Phenolphthalein is a synthetic organic compound that serves as a common acid-base indicator. It belongs to a class of compounds known as phthalein dyes and functions as a weak acid dissolved in an alcoholic solution for chemical analysis. The indicator visibly signals a change in the acidity or alkalinity of a solution. This visual cue is caused by a structural rearrangement of the molecule when the concentration of hydrogen ions shifts dramatically.
The Specific pH Range and Color Results
Phenolphthalein provides a sharp, noticeable visual indication of the transition from neutral or acidic conditions to basic conditions. The molecule remains completely colorless in any solution that is acidic or slightly neutral, generally existing below a pH of approximately 8.2.
As the solution’s alkalinity rises, the color change occurs between pH 8.2 and pH 10.0. Within this transition zone, the solution shifts from clear to a pale pink, deepening to a vibrant fuchsia or magenta color. The full, saturated pink color is present once the solution’s pH reaches 10.0 or higher.
The appearance of the first permanent hint of pink color indicates that the solution has crossed the pH 8.2 threshold. If a solution surpasses a pH of about 12 or 13, the indicator may become colorless again due to a further structural change.
How Molecular Structure Causes the Color Shift
The dramatic shift from colorless to pink is rooted in the chemical structure of the phenolphthalein molecule and its interaction with hydrogen ions. In acidic or neutral solutions, the molecule exists in a form that includes a closed ring structure called a lactone. This arrangement prevents the molecule from absorbing light in the visible spectrum, making the solution appear transparent.
When the solution becomes basic, the increased concentration of hydroxide ions causes the molecule to lose hydrogen ions in a process called deprotonation. This removal triggers the breaking of the internal lactone ring structure. The molecule then rearranges itself into a different form known as the quinoid structure.
The formation of the quinoid structure results in extended conjugation, a larger system of alternating single and double bonds. This allows the electrons within the molecule to move more freely, changing how the compound interacts with light. The new structure absorbs light at approximately 552 nanometers, corresponding to the green region of the visible spectrum. By absorbing green light, the molecule reflects the complementary color, which is perceived as pink or fuchsia. This molecular rearrangement is entirely reversible.
Practical Uses in Acid-Base Chemistry
The primary application of phenolphthalein is in analytical chemistry, specifically as an indicator in acid-base titrations. Titration is a laboratory technique used to determine the unknown concentration of a substance by precisely adding a solution of a known concentration. Phenolphthalein is added to signal the end point of the reaction.
Its sharp transition point, which is close to the neutral pH of 7, makes it useful for titrations involving strong bases and weak acids, or strong acids and strong bases. The color change indicates the equivalence point, the moment the acid and base have completely neutralized each other. Phenolphthalein has also been used in construction to test the carbonation of concrete, a measure of the material’s alkalinity. Historically, it was used in medicine as an active ingredient in laxatives, though this use has been discontinued.