pH/pOH Calculator

pH and pOH: pH = -log₁₀[H⁺]

pH measures how acidic or basic an aqueous solution is: pH = -log₁₀[H⁺], with [H⁺] in mol/L. The scale typically runs from 0 to 14, but very strong acids can go below 0 and very strong bases above 14. At 25 °C, pure water has [H⁺] = 10⁻⁷ → pH = 7 (neutral); acidic solutions have pH < 7 and basic ones pH > 7. The companion is pOH = -log₁₀[OH⁻], and at 25 °C they obey pH + pOH = 14. Each whole pH unit means a 10× change in [H⁺]: pH 4 is 10× more acidic than pH 5 and 100× more acidic than pH 6. Example: [H⁺] = 10⁻⁴ → pH = 4 (acidic, similar to black coffee). Reference values: soft drinks ≈ 2.5; stomach acid 1.5-3.5; blood is buffered to 7.35-7.45 (acidosis below 7.35 is dangerous); soap 9-10; household bleach ≈ 13. Indicators (litmus paper, phenolphthalein) and pH meters are used to measure it; in physiology, the bicarbonate buffer (HCO₃⁻/CO₂) keeps blood pH stable.

Applications

Drinking water treatment (in Brazil, ANVISA 888/2021 requires pH 6.0-9.5), aquaculture and fish farming, agriculture (soil correction with lime to raise pH), winemaking and brewing, swimming pool maintenance (ideal 7.2-7.6 to keep chlorine effective and avoid eye irritation), pharmaceuticals, and chemical industry process control.

FAQ

Can pH be negative or above 14? Yes. Very concentrated strong acids (e.g., 12 M HCl) can show negative pH, and very concentrated strong bases can exceed 14. The 0-14 range is just typical for dilute aqueous solutions.

Does pH + pOH = 14 always hold? Only at 25 °C. The relation comes from Kw (water's ion product), which changes with temperature: at 100 °C, Kw rises and the sum drops to about 12.

What's a buffer solution? A mixture (weak acid + its conjugate base) that resists pH changes when small amounts of acid or base are added. Blood uses the HCO₃⁻/CO₂ buffer to stay near 7.4.