The pH formula measures the acidity or basicity of a solution on a scale from 0 to 14. pH is one of the most important measurements in chemistry, biology, environmental science, and medicine.
pH = -log[H⁺]
Negative logarithm of hydrogen ion concentration
Units: Unitless (dimensionless)
Range: 0 (most acidic) to 14 (most basic)
Neutral: pH = 7 at 25°C
Units: M (mol/L or molar)
Meaning: Concentration of H⁺ ions in solution
💡 Also written as [H₃O⁺] (hydronium ion)
The common logarithm, not natural log (ln)
pH 0-6
ACIDIC
More H⁺ ions
pH 7
NEUTRAL
Equal H⁺ and OH⁻
pH 8-14
BASIC
More OH⁻ ions
[H⁺] = 10-pH
Reverse calculation
pOH = -log[OH⁻]
For bases
pH + pOH = 14
At 25°C
pH = 14 - pOH
Quick conversion
pH = -log[H⁺]
pH = -log(1.0 × 10⁻³)
pH = -(-3) = 3
Answer: pH = 3 (acidic solution)
It's -log[H⁺], not just log[H⁺]. The negative makes pH values positive.
pH uses log base 10, not ln (natural log). Make sure your calculator is in log mode.
Lower pH = more acidic. Higher pH = more basic. pH 7 is neutral.
pH = -log[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. It measures acidity on a 0-14 scale.
At 25°C, water's ion product is 10⁻¹⁴. Since pH + pOH = 14, the practical range is 0-14, though values outside this range exist for very strong acids/bases.
For powers of 10: If [H⁺] = 10⁻ⁿ, then pH = n. Example: [H⁺] = 10⁻⁵ → pH = 5.
pH measures [H⁺] while pOH measures [OH⁻]. They're related: pH + pOH = 14 at 25°C.
Pure water has [H⁺] = [OH⁻] = 10⁻⁷ M at 25°C, giving pH = -log(10⁻⁷) = 7.