pH Calculator
Calculate pH, pOH, [H⁺], and [OH⁻] concentrations for acids and bases.
What Is the pH Calculator?
The pH Calculator converts between pH, hydrogen ion concentration [H⁺], hydroxide ion concentration [OH⁻], pOH, and acid/base dissociation constants (Ka and Kb). Enter any one value to instantly compute all related acid-base quantities for aqueous solutions at 25°C.
Formula
How to Use
Enter a pH value (0-14), hydrogen ion concentration in mol/L, hydroxide concentration in mol/L, or pOH. The calculator computes all related values and indicates whether the solution is acidic (pH < 7), neutral (pH = 7), or basic (pH > 7). Optionally enter Ka or Kb of an acid or base to find solution pH.
Example Calculation
pH = 3: [H⁺] = 10⁻³ = 0.001 mol/L. pOH = 14−3 = 11. [OH⁻] = 10⁻¹¹ mol/L. Acidic. pH = 11: [H⁺] = 10⁻¹¹, [OH⁻] = 10⁻³ = 0.001 mol/L. Basic. Stomach acid pH ≈ 2, blood pH ≈ 7.4.
Understanding pH
The pH scale is one of chemistry's most important measurement systems, describing the acidity or alkalinity of aqueous solutions. Introduced by Søren Peder Lauritz Sørensen in 1909, pH quantifies the concentration of hydrogen ions — the measure of how many H⁺ ions are present per liter of solution, expressed on a convenient 0-14 logarithmic scale.
The logarithmic nature of pH has important practical implications: blood pH must be maintained between 7.35 and 7.45 (a 0.1 unit range) for normal function — this seemingly small range actually represents a 25% change in H⁺ concentration. Enzyme activity, protein structure, chemical reaction rates, and solubility all depend critically on pH, making its measurement and control essential in chemistry, biology, and medicine.
pH control is critical in industrial processes (plating, water treatment, paper manufacturing), agriculture (soil pH affects nutrient availability; most crops prefer 6-7), food science (fermentation pH, food preservation, baking soda/acid reactions), medicine (blood pH disorders: acidosis pH < 7.35, alkalosis pH > 7.45), and aquariums and swimming pools where pH directly affects organism health and disinfection efficacy.
Frequently Asked Questions
What does pH mean?
pH = potential of hydrogen. It measures the concentration of hydrogen ions (H⁺) in solution on a logarithmic scale. pH 7 is neutral (pure water); below 7 is acidic; above 7 is basic (alkaline).
Why is the pH scale logarithmic?
Hydrogen ion concentrations in natural systems span 15 orders of magnitude (10⁻¹⁵ to 10⁰ mol/L). A logarithmic scale compresses this enormous range to a convenient 0-14 scale. Each unit change in pH represents a 10× change in [H⁺].
What is the pH of common substances?
Battery acid: pH 0. Stomach acid: pH 2. Lemon juice: pH 2-3. Coffee: pH 5. Pure water: pH 7. Blood: pH 7.4. Baking soda: pH 9. Milk of magnesia: pH 10. Bleach: pH 12-13.
Why does pH + pOH = 14?
At 25°C, the water dissociation constant Kw = [H⁺][OH⁻] = 10⁻¹⁴. Taking −log₁₀ of both sides: pH + pOH = 14. This relationship only holds at 25°C; at higher temperatures, Kw increases and the sum changes.
Is this calculator free?
Yes, completely free with no account required.