Acids, Bases, and Indicators: Unveiling the Science Behind Colourful Chemistry

Acid-base indicators are essential tools in chemical analysis, used to determine the acidity or basicity of a solution by undergoing distinct color changes at specific pH levels.1 These indicators are either weak acids or weak bases that exhibit different colors in their protonated and deprotonated forms, making them invaluable in laboratory experiments, pharmaceutical formulations, and industrial applications.2 In chemical analysis, precise identification of pH is crucial for various applications, including drug formulation, quality control, and titration experiments. Indicators provide a quick and cost-effective method to assess pH changes without the need for sophisticated instruments. They are widely used in pharmaceuticals to ensure drug stability, in food industries for quality control, and even in environmental monitoring to detect pollution levels.3. Acid-base indicators function based on the principles of chemical equilibrium. When an indicator is added to a solution, it establishes an equilibrium between its acid and base forms. As the pH of the solution changes, the ratio of these forms shifts, leading to a visible color transition. For instance, phenolphthalein turns pink in basic solutions but remains colorless in acidic conditions, making it a popular choice for titrations.4 Similarly, litmus changes from red in acidic solutions to blue in basic solutions, offering a simple yet effective way to test pH. Beyond these classical indicators, the concept of olfactory indicators adds an intriguing dimension to acid-base analysis.5 Certain substances, like vanilla or onion extract, change their smell in acidic and basic conditions, providing an alternative sensory approach to pH detection. The study of acid-base indicators is deeply rooted in the theories of acid-base chemistry, including Arrhenius, Brønsted-Lowry, and Lewis theories, which explain how acids and bases interact with these indicators.6 Their versatility extends beyond the laboratory, playing a significant role in medical diagnostics, pharmaceuticals, and industrial applications. This article delves into the fascinating world of acid-base indicators, exploring their classification, mechanisms, and real-world applications, offering insights that are essential for both undergraduate and postgraduate pharmacy students.  

Classification of Indicators

1. Natural Indicators: These are derived from plant sources and change color based on the pH of the solution.^7,8 For example, Litmus, extracted from lichens, turns red in acidic solutions and blue in basic solutions, making it a classic pH indicator used in laboratories. Turmeric, a kitchen staple, remains yellow in acidic conditions but turns reddish-brown in basic environments, commonly observed when soap comes in contact with turmeric stains. Red cabbage extract is a natural, eco-friendly pH indicator that changes color based on acidity or basicity due to anthocyanins. It turns reddish-pink in acids, purple in neutral solutions, and green to yellow in bases.
2.  Synthetic Indicators: These are laboratory-prepared compounds with distinct and sharp color changes at specific pH ranges.⁸ Example: Phenolphthalein is colorless in acidic solutions but turns pink in basic solutions, making it widely used in titrations involving strong bases like sodium hydroxide. Methyl orange shifts from red in acidic conditions to yellow in basic solutions, making it useful in detecting endpoint transitions in acid-base titrations.
3. Olfactory Indicators: These indicators work by altering their smell rather than their color in acidic or basic environments.⁸ Example: Onion extract loses its pungent smell in basic solutions but retains it in acidic conditions, offering a unique way to detect pH changes. Vanilla essence loses its characteristic sweet aroma in alkaline solutions, making it another unconventional but effective acid-base indicator. Clove loses its characteristic smell in basic solutions but retains it in acidic conditions, making it an effective olfactory indicator.

6. Theories of Acid-Base Indicators

Ostwald’s Theory: As per Ostwald's theory, acid-base indicators work using the concept of ionization and color change.^9,10 According to this theory, indicators are typically weak acids or weak bases that exist in equilibrium between their ionized and unionized forms. The unionized form (HIn) and its ionized form (In?) have different colors. The color change usually occurs at a particular pH, known as the _pKa of the indicator.

• In acidic solutions:           Hin ? H⁺+In−
  Excess H⁺ shifts the equilibrium left, showing the colour of HIn.
• In basic solutions: InOH ? In⁺ + OH⁻
  Excess OH⁻ shifts the equilibrium right, showing the color of In⁺.
• For example, phenolphthalein is colorless in acidic medium (unionized form) and pink in basic medium (ionized form). Methyl Orange is red in acidic medium and yellow in basic medium.

Quinonoid Theory: According to this theory, the color change is due to the structural transformation of the indicator molecule, specifically the formation of a quinonoid structure.^9,10 The Quinonoid theory provides a structural explanation for indicator color changes by highlighting the shift between benzenoid and quinonoid forms. This theory complements Ostwald’s theory by adding a molecular perspective.

As per this theory, acid-base indicators exist in two tautomeric forms- Benzenoid form is often colorless or lightly colored and Quinonoid form is intensely colored. When the pH changes, the structure shifts between the benzenoid and quinonoid form. The quinonoid form has a conjugated double-bond system, which absorbs visible light, resulting in a color change. In an acidic medium, the indicator may predominantly exist in the benzenoid form, while in a basic medium, it may shift to the quinonoid form. For example, Phenolphthalein Benzenoid form is Colorless in acidic medium, while its Quinonoid form is pink in basic medium. The quinonoid form of Methyl Orange is red in acidic medium, while its benzenoid form is yellow in basic medium.

7. Applications of Indicators

A. Daily Life

Acid-base indicators are not just for laboratories, they play a significant role in various aspects of our daily lives.^11-13 Indicators are practical tools that simplify decision-making in homes, agriculture, healthcare, and environmental monitoring. Details were provided in Table 1.

B. Identification of Chemicals

Indicators are widely used to identify chemicals, particularly acids, bases, and neutral substances. They help detect the nature of a substance by showing a visible color change when added to a solution.^8,9 Indicators offer a quick and reliable way to identify acids, bases, and neutral substances. Whether in labs, industries, or homes, they make chemical detection simple and effective (Table 2).

C. Food Industry

Indicators play a crucial role in the food industry to monitor quality, safety, and freshness. From detecting spoilage to ensuring pH balance, they provide quick and reliable results. ^{12, 14} From ensuring safety to enhancing product quality, indicators are essential in the food industry (Table 3). They offer a simple yet effective way to maintain consumer trust and satisfaction.

Indicators play a significant role in the chemical industry by ensuring accurate monitoring, quality control, and safety. They are essential for detecting changes in pH, chemical reactions, and the presence of specific compounds. ^{15, 16} Whether it’s monitoring reactions or detecting pollutants, they keep operations running smoothly (Table 4).

E. Pharmaceutical Industry

In the pharmaceutical industry, indicators play a vital role in quality control, product formulation, and research. They provide visual confirmation of chemical changes, ensuring safety and efficacy in drug development. Indicators ensure pharmaceutical products meet strict safety and quality standards.^{17, 18} From formulation to final testing, they’re a small but powerful tool in drug manufacturing (Table 5).

F. Other Applications

Apart from their applications in daily life, chemical identification, the food industry, the chemical industry, and the pharmaceutical industry, indicators are used in various other fields. Indicators are versatile tools across multiple industries, providing essential insights for safety, quality, and research purposes.^19-21 Here are some notable applications mentioned under Table 6.