3 hydroxy 2 naphthoic acid

For synthesis, the IL precursor trihexyltetradecylphosphonium chloride (95%) and 3-hydroxy-2-naphthoic acid (98%) were purchased from Sigma-Aldrich (USA) along with potassium hydroxide (p.a., ≥86%) and the solvents methanol (HPLC grade, ≥99.9%) and dichloromethane (p.a., ≥99.9%). Element standard solutions of La, Ce and Lu (1000 mg L⁻¹ ± 4) in 2% (w/w) HNO₃ for the preparation of the extraction solutions were purchased from Sigma-Aldrich (USA), Nd and Ho (999 μg mL⁻¹ ± 5) in 5% (w/w) HNO₃ from AlfaAesar. The Pt standard solution (1000 mg L⁻¹ in 10% (w/w) HCl) used for sample preparations before TXRF measurements was purchased from VWR. A 3.0 g L⁻¹ polyvinyl alcohol (PVA) solution was prepared from hydrolysed polyvinyl alcohol (86–89%) with low molecular weight (AlfaAesar). For pH adjustment and dilutions, we used sodium hydroxide (50% (w/w) in H₂O, extra pure, Acros Organics) and nitric acid (>68%, PrimarPlus-trace analysis grade, Fisher Scientific). Ultra-pure water of resistivity >18.2 MΩ cm was obtained from a Millipore Milli-Q apparatus (Merck Millipore, USA).