Química analítica básica: representações gráficas dos equilíbrios iônicos

Abstract

Most chemistry students have always been induced to think and work ionic equilibrium in aqueous solution using traditional algebraic treatment, as in the examples recently shown for acid-base equilibria[1-3]. However, the exact algebraic solution of this type of problem often requires the resolution of higher polynomials. For example, the exact pH calculation of a simple aqueous solution of a weak monoprotic acid requires the solution of a cubic equation and in the case of a triprotic acid (eg: phosphoric acid), the solution of a fifth degree polynomial[1,2]. This induces the use of approximations to simplify the work, which always leads to approximate results. Another procedure proposed by Birk [4], which employs a reaction extension-based matrix formulation, can also be used for this purpose, but it is a method that requires more in-depth knowledge of chemistry and linear algebra, which is why it is not presented in textbooks and will not be discussed in this paper either. An alternative and easier approach is the use of numerical calculations and along with the graphical presentation of results, a procedure greatly facilitated by the use of spreadsheets. This provides an overview of the acid-base system, reinforces important principles and allows treatment of a wide range of chemical systems.