Qualitative Measurement of pH and Mathematical Methods for the Determination of the Equivalence Point in Volumetric Analysis

Abstract

In this article is presented a gradual path on the pH making use of the laboratory. The idea arises from the consideration that the laboratory does not have to be the place of the ritual repetition of standardized procedures, often carried out with materials and instruments prepared, but a method of approach to phenomena and situations that promote the active participation of students. We move from a qualitative measure of the pH of acidic solutions, basic and neutral, to quantitative measure of the equivalent point of an acid-base titration. The methodology used by the teachers is based on learning by doing: few notions of theory to learn the fundamental concepts and many laboratory activities that help the students to produce reports on their activity. The qualitative measurement of pH, using as an indicator the red onion of Tropea (Italy), was carried out in a lower secondary school, while the volumetric analysis has been at the center of lab activity addressed to fourth class of a technical institute. In particular, the attention has been focused on acid-base titrations and how to determine the equivalence point. In both school levels, the primary objective was the acquisition of the fundamental principles and practical skills relating to the methods of analysis. All activities have been selected and organized in order to constitute a valid and mutual integration with the theory, not only as a moment of experimental verification of what the students have learned in theory, but also, where possible, as a basic tool to derive laws, principles theoretical models starting from the experience.

Influence of the polymer structure of polycarboxylate-based superplasticizers on the intercalation behaviour in montmorillonite clays

“…The first point is attributed to the HCl surplus and the second point (located close to pH 9) is attributed to the neutralization of PCE carboxylic groups. The points of equivalence are identified using the method of incremental ratios [12]. The total PCE anionic charge is calculated using the difference in the KOH volume corresponding to the first and the second inflexion points.…”

Preparation and clay tolerance research of a polycarboxylate superplasticizer modified by phosphonate group

“…The acidified sample was titrated using a ZDJ-4A automatic potential titrator with 0.5 mol/L KOH solution until the pH reached 12~13, which two inflection points corresponding to the chemical equilibrium were observed. The anionic charge density of PCE can be calculated based on the volume difference of KOH solution at the two inflection points [13].…”

Evaluación de la acidez en vinagres comercializados en la República Dominicana

“…Estas se obtienen a partir de los datos para cada punto de las curvas de titulación (pH versus volumen). Una vez construido el gráfico se identifica el punto de equivalencia de la titulación, con un cambio máximo para este punto (Checchetti & Lanzo, 2015).…”

“…La cuantificación de la acidez en vinagres puede realizarse por varias metodologías, las cuales abarcan desde los métodos clásicos de análisis hasta los instrumentales, donde se incluyen potenciométricos, cromatográficos, entre otros (Rudolph, 1960;Kotani et al, 2003;Sanarico, Motta, Bertolini & Antonelli, 2003;Regmi, Palma, & Barroso, 2012;McPherson, 2014). Desde el siglo xvii los métodos volumétricos se han usado para cuantificar la acidez en muestras de vinagres (McPherson, 2014), en la actualidad, estos últimos se combinan con técnicas potenciométricas, para mejorar la calidad de los resultados (Nielsen, 2017;Checchetti & Lanzo, 2015). De esta forma, se mejora la precisión del método a utilizar y en el caso de las valoraciones de vinagres coloreados se disminuyen los errores causados por la imprecisión al determinar los cambios de color en las volumetrías clásicas (Kotani et al, 2003).…”