pH Calculator

pH (potential of Hydrogen) is a logarithmic scale measuring how acidic or alkaline a solution is. It ranges from 0 to 14, with 7 being neutral at 25°C.

The pH formula: pH = −log₁₀[H⁺]

Where [H⁺] is the molar concentration of hydrogen ions in moles per liter (mol/L).

Reverse formula — finding H⁺ concentration from pH: [H⁺] = 10^(−pH)

pOH and the relationship: pOH = −log₁₀[OH⁻] pH + pOH = 14 (at 25°C)

Worked example: A solution has [H⁺] = 0.001 mol/L = 10⁻³ mol/L pH = −log₁₀(10⁻³) = −(−3) = pH 3 (acidic)

If pH = 9: [H⁺] = 10⁻⁹ = 0.000000001 mol/L pOH = 14 − 9 = 5

Why logarithmic? The log scale means each whole number step represents a 10× change in acidity. pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.

pH reference scale:

Buffer solutions resist changes in pH when small amounts of acid or base are added. They are critical in biology — human blood maintains pH between 7.35 and 7.45 very tightly.

The scale is logarithmic, which is easy to underestimate

Each pH unit is a factor of ten in hydrogen-ion concentration. Lemon juice at pH 2 is not “twice as acidic” as tomato juice at pH 4, it is a hundred times more concentrated in H⁺. Moving from pH 7 to pH 4 is a thousandfold change. This is why small pH shifts in a swimming pool or an aquarium matter so much more than the numbers suggest, and why blood is held between 7.35 and 7.45 rather than anywhere near a whole unit of slack.

Outside 0 to 14

The 0-14 range is a convention, not a limit. Concentrated hydrochloric acid at 12 mol/L has a negative pH, and saturated sodium hydroxide goes above 15. The scale runs from 0 to 14 in ordinary use because that is the range water itself allows at 25 °C, where pH and pOH must sum to 14.

That sum is temperature-dependent, and only exact at 25 °C. At body temperature water’s neutral point is 6.81 rather than 7.00, so blood at 7.40 is less alkaline relative to neutral than the room-temperature scale makes it look, though it is still genuinely alkaline.