General Chemistry
pH Scale
Logarithmic measure of acidity — from very acidic (0) to very basic (14)
pH is a logarithmic measure of hydrogen ion concentration: pH = -log[H⁺]. Scale typically 0-14 for aqueous solutions. pH 7: neutral (pure water). pH < 7: acidic. pH > 7: basic. Each unit = 10× change in [H⁺]. Examples: stomach acid pH 1, lemon juice 2, coffee 5, pure water 7, baking soda 9, ammonia 11, household bleach 13. Critical for biology (blood pH 7.35-7.45, tightly regulated), agriculture (soil pH affects nutrients), industry (corrosion, processes). pOH analogous: pH + pOH = 14 at 25°C.
- pH formulapH = -log[H⁺]
- NeutralpH 7 (at 25°C; pure water)
- AcidicpH < 7
- BasicpH > 7
- Scale changeEach unit = 10× [H⁺] change
- Body pHBlood 7.35-7.45 (tightly regulated)
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Why pH matters
- Biology. Enzymes, proteins, blood pH critical.
- Agriculture. Soil pH affects nutrient availability.
- Aquaculture. Fish pH tolerance.
- Brewing. Yeast pH preferences.
- Industrial. Corrosion, processing.
- Pool chemistry. Comfort, equipment.
- Skincare. Skin pH ~5.5; soap effects.
Common misconceptions
- pH is linear scale. Logarithmic — each unit is 10×.
- Pure water always pH 7. pH 7 only at 25°C.
- pH only 0-14. Can extend beyond at extreme conditions.
- Neutral means no ions. [H⁺] = [OH⁻]; ions present.
- Acid pH = strong acid. Concentration matters.
- pH = acidity. Measures [H⁺]; relate but distinct.
Frequently asked questions
What is pH?
Negative logarithm of hydrogen ion concentration. pH = -log[H⁺] (or -log[H₃O⁺] more accurately). For [H⁺] = 10⁻⁷ M: pH = 7. Defined by Søren Sørensen (1909). Scale typically 0-14, but extreme conditions can go beyond. Logarithmic: pH 4 is 10× more acidic than pH 5; pH 3 is 100× more acidic than pH 5.
How is pH measured?
(1) pH meter — glass electrode measures voltage related to [H⁺]; most accurate. (2) Indicators — color changes with pH (litmus, universal indicator). (3) pH paper — strips with multiple indicators. (4) Modern: combination electrodes, ion-selective field-effect transistors. Calibrated using buffer solutions at known pH (e.g., 4.00, 7.00, 10.00).
What's pOH?
Analogous for hydroxide. pOH = -log[OH⁻]. At 25°C: pH + pOH = 14 (because Kw = [H⁺][OH⁻] = 10⁻¹⁴). Convenient for basic solutions: pH 12 = pOH 2 = [OH⁻] 0.01 M. Calculate pH from base concentration via pOH first if easier.
Why is blood pH so tightly regulated?
Enzymes are pH-sensitive. Optimal range 7.35-7.45 (slightly basic). Outside: enzyme function impaired. Acidosis (pH < 7.35): from CO₂ buildup, kidney failure, diabetes. Alkalosis (pH > 7.45): from hyperventilation, vomiting. Buffers (bicarbonate, phosphate, proteins) maintain pH. Even small deviations dangerous; pH < 7 or > 8 fatal.
What about strong vs weak acids?
Strong: completely dissociate. HCl 0.1 M → [H⁺] = 0.1 M → pH = 1. Weak: partial dissociation. CH₃COOH 0.1 M → [H⁺] ≈ 0.0013 M → pH ≈ 2.9 (much higher than strong). pKa = -log Ka measures dissociation strength. Strong acids: HCl, H₂SO₄, HNO₃ (pKa < 0). Weak: acetic, citric (pKa > 0).
How does pH affect biology?
(1) Enzymes have optimal pH (pepsin: pH 1.5-2; trypsin: pH 8). (2) Protein folding pH-dependent (ionizable side chains). (3) Drug absorption affected by stomach/intestinal pH. (4) Blood buffer system carbonic acid/bicarbonate maintains pH. (5) Cellular pH ~7.4 (cytoplasm); lysosomes pH 4.5; mitochondria 7.5-8.
What's pH of common substances?
Battery acid: 0. Lemon juice: 2. Vinegar: 3. Coffee: 5. Milk: 6.5. Pure water: 7. Blood: 7.4. Sea water: 8.1. Baking soda: 9. Ammonia: 11. Bleach: 12.5. Drain cleaner: 14. pH varies even among similar substances depending on dilution and composition.