pH

pH is a logarithmic scale, typically ranging from 0 to 14, that measures how acidic or basic an aqueous solution is. It is derived directly from the molarity of hydrogen ions (H+) dissolved in the solution — the higher the hydrogen ion concentration, the lower (more acidic) the pH value. Pure water at 25°C has a pH of 7, which is considered neutral because hydrogen and hydroxide ion concentrations are exactly equal.

Solutions with a pH below 7 are acidic, meaning they contain a higher concentration of hydrogen ions than pure water, while solutions above 7 are basic (or alkaline), meaning hydroxide ions dominate. Because the scale is logarithmic, each single-unit drop in pH represents a tenfold increase in hydrogen ion concentration — a solution of pH 3 is ten times more acidic than one at pH 4, and one hundred times more acidic than one at pH 5.

pH governs an enormous range of chemical and biological processes, from enzyme activity in the human body to the effectiveness of buffer solutions used in labs and industry. The Hydrogen Ion Concentration Calculator and Buffer pH Calculator both let you move quickly between hydrogen ion concentration, pH, and buffer composition.

pH = -log10[H+]

Where [H+] is the molar concentration of hydrogen ions in the solution, expressed in mol/L.

To reverse the calculation and find hydrogen ion concentration from a known pH:

[H+] = 10^(-pH)

Suppose a solution has a hydrogen ion concentration of 1 × 10^-5 mol/L.

pH = -log10(1 × 10^-5) = -(-5) = 5

This means the solution is acidic. Now suppose you're told a different solution has a pH of 9. Working backward:

[H+] = 10^(-9) = 1 × 10^-9 mol/L

That's a much lower hydrogen ion concentration, confirming pH 9 is basic. Try both directions of this calculation with the Hydrogen Ion Concentration Calculator.

• pH is built on molarity: The entire pH calculation depends on knowing the molarity of hydrogen ions in solution, so any error in concentration measurement directly shifts the calculated pH.
• Each pH unit is a tenfold jump: Because the formula uses a base-10 logarithm, moving from pH 4 to pH 5 represents a tenfold decrease in hydrogen ion concentration, not a linear tenth of the change.
• Buffers stabilize pH: A buffer solution is specifically formulated to resist pH changes when small amounts of acid or base are added, which is essential in biological systems and lab reagents.
• Neutral pH depends on temperature: While pH 7 is neutral at 25°C, the neutral point shifts slightly at other temperatures because the ion product of water itself is temperature-dependent.
• pH affects reaction equilibria: Many reactions involving acids and bases are described by an equilibrium constant, and the position of that equilibrium is often directly tied to the solution's pH.