INFLUÊNCIA DA FORÇA IÔNICA NA ESTIMATIVA DE pKa POR MÉTODO VOLTAMÉTRICO

AUTOR(ES)

Liberato, Priscila A.; Silva, Astréa F. de S.; Okumura, Leonardo L.; Diniz, Juliana A.; Oliveira, André Fernando de

FONTE

Quím. Nova

DATA DE PUBLICAÇÃO

2020-12

RESUMO

The voltammetry is an important analytical technique and also useful for pKa determination. It is a usual procedure of this technique to employ inert electrolyte concentration 100 times higher than redox compound concentration further a pH buffer. However, the ionic strength changes with the solution pH. In this work, we study the consequences of this procedure by using a model system, aniline. A differential pulse voltammetry was used with a glass carbon electrode in 4.0 x 10-3 mol L-1 aniline, 0.04 mol L-1 Britton-Robinson Buffer, NaNO3 as inert electrolyte and pH from 2.0 up to 7.0. The first oxidation peak (0.67V ca.) was used. The pKa determination was performed by using intersection lines at Epeak versus pH. This method was also theoretically discussed for both the oxidant or reducing compound with Brönsted acid-base behavior. For the cited procedure, the ionic strength changed from 0.418 to 0.528 mol L-1, and the experimental pKa (5.691) was far from theoretical one (4.722), still a good fit of lines to experimental data. Using the correct procedure, the experimental pKa (4.938) was near the theoretical one. Therefore, we have proved the need for a more careful adjust of ionic strength in this application.

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