AN-T-250
2026-06
Potentiometric analysis of rare earth elements (REEs)
Accurate and precise back-titration of rare earth elements with the copper-selective electrode
Summary
Rare earth metals are comprised of 17 elements including the lanthanide series as well as the elements scandium and yttrium. These REEs are primarily used in batteries, nanotechnology, photovoltaics, medical technology, aerospace, and military technology.
The concentrations of REEs in ore and rock must be measured to determine the viability of a rare earth deposit. After crushing, the ores are dissolved, separated, and purified, and the REEs, along with other elements, are monitored throughout this stage. Determining the mass fraction of rare earth metals here is crucial and involves significant effort.
This Application Note presents a rapid and precise back-titration method using the Cu-ISE, which clearly separates a number of rare earth metals, even when they are combined with other elements, and enables them to be analyzed with nearly 100% recovery.
Introduction
Alongside gravimetry, rare earth metals titration is an absolute method which is often used as a reference for chromatographic and spectroscopic methods. As sample preparation for titration and ICP (inductively coupled plasma) is virtually identical, titration offers unbeatable cost and effort advantages in this case. Furthermore, back-titration is highly flexible and can be adapted to individual customer samples. Titration can therefore certainly be used as a rapid on-site analytical method.
A standardized ethylenediaminetetraacetic acid (EDTA) solution is added in excess to the dissolved and buffered rare earth sample (Ln3+). An EDTA-metal complex then forms. Any unused EDTA is then back-titrated with copper sulfate.
The following reactions offer a highly simplified description of the titration:
Ln3+ + EDTA → Ln(EDTA)
CuSO4 + EDTA(excess) → Cu(EDTA)
The amount of rare earth content can be calculated based on the amount of copper sulfate used.
Sample and sample preparation
This application is demonstrated on yttrium, scandium, lanthanum, cerium, and neodymium standard solutions. In addition, testing was performed on a mixture of lanthanum and scandium, as well as a synthetic mineral sample that closely resembles steenstrupine.
No sample preparation is required.
Experimental
The determination is carried out using an OMNIS Sample Robot S – WSM, an OMNIS Professional Titrator equipped with OMNIS Dosing Modules, as well as a copper-selective electrode (Figure 1).
An appropriate amount of sample is weighed into the titration beaker, and acetate buffer as well as standardized EDTA solution are added. After a waiting time, the solution is titrated until after the first or second equivalence point with standardized CuSO4 solution.
Results
As shown in Table 1, the results for the REE standards present high recovery rates and confirm the robustness of the back-titration.
Table 2 shows the results for the lanthanum and scandium sample mixture. This demonstrates the potential for separating the two rare earth elements, with good recovery rates for both.
The result of the synthetically made REE mineral «steenstrupine» with a mass concentration of 8.000 g/L of cerium and an approximate molecular composition of Na7.9HXCe6Mn1.6Fe1.8Zr0.3P4.5Si1.7Cl4.4N7.9S6OY is summarized in Table 3.
An exemplary titration curve of a mixture of lanthanum and scandium is given in Figure 2.
| Sample (n = 6) | Content (g/L) | Recovery (%) |
|---|---|---|
| Yttrium | 10.07 | 100.9 |
| Scandium | 10.07 | 100.6 |
| Lanthanum | 13.88 | 99.6 |
| Cerium | 16.01 | 100.1 |
| Neodymium | 10.06 | 100.6 |
| Sample (n = 2) | Content (g/L) | Recovery (%) |
|---|---|---|
| Lanthanum | 6.88 | 98.7 |
| Scandium | 5.13 | 102.4 |
| Sample (n = 3) | Content Ce(III) (g/L) | Recovery (%) |
|---|---|---|
| Steenstrupine | 7.93 | 99.1 |
Conclusion
Back-titration is a cost-effective and precise alternative to conventional analytical methods, such as ICP, for determining the presence of rare earth elements. As a method of determination, potentiometric titration is flexible in handling the wide variety of REE minerals. Thanks to optimization of the analytical matrix and the use of complex chemistry, it can even separate certain REEs from each other in mixtures.
The automated system with the OMNIS Sample Robot S – WSM equipped with an OMNIS Titrator, OMNIS Dosing Modules, and copper-selective electrode, impresses with its high level of professionalism and offers flexible analyses combined with high-end software.
References
- Misumi, S.; Taketatsu, T. Complexometric Titration of Rare Earth Elements. Dissolution of the Rare Earth Oxalate with Ethylenediaminetetraacetic Acid and Back Titration with Magnesium Sulfate. bull. Chem. Soc. Jpn. 1959, 32 (8), 873–876. DOI:10.1246/bcsj.32.873
- Seel, F. Die Komplexometrische Titration. Die Chemische Analyse. Band 45. Von G. Schwarzenbach Und H. Flaschka. Ferdinand Enke Verlag, Stuttgart 1965. 5. Neubearbeitete Aufl., XVI, 339 S., 42 Abb., 12 Tab., 48,– DM. Angewandte Chemie 1966, 78 (8), 455–455. DOI:10.1002/ange.19660780817
- Chelates in Analytical Chemistry – A Collection of Monographs; Flaschka, H. A., Barnard, A. J., Eds.; Marcel Dekker, Incorporated, 1967; Vol. 3.
- Krebs, D.; Furfaro, D. Concentrated Hydrochloric Acid Leaching of Greenland Steenstrupine to Obviate Silica Gel Formation. In Rare Earth Elements - Emerging Advances, Technology Utilization, and Resource Procurement; IntechOpen, 2022. DOI:10.5772/intechopen.107012
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