The magnitude of the double-double /tetrad/ effect in unit cell volumes has been determined for a number of simple lanthanide compounds of the LnmXn type. The effect increases with decreasing electronegativity of the X atom, which indicates that the effect originates in the partially covalent character of the Ln-X bond.
Two models of the extraction process have been proposed. In the first model it is assumed that the partitioning neutral species
is at first formed in the aqueous phase and then transferred into the organic phase. The second model is based on the assumption
that equivalent amounts of cations and anions are at first transferred from the aqueous into the organic phase and then associated
to form a neutral molecule. The role of the solubility parameter in extraction and the relation between the solubility of
liquid organic substances in water and the partition of complexes have been discussed. The extraction of simple complexes
and complexes with organic ligands has been discussed using the first model. Partition coefficients have been calculated theoretically
and compared with experimental values in some very simple cases. The extraction of ion pairs has been discussed using the
partition-association model and the concept of single-ion partition coefficients.
Single-ion partition coefficients have been calculated for NBu
and I− in the system water—chloroform. The standard free energy of transfer of each ion was separated into an electrostatic and
a neutral term. The electrostatic term was calculated with the aid of the Born equation. In the case of I− the neutral term was calculated with the aid of the xenongas assumption, whereas in the case of NBu
the tetrabutylmethane assumption was used. The results of the calculation were compared with the experimentally found partition
coefficient of the completely dissociated ion-pair NBu4I.
The investigations carried out till now and presented in this paper show that apart from the well known itinerant properties of yttrium in respect to free energy of complex formation, also actinides(III) change their position in the lanthanide series in respect to G. It has also been shown that yttrium and actinides exhibit itinerant behaviour in respect to unit cell volumes. Evidence has been presented that delocalization of 4f and 5f orbitals is the reason for the two types of migratory properties. Since the itinerant behaviour of yttrium and actinides(III) in respect to stability constants (free energies of complex formation) is the basis for yttrium-lanthanides and lanthanides-actinides group separations, a better qualitative understanding of the mechanism involved may contribute to the development of more efficient separation procedures.
The position of Pu/III/ within lanthanides in respect to G0, H0 and S0 of complex formation with nitrate and thiocyanate ligands was determined by the extraction method. It was found that in respect to G0, Pu/III/ is a light pseudolanthanide for nitrate ligands and a heavy pseudolanthanide for thiocyanate ligands. A comparison of the positions of Pu/III/ and Am/III/ in respect to G0, H0 and S0 shows that the radius of plutonium is greater than that of americium in the An/NO3/
complex and smaller in the An/NCS/3/TBP/n complex. The increase in the radii between plutonium and americium in the thiocyanate complex points out to a contribution from 5f orbitals to bonding.
Extractive purification of boric acid from radioactive corrosion and fission products dissolved in aqueous solutions modelling
nuclear reactor coolants has been studied. Aliphatic 1,3-diols containing 8 and 9 carbon atoms per molecule were used as extractants
fro boric acid. The behaviour of some representative corrosion and fission products as well as various factors affecting their
distribution between the organic and aqueous phases have been investigated under the conditions of boric acid extraction.
Conditions for the effective separation of boric acid from most of the radioactive contaminants are presented.