Class 12th Chemistry Projects for 2009 Exams
Measuring the Amount of Acid in Vinegar by Titration with an Indicator Solution

Objective

The goal of this project is to determine the amount of acid in different types of vinegar using titration with a colored pH indicator to determine the endpoint.

Introduction

Vinegar is a solution made from the fermentation of ethanol (CH₃CH₂OH), which in turn was previously fermented from sugar. The fermentation of ethanol results in the production of acetic acid (CH₃COOH). There are many different types of vinegar, each starting from a different original sugar source (e.g., rice, wine, malt, etc.). The amount of acetic acid in vinegar can vary, typically between 4 to 6% for table vinegar, but up to three times higher (18%) for pickling vinegar (Wikipedia contributors, 2007).

In this project, you will determine the amount of acid in different vinegars using titration, a common technique in chemistry. Titration is a way to measure the unknown amount of a chemical in a solution (the titrant) by adding a measured amount of a chemical with a known concentration (the titrating solution). The titrating solution reacts with the titrant, and the endpoint of the reaction is monitored in some way. The concentration of the titrant can now be calculated from the amount of titrating solution added, and the ratio of the two chemicals in the chemical equation for the reaction. Let's go through the process with a specific example: the titration of acetic acid. But before we go over titration, here is a quick review of the chemistry of acids and bases.

It all has to do with hydrogen ions (abbreviated with the chemical symbol H⁺). In water (H₂O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxyl ions (OH⁻). The "lost" hydrogen ions join up with water molecules to form hydronium ions (H₃O⁺). By convention (and for simplicity in writing chemical equations), hydronium ions are referred to as hydrogen ions H⁺. In pure water, there are an equal number of hydrogen ions and hydroxyl ions. The solution is neither acidic or basic.

An acid, like acetic acid, is a substance that donates hydrogen ions. When acetic acid is dissolved in water, the balance between hydrogen ions and hydroxyl ions is shifted. Now there are more hydrogen ions than hydroxyl ions in the solution. This kind of solution is acidic.

A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxyl ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxyl ions than hydrogen ions. This kind of solution is alkaline.

To measure the acidity of a vinegar solution, you can add enough hydroxyl ions to balance out the added hydrogen ions from the acid. The hydroxyl ions will react with the hydrogen ions to produce water. In order for a titration to work, you need three things:

1. a titration solution (contains hydroxyl ions with a precisely known concentration),

2. a method for delivering a precisely measured volume of the titrating solution, and

3. a means of indicating when the endpoint has been reached.

For the titrating solution, you'll use a dilute solution of sodium hydroxide (NaOH). Sodium hydroxide is a strong base, which means that it dissociates almost completely in water. So for every NaOH molecule that you add to the solution, you can expect to produce a hydroxyl ion.

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Courtesy : Sciencebuddies.org