pH & Buffer Solution Calculator

pH & Buffer Guide

pH Basics

pH = −log₁₀[H⁺]. Where [H⁺] is the hydrogen ion concentration in mol/L. Scale: 0-14 in water. Below 7: acidic. Equal 7: neutral (pure water at 25°C). Above 7: alkaline/basic. Each pH unit = 10× change in [H⁺]. pH 4 has 10× more H⁺ than pH 5, 100× more than pH 6. Strong acids (HCl, HNO₃, H₂SO₄) fully dissociate: pH = −log(concentration). 0.1 M HCl: pH = 1. 0.01 M HCl: pH = 2. Strong bases (NaOH, KOH) fully dissociate: pOH = −log(concentration). pH = 14 − pOH. 0.1 M NaOH: pH = 13. Weak acids/bases

Weak Acids and Ka

Weak acids only partially ionise: HA ⇌ H⁺ + A⁻. Acid dissociation constant: Ka = [H⁺][A⁻] / [HA]. pKa = −log(Ka). Common pKa values: acetic acid (vinegar): 4.76. Carbonic acid: 6.35 (first dissociation). Phosphoric acid: 2.15, 7.20, 12.35 (three protons). Citric acid: 3.13, 4.76, 6.40. For weak acid pH: pH ≈ ½(pKa − log[HA]) — approximation works when [HA] >> Ka. Example: 0.1 M acetic acid: pH ≈ ½(4.76 − log 0.1) = ½(4.76 + 1) = 2.88. Verify: 0.1 M acetic acid pH measured ≈ 2.87 ✓

Buffers and Henderson-Hasselbalch

Buffer: solution that resists pH change when small amounts of acid or base are added. Consists of weak acid + its conjugate base (or weak base + its conjugate acid). Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). Where [A⁻] = conjugate base concentration, [HA] = weak acid concentration. Buffer pH = pKa when [A⁻] = [HA]. Most effective buffering range: pKa ± 1. Examples: acetate buffer (acetic acid + sodium acetate): effective pH 3.76-5.76. Phosphate buffer (H₂PO₄⁻ + HPO₄²⁻): effectiv

Biological pH Regulation

Blood pH: maintained at 7.35-7.45. Tiny range — outside it causes serious illness or death. Below 7.35: acidosis. Above 7.45: alkalosis. Buffer systems in blood: carbonic acid/bicarbonate: HCO₃⁻ + H⁺ ⇌ H₂CO₃ ⇌ CO₂ + H₂O. Lungs control CO₂ removal. Kidneys regulate bicarbonate. Phosphate buffer: minor blood role, important in intracellular fluids and urine. Protein buffer: amino acid side chains (especially histidine) buffer within proteins and haemoglobin. Cellular pH: tightly regulated. Lysosom