutions, where the adsorption of the leuco form of methylene blue is insignificant. The height of this peak rapidly reduces to zero with increase in the alternating-current frequency. 

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