pH Scale Notes

pH Scale: Understanding Pondus Hydrogenii

Introduction to pH

  • The pH scale measures the potential hydrogen, or power of hydrogen, in a substance.
  • It is a logarithmic scale, based on powers of 10, similar to metric prefixes.

Logarithmic Nature of the pH Scale

  • The pH scale typically ranges from 0 to 14, although it can extend beyond these values.
  • Each step on the pH scale represents a tenfold change in hydrogen ion concentration.
  • Moving one unit on the scale indicates a change in hydrogen concentration by a factor of 10.

Understanding pH Values

  • Neutral pH:
    • Pure, distilled water has a neutral pH of 7.
    • This corresponds to a hydrogen ion concentration of 10^{-7} molar.
  • Acidic pH:
    • pH values less than 7 are acidic.
    • For example:
      • pH 6: 10^{-6} moles per liter of hydrogen ions
      • pH 5: 10^{-5} moles per liter of hydrogen ions
      • pH 4: 10^{-4} moles per liter of hydrogen ions
  • Basic (Alkaline) pH
    • pH values greater than 7 are basic.
    • For example:
      • pH 8: 10^{-8}
      • pH 9: 10^{-9}

Acids and Bases on the pH Scale

  • Strong Acids:
    • pH range of approximately 0 to 2.
  • Weak Acids:
    • pH range from just above 2 to just below 7. The closer to 7, the weaker the acid.
  • Strong Bases:
    • pH range of approximately 12 to 14.
  • Weak Bases:
    • pH range from just above 7 to just below 12. The closer to 7, the weaker the base.

Common Substances and Their pH Values

  • Acids:
    • Car battery (sulfuric acid): pH of 0
    • Stomach acid: pH of about 1
    • Lemon juice: pH of about 2
    • Vinegar: pH of about 3
    • Tomatoes, acid rain, orange juice: pH of about 4
    • Coffee: pH of about 5
    • Pure rain: pH of about 6 (slightly acidic due to dissolved CO_2)
    • Egg yolks, milk: Just barely acidic
  • Bases:
    • Blood, saliva, tears: pH of about 8
    • Seawater: Slightly basic
    • Baking soda: pH around 9
    • Ammonia: pH of about 11
    • Bleach: pH of about 13
    • Lye (sodium hydroxide): pH of 14, used to make soap

Equations for pH Calculation

  • Equation 1: Calculating pH from hydrogen ion concentration
    • pH = -\log[H^+] (where [H^+] is the hydrogen or hydronium concentration)
  • Equation 2: Calculating hydrogen ion concentration from pH
    • [H^+] = 10^{-pH}

Examples of pH Calculations

  • Blood pH:
    • pH of blood = 7.4
    • Hydrogen ion concentration: [H^+] = 10^{-7.4} = 3.98 \times 10^{-8} molar
  • Stomach pH:
    • pH of stomach acid = 2
    • Hydrogen ion concentration: [H^+] = 10^{-2} = 0.01 molar or 1 \times 10^{-2} molar

Comparison of Hydrogen Ion Concentrations

  • Comparison of hydrogen concentration in the stomach vs. bloodstream illustrates the power effect.
  • Stomach: 1 \times 10^{-2} molar
  • Bloodstream: 3.98 \times 10^{-8} molar
  • The vast difference demonstrates how sensitive our bodies are to pH and the importance of maintaining proper pH levels in different bodily systems.