Kinetics of the Color-Fading Reaction of Phenolphthalein in Alkaline Solution Studied with a Simple and Economical Device

A simple photometric device was built to measure the kinetics of the title reaction. The device consisted of a green light-emitting diode (centered at 520 nm), a sample vial, a photodiode, and an Arduino microcontroller board. These parts were mounted on a black poly(lactic acid) holder which was manufactured by utilizing three-dimensional printing. Adequate sensitivity and dynamic range (linear detection range from 2 × 10[superscript -3] to 0.9 in absorbance, base 10) and time resolution (ca. 6 × 10[superscript -3] s) have been achieved. The time-resolved absorbance of the pink form of phenolphthalein was recorded automatically. The time scale of the forward reaction can be controlled by [OH[superscript -]]. Evidence of reaching a chemical equilibrium was observed at conditions of higher concentration of phenolphthalein and lower [OH[superscript -]]. We also demonstrated direct observation of the slow reversed reaction (taking ca. hours). Factors affecting the reaction kinetics like the ionic strength and organic solvents have also been tested. For a laboratory class, the basic goal may include the kinetics of the forward reaction and a simple description of the chemical equilibrium. If time allows, the reversed reaction (color reappearing) may be demonstrated. In a test run, the participating high school students positively agreed that this activity had increased their related ability, while a minor portion of them were less confident on more complicated knowledge items like using the Beer-Lambert law, measuring the lifetime, and knowing the evidence of the reverse reaction.