pH Calculators

What Is pH?

pH stands for potential of hydrogen and is defined as the negative base-10 logarithm of the hydrogen ion concentration in a solution:

pH = −log₁₀[H⁺]

Where [H⁺] is the molar concentration of hydrogen ions (or hydronium ions, H₃O⁺) in mol/L. Because the scale is logarithmic, a solution at pH 3 has 10× more H⁺ than a solution at pH 4, and 100× more than pH 5.

The pH Scale

• pH 0–6.9: Acidic — more H⁺ than OH⁻ ions (e.g., stomach acid ~pH 1.5, lemon juice ~pH 2.5, coffee ~pH 5)
• pH 7.0: Neutral — equal concentrations of H⁺ and OH⁻ (pure water at 25°C)
• pH 7.1–14: Basic (alkaline) — more OH⁻ than H⁺ (e.g., blood ~pH 7.4, seawater ~pH 8.1, bleach ~pH 12.5)

How to Calculate pH

Strong Acids

Strong acids fully dissociate in water. For a 0.01 M HCl solution: [H⁺] = 0.01 M → pH = −log(0.01) = 2.

Strong Bases

First calculate pOH: pOH = −log[OH⁻]. Then use: pH + pOH = 14 (at 25°C). For 0.001 M NaOH: pOH = 3 → pH = 11.

Weak Acids

Weak acids partially dissociate. For dilute solutions where Ka ≪ C, approximate [H⁺] ≈ √(Ka × C), then take the negative log.

Buffer Solutions

Use the Henderson-Hasselbalch equation:

pH = pKa + log([A⁻] / [HA])

Where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. This is the standard formula for buffer pH calculations in biology and biochemistry.

pH in Biology

Human blood is maintained at pH 7.35–7.45 by bicarbonate and protein buffer systems. Deviations beyond this narrow range cause acidosis (pH < 7.35) or alkalosis (pH > 7.45), both of which disrupt enzyme function and oxygen transport.

Most mammalian cell cultures grow optimally between pH 7.2 and 7.4. Cell culture media use bicarbonate-CO₂ buffer systems that automatically maintain pH when cells are incubated with 5% CO₂.

Measuring pH in the Lab

A calibrated pH meter with a glass electrode is the most accurate method, typically giving ±0.01 pH units after two- or three-point calibration with standard buffers (pH 4.0, 7.0, and 10.0). pH indicator strips and paper give quick estimates (±0.5 pH units) but lack the precision needed for most scientific work.