Urea in widely employed as a nitrogen-release fertilizer with more than 90% of urea production destined for agricultural applications[1]. Urea is also known to form complexes with fatty acids[2], which have been employed for separation of complex mixtures and purification processes[3]. In this application note, we present the quantification of the concentration of urea in ethanol by Raman Spectroscopy and show how this method can be employed for determining the percentage of urea in a solid inclusion compound with stearic acid[4].

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Chemicals: Urea (Aldrich, >99% ); Stearic acid (SA) (Aldrich, >95%); Ethanol (Soria)

Instrument: i-Raman® Plus 785S

Stock solutions of urea (0.0420 gr/gr ethanol) and SA (0.04126 gr/gr ethanol) were prepared. Standard solutions were prepared by mixing these stock solutions in different proportions in order to obtain urea concentrations between 0 and 0.042 gr/gr ethanol and keeping the total mass concentration (urea + SA) approximately constant. Raman spectra of 0.5 mL of each solution in aluminum containers were measured employing a 785 nm laser (power: 30%, ~ 90 mW) and 5000 msec of acquisition time (20 repetitions). No effects of heating or evaporation were observed. Spectra of the standard solutions that were background corrected employing the BWSpec® software are shown in Figure 1.

Spectra were normalized by employing the intensity of the ethanol band at 1049-1050 cm^-1. Normalized spectra (Figure 2) clearly show that the only appreciable change due to the increment in relative urea concentration occurs at the band at 996-997 cm^-1, which is attributed to urea. This band corresponds to the symmetric C-N stretching[5] that is experimentally and theoretically reported at about 1010 cm^-1 for the solid urea,[6] but shifts to lower wavenumbers in solution.[5,7]

For quantification purposes, spectra were deconvoluted, fitting the experimental results in the region 950-1200 cm^-1 by 4 Lorentzian functions. These curve fitting results are shown in Figure 3 for some of the standard solutions.

The ratio of the intensities of the fitted peaks assigned to urea at 996 cm^-1 (peak 1, a1) and ethanol at 1049 cm^-1 (peak 2, a2) was employed as the analytical parameters. The dependence of this ratio on the urea concentration of the samples is presented in Figure 4. The calibration curve plotted in this figure shows a good linear behavior which indicates this parameter could be employed for urea quantification.

For the determination of the urea content of real samples containing both urea and SA, the solid sample was dissolved in ethanol (0.04299 gr/gr ethanol) and Raman spectra were recorded in the same conditions. From the values of the ratio a1/a2 for this sample, obtained by fitting of the peaks at 996 and 1049 cm^-1 (Figure 5), the urea concentration of the solution was determined to be 0.03274 gr urea /gr ethanol. Thus, the urea content of the sample resulted to be 76 % w/w. This value is consistent with other reported values for the inclusion compounds formed by stearic acid and urea (ca. 75 %).[2,4]

1. J. H. Meessen, H. Petersen, in Ullmann’s Encycl. Ind. Chem., Wiley-VCH Verlag GmbH & Co. KGaA, 2000.
2. H. Schlenk, R. T. Holman, J. Am. Chem. Soc. 1950, 72, 5001–5004.
3. D. G. Hayes, Y. C. Bengtsson, J. M. Van Alstine, F. Setterwall, J. Am. Oil Chem. Soc. 1998, 75, 1403–1409.
4. A. Strocchi, G. Bonaga, Chem. Phys. Lipids 1975, 15, 87–94.
5. R. Keuleers, H. O. Desseyn, B. Rousseau, C. Van Alsenoy, J. Phys. Chem. A 1999, 103, 4621.
6. B. Rousseau, C. Van Alsenoy, R. Keuleers, H. O. Desseyn, J. Phys. Chem. A 1998, 102, 6540–6548.
7. D. Gangopadhyay, S. K. Singh, P. Sharma, H. Mishra, V. K. Unnikrishnan, B. Singh, R. K. Singh, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2016, 154, 200–206.