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Scadden and Ballou  have employed acid phosphates for the first time, and reported the preferential extraction of the yttrium group rare earths in di-w-butyl phosphoric acid (7&-C4H90)2P0(0H) over the lower Z rare earths.
By substituting different organic groups, a series of extractants of general formula (G0)2P0(0H), (abbreviated as HDGP) where G is a generalized group, can be prepared. Di(2-ethylhexyl) ester (HDEHP) as an extractant has been investigated by Peppard et al.  mainly because of its low water solubility and greater stability towards hydrolysis. It has been proposed that the class of HDGP compounds is dimeric  (Fig. 1) and subsequent studies  showed the possible mechanism (far below saturation) to be as follows.
Methods of Separation of Individual Rare Earth Elements
GO O—H—O OG GO O—H—O OG
\ / \ / \ / \ /
P P P P
/ \ ✓ \ / \ / \
GO O—H—O OG GO O O OG
Fig. 1. (a) Possible dimeric nature of HDGP extractant, (b) The M3+-extracted
M(aqu) + 3 (HDGP)2 (OTg) ^ M [H(DGP)2] 3 (org) + 3H(aqU) (4)
A plot of log k vs. Z for the HDEHP—HC1 system gave a straight line of slope 2.5.
Other extractants, such as mono(2-ethylhexyl)phosphoric acid, mono [j?(l,l,3,3-tetramethylbutyl)phenyl] phosphoric acid and an isooctyl pyrophosphate  of unknown composition have been used in the separation process.
A ligroin solution of monooctyl- a-anilinobenzylphosphonate was used  to separate a mixture of Eu3+, Tb3 u and U6+. The course of the extraction was followed by using radioactive tracers, 152Eu and 160Tb.The separation factor (/?) for Eu is in the range of 103 to 104.
(ii) Ion exchange. —The first series of papers [68—76] on the separation of rare earths on ion exchange columns, using citric acid buffered with ammonium citrate as eluant, appeared in 1947. Since then this method has been much improved, and many complexing agents have been employed. Separation of very small amounts of rare earths as well as production of highly purified individual rare earths have been achieved by the ion exchange technique. Elution chromatography is better for analytical purposes, whereas for the production of gross quantities the displacement chromatographic technique is found to be the best.
In elution chromatography the adsorbed ion and the eluant ion are the same. The eluant always has a smaller affinity for the ion exchange resin than the ions being separated. In this technique the concentration of the sample is much less than the concentration of the eluant ions.
The displacement chromatographic technique makes use of a third ion with which the ion exchange column is impregnated. The sample containing the ions to be separated is adsorbed at the top of the column and is eluted with a simple salt solution having greater affinity for the resins than the ions being separated. Thus as the eluant progresses through the column, the ionic species having less affinity is displaced.
The separation of adjacent rare earths by simple displacement chromatography using cation exchange resins is not feasible because all rare earths exhibit almost the samo affinity for the resins, excluding chelating resins. The separation factors (ft) for adjacent rare earths are nearly unity. However, to enhance separation a chelating agent possessing the following characteristics is usually used: a) the reagent must be selective in its chelating tendencies, b) it must form rare earth chelates of sufficient stability to permit replacement by ammonium ion or alkali metal ion from the resin, and c) the reagent must not form complexes of very great stability which would hamper the cation exchange process.
Cation exchangers show greatest affinity for trivalent ions than for divalent or monovalent ions. In the rare earth series, with like charge, the affinity for the resin decreases with increasing ionic radii. The observed series is as follows
La3+ > Ce3+ > Pr3-*- > Nd3+ > Sm3+ > Eu3+ > Gd3+ > Tb3+ >
Dy3+ > Y3+ > Ho3+ > Er3+ > Tm3+ > Yb3^ > Lu3+ > Sc3+ >
Ba2+ > Sr2+ > Ca2+ > Mg2+ > Be2+ > Cs+ > Rb+ > NH^ >
K+ > Na+ > H+ > Li+
A quick separation of 152Eu — 241 Am mixture with ammonium lactate as eluant has been described . The influence of the flow rate of the eluant and the particle size of the ion exchanger has been studied. Katex KP-1, a polystyrol cation exchange resin was used to separate 152Eu, 154Eu and 147Pm mixture with success .
The separation of rare earths by anion exchange resins depends on somewhat different factors for a given chelating agent than to the cation exchange resins. For EDTA in an anion exchange process the order of elution is not the inverse order of the stability constants as in cation exchange process, rather 
Lu3+, Yb3+, Tm3+ ^ Sc3-4-, Er3* Y3+, Ho3+, La3+, Dy3+, Ce3f, Tb3+, Pr3*, Nd3+, Gd3+, Pm3+, Sm3+, Eu3+
The distribution coefficient increases from La3+ to Lu3+ and then decreases with increasing Z. When citrate ion is used as eluant the elution sequence is the reverse of the order for cation exchange resins .