Química analítica básica: generalizando os conceitos de ácidos e bases

Abstract

Interactions between solutes and solvents are complex processes involving the release or absorption of heat and changes in the solute environment and in the structure and properties of the solvent. Thus, dissolution and solvation processes can be considered primarily as chemical reactions. In particular, in the aqueous solution solvation process, the solvent molecules are associate with the ions in solution, forming layers around the central ion [1].
It is observed that some salts, such as NaCl, when dissolved in water to form a homogeneous solution, produce a solution with neutral pH. However, other salts result in basic or acidic aqueous solutions. In fact, when salts of strong bases with weak acids and strong bases with weak acids are dissolved in water, the final solution will have a pH different of 7.00. In the first case the final solution will be basic and these salts are called salts of basic reaction, while in the second case the resulting solutions are acidic and the salts known as salts acid reaction. In addition to acetates, the most common examples cited in textbooks as examples of basic reaction salts are alkali metal carbonates and phosphates (eg Na₂CO₃ and NaH₂PO₄), while NH₄Cl is the most common example among acid reaction salts. This phenomenon is known as hydrolysis (“water cleavage”) and is used in textbooks to describe the acidic or basic properties that many salts impart to the solution when dissolved in water. However, anions such as acetate, carbonate, phosphate and cyanide, among others, are actually only weak bases, just like ammonia, except for their charges. The same is valid for cations, such as ammonium ions, which should be treated only as weak acids. Therefore, this problem boils down to a purely acid-base issue and should be treated accordingly. For this reason these concepts will be reviewed and discussed in this article.