Phenolphthalein: a Dance of Colors in Acidity

Introduction

The experiment conducted in this laboratory report aimed to investigate the characteristics and behavior of the compound phenolphthalein. Phenolphthalein is an organic compound extensively used as an acid-base indicator due to its remarkable color change properties. By examining the properties of phenolphthalein in different solutions and under varying conditions, a deeper understanding of its behavior as an indicator was sought. This report will describe the experimental procedure, the obtained results, and the implications of these findings.

Experimental Procedure

The experiment commenced by preparing different solutions of known concentrations of acids and bases. Specifically, hydrochloric acid (HCl), sodium hydroxide (NaOH), acetic acid (CH3COOH), and ammonia (NH3) solutions were utilized. Using a pipette, specific volumes of these solutions were transferred to separate test tubes for analysis.
To each test tube, a few drops of phenolphthalein solution were added. This allowed the observation of any visible color change upon the addition of acids or bases. The pH values of the acidic and basic solutions were measured using a pH meter to establish the correlation between the color changes observed and the changes in pH.
The second part of the experiment involved titration of an unknown solution with NaOH. The phenolphthalein indicator was added to the unknown solution, which initially exhibited an acidic color. Sequential additions of NaOH were made while carefully monitoring the color changes. The experiment concluded when the solution turned from pale pink to a permanent bright pink color.

Results

The study of phenolphthalein's behavior as an indicator revealed intriguing findings. Observations were made regarding the color changes of phenolphthalein when exposed to different acidity or basicity levels. Initially, the phenolphthalein solution was colorless, indicating its neutrality. However, upon the addition of hydrochloric acid, the solution turned slightly pink, indicating a slightly acidic pH.
When sodium hydroxide, a strong base, was added to the solution, the color changed dramatically to a vibrant pink, indicating a basic pH. The same phenomenon was observed when acetic acid, a weak acid, was added. However, upon adding ammonia, a weak base, no apparent color change was observed.
During the titration process, the unknown solution turned from colorless to pale pink as NaOH was added gradually. As the pH increased, the pink color intensified until it reached a point where the solution turned permanently bright pink. The volume of NaOH consumed during this process was then used to calculate the concentration of the unknown solution.

Discussion and Interpretation

The observed color changes during the experiment can be interpreted in terms of phenolphthalein's acid-base characteristics. Phenolphthalein is a weak acid with a pKa value of approximately 8.2. In an acidic solution, the phenolphthalein molecule remains uncharged and colorless. However, as the pH approaches its pKa value, phenolphthalein starts to dissociate and changes to a pink color.
When phenolphthalein is exposed to a basic solution, it readily reacts with hydroxide ions (OH-) to form a negatively charged ion. This ion is responsible for the strong pink color observed. This behavior is attributed to the phenolphthalein molecule's structure, where the addition of hydroxide ions leads to the formation of a conjugate base.
In the titration process, the added NaOH gradually neutralized the acidic unknown solution, resulting in the observed color changes. The endpoint of the titration was reached when the solution became permanently pink, indicating neutralization. From the volume of NaOH consumed, the concentration of the unknown solution could be determined.

Conclusion

In conclusion, this laboratory experiment aimed to investigate the characteristics of phenolphthalein as an acid-base indicator. The observations made during the experiment highlighted the color changes associated with varying pH levels. Phenolphthalein exhibited a colorless hue in neutral solutions, became slightly pink in weakly acidic solutions, and turned bright pink in both strong acidic and basic solutions.
The experiment demonstrated the ability of phenolphthalein to act as an effective and sensitive acid-base indicator. By utilizing a pH meter and titration methods, the concentration of an unknown solution could be determined. This experiment not only enhanced our understanding of phenolphthalein's behavior but also provided a valuable opportunity to reinforce fundamental concepts related to acid-base chemistry.