Sodium selenate is an important inorganic compound, but lacks reliable and accurate physico-chemical and spectral characterization information. This article described the in-depth physicochemical, thermal, and spectroscopic characterization of sodium selenate using various analytical techniques. The powder X-ray diffractogram showed well- defined, narrow and sharp peaks revealed that sodium selenate is crystalline in nature. The crystallite size was found to be in the range of 28.75 to 49.97 nm. The average particle size was found to be of 3.93 (d10), 14.44 (d50), and 40.648 (d90) µm with an average surface area of 0.676 m2/g. The differential scanning calorimetry showed the endothermic inflation at 588.81 °C with the latent heat of fusion 103 J/g. The thermogravimetric analysis revealed two steps of the thermal degradation process. Similarly, the differential thermogravimetric analysis exhibited the major peaks in the thermogram and disclosed Tmax at 852.65 °C. The UV-visible spectrum showed maximum absorbance at 205.1 nm (λmax). This indicated sodium selenate thermally more stable in nature. The Fourier transform infrared spectrum showed a peak at 888 cm-1 due to the Se-O stretching. This information would be very much useful in the field of nutraceuticals/ pharmaceuticals and other industries using sodium selenate as an ingredient.
Sodium selenate (Na2SeO4) is an important inorganic compound as a source of a trace essential nutritional element “selenium” and plays as an ingredient in multivitamins and livestock feeds (1-3). Selenium (Se) was investigated and observed to have the capability to delay the Alzheimer’s symptoms as well as the initiation of skin aging (4, 5). It also protects various enzymes against the heavy metals toxicity such as Pb, Al, Cu, and Cd (6). Various other role of selenium on investigation found as an antioxidant (7), anti-cancer (8), and heart disease (4). Sodium selenate has the other applications in glass (8), pesticide, and some fungicides (9, 10) manufacturing industry. Chronic exposure or excessive dose of sodium selenate (1.6 mg/kg of body wt.) is toxic to human or animals and may cause severe lung, kidney, and liver damage (2, 3). Selenium (Se) bioavailability and toxicity depend on their chemical form or speciation. The toxicity of most forms of the selenium is low and sodium selenate exists in the highest +6 oxidation state, thus is stable under alkaline and other oxidizing conditions (11). Selenium is more efficiently absorbed in the form of selenate than as selenite (94% versus 59%) (12).
Powder X-ray diffraction (PXRD), particle size distribution (PSD), differential scanning calorimetry (DSC), thermogravimetric/differential thermogravimetric analysis (TGA/DTG), ultraviolet-visible (UV-vis) spectroscopy, and Fourier transform infrared (FT-IR) spectrometry techniques can be used to solve the several problems experienced in the industries for pharmaceutical and/or nutraceutical formulation and development. To the best of our knowledge, a complete study on the physicochemical, thermal, and spectroscopic characterization of sodium selenate using these all techniques together is not yet reported, although sodium selenate has a wide range of applications in nutraceuticals, pharmaceuticals and other heavy industries (8-10, 12, 13). The physicochemical properties such as crystallite type and size, particle size, surface area, thermal properties, etc. that have the influence on its absorption, dissolution, and bioavailability have a lot of importance in formulation research and development (14). Therefore, the physico-chemical, thermal, and spectroscopic characterization of sodium selenate using PXRD, PSD, DSC, TGA/DTG, UV- Vis spectroscopy and FT-IR techniques were carried out and explored in-depth in this study.
2. Materials and Methods
Sodium selenate was procured from Alfa Aesar, USA. All other chemicals used in the experiment were of analytical grade available in India.
2.2 Methods of analysis
The sodium selenate sample was evaluated using the various analytical techniques for its physicochemical and spectroscopic characterization.
2.2.1 Powder X-ray diffraction (PXRD) analysis
The XRD analysis was accomplished on PANalytical X’pert Pro powder X-ray diffractometer system. The X-ray of wavelength (λ) 1.54056 Å was used. The data were collected in the form of a chart of the Bragg angle (2θ) vs. intensity (counts/second), and a detailed table containing information on peak intensity counts, d-value (Å), relative intensity (%), full-width half maximum (FWHM) (θ°) (Table 1). From the XRD results, the crystallite size (G) was calculated using X’pert data collector and X’pert high score plus processing software. The method was based on the width of the diffraction patterns obtained in the X-ray reflected the crystalline region. The crystallite size (G) was calculated by using the following Scherrer formula (15, 16):
G = kλ/(bCosθ)
Where, k is the equipment constant (0.5), λ is the X-ray wavelength (0.154 nm); b in radians is the FWHM of the peak and θ the corresponding Bragg angle.
The sample was prepared using 500 mg of sodium selenate. The sample holder ring was fixed to the sample preparation table, and the sample was prepared by back loading technique using sample preparation kit. Then it was spread with sufficient quantity in the holder ring to fill the ring cavity. The sample was pressed down using powder press block, and the surplus powder was scraped using a glass slide to get a densely packed specimen. The bottom plate was positioned onto the holder ring and clamp in position. The sample holder was removed from the sample preparation table by turning it upside down. A smooth surface of the sample was obtained to ensure the optimum results (16).
Bragg angle (°2θ)
Rel. Int. (%)
Crystallite size (G, nm)
Rel. Int.: Relative intensity; FMHM: Full width at half maximum.
Table 1: Powder X-ray diffraction data with Bragg angle, d-spacing, relative intensities, areas, and crystallite size analysis of sodium selenate.
2.2.2 Particle size distribution (PSD) analysis
The average particle size and particle size distribution were analyzed using Malvern Mastersizer 2000, UK with a detection range between 0.01 µm to 3000 µm. The sample was prepared with 0.15 g of sodium selenate added with 2 drops of span 85 and 10 mL of sunflower oil. The suspension was mixed with the mixer for 2 minutes. The sample unit was filled with the dispersant medium (sunflower oil) and operated the stirrer at 2500 rpm. Alignment of the optics was done and taken the background measurement. After the background measurement, the sample was added to the sample unit with constant monitoring the obscuration and stopped the addition of sample when the obscuration reached in between 10 and 20%. When the obscuration was stable, the measurement was taken six times. The histograms of the measurement were recorded. The particle size (μm) for at below 10% level (d10), 50% level (d50), and 90% level (d90) were calculated from the histogram (17) and presented in Table 2. The calculations were done by using software Mastersizer 2000.
One common measure of the cumulative distribution is the span (d90 – d10), and the relative span is a common calculation to quantify distribution width: (18, 19)
Relative span = (d90 – d10)/d50
Surface area (m2/g)
Table 2: Particle-size distribution (d10, d50 and d90) and surface area analysis of sodium selenate.
2.2.3 Differential scanning calorimetry (DSC)
The thermal properties of sodium selenate were analyzed using the DSC Q20 (TA instruments, USA) differential scanning calorimeter. 2.70 mg of sample was weighed and sealed in aluminium pans and equilibrated at 25 °C. Then the sample was heated up to 600 ºC at the heating rate of 10 ºC/min under nitrogen gas as purge atmosphere with a flow rate of 50 mL/min. The values of onset, endset, peak temperature, peak height (mJ or mW), peak area, and change in heat (J/g) for each peak were recorded.
The TG analysis was performed using instruments TGA Q50 (TA instruments, USA) at a heating rate of 10 ºC/min from room temperature, i.e. 25 ºC to 900 ºC under a nitrogen atmosphere (sample mass 11.86 mg on a platinum pan). In TGA, the weight loss for each step was recorded in grams as well as the percent loss with respect to the initial weight along with, the onset, endset, and peak temperature for each step were recorded. Similarly, in DTG, the onset, endset, peak temperature, and change in heat (J/g) of each peak were recorded.
The UV-vis spectroscopic analysis of sodium selenate was carried out using Shimadzu UV-2450 (Shimadzu, Japan) with UV probe. The spectrum was recorded using 1 cm quartz cell that has a slit width of 1.0 nm. The wavelength ranges chosen for recording the spectrum were 190-800 nm. The absorbance spectrum (in the range of 0.2 to 0.9) and absorbance maximum (λmax) were recorded.
FT-IR spectroscopy of sodium selenate was performed on Spectrum two (Perkin Elmer, USA) Fourier transform infrared spectrometer with the frequency array of 400-4000 cm-1 by using pressed KBr disk technique. The compound was run as pressed disks using 300 mg of KBr as the diluents and 2.06 mg of sodium selenate sample.
3. Results and Discussion
3.1 Powder X-ray diffraction (PXRD) analysis
Powder X-ray diffraction provides the characteristic diffraction pattern from the powder material which is often easier and more convenient method than individual crystal diffraction (20). The diffractogram of sodium selenate showed the XRD patterns (Figure 1). The intensity data were in Bragg’s angle (2θ) range of 10-100º (Figure 1). The PXRD patterns showed well-defined, narrow, sharp, significant peak at 2θ position equal to 18.43, 22.55, 27.30, 28.43, 31.05, 32.68, 37.26, 47.40, 48.84, 52.99, 57.2, and 68.31º. This observation represented the crystalline nature sodium selenate. The d-spacing obtained are 4.81, 3.94, 3.27, 3.14, 2.88, 2.74, 2.45, 1.92, 1.86, 1.73, 1.61, and 1.37 Å (Table 1), which help in identification of sodium selenate, because each compound has a set of unique d-spacing (21, 22). The d-spacing are in the range of 4.81 to 1.37 Å and decreased with increased 2θ values. Typically, this could be a set of standard d-spacing information in order to achieve the identification of the unknown samples.
The peak intensity (I) was reported as peak height intensity (Table 1) and the relative intensity was recorded as the ratio of the peak intensity to that of the strongest peak (I1) (relative intensity = I/I1 x 100). The integrated peak intensities also calculated and the average peak area calculated for the sodium selenate was 61.846 (Table 1). The relative peaks intensities could be altered due to the texture (i.e. preferred crystallographic orientation) in the sample (23).
The full-width half maximum (FWHM) is a very important factor for the calculation of crystallite size using the Scherrer equation (G = kλ/bCosθ) (15). The crystallite size was found to be in the range of 28.75 to 49.97 nm (Table 1). The average crystallite size of sodium selenate was calculated to be 41.25 nm (Table 1). Crystal lattice strain has the impact on the crystallite size (20). However, the PXRD analysis of sodium selenate is a rapid, non-destructive important technique and these data would be much useful in all stages of nutraceuticals, and pharmaceuticals research and development.
Figure 1: Powder X-ray diffraction patterns of sodium selenate.
3.2 Particle size distribution (PSD) analysis
The particle size of sodium selenate was investigated, and the results are presented in Table 2. The distribution width cited three values on the X-axis, the d10, d50, and d90 diameter as shown in Figure 2. The experiment was repeated for six- time, and the average of particle size and surface area were calculated from the data (Table 2). The average particle size of the sodium selenate powder sample was 3.93 (d10), 14.44 (d50), and 40.65 µm (d90) (Table 2). The PSD analysis indicated the average surface area of sodium selenate of 0.676 m2/g (Table 2). Surface area and energy of a particle play an important role in the dissolution of a formulation in a medium/solvent. The calculated span value was to be 36.71 µm, and relative span value was 2.54 particularly for this sodium selenate powder. The significance of the span value describes relative dispersion and could be applied to all particle distributions, and deviations often under d10 and over d90 (19, 20). The particle size distribution and its area play a significant role in nutraceuticals/pharmaceutical formulation development and research (24).
Figure 2: Particle size distribution (d10, d50, and d90) histogram of sodium selenate.
The thermal analysis of sodium selenate provided the decomposition temperature and latent heat of fusion and the results are presented in Figure 3. The DSC thermogram of sodium selenate represented a very sharp endothermic peak at 588.81 ºC and the ∆H fusion was about 103 J/g. The onset, peak, and endset temperature to the latent heat of fusion was 584.41, 588.81, and 596.19 ºC, respectively. The enthalpy of melting/fusion is the heat energy required for melting, i.e. for breaking down the sodium selenate crystalline lattice. The absence of glass transition temperature in the DSC thermogram proved the crystal nature of sodium selenate (25).
Figure 3: DSC thermogram is representing the decomposition temperature (°C) and latent heat of fusion (∆H in J/g) of sodium selenate.
The TGA and DTG thermograms of sodium selenate are shown in Figures 4a and 4b, respectively. The TGA analysis revealed the changes in physical and chemical properties of sodium selenate as a function of increasing temperature. The TGA thermogram showed two steps of thermal degradation process (Figures 4a). The sodium selenate sample had lost 0.14% (0.013 mg) and 6.73% (0.657 mg) in the first and second step of degradation respectively, of their total original weight (Figure 4a). The major weight loss was in the second step of degradation in the temperature range of 101.01 to 896.18 °C. The result indicated sodium selenate was highly thermally stable compound due to inorganic in nature.
The DTG analysis exhibited one small and one major peak in the thermogram (Figure 4b) of sodium selenate. The peak at 97.48 °C might be due to the trapped water during the experimentation, which was supported by the DSC and TGA data. The maximum temperature (Tmax) was at 852.65 °C. The onset, peak, and endset temperature were 757.89, 852.65, and 896.18 °C, respectively. These results indicated that sodium selenate is thermally stable compound for higher temperature (i.e. 852.65 °C). These data would be helpful for pharmaceuticals/nutraceuticals and other industries dealings with sodium selenate (26-27).
Figure 4: TGA (a) and DTG (b) thermograms of sodium selenate showing the degradation steps and temperature.
The UV-visible spectrum of sodium selenate is shown in Figure 5. It has been reported that the UV absorbance occurred due to the excitation of electrons from highest energy occupied molecular orbital (HOMO) to lowest energy unoccupied molecular orbital (LUMO) (28). The UV spectrum of the sodium selenate sample exhibited the maximum absorbance at 205.1 nm (λmax). The λmax at 205.1 nm was indicated the absorbance maxima of 2.5628. The information of UV-visible characterization data would be helpful in analytical chemistry for the qualitative and quantitative analysis of the analyte in samples containing sodium selenate.
The FT-IR spectrum of sodium selenate is presented in Figure 6. The spectrum showed a broad peak at 3541 cm-1 and 1723 cm-1 might be due to the O-H stretching and bending vibration respectively, due to the trapped water molecule, which is incorporated into the lattice structure of the crystalline inorganic compounds and produced the characteristic absorption bands. The information was supported by the DSC, TGA, and DTG analysis report. The spectrum showed the strong and sharp peak at 888 cm-1 (Figure 6) due to the Se-O stretching in the fingerprint region of sodium selenate. The pattern of the spectrum was closely matched with the reported literature (29). The characteristic FT-IR spectroscopic information would be helpful in order to quick, qualitative establishment of sodium selenate in various samples.
Figure 6: Fourier transform infrared (FT-IR) spectrum of sodium selenate.
The current study effectively investigated in-depth physicochemical, and spectroscopic characterization of sodium selenate using XRD, PSD, DSC, TG/DTG, UV-vis, and FT-IR analytical technique. The sodium selenate PXRD patterns showed significant narrow, and sharp; peak at Bragg’s angle (2θ) position, indicated the crystalline nature of sodium selenate. The crystallite size was found to be in the range of 28.75 to 49.97nm. The average particle size of sodium selenate was 3.93 (d10), 14.44 (d50), and 40.65 µm (d90) and average surface area of 0.676 m2/g. The calculated span and relative span value were 36.71 µm and 2.54, respectively. The DSC thermogram of sodium selenate showed endothermic inflation at 588.81 °C. The overall thermal characterization indicated sodium selenate has the long range melting/degradation temperature implies very high thermal stability. The UV-visible spectrum showed the highest absorbance maxima (λmax) at of 205.1 nm. The FT-IR spectrum showed the strong and sharp peak at 888 cm-1 due to the Se-O stretching. These in-depth, comprehensive, physico-chemical and thermal solid state characterization information of sodium selenate would be a useful asset. This can set a standard for scientific qualitative and quantitative analysis of nutraceuticals, pharmaceuticals and other industries utilizing sodium selenate as an ingredients.
The authors gratefully acknowledged to GVK Biosciences Pvt. Ltd., Hyderabad, India for their support.
6. Conflict of Interest
Authors confirm that this article content has no conflict of interest.
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