We aimed to develop and validate rapid, simple and green non extractive spectrophotometric methods for the determination of valsartan in tablet dosage form.

A double–beam Shimadzu UV-Visible spectrophotometer, with spectral bandwidth of 1 nm wavelength accuracy ±0.5 nm, Model –UV 1800 (Japan), Software UV-Probe 2.62, and a pair of 1 cm matched quartz cells, was used to measure absorbance of the resulting solution. Designed in accordance with the governing Japanese and European Pharmacopoeia, the new UV-1800 UV-VIS spectrophotometer achieves a resolution of 1 nm, the highest in its class, in a compact design.

All the chemicals used were of analytical reagent grade.

The valsartan was provided by Sigma-Aldrich (≥ 98%, HPLC).

Bromophenol blue (Honeywell Fluka) was prepared as 1.3 × 10^-3 M methanol solution.

Methyl red (Honeywell Fluka) was prepared as 3.7 × 10^-3 M ethanol solution.

Spectrophotometric method for the determination of valsartan with BPB

Proposed procedure for the determination of valsartan with BPB

24.54 mg of CRS valsartan was transferred into a 25 mL volumetric flask with 15 mL methanol. The mixture was shaken and diluted to volume with methanol. Aliquot 0.4 mL was added to 1.0 mL of 1.3 × 10^-3 M methanol solution of BPB. The volume 25.0 mL was made up to the mark by adding methanol. The absorbance was measured at 424 nm against the reagent blank, which was similarly prepared by omitting the drug. The calibration curve was performed by plotting the measured absorbance values versus concentration.

Procedure for pharmaceutical formulation for the determination of valsartan with BPB

Twenty tablets were accurately weighed and powdered. A quantity of powder containing 24.54 mg of valsartan was transferred into a 25 mL volumetric flask with 15 mL methanol. The mixture was shaken for 15 min, diluted to volume with methanol and then filtered using 0.2 µm Nylon filter membrane. Aliquot 0.4 mL was added to 1.0 mL of 1.3 × 10^-3 M methanol solution of BPB. The volume 25.0 mL was made up to the mark by adding methanol. The absorbance was measured at 424 nm against the reagent blank, which was similarly prepared by omitting the drug. The calibration curve was performed by plotting the measured absorbance values versus concentration.

Spectrophotometric method for the determination of valsartan with MR

Proposed procedure for the determination of valsartan with MR

11.73 mg of CRS valsartan was transferred into a 25 mL volumetric flask with 15 mL ethanol. The mixture was shaken and diluted to volume with ethanol. Aliquot 0.3 mL was added to 0.5 mL of 3.7 × 10^-3 M ethanol solution of MR. The volume 25.0 mL was made up to the mark by adding ethanol. The absorbance was measured at 494 nm against the reagent blank, which was similarly prepared by omitting the drug. The calibration curve was performed by plotting the measured absorbance values versus concentration.

Procedure for pharmaceutical formulation for the determination of valsartan with MR

Twenty tablets were accurately weighed and powdered. A quantity of powder containing 11.73 mg of valsartan was transferred into a 25 mL volumetric flask with 15 mL ethanol. The mixture was shaken for 15 min, diluted to volume with ethanol and then filtered using 0.2 µm Nylon filter membrane. Aliquot 0.3 mL was added to 0.5 mL of 3.7 × 10^-3 M ethanol solution of MR. The volume 25.0 mL was made up to the mark by adding ethanol. The absorbance was measured at 494 nm against the reagent blank, which was similarly prepared by omitting the drug. The calibration curve was performed by plotting the measured absorbance values versus concentration.

The developed analytical methods were validated in accordance with the requirements of ICH Q2 (ICH 2005) on the following parameters: selectivity, linearity, limits of detection and quantification, range of application, accuracy, precision and robustness.

A non extractive binary complexes between valsartan and bromophenol blue (BPB) and methyl red (MR) were formed based on ion-pair associates. This might be due to the electron-donating groups (tetrazole and amino butanoic acid) present in the valsartan structure. Valsartan forms ion-pair complex selectively with the dyes, as indicated by the formation of a coloured complex with BPB at pH 5.5 with λmax at 424 nm and MR at pH 4.3 with λmax at 494 nm. These ion pair associates of valsartan with the anion of dye were slightly soluble in water, but under the optimized experimental conditions, they become freely soluble and neither needed an extraction into organic solvents nor addition of non-ionic surfactants. Absorption spectra of valsartan with BPB and MR are presented in Figures 2, 3.

Figure 2. 

Absorption spectra of valsartan with BPB

Figure 3. 

Absorption spectra of valsartan with MR.

Figure 4. 

Absorption spectra of BPB and valsartan with BPB in different solvents.

Figure 5. 

Absolute values ∆ A of the difference in the absorption of valsartan with BPB in different solvents with the absorption of BPB in different solvents.

Figure 6. 

Absorption spectra of MR and valsartan with MR in different solvents.

Figure 7. 

Absolute values ∆ A of the difference in the absorption of valsartan with MR in different solvents with the absorption of MR in different solvents.

Scheme 1. 

Proposal of the reaction pathway between valsartan and BPB with formation Valsartan-Bromphenol blue complex.

Scheme 2. 

Proposal of the reaction pathway between valsartan and MR with formation Valsartan-Methyl red ion pair associates.

Optimisation of conditions is necessary for rapid and quantitative formation of colored ion-pair complexes with maximum stability and sensitivity. Maximal absorbance was observed in methanol solution with BPB and ethanol solution with MR and while chloroform, propanol, acetonitrile and ethylacetate were not suitable. Absorption spectra of valsartan with BPB and MR in different solvents are presented in Figures 4, 6. The absolute values ∆ A of the difference in the absorption of valsartan with dyes in different solvents with the absorption of dyes in different solvents are presented in Figures 5, 7.

To establish the analytical sensitivity of valsartan with BPB and MR, the sensitivity of the reactions were calculated. The molar absorption index (ε) was 4.43 × 10³ for BPB and 2.36 × 10⁴ for MR, the specific absorption (a) was 1.07 × 10^-1 for BPB and 5.4 × 10^-1 for MR, and the Sendel coefficient (Ws) was 9.83 × 10^-2 for BPB and 1.84 × 10^-2 for MR. As a result of calculations of analytical indicators of sensitivity of reactions it was established that reaction of valsartan with MR has higher sensitivity, than reaction of valsartan with BPB that was testified by high value of molar coefficient of absorption and low value of an opening minimum.

The stoichiometry of the reactions was determined using Job’s method of continuous variation (Job P. 1936). Master equimolar solutions of 1.3 × 10^-3 M BPB and 3.7 × 10^-3 M MR with valsartan were prepared. The method revealed 1:1 ratio for both dyes (dye:drug). The results obtained from molar ratio studies were in agreement with the suggested reaction mechanism (scheme 1, 2) (Figures 8, 9).

Linearity

Beer’s law limit, molar absorptivity, detection limit, regression equation and correlation coefficient were obtained by least square treatment of results (ICH 2005). The linear relationship was found between absorbance at λmax and concentration of drug in the range 8–24 µg/mL for BPB and 4–20 µg/mL for MR. Regression analysis of Beer’s law plot at 424 nm yielded the regression equation, y = 0.0102x + 0.1636(BPB) and at 494 nm y = 0.0222x – 0.0063 (MR). High values of correlations coefficient (R²=0.9988 (BPB) and R²=0.9991 (MR)) and small values of intercept validated the linearity of calibration curve and obedience to Beer’s law. Calibration curves are presented in Figures 10, 11.

The ICH guidelines were followed in order to determine the LOD and LOQ. Accordingly, the method based on the standard deviation of the response and the slope has been applied, so that 3.3 and 10 times the standard deviation values of y-intercept of regresion line and the regression equation were used to calculate the LOD and LOQ. The LOD and LOQ values were calculated to be 1.03 µg/mL and 3.43 µg/mL respectively (BPB) and 0.68 µg/mL and 2.26 µg/mL respectively (MR).

Figure 8. 

Plot of Job’s method for determination of molar ratio of the formed Valsartan-BPB ion pair complex.

Figure 9. 

Plot of Job’s method for determination of molar ratio of the formed Valsartan-MR ion pair complex.

Figure 10. 

Calibration curve for the determination of valsartan with BPB.

Figure 11. 

Calibration curve for the determination of valsartan with MR.

The proposed methods were tested in order to assess its selectivity using the artificial mixture for analysis. It has been confirmed that the measured absorbance was only produced by the analyte. A synthetic mixture was prepared, containing valsartan (160 mg), lactose monohydrate, microcrystalline cellulose, povidone, croscarmellose sodium, colloidal anhydrous silica, magnesium stearate, hypromellose, titanium dioxide. The extract was yielded according to the procedure that was described for tablets and subsequently analyzed using the procedure previously described. The replicate analysis (n = 5) for a concentration level of 16 µg/mL valsartan has yielded the % valsartan recovery at 100.14 ± 1.03 (BPB) and for a concentration level of 12 µg/mL has yielded the % valsartan recovery at 100.27 ± 1.15 (MR), and thus reveled that the inactive ingredients did not interfere with valsartan determination.

Intra-day and inter-day precision values have been calculated by replicate analysis (n = 5) of calibration standard, at three different concentration levels, during the same day, and then during 5 consecutive days. The RSD (%) values of intra-day and inter-day measurements have indicated a good precision. (Table 1). Accuracy, defined as the closeness between the reference and the found values, has been evaluated, on the other hand, as percentage relative error between the measured and theoretical concentration of valsartan. The results are presented in Table 1, and show good accuracy for developed methods.

The evaluation of robustness was carried out at the stage of development of spectrophotometric methods for the determination of valsartan during the establishment of optimal conditions for the course of reactions and determination of factors that may affect the optical density (volume of BPB and MR and stability of solutions over time). It was found that fluctuations in the volume of BPB and MR within ± 10% did not affect the results of the study (Figures 12, 13, Tables 2, 3) and the studied solutions were stable for at least 45 minutes (Figures 14, 15).

Figure 12. 

Absorption spectra of BPB and valsartan with BPB under conditions of studying the amount of added BPB.

Figure 13. 

Absorption spectra of MR and valsartan with MR under conditions of studying the amount of added MR.

Figure 14. 

Graph of the dependence of the adsorption of the reaction product of valsartan with BPB on time.

Figure 15. 

Graph of the dependence of the adsorption of the reaction product of valsartan with MR on time.

The proposed methods were applied for the quantification of valsartan in tablets pertaining to three commercial formulations. The results as presented in Table 4 reveal no significant differences between the proposed methods. The Student’s t- and the F-values at 95 % confidence level are less than the theoretical one, but nevertheless confirming a good agreement between the results obtained by the proposed methods.

An important aspect of modern pharmaceutical analysis is compliance with the principles of green chemistry, which must be taken into account when developing analytical methods for the determination of valsartan in substances and drugs (Van Aken K. et al. 2006; Peleshok K. et al 2021a, 2021b; Gałuszka A. et al. 2012; Shulyak N. et al 2021; Yuryeva O. et al 2018; Polyauk O. and Logoyda L. 2017; Mykhalkiv M. et al. 2018; Logoyda L. et al. 2018). Table 5 summarizes the results of developed methods found to be an excellent green analysis with a score of 89 for both.