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 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.

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.

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.

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^-

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.