The pH Scale

The pH scale (potential of hydrogen) tells how acidic or basic a solution is.

The pH scale transforms an invisible sea of ions into a simple number. It measures the concentration of hydrogen ions (H⁺) — specifically hydronium (H₃O⁺) — in water.

The scale runs from 0 to 14 and is logarithmic, meaning each whole number represents a tenfold change in acidity.

• Low pH (0–6): Acidic → More H⁺ ions.
• pH 7: Neutral → Pure water.
• High pH (8–14): Basic (alkaline) → Fewer H⁺ ions, more OH⁻ ions.

The Formula (Simplified)

pH = –log₁₀[H⁺]

Because of the negative log, a high H⁺ concentration gives a low pH. For example:

• [H⁺] = 0.1 M (10⁻¹) → pH = 1 (very acidic)
• [H⁺] = 0.0000001 M (10⁻⁷) → pH = 7 (neutral)

Thus, a solution at pH 3 is 10 times more acidic than one at pH 4, and 100 times more than pH 5.

Common pH Values in Everyday Life

How Do We Measure pH?

• Litmus paper: Turns red in acid, blue in base. Quick but imprecise.
• pH indicator solutions: Liquids that change color across specific ranges (e.g., phenolphthalein turns pink above pH 8).
• pH meter: An electronic probe that gives a precise digital reading — standard in labs.

Why Does pH Matter?

• Biology: Human blood stays tightly between pH 7.35–7.45. A shift of 0.2 can be dangerous. Stomach acid (pH ~1.5) aids digestion.
• Agriculture: Most crops prefer slightly acidic soil (pH 6–7). Too low? Lime (a base) raises it. Too high? Sulfur lowers it.
• Industry: Swimming pools need pH 7.2–7.8 to prevent eye irritation and protect equipment. Wastewater treatment adjusts pH to precipitate heavy metals.
• Oceans: Rising CO₂ makes seawater more acidic (ocean acidification), threatening coral and shellfish.