Since cinnamic acid can be produced from cinnamaldehyde by acid-catalyzed oxygenation, the experimental procedure for the preparation of cinnamic acid is as follows:

1. Prepare cinnamaldehyde and dilute nitric acid solution.

2. Cinnamaldehyde is added to the dilute nitric acid solution with stirring, heating, and oxygen is introduced to make cinnamaldehyde oxidized to cinnamic acid.

3. The cinnamic acid is precipitated, filtered and washed with water to obtain dry cinnamic acid crystals.

4. The cinnamic acid crystals obtained are characterized, such as by determining their melting point and infrared spectrum.