Experiment 9 - pH, Acid-Base Indicators, Buffers

Objectives

1. To standardize a pH meter and then use it to determine the pH of various solutions.
2. To determine the pH at which various indicators change colour.
3. Tto compare the pH changes which occur when small quantities of acid and base are added to equal volumes of water and buffer solutions.

Introduction

The pH Scale

The pH scale is used to measure the acidity and basicity of a solution. Knowing the concentration of hydronium, H₃O⁺, ions in a solution, the pH can be calculated using the formula

pH = - log [H₃O⁺]

where [H₃O⁺] is the concentration of H₃O⁺ ions in moles/L.

Acids are substances, which react with water to produce hydronium ions, H₃O⁺. There are two types of acids:

1. Strong acids such as HCl and HNO₃ react essentially completely with water to produce hydronium ions.
2. Weak acids such as CH₃COOH and carbonic acid, H₂CO₃, only a small fraction of the molecules produce hydronium ions.

Likewise with bases, there are two types of bases:

1. Strong bases such as NaOH and KOH react essentially completely with water to produce hydroxide, OH^- ions.
2. Weak bases such as NH₄OH, only a small fraction of the molecules produce hydroxide, OH^- ions.

The pH of strong and weak acids or strong and weak bases may be measured with a pH meter.

Acid-Base Indicators

The pH of solutions may also be measured using acid-base indicators. These substances change colour at certain pH values.

Buffers

Buffers are solutions which resist pH changes when small quantities of acid or base are added to them. Buffers are made by mixing weak acids and their salts. For example, a buffer can be prepared by dissolving CH₃COONa in CH₃COOH solution. Likewise, buffers are made by mixing weak bases and their salts. For example, a buffer can be prepared by dissolving NH₄Cl in a NH₄OH solution.