Abstract A spectrophotometric method for the determination of selenium(IV) is described, based on the selenium-catalyzed oxidation of methyl orange with H₂O₂ in a nitric acid medium. As the redox reaction proceeds, the orange color of methyl orange is decolorized. The values of logA₀/A show a good linear relationship with the amounts of selenium(here A₀ and A stands for the absorbance of uncatalyzed reaction and catalyzed reaction, respectively ).The detection range is 18.8-65.7mg L^-1 and the sensitivity is 0.876mg L^-1.The method had been used to determine trace Se (IV) in high- Se tea with satisfactory results.
Keywords Catalytic kinetic spectrophotometry, Trace selenium, High-Se tea

1. EXPERIMENTAL
1.1 Reagents and instrumentation          
Se(IV) standard solutions were prepared by dissolving 0.1000g of pure Se powder(99.9%) in 10mL of nitric acid and diluting to 100mL in a volumetric flask. A 1.3×10⁴mg L^-1(5.00×10^-4mol L^-1) solution of methyl-orange, a 1.665×10⁸mg L^-1 solution of hydrogen peroxide and a solution of 6.3×10⁶mg L^-1(1.0mol L^-1) nitric acid were used. Analytical grade reagents and doubly distilled water were used in the experiment. UV absorbency was assayed by a WFZ800-D₃A spectrophotometer(made in Beijing second optical instrument factory). High-Se tea (Hubei Province Enshi city ).
1.2 Experimental method
An appropriate amounts of Se(IV) standard solution was added into a 10-mL test tube, which contains 0.8mL of 1.3×10⁴mg L^-1 methyl-orange solution, 1.0mL of 6.3×10⁶mg L^-1nitric acid solution and 2.0mL of 1.665×10⁸ mg L^-1hydrogen peroxide solution. Another tube contains the same solution but no Se(IV) was used as a blank. The two tubes were placed into a boiling water bath for 12min. Then the tubes were transferred immediately to a bath of flowing water and kept for 3min (to be balanced with the room temperature.), after which the absorbance A of the catalyzed solution and the absorbance A₀ of uncatalyzed solution were measured respectively in a 1-cm cell at 510nm against a re-distilled water reference, and log (A₀/A) was calculated.

2. RESULTS AND DICUSSION
2.1 Effect of reagent value             
The reduction reaction is optimal in dilute nitric acid, and 1.0mL of HNO₃ was used on the basis of the maximum value of log (A₀/A) (Table1). The use of 0.8mL methyl-orange solution was found to provide the maximum value of log (A₀/A) (Table2). The use of 2.0mL H₂O₂ afforded the best result on the basis of the same criterion (Table3).

Table1 The value of log (A₀/A) at different amounts of HNO₃.

(C_Se(IV) =33mg L^-1, C_MO=1.3×10⁴mg L^-1, reaction time:12min , temp. boiling water)

Table2 The value of log (A₀/A) at different amounts of MO.

(C_Se(IV) =33mg L^-1, C_HNO3=6.3×10⁶mg L^-1, reaction time:12min , temp. boiling water)

Table3 The value of log (A₀/A) at different amounts of H₂O₂.

(C_Se(IV) =33mg L^-1, C_MO=1.3×10⁴mg L^-1, C_HNO3=6.3×10⁶mg L^-1, reaction time:12min , temp. boiling water)

2.2 Effect of reaction temperature and time
It was proved that in this system the reaction did not occur at room temperature. The rate of oxidation of methyl orange by H₂O₂ increased as the reaction temperature was raised up to 89ºC. The reaction was catalyzed by Se(IV) at high temperature and the value of log (A₀/A) increased with temperature. The reaction temperature was fixed at boiling water in order to obtain maximum value of log (A₀/A). Reaction time from 10min to 13min was investigated with the other conditions fixed, data showed that 12min was the optimum reaction time.
2.3 Termination of the reaction
It is worthwhile to note that the oxidation reaction of methyl orange will be stopped at room temperature. Therefore, the cooling of the reaction solution with flowing tap water for 3 minutes was efficient for the termination of the reaction.
2.4 The sensitivity of the method            
The standard curve was obtained under the optimum conditions described above (Table4). A plot of log (A₀/A) versus C_Se(IV) was linear in the range 18.8-65.7mg L^-1with the following equation: log (A₀/A)=0.014C_Se(IV)（mg L^-1）-0.010(r=0.9996). According to L=1(l stand for thickness of the cell) and log (A₀/A)=0.001, the sensitivity of the method was 0.786 mg L^-1.

Table4 Data of standard curve

(C_MO=1.3×10⁴mg L^-1, C_HNO3=6.3×10⁶mg L^-1, reaction time:12min, temp. boiling water)

2.5 Determination of the reaction order with methyl-orange and hydrogen peroxide^[8]   
The scheme of the Se(IV) catalyzed reduction of methyl-orange with H₂O₂ was as follows:
MO (red color)+H₂O₂ oxidation product (colorless)
The rate of discoloring of MO was written as follows:
-dC_mo/dt=kC^a_mo C^b_Se(IV)C^g_H2O2C^d_H+ (1)
The alteration of C^g_H2O2 and C^d_H+ in the reaction can be ignored and b=1 according to the characteristic of the catalytic reaction^[9]. We obtain equation (2) from equation (1):
-dC_mo/dt=k₁C^a_moC_se(IV) (2)
According to the Beer's Law A=e l C_mo equation (2) was transferred as
-dA/dt=k₁'A^aC_Se(IV) (3)
At constant C_Se(IV) can be got equation (4)
-dA/dt=k_obsA^a or log (-dA/dt)= a logA + logk_obs (4)
dA/dt was substituted for DA/Dt and stands for the average absorbance within a constant period. The plot of -log (DA/Dt) vs. -log was a good straight line with equation -log (DA/Dt)=0.984 log+1.097 (r=0.999). So the reaction order with methyl-orange and hydrogen peroxide is unity.
2.6 Determination of the activation energy ^[8]
Under the optimum conditions, at the range temperature of 83-98ºC, the observed rate constants k_obs were gained. According to Arrhenius of equation, the linear regression equation of lnk_obs vs. 1/T was lnk_obs= -16.52×10³(1/T)+42.28 (r=0.997), from which the observed activation energy was obtained as 137.35 kJ mol^-1.
2.7 Interference Studies
Within +5% relative error, on the determination of 33mg L^-1 Se(IV) by this method according to recommended procedure, 3.0×10⁴ times of La³⁺、Zn²⁺ 、Ni²⁺、Ca²⁺ 、Co²⁺, 2.6×10⁴ times of Ag⁺ , 3.2×10³ times of Mg²⁺, 2.5×10 ³ times of Al³⁺, 2.0×10³ times of NH₄⁺,300 times of Ni²⁺, 30 times of S₂O₃²- can coexist with Se(IV)，Pb²⁺ 、Cd²⁺、NO₂^- were not be permitted to exist because of their strong interference effects.

3. Determination of Se(IV) in the sample^[10-11]         
In order to test the described method, selenium content was determined in a kind of high-Se-tea sample. Sample dissolution was carried out by the procedure described by Zhou ^[10]. About 1.5g of well-mixed tea powder was weighed accurately into a 50mL flask, and 30mL of concentrated HNO₃ was added. The mixture was kept at room temperature overnight, then heated at 150ºC for 3 hours, after which a mixture of 10mL HNO₃-HClO₄(both concentrated, 3:1) was added. The resulting mixture was concentrated to 1-2mL at 150ºC, then was cooled at room temperature. The above concentrated solution was diluted to 10-15mL with water, and removed the tolerated ions by using cation exchange resin^[11]. The eluate was diluted to 50mL with distilled water and saved for analysis. The results showed the presence of 202.3mg g^-1selenium (n=5) in the high-Se tea. Recovery was 98.6%-102.4%.
    In conclusion, the existing catalytic-spectrophotometric methyl orange for Se(IV) determination has been modified to increase its interference tolerance. This method has been applied to Se(IV) analysis in high-Se tea with satisfactory results.

REFERENCES        
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[7] Wang Y L, Hou J G, Zhang C X. Journal of Northwest Normal University (Natural Science) (Xibei Shifan Daxue Xuebao), 1999, 35 (2): 44-46.
[8] Zhang Z H, Liu J Y, Xiong L L, Acta Chimica Sinica (Huaxue Xuebao), 1989,47: 808-811.
[9] Liu Y Q, Modern Instrumental Analysis(Chinese Agriculture & Technology Publishing Co. Beijing), 1998, 429.
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[11] M F Mousavi, A R Ghiasvand & A R Jahanshahi, Talanta, 1998, 46: 1011.

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