Module 3: pH Notes

2.4 Acids and Bases

  • pH is a measure of hydrogen ion concentration in a solution.
  • When water ionizes or dissociates, it releases an equal number of hydrogen ions (H⁺) and hydroxide ions (OH⁻).
  • Acids are substances that dissociate in water, releasing hydrogen ions (H⁺).
  • Bases are substances that either take up hydrogen ions (H⁺) or release hydroxide ions (OH⁻).

The pH Scale

  • The pH scale is used to indicate the acidity or basicity (alkalinity) of a solution.
  • Values range from 0 to 14.
  • 0 to < 7 = Acidic
  • 7 = Neutral
  • > 7 to 14 = Basic (alkaline)
  • The scale is logarithmic: each unit change in pH represents a 10-fold change in hydrogen ion concentration.
  • Examples:
    • A solution with pH 4 is 10× more acidic than a solution with pH 5.
    • A solution with pH 5 is 100× more acidic than a solution with pH 7.
  • Expressions to relate pH and hydrogen ion concentration:
    • \text{pH} = -\log_{10}([H^+])
    • [H^+] = 10^{-\text{pH}}
  • Note on biological relevance: pH reflects acidity/basicity of the environment, which affects enzyme activity, protein folding, and metabolic processes.

Buffers and pH

  • A buffer is a chemical or a combination of chemicals that keeps pH within normal limits.
  • Maintaining pH is essential for the health of organisms; body fluids must stay within narrow pH limits.
  • Human blood is normally pH 7.4 (slightly basic).
  • If blood pH drops below 7.0, acidosis results.
  • If blood pH rises above 7.8, alkalosis results.
  • Example: the carbonic acid buffer system can dissociate and re-form to resist pH changes:
    • Dissociation/association reaction: H2CO3 \rightleftharpoons H^+ + HCO_3^-
  • Buffers work by neutralizing added acids or bases to minimize changes in pH.
  • Practical implications: Buffering systems are crucial in physiology, medicine, and environmental contexts where pH stability is necessary.

Connections and Implications

  • The pH scale is foundational for understanding biochemistry, physiology, and pharmacology.
  • Changes in pH can affect protein structure, metabolic pathways, and the function of ion channels and enzymes.
  • Acid-base homeostasis involves buffers (like the bicarbonate system), respiratory compensation, and renal regulation in humans.
  • Real-world relevance: medical conditions such as metabolic or respiratory acidosis/alkalosis illustrate the importance of pH balance for survival.

Quick reference: Key numbers and formulas

  • pH scale range: 0 \le pH \le 14
  • Neutral pH: pH = 7
  • Blood pH normal range (human): around pH \approx 7.35 \text{ to } 7.45 (note: the slide states 7.4 as normal)
  • Acidosis threshold: pH < 7.0
  • Alkalosis threshold: pH > 7.8
  • Hydrogen ion concentration relation:
    • [H^+] = 10^{-\text{pH}}
    • \text{pH} = -\log_{10}([H^+])
  • Carbonic acid buffer equilibrium: H2CO3 \rightleftharpoons H^+ + HCO_3^-

End notes

  • The material emphasizes how pH is measured, what constitutes acids and bases, how the pH scale is structured, and why buffers are essential for maintaining life-supporting pH levels in organisms.