Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
Phys Chem Chem Phys. 2019 Sep 11;21(35):19172-19180. doi: 10.1039/c9cp01392j.
Iron is the most abundant transition metal in the earth's crust and is important for the proper functioning of many technological and natural processes. Despite the importance, a complete microscopic understanding of the hydration of ferric ions and water mediated ion pairing has not been realized. Hydrated Fe(iii) is difficult to study due to the process of complexation to the anion and hydrolysis of the hydrating water molecules leading to a heterogeneous solution with diverse speciation. Here, ferric chloride and nitrate aqueous solutions are studied using polarized Raman spectroscopy as a function of concentration and referenced to their respective sodium salt or mineral acid. Perturbed water spectra (PWS) were generated using multivariate curve resolution-alternating least squares (MCR-ALS) to show the residual spectral response uniquely attributable to the hydration of ferric speciation. The hydrogen bonding network associated with the hydrating water molecules in ferric chloride solutions are found to be more similar to hydrochloric acid solutions, whereas in ferric nitrate solutions, the network behaves more similar to sodium nitrate, despite increased acidity. Thus, in the FeNO3 and FeCl3 solutions, ion pairing and coordination, respectively, are significantly influencing the hydration spectra signature. These results further reveal concentration dependent changes to the hydrogen bonding network, hydrating water symmetry, and changes to the relative abundance of solvent shared ion pairs that are governed primarily by the ferric salt identity.
铁是地壳中含量最丰富的过渡金属，对许多技术和自然过程的正常运行都很重要。尽管其重要性不言而喻，但人们尚未完全从微观角度理解三价铁离子的水合作用以及水介导的离子对形成。由于与阴离子的络合作用以及水合水分子的水解，导致形成具有不同形态的多相溶液，因此难以研究水合的三价铁。本文使用偏振拉曼光谱法研究了氯化铁和硝酸铁水溶液，作为浓度的函数，并参考了它们各自的钠盐或矿物酸。使用多元曲线分辨-交替最小二乘法（MCR-ALS）生成受扰水光谱（PWS），以显示唯一归因于铁形态水合作用的剩余光谱响应。研究发现，在氯化铁溶液中与水合水分子相关的氢键网络与盐酸溶液更为相似，而在硝酸铁溶液中，尽管酸度增加，网络的行为与硝酸钠更为相似。因此，在 FeNO3 和 FeCl3 溶液中，分别是离子对和配位作用显著影响了水合光谱特征。这些结果进一步揭示了氢键网络、水合对称性以及溶剂共享离子对的相对丰度随浓度的变化，这些变化主要由铁盐的特性决定。
