Fading of Phenolphthalein in NaOH - UV Vis Spectroscopy

This application note measures the rate of reaction of the fading of phenolphthalein in sodium hydroxide (NaOH) using UV Vis spectrophotometry. To find out more, please proceed to download the application note below.

How is the rate of reaction of phenolphthalein and NaOH measured?

Phenolphthalein is commonly used as a color indicator to check changes in pH, since it changes its color from colorless to pink at a pH of over 8.

When used with a strong base, such as NaOH, then phenolphthalein fades. The topic of this application is to calculate the reaction rate using a UV Vis Spectrophotometer.

The rate of the fading of phenolphthalein depends on ionic strength or the ion concentration of the reaction solutions. To maintain a constant ionic strength, the NaOH dilutions will be made with 0.3M NaCl.

The fading of phenolphthalein is a second order reaction, because the reaction of phenolphthalein (abbreviated P) with hydroxide (OH^-) to the colorless compound POH leads to changes in the concentration of both OH^- and phenolphthalein:

P + OH^- → POH

The rate law for this reaction is:

rate = v = - k [OH^-] * [P]

where k is a positive number with unit s^-1.

The concentration of OH^- ions is very large compared to the concentration of phenolphthalein. Therefore the OH- concentration can be considered as constant and the reaction is a "pseudo" first order. In a first order kinetic reaction, a graph of the natural log (ln) of the concentration versus time is linear, and the slope of the graph corresponds to the rate.

v' = - k' * [P]

The negative sign for the rate constant (k and k') is used because the reactants are consumed.

For full equipment, method, and results analysis information, please proceed to download the application note.