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Abstract--Methylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB. A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. These results suggest that dimers are formed in the contact region between two aggregating particles.

An alternative, simple, and reliable FIA amperometric method for ranitidine quantification was developed. A working glassy carbon electrode and a Ag|AgCl|NaCl (3 M) reference electrode were used. The determination is based on the electrochemical oxidation of ranitidine at 1.2 V in phosphate buffer solution (pH 7.2). The determination of ranitidine in the range of 1 × 10−6 to 2 × 10−5 M was possible with a sampling frequency of 120 h−1. The method was validated and satisfactorily applied to the determination of ranitidine in pharmaceutical formulations with a low limit of detection (6 × 10−7 M) and a quantitative recovery with no interferences from excipients. The results compare favorably with those obtained by the liquid chromatographic method of the U.S. Pharmacopoeia.