Basic analytical chemistry: use of Gran Plot in acid-base volumetry

Abstract

The Gran method, used to linearize  titration curves, have also been proved to be more comprehensive and useful in several situations, from the simplest cases, such as the titrations between acids and bases, to more complex ones, such as those involving the formation of precipitates and / or oxidation-reduction reactions. It is simpler and easier to be applied, since it does not need (and should not) to reach the end point of the titration, implying that its use will require less experimental points. In fact, only those points obtained before or after the titration end point should be used, more specifically, those in the linear portion of the Gran Plot. In addition to being more versatile, it can be applied in many other limit situations, such as analytical processes involving highly diluted reagents and / or small volumes of titrant or titrate. Despite all these apparent advantages, Gran's method has not been highlighted in textbooks and educational articles, perhaps because of the current need for laboratory certification, which would result in the frequent use of automated instruments. However, since it is always desirable the inclusion of alternative methods of analysis in the basic training of a professional in Chemistry, this article will show how to use experimental data to calculate the first and second derivative in the determination  of the end point in a potentiometric titration, comparing with the Gran II method. Also, to illustrate the practical use of the Gran Plot, this article also includes the suggestion of a simple didactic experiment, along with numerical results obtained during the laboratory practice.