**5. Conclusions**

It was found that the logarithmic distribution co-e fficient λ for precipitation between a pair of rare earths was fairly close to the ratio of the equilibrium constants for the step from mono-oxalato complex to the rare earth oxalate salt. This model relates the total concentration of rare earths (mostly free rare earth ions) to the fractions precipitating into the solid, so it is unclear why only the last step of the precipitation counts for λ. There appears to be no simple kinetic explanation either, with λ being derived from thermodynamic terms, and the curious result that a higher stability of mono-oxalato complex results in less precipitation.

**Author Contributions:** H.W. was responsible for designing and conducting the experiments and drafting. Y.-K.L. provided academic supervision and feedback. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Acknowledged is the Minerals Research Institute of Western Australia for its funding of this project. Also acknowledged is the Australian Government Research Training Program for its funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
