Physico-chemical investigations on rare earth compounds

Abstract

The work on the hydrolysis of lanthanum ion described in this thesis has revealed the existence of the following hydroxy complexes: LaOH²?, La(OH)??, La?OH³? and La?OH??. The various reactions that occur in this system along with the corresponding equilibrium constants have been given (p. 87-). The equilibrium constants have been computed by different methods, which include extrapolation, curve fitting, elimination and the core + links techniques. The large decrease in the n value at constant pH with an increase in the concentration of the metal is a unique feature of this system. The interpretation of the mechanism of the formation of the various complexes in a system of this type has been given for the first time in the present work. New mathematical formulations have been derived based on a new theory proposed for the decrease of n with increasing metal concentration. In addition, some of the expressions derived by the earlier workers (Sillén, Ahrland and others) have been modified so as to be applicable to the present system. The assumptions and limitations of the various mathematical methods employed have been critically examined. On the basis of the results presented in this thesis, the following mechanism for the hydrolysis of lanthanum at low concentrations and low pH values has been proposed: La³? + H?O ? LaOH²? + H? La³? + 2H?O ? La(OH)?? + 2H? The values of n in the true mononuclear curve are quite high (Fig. -). The formation curve even for 1 mM lanthanum is sufficiently below the true mononuclear curve, indicating polynuclear complex formation. The species present on the ‘mononuclear wall’ have been shown to be LaOH²? and La(OH)??. As the concentration of lanthanum is increased, La?OH³? and La?OH?? get formed as shown below: La(OH)?? ? LaOH²? + OH? 2LaOH²? ? La?OH³? + OH? 3LaOH²? ? La?OH?? + 2OH? The above mechanism is consistent with the sharp fall in the value of n at constant pH and the drift towards higher n at constant pH as the concentration of metal is increased. It may be stated that the above reactions occur when the concentration of lanthanum is below 5 mM, particularly at low pH values. When the concentration of lanthanum is increased beyond 5 mM, the polynuclear complexes are formed in accordance with the core + links theory. It has been found that the parallel region corresponds to ‘core + links’ complex of the type La?(OH)??? [= La?(OH)?·H?O?]. It has been concluded that the core + links theory cannot be applied to give further information about the complexes, with the present data. The various hydrolytic equations and the constants that have been deduced from the data given in this thesis are summarized below: Mononuclear Species: 1.a. La³? + H?O ? LaOH²? + H? 1.b. La³? + OH? ? LaOH²? 2.a. La³? + 2H?O ? La(OH)?? + 2H? (Values corrected as: 5.94 ± 0.02, 12.99 ± 0.07, 7.13 ± 0.07, 7.54 ± 0.02, 8.45 ± 0.08, 5.86 ± 0.05, 13.04 ± 0.07, 7.19 ± 0.07, 5.85 ± 0.06) a. obtained by limiting technique (p. 56) b. obtained by curve?fitting method (Schwarzenbach’s) (p. 69) c. obtained by Sillén’s curve?fitting method (p. 71)

The present work on the hydrolysis of cerium has revealed the existence of the following complexes: CeOH²?, Ce?OH³?, Ce?OH?? and possibly Ce?(OH)???. In contrast to the lanthanum system, Ce(OH)?? is not produced under the experimental conditions. It has been found that polynuclear complexes predominate even at the lowest concentration of cerium studied. The core + links theory of Sillén has been modified to make the theory applicable to cases where t is a fraction. Equilibrium constants of the reactions in which the various complexes are formed have been calculated by different methods. It is interesting to note that in the case of lanthanum and cerium hydrolytic systems the corresponding polynuclear complexes are formed by two entirely different mechanisms. In the case of cerium, like many other cations, polynuclear complexes are formed in addition to the mononuclear ones, whereas in the case of lanthanum they are produced at the expense of the mononuclear ones. On the basis of the results presented in this thesis, the following mechanism for the formation of various complexes are as follows: As has already been pointed out, the main purpose of the present work was to inquire into the possibility of using lithium rare?earth double sulphate, if formed, for purposes of fractionation of rare earths and secondly, to fill up the gap in the existing literature on rare?earth–alkali–sulphates systems. The results presented in the present work indicate that the double sulphates are not produced in case of lithium, thus belying our expectations. The main feature of this investigation is the sharp contrast that this system exhibits as compared to the other alkali sulphates. Normally lithium is expected to differ in its properties as the first member of the periodic group from the rest of the alkali elements. It is also known that the first member of a group resembles the second member in the neighbouring group in at least some of its properties. These well?known ideas have been borne out by this study also. Magnesium sulphate resembles lithium sulphate in not forming any double sulphates with cerous sulphate. Regarding the mutual solubility effects noticed in this investigation, it may be pointed out that no satisfactory explanation can be given with the present knowledge of the theory of concentrated solutions.