Shupack S I
Department of Chemistry, Villanova University, PA 19085.
Environ Health Perspect. 1991 May;92:7-11. doi: 10.1289/ehp.91927.
Chromium, named for its many-colored compounds, exists in the oxidation states of -2 to +6 inclusively. The compounds exhibit a wide range of geometries including square planar, tetrahedral, octahedral, and various distorted geometries. Chromium is found in nature principally as the chromite ore FeCr2O4 in which chromium is in the +3 state. The existence of a particular oxidation state is dependent on many factors including pH, redox potentials, and kinetics. Thermodynamically, +3 and +2 are the most stable states, while the +3 and +6 oxidation states are the most common ones found in aqueous solution. Kinetically, chromium +3 is substitutionally inert: for water exchange k(sec-1) = 2.5 x 10(-6), due to the presence of the half-filled d(t2g)3.4A2g state. On the other hand, protonation/deprotonation is quite rapid. Polymerization is very slow but is promoted at higher pHs; acid cleavage of the protonated oligomers is also quite slow. Chromium +6 as the chromate ion is strongly oxidizing at low pHs and less so in basic solution. The chromate ion does form some polyacids and polyanions. These factors must be considered in analyzing samples for total chromium and for the amounts of each oxidation state.
铬因其多种颜色的化合物而得名，其氧化态范围为 -2 至 +6（含）。这些化合物呈现出广泛的几何构型，包括平面正方形、四面体、八面体以及各种扭曲的几何构型。铬在自然界中主要以铬铁矿 FeCr₂O₄ 的形式存在，其中铬处于 +3 价态。特定氧化态的存在取决于许多因素，包括 pH 值、氧化还原电位和动力学。从热力学角度来看，+3 和 +2 是最稳定的状态，而 +3 和 +6 氧化态是在水溶液中最常见的。从动力学角度来看，铬 +3 是取代惰性的：对于水交换，k(秒⁻¹) = 2.5 × 10⁻⁶，这是由于存在半充满的 d(t₂g)³.⁴A₂g 状态。另一方面，质子化/去质子化相当迅速。聚合非常缓慢，但在较高 pH 值下会被促进；质子化低聚物的酸裂解也相当缓慢。铬 +6 作为铬酸根离子在低 pH 值下具有强氧化性，在碱性溶液中氧化性较弱。铬酸根离子确实会形成一些多酸和多阴离子。在分析总铬以及每种氧化态的含量的样品时，必须考虑这些因素。
