Quantification of Urea in Ethanol by Raman Spectroscopy

Significance of the Topic

The quantification of urea in ethanol addresses critical needs in agriculture and chemical processing. Urea, a key nitrogen fertilizer, also forms inclusion complexes with fatty acids such as stearic acid. Accurate measurement of urea concentration supports quality control in fertilizer production, purification processes, and the characterization of inclusion compounds.

Objectives and Study Overview

This work aims to establish a simple, rapid Raman spectroscopy method to quantify urea dissolved in ethanol and to determine urea content in a solid inclusion compound with stearic acid. A calibration model is developed for urea concentrations up to 0.042 g urea per gram ethanol, and the method is applied to real samples of a urea–stearic acid binary system.

Methodology

Stock solutions of urea and stearic acid in ethanol were prepared at known concentrations. Calibration standards covering urea from 0 to 0.042 g/g ethanol (total solute mass kept constant) were measured in 0.5 mL aluminum containers. Raman spectra were collected with a 785 nm laser (~90 mW, 30% power) using 5 s acquisitions repeated 20 times. Background correction and baseline subtraction were performed using dedicated software. Spectra were normalized on the ethanol band at 1049 cm⁻¹ and analyzed in the 950–1200 cm⁻¹ range by fitting four Lorentzian peaks.

Used Instrumentation

• i-Raman® Plus 785S portable Raman spectrometer equipped with a TE-cooled CCD detector and fiber probe.
• BCR100A Raman cuvette holder for 9.5 mm probes, featuring a precision mirror and light trap.

Main Results and Discussion

Normalized spectra revealed that only the urea symmetric C–N stretching band around 996–997 cm⁻¹ varied significantly with concentration. The intensity ratio of this urea band to the ethanol reference at 1049 cm⁻¹ exhibited excellent linearity (R² ≈ 0.99) across the calibration range. Application to a dissolved urea–stearic acid sample yielded a urea concentration of 0.03274 g/g ethanol, corresponding to 76 % w/w urea—consistent with literature values for such inclusion compounds.

Benefits and Practical Applications

This Raman-based protocol offers rapid, non-destructive quantification with minimal sample preparation. It is suitable for on-site or laboratory analysis, providing reliable monitoring of urea in fertilizers, inclusion complexes, and process streams.

Future Trends and Potential Applications

Future developments may include extending the method to other solvent systems and analytes, lowering detection limits through advanced multivariate calibration, and integrating portable Raman devices into continuous process control and real-time quality assurance.

Conclusion

A straightforward Raman spectroscopy approach was demonstrated for quantifying urea in ethanol and in urea–stearic acid inclusion compounds. The method shows strong linearity, robustness against matrix effects, and practical applicability for industrial and research settings.

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