含义Meaning
活度是“有效浓度”，使真实体系的化学势保持理想体系的简单形式；理想时 γ = 1。
Activity is the ‘effective concentration’ that keeps the ideal form of the chemical potential; γ = 1 when ideal.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(a_i\) | 活度（无量纲）activity (dimensionless) | 1 |
| \(\gamma_i\) | 活度系数activity coefficient | 1 |
| \(c^\circ\) | 标准浓度 1 mol/Lstandard concentration, 1 mol/L | mol·L⁻¹ |
推导Derivation
理想稀溶液中 \(\mu_i = \mu_i^\circ + RT\ln(c_i/c^\circ)\)。
For an ideal dilute solution \(\mu_i = \mu_i^\circ + RT\ln(c_i/c^\circ)\).
真实溶液把一切非理想性归入 \(\gamma_i\)：\(RT\ln\gamma_i = \mu_i - \mu_i^{\text{ideal}}\)。
Real solutions put all non-ideality into \(\gamma_i\): \(RT\ln\gamma_i = \mu_i - \mu_i^{\text{ideal}}\).
纯固体、纯液体活度为 1；理想气体用 \(p/p^\circ\)。
Pure solids and liquids have a = 1; ideal gases use \(p/p^\circ\).
适用条件与局限Assumptions & limitations
单个离子的活度系数无法实验测定，只能测平均值 \(\gamma_\pm\)。
Single-ion activity coefficients are not measurable; only \(\gamma_\pm\) is.
标准态必须明确（摩尔分数、质量摩尔浓度或物质的量浓度标度的 γ 数值不同）。
Specify the standard state — γ differs between mole-fraction, molality and molarity scales.