Kinetic Spectrophotometric Method for Determination of Vanadium in Syrian Oil Using 1, 5-Diphenylcarbazide Reagent

. New, sensitive, accurate and inexpensive kinetic spectrophotometric method was developed and validated for the determination of V(V) in crude oil and its heavy product. The method is based on the formation of colored complex between V(V) and 1, 5-diphenylcarbazide (DPCI). The method involves the reaction of V(V) with DPCI in presence of buffer solution of pH 6.8 to form colored complex with λ max at 530 nm. The fixed time method (2000 Sec) was adopted for constructing the calibration curve. The linearity range was found to be (2-12) mg/L. The correlation coefficient was 0.9999 and the limit of detection was found to be 0.349 mg/L. The method is feasible with the calibration equations obtained, which makes this method more applicable. Statistical treatment of the experimental results indicates that the method is precise, selective and accurate. The method was successfully applied for determination of V(V) in Syrian crude oil and its heavy product.

DPCI was 98.0%, standard of V(V) was obtained from CHEM-LAB. Stock standard solution of V(V)1000 mg/L (Belgium), acetone was obtained from Panreac with purity 99.5%, soduim hydroxide was obtained from POCH SA (Poland) with purity 99.0%, Sulfuric acid was obtained from CHEM-LAB with purity 98%. Stock standard solution of DPCI (2.06x10 -2 M) was prepared by dissolving 0.5 gr of DPCI in 0.5 mL of 0.5 N H 2 SO 4 solution and diluting to 100 mL with acetone. Buffer solution pH 6.8 (Britton 1 M) boric acid (1M), acetic acid (1M) and phosphoric acid (1M) were prepared by dissolving 62.41 mL boric acid, 57.43 mL acetic acid and 67.42 mL phosphoric acid in 1000 mL water and adjusting to pH 6.8.

Procedure for fixed time method
Aliquots of 2-12 mg/L of standard V(V) solution (0.5-3.0 mL, 100 mg/mL) were transferred into a series of 25 mL calibrated volumetric flasks. Then 6 mL of 2.0x10 -3 M DPCI solution and 2.5 mL of buffer solution (pH 6.8) were added. The volume was made up to the mark with water and the absorbance was measured after 2000 Sec at 530 nm against the reagent blank. Concentrations of V(V) in crude oil and its heavy product were determined from the calibration curve.

Procedure for Oil forms
Wet ashing method was used for preparation of the oil samples [16,17]. Three samples were mixed thoroughly (powdered in the case of coke sample). An amount corresponding to 10-50g of samples were weighed, dissolved with (10-20) mL of 98% H 2 SO 4 solution. The samples were heated until the white fumes were stopped. The Samples are placed in ashtray at 500°C for 24-72 hour. Subsequently, the ash was dissolved with (2 mL of 98% H 2 SO 4 , 2 mL of 37% HCl and 2 mL of 69% HNO 3 ). The samples solutions were heated until the white fumes were stopped and diluting to 100 mL with double distilled water. The resulting solution was used for analysis by the recommended procedures in the concentration range mentioned above.

Absorption spectra
V(V) was found to react with DPCI in an acid medium at 25°C producing an pink colored product which absorbs maximally at 530 nm as shown in Figure 1. The increase in the intensity of the color by time was used as a basis for a useful kinetic method for the determination of V(V) in crude oil and its heavy product.  International Letters of Chemistry, Physics and Astronomy Vol. 67 43

Optimization of reaction conditions
The spectrophotometric properties of the colored product as well as the different experimental parameters affecting the color development and its stability were studied and optimized by changing each variable in turn, while keeping all others constants. In all experiments, V(V) concentration was constant 12 mg/L. The effect of Buffer solution (Britton 1 M) concentration on the reaction was studied over the range 2×10 -2 -14×10 -2 M, in the final concentration. The maximum absorbance was obtained at concentration of 10×10 -2 M (when 2.5 mL of 1 M Britton was added). The influence of DPCI was investigated between 2.0×10 -4 and 5.6×10 -4 M in the final concentration (6 mL of 2.0×10 3 M DPCI) was chosen for all subsequent experiments ( Figure 2).

Quantitation methods
As the intensity of the color increased at 530 nm with time ( Figure 3) this was used as the basis for a useful kinetic method for the determination of V(V). The rate data, rate constant, fixed absorbance and fixed time methods [18,19] were carried out and the most suitable analytical methods were chosen regarding the applicability, sensitivity, the values of the intercept and correlation coefficient (r).

Rate data method:
The rate data of reaction would follow a pseudo order rate constant and obeyed the following rate equation: Where ν is the reaction rate, A is the absorbance, ∆A=At 2 -At 1 , t is the measuring time, Δt=t 2 -t 1 , k' is the pseudo order rate constant, C is the concentration of the V(V) mol/L and n is the order of the reaction. A calibration curve was constructed by plotting the logarithm of the initial rate of reaction (log ν) versus logarithm of V(V) concentration (log C), which showed a linear relationship over the concentration range of (2-12)mg/L (Figure 4).

Rate constant method:
The logarithm of the absorbance of reaction versus time for each concentration of V(V) which studied over the concentrtion rang of 2-12 mg/L was calculated. Graphs of log absorbance versus time for V(V) concentration in the range of 8-12 mg/L ( 1.570x10 -4 -2.355x10 -4 ) were plotted and all appeared to be rectilinear. Pseudo order rate constant (k') corresponding to different V(V) concentrations were calculated from the slopes multiplied by -2.303 and are presented in (Figure 5). Regression of C versus k' gave the following equation: k' = 0.2935 C-0.00013 (r = 0.7506).

Fixed absorbance method:
Reaction rate data were recorded for different V(V) concentrations in the range 6-12mg/L (1.177x10 -4 -2.355x10 -4 M). A preselected value of the absorbance 0.3 was fixed and the time was measured in the seconds. The reciprocal of time (1/t) versus the initial concentration of V(V) was plotted ( Figure 6) and the following equation of calibration graph was obtained.

Fixed time method:
At preselected fixed time, the absorbance of pink colored solution containing varying amounts of V(V) was measured at 25°C and 530 nm. Calibration graphs were constructed by plotting the absorbance against the initial concentration of V(V) at fixed time 0-3600Sec (60 min). The regression equations, correlation coefficients and linear ranges are given in (Table 1). It is clear that, the slope increases with the time and the most acceptable value of r was obtained for a fixed time of 2000 Sec (Figure 7). Therefore, the fixed time of 2000 Sec was utilized for assay of V(V) concentration.

Calibration graph
Correlation coefficient, intercept and slope values for the calibration data calculated using the least squares method [20]. It is clear that, both the slopes and intercepts increase with the time. The most acceptable values of the correlation coefficient of reaction products were obtained at 2000 Sec as fixed time, which was, chosen as the most suitable time interval for measurements ( Table 2). After optimizing the reaction conditions, the fixed time method was applied to the determination of V(V) in crude oil and its heavy product over the concentration range of 2-12 mg/L. The minimum level at which the investigated compound can be reliably detected (limit of detection, LOD) was determined experimentally for fixed time (2000 Sec) method, was found to be 0.349 mg/L. Correlation coefficient, r 0.9999 a A = mC+b, where A is the absorbance and C is the concentration in mg/L.

Accuracy and precision
The accuracy and precision of the proposed method was carried out by three determinations at four different concentrations. Percentage relative standard deviation (RSD %) as precision and percentage recovery as accuracy of the suggested method were calculated and showed in Table 3. The values of relative standard deviations for different concentrations of V(V) determined from the calibration curves. These results of accuracy and precision show that the proposed method has good repeatability and reproducibility.

Application to the Oil forms
The average percent recoveries obtained, indicated good accuracy of the method. The result obtained for the analysis of the studied V(V) in oil forms was compared with those obtained with the Atomic Absorption method ( Table 4). The proposed method is simple sensitive and reproducible and can be used for routine analysis of V(V) in model form and in oil forms. The proposed method was applied for determination of V(V) in crude oil and its heavy product. The results of analysis of the oil forms are shown in (Table 4).

-CONCLUSION
The developed kinetic spectrophotometric method for the determination of V(V) was sensitive, accurate and precise and hence can be used for the routine analysis of V(V) in model oil and oil forms with a limit of detection of 0.349 mg/L. The method was based on the reaction of V(V) with DPCI. The fixed time method for the proposed kinetic spectrophotometric method can be easily applied to the determination of V(V) in model oil and oil forms. The proposed method is compared with the previously reported method in terms of accuracy and precision. The proposed method is more selective and higher sensitivity than the sophisticated spectrophotometric techniques and similar reported methods.