pH Calculator
The pH and pOH from the hydrogen ion concentration.
Result
pH
4.00
pOH
10.00
How it works
The pH scale measures how acidic or basic a solution is: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in moles per litre. Pure water at 25 °C has pH 7 — the neutral point where acid and base exactly balance. The minus sign and the logarithm together mean the scale runs backwards and compresses enormously. Lower pH is more acidic, and each single unit is a tenfold change in concentration: pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6. That is why a small pH shift in blood or soil is a large chemical event. pOH is the mirror measure for hydroxide ions, and the two always sum to 14 in water at 25 °C — so pH 3 implies pOH 11. Familiar landmarks: stomach acid near 1.5, lemon juice 2, coffee 5, blood 7.4, seawater 8.1, bleach 13.
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Frequently asked questions
What is a neutral pH?
7 at 25 °C, where [H⁺] = 1 × 10⁻⁷ mol/L.
What is pOH?
The hydroxide counterpart; at 25 °C pH + pOH always equals 14.
Why does one pH unit mean a factor of ten?
Because the scale is logarithmic — it is built on powers of ten. pH 5 means a hydrogen-ion concentration of 10⁻⁵ mol/L, pH 4 means 10⁻⁴, which is ten times more. The log compresses a billion-fold range into a 0–14 dial.
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