Thursday, February 10, 2022

Preparation of phenolphthalein indicator solution

Learn the procedure for making phenolphthalein indicator solutions.

The compound that change color when exposed to acidic or basic solutions are called indicator. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

Phenolphthalein (C20H14O4) is a chemical compound often written in shorthand notation as "HIn" or "phph" that belongs to the phthalein family. Phenolphthalein can be formed by condensation of phthalic anhydride with two equivalents of phenol under acidic conditions.

It comes in a yellowish-white to light orange fine crystalline powder. It is a weak acid that, in solutions, can lose H+ ions.

How to prepare phenolphthalein indicator for titration:

  • Weigh accurately 01.00 gm of phenolphthalein and pour it into a 100.00 ml volumetric flask containing 50.00 ml of 95% ethanol, mix it.
  • Once it is dissolved, dilute to 100.00 ml with ethanol.
  • The concentration of the prepared solution is about 10 g/L.

Phenolphthalein has a pKa of 9.7 and the pH range is 8.2 to 10. It is colorless in acidic solutions, while in a basic condition it turns pink. In acid-base titration, is frequently used as an indicator.

References:
  1. Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, The Free Encyclopedia. Available Here
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.

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Preparation of m-cresol purple indicator solution

Learn the procedure for making cresol purple indicator solution.

The compound that change color when exposed to acidic or basic solutions are called indicator. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

Meta-cresol purple (C21H18O5S), commonly known as m-cresol purple or m-cresolsulfonphthalein, is a triarylmethane dye and a pH indicator. It comes in a green or brick-red powder that is acidic and hydrophobic in nature, with slightly soluble in water and ethanol. It is primarily used as an acid-base indicator and in spectrophotometric pH measurements. It's also used to stain epithelial cells and proteins.

How to prepare cresol purple indicator for titration:

  • Weigh accurately 40 mg of m-cresol purple and pour it into a 100.00 ml volumetric flask containing 02.08 ml of 0.05N NaOH solution and 20.00 ml of 95% ethanol.
  • Once it is dissolved, dilute to 100.00 ml with distilled water.
  • The concentration of the prepared solution is about 0.4 g/L.

m-cresol purple has a pKa of 8.3 and it has two useful pH ranges are 1.2 - 2.8 (red to yellow) and 7.4 - 9.0 (yellow to violet). It changes color in both pH ranges: below pH 1.2 it turns red, above pH 2.8 it turns yellow, and below pH 7.4 it turns yellow, above pH 9.0 it turns violet.


References:
  1. Wikipedia contributors. (2021, October 3). Metacresol purple. In Wikipedia, The Free Encyclopedia. Available Here:
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.

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Preparation of phenol red indicator solution

Learn the procedure for making phenol red indicator solution.

The compound that change color when exposed to acidic or basic solutions are called indicator. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare phenol red indicator for titration:

  • Weigh accurately 100 mg of phenol red and pour it into a 100.00 ml volumetric flask containing 02.82 ml of 0.05N NaOH solution and 20.00 ml of 95% ethanol, warm it.
  • After the solution has been effected, dilute to 100.00 ml with distilled water.
  • The concentration of the prepared solution is about 1 g/L.

Phenol red (C19H14O5S) is a synthetic phenolic compound also known as phenolsulfonphthalein or PSP, which acts as a weak acid in a solution. Phenol red exists as a red crystal that is stable in air, it is a water-soluble dye used as an acid pH indicator in different medical and cell biology tests.

Phenol red has a pKa of 8.0 and the pH range is 6.8 to 8.2. In an acidic medium (below pH 6.8) it turns yellow, while in a basic condition (above pH 8.2) it turns bright pink.

Bromothymol blue, thymol blue, bromocresol purple, thymolphthalein, and phenolphthalein are all indicators that have a similar structure to phenol red.



References:
  1. Wikipedia contributors. (2022, January 1). Phenol red. In Wikipedia, The Free Encyclopedia. Available Here:
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.
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Wednesday, February 9, 2022

Preparation of bromothymol blue indicator solution

Learn the procedure for making bromothymol blue indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare bromothymol blue indicator for titration:

  • Weigh accurately 50 mg of bromothymol blue and pour it into a 100.00 ml volumetric flask containing 04.00 ml of 0.05N sodium hydroxide solution and 05.00 ml of 95% ethanol, warm it.
  • After the solution has been effected, add 45.00 ml of 95% ethanol and dilute to 100.00 ml with distilled water.
  • The concentration of the prepared solution is about 0.5 g/L.

Bromothymol blue (C27H28Br2O5S) is also known as bromothymol sulfone phthalein and BTB, which acts as a weak acid in a solution. It used as is a pH indicator and for measuring the presence of carbonic acid in a liquid. BTB is a suitable choice for any titration with a near-neutral equivalence point.

Bromothymol blue has a pKa of 7.1 and the pH range is 6.0 to 7.6. In an acidic medium (below pH 6.0) it turns yellow, while in a basic condition (above pH 7.6) it turns blue, and green in a neutral solution.

Bromothymol blue is prepared by the addition of elemental bromine with thymol blue in a glacial acetic acid solution. It comes in the form of a powder, crystals, or chunks. It might seem yellow or blue depending on whether it is protonated or deprotonated.


References:
  1. Wikipedia contributors. (2022, January 25). Bromothymol blue. In Wikipedia, The Free Encyclopedia. Available Here:
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.
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Preparation of methyl red indicator solution

Learn the procedure for making methyl red indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare methyl red indicator for titration:

  • Weigh accurately 50.00 mg of methyl red and pour it into a 100.00 ml volumetric flask containing 01.86 ml of 0.1 M sodium hydroxide solution and 05.00 ml of 95% ethanol, warm it.
  • After the solution has been effected, add 45.00 ml of 95% ethanol and dilute to 100.00 ml with distilled water.
  • The concentration of the prepared solution is about 0.5 g/L.

Methyl red (C15H15N3O2), commonly known as C.I. Acid Red 2, is a pH indicator that comes in a dark red crystalline powder. It can be prepared by diazotizing anthranilic acid and then reacting it with dimethylaniline.

It has a pKa of 5.0 and the pH range is 4.2 to 6.2. In an acidic medium (less than 4.4), methyl red turns red, while in a basic condition (higher than 6.2), it turns yellow, and shows orange color in between, with a pKa of 5.1.

Methyl red can be used as an indicator in acid-base titrations involving the strong base and weak acid. It is also used as a dye for the food, leather, and textile industries.



References:
  1. Wikipedia contributors. (2021, November 19). Methyl red. In Wikipedia, The Free Encyclopedia. Available Here:
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.
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Preparation of bromocresol green indicator solution

Learn the procedure for making bromocresol green indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

Bromocresol green (C21H14Br4O5S) is a triphenylmethane-based dye. It is a sulfonphthalein, which is a type of dye. It has a pKa of 4.7 and the pH range is 3.8 to 5.4. It commonly dissolves in ethanol or water, but can also be dissolved in benzene and diethyl ether.

How to prepare bromocresol green indicator for titration:

  • Weigh accurately 0.1 g of bromocresol green and pour it into a 100 ml volumetric flask containing 0.58 ml of 0.05 N NaOH solution and 05.00 ml of rectified spirit/ethanol, warm it.
  • After the solution has been effected, add 10.00 ml of rectified spirit/ethanol and dilute to 100 ml with distilled water.
  • The concentration of the prepared solution is about 1 g/L.
Bromocresol green ionizes in an aqueous solution to produce the green monoanionic form (yellow), which deprotonates at higher pH and produces the dianionic form (blue), which is stabilized by resonance.

In an acidic medium (less than 3.8), bromocresol green turns yellow, while in a basic condition (higher than 5.4), it turns green. It's commonly used in strong acid+weak base titration, for electrophoresis, and in microbial growth media.



References:
  1. Law.resource.org. 2022. [online] Available Here:
  2. Wikipedia contributors. (2022, January 5). Bromocresol green. In Wikipedia, The Free Encyclopedia. Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here:
  4. Sabnis, R. W. (2008). Handbook of Acid-Base Indicators. Boca Raton, FL: CRC Press.

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Preparation of methyl orange indicator solution

Learn the procedure for making methyl orange indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare methyl orange indicator for titration:

  • Weigh accurately 0.1 g of methyl orange and dissolve in 50 ml of 95% ethanol in a volumetric flask, and properly mix it. Once it has completely dissolved, make up the volume up to 100 ml with ethanol.
  • The concentration of the prepared solution is about 1 g/L.

Methyl orange (C14H14N3NaO3S) is an organic compound that an azo dye prepared from sulfanilic acid and N,N-dimethylaniline. It has a pKa of 4.2 and the pH range is 3.1 to 4.4.

In an acidic medium, methyl orange turns red, while in a basic condition, it turns yellow and in neutral solution it shows orange colour. It's commonly used in acid titration since it changes color at the pKa of a mid-strength acid.

Methyl orange, unlike a universal indicator, does not have a wide range of color changes, however has a sharp end point.

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Tuesday, February 8, 2022

Preparation of methyl yellow indicator solution

Learn the procedure for making methyl yellow indicator solution.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare methyl yellow indicator for titration:

  • Weigh accurately 0.1 g of dimethyl yellow and dissolve in 50 ml of 95% ethanol in a volumetric flask, and properly mix it. 
  • Once it has completely dissolved, make up the volume up to 100 ml.
  • The concentration of the prepared solution is about 1 g/L.

Dimethyl yellow (4-dimethylaminoazobenzene -C6H5N2C6H4N (CH3)2 is an organic compound that an azo dye derived from dimethylaniline used as an acid-base indicator. It has a pKa of 3.3 and the pH range is 2.9 to 4.0.

Methyl yellow shows red in an aqueous solution with a low pH, it undergoes a transition between pH 2.9 and 4.0, turning yellow above pH 4.0. Generally, it is used in the titration of a weak base with a strong acid.



References:
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Preparation of thymol blue indicator solution

Learn the procedure for making thymol blue indicator solution.

The compound that change color when exposed to acidic or basic solutions are called indicator. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.

Most pH indicators are weak acids or bases. For titration, you must select a pH indicator that changes color only at the equivalence point.

How to prepare thymol blue indicator for titration:

  • Weigh accurately 0.1 g of thymol blue and pour it into a 100 ml volumetric flask containing 2.15 ml of 0.05 N NaOH solution and 05.00 ml of 95% ethanol, warm it. 
  • After the solution has been effected, add 25.00 ml of 95% ethanol and dilute to 100 ml with distilled water.
  • The concentration of the prepared solution is about 1 g/L.

Thymol blue (Thymolsulfonephthalein- C27H30O5S) is a crystalline powder that is brownish-green or reddish-brown in color and is used as a pH indicator. It has a pKa of 0.8 and pH ranges of 1.2-2.8 and 8.0–9.6. At pH 1.2–2.8, it changes from red to yellow, while at pH 8.0–9.6, it transitions from yellow to blue.

Thymol blue is a weak acid that is soluble in alcohol and dilute alkali solutions but not in water. It is also usually a component of the universal indicator.


References:
  1. Wikipedia contributors. (2021, December 12). Thymol blue. In Wikipedia, The Free Encyclopedia. Available Here:
  2. Law.resource.org. 2022. [online] Available Here:
  3. Dr John Elfick, j., 2022. Learn how to prepare useful acid-base indicators. Uq.edu.au. Available Here: 
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Monday, February 7, 2022

Difference between phenolphthalein and methyl orange indicator

Phenolphthalein and methyl orange are acid-base indicators that are chosen based on the neutralization reaction that occurs, however they have some differences, lets check them.

The compounds that change color when exposed to acidic or basic solutions are called indicators. Color indicators are commonly used to measure pH and may be added to the reaction mixture to identify the titration endpoint/equivalence point.


Acid-base titration indicators:

Acid-base titrations employ pH chemical indicators that change color over a range of pH values, rather than a single pH value. Generally, we use phenolphthalein and methyl orange to determine the endpoints of acid-base titrations. The selection of a certain indicator for titration is decided based on a plot known as the pH curve, which is typical of a particular reaction.

Methyl orange indicator:

Methyl orange is a weak acid, a synthetic indicator that provides endpoints for a pH range of 3.1 to 4.4. Its color is yellow in weak acidic or basic conditions (pH>4.4) and turns red in a highly acidic environment (pH<3.1). Its color is orange in the detection range of 3.1 to 4.4 pH.

To indicate the equivalence point, when the pH of the solution falls into the detection range, its color changes from yellow to orange, and when the pH of the solution raises into the detection range its color is red to orange.

Phenolphthalein indicator:

Phenolphthalein is a weak acid, a synthetic indicator that provides endpoints for a pH range of 8.3 to 10. It is colorless in strongly acidic and weak basic conditions (pH<8.3), and it turns pink in the strong basic environment (pH>10). Its color is pale pink in the detection range (pH: 8.8 to 10).

To indicate the equivalence point, when the pH of the solution rises into the detection range the color changes from colorless to pale pink, and when the pH of the solution falls toward the detection range its color changes from pink to pale pink

Difference between phenolphthalein and methyl orange indicator:

  • Phenolphthalein is a colorless, weak acid that dissociates in water to form pink anions, while methyl orange is a weak acid that dissociates in water to form orange-neutral molecules.
  • Phenolphthalein is suitable for titration of a weak acid against a strong base, while methyl orange is a useful chemical indicator for titration of a weak acid against a strong base.
  • The pKa of phenolphthalein is 9.4, while the pKa of methyl orange is 3.5.
  • The phenolphthalein indicator has a pH range of 8.2 to 10.0, while the methyl orange indicator has a pH range of 3.1 to 4.4.
  • In terms of color change due to pH, phenolphthalein changes from colorless to pink at pH 8.2, while methyl orange changes at pH 4.
  • In acid, the methyl orange indicator turns red, while phenolphthalein remains colorless.
  • In a basic solution, methyl orange turns yellow, while phenolphthalein turns pink.

References:
  1. Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, The Free Encyclopedia. Available Here
  2. Wikipedia contributors. (2021, September 27). Methyl orange. In Wikipedia, The Free Encyclopedia. Available Here
  3. “Acid-Base Indicators.” Chemguide: Helping You to Understand Chemistry - Main Menu, Available Here
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Why is phenolphthalein better than methyl orange?

Phenolphthalein changes color when the pH rises over 07.00, and methyl orange changes color at a pH of 3.4. As a result, methyl orange is used when titrating a strong acid with a weak base, while phenolphthalein best option than methyl orange when titrating a weak acid with a strong base.

The pH of the equivalence point for an acid-base titration determines which indicator to use. The indicator that changes color at or near the point of equivalence should be selected. The titration curve will be steep at this point, and you will get a good endpoint.

Phenolphthalein is a synthetic indicator that is colorless in an acidic medium and pink in basic conditions. Methyl orange is also a synthetic indicator that shows various colors in acid, base, and neutral substances. As the pH of the solution increases, it changes color from red to orange to yellow.

Both phenolphthalein and methyl orange are acid-base indicators that are chosen based on the neutralization reaction that occurs. A phenolphthalein indicator is used to do a strong acid-strong base titration because it changes color over a pH range of 8.3 to 10.

Methyl orange is a pH indicator that ranges from 3.1 and 4.4. Since its pH range does not include the pH equivalence point (~8) for the reaction, it is not a good indicator for the neutralization reaction between a weak acid and a strong base (eg. Oxalic acid vs. sodium hydroxide).
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Friday, February 4, 2022

Why does phenolphthalein turn pink?

The formation of ions through a chemical reaction causes the solution to turn pink. In acid-base titrations, phenolphthalein is commonly employed as an indicator. In the presence of an acid, it becomes colorless, while in the presence of a base, it becomes pink.

The chemical compound phenolphthalein (C20H14O4) belongs to the phthalein family. It was discovered in 1871 by Adolf von Baer, a German scientist, who created it by fusing phenol and phthalic anhydride in the presence of H2SO4 or ZnCl2. It is related to triphenylmethane dyes.

The pH range of the phenolphthalein indicator is from 08.20 to 10.00. It is colorless over an acidic pH range, although it turns pink at pH 8.2 and bright magenta at pH 10.00 and higher. It is a component of the universal indicator, along with methyl red, bromothymol blue, and thymol blue.

Why does the phenolphthalein turn pink?

Phenolphthalein (HIn) is a slightly acidic compound that dissociates into H+ and In- ions in an aqueous solution. The concentration of In-ions in the solution gives the solution its pink color.

The solution is colorless because the concentration of In- in the solution is very low under acidic conditions and the concentration of H+ is quite high.

Likewise, under basic conditions, the concentration of H+ ions is extremely low while the In-concentration is high, resulting in a pink-colored solution.

For example, using phenolphthalein as an indicator, titration of 0.1N sodium hydroxide against 0.1N hydrochloric acid.

In a conical flask, 10 ml HCL and 2-3 drops of phenolphthalein were added. At this point, no titrant (NaOH) is added to the mixture; hence the phenolphthalein is colorless because it is in an acidic environment.

The 0.1N NaOH solution is now being used to titrate this solution. It is completely neutralized as soon as we add about 10 ml of the titrant. At this stage, the phenolphthalein indicator is in a neutral state and gives the solution a very pale pink color. This is an equivalence point of the reaction.

Following the equivalence point, we add a few ml of NaOH, and the addition of phenolphthalein occurs under basic conditions, giving the solution a pink color.

Why is phenolphthalein pink in the basic solution? 

The ionization of phenolphthalein increases dramatically in an alkaline medium.  The OH- ions from the alkali continuously remove H+ ions released from HPh.  As a result, the concentration of Ph ions in the solution increases, making the solution pink. Therefore phenolphthalein becomes pink in the presence of basic solution.

Why does the pink color of phenolphthalein fade away?

Right after the endpoint of the titration, the basic solution slowly absorbs CO2 from the air and becomes more acidic, causing the color to fade from pink to colorless. When titrating strong or weak acids with a strong base, phenolphthalein is one of the most commonly used indicators.
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How does the phenolphthalein indicator work?

Phenolphthalein is an acid-base indicator that turns pink in the presence of a weak base and is used to test the pH of a solution. The number of anions increases when the pH of the solution is 8.2 or higher, causing the solution to turn pink.

Indicators are compounds that change color when exposed to acidic or basic solutions. Color indicators are commonly used to measure pH and can be used to identify the titration endpoint/equivalence point by adding them to the reaction mixture.

Several types of indicators are employed in various forms of titration, including acid-base, redox, precipitation, and complexometric. Each has its own pKa and pH range and produces a different color at different pH levels.


What is phenolphthalein?

Phenolphthalein (C20H14O4) is a chemical compound that belongs to the phthalein family. It was discovered in 1871 by the German scientist Adolf von Baer, who made it by fusing phenol and phthalic anhydride in the presence of H2SO4 or ZnCl2 and is closely related to triphenylmethane dyes.

It is a fine crystalline powder that is yellowish-white to bright orange in color. Its pKa is 9.3, which is mildly soluble in water, and when dissolved in alcohol, it is used as an indicator for acid-base titration.

The pH range of the phenolphthalein indicator is from 08.20 to 10.00. The chemical is colorless over a wide range of acidic pH values, although it turns pink at pH 8.2 and bright magenta at pH 10.00 and higher. Along with methyl red, bromothymol blue, and thymol blue, it is also a component of the universal indicator.

How does phenolphthalein indicator work?

Ionization is the chemical reaction that causes the compound's color to change. Furthermore, ionization happens when a molecule loses or gains an electron, resulting in a positive or negative charge on the molecule. 

The ionized molecule attracts the opposite charge while repelling the same charge. It also has an affects on molecular shape. Also, the charge determines how it will react to light.

Generally, phenolphthalein solution appears colorless when all colors of light flow through it. However, when it comes into contact with the alkaline condition, it begins to block the blue color of the light spectrum, turning pale pink. 

The stronger the alkaline solution, the more the phenolphthalein molecules will change, resulting in a darker color.

Structure of phenolphthalein:

It has 3 hexagonal structures, 1 pentagonal structure, 2 alcoholic groups, and 1 ketone group in its structure. Phenolphthalein also has a structure that includes carbon, hydrogen, and oxygen chains.

Depending on whether it is in alkali (pink) or acid (colorless) solution, the phenolphthalein indicator has two different structures. Both structures absorb light in the ultra-violet (UV) range, which the human eye cannot see. On the other hand, the pink form is also absorbed in the visible light spectrum.

The structure of the pink form of the phenolphthalein indicator  is responsible for the absorption of visible light. The electrons in the molecule are more delocalized due to ionization than in the colorless form. In short, delocalization occurs when electrons in a molecule are not associated with a single atom but are spread among several atoms.

The energy gap between molecular orbital shifts as delocalization increases. The energy required to jump an electron to a higher orbital is less. In solution, the human eye detects a pink color. The phenolphthalein indicator changes more as the solution becomes more alkaline, resulting in a deeper pink color.



References:
  1. Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, Available Here
  2. ‘Why Does Phenolphthalein Change Color?’ Sciencing, Updated February 16, 2020 By Rosann Kozlowski, Available Here
  3. Britannica, The Editors of Encyclopaedia. "phenolphthalein". Encyclopedia Britannica, 23 Nov. 2018, Available Here
  4. Theory-of-acid-base-indicators.pdf, Dr. Pankaj Kumar Rastogi, Session-2018-19, Available Here

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Thursday, February 3, 2022

What is phenolphthalein and why is it used

Phenolphthalein (C20H14O4) is an acid-base indicator that belongs to the phthalein family of chemical compounds. It also is used as a component of the universal indicator, along with methyl red, bromothymol blue, and thymol blue.

Compounds that change color when exposed to acidic/basic solutions are known as indicators. Color indicators are widely used to detect pH and can be added to the reaction mixture to determine the titration endpoint/equivalence point.

In chemistry different types of titration, including acid-base, redox, precipitation,  complexometric, and several types of indicators are used. Each one has its pKa, pH range, and produces a particular color at different pH values. The two theories that explain the acid-base indicator are Ostwald's theory and the quinonoid theory.


What is phenolphthalein?

Phenolphthalein (C20H14O4) is an indicator that belongs to the phthalein family of chemical compounds. It's a yellowish-white to light orange thin crystalline powder. It has a pKa of 9.3, is slightly soluble in water, and is prepared as an indicator for acid-base titration experiments when dissolved in alcohol. It appears colorless in the liquid form up to pH 8.5, after which it changes from pink to dark red.

Phenolphthalein which is closely related to the triphenylmethane dyes was discovered in 1871 by German scientist Adolf von Baeyer, who made it by fusing phenol and phthalic anhydride in the presence of sulfuric acid (H2SO4) or zinc chloride (ZnCl2).

Structure of phenolphthalein:


phenolphthalein indicatorThe chemical compound phenolphthalein has the formula C20H14O4 and is commonly abbreviated as "HIn" or "phph" in shorthand notation.

It has 3 hexagonal structures, 1 pentagonal structure, 2 alcoholic groups, and 1 ketone group in its structure. Phenolphthalein also has a structure that includes carbon, hydrogen, and oxygen chains.

In- is the ionic form of phenolphthalein, while HIn is the molecular form. In the basic solution, the molecule loses a hydrogen ion and becomes a negative ion. When titrating weak acids with strong bases, phenolphthalein is the ideal indicator.

The ionic form of phenolphthalein is In-, while the molecular form is HIn. In the basic solution, the molecule loses a hydrogen ion and becomes a negative ion.

Properties of phenolphthalein:


Molecular Weight/ Molar Mass: 318.32 g/mol

Formula: C20H14O4

Density: 1.277 g/cm3

Appears: White powder

Melting Point: 258–263°C

FAQ (Frequently Asked Questions):


Phenolphthalein is which type of indicator?

Phenolphthalein (C20H14O4) is a phthalein-family chemical molecule that is commonly used as an acid-base indicator.

What is the phenolphthalein indicator made of?

Under acidic conditions, phthalic anhydride and two equivalents of phenol can be condensed to prepare phenolphthalein.

Is phenolphthalein a natural indicator?

Phenolphthalein is a synthetic indicator, not a natural indicator, which is colorless in an acidic medium but pink in a basic medium.

How does phenolphthalein work as an indicator?

Phenolphthalein changes color when the molecule undergoes a rearrangement in the structure, transforming it from a colorless molecule to a colorful ion.

Why do we use phenolphthalein as an indicator in acid-base titration?

Phenolphthalein is chosen because it changes color in the pH range of 8.3 to 10, it is inexpensive and commonly available. The solution turns pink from colorless when the concentration of OH- ions increases somewhat, indicating the endpoint. In basic solution, it will appear pink; in acidic solution, appears colorless.

What is the pH range of phenolphthalein to produce permanent pink?

The pH range for the phenolphthalein indicator is 08.20 to 10.00. The chemical is colorless over a wide range of acidic pH values but begins to turn pink at pH 8.2 and continues to a brilliant magenta at pH 10.00 and higher.

What color is phenolphthalein indicator in a neutral solution?

Phenolphthalein is colorless in a neutral solution, as it begins to changes from a colorless to a pink solution at pH = 8.3, whereas the pH of a neutral solution is 7.00.

What color is phenolphthalein indicator in an acidic solution?

Phenolphthalein is colorless in acidic conditions, as does not react with acid. It is a weak acid that reacts only with strong bases and changes color.

What color is phenolphthalein indicator in a basic solution?

In basic solutions, phenolphthalein turns pink due to the formation of ions, which makes it very easy to visualize the stoichiometric end-point.

How do you make a 1.0 percent w/v solution of phenolphthalein indicator from phenolphthalein?

Take 01.00 g of phenolphthalein (C20H14O4) and dissolve in 100 ml of ethanol (95 percent).in a volumetric flask, and properly mix it. Once it has completely dissolved, transfer it to an indicator bottle.

Which indicator is best for weak acid strong base titration?

Phenolphthalein (pH range 8.3 - 10.0) or thymol blue (pH range 8.0 - 9.6) would be good indicators for titration of the weak acid CH3COOH and the strong base NaOH,

Why is methyl orange used as an indicator?

Methyl orange, with a pKa of 3.8, is an often-used pH indicator in titration because of its clear and distinct color variance in different pH ranges of 3.1 - 4.4.


References:

  1. Wikipedia contributors. (2022, January 23). Phenolphthalein. In Wikipedia, The Free Encyclopedia. Available Here
  2. ‘Why Does Phenolphthalein Change Color?’ Sciencing, Available Here
  3. Britannica, The Editors of Encyclopaedia. "phenolphthalein". Available Here
  4. Theory-of-acid-base-indicators.pdf, Dr. Pankaj Kumar Rastogi, Session-2018-19, Available Here
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