\[K_\text{aw} = \frac{c_\text{air}}{c_\text{water}} = \frac{H}{RT},\qquad H = \frac{p}{c_\text{water}}\ (\mathrm{Pa\,m^3\,mol^{-1}})\]
含义Meaning
污染物在空气与水之间平衡分配的比值。K_aw 大（如氯乙烯、三氯乙烯）的物质易从水中挥发；小（如苯酚、农药）的易留在水中。用于评价污染物的环境归趋。
The equilibrium ratio of a chemical’s air and water concentrations. High K_aw (vinyl chloride, TCE) means it volatilises from water; low (phenol, many pesticides) means it stays dissolved. Used in environmental fate assessment.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(K_\text{aw}\) | 无量纲气–水分配系数dimensionless air–water partition coefficient | 1 |
| \(H\) | 亨利定律常数Henry’s-law constant | Pa·m³·mol⁻¹ |
推导Derivation
亨利定律 \(p = Hc_\text{water}\)。
Henry’s law p = Hc_water.
气相浓度 \(c_\text{air} = p/RT\)，相除得 \(K_\text{aw} = H/RT\)。
With c_air = p/RT.
估算：\(H \approx p^*/S\)（纯物质蒸气压 / 水中溶解度），适用于难溶物。
Estimate H ≈ p*/S for sparingly soluble compounds.
适用条件与局限Assumptions & limitations
稀溶液；温度依赖显著（范特霍夫形式）；离子化物质要用中性形态分数校正。
Dilute solutions; strongly T-dependent; correct ionisable compounds for speciation.
例题Worked example
苯 H ≈ 557 Pa·m³/mol：25 °C 时 K_aw = 557/(8.314×298) = 0.22。
Benzene: K_aw ≈ 0.22 at 25 °C.