Tabla de Contenidos
Phenolphthalein is a slightly acidic organic compound with the molecular formula C 20 H 14 O 4 , which is partially soluble in water. Its aqueous solutions are completely colorless when the pH is neutral or acidic, but it exhibits a characteristic deep pink coloration in solutions with a pH of 8.3 or higher. These characteristics make phenolphthalein a suitable substance to be used as a pH indicator in acid-base titrations of strong acids and bases, and some weak acids.
phenolphthalein structure
Phenolphthalein is an aromatic phenolic compound that has three independent benzene rings, two of which each have hydroxyl groups in the para position (converting them to phenolic groups) and one of which is fused to a 5-membered lactone (a cyclic ester). The structure is shown in the following figure:
This basic structure corresponds to a derivative of furan called isobenzofuranone, so the IUPAC systematic name for phenolphthalein is 3,3-bis(4-hydroxyphenyl)isobenzofuran-1(3H)-one.
Like all phenols, the hydroxyl groups attached to the aromatic rings are more acidic than the hydroxyl groups of alcohols and than those of water, thus making phenolphthalein a weak diprotic acid. As will be seen later, the loss of protons from these hydroxyl groups produces some changes in the structure of phenolphthalein that lead to the color change observed in the conjugate base.
The turning point of phenolphthalein
Like all acid-base indicators of an acidic nature, which we can represent with the generic formula HIn, phenolphthalein reacts by releasing a proton or transferring it to a suitable base and becoming the ion corresponding to the conjugate base, In – . It is a reversible acid dissociation reaction associated with an equilibrium constant or, in this case, an acidity constant of 10 -9 (pKa = 9). The reaction is:
The equilibrium constant for this reaction is given by:
This equation can be rearranged to:
At typical concentrations of the indicators, the color is generally observable when the concentration of the ionized phenolphthalein is around just one-tenth the concentration of the protonated species, and the change is no longer observable when the ionized species has a concentration around 10 times greater than that of the neutral spice.
In other words, the range corresponding to the color change is observed when the ratio between the concentrations of In – and HIn goes from approximately 0.1 to 10, which implies that the pH changes from:
Or what is the same:
Since the pKa of phenolphthalein is 9, this implies that the pH range for the color change is between 8 and 10, although in some references the range is reduced to 8.2 – 9.8.
At more extreme pH values, such as near 0 or 14, different acid-base reactions occur in which other color changes occur. However, the extremes of these pH values make these reactions unsuitable for most applications.
Why does the color change occur?
Substances that have visible colors generally have a part of the molecule that is capable of absorbing visible light. This part of the molecule is called a chromophore. Most chemical compounds are capable of absorbing light or electromagnetic radiation of some wavelength. However, most of them are only capable of absorbing high-energy ultraviolet light, since the difference in energy between the higher-energy occupied molecular orbital (HOMO) and the lower-energy unoccupied molecular orbital ( LUMO) is very high.
This happens, for example, in the case of neutral phenolphthalein. The fact that it is colorless means that it allows all visible light to pass through, that is, it does not absorb it. Instead, when ionized, a system of conjugated double bonds is formed containing 16 pi electrons moving freely along 15 atomic centers along two of the three benzene rings of the molecule, as can be seen in the following equation (in which some intermediate conversions are omitted).
This conjugation of many double bonds leads to the formation of a greater number of bonding and anti-bonding molecular orbitals that reduces the energy gap between the HOMO-LUMO orbitals, thus reducing the energy required to excite an electron in the molecule. Thus, electronic transitions are created that allow the chromophore to absorb light of a longer wavelength.
In the case of phenolphthalein, being ionized, it intensely absorbs light of approximately 550 nm, corresponding to a color between green and yellow. As a consequence, the solution appears as the complementary color that is between pink and magenta.
Uses of phenolphthalein
Phenolphthalein has been known for hundreds of years, so it has many applications. However, the most common are the following:
End point indicator in acid-base titrations
Due to its color transition between pH 8.2 and 9.8, phenolphthalein is suitable as an acid-base indicator in the following acid-base titrations:
- Titrations of strong acids with strong bases.
- Titrations of weak acids with strong bases.
- Titrations of strong bases with strong acids.
However, phenolphthalein is not suitable as an indicator in strong acid-weak base titrations , since the pH range in which phenolphthalein color change occurs usually falls in the zone where the buffer is formed. or buffer during these titrations, rather than near the equivalence point.
This causes the phenolphthalein end point of a strong acid weak base titration to be reached well before the equivalence point, thus producing a very high under-error in the titration.
As a pH indicator in bacterial cultures
Phenolphthalein diphosphate is used in microbiology as a pH indicator in some selective culture media to identify acid phosphatase-positive bacteria. In these cases it is used in the form of a derivative called phenolphthalein diphosphate dissolved in an alkaline buffer. If the bacterium expresses acid phosphatase, it hydrolyzes the phosphate groups, releasing phenolphthalein and producing the color change to pink.
Kastle-Meyer test reagent
The Kastle-Meyer test is a quick and easily implemented forensic test that reveals the presence of hemoglobin in a sample and thus helps confirm whether a forensic sample contains blood. In addition to hemoglobin, some other substances such as certain metals and some vegetables react with phenolphthalein in the Kastle-Meyer test, giving many false positives, so the test cannot be used as definitive of the presence of blood in a sample. crime scene. However, a negative test does rule out the presence of hemoglobin, so this is usually used as a first rapid test that, if positive, requires the application of a more specific and selective test.
In pharmacology as a laxative
Since the beginning of the 20th century it has been known that phenolphthalein is capable of acting as a cathartic laxative. It does so by acting on the enteric nervous system where it stimulates the production of nitric oxide, which prevents the intestines from reabsorbing water, sodium and chloride ions from the feces, thus facilitating bowel movements. However, this compound has been discontinued as a laxative due to its undesirable side effects, including even cancer and loss of bowel function.
In medicine as a diagnostic agent
The color of phenolphthalein in basic medium is used as a diagnosis of renal function, particularly in the study of residual urine in the bladder. It is also a common cause of red urine in patients who overuse phenolphthalein-derived laxatives.
References
Cantor B., D.A. (2018, August 11). phenolphthalein . Interactive and fun science. https://cienciainteractivaydivertida.wordpress.com/2017/11/01/fenolftaleina/
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed .). McGraw-Hill Education.
EcuRed. (nd). Phenolphthalein – EcuRed . Collaborative encyclopedia in the Cuban network. https://www.ecured.cu/Fenolftale%C3%ADna
phenolphthalein . (sf-a). Laboratorium Discounter. https://www.laboratoriumdiscounter.nl/en/chemicals/az/f/fenolftaleina/
phenolphthalein . (sf-b). Chemistry.is. https://www.quimica.es/enciclopedia/Fenolftale%C3%ADna.html
Hanai, T. (2016). Quantitative Evaluation of Dissociation Mechanisms in Phenolphthalein and the Related Compounds . J. Comput. Chem. Jpn., 15.1. 13–21. https://www.researchgate.net/publication/301942181_Quantitative_Evaluation_of_Dissociation_Mechanisms_in_Phenolphthalein_and_the_Related_Compounds
Herrera C., D., Fernández P., C., Coria A., GA, Manzo D., J., Aranda A., GE, & Hernández A., ME (2015, October 16). Phenolphthalein laxatives and their relationship with the development of cancer . mediagraphic.com. https://www.medigraphic.com/pdfs/veracruzana/muv-2015/muv152f.pdf
Hornerov, M. (2015, October 11). Kastle–Meyer test . PHYSICS AND CHEMISTRY. https://cuadernofyq2015sp3e13.wordpress.com/2015/10/11/prueba-de-kastle-meyer/
Khan Academy. (2015). Conjugation and color . Khan Academy. https://www.khanacademy.org/science/organic-chemistry/spectroscopy-jay/uv-vis-spectroscopy/v/conjugation-and-color-1
AGB High School. (nd). INDICATORS ACID-BASE ASSESSMENTS . liceoagb.es. https://www.liceoagb.es/quimigen/acibas11.html
Restrepo V., CA (nd). Basic Nephrology 2 – Chapter 2: Laboratory tests in nephrology . Asocolnef.com. http://asocolnef.com/wp-content/uploads/2018/03/Cap02.pdf