6.1 Acid-Base Theories and pH

Acid-base theories are fundamental to understanding chemical reactions in biological systems. From Arrhenius to Lewis, these concepts explain how substances interact, donate or accept protons, and form bonds in aqueous solutions.

pH and pOH calculations quantify acidity and basicity, crucial for maintaining balance in living organisms. These measurements, along with Ka values and water autoionization, help us grasp the delicate equilibria that sustain life processes.

Acid-Base Theories

Acid-base theories

• Arrhenius theory defines acids as substances releasing H+ ions in water, bases release OH- ions (hydrochloric acid, sodium hydroxide)
• Brønsted-Lowry theory broadens definition: acids donate protons, bases accept protons, forming conjugate acid-base pairs (ammonia accepting H+ from water)
• Lewis theory further expands concept: acids accept electron pairs, bases donate electron pairs, forming coordinate covalent bonds (boron trifluoride accepting electrons from ammonia)

pH and pOH calculations

• pH measures acidity: negative log of H3O+ concentration $pH = -log[H_3O^+]$ (lemon juice pH ~2)
• pOH measures basicity: negative log of OH- concentration $pOH = -log[OH^-]$ (bleach pOH ~4)
• pH and pOH sum to 14 at 25℃, allowing calculation of one from the other
• Convert between concentration and pH/pOH using $[H_3O^+] = 10^{-pH}$ and $[OH^-] = 10^{-pOH}$ (blood pH 7.4 corresponds to [H3O+] = 3.98 × 10^-8 M)

pH, pOH, and Ka relationships

• Ka (acid dissociation constant) measures acid strength $Ka = \frac{[H_3O^+][A^-]}{[HA]}$ (acetic acid Ka = 1.8 × 10^-5)
• pKa (negative log of Ka) often used instead of Ka for convenience
• Henderson-Hasselbalch equation relates pH, pKa, and concentrations: $pH = pKa + log(\frac{[A^-]}{[HA]})$
• Buffers resist pH changes, composed of weak acid and conjugate base (phosphate buffer in blood)

Water autoionization in equilibria

• Water molecules react, producing H3O+ and OH- ions: $H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-$
• Kw (ion product of water) equals [H3O+][OH-], constant 1.0 × 10^-14 at 25℃
• Neutral solutions: [H3O+] = [OH-] = 1.0 × 10^-7 M, pH = 7 (pure water)
• Acidic solutions: [H3O+] > [OH-], pH < 7 (stomach acid pH ~2)
• Basic solutions: [H3O+] < [OH-], pH > 7 (baking soda solution pH ~9)