pH from [H+] Calculator

pH from hydrogen ion concentration [H+]

Take the hydrogen ion concentration, apply a base-10 log, flip the sign, and you have pH: pH = -log₁₀([H⁺]), where [H⁺] is in mol/L (M). Pure water at 25°C sits at [H⁺] = 10⁻⁷ M, which works out to pH 7, the neutral point. Bump that up to [H⁺] = 10⁻³ M and you land at pH 3, roughly as acidic as vinegar. Because the scale is logarithmic, one pH unit means a tenfold swing in concentration, so pH 4 is ten times more acidic than pH 5. A few familiar values: human blood holds around 4×10⁻⁸ M (pH 7.4, kept on a very tight leash), stomach acid runs near 10⁻² M (pH 1-2), and seawater sits around 10⁻⁸·¹ M (pH 8.1). Its mirror quantity is pOH = -log[OH⁻], and at 25°C the two satisfy pH + pOH = 14.

Applications

It shows up in analytical chemistry and titration, in clinical physiology (arterial blood gas, acid-base balance), and in water treatment, where chlorination does its best work between pH 6 and 9. Pool owners aim for 7.2-7.6. Winemakers and brewers watch it closely. Aquarists track it too (freshwater 6.5-7.5, marine 8.1-8.4), and on the farm soil acidity decides how well plants take up nutrients.

FAQ

Can pH be negative or above 14? It can. A very concentrated strong acid such as 12 M HCl pushes pH below 0, and concentrated bases climb past 14. The 0-14 window is just the range you meet most often, not a hard limit.

Why is pH 7 neutral? At 25°C pure water self-ionizes until [H⁺] = [OH⁻] = 10⁻⁷ M, and that balance point is what we call neutral. Change the temperature and the neutral point drifts a little.

What if [H+] = 0? That never happens in water. Autoionization always keeps [H⁺] at roughly 10⁻⁷ M or higher, so pH stays defined and finite no matter what.