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.
There are three major reasons:
The fact that students cannot visualize the processes, and not seeing their relevance to the real world can make studying it tough.
Acids have been defined in three ways namely:
Bases have three definitions that are basically the opposite of acid as follows:
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.
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?
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.
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.
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.
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.
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-
Procedure:
Phenolphthalein preparation:
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:
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:
Safety precautions:
NB: while performing this procedure, protective gear must be worn to avoid accidental contact with the reagent.
Memory aids can be very helpful in understanding complex topics. An example is
Pink Pants: Phenol stain test Pink
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.
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