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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 15 (2010) pp 90-99

The Midpoint Method: a Shortcut Approach to Calculate the pH

M. José Ruiz-Ángel, José R. Torres-Lapasió and M. Celia García-Álvarez-Coque*

Departament de Química Analítica, Universitat de València, c/Dr. Moliner 50, 46100 Burjassot, Spain,
Received January 11, 2008. Accepted November 8, 2009.

Published: 27 January 2010

Abstract. A shortcut intuitive graphical approach for pH calculation that yields errors usually below 0.1–0.2 pH units is described. The approach is valid for solutions of monoprotic weak systems, or polyprotic systems with isolated successive equilibria, and is useful for students and professionals without special mathematical and chemical skills, or for situations where a rapid estimation of pH is required, since the calculation time will never exceed a few seconds. The midpoint method is reduced to finding the central point in a segment, where the endpoints are easily established. The following situations are considered: solutions of acidic and basic species in mono- and polyprotic systems, intermediate species in a polyprotic system, salts of metal ions with inert anions, ammonium or amine salts with anions showing acid-base behaviour, and mixtures of the acidic species of a monoprotic system with the basic species of another. Diagrams that show the expected bias in these calculations are given, which allow correcting the approximate pH value to obtain the accurate one, when needed.

Key Words: In the Classroom; general chemistry; analytical chemistry; acid-base equilibria, calculation of pH; shortcut

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