pH Determination Under Unconventional Conditions of Temperature and Ionic Strength
DOI:
https://doi.org/10.29356/jmcs.v69i2.2160Keywords:
Chemical equilibria, apparent constant, pH determination, Van’t Hoff, Debye-HückelAbstract
Abstract. The pH in an aqueous solution is a relevant parameter in many fields of chemistry, and its determination is not trivial when factors such as temperature and ionic strength are considered. In multicomponent systems, this situation becomes significant. Even in simple systems, there are variations of up to 1.813 pH units in CH3COOK solutions when thermodynamic constants are used instead of apparent constants to calculate it. In this study, we propose a methodology that investigates the influence of these variables on the apparent dissociation constants of water and acetic acid, as well as their impact on the pH measurement of solutions prepared from CH3COOH and a salt of its conjugate base. Non−linear adjustments were carried out using a polynomial analogous to the Van't Hoff equation to establish a relationship between the thermodynamic constants of formation and the wide temperature range proposed. Furthermore, the influence of the ionic medium was considered when correcting the activity coefficients using the extended model of the Debye−Hückel equation. This approach enabled a detailed description of the set of apparent formation constants, which were directly applied in the formal pH calculation without approximations. These variations were represented on response surfaces and interpolated to the proposed operating conditions. The successful correlation between the theoretical results and those obtained experimentally through potentiometric measurements confirmed a harmonious relationship between both data sets. The described methodology offers a novel alternative for calculating pH in multicomponent systems, including real samples, in unconventional conditions of temperature and ionic strength.
Resumen. El pH en una disolución acuosa es un parámetro relevante en muchos campos de la química, y su determinación no es trivial cuando se consideran factores como la temperatura y la fuerza iónica. En sistemas multicomponente, esta situación se vuelve significativa. Incluso en sistemas simples, existen variaciones de hasta 1.813 unidades de pH en disoluciones de CH3COOK cuando se utilizan constantes termodinámicas en lugar de constantes aparentes para calcularlo. En este trabajo, se propone una metodología que indaga en la influencia de estas variables sobre las constantes de disociación aparentes del agua y del ácido acético, así como su impacto en la medición del pH de soluciones preparadas a partir de CH3COOH y una sal de su base conjugada. Se realizaron ajustes no lineales utilizando un polinomio análogo a la ecuación de Van't Hoff para establecer una relación entre las constantes termodinámicas de formación y el amplio rango de temperaturas propuesto. Además, se consideró la influencia del medio iónico al corregir los coeficientes de actividad mediante el modelo extendido de la ecuación de Debye−Hückel. Este enfoque permitió una descripción detallada del conjunto de constantes de formación aparentes, que se aplicaron directamente en el cálculo formal del pH sin aproximaciones. Estas variaciones se representaron en superficies de respuesta y se interpolaron a las condiciones de operación propuestas. La correlación exitosa entre los resultados teóricos y los obtenidos experimentalmente mediante mediciones potenciométricas confirmó una relación armoniosa entre ambos conjuntos de datos. La metodología descrita ofrece una alternativa novedosa para el cálculo del pH en sistemas multicomponentes, incluidas muestras reales, en condiciones no convencionales de temperatura y fuerza iónica.
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