Acid-Base Titration: Principle, Uses, Types, and Drug Examples

Acid-base titration is one of the most fundamental and widely used analytical techniques in pharmaceutical and chemical sciences. It is simple, accurate, and highly useful for determining the strength, purity, and content of acids and bases, including many drug substances.

Principle of Acid-Base Titration

The principle of acid-base titration is based on the neutralization reaction between an acid and a base.

Acid + Base → Salt + Water

• In this process, a solution of known concentration (titrant) is added to a solution of unknown concentration (analyte) until neutralization occurs.
• The endpoint of the titration is detected using an indicator or a pH meter, which shows a sudden change in color or pH.
• The equivalence point is the theoretical point where the amount of titrant is exactly equal to the amount of analyte.

Indicators in Acid-Base Titration

Indicators are substances that change color at or near the equivalence point.

• Phenolphthalein → Colorless in acidic solution, pink in alkaline solution (used in strong acid vs strong base titration).
• Methyl orange → Red in acidic solution, yellow in alkaline solution (used in strong acid vs weak base titration).
• Bromothymol blue → Yellow in acidic solution, blue in alkaline solution (used in strong acid vs strong base titration).

Selection of indicator depends on the pH range of equivalence point.

Types of Acid-Base Titrations

Depending on the nature of acid and base involved, acid-base titrations are classified as:

1. Strong Acid vs Strong Base
   • Example: HCl vs NaOH
   • Sharp pH change → Phenolphthalein, Methyl orange can be used.
2. Strong Acid vs Weak Base
   • Example: HCl vs NH₄OH
   • Equivalence point in acidic range → Methyl orange preferred.
3. Weak Acid vs Strong Base
   • Example: Acetic acid vs NaOH
   • Equivalence point in alkaline range → Phenolphthalein is suitable.
4. Weak Acid vs Weak Base
   • Example: Acetic acid vs Ammonium hydroxide
   • No sharp pH change → Indicators are unsuitable, pH meter is used.

Uses of Acid-Base Titration in Pharmacy

1. Assay of Drugs
   • Standardization of acidic or basic drug solutions.
   • Estimation of drug content in formulations.
2. Purity Testing
   • Determination of impurities that act as acids or bases.
3. Identification
   • Confirming the presence of functional groups (acidic or basic).
4. Standardization of Reagents
   • NaOH, HCl, H₂SO₄, and other volumetric solutions are standardized by titration.

Drug Examples Assayed by Acid-Base Titration

• Acidic drugs:
  • Ascorbic acid (Vitamin C)
  • Aspirin (Acetylsalicylic acid)
  • Sulfonamides
• Basic drugs:
  • Atropine sulfate
  • Ephedrine hydrochloride
  • Codeine phosphate
• Other examples:
  • Sodium bicarbonate (as a base)
  • Magnesium hydroxide in antacids

Conclusion

Acid-base titration remains a cornerstone in pharmaceutical analysis due to its simplicity, accuracy, and reliability. By choosing the correct indicator and titration type, it can be effectively applied to assay numerous acidic and basic drugs, ensuring their quality and safety for therapeutic use.

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