Abstract:

Titrimetric methods remain very important in the curriculum of chemistry students. They are classical quantitative methods of analysis directly related to dissolution equilibria, which continue to be used in quality control. In general, titrimetries are fast and simple analysis methods that require only glassware for their implementation, so that they are easily adaptable to any practice laboratory. Direct titrimetries are usually carried out in these educational practices in which the titrant is added directly to the sample using a chemical indicator suitable for the detection of the equivalence point. However, in very few cases, the student understands all the conditions in which the titrimetry must be developed. This is especially pronounced in the case of titrating a mixture of two or three analytes. This is because in this type of practice a behavioral teaching model is followed. A systematic study of this topic is necessary in order to offer students a tool that can be used to understand and design different titrimetries. The Prof. Gaston Charlot, in the 1940s, offered an interesting approach of ionic chemical reactions based on a very simple model (donor-acceptor model), which in turn was based on Brønsted and Lowry theory [1]. The model proposed by G. Charlot has been related with the constructivism used nowadays for teaching of sciences [2]. Subsequently, it was complemented by A. Ringbon [3].

In this work, using Charlot´s approach, an acid-base titration is designed. A study case is presented: a mixture of two substances with acid-base characteristics, arginine hydrochloride (diprotic acid, pK1 = 1.8 and pK2= 8.9) and histidine hydrochloride (triprotic acid, pK1 = 1.7, pK2 = 6.0 and pK3 = 8.8). Therefore, as a starting point, a strong base (NaOH) as titrant and 0.1 M concentrations are used.

References:
[1] G. Charlot, Cours de chimie analytique génerále, Masson, Paris, 1969.
[2] M.R. Gómez-Moliné, A. Rojas-Hernández, M.T. Ramírez-Silva, Educación química, 2009, 20, 192-197, El constructivismo y la química analítica del profesor Gaston Charlot, http://www.scielo.org.mx/pdf/eq/v20n2/v20n2a14.pdf
[3] A. Ringbom, Complexation in analytical Chemistry, J. Wiley and Sons Inc., 1963.

Keywords:

Educational practices, systematic design of titrimetry, ionic equilibrium, Charlot´s model.