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<p>Thorough Raman spectroscopic measurements have been made of aqueous solutions of Ce(ClO<sub>4</sub>)<sub>3</sub>, Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>, La<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>, Na<sub>2</sub>SO<sub>4</sub>(<i>aq</i>), and ternary Ce(ClO<sub>4</sub>)<sub>3</sub>/Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> down to 50&#xa0;cm<sup>−1</sup> and in some cases to minimal concentrations (≥ 5·10<sup>−4</sup>&#xa0;mol·kg<sup>−1</sup>) at 22&#xa0;°C. In dilute Ce(ClO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) solutions, the breathing mode <i>ν</i><sub>1</sub>–CeO<sub>9</sub> of [Ce(OH<sub>2</sub>)<sub>9</sub>]<sup>3+</sup>(<i>aq</i>) has been observed at 344&#xa0;cm<sup>−1</sup> as a weak, strongly polarized band, signifying that no contact ion pairs (IPs) are formed. In contrast, in Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>), the <i>ν</i><sub>1</sub> breathing mode of the [Ce(OH<sub>2</sub>)<sub>7</sub>(SO<sub>4</sub>)]<sup>+</sup> species appears at 320&#xa0;cm<sup>−1</sup>. Simultaneously, the <i>ν</i><sub>1</sub>–SO<sub>4</sub><sup>2−</sup> mode at 981&#xa0;cm<sup>−1</sup> shows a band component at 992&#xa0;cm<sup>−1</sup> assigned to the ligated sulfate of the sulfato-complex species. The ligated sulfate also shows specific bands in the deformation region and the antisymmetric stretching region.</p><p>After subtraction of the sulfato-complex band at 992&#xa0;cm<sup>−1</sup>, the <i>ν</i><sub>1</sub>–SO<sub>4</sub><sup>2−</sup> band in Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) showed systematic differences from that in Na<sub>2</sub>SO<sub>4</sub>(<i>aq</i>). This is consistent with a previously undetected <i>ν</i><sub>1</sub>–SO<sub>4</sub><sup>2−</sup> band at 982.6&#xa0;cm<sup>−1</sup> that can be assigned to the presence of ion pairs Ce<sup>3+</sup> (H<sub>2</sub>O)<sub>n</sub>SO<sub>4</sub><sup>2−</sup> (<i>n</i> = 1,2) with interposed water molecules. In a solution with a high Ce<sup>3+</sup>/SO<sub>4</sub><sup>2−</sup> concentration ratio, a further <i>ν</i><sub>1</sub>SO<sub>4</sub><sup>2−</sup> mode at 1002.5&#xa0;cm<sup>−1</sup> was observed and assigned to a 1:2 complex.</p><p>Quantitative Raman spectroscopy on Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) solutions (0.00316–0.1627&#xa0;mol·kg<sup>−1</sup>), taking all the 1:1 sulfate species associated with Ce<sup>3+</sup>, resulted in the overall association constant as log<sub>10</sub><i>K</i><sub>A</sub><sup>0</sup> equal to 3.58 ± 0.1 at 22&#xa0;°C. A re-evaluation of the study of La<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) solutions in which the occurrence of sulfato-complex species, and including the ion pairs of the type La<sup>3+</sup>(H<sub>2</sub>O)<sub>n</sub>SO<sub>4</sub><sup>2−</sup> (<i>n</i> = 1,2), leads to a log<sub>10</sub><i>K</i><sub>A</sub> at 3.59 ± 0.1 (at 22&#xa0;°C). Furthermore, the third step of the ion association process, <i>K</i><sub>3</sub>, was determined at 3.2 and the re-determined <i>K</i><sub>3</sub> value in La<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) solutions at 1.6. The Raman spectroscopic results on Ce<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) and La<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) contradict the claim of a recent DRS study, disputing the existence of sulfato-complex speciation in La<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(<i>aq</i>) solutions.</p>

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A Raman Spectroscopic Investigation on Ce2(SO4)3(aq) and La2(SO4)3(aq)—Ion Pairs and/or Sulfato-Complex Formation?

  • 期刊论文

  • 发表日期：

    2025年12月26日

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  • Wolfram Rudolph

摘要

Thorough Raman spectroscopic measurements have been made of aqueous solutions of Ce(ClO4)3, Ce2(SO4)3, La2(SO4)3, Na2SO4(aq), and ternary Ce(ClO4)3/Ce2(SO4)3 down to 50 cm−1 and in some cases to minimal concentrations (≥ 5·10−4 mol·kg−1) at 22 °C. In dilute Ce(ClO4)3(aq) solutions, the breathing mode ν1–CeO9 of [Ce(OH2)9]3+(aq) has been observed at 344 cm−1 as a weak, strongly polarized band, signifying that no contact ion pairs (IPs) are formed. In contrast, in Ce2(SO4)3(aq), the ν1 breathing mode of the [Ce(OH2)7(SO4)]+ species appears at 320 cm−1. Simultaneously, the ν1–SO42− mode at 981 cm−1 shows a band component at 992 cm−1 assigned to the ligated sulfate of the sulfato-complex species. The ligated sulfate also shows specific bands in the deformation region and the antisymmetric stretching region.

After subtraction of the sulfato-complex band at 992 cm−1, the ν1–SO42− band in Ce2(SO4)3(aq) showed systematic differences from that in Na2SO4(aq). This is consistent with a previously undetected ν1–SO42− band at 982.6 cm−1 that can be assigned to the presence of ion pairs Ce3+ (H2O)nSO42− (n = 1,2) with interposed water molecules. In a solution with a high Ce3+/SO42− concentration ratio, a further ν1SO42− mode at 1002.5 cm−1 was observed and assigned to a 1:2 complex.

Quantitative Raman spectroscopy on Ce2(SO4)3(aq) solutions (0.00316–0.1627 mol·kg−1), taking all the 1:1 sulfate species associated with Ce3+, resulted in the overall association constant as log10KA0 equal to 3.58 ± 0.1 at 22 °C. A re-evaluation of the study of La2(SO4)3(aq) solutions in which the occurrence of sulfato-complex species, and including the ion pairs of the type La3+(H2O)nSO42− (n = 1,2), leads to a log10KA at 3.59 ± 0.1 (at 22 °C). Furthermore, the third step of the ion association process, K3, was determined at 3.2 and the re-determined K3 value in La2(SO4)3(aq) solutions at 1.6. The Raman spectroscopic results on Ce2(SO4)3(aq) and La2(SO4)3(aq) contradict the claim of a recent DRS study, disputing the existence of sulfato-complex speciation in La2(SO4)3(aq) solutions.