pH Calculator

In "pH from [H+]" mode, pH = -log10(concentration in mol/L). Any concentration above 1 mol/L makes the log10 term positive, which the leading minus sign flips negative -- so a very concentrated acid (above roughly 1 molar in H+) genuinely does produce a negative pH mathematically. This is correct chemistry, not a calculator error, though such high concentrations are uncommon outside concentrated industrial acids. This specific behavior is mode-specific: the same concentration above 1 mol/L entered in "pOH from [OH-]" mode instead pushes pH above 14, not below 0.

At the standard reference temperature of 25C, pH + pOH always equals 14, a consequence of water's own ion-product constant. That means once you know either value you can find the other by simple subtraction, which is exactly what this calculator does internally in every solve mode.

It buckets the computed pH into three bands: Acidic below pH 6.5, Basic above pH 7.5, and Neutral in the small window between them (pure water sits at pH 7). The bands are intentionally a bit wider than a single point at 7 to account for the practical reality that "neutral" readings rarely land on exactly 7.0000.

No -- only the field relevant to your chosen Solve For mode is used. Solving pH from [H+] or pOH from [OH-] uses only the Concentration field; solving [H+] from pH uses only the pH Value field. The unused field in each mode has no effect on the result.

Because pH is a logarithmic scale: pH = -log10[H+], so reversing the log means each whole-number step in pH corresponds to a power-of-ten change in the underlying hydrogen ion concentration. A solution at pH 4 has 10 times the [H+] of one at pH 5, and 100 times the [H+] of one at pH 6 -- the same tenfold relationship holds for [OH-] and pOH. That's why this calculator's [H+] and [OH-] outputs need so many decimal places to render correctly across the full pH range: the underlying values span many orders of magnitude even though pH itself only moves in small, easy-to-read numbers.