pH Calculator

Calculate pH, pOH, hydrogen ion concentration, and hydroxide ion concentration. For strong/weak acids and bases.

Basic pH/pOH
Strong Acid/Base
Weak Acid/Base
Calculating...
pH Calculation Results
pH = 7.0
Acidic Basic

This online pH calculator is designed to determine the pH of an aqueous solution of a given compound. You can select any acid or base from the chemical list, or use a known value as the dissociation constant Ka or Kb. The concentration of the substance can be specified in molarity or mass per unit volume.

Compound
Concentration
pH

Understanding pH and Acid-Base Chemistry

pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration:

pH = -log₁₀[H⁺]

Key Insight: The pH scale ranges from 0 to 14, with 7 being neutral. Solutions with pH less than 7 are acidic, while those with pH greater than 7 are basic.

pH and pOH Relationship

pOH is similarly defined for hydroxide ion concentration:

pOH = -log₁₀[OH⁻]

At 25°C, the product of hydrogen and hydroxide ion concentrations is constant:

[H⁺][OH⁻] = 1.0 × 10⁻¹⁴ = Kw

This leads to the relationship:

pH + pOH = 14

Strong vs. Weak Acids and Bases

1

Strong Acids/Bases: Completely dissociate in water. For strong acids, [H⁺] equals the initial acid concentration. For strong bases, [OH⁻] equals the initial base concentration.

2

Weak Acids/Bases: Partially dissociate in water. The extent of dissociation is described by the acid dissociation constant (Ka) for acids or base dissociation constant (Kb) for bases.

Common Acid-Base Constants

Acid/Base Formula Ka or Kb pKa or pKb
Hydrochloric Acid HCl Strong acid -
Acetic Acid CH₃COOH 1.8 × 10⁻⁵ 4.74
Formic Acid HCOOH 1.8 × 10⁻⁴ 3.74
Carbonic Acid H₂CO₃ 4.3 × 10⁻⁷ 6.37
Sodium Hydroxide NaOH Strong base -
Ammonia NH₃ 1.8 × 10⁻⁵ 4.74

pH Calculation Methods

Strong Acids

For a strong acid HA with concentration C: [H⁺] = C, pH = -log(C)

Strong Bases

For a strong base BOH with concentration C: [OH⁻] = C, pOH = -log(C), pH = 14 - pOH

Weak Acids

For a weak acid HA with concentration C and Ka: [H⁺] ≈ √(Ka × C), pH = -log[H⁺]

Weak Bases

For a weak base B with concentration C and Kb: [OH⁻] ≈ √(Kb × C), pOH = -log[OH⁻], pH = 14 - pOH

Practical Application: pH calculations are essential in many fields including chemistry, biology, environmental science, medicine, and industry. Understanding pH helps in buffer preparation, water treatment, pharmaceutical development, and many other applications.

Frequently Asked Questions

pH is critical in biological systems because enzymes and other proteins have optimal pH ranges for activity. Blood pH is tightly regulated around 7.4, and deviations can be life-threatening. Cellular processes, membrane transport, and metabolic reactions are all pH-dependent.

pH is a measure of the hydrogen ion concentration in a solution. pKa is the negative logarithm of the acid dissociation constant (Ka) and is a property of a specific acid. When pH = pKa, the acid is half-dissociated. The relationship between pH and pKa is described by the Henderson-Hasselbalch equation.

Temperature affects the autoionization constant of water (Kw). At 25°C, Kw = 1.0 × 10⁻¹⁴, so neutral pH is 7.0. At higher temperatures, Kw increases, so neutral pH decreases (e.g., at 50°C, neutral pH is about 6.63). The pH scale remains 0-14, but the neutral point shifts.

The logarithmic pH scale is used because hydrogen ion concentrations in aqueous solutions vary over many orders of magnitude (from about 10⁰ M to 10⁻¹⁴ M). A linear scale would be impractical. The logarithmic scale compresses this wide range into manageable numbers from 0 to 14, where each unit change represents a 10-fold change in [H⁺].

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base (or weak base and its conjugate acid). Buffers work by converting strong acids or bases into weak ones through chemical reactions, minimizing the change in [H⁺]. The effectiveness of a buffer is greatest when pH is close to the pKa of the acid.