EXPT 9:
pH, Acid-Base, Indicators,
Buffers

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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.



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