Differential Pulse Anodic Stripping Voltammetric Analysis of Selenium (IV) at a Gold Electrode Modified with O-Phenylenediamine- Nafion

 

Abdul Aziz Ramadan*,  Hasna Mandil**,  Abdulrahman Shikh-Debes

Dept. of Chemistry, Faculty of Sciences, Aleppo University, Syria

 

ABSTRACT:

Differential pulse anodic stripping voltammetric analysis (DPASVA) of selenium(IV) using a gold electrode modified with O-Phenylenediamine-nafion (GEMO-PN) has been studied. Various parameters (electrolyte, deposition time, pulse duration, pulse amplitude, etc.) affecting the Se(IV) determination were examined.  Selenium(IV) was determined in an aqueous HClO₄ (0.2 M) medium of pH 0.22 at an accumulation potential of -350 mV (vs. Ag/AgCl) and an accumulation time of 200 s. Under the optimum conditions, liner calibration graph, I_p=f(C_Se⁴⁺), was obtained in the concentration ranges of 5x10^-8 M (3.948 ng/mL) to 1x10^-6 M (78.96 ng/mL) with relative standard deviations (RSD)3.8%, and the detection limit was 0.048 ng/mL. This method showed a good accumulation efficiency for selenium and good resistance to interferences from metal ions as well as those associated with selenium in pharmaceuticals. The results for the determination of Se⁴⁺ using (GEMO-PN) were more sensitive and accurate than that obtained using bare gold electrode; the sensitivity was increased about 2000 times.

 

 

 

INTRODUCTION:

A new sensitive and selective pulse polarographic method for the determination of Se (IV) next to interfering ions is described. This method is based on the highly sensitive peak formed by the reaction of Se (IV) with o-phenylenediamine in Britton-Robinson buffer solution (pH 2.5) at 0.08 V (vs. Ag/AgCl). A linear calibration graph is obtained for Se(IV) at a concentration of 5.0 x 10⁻⁸ to 6.8 x 10⁻⁵ mol/L with a correlation coefficient of 0.9988 [1]. The adsorptive voltammetric behavior of Se (IV) on the hanging mercury drop electrode (HMDE) in the presence of O-PDA has been investigated. Experimental results show that the Se (IV) complex with O-PDA can be adsorbed on the surface of the HMDE, yielding two peaks at -0.13 V and -0.62 V (vs. Ag/AgCl).

 

The detection limit of the adsorption voltammetry, established from the second peak at -0.62 V, is as low as 4×10⁻¹⁰ mol/L [2]. The O-phenylenediamine in situ modified electrode, which was used to detect the trace selenium in the human body was studied. The detection limit is 3.2× 10^-9 mol/L and the average percentage of recovery is 97.0±4.3% [3].

 

Determination of Se⁴⁺ by pulse anodic stripping voltammetry with constant amplitude of negative polarity (SVPNP) using a Vitamin E–Nafion modified gold electrode has been studied. Selenium (IV) was determined in a aqueous HClO₄ medium (pH=1.1) at an accumulation potential of -240 mV and an accumulation time of 300 s. The analytical signal was linear from in the concentration ranges of 1x10^-7 to 8x10^-6  mol/L with relative standard deviations (RSD) 5.2% [4].

 

Differential pulse anodic stripping voltammetric analysis of selenium (IV) using a gold electrode modified with 3,3′-diaminobenzidine.4HCl-nafion (GEMDN) has been studied. Selenium (IV) was determined in an aqueous HClO₄ (0.2M) medium of pH 0.22 at an accumulation potential of -200 mV and an accumulation time of 200 s. Under the optimum conditions, liner calibration graph was obtained in the concentration ranges of 5x10^-9 M to 2x10^-6 M with RSD4.6% [5].

 

Differential pulse anodic stripping voltammetric determination of selenium (IV) using a vitamin E-Nafion modified gold electrode has been studied. Selenium (IV) was determined in an aqueous HClO₄ medium of pH 1.10 at an accumulation potential of -240 mV and an accumulation time of 300s. Under the optimum conditions, liner calibration graph was obtained in the concentration ranges of 5×10^-8 - 1×10^-5 mol/L with relative standard deviations (RSD) 4.5 %. This method shows that the results for the determination of Se⁴⁺ using vitamin E-Nafion modified gold electrode were more sensitive and accurate than that obtained using bare gold electrode. The sensitivity was increased about 2000 times [6].

 

A simple, direct and very sensitive differential pulse anodic stripping voltammetric  analysis (DPASVA) of selenium (IV) in bulk and in dosage formulations using a gold electrode multi-modified with a mixture of {3,3′-diaminobenzidine.4HCl (Dab) and vitamin E (V_E) –Nafion} (GEMD_abV_EN) has been studied [7].

 

Differential pulse anodic stripping voltammetric analysis (DPASVA) of selenium (IV) using a methylene blue-Nafion modified gold electrode (MBNMGE) has been studied. Selenium (IV) was determined in an aqueous HClO₄ medium (1.0 M) at an accumulation potential of -240 mV and an accumulation time of 300s. The peak height was measured at 1030-1060 mV. The calibration graph for Se (IV) under optimized conditions was linear in the range from 1×10^-8 to 1×10^-6 mol/L (0.79 ng/mL  to 79 ng/mL). The detection limit was 5.0 × 10^-9 mol/L [8].

 

In the present work, DPASVA of selenium(IV) using a gold electrode modified with O-PDA -Nafion (GEMO-PN) has been applied.

 

Experimental:

Reagents:

Nafion perfluorinated ion-exchange resin in ethanol (3% v/v) was purchased from Aldrich. O-Phenylenediamine, mol. mass 108.144 g (Scheme 1) was from SIGMA. H₂SeO₃ and all other reagents were of analytical grade from Merck. An HClO₄ solution 0.20M  was used at pH=0.22. A stock solution (a) of Se (IV): 789.6 µg/mL (0.01 mol/L) and A stock solution (b) of Se (IV): 7.896 µg/mL (0.1 mmol/L) were prepared using HClO₄ solution. A working solution for voltammetric investigations was prepared by dilution of the stock solution of Se (IV)  (a or b) with HClO₄ solution.

 

 

Scheme1: O-Phenylenediamine, (C₆H₈N₂) or C₆H₄(NH₂)₂, O-PDA

 

Apparatus:

A polarograghic analyzer, model PRG-5 (Tacussel), with increasing amplitude pulses was used for differential detection of current and for superimposing constant amplitude pulses of negative or positive polarity and pulses of linearly increasing amplitude as the source of scanning voltage. A programmer model Polarmax-78, and a recorder model Ecosript (Tacussel) were also used. A rotating disk gold electrode (RDGE) model DI-65-14 was used as a working electrode. The reference electrode was Ag/AgCl model BJC. The solution was stirred with a rotating electrode and was kept in a thermostat at 25^oC. The diluter pipette model DIP-1 (Shimadzu), having 100 µL sample syringe and five continuously adjustable pipettes covering a volume range from 20 to 5000 µL (model PIPTMAN P, GILSON), were used for preparation of the experimental solutions, And a micro Syringe which size 5µL

 

Preparation of modified gold electrode:

Gold electrode was first polished, rinsed with deionized water and ultrasonicated successively in a 1:1 aqueous solution of nitric acid and an ethanol solution each for 3 min and then dried. A modified solution was prepared by putting 4.75 mL of O-PDA (10 mg/mL) and 3 mL of Nafion–Ethanol solution (10% v/v) in 10 mL volumetric flask, then the volume was diluted to the mark with ethanol (this solution contents 4.75 mg/mL O-PDA and 3% v/v Nafion). A modified Gold electrode was prepared by placing 5µL from modified solution onto the dry electrode with a micro syringe. The electrode was dried to evaporate the solvent and rinsed with deionized water.

 

^{Sample preparation:}

^{A commercial formulations (as tablet) were used for the analysis of} Se(IV) ^{by using} DPASVA ^with (GEMO-PN)^{. The pharmaceutical formulations were subjected to the analytical procedures:}

(1)    DamVita Silver Plus:

Tablets, Ultra Medica^, Sydnaya–SYRIA, Each tablet contains: 70 µg Selenium.

 

(2)    Daily-Vit:

Tablets, Biomed^, Damascus–SYRIA, Each tablet contains: 70 µg Selenium.

 

(3)    Adult Vit Silver:

Tablets, Afhamea^, Hama–SYRIA, Each tablet contains: 25 µg Selenium.

Three tablets of each studied pharmaceutical formulations were placed in the crucible of platinum, burning it until the flame  was ended, then crushed and dissolved with 10 mL nitric acid (65%). After that, it was heated until the drought, then dissolved with HClO₄ solution and filtered over a 100 mL flask and diluting to 100 mL with HClO₄ solution. Three stock solutions of pharmaceuticals: DamVita Silver Plus, Daily-Vit and Adult Vit Silver, which content: 2100, 2100 and 750 ng/mL of Se(IV), respectively.

 

Working solutions of pharmaceuticals:

These solutions were prepared by diluting 1.19, 1.19 and 3.33 mL of stock solutions of pharmaceuticals respectively to 100 mL with HClO₄ solution (each one content 25 ng/mL selenium). 

 

Working standard additions solutions of pharmaceuticals:

These solutions were prepared as the follows: same mentioned volumes of stock solutions of pharmaceuticals with 0.000, 0.100,  0.200, 0.400 and 0.600 mL from stock solution (b) of selenium and diluting to 100 mL with HClO₄ solution; each one content 25 ng/mL selenium (from  pharmaceutical formulations) with 7.896, 15.792, 31.584 and 47.376 ng/mL selenium from standard additions solutions of Se (IV),  respectively.

 

Procedure:

A 10 mL volume of a working solution containing an appropriate concentration of Se(IV) was transferred into an electrochemical cell. The accumulation potential               (-350 mV) was applied to the modified electrode for a certain time. The potential was then scanned from +400 to +1250 mV by differential pulse anodic stripping voltammetry using the auto-scan facility. The peak height was measured at 980-1000 mV.

 

Results and discussion:

Voltammetric behavior:

The differential pulse anodic stripping voltammograms using the procedure described above with a bare Au electrode (C_Se(IV) ≥1x10^-4 mol/L, RSD=6.8%), while an electrode modified with (GEMO-PN) shows that the peak potential shifted slightly from 980 mV to 1000 mV and the sensitivity increased (C_Se(IV) ≥5x10^-8 mol/L) when the (GEMO-PN) was introduced to modify the coating.

 

Effect of modified electrode composition:

The effect of the Nafion and O-Phenylenediamine concentrations in modified solution for formation gold electrode modified with O-Phenylenediamine-nafion (GEMO-PN) on the peak current were studied. The peak current reached its maximum when the concentration Nafion is 3% v/v and O-PDA is 4.75 mg/mL.

Effect of the accumulation potential:

The dependence of the differential pulse anodic stripping peak current on the accumulation potential was examined. It was found that the maximum response for selenium (IV) occurs with accumulation potentials equal to -0.350 V, (Figure 1).

 

Effect of accumulation time:

The dependence of the peak current on the accumulation time for Se (IV) concentrations was studied. The peak current increases with increasing accumulation time. The current is nearly linear from 50 to 450 s. Various parameters (electrolyte, accumulation time, accumulation potential, pH solution, scan rate, waiting time, stirring speed of electrode, initial potential, final potential and composition of modified solution) affecting the Se (IV) determination were examined, see (Figure 2). The optimum parameters for DPASV determination of selenium (IV) were selected and presented in the Table 1.

 

 

Fig.1. Effect of accumulation potential on differential pulse anodic stripping  voltammograms of Se(IV) 78.96 ng/mL using O-PDA–Nafion modified gold electrode, pH=0.22, scan rate 10 mV/s and temperature 25°± 0.5°C).

 

 

Fig.2. Effect of accumulation time on differential pulse anodic stripping voltammograms of Se(IV) 78.96 ng/mL using O-PDA–Nafion modified gold electrode (accumulation potential -350 mv, pH=0.22, scan rate 10 mV/s and temperature 25°± 0.5°C).

Table 1: The optimum parameters established for differential pulse anodic stripping voltammetric determination of selenium(IV)

parameters	Operating modes
Accumulation (deposition) time	200 s or 300 s
Accumulation potential	-350 mV
Supporting electrolyte	0.20 M HClO4
Indicator electrode	Rotating disk gold electrode (RDgE)
pH solution	0.22
Modified electrode composition	4.75mg/mL O-PDA and 3% v/v Nafion-Ethanol
Waiting time	5 s
Initial potential	+400 mV
Final potential	+1250 mV
Scan rate	10 mV/s
Stirring speed	1000 rpm
Temperature of solution	25°± 0.5°C

Analytical results:

The analytical curves, I_p=f (C_Se(IV)) for the determination of Se(IV) in presence of 0.20 M HClO₄ on the modified electrode (GEMDN) by DPASVA showed linear proportionality over the concentration range from 3.948 to 78.96 ng/mL (Figure 3). In this method we determined a very low concentration 3.948 ng/mL (5×10^-8 M) of Se(IV) on the O–PDA-Nafion modified gold electrode with relative standard deviation not exceed ±3.8% (Table 2). This method gives accurate and sensitive results compared with obtained results using bare gold electrode; the sensitivity was increased about 2000 times.

 

 

 

Fig.3. (a) Differential pulse anodic stripping voltammograms of Se(IV) on O–PDA Nafion modified gold electrode at pH=0.22 when C_Se(IV): 1- electrolyte,  2- 3.948, 3- 7.896, 4- 15.792, 5- 31.584, 6- 47.376, 7-63.168, and 8-78.96 ng/mL. (b) I_p = f (C_Se(IV)) for the determination of Se(IV) in presence of 0.20 M HClO₄ by DPASVA using a  O-PDA-Nafion modified gold electrode (accumulation time 200 s and 300 s , accumulation potential -350 mV, pH=0.22, scan rate 10 mV/s and temperature 25°± 0.5°C).

 

Table 2: Determination of selenium(IV) by DPASVA on a o-PDA–Nafion modified gold electrode (accumulation time 200 s, accumulation potential -350 mV, pH=0.22, scan rate 10 mV/s, temperature 25°± 0.5°C and n=5, t=2.776).

 

APPLICATIONS:

Many applications for the determination of Se(IV) in some pharmaceutical preparations by DPASVA on a O-PDA–Nafion modified gold electrode using the optimum parameters were proposed. Standard addition curves for determination of Se(IV) in different pharmaceutical preparations (DamVita Silver Plus, Daily-Vit and Adult Vit Silver) were used. The standard addition curve of DamVita Silver Plus (70 mg/tab.) was showed in (Figure 4), as an example. Regression equations and correlation coefficients were included in Table 3. Standard addition curves for determination of Se(IV) in different pharmaceutical preparations were used. The amount (m) of Se(IV)  in one tablet by mg/tab calculated from the following relationship: m = h. m', where: m' is the amount of Se(IV)  in tablet, which calculated from the standard additions curve according to the following regression equation: y=a.x+b; when y=0; m'=x= b/a= intercept/slope (ng/mL) and  (h)  conversion factors are equal  to 2.8, 2.8 and 1.0 for all pharmaceuticals  content 70, 70 and 25 µg/tab, respectively. The results of quantitative analysis for Se(IV) in the pharmaceutical preparations using this method were included in Table 4. The proposed method was simple, economic, accurate and successfully applied to the determination of Se(IV)  in pharmaceuticals. The results obtained agree well with the contents stated on the labels.

 

Table 3: Regression equations and correlation coefficients for determination of C_Se(IV) in pharmaceutical preparations using DPASV on a O-PDA–Nafion modified gold electrode (accumulation time 200 s, accumulation potential -350 mV, pH=0.22, scan rate 10 mV/s, temperature 25°± 0.5°C and n=5).

Pharmaceutical preparations	CSe(IV) in tab, µg	Operating modes
		Regression equations*	Correlation coefficients	m', ng/mL	Amount of Se4+ (m), µg/tab.
DamVita Silver Plus tablets, Ultra Medica, Sydnaya–SYRIA	70	y = 0.2947x + 7.5905	R2=0.9992	25.76	m = 2.8 m'=72.13
Daily-Vit tablets, Biomed, Damascus–SYRIA	70	y = 0.2927x + 7.4509	R2=0.9996	25.45	m = 2.8 m'=71.26
Adult Vit Silver tablets, Afhamea, Hama–SYRIA	25	y = 0.2923x + 7.1198	R2=0.9998	24.36	m = 1.0 m'= 24.36

_*y= n A, x= C_Se(IV) (ng/mL)= m' = ^{intercept/slope.}

 

 

Table 4: Determination of Se(IV)  in pharmaceutical preparations using DPASV on a O-PDA–Nafion modified gold electrode (accumulation time 200 s, accumulation potential -350 mV, pH=0.22, scan rate 10 mV/s, temperature 25°± 0.5°C and n=5).

Commercial name	Contents, µg/tab.	, µg/tab.	RSD%	Recovery %
DamVita Silver Plus tablets, Ultra Medica, Sydnaya–SYRIA	70	72.13	3.0	103.04
Daily-Vit tablets, Biomed, Damascus–SYRIA	70	71.26	2.8	101.80
Adult Vit Silver tablets, Afhamea, Hama–SYRIA	25	24.36	3.4	97.44

 

 

 

Fig. 4: The standard addition curve for determination of Se(IV) in  DamVita Silver Plus (70 mg/tab.)using DPASV on a o-PDA–Nafion modified gold electrode (accumulation time 200 s, accumulation potential -350 mV, pH=0.22, scan rate 10 mV/s, temperature 25°± 0.5°C and n=5).

 

Validation of Proposed Method:

The developed method for simultaneous estimation of Se(IV) has been validated in accordance with the International Conference on Harmonization guidelines (ICH) [9].

 

Selectivity:

Selectivity test determines the effect of excipients on the assay result. To determine the selectivity of the method, standard solution of Se(IV), commercial product solution and blank solutions were analyzed. The results of the tests proved that the effect of the presence of common excipients such as; starch, lactose, glucose, sucrose, and gum acacia no interference was introduced by any of them.

 

Linearity:

In the proposed methods, linear plots (n= 5) with good correlation coefficients were obtained in the concentration ranges of 3.948-78.960 ng/ml for Se(IV), linearity equations obtained were y=0.2968x+0.0243 (R²=0.9997).

 

Precision and Accuracy:

The precision and accuracy of proposed method was checked by recovery study by addition of standard Se(IV) solution to pre-analyzed sample solution at three different concentration levels  (80%, 100% and 120%) within the range of linearity for Se(IV). The basic concentration level of sample solution selected for spiking of the Se(IV) standard solution was 31.584 ng/mL. The proposed method was validated statistically and through recovery studies, and was successfully applied for the determination of Se(IV) in pure and dosage forms with percent recoveries ranged from 98.8% to 101.5%, Table 5.

 

Table 5: Results of recovery studies (n=5).

Level	% Recovery
80%	99.8
100%	101.5
120%	100.4

 

Repeatability:

The repeatability was evaluated by performing 10 repeat measurements for 7.896 ng/mL of Se(IV) using the studied method under the optimum conditions in two concentration ranges. The found amount of Se (IV) (± SD) was 7.998± 0.27 ng/mL and the percentage recovery was found to be 101.29 ± 3.6 with RSD of 0.034. These values indicate that the proposed method has high repeatability for Se(IV) analysis.

 

Sensitivity (LOD and LOQ):

The limits of detection (LOD) and quantitation  (LOQ) were determined using the formula: LOD or LOQ =jSD/b, where j=3.3 for LOD and 10 for LOQ, SD is the standard deviation of the intercept, and b is the slope. The values of LOD and LOQ for Se(IV) are 0.045 and 0.136, respectively

 

Robustness:

The robustness of the method adopted is demonstrated by the constancy of the current peak (IP) with the deliberated minor change in the experimental parameters such as the change in the concentration of excipients, temperature (25±5^oC), pH (0.22±0.01), accumulation potential (-350±5 mV) and C_HClO4 (0.20±0.01 mol/L), Table 6. indicates the robustness of the proposed method. I_p was measured and assay was calculated for five times.

 

Specificity: The specificity of the method was ascertained by analyzing standard Se(IV) in presence of excipients. There was no interference from the common excipients.  

 

Table 6: Robustness of the proposed DPASVA method.

* n=5.

 

The homogenization of tablets:

The homogenization of tablets in terms of the weight and the amount of drug was studied. We found that the mean weight and amount drug in the tablets was 1.509 ± 0.009 g (i.e. ±0.596%), 1.515 ± 0.015 g (i.e. ±0.990%) and 1.413 ± 0.013 g (i.e. ±0.958%) for DamVita Silver Plus tablets, Daily-Vit tablets and  Adult Vit Silver tablets, respectively. While the mean amount drug in the tablets was 72.13 ± 2.16 µg (i.e. ±3.0%), 71.26 ± 1.99 µg (i.e. ±2.8%) and 24.36 ± 0.83 µg (i.e. ±3.4%) for DamVita Silver Plus tablets, Daily-Vit tablets and  Adult Vit Silver tablets, respectively; which shows that homogeneity of tablets is good.

 

Conclusion:

Differential pulse anodic stripping voltammetric  Analysis (DPASVA) of selenium(IV) using a gold electrode modified with O-Phenylenediamine-nafion (GEMO-PN) has been studied. Selenium(IV) was determined in an aqueous HClO₄ (0.2M) medium of pH 0.22. Under the optimum conditions, liner calibration graph, I_p=f(C_Se⁴⁺), was obtained in the concentration ranges of 3.948 to 78.96 ng/mL  with relative standard deviations (RSD)3.8%, and the detection limit 0.045 ng/mL. This method showed a good accumulation efficiency for selenium and good resistance to interferences from metal ions as well as those  associated with selenium in pharmaceuticals. The results for the determination of Se⁴⁺ using (GEMO-PN) were more sensitive and accurate than that obtained using bare gold electrode; the sensitivity  was increased about 2000 times.

 

References:

1      Inam R, Toprak Ç. Determination of selenium(iv) using o-phenylenediamine by differential pulse polarography/interference of metal ions. Turk j chem 2004; 28(3):299-309.

2      Wang J, Sun C. Adsorption voltammetry of selenium in the presence of phenylenediamine (o-pda). J.electroanal chem 1990; 291(1-2):59-66.

3      Zhihong L, Wenjian Q.  Determination of trace selenium in the human body by stripping voltammetric method with o-phenylene diamine in situ. Acta universitatis medicinae tongji 1992; 21(6):382-384.

4      Ramadan AA, Mandil H, Ozoun A,  Determination of Se(iv) by pulse anodic stripping voltammetry with a vitamin E – nafion modified gold electrode. Res J of Aleppo Univ, Syria, 2008;61:149- 184.

5      Ramadan A A.; Mandil H.; Shikh-Debes A A.,Differential Pulse Anodic Stripping Voltammetric Determination of Selenium(IV) at a Gold Electrode Modified With 3,3′-Diaminobenzidine.4HCl-Nafion, Int J Pharm Pharm Sci , 2014;(6):148-153.

6      Ramadan AA, Mandil H, Ozoun A, Differential pulse anodic stripping voltammetric determination of selenium(iv) with a vitamin e–nafion modified gold electrode. Asian j chem, 2011; 23:843-846.

7      Ramadan A A.; Mandil H.; Shikh-Debes A A., Development and validation of differential pulse anodic stripping voltammetric analysis of selenium(iv) in bulk and in dosage formulations at a gold electrode multi-modified with a mixture of 3,3′-diaminobenzidine. 4HCl and vitamin E., Int J Pharm Pharm Sci, 2017; 9(7):97-102.

8      Ramadan AA, Mandil H, Ozoun A, Differential pulse anodic stripping voltammetric determination of selenium(iv) with a methylene blue-nafion modified gold electrode. Asian J Chem, 2012; 24: 391-394.

9      ICH: proceedings of the international conference on harmonization of technical requirement of registration of pharmaceuticals for human use (ICH harmonized tripartite guidelines); 2000.

 

 

 

Accepted on 04.04.2018          © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(5):2030-2035.