# Separating lanthanides using ion-exchange method

Following statements are on Separation of rare earth metals (lanthanides) via in-exchange method

The tripositive ions are strongly sorbed by cation exchange resin: $$\ce{La^{3+}}$$, the largest is most strongly held; $$\ce{Lu^{3+}}$$, the smallest is least strongly held. Elution of $$\ce{HCl}$$, therefore removes $$\ce{Lu^{3+}}$$ first and $$\ce{La^{3+}}$$ last.

Generally, the smallest cation forms the strongest bond, so shouldn't $$\ce{Lu^{3+}}$$ be most strongly held? The most common cation exchangers, such as $$\ce{RSO3^-Na+}$$, where $$\ce{R}$$ is a cross-linked matrix are hard bases, so shouldn't according to HSAB principle also, $$\ce{Lu^{3+}}$$ be most strongly held?

Follow-on excerpt,

The separation is however not good, particularly between the heavier lanthanides, but is markedly improved by the addition of complexing agents such as $$\ce{EDTA}$$, or ammonium citrate/citric acid buffer to the eluting solution. The citrate ion complexes most strongly with the smaller elements, i.e. with the heavier elements.

Now, the two compounds (cation-exchanger and complex) helds strongly different elements, one smaller in size and other larger in size. How will presence of two enhance separating property? Also here ammonium phosphate is mono-, di- or tribasic?

Following graphs are also given

Please explain the meaning and significance of the graphs.

• What do you want us to do, exactly ? We are not used to solve all your problems without you having at least tried to answer. We may help you. But we will not all the job, and you nothing. So try to solve some part of your questions. We will help you – Maurice Mar 25 at 12:42
• @Zenix Yes there are ion exchange resins with sulfonate groups to soften water. This is plausible to interact with hard cations (HSAB). An ion exchange resin may be altered, though, for an exchange of any combination of (hard / soft) (metal cation, small anion) may use other groups (e.g., carboxylic acids, amino acids, alkylated ammonium groups) instead to purify water; solution of salts, sugars, proteins, etc. pp. See a list of resins available in bulk (e.g. by Rohm & Haas, lenntech.com/products/resins/rohm-haas/…), which is a selection of a few. – Buttonwood Mar 25 at 13:00
• @Maurice these aren't my homework problem, my queries that came when I was reading about separation of lanthanides using ion exchange resin. If I could figure them out, why would've I asked you? It would we gr8, if u could answer any part.. – Zenix Mar 25 at 14:05

You must be reading this from really old ion exchange references. It is always good to mention the resources and references for other readers.

Ion-exchange retention can be very roughly approximated with Coulomb's law, which says that the force of interaction between two "ions" is directly proportional. The force of interaction is inversely proportional to the square of the hydrated radius of the ions. Here the word interaction means attraction or repulsion.

So you have to look up the hydrated radii of the lanthanides and try to see a trend on a simple sulfonated cation exchanger.

You second point is a misconception. The "trick" to add complexing agents works when you wish to alter the sign of the species you wish to separate. For example Cu(II) has a +2 charge, if I complex it with EDTA, the [Cu(EDTA)] has a charge of -2 Think about the Coulomb's law now. Can we use a cation exchanger now? This time we must try an anion exchanger.

One can still use a cation exchangers, in that case the complexed metallic anions will be repelled out and those ions which did not complex well will stay on the resin.

• This was shared by my chemistry (school) teacher, (lockdown due to COVID-19) can't say if he took it from really old reference. Also I didn't get the graphs, please explain them too. What about the citrates? If we take ammonium citrate, polarity (sign of charge) on complex won't change.. – Zenix Mar 25 at 15:22
• I cannot explain anything from the graphs without seeing the whole figure captions. Post full pictures and all the context. – M. Farooq Mar 25 at 15:24
• Caption is just "Elution of tripositive lanthanide ions". It was given at the end, without any reference :( – Zenix Mar 25 at 15:28
• Sorry, then nobody can say anything. You will have to search. Is it an academic assignment or a practical problem? Please provide the context. – M. Farooq Mar 25 at 15:33
• Okay, makes sense now. I think the teacher just needs you to learn the concept of ion-exchange process rather than going into the nitty-gritty details. Just keep the facts I told you in your mind. You would probably need a semester of PhD level separations course to go the depth. – M. Farooq Mar 25 at 15:42