\[\mathrm{pH} = -\lg a_{\mathrm H^+} \approx -\lg\frac{[\mathrm H^+]}{c^\circ},\qquad \mathrm{pOH} = -\lg[\mathrm{OH^-}]\]
含义Meaning
氢离子活度的负常用对数（索伦森，1909）。每差 1 个 pH 单位，[H⁺] 相差 10 倍。
The negative decimal logarithm of hydrogen-ion activity (Sørensen, 1909); each pH unit is a tenfold change in [H⁺].
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(a_{\mathrm H^+}\) | 氢离子活度hydrogen-ion activity | 1 |
| \([\mathrm H^+]\) | 氢离子浓度（实指 H₃O⁺）hydrogen-ion concentration (really H₃O⁺) | mol·L⁻¹ |
推导Derivation
[H⁺] 跨越十几个数量级，取对数便于表示。
[H⁺] spans many orders of magnitude, so a log scale is used.
严格定义用活度；稀溶液中 γ ≈ 1，可用浓度近似。实际 pH 由玻璃电极对标准缓冲液校准测定（操作定义）。
The strict definition uses activity; in dilute solution concentration suffices. Practically pH is measured with a glass electrode calibrated against standard buffers.
适用条件与局限Assumptions & limitations
强酸浓溶液中活度系数偏离 1，pH 可小于 0；单个离子活度无法严格测定，pH 标度是约定的。
In concentrated acid pH can be below 0; single-ion activity is not strictly measurable, so the pH scale is conventional.
纯水中性 pH = 7 只在 25 °C 成立（Kw 随温度变化）。
Neutral pH = 7 only at 25 °C.