Determination of the rare earth element neodymium in ore by UV-Vis spectrophotometry

Introduction

China possesses the world’s largest reserves of rare earth elements; however, if these are not developed and utilised properly, they can pose significant risks to the environment, living organisms and human health. Rare earth elements and their compounds, as specialised materials, are now widely used in cutting-edge fields of modern technology. Currently, the primary methods for detecting rare earth elements include atomic emission spectroscopy, spectrophotometric analysis and mass spectrometry. Among these, spectrophotometry offers advantages such as simple operation, rapid and efficient analysis, high sensitivity, good selectivity, low cost and a wide range of applications. When combined with a highly selective colouring agent, it can be used to analyse samples with complex compositions. The main factors influencing spectrophotometric analysis include: wavelength of incident light, temperature, concentration, acidity, chemical composition, colouring agent and reaction time.

UV-Vis spectrophotometry is widely used in the analysis of rare earth element content. Azoarsenite(III) reacts with rare earth elements to produce a colour, and the colour development is essentially consistent, making it suitable for the determination of rare earth samples. In hydrochloric acid medium, the colour reaction between rare earth elements and azoarsenite(III) under weakly acidic conditions is highly sensitive, forming a stable blue-violet complex. Consequently, the content of rare earth elements can be directly determined using UV-Vis spectrophotometry. This study first identified the colouring agent and sensitiser; subsequently, using azoarsenite(III) as the colouring agent and sodium dodecylbenzenesulphonate as the sensitiser, various influencing factors were investigated to determine the optimal experimental conditions. Finally, analyses were conducted on ore samples, yielding satisfactory results.

1. Experimental Section

1.1 Apparatus and Reagents

Apparatus: UV-Vis spectrophotometer; analytical balance; pH meter; volumetric flasks, beakers, pipettes, and cuvettes.

Reagents: Nd₂O₃; dimethylphenol orange; chrome black T; chrome azure S; 1,2-(2-pyridylazo)-2-naphthol; 1,10-phenanthroline; copper reagent; ZrOCl₂·8H₂O; alizarin red; Methyl orange; Methylene blue; Sodium dodecylbenzenesulphonate; Sodium dodecylsulphonate; Naphthol green; Aluminium reagent; Sodium fluorescein; Magnesium reagent I; Tween X-100; β-Cyclodextrin; Tween 80; Hexadecyltrimethylammonium bromide; Mg(NO₃)₂·6H₂O; Cu(NO₃)₂·3H₂O; Zn(CH₃OO)₂; Ba(NO₃)₂; Ni(NO₃)₂·6H₂O; CaCl₂.

1.2 Preparation of Solutions

Neodymium(III) standard solution (1.00 × 10² mg/L): Accurately weigh 0.0592 g of Nd₂O₃ into a dry, clean beaker; add a small amount of hydrochloric acid to dissolve it, then transfer the solution to a 500 mL volumetric flask; make up to the mark with distilled water, shake well and set aside. Dilute to 10 mg·L⁻¹ when in use.

Azoarsenite(III) solution (20 mg·L⁻¹): Accurately weigh 0.0211 g of azoarsenite(III) into a dry, clean beaker; add a small amount of distilled water to dissolve, then transfer to a 100 mL volumetric flask; make up to the mark with distilled water, shake well and set aside.

Sodium dodecylbenzenesulphonate (10 g/L): Accurately weigh 1.0035 g of sodium dodecylbenzenesulphonate into a dry, clean beaker; add distilled water to dissolve, then transfer to a 100 mL volumetric flask; make up to the mark with distilled water, shake well and set aside.

Hydrochloric acid (0.01 mol/L): Using a measuring cylinder, transfer 0.5 mL of 2 mol/L hydrochloric acid to a clean, dry beaker; add a small amount of distilled water, then transfer to a 100 mL volumetric flask; make up to the mark with distilled water, shake well and set aside.

1.3 Experimental Method

First, pipette 2.00 mL of a neodymium standard solution (10 mg/L) into a 25 mL colorimetric tube, then add, in sequence, 2.30 mL of a 20 mg/L azobenzene solution, 0.5 mL of a 10 g/L sodium dodecylbenzenesulphonate solution, 0.1 mL of 0.01 mol/L hydrochloric acid, and dilute to the mark with distilled water. Shake well and leave to stand for 10 minutes. Finally, using a blank reagent as the reference, measure the absorbance of the complex in a 1 cm pathlength quartz cuvette at a wavelength of λ = 652 nm using a UV spectrophotometer. Plot a standard curve relating the concentration of neodymium(III) ions to the absorbance of the complex, and determine the content of neodymium(III) in the ore.