Overview
This lecture explains the concept of pH, how it measures acidity and basicity in aqueous solutions, and why buffers are important for biological systems.
The pH Scale and Measurement
- pH measures the acidity or basicity of a water-based solution through hydronium ion (H₃O⁺) concentration.
- The pH scale converts hydronium concentration into the negative base 10 logarithm, simplifying wide concentration ranges.
- The pH scale usually ranges from 0 (most acidic) to 14 (most basic/alkaline), with 7 as neutral.
- Pure water has a neutral pH of 7, meaning equal concentrations of hydronium and hydroxide ions.
- Substances with pH below 7 are acidic, donating protons to the solution.
- Substances with pH above 7 are alkaline (basic), accepting protons and lowering hydronium concentration.
Examples of Acidic and Alkaline Solutions
- Acidic examples: coffee (pH ~4.5), lemon juice (pH ~2.5), gastric acid (pH ~1.5).
- Alkaline examples: saltwater (pH ~8), soapy water (pH ~12).
- Some substances, like carborane, can have extreme pH values beyond the typical range.
Biological Importance of pH and Buffers
- Most biological functions require near-neutral pH (about 7.35–7.45).
- Blood pH (~7.4) is stabilized by a buffer system including carbonic acid, bicarbonate ions, and carbon dioxide.
- Bicarbonate buffers also protect the intestine from acidic chyme during digestion.
Key Terms & Definitions
- pH — Negative logarithm (base 10) of hydronium ion concentration; measures acidity/basicity.
- Acidic — Solution with pH below 7; high hydronium ion concentration; donates protons.
- Alkaline (Basic) — Solution with pH above 7; low hydronium ion concentration; accepts protons.
- Buffer — Solution that minimizes pH changes by neutralizing added acids or bases.
- Hydronium ion (H₃O⁺) — Ion representing acidic hydrogen in water.
- Hydroxide ion (OH⁻) — Ion representing basic character in water.
Action Items / Next Steps
- Review the chemical equation for pH: pH = -log₁₀[H₃O⁺].
- Study common buffer systems in the human body.
- Prepare examples of solutions and estimate their pH.