Chemistry

Corresponding Member of the USSR Academy of Sciences A. V. Nikolaev, A. I. Ryabinin,
Yu. A. Afanas’ev

On the Mechanism of Extraction of Cerium(IV) Nitrate from Nitric-Acid Solutions by Tributyl Phosphate (TBP)

The extraction of cerium(IV) nitrate by TBP has been studied by a number of authors from different points of view \(^{(1–7)}\), including technological and radiochemical aspects \(^{(2,4,6)}\). However, the available data are insufficient for a complete description of the system \(\mathrm{Ce(NO_3)_4 — HNO_3 — H_2O — TBP}\), which would make it possible to judge reliably the mechanism of extraction of \(\mathrm{Ce(NO_3)_4}\) by tributyl phosphate.

Fig. 1. Quaternary extraction system \(\mathrm{Ce(NO_3)_4 — HNO_3 — H_2O — (C_4H_9O)_3PO}\) at \(25^\circ\)

At present there are two points of view: some authors \(^{(8)}\) believe that \(\mathrm{Ce(NO_3)_4}\) passes into TBP, while others \(^{(6)}\) believe it is \(\mathrm{H_2[Ce(NO_3)_6]}\). Earlier, one of us \(^{(9,10)}\) showed that studying the course of extraction rays makes it possible to judge the composition of the extracted compound. We have used this method to investigate the system named above. Cerium(IV) was determined by potentiometric titration with hydrogen peroxide \(^{(11)}\); nitric acid was titrated with a sodium hydroxide solution. The extraction was carried out at a temperature of \(25 \pm 0.05^\circ\).

Figure 1 presents the course of the extraction rays (I–VII) in the system \(\mathrm{Ce(NO_3)_4 — HNO_3 — H_2O — TBP}\), and the isolines of the distribution coefficients—

Ce(NO₃)₄ line ($K_p$ from 0.95 to 4.9), as well as the saturation line of Ce(NO₃)₄ in the aqueous phase (BC). Most of the extraction rays are rectilinear over a wide range of concentrations of cerium(IV) nitrate and nitric acid. If cerium passed into the organic phase in the form of the complex acid H₂[Ce(NO₃)₆], then the extraction rays would converge at point A, which corresponds to the composition of this acid.

Thus, for the system (C₂H₅)₂O—H₂O—FeCl₃—HCl, all extraction rays converge at the point of composition H₂[FeCl₆]·6H₂O [10]. As is seen from Fig. 1, not a single ray arrives at this point. The course of rays I–IV indicates that nitric acid passes into the organic phase in different (nonstoichiometric) amounts, which depend on its concentration in the aqueous phase (see also Table 1). Rays V and VI converge at point D (100% Ce(NO₃)₄), and this unambiguously shows that cerium(IV) is extracted in the form Ce(NO₃)₄. On rays VI and VII, at a cerium(IV) nitrate concentration of ~30%, a bend is observed and, as is seen from the data of Table 1, a deficiency of nitric acid appears in the organic phase. This apparently indicates that, at the indicated cerium concentration and an HNO₃ concentration not higher than 15%, basic salts of cerium(IV) pass into the organic phase along with Ce(NO₃)₄. The bending of rays V, VI, and VII toward the H₂O corner indicates that, at small amounts of cerium(IV) in the aqueous phase, the salting-out of HNO₃ from the organic phase by it ceases. At these same points, with a low concentration of cerium(IV), its extraction is maximal ($K_p$ reaches 400 and 500).

Table 1

Composition and specific gravities of equilibrium phases

The isolines of the distribution coefficients of cerium(IV) over the entire field of the system are practically rectilinear, and the angle of their inclination to the cerium axis is close to 90°. This indicates that HNO₃ has little effect on the extraction of cerium(IV) nitrate.

Analysis of the data presented in Table 1 also confirms that in the organic phase there is no constant ratio between the amounts of Ce(NO₃)₄ and HNO₃. Consequently, only their joint extraction takes place, and not the extraction of any compound between them.

From consideration of the composition of the equilibrium organic phases at a cerium(IV) concentration in the aqueous phase of more than 1 mol/l, it is seen that Ce(NO₃)₄ : TBP = 1 : 2, i.e., a solvate of composition Ce(NO₃)₄·2TBP is formed. The distribution coefficients of nitric acid in most cases do not exceed 0.2. Such low extraction of nitric acid is explained by its salting out by ce—

of cerium(IV) from the organic phase into the aqueous phase, which is analogous to the effect of cerium(IV) on the extraction of thorium \((^{7})\) and rare earths \((^{2,4})\).

The data presented indicate the possibility of successfully extracting cerium(IV) from solutions 1–2 \(M\) in nitric acid.

Institute of Inorganic Chemistry
Siberian Branch of the Academy of Sciences of the USSR

Received
27 II 1963

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