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.

O coeficiente de atividade molar individual do íon fluoreto (yF) foi determinado a 25° C para valores de força iônica entre 0,100 and 3,00 mol L-1 (NaClO4) usando-se um eletrodo íon-seletivo. A dependência do coeficiente de atividade com a força iônica é explicada pela equação ΦF = log yF = 0,2315I - 0,041I². A função ΦF(I), foi associada a outras funções estimadas em trabalhos anteriores, por exemplo,ΦCu(I) para cobre, e ΦH(I) para íon hidrogênio, possibilitando a estimativa de constantes de estabilidade estequiométricas e termodinâmicas tanto para equilíbrios de protonação de alguns haletos e pseudo-haletos como para as constantes de formação de complexos 1:1 de metais bivalentes com fluoreto e pseudo-haletos. O procedimento de cálculo proposto é consistente com dados experimentais criticamente selecionados da literatura. Verificou-se que é possível usar ΦF(I) na previsão de parâmetros de equilíbrio termodinâmicos independentemente do caráter HSAB de Pearson .

The molar single ion activity coefficient (yF) of fluoride ions was determined at 25 °C and ionic strengths between 0.100 and 3.00 mol L-1 NaClO4 using an ion-selective electrode. The activity coefficient dependency on ionic strength was determined to be ΦF = log yF = 0.2315I - 0.041I². The function ΦF(I), combined with functions obtained in previous work for copper (ΦCu) and hydrogen (ΦH), allowed us to make the estimation of the stoichiometric and thermodynamic protonation constants of some halides and pseudo-halides as well as the formation constants of some pseudo-halides and fluoride 1:1 bivalent cation complexes. The calculation procedure proposed in this paper is consistent with critically-selected experimental data. It was demonstrated that it is possible to use ΦF(I) for predicting the thermodynamic equilibrium parameters independently of Pearson's hardness of acids and bases.