5 Ways To Get Students Energized About Phthalein-Dye Test

Color change has long fascinated people. Traditionally used for discoloration, phthalein dyes are the main class of pH indicators. The Phthalein dye test is a test to determine the presence of phenolic functional groups.

Figure 1: An image showing the analytical chemistry lab available in the phthalein-dye test simulation from Labster. The simulation is useful for High School and University/College courses.

Alcohols are functional groups that have a hydroxy group (-OH) bonded to a saturated carbon. When a hydroxy group is attached to an aromatic ring, the functional group is referred to as a phenol. Note that phenol is the name of the functional group and the simplest compound in this group. Phenol is a hydroxyl group (-OH) of an aromatic ring or simply a hydroxyl derivative of an aromatic compound. Phenols are one of the most versatile and important industrial organic chemicals.

Read on for some thoughts on why this can be a challenging topic for teachers and students alike, five suggestions to help change that, and thoughts on why a virtual lab could make things easier.

Why can a phthalein-dye test be difficult for students to grasp

There are three major reasons:

1. These are complex reactions that occur at a molecular level

The fact that students cannot visualize the processes, and not seeing their relevance to the real world can make studying it tough.

2. Requires basic knowledge of acids, bases, and alkalis

Acids have been defined in three ways namely:

1. An acid increases the number of hydrogen ions when dissolved in water. (The Arrhenius definition).
2. An acid is a species that donates a proton or hydrogen ion (H+). (The Brønsted-Lowry definition).
3. An acid is a species that accepts an electron pair. (The Lewis definition)

Bases have three definitions that are basically the opposite of acid as follows:

1. A base increases the amount of hydroxide ions when dissolved in water. (The Arrhenius definition) 
2. A base is a species that accepts a proton or hydrogen ion (H+). (The Brønsted-Lowry definition)
3. A base is a species that donates an electron pair. (The Lewis definition)

Alkalis are bases that are soluble in water. For example, both copper oxide and sodium hydroxide are bases, but only sodium hydroxide is an alkali because it dissolves in water.

3. Understanding pH indicators and how they work can seem complicated

A pH indicator is a substance added to a solution to visually measure the approximate pH. The color of the pH indicator solution depends on the pH of the solution. For example, an indicator can be red in an acidic solution and blue in a basic solution. pH indicators are often used in laboratories to mark the end point of a titration. You can also use it outside the laboratory to measure the acidity of soil or water. Many pH indicators are available and each indicator has a different color change at different pH levels. Some examples of pH indicators are thymol blue, phenolphthalein, and thymolphthalein. 

Natural pH indicators:

All of the naturally occurring pH indicators in Figure 3 have a pH-dependent color change, but the mechanism that causes the color change varies between them. The color of a hydrangea flower depends on whether there are aluminum ions in the petals. In acidic soils, aluminum ions in the soil can be absorbed by hydrangea leaves, turning them blue. On the other hand, curcumin, anthocyanins, and litmus are substances that change their chemical structure when there is a change in pH, and this change in chemical structure causes a color change.

Figure 3: Naturally sourced pH indicators.

How Do pH Indicators Work?

1. The pH changes the shape of the indicator molecule. Acids and bases are solutions with lots of free H+ or OH- ions. When mixed with an indicator, these reactive ions add or remove from the molecule. With new atoms, charges, and bonds, molecules take on new shapes.
2. The shape of the molecule determines its color. White light is made up of all the different frequencies and colors of light. As it passes through the sample, light is continuously reflected, absorbed, and re-emitted. The amount absorbed by each frequency of light depends on the shape of the molecule and the bonds it forms. So the pH changes shape, then the shape changes color.

The Phthalein Dye Test:

Most phenols give this exact test and show the right results. Its purpose is to detect the presence of phenolic functional groups in a given sample. Its mode of activity is that phenol condenses on heating with phthalic anhydride in the presence of concentrated sulfuric acid which then forms phenolphthalein. Phenolphthalein gives a pink-colored compound when reacted with a limited amount of sodium hydroxide, whereas more sodium hydroxide produces a colorless compound.

5 ways to make the phthalein-dye test a more approachable topic

With these points in mind, here are five things you can build into your phthalein-dye test class to make them more engaging, approachable, and fun for you and your students.

1. Show the people behind the science

Adolf von Bayer, fully Johann Friedrich Wilhelm Adolf von Bayer, (born 31 October 1835 in Berlin, Prussia [now in Germany] – died 20 August 1917 in Starnberg near Munich, Germany), German research chemist who synthesized indigo (1880) and formulated its structure (1883). He received the Nobel Prize in Chemistry. Bayer studied with Robert Bunsen, but August Kekulé had a greater influence on his development. He received his doctorate in 1858 at the University of Berlin, became a private lecturer in 1860, and headed the chemistry laboratory at the Berlin Industrial Institute until 1872, where he founded the chemistry laboratory in which many young chemists were trained by later fame. In 1881 the Royal Society of London awarded him the Davy Medal for his work with indigo. His collection of scholarly works was published in 1905 for his 70th birthday. He not only invented indigo dye but also phthalein dye.

2. Relate it to the real world 

Phenylene dye was used as a laxative until it was banned because of its carcinogenic effect. The most widely used method for detecting liver damage in animals is sulfobromophthalein (BSP), an anionic phthalate dye. After intravenous administration, BSP remains in the systemic circulation; it is taken up by the liver and excreted in the bile. The use of BSP as a liver function test depends on delaying its clearance from the blood in the presence of liver dysfunction.

3. Explain the test in a way that is easy to understand

When the subject is as complex and abstract as the phthalein dye test, it is helpful to explain the material and procedures clearly.

Materials Required-

1. Test tube, 
2. Unknown organic compound,
3. Phthalic anhydride,
4. Concentrated Sulfuric acid, 
5. Sodium hydroxide, 
6. Distilled water, 
7. Burner, 
8. Oil bath, 
9. Pipette etc

Procedure: 

Phenolphthalein preparation:

1. Weigh 0.1 g each of phenol powder and phthalic anhydride powder into a test tube.
2. Dissolve the solid in sulfuric acid (solvent and catalyst).
3. Heat the solution in a water bath (to provide energy for the reaction). The product of this reaction is a phenolphthalein solution.

Phenolphthalein is used as a pH indicator with a color change from clear to light pink around pH = 8. From this, we can distinguish positive results. Observing the color change:

• Measure 15 mL of NaOH into a beaker.
• Using a pipette, add a few drops of the phenolphthalein solution to the beaker. 
• A positive result is indicated by a color change. If the reaction color in the mixture turns pink, red, green, or blue, then this indicates the presence of a phenol group in the given organic compound. The color of the solution and the pH at which this occurs depend on the phenol used initially. For example, alpha-naphthol produces naphtholphthalein, which changes color from pink to green at pH = 9.

Figure 4: The reactions in the phthalein-dye test. Top: Phenol reacts with phthalic anhydride in a heated acidic environment to make phenolphthalein. Bottom: Phenolphthalein changes in structure when the base is added, causing the color change. 

Some phenolic compounds and their color changes include:

• phenol = pink 
• m-cresol = blue 
• o-cresol = red, 
• Naphthol = green

Safety precautions:

1. Phenol is toxic and corrosive and must be handled with care.
2. Concentrated sulfuric acid must be handled with extreme care.
3. Some phenols are strong acids that contain an electron-withdrawing group and are also soluble in sodium bicarbonate solution

NB: while performing this procedure, protective gear must be worn to avoid accidental contact with the reagent.

4.  Make it stick with word-play

Memory aids can be very helpful in understanding complex topics. An example is

Pink Pants: Phenol stain test Pink

5. Highlight the use of virtual lab simulations

A unique way to teach phthalein-dye tests is through a virtual laboratory simulation. At Labster, we’re dedicated to delivering fully interactive advanced laboratory simulations that utilize gamification elements like storytelling and scoring systems inside an immersive and engaging 3D universe.

Check out the Labster phthalein-dye test simulation that allows students to learn about chemical reactions through active, inquiry-based learning. In the simulation, students will go on a mission to learn why chemicals are the color they are and put this all to use in the phthalein-dye test for phenols.