pH Calculator
Calculate pH, pOH, and [H⁺] for any acid or base. Free online pH calculator with ion concentrations, acid-base classification, and visual charts for students.
About This Calculator
The pH Calculator computes pH, pOH, hydroxide ion concentration [OH⁻], and acid-base classification from hydrogen ion concentration [H⁺]. Simply enter the H⁺ concentration in mol/L to get the complete acid-base profile including the logarithmic pH value, the corresponding pOH, and whether the solution is acidic, basic, or neutral. This calculator is ideal for chemistry students, lab professionals, researchers, and anyone working with acid-base chemistry who needs quick and accurate pH calculations without manual logarithm computations.
The fundamental formula is pH = -log₁₀[H⁺], where [H⁺] is the molar concentration of hydrogen ions. At 25°C, pure water has a pH of exactly 7, which is considered neutral. The pH scale is logarithmic, meaning each whole unit represents a tenfold change in hydrogen ion concentration. From pH, we derive pOH = 14 − pH and [OH⁻] = 10⁻ᵖᴼᴴ using the ion product of water Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ at 25°C.
The Autoionization constant Kw varies with temperature: at 0°C, Kw ≈ 1.14×10⁻¹⁵ (neutral pH ≈ 7.47), while at 100°C, Kw ≈ 5.5×10⁻¹³ (neutral pH ≈ 6.14). Most pH meters include automatic temperature compensation (ATC) to correct for this variation. In environmental science, pH monitoring of natural water bodies helps determine pollution sources and ecosystem health. In medicine, blood pH is tightly regulated between 7.35 and 7.45 — deviations indicate acidosis or alkalosis.
Regional Notes
India (IN): pH measurements are widely used in water quality testing, soil analysis for agriculture, and effluent monitoring. The Bureau of Indian Standards (BIS) specifies pH ranges for drinking water (6.5–8.5).
United States (US): The EPA regulates pH in public water systems (6.5–8.5). pH testing is standard in wastewater treatment, food production (FDA), and clinical labs using blood gas analyzers.
United Kingdom (UK): The Drinking Water Inspectorate (DWI) mandates pH 6.5–9.5 for potable water. pH measurement is routine in the NHS for blood gas analysis and in environmental monitoring by the Environment Agency.
Frequently Asked Questions
What is pH?
pH = -log10[H⁺], where [H⁺] is hydrogen ion concentration in mol/L. pH 7 is neutral, below 7 is acidic, above 7 is basic.
What is the relationship between pH and pOH?
pH + pOH = 14 at 25 degC. This comes from Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ at 25 degC.
What is a strong acid vs weak acid?
Strong acids (HCl, H2SO4) dissociate completely. Weak acids (CH3COOH) partially dissociate, establishing equilibrium with Ka values.
What is the pH of common substances?
Stomach acid pH ≈ 2, lemon juice ≈ 2.5, vinegar ≈ 3, coffee ≈ 5, milk ≈ 6.5, blood ≈ 7.4, seawater ≈ 8, bleach ≈ 12.
How does temperature affect pH?
Kw increases with temperature. At 0 degC neutral pH ≈ 7.47; at 100 degC neutral pH ≈ 6.14. pH meters compensate for temperature.
How do you calculate pH from hydrogen ion concentration?
Take the negative base-10 logarithm of the H⁺ concentration: pH = -log₁₀[H⁺]. For example, if [H⁺] = 0.0001 mol/L, then pH = -log₁₀(0.0001) = 4.
What is a buffer solution and how does it resist pH changes?
A buffer solution contains a weak acid and its conjugate base (or weak base and its conjugate acid). It resists pH changes by neutralizing added acid or base. The Henderson-Hasselbalch equation pH = pKa + log([A⁻]/[HA]) describes buffer pH.
What pH range is safe for drinking water?
Drinking water should ideally have a pH between 6.5 and 8.5. In India, BIS specifies 6.5–8.5; in the US, the EPA recommends the same range; in the UK, DWI allows 6.5–9.5. Extremely low or high pH can indicate contamination or corrosion issues.