Why a Titrated Fuschia Sample Turns Colorless When Breath is Blown Into It

Phenolphthalein, a commonly used pH indicator, is renowned for its ability to signal the neutralization of an acid with a base by turning colorless samples fuchsia (a light, pinkish-red). However, a common observation in chemistry experiments is that when a titrated fuschia sample is exposed to one's breath, it turns colorless. This phenomenon is not merely a coincidence; it is rooted in the chemistry of carbonic acid formation due to the presence of carbon dioxide in the inhaled breath. In this article, we will delve into the science behind this intriguing observation and explore how carbon dioxide induces a reversal of the pH change previously achieved.

Understanding Phenolphthalein and Titration

Phenolphthalein is a weak acid derivative of phthalic acid and functions as a pH indicator within a specific pH range, typically from pH 8.2 to 10.0. When in an acidic environment or below this pH range, phenolphthalein remains colorless. Conversely, when the pH exceeds the upper limit of the range, the indicator shifts to a pink color, becoming visible to the naked eye.

Titration is a fundamental technique in analytical chemistry used to determine the concentration of a substance in solution. In this context, it involves gradually adding a solution of known concentration (standard solution) to a solution of unknown concentration (sample solution) until the equivalence point is reached. The endpoint of the titration is visually identified by the indicator's color change, signifying the completion of the chemical reaction.

The Role of Carbon Dioxide in the Reaction

When you exhale onto a titrated fuschia sample, the carbon dioxide in your breath interacts with the water in the sample. This interaction sets off a series of chemical reactions that can potentially reverse or partially reverse the neutralization reaction that was previously established. To understand this in detail, let's break down the steps involved:

Inhalation and Exhalation: The process of breathing involves taking in oxygen and exhaling a mixture of air, carbon dioxide, and other trace gases. The exhaled air contains up to 5% to 6% carbon dioxide, depending on the individual and their state of health. Carbon Dioxide and Water: Upon breathing onto the phenolphthalein solution, the carbon dioxide from your exhalation comes into contact with the water in the sample. This sets in motion a chemical reaction involving the formation of carbonic acid (H2CO3) according to the following equation:

H2O CO2 → H2CO3

Formation of Carbonic Acid: The carbonic acid (H2CO3) formed in the solution is a weak acid, meaning it is partially ionized in water. The ionization of carbonic acid can be expressed as:

H2CO3 ? HCO3- H

Acid-Base Reaction Reversal: As carbonic acid forms, it can react with the base present (phenolphthalein in its base form) to re-establish an acidic environment. This process can be described as: H2CO3 OH- → H2O HCO3- H2O H2CO3 → H2CO3 HCO3-

These reactions ultimately result in a decrease in pH, causing the pink color of phenolphthalein to disappear. The extinguishing of the color signalizes the re-establishment of an acidic environment in the solution.

Enhancing the Experiment with Baking Soda

To better understand the underlying chemistry and the effect of breathing on the titration, you can conduct a complementary experiment using baking soda (sodium bicarbonate, NaHCO3). Baking soda is a base known for its ability to neutralize acids. When you mix baking soda with water, it forms a basic solution that can be used to reverse a previously titrated fuschia phenolphthalein solution to its original colorless state. This experiment not only reinforces the principles of acid-base reactions but also provides a clear visual representation of the titration process.

Mixing Baking Soda and Water: Dissolve a small amount of baking soda in water to create a solution with a noticeable basic taste. The concentration of the solution should be sufficient to bring the pH above the range where phenolphthalein is colorless. Application to the Titrated Solution: Gently add the baking soda solution to the titrated fuschia sample. As the basic solution is introduced, the pink color of phenolphthalein should gradually return, signaling the restoration of a basic pH environment.

Conclusion and Further Experiments

The phenomenon of a titrated fuschia sample turning colorless when breath is blown into it is a fascinating example of the interplay between acid-base chemistry and respiratory physiology. By understanding the role of carbon dioxide and the formation of carbonic acid, we can appreciate the dynamic nature of pH changes in various solutions. Furthermore, the use of baking soda in such experiments not only enhances our comprehension of acid-base reactions but also provides a practical method for visualizing these complex interactions.

For interested readers and students of chemistry, these experiments can be expanded to include a variety of scenarios, such as examining the effect of varying concentrations of carbon dioxide or the influence of other gases (like ammonia) on the pH of solutions. Such explorations can deepen one's insight into chemical kinetics, equilibrium, and the principles of titration.