The phenolphthalein is an organic dye, which is in turn a weak diprotic acid, used in many volumetric determinations as an acid-base indicator. That is, if it is a diprotic acid, in solution you 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 evaluated.
In basic medium (pH> 8) phenolphthalein is pink, which can be intensified to a purplish red (as illustrated in the following image). To be used as an acid-base indicator, it must not react more quickly with OH - of the medium that the analytes to determine.
Furthermore, since it is a very weak acid, the presence of the -COOH groups is discarded and, therefore, the source of the acid protons are two OH groups linked to two aromatic rings.
Index
- 1 Formula
- 2 Chemical structure
- 3 Applications
- 3.1 Indicator function
- 3.2 Uses in medicine
- 4 Preparation
- 5 Properties
- 6 References
Formula
Phenolphthalein is an organic compound whose condensed chemical formula is C twenty H 14 OR 4 . Although it may not be enough to discover what organic groups you have, you can calculate the insaturations from the formula, to begin to elucidate your skeleton.
Chemical structure
The structure of phenolphthalein is dynamic, which means that it undergoes changes depending on the pH of its environment. In the upper image the structure of phenolphthalein in the range of 0 It is the pentagonal ring that undergoes the greatest modifications. For example, in 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 links. Here, the new negative charge is located in the -COO group - , which"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 delocalised 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 wavelength of the yellow color, reflecting the pink color that reaches the eyes of the spectator. Phenolphthalein has a total of four structures. The previous two are the most important in practical terms and are abbreviated as: H 2 In and In 2- .
Phenolphthalein is used in chemical analysis as a visual indicator in the determination of the equivalence point in neutralization reactions or acid-base titrations. The reagent for these acid-base titrations is prepared at 1% dissolved in 90% alcohol. Phenolphthalein has 4 states: - In a strongly acid medium it has an orange color (H 3 In + ). - By increasing the pH and becoming slightly basic, the solution becomes colorless (H 2 In). - In the anionic form when losing the second proton, a color change is generated in the solution from colorless to violet red (In. 2- ), this as a consequence of the pH increase between 8.0 to 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 changes the pH to a value between 8.5 to 9. Also, the change of coloration is very abrupt; that is, that the anion In 2- Pink is produced at high speed. Consequently, this allows him to be a candidate as an indicator in many volumetric determinations; for example, that of a weak acid (acetic acid) or strong acid (hydrochloric acid). Phenolphthalein was used as a laxative agent. However, there is a scientific literature indicating that some laxatives containing phenolphthalein as an active ingredient - which acts by inhibiting the absorption of water and electrolytes in the large intestine by promoting evacuations - could have negative effects. The prolonged use of these medicines containing phenolphthalein is associated with the production of various disorders in intestinal function, pancreatitis and even cancer, produced mainly in women and in the animal models used for the pharmacological study of this chemical compound. The chemically modified phenolphthalein, to then transform it into its reduced state, is used as a reagent in forensic tests that allow to determine the presence of hemoglobin in a sample (Kastle-Meyer test), which is inconclusive due to the presence of false positives . It is formed from the condensation of phthalic anhydride with phenol, in the presence of concentrated sulfuric acid, and from a mixture of aluminum and zinc chlorides as catalysts of the reaction:
Electrophilic aromatic 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 electron-rich oxygen atom, which is able to make any free pair of them travel through the"electronic circuit"of the ring. On the other hand, the carbon of the C = O group of phthalic anhydride is very unprotected, because the phthalic ring and the oxygen atoms subtract electron density, thus carrying a positive partial charge. The phenolic ring, rich in electrons, attacks this electron-poor carbon, incorporating the first ring to the structure. This attack occurs preferentially at the opposite end of the carbon bound to the OH group; this is the position - for . The same happens with the second ring: it attacks the same carbon and from this a molecule of water generated thanks to the acid 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).
Its physical appearance is that of a white solid with triclinic crystals, often agglomerated or in the form of rhombic needles. It is odorless, denser than liquid water (1,277 g / mL at 32ºC), and very little volatile (estimated vapor pressure: 6,7 x 10 -13 mmHg). It is very poorly 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 262.5 ° C and the liquid has a boiling point of 557.8 ± 50.0 ° C 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. His pKa is 9.7 at 25 ºC. This results in a very low tendency to dissociate in aqueous medium: H 2 In (ac) + 2H 2 O (l) In 2- (ac) + 2H 3 OR + This is a balance in aqueous medium. However, the increase in OH ions - in the solution decreases the present amount of H 3 OR + . Consequently, the balance shifts to the right, to produce more H 3 OR + . In this way your initial loss is rewarded. As more base is added, the balance keeps moving to the right, and so on until there is nothing left of the species H 2 In. At this point, the species In 2- color the solution pink. Finally, phenolphthalein decomposes on heating, emitting acrid and irritating smoke. Applications
Indicator function
Uses in medicine
Preparation
Properties
References