Experiment 7 - Acetic Acid Content of Vinegar

Objectives

1. to use a standard solution of hydrochloric acid, HCl, to determine the concentration of a base solution.
2. to use the standardized base solution to determine the concentration of acetic acid in vinegar.

Introduction

Acid-base titrations are carried out to determine the concentration of an unknown acid or base solution. A titration involves two solutions:

Solution 1 is placed in a buret. It is known as the titrant.
Solution 2 is placed in an Erlenmeyer flask.

One of the two solutions must be of known concentration. The solution of known concentration is referred to as the standard solution.

In a titration, the titrant is slowly added to the solution in the Erlenmeyer flask until the equivalence point is reached. If the base is the titrant, the equivalence point is the theoretical point that is predicted by the balanced equation such that enough base has been added to completely neutralize the acid in the Erlenmeyer flask. Or, if the acid is the titrant, the equivalence point is the theoretical point that is predicted by the balanced equation such that or enough acid has been added to completely neutralize the base in the Erlenmeyer flask.

How do we know when "enough" acid (or base) from the buret has been added to the base (or acid) in the Erlenmeyer flask?

In Part A, the neutralization reaction, which occurs during the titration is:

HCl is the standard solution. (Note: The concentration of the acid solution will be written on the bottle. Be sure to copy down the concentration of the acid solution before you leave the lab. ) A known volume of the standard solution is pipetted in an Erlenmeyer flask. A few drops of phenolphthalein is added to the acid solution. At the beginning of the titration, the acid solution in the Erlenmeyer flask is colourless. The titrant,which is the NaOH solution, is slowly added to the acid until the acid solution is slightly pink in colour. When the colour change is achieved, the endpoint of the titration is reached and no more titrant should be dispensed into the Erlenmeyer flask.

At the equivalence point in the titration:

From the titration in Part A the concentration of the NaOH solution can be determined. This NaOH solution will be the standard solution that is used to determine the concentration of acetic acid in vinegar in Part B.

Vinegar is an acetic acid, CH₃COOH, solution. In Part B, you will be given a sample of white vinegar. You will be determining the concentration of acetic acid of the vinegar sample by carrying out a titration using the NaOH solution from Part A.

In Part B, the neutralization reaction, which occurs during the titration is:

The titrant, which is the NaOH solution from Part A, is slowly added to the vinegar until the acid solution is slightly pink in colour. When the colour change is achieved, the endpoint of the titration is reached and no more titrant should be dispensed into the Erlenmeyer flask.