Miscellaneous

Titration of an Acid

The object of this experiment is to determine the molar concentration of a sample of phosphoric acid (H3PO4) of unknown concentration. We will use quantitative analysis for this determination. Often, this determination is accomplished through a titration of the sample with a solution of another substance whose concentration is known. These substances must react with each other, of course. Some physical or chemical response must be coupled to the titration to signal the exact completion of the reaction. An indicator can sometimes be used. The indicator chosen will have one color before the reaction is complete and another color when completion occurs. After the indicator is added to the sample solution, the solution of known concentration is delivered carefully from a buret until the indicator changes color.

We will determine the concentration of phosphoric acid by titrating a sample of phosphoric acid with a solution of sodium hydroxide (NaOH). These substances react readily according to the following equation.

H3PO4 + 2NaOH ----> Na2HPO4 + 2H2O (1)

Thymol blue will be used as the indicator. This indicator turns from yellow to blue signaling the end point of the titration. You will do a trial titration to find the approximate endpoint before you do a pair of exact titrations. These titrations, the molarity of the solution of sodium hydroxide, and the stoichiometry of the reaction will provide all that you need to determine the molarity of the phosphoric acid.

Procedure

A. Getting Started

l. Use the following format for your data table. Label the columns: Trial 1, Trial 2 and Trial 3. Label the rows: Volume acid, Final buret reading, Initial buret reading and Total volume sodium hydroxide.

2. Obtain a sample of phosphoric acid of unknown concentration. Record the number.

B. Doing the Trial Titration

1. Rinse a buret 3 times with 3 mL portions of the sodium hydroxide solution, discard each portion into a waste beaker and then fill the buret with the 0.1 M NaOH solution. Cover the solution of sodium hydroxide when it is not in use because it will react with the CO2 in air. Completely fill the buret with the sodium hydroxide solution and remove any air in the tip by running out some of the liquid into the waste beaker. Make sure that the lower part of the meniscus is between zero and 1.00 mL on the buret. Allow the buret to stand for at least 30 seconds before reading the exact position of the meniscus. Remove any hanging drop from the buret tip by touching it to the side of the waste beaker. Record the initial buret reading and the molarity of the sodium hydroxide. Remember, read the buret to the closest 0.01 mL.

2. Rinse a 10.00 mL pipet with at least two 2 mL of your phosphoric acid and then pipet 10.0 mL of phosphoric acid into a clean 250-mL Erlenmeyer flask. Add about 20-mL of distilled water from a clean graduated cylinder.