INFLUENCE OF IONIC STRENGTH IN DETERMINATION OF pK<sub>A</sub> BY VOLTAMMETRY

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

doi:10.21577/0100-4042.20170636

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Artigo • Quím. Nova 43 (10) • Dec 2020 • https://doi.org/10.21577/0100-4042.20170636 copy

INFLUENCE OF IONIC STRENGTH IN DETERMINATION OF pK_A BY VOLTAMMETRY

Authorship SCIMAGO INSTITUTIONS RANKINGS

The voltammetry is an important analytical technique and also useful for pK_a 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 pK_a determination was performed by using intersection lines at E_peak 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 pK_a (5.691) was far from theoretical one (4.722), still a good fit of lines to experimental data. Using the correct procedure, the experimental pK_a (4.938) was near the theoretical one. Therefore, we have proved the need for a more careful adjust of ionic strength in this application.

Keywords:
voltammetry; aniline; pK_a ; ionic; strength

Sistema ácido base	Fração de Equilíbrio	Inclinação (I) da reta		$\frac{I (antes)}{I (depois)}$
Sistema ácido base	Fração de Equilíbrio	Antes de pK_a	Após o pK_a	$\frac{I (antes)}{I (depois)}$
Apenas a espécie oxidada	α_o	$S' \frac{m}{n}$	$S' \frac{(m + b)}{n}$	$\frac{m + b}{m}$
Apenas a espécie oxidada	α₁	$S' \frac{(m + b)}{n}$	$S' \frac{m}{n}$	$\frac{m}{m + b}$
Apenas a espécie reduzida	α_o	$- S' \frac{m}{n}$	$- S' \frac{(m + b)}{n}$	$\frac{m + b}{m}$
Apenas a espécie reduzida	α₁	$- S' \frac{(m + b)}{n}$	$- S' \frac{m}{n}$	$\frac{m}{m + b}$

Condição	Região	Inclinação (I) / V	Constante / V	R2^a a Coeficiente de determinação.	$\frac{I (antes)}{I (depois)}$
A	antes do pKa	-0,0368 ± 0,0007	0,8740 ± 0,0023	0,988	1,41 ± 0,07
A	depois do pKa	-0,0261 ± 0,0017	0,8131 ± 0,0097	0,983	1,41 ± 0,07
B	antes do pKa	-0,0443 ± 0,0037	0,900 ± 0,011	0,957	1,64 ± 0,38
B	depois do pKa	-0,027 ± 0,010	0,820 ± 0,054	0,969	1,64 ± 0,38

Condição	μ /(mol L^-1)	pK_a experimental^a a Estimado pela intersecção das retas ajustadas.	pK_a teórico^b b Calculado pela Equação 33.	% erro
A	0,428 < m < 0,518	5,691	4,708 < pK_a < 4,725	21 - 20
B	0,500	4,938	4,722	4,6

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INFLUENCE OF IONIC STRENGTH IN DETERMINATION OF pKA BY VOLTAMMETRY

INFLUENCE OF IONIC STRENGTH IN DETERMINATION OF pK_A BY VOLTAMMETRY