pH Scale, Importance, and Indicators

pH Scale  

- The pH scale ranges from 0 to 14 and is used to determine the strength of acids and bases.  

- Strong acids: pH 1–4  

- Weak acids: pH 5–6  

- Neutral: pH 7  

- Weak bases/alkalis: pH 8–10  

- Strong bases/alkalis: pH 11–14  

Importance of pH  

Maintaining the correct pH is essential for proper functioning of systems:  

- Small intestine: Maintained at an alkaline medium for effective digestion.  

- Soil: Optimal pH for plant growth is 7–8.  

  - If soil becomes too acidic, it is treated with slaked lime (Ca(OH)₂) to neutralize acidity.  

Indicators  

- Definition: Indicators are weak organic acids or bases that change color depending on the pH of the solution.  

- Application: Used to determine whether a substance is acidic or alkaline.  

Examples of Indicators:  

- Methyl Orange  

- Phenolphthalein  

- Litmus  

- Methyl Red  

- Congo Red  

Color Changes:  

- Methyl Orange:  

  - Acidic medium → Red/Pink  

  - Basic medium → Yellow  

- Phenolphthalein:  

  - Acidic medium → Colorless  

  - Basic medium → Red/Pink  

Definitions  

- pH:  

  pH = -log H+ 

  Defined as the negative logarithm of the hydrogen ion (or hydronium ion) concentration of a solution.  

- pOH:  

  pOH = -log OH-

  Defined as the negative logarithm of the hydroxide ion concentration of a solution.