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An adsorption study on a strong base anion exchange resin from dilute HNO3 and aliphatic alcohols showed  that, with the exception of Eu (in CH3OH), the rare earth adsorption from HN03-alcohol solutions decreased with increasing length of the primary carbon chain in the alcohol concerned.
A combined anion exchange-solvent extraction procedure for separating trace amounts of Sm, Eu, Gd, Dy and Er from TI1F4 resulted 
Methods of Separation of Individual Rare Earth Elements
in the recovery of ~ 98%. A review on anion exchange adsorption onto strong base resins has recently been published .
Mabcus and Nelson  used concentrated LiN(>3 solution on an anion exchange resin, Dowex 1 (X10) (325 mesh), at 78° C to separate Ce, Pr, Nd, Sm, Eu and Yb. Both stepwise and concentration gradient (6 to 3M LiNCfe) elution were tried.
Eluants.—Citric acid buffered with ammonium citrate is the first eluant to be used in the separation of rare earths by ion exchange process. It is also the most extensively [85—89] investigated eluant. At low pH the individual rare earths move down the column at different rates. A plot of volume of eluted portion vs. concentration shows typical bell-shaped curves (Fig. 2) with widely spaced maxima characteristic of elution chromatography, although the system makes use of a chelating agent.
Fig. 2. A typical elution plot
The yttrium group rare earths were separated by Ketelle and Boyd  on a column of Dowex 50 (270—325 mesh) using 5% (0.25 M) citric acid at pH 3.28. Mixtures of Eu—Sm and Y—Tb took 8 hr. for complete separation  with 0.5 M citrate at pH 3.04 (87° C) and 0.24 M lactate at pH 5. Cunninghame et al.  reduced the separation time to 4 hr. using 1M lactate (pH 3.25) at 87° C for a mixture of Y—Pr— Nd—Pm—Sm—Eu with Zeokarb 225.
Besides citric acid, a number of other complexing agents including hydroxy acids, amino acids and polyaminopolycarboxylic acids have been used to resolve rare earth mixtures. Among the hydroxy acids— glycolic [90, 92, 93], a-hydroxyisobutyric [94—96], lactic [90, 91, 97—100], malic [90, 101] and tartaric  and among the amino and polyaminopolycarboxylic acids—aminoaectic , nitrilotriaectic (NTA) [102—110], ethylenediaminetetracetic (EDTA) [106, 107, 111— 215], A^-hydroxyethylethylenediaminetriacectic (HEDTA) [114,118,119] are the commonly used eluants for both micro and macro scales of separation.
Due to the limited solubility of NTA and EDTA in their acid forms it is somewhat difficult to carry out elutions on H+ form resins. Sped-deno et al.  suggested the use of a retaining ion, which formed stable, soluble complexes with the eluant, to impregnate the resin column. They proposed Fe3+, Cu2+ and Ni2+ as retaining ions; of these Cu2+ has been most widely used [115—117, 119]. Wolf and Massonne  used Zn2+ for the elution with NTA because the Zn(NTA) complex is more soluble than the Cu(NTA) complex. Recently Fuger  proposed the use of the EDTA chelate of the same ion with which the ion exchange resin is pretreated as eluant. This has been very useful in the case of radioactive tracers.
(iii) Paper chromatography.—Cerrai and Testa  separated some rare earths by ascending paper chromatographic technique using CRL/I papers treated with 0.1 Mdi(2-ethylhexyl)orthophosphoricacid(HDEHP). Under the same conditions an increase in Rf value is noted with decreasing Z of the rare earths. For a given element the Rf value increases with the molarity of eluting acid (HC1) and with temperature, but decreases with the concentration of HDEHP. The following Rf values for 1M HC1 eluant have been reported.
M3+ La Ce Pr Sm Eu Gd Dy Er Yb Y Sc
Rf 0.92 0.91 0.90 0.76 0.64 0.51 0.12 0.06 0.03 0.07 0.0
Later Cerrai and Triulzi  used a mixture of 3M HNO3 and 80% CH3OH as solvent on Whatman cellulose paper DE-20 to separate La, Ce, Pr, Nd, Sm and Gd. Whatman DE-20 is a cellulose anion exchanger containing amine groups bonded to the cellulose matrix.
The application of cellulosic anion exchanger in the separation of trace amounts of rare earths has also been investigated. Diethylaminoe-thyl cellulose paper and 0.026M citric acid were found to be the most satisfactory. A separation factor of 2.6 between Eu andCe was obtained . It has been found  that a mixture of HC1 and various aliphatic alcohols can be successfully used as eluant for the separation of rare earths by paper chromatography (Whatman No. 1).
(iv) Gas chromatography. — The 2,2,6,6-tetramethyl-3,5-heptanedione complexes of the rare earths, M(THD)3, are quite stable and also volatile. There is an appreciable and significant difference in the volatilities of the complexes. The complexes are monomeric in benzene. Eisentraut and Sievers  have recently separated these complexes by gas chromatographic method. Fig. 3 a shows a gas chromatographic separation of a mixture of the complexes in benzene solution. It is found that the com-