Ph and Buffer Solutions

Saint Louis University
Madrid Campus

Introduction to Chemistry II Laboratory Session Report Spring 2010

Yolatl Ruiz de Gordoa Ph.D. Carl Gustaf Saluste

Experiment No. 10! April 21, 2010 pH and Buffer Solutions !

Ruiz de Gordoa, Yolatl

Abstract: This lab session objectives were to learn how to measure the pH of a

solution and to understand the operation of buffer systems. The measurements of pH were correctly performed and the experimental values achieved agreed with the expected by the theory, therefore, the speed reducing effect of buffer solutions was proven. Introduction: The experiment was based upon the Brønsted-Lowry theory, which is an acid-base theory, proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. In this system, Brønsted acids and Brønsted bases are defined, by which an acid is a molecule or ion that is able to lose, or "donate" a hydrogen cation (proton, H+), and a base is a species with the ability to gain or "accept" a hydrogen cation (proton). Thus, the general formula for acid-base reactions according to the Brønsted-Lowry definition is: AH + B → BH+ + A− Applying this, to the element HCl used in the experiment: HCl (acid) + H2O (base) H3O+ (acid) + Cl− (base)

This can be described by an acid dissociation constant, pKa:

Which plays an essential role in the Henderson-Hasselbalch equation used to know the pH of the solution:

Experiment No. 10! !

Ruiz de Gordoa, Yolatl

Buffer solutions are aqueous solutions consisting of a mixture of a weak acid and

its conjugate base or a weak base and its conjugate acid. They have the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range; an example of a buffer solution is blood. pH meters are electronic...