What is phenolphthalein?

The phenolphthalein is an organic dye, which is itself a weak acid diprotic, used in many titrimetric determinations as acid-base indicator. That is, if it is a diprotic acid, in solution it can lose two H + ions , and to be an indicator it must have the property of being colorful in the pH range that is being assessed.

In basic medium (pH> 8), phenolphthalein is pink in color, which can intensify to a purplish red. To be used as an acid-base indicator, it must not react faster with the OH – in the medium than the analytes to be determined.

Furthermore, as it is a very weak acid, the presence of the -COOH groups is ruled out and, therefore, the source of the acidic protons are two OH groups linked to two aromatic rings.


Phenolphthalein is an organic compound whose condensed chemical formula is C 20 H 14 O 4 . Although it may not be enough to discover what organic groups it has, unsaturations can be calculated from the formula, in order to begin to elucidate its skeleton.

Chemical structure

Structure of phenolphthalein

The structure of phenolphthalein is dynamic, which means that it undergoes changes depending on the pH of its environment. The upper image illustrates the structure of phenolphthalein in the range of 0 <pH <8. It has four rings, of which three are aromatic and hexagonal, and one is pentagonal.

It is the pentagonal ring that undergoes the greatest modifications. For example, in a basic medium, when one of the OH groups of the phenolic rings is deprotonated, its negative charge (-O  ) is attracted by the aromatic ring, “opening” the pentagonal ring in the new arrangement of its bonds.

Here, the new negative charge is located at -COO  , which is “detached” from the pentagonal ring.

Then, after increasing the basicity of the medium, the second OH group of the phenolic rings is deprotonated and the resulting charge is delocalized throughout the molecular structure.

The lower image summarizes the result of the two deprotonations in basic medium. It is this structure that is responsible for the familiar pink coloration of phenolphthalein.


The electrons that “travel” through the conjugated π system (represented by the resonant double bonds) absorb in the visible spectrum, specifically at the yellow wavelength, reflecting the pink color that reaches the viewer’s eyes.

Phenolphthalein has a total of four structures. The previous two are the most important in practical terms and are represented briefly as: H 2 In and In 2- .

Uses / applications of phenolphthalein

Adding alkaline solution to neutral solution with phenolphthalein indicator

Indicator function

Phenolphthalein is used in chemical analysis as a visual indicator in determining the equivalence point in neutralization reactions or acid-base titrations. The reagent for these acid-base titrations is prepared 1% dissolved in 90% alcohol.

Phenolphthalein has 4 states:

  • In a strongly acidic medium it has an orange color (H 3 In + ).
  • As the pH increases and becomes slightly basic, the solution becomes colorless (H 2 In).
  • In the anionic form, when the second proton is lost, a color change is generated in the solution from colorless to purplish red (In 2- ), this as a consequence of the increase in pH between 8.0 and 9.6.
  • In a strongly basic medium (pH> 13), the coloration is colorless (In (OH) 3- ).

This behavior has allowed the use of phenolphthalein as an indication of the carbonation of concrete, which causes the pH to vary to a value between 8.5 to 9.

Also, the color change is very abrupt; that is, the pink anion In 2- is produced at high speed. Consequently, this allows it to be a candidate as an indicator in many volumetric determinations; for example, that of a weak acid (acetic acid) or strong (hydrochloric acid).

Uses in medicine

Phenolphthalein was used as a laxative agent. However, there is a scientific literature indicating that some laxatives that contain phenolphthalein as an active ingredient —which works by inhibiting the absorption of water and electrolytes in the large intestine, promoting evacuations— could have negative effects.

Prolonged use of these drugs containing phenolphthalein is associated with the production of various disorders in intestinal function, pancreatitis and even cancer, mainly produced in women and in the animal models used for the pharmacological study of this chemical compound .

Chemically modified phenolphthalein, to later transform it into its reduced state, is used as a reagent in forensic tests that allow determining the presence of hemoglobin in a sample (Kastle-Meyer test), which is not conclusive due to the presence of false positives .

Synthesis of phenolphthalein

It is formed from the condensation of phthalic anhydride with phenol, in the presence of concentrated sulfuric acid, and of a mixture of aluminum and zinc chlorides as reaction catalysts:


Aromatic electrophilic substitution is the mechanism that governs this reaction. What does it consist of? The phenolic ring (the molecule on the left) is negatively charged thanks to the oxygen atom rich in electrons, which is capable of making any free pair of them go through the “electronic circuit” of the ring.

On the other hand, the carbon of the C = O group of phthalic anhydride is highly unprotected, due to the fact that the phthalic ring and the oxygen atoms subtract electronic density from it , thus carrying a positive partial charge. The electron-rich phenolic ring attacks this electron-poor carbon, incorporating the first ring into the structure.

This attack occurs preferentially at the opposite end of the carbon linked to the OH group; this is the position – stop .

The same happens with the second ring: it attacks the same carbon and from this a water molecule generated thanks to the acidic medium is released.

In this way, phenolphthalein is nothing more than a phthalic anhydride molecule that has incorporated two phenolic rings in one of its carbonyl groups (C = O).

Properties of phenolphthalein

Its physical appearance is that of a white solid with triclinic crystals, often agglomerated or in the shape of rhombic needles. It is odorless, denser than liquid water (1.277 g / mL at 32 ºC), and very little volatile (estimated vapor pressure: 6.7 x 10 -13 mmHg).

It is very slightly soluble in water (400 mg / l), but very soluble in alcohols and ether. For this reason it is recommended to dilute it in ethanol before being used.

It is insoluble in aromatic solvents such as benzene and toluene, or in aliphatic hydrocarbons such as n-hexane.

It melts at 262.5ºC and the liquid has a boiling point of 557.8 ± 50.0ºC at atmospheric pressure. These values ​​are indicative of strong intermolecular interactions. This is due to the hydrogen bonds, as well as the frontal interactions between the rings.

Its pKa is 9.7 at 25ºC. This translates into a very low tendency to dissociate in aqueous medium:

2 In (aq) + 2H 2 O (l) <=> In 2- (aq) + 2H 3 O +

This is an equilibrium in an aqueous medium. However, increasing the OH  ions  in the solution decreases the amount of H 3 O + present .

Consequently, the equilibrium shifts to the right, to produce more H 3 O + . This is how your initial loss is rewarded.

As more base is added, the equilibrium continues to shift to the right, and so on until there is nothing left of the H 2 In species . At this point, the In 2 species  turns the solution pink.

Lastly, phenolphthalein breaks down when heated, emitting acrid and irritating smoke.

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