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Free acidity is an important parameter especially in the presence of hydrolysable ions. Several methods have been developed for the determination of free acidity, attributing due importance to the accuracy and the precision of the measurement with the aim of the easiness of the methodology as well as post-measurement recovery in mind. This review covers important methods for the determination of free acidity with emphasis on actinide containing solutions, reported in the literature over the past several decades classifying them into different categories., (© 2013 Elsevier B.V. All rights reserved.)

The extraction behavior of U(VI), Pu(IV) and Am(III) from nitric acid medium by a solution of N,N-dimethyl-N,N-dioctyl-2-(2-hexyloxyethyl)malonamide (DMDOHEMA) in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C(4)mimNTf(2)), was studied. The distribution ratio of these actinides in DMDOHEMA/C(4)mimNTf(2) was measured as a function of various parameters such as the concentration of nitric acid, DMDOHEMA, NTf(2)(-), alkyl chain length of ionic liquid. The extraction of actinides in the absence of DMDOHEMA was insignificant and the distribution ratio achieved in conjunction with C(4)mimNTf(2), was remarkable. The separation factor of U(VI) and Pu(IV) achieved with the use of DMDOHEMA, ionic liquid was compared with Am(III) and other fission products. The stoichiometry of the metal-solvate was determined to be 1:2 for U(VI) and Pu(IV) and 1:3 for Am(III)., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The retention behaviour of uranium and thorium was investigated on modified reverse phase supports using 3-oxo-pentanedioicacid bis-[bis-(2-ethyl-hexyl)-amide (OPAEHA), 3-oxo-pentanedioicacid bis diisobutyl amide (OPAIBA) and bis-2-ethylhexyl succinamic acid (BEHSA). alpha-Hydroxy isobutyric acid (alpha-HIBA) was employed as the complexing reagent for elution. Elution profiles of uranium and thorium were studied as a function of the modifier concentration, mobile phase composition and its pH. Based on these investigations, a novel high performance liquid chromatography (HPLC) based separation technique was developed using BEHSA modified support for the isolation and quantitative determination of lanthanides as a group in uranium matrix. Hundreds of samples obtained from pyrochemical reprocessing of molten salts containing lanthanides in uranium matrix (e.g. 1:20,000) were separated and determined within 7 min using the coated support. The advantage of the present HPLC technique lies in the simultaneous separation and assay of total lanthanides and uranium whereas other analytical methods necessitate the separation of uranium matrix prior to lanthanide assay.

The retention behavior of uranium, thorium and lanthanides has been investigated with amide modified reversed phase C(18) supports using alpha-hydroxy isobutyric acid (alpha-HIBA) as the mobile phase. Four structurally different amide moieties namely, 4-hydroxy-N,N-dihexyl butyramide (4HHBA), 4-hydroxy-N,N-di-2-ethylhexylhexanamide (4HEHHA), bis(N,N,N',N'-2-ethylhexyl)malonamide (B2EHM) and N-methyl-tris(dihexylcarbamoyl-3-methoxy)pivolamide (MTDCMPA) have been synthesized and studied. Among the various amide coated columns, the supports modified with 4HHBA, B2EHM and MTDCMPA exhibit an interesting retention for uranium and thorium, which is different from 4HEHHA modified support. The retention time for uranium and thorium increases with increasing amide concentration for 4HHBA, B2EHM and MTDCMPA supports, while the same decreases with increasing 4HEHHA content. However, the separation factor for uranium and thorium is greater on a 4HEHHA support, compared to an unmodified C(18) column, reflecting the amide's preferential complexation of uranium over thorium. Columns modified with 4HHBA, B2EHM and MTDCMPA exhibit relatively higher retentions for lanthanides. However, MTDCMPA modified support shows a different elution profile for lanthanides compared to 4HHBA, and B2EHM modified columns. Individual separations of heavier lanthanides, i.e., from gadolinium to lutetium also have been achieved using 4HHBA and B2EHM modified supports. The influence of modifier content, mobile phase concentration and its pH on the retention of metal ions has also been studied. Based on these investigations, an efficient high performance liquid chromatographic method (HPLC) has been developed for the rapid separation of uranium from thorium as well as for the individual separation of heavier lanthanides.

Spectrophotometric determination of microgram amounts of U(VI) with 2-(5-Bromo-2-Pyridylazo)-5-diethylaminophenol (Br-PADAP), originally developed by Johnson and Florence has been modified to enable the determination of U(VI) in the presence of a large excess of Th(IV). The effects of thorium, tri-n-butyl phosphate (TBP) and ethanol on the estimation of uranium have been studied in detail and are presented in this paper. This modified method can be applied for the analysis of U(VI) both in aqueous and organic samples containing a large excess of Th(IV) (Th:U = 10000:1).