Mechanochemistry with Phenolphthalein and Base

Co-Authored by Dean J. Campbell*, Ali Patel*

*Bradley University, Peoria, Illinois

Phenolphthalein has been used extensively as a pH indicator.¹ Although the compound actually can exhibit multiple color transitions, it is best known for turning from colorless to pink in aqueous solution as the pH is raised above 8.3.¹ The equilibrium associated with this transition is shown in Figure 1.

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Figure 1. Structures of (LEFT) colorless phenolphthalein and (RIGHT) the pink deprotonated form of the indicator.

Phenolphthalein will also change color when ground together with base in dry conditions. It is important to note that “dry” here refers to samples that are in contact with ambient air that contains some water vapor. The color change reactions take place with little or no water available as solvent – there are no obvious solutions involved. The base chosen to demonstrate this reaction in this context is magnesium hydroxide. Since the dry reaction occurs when the reactants are ground together, this dry reaction then becomes a simple visual demonstration of mechanochemistry. Mechanochemistry can be described as “chemical synthesis enabled or sustained by mechanical force”.² Simply mixing the powdered compounds together, e.g., by gentle shaking, does not produce the chemical reaction that causes the indicator color to change. There has to be more energy imparted by grinding or collisions to produce the desired result.

Demonstrations

In one version of the demonstration, white solid phenolphthalein and white solid magnesium hydroxide are ground together with a mortar and pestle. The initially white solid turns to a pink color. It appears that 0.2 grams of each compound are sufficient for the demonstration. After grinding, addition of water as solvent to the mixture at least partially dissolves the mixture, producing a more intense pink color. Figure 2 and Video 1 illustrates this demonstration.

grinding_phenolphthalein_.png

Figure 2. Grinding solid phenolphthalein and solid magnesium hydroxide together using a mortar and pestle. (LEFT) Before grinding, the compounds are white. (MIDDLE) After grinding, the mixture is pink. (RIGHT) After addition of water, the mixture becomes a darker pink.

Video 1. Grinding solid phenolphthalein and solid magnesium hydroxide together using a mortar and pestle. ChemDemos YouTube Channel (accessed 7/22/2024).

In another version of the demonstration, white solid phenolphthalein and white solid magnesium hydroxide are added to an empty, dry 12 oz. (355 mL) polyethylene terephthalate soda bottle. The mixture stays white until iron spheres are added and bottle is capped and vigorously shaken. The initially white solid turns to a pink color. The demonstration described in Figure 3 and Video 2 used 0.5 g of phenolphthalein, 0.5 g of magnesium hydroxide, and five half-inch (12.7 mm) iron spheres. Longer shaking time produces darker colors, e.g., shaking for 50 seconds produces a darker shade of pink than 25 seconds of shaking. After shaking, addition of water to the mixture as a solvent at least partially dissolves the mixture, producing an even more intense pink color.

grinding_with_soda_bottle.png

Figure 3. Grinding solid phenolphthalein and solid magnesium hydroxide together using iron spheres in a shaken soda bottle. (LEFT) Before grinding, the compounds are white. (RIGHT) After grinding, the mixture is pink.

Video 2. Grinding solid phenolphthalein and solid magnesium hydroxide together using iron spheres in a shaken empty soda bottle. ChemDemos YouTube Channel (accessed 7/22/2024).

Comparing the utility of the demonstrations, a colorful reaction mixture smeared across the open bowl of a mortar might be more visible to spectators than a powder in a plastic bottle. A mortar can also be more robust and likely more reusable than a plastic bottle. On the other hand, the mortar and pestle system is more open to the surroundings than the plastic bottle and thus chemical species might have more opportunity to escape the container.

Discussion

These simple demonstrations can be connected to concepts associated with acids, bases, and indicators. Because they are mechanochemical processes, they can be connected to the field of Green Chemistry. One of the Twelve Principles of Green Chemistry is Safer Solvents and Auxiliaries.³ There is an increase in interest in reactions using mechanochemistry because they do not require solvents during the reaction.² Having no or at least less solvents to purchase, purify, store, dispose, etc. are attractive prospects. In the phenolphthalein and base mechanochemistry demonstrations, there are no solvents such as water to be used during the reactions. Water or other solvents are also not necessary to recover the raw product. Solvents such as water and/or alcohols could still potentially be used to clean up the demonstration equipment. These demonstrations also provide a springboard for further discussions of concepts of toxicology, environmental chemistry, and Green Chemistry. Two additional Principles of Green Chemistry are Designing Safer Chemicals and Design for Degradation.³ It would be desirable to find the least hazardous alternatives to phenolphthalein and magnesium hydroxide for use and disposal that still show a visible color change upon grinding together. Some considerations associated with these compounds include the fact that phenolphthalein has been used internally in laxatives, but cancer concerns have led to decreases in this application for the compound.¹ Also, various base species can react with phenolphthalein to produce a color change, but magnesium hydroxide is sufficiently safe to be ingested as an aqueous suspension for use as an antacid and laxative. The hazard statements given in the Safety section below suggest that there is room for improvement with respect to chemical safety.

It also seems that there might be an opportunity to use mechanochemistry in off-Earth settings. In space, or on the Moon, Mars, or other worlds, solvents such as water could be incredibly costly to acquire, transport, and store. Being able to employ at least some processes using mechanochemistry could be an attractive alternative to solvents in these challenging environments.  

Safety. Precautions, including proper personal protective equipment such as goggles and gloves, should be used when working with these demonstrations. Avoid spilling the compounds on clothing. Always wash your hands after completing the experiments and demonstrations.

PubChem gives the following GHS hazard statements regarding phenolphthalein (CAS# 77-09-8):⁴

• H341 Suspected of causing genetic defects (Warning: Germ cell mutagenicity)
• H350 May cause cancer (Danger: Carcinogenicity)
• H361 Suspected of damaging fertility or the unborn child (Reproductive toxicity)

PubChem give the following GHS hazard statements regarding magnesium hydroxide (CAS# 1309-42-8):⁵

• H302 Harmful if swallowed (Warning: Acute toxicity, oral)
• H315 Causes skin irritation (Warning: Skin corrosion/irritation)
• H317 May cause allergic skin reaction (Warning: Sensitization, skin)
• H318 Causes serious eye damage (Danger: Serious eye damage/irritation)
• H319 Causes serious eye irritation (Warning: Serious eye damage/irritation)
• H332 Harmful if inhaled (Warning: Acute toxicity, inhalation)
• H335 May cause respiratory irritation (Warning: Specific target organ toxicity, single exposure; respiratory tract irritation)

Acknowledgements This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium.

References

1. May, P. Molecule of Month: Phenolphthalein. https://www.chm.bris.ac.uk/motm/phenolphthalein/phphh.htm (accessed July 22, 2024).
2. Do, J.-L.; Friščić, T. “Mechanochemistry: A Force of Synthesis.” ACS Cent. Sci., 2017, 3, 13-19. 
3. Compound Interest. The Twelve Principles of Green Chemistry: What it is, & Why it Matters. https://www.compoundchem.com/2015/09/24/green-chemistry/ (accessed July 22, 2024).
4. National Library of Medicine. PubChem. Phenolphthalein (Compound). https://pubchem.ncbi.nlm.nih.gov/compound/4764#section=Safety-and-Hazards (accessed July 22, 2024).
5. National Library of Medicine. PubChem. Magnesium Hydroxide (Compound). https://pubchem.ncbi.nlm.nih.gov/compound/73981#section=Safety-and-Hazards (accessed July 22, 2024).