Construção de espectrofotômetro UV-VIS multicanal e aplicações no estudo da cinética de reações químicas: uma proposta de plataforma analítica e educacional

Abstract

The UV-VIS spectrophotometer is the most widely used instrument in analytical laboratories around the world. Due to the lack of investment in Brazilian education, it is still possible to observe a great deficiency of infrastructure in teaching laboratories. As an alternative to the acquisition of this commercial equipment, the present work presents the construction and validation of a portable and low cost multichannel UV-VIS spectrophotometer. The instrument has the construction scheme of a single-beam spectrophotometer, using a CMOS (Complementary metal-oxide-semiconductor) camera as a multi-channel detector, a 1000 lines/mm holographic diffraction grating and a thermostated cell for the sample being this the innovative component compared to commercial instruments. The assembled device costs approximately US$ 95.00, its dimensions were 23cmx13cmx06cm, providing easier accommodation and transport. For validation, a potassium permanganate solution was used as a chemical specie due to its characteristic absorption spectrum. Thus, calibration tests and analytical validation of the equipment were performed by constructing an analytical curve with concentration solutions from 2.5x10-5 to 2.0x10-4 mol L-1. As results, figures of merit were obtained: linear correlation coefficient (R2) 0.997, detection limit 0.331 mg L-1, quantification limit 1.10 mg L-1, measurement dispersion of 1.7%, spectral resolution 0.14 nm, acquisition speed of up to 120 spectra/s. In addition, the molar absorptivity for potassium permanganate of 2.554 x103 Lmol-1cm-1 was calculated, being consistent with values reported in the literature. The equipment was also used to monitor the kinetics of chemical reactions. The investigated reaction was the oxidation of sucrose by potassium permanganate in an alkaline medium, known as the “chemical chameleon”. From the results obtained, the linear behaviour in the Arrhenius equation was observed between temperatures from 30°C to 40°C, being possible to calculate the kinetic parameters Ea=13,932.03 J; ∆H# (308.15K) = 11,370.07 J; ∆S# (308.15K) = -190.43 J; ∆G# (308.15K) = 70,052.13 J. Therefore, the manufactured equipment can be considered a promising miniaturized analytical platform, accessible and also with a performance comparable to commercial equipment. In addition, this equipment can be applied as a good teaching tool due to its ability to work with many fundamental concepts of chemistry.