spectrophotometric determination of trace cadmium in ...

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reagent, 3,5 bis(4 phenylazophenylaminodiazo)benzo ... diazoaminobenzene 4 vinylpyridine, (commonly abbre viated as Poly Cd(II) .... dimethylaminophenol.

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 1, с. 34–39

ОРИГИНАЛЬНЫЕ СТАТЬИ УДК 543

SPECTROPHOTOMETRIC DETERMINATION OF TRACE CADMIUM IN VEGETABLES WITH 3,5BIS(4PHENYLAZOPHENYLAMINODIAZO)BENZOIC ACID © 2011 г. Shuangming Meng, Buqin Jing, Yueqin Fan, Yongwen Liu, Yong Guo Department of Chemistry and Chemical Engineering, Shanxi Datong University Datong 037009, People’s Republic of China Received 19.08.2009; in final form 12.05.2010

A new highly sensitive and selective chromogenic reagent, 3,5bis(4phenylazophenylaminodiazo)benzoic acid (BPPABA) has been synthesized and applied to the determination of trace cadmium(II) in vegetables. The method is based on the color reaction between BPPABA and cadmium(II). In the presence of Triton X100, cadmium(II) reacts with BPPABA in Na2B4O7–NaOH buffer solution (pH 10.5), forming red complex with maximum absorp tion at 530 nm. Under the optimal conditions, Beer’s law is obeyed within 0–12 μg of cadmium within 25 mL of solution, and the apparent molar absorptive coefficient of the complex is 2.8 × 105 L/mol cm. The detection limit and the relative standard deviation were found to be 0.92 μg/L and 1.0%, respectively. Interference of foreign ions was also investigated. Most of the metal ions are tolerated in considerable amounts except for Hg(II), Cu(II) and Ni(II). To eliminate the interference of foreign ions, metal ion imprinted polymer technique was utilized. Keywords: 3,5bis(4phenylazophenylaminodiazo)benzoic acid (BPPABA), spectrophotometry, cadmium, vegetables.

With ecological and health problems attracting high attention, it is more and more important that trace metals such as Cd(II), Hg(II), Cu(II) etc. can be determined in biological samples. Cadmium is a nonessential element for plants and could be uptaken and accumulated by plant tissues. Cadmium is very harmful to human health when it enters the food chain [1–5]. So the determina tion of cadmium in vegetables has an increasing impor tance. Many methods have been used or the determina tion of cadmium, such as atomic absorption spectrome try [6], inductively coupled plasma atomic emission spectrometry [7], and mass spectrometry [8], Xray fluo rescence spectrometry [9], neutron activation analysis [10], differential pulse anodic stripping voltammetry [11]. Still spectrophotometry is an important method for the determination of cadmium due to its simplicity and low cost.

venient and efficient methods for the determination of cadmium. To eliminate interference of foreign ions, metal ion im printed polymer technique was utilized to enrich and sep arate cadmium in vegetables, which showed and has the satisfactory results. Metal ion imprinted polymer has good selectivity because it has special memory for specific parti cles [24–26]. Metal ion imprinted polymer PolyCd(II) diazoaminobenzene4vinylpyridine, (commonly abbre viated as PolyCd(II)DAABVP) [27] can selectively ex tract cadmium ion with satisfactory results. EXPERIMENTAL Instruments. The elemental analyses for C, H and N were performed on a PerkinElmer 2400 analyzer. IR spectra were obtained for KBr pellets on a FTIR8400 spectrophotometer in the 4000–400 cm–1 range. Absor bance and absorption spectra were measured on a LAMBDA35 spectrophotometer with 1 cm pathlength. A model pHS3C pHmeter (Shanghai Lida Instrument Factory, China) was used for the pH adjustment. The flow rate of liquid through columns was controlled using a model BT00600 M peristaltic pump (Baoding La onger Precision Pump Co.Ltd.). Sample digestion was carried out in a model MDS2002A microwave (Shang hai Xinyi microwave Co.Ltd.) Reagents and solution. Unless otherwise stated, all water used was 18 MΩ cm distilled deionized (DDI) water purified with a MilliQ system (Millipore, USA).

Triazene reagents are well suited for spectrophoto metric determination of cadmium. Each reagent has its advantages and disadvantages with respect to sensitivity, selectivity, color reaction and linear range. Some of the available spectrophotometric methods for the cadmi um(II) are listed in Table 1. In this paper, a new triazene reagent, 3,5bis(4phenylazophenylaminodiazo)benzo ic acid (BPPABA) has been synthesized, and optimum conditions for its color reaction with cadmium are dis cussed in detail. The new method for the spectrophoto metric determination of cadmium with BPPABA has been proposed and applied for the determination of cad mium in vegetables. This method is one of the most con 34

SPECTROPHOTOMETRIC DETERMINATION OF TRACE CADMIUM

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Table 1. Comparison of analytical performance of various spectrophotometric methods for determination of cadmium Reagents

ε × 10–5, L/mol cm

λmax, nm

Conditions

Remark

Refer ence [12]

1.2 M NH3

Wide linear range, many ions interfere with color reaction Low sensitivity, separation ofinterfering ions with anion exchange resin Wide linear range, many ions interfere with color reaction Low sensitivity, extraction with MIBK

pH 10.1

Low sensitivity

[16]

2,6Dibromo4nitrophenyl dia zoaminoazobenzene 4Formylphenyldiazo aminoa zobenzene oChloropnitrophenyl diaz oaminoazobenzene ohydroxyphenyldiazo aminoa zobenzene 4Fluoro4'fluorophenyl diaz oaminoazobenzene pNitrophenyl diazoaminoa zobenzene

1.52

500

pH 8.5

1.33

525

pH 9.5

1.5

503

pH 7.5–8.1

1.5

520

1.17

490

1.19

1,4,8,11Tetraza1,4,8,11tet ramethylcyclotetradecane 2[2(5Bromopyridine)azo]5 dimethylaminophenol 2[(5Bromo2pyridine)azo] 5diethylaminophenol 3Bromo4(4nitrophenyl diaz oamino)azobenzene oMethylbenzenediazo aminoa zobenzene pNitrodiazoaminoazobenzene 3,5bis(4Phenylazophenyl ami nodiazo)benzoic acid

1.1

480(560) 0.2–0.3 M KOH Low sensitivity, strong alkaline medium with toxic KCN as masking reagent and formaldehyde as demasking reagent 550 NaOH Low sensitivity, extraction with CHCl3

1.41

555

pH 8–10.5

1.39

556

pH 9–11

Low sensitivity and extraction with trimeth [19] ylbutanol In 50% ethanol medium [20]

1.56

495

pH 8.8

Narrow linear range

[21]

1.68

524

0.08 M NH3

Narrow linear range

[22]

0.814 2.8

480 530

pH 10.3 pH 10.5

Low sensitivity High sensitivity and wide linear range

[23] This work

[13] [14] [15]

[17]

[18]

All reagents used were of analytical grade and all solu tions were prepared with DDI water. Standard labware and glassware were repeatedly cleaned with HNO3 and DDI water, according to a published procedure [28].

ter were mixed in 250 mL beaker and cooled below 0°C in ice water (solution B). While stirring, the solution A was diazotized by dropwise adding solution B (diazotized solution C).

Stock solutions (1.0 g/L) of the elements were pre pared as follows: specpure cadmium (99.99%) was dis solved in HNO3 and further diluted prior to use. Triton X100 (3%, v/v) was obtained by dissolving 15 mL of Tri ton X100 in 500 mL of water; Na2B4O7–NaOH buffer solution was prepared by dissolving 0.05 mol of Na2B4O7 in water and further added 0.1 M NaOH to the values of pH required, which were adjusted by pHS3C pH meter; BPPABA DMF solution (0.2 g/L) was prepared by dis solving 0.1 g of BPPABA in 500 mL of DMF; Poly Cd(II)DAABVP was prepared according to the pub lished procedure [27].

3.94 g of paminoazobenzene were dissolved in 150 mL of absolute ethanol and cooled below 0°C. The diazotized solution C was added while stirring the result ing adjusted to pH 4–5 with 10% NaOH and left to stand for 2 h. Adjusting the solution to pH 6–7 with saturated sodium carbonate lead to a large dark red precipitate which was left to stand overnight. The precipitate was fil tered, washed with a 1 : 1 (v/v) ethanol/water 3 times and distilled water 3–5 times to obtain dark red crude BPPABA (Fig. 1). Crude BPPABA was recrystallized with acetone, separated on a silica gel column with a mixture of cyclohexane, petroleum ether and ethylace tate (3 : 3 : 1) as eluent, and distilled under low pressure to obtain the pure product with the following character istics: IR (KBr): 3443 cm–1(–NH–); 1700 cm–1(C=O); 1600 cm–1, 1516 cm–1, 1450 cm–1(phenyl); 1630 cm⎯1 (⎯N=N–); 3060 cm–1(Ar–H). The elemental analysis for C31H21N10O2: calculated C = 65.84%, H = 3.72%, N = = 24.78%, found C = 65.63%, H = 3.73%, N = 24.64%.

Synthesis and purification of 3,5bis(4phenylazophe nylaminodiazo)benzoic acid (BPPABA). To 40 mL of ab solute ethanol containing 1.52 g (0.01 mol) of 3,5diami nobenzoic acid, 10% NaOH solution was added drop wise to dissolve the 3,5diaminobenzoic acid. Then 10 mL aqueous solution containing 1.4 g NaNO2 were dropped in this mixture, and solution cooled below 0°C (solution A). 20 mL of hydrochloric acid and 5 mL of wa ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ

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COOH

N N

NH N N

N N NH

N N

Fig. 1.The structure of BPPABA.

General procedures. Column procedure. We have syn thesized the polyCd(II)DAABVP based on the previ ous work [27]. The stopcock of the glass column (100 mm in length and 10 mm in diameter) was covered with a fritted glass disc. A small amount of glass wool was placed on the disc to prevent any loss of the resin beads during sample loading. 500 mg of polyCd(II) DAABVP were slurred in water and then poured into the column. The sorbent was treated successively with 4.0 M HCl and water. The column was preconditioned by pass ing a blank solution and then a solution containing 10 μg of Cd(II) in a volume of 100 mL at a flow rate of 0.5 mL/min (controlled by a peristaltic pump) after ad justing pH 5. The column was washed with 15 mL of DDI water, and Cd(II) was eluted with 0.2 M HCl and 2 mL of DDI water and then determined by FAAS. Procedure for the determination of cadmium (proce dure A). No more than 10 μg of cadmium was trans ferred into a 25 mL calibrated flask. 1.5 mL of 3.0% Triton X100, 3.0 mL of Na2B4O7–NaOH buffer solu tion (pH 10.5) and 1.5 mL of 0.2 g/L BPPABA DMF so lution were added successively. The solution was diluted to the mark with water, mixed well and left for 5 min. The absorbance at 480 nm was measured in a 1 cm cell against a reagent blank. 2 0.7

3

Absorbance

0.6 1

0.5 0.4 0.3 0.2 0.1 0 350

400

450 500 Wavelength (nm)

550

600

Fig. 2. Spectra of BPPABA against water at different pH values. 1 – pH 7, 5 mL of Na2HPO4/NaH2PO4; 2 – pH 10.5, 5 mL of Na2B4O7–NaOH buffer solution; 3 – pH 11, 5 mL of 0.001 M NaOH. Other conditions: 1.5 mL of 3.0% Tritonx100 and 1.5 mL of 0.2 g/L BPPABA in DMF; total volume of the solution 25 mL.

Procedure for the determination of cadmium in vegeta bles (Procedure B). The vegetables were washed with wa ter, dried in a forceddraft oven at 70°C to constant mass and ground to a fine powder. A suitable weight was weight (2.0 g of dry material) was placed into a 100 mL claisen distilling flask, and 15 mL of HNO3 were added, the flask was put into microwave oven and digested for 10 min at 50% power and for 15 min at 100% power. Then the flask was taken out and cooled to room temperature before an other 15 mL HNO3 and 2 mL H2O2 were added and the flask was left to stand for 20 min. Then it was placed in the microwave oven and irradiated for 30 min at 100% power, taken out and cooled to room temperature. Finally 2 mL HNO3 were added and the flask and was left to stand for 15 min. The solution was neutralized to pH 4⎯5 with sol id Na2CO3 to obtain the sample solution. The sample solution was passed through the column packed with polyCd(II)DAABVP (slurred in water be fore) at a flow rate of 0.5 mL/min (controlled by a peristal tic pump) after adjusting pH 5.0. The column was washed with 15 mL water and eluted with 5 mL of 0.2 M HCl and 2 mL water. The solution was transferred into a 25 mL cal ibrated flask and further treated as in procedure A. RESULTS AND DISCUSSION Physicochemical properties of BPPABA. BPPABA, an orange yellow solid, is easily soluble in ethanol and ac etone and slightly soluble in water. The spectra of BPPABA at different acidities are shown in Fig. 2. It can be seen that BPPABA is yellowish in weak alkaline medi um (pH value between 7.0 and 10.5) with one absorption peak at 440 nm, and becomes purple in an alkaline medi um (pH > 10.5), the maximum absorption wavelength shifting to 525 nm. Absorption spectra. Under the experimental condi tions, the absorption spectra of BPPABA and the Cd(II) BPPABA complex were scanned (Fig. 3). The absorption maximum of BPPABA was at 440 nm, whereas the Cd(II)BPPABA complex gave an absorption peak at 530 nm. The contrast of the two peaks was 90 nm, which could be easily distinguished. Therefore, the 530 nm was chosen as the determination wavelength. Effects of medium and acidity. The effect of pH of the buffer solution on the Cd(II)BPPABA complex forma tion was tested as shown in Fig. 4. It was found that the absorption of the Cd(II)BPPABA complex was optimal in a Na2B4O7–NaOH buffer solution (pH 9.0–11.5). Considering both selectivity and rate of the reaction, the

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action is completed in 3 min to reach maximum absorp tion. The absorption of complex remains stable for 24 h.

0.5 2 0.4

Effect of surfactants. Since it was not easy to dissolve BPPABA in water, it was difficult to observe a reaction of BPPABA with Cd(II) in water without a surfactant. In order to improve the reaction of BPPABA with Cd(II), an appropriate surfactant should be selected carefully. It was found that different types of surfactants exhibited dif ferent effects on the absorption of the system. Many ex periments indicated that the complex formation reaction of BPPABA and Cd(II) was not good in the presence of anionic surfactants such as sodium lauryl sulfate or cat ionic surfactants such as triethanolamine.

1

A

0.3

0.2

0.1

0 350

450

400

λ, nm

500

550

600

Fig. 3. Absorption spectra of the BPPABA against water (1) and BPPABACd complex against reagent blank (2). Other conditions c 2+ = 1.65 × 10–6 M; 1.5 mL of 3.0% Cd

Triton X100 and 1.5 mL of 0.2 g/L BPPABA in DMF, 3.0 mL pH 10.5 Na2B4O7–NaOH buffer solution in 25 mL solution.

addition of 3.0 mL of Na2B4O7–NaOH buffer solution (pH 10.5) was recommended. Effect of amount of BPPABA. The amount of BPPABA has a significant effect on the sensitivity of the color reaction. The results are shown in Fig. 5. 1.5– 2.0 mL of 0.2 g/L BPPABA DMF solution provided maximum and constant absorption of the complex. The reaction had low sensitivity below 1.5 mL of BPPABA, and the solution became turbid above 2.0 mL. Hence, 1.5 mL BPPABA DMF solution were used. Effect of time and the stability of complex. The color reaction of cadmium with BPPABA is very fast. The re

Nonionic surfactants such as Tween80, Triton X100 and octylphenyl ether enhanced the absorbance of the complex remarkably. Triton X100 provided the highest sensitivity and lowest background signals, and was there fore selected. The effect of the concentration of Triton X100 on the molar absorptivity of the system was studied. The addition of 1.5 mL of 3.0% Triton X100 was suggest ed for subsequent experiments. Analytical characteristics. The calibration curve was constructed in the usual way according to proce dure A. Beer’s law was obeyed for 0–12 μg in 25 mL of solution at 530 nm. The apparent molar absorptivity of the complex was 2.8 × 105 L/mol cm. The detection limit and the relative standard deviation were found to be 0.92 μg/L and 1.0%, respectively. The equation of the line obtained by a leastsquares treatment was A = 0.0991C (μg/25mL) + 0.0029 with a correlation coefficient of 0.994. Effect of foreign ions. The effects of various foreign ions on the determination of 10 μg of Cd(II) in 25 mL of solution were studied under optimum conditions. Various amounts of foreign ions were prepared, and the procedure for the determination of Cd(II) was fol lowed. With a relative error of less than ±5%, the toler ated limits for various foreign ions are listed in Table 2.

0.5

0.5

0.4

0.4

Absorbance

Absorbance

37

0.3 0.2

0.3 0.2 0.1

0.1

0 1.0

0 8

9

10

11

12

13

pH

Fig. 5. Effect of the amount of BPPABA on absorbance of Cd(II)BPPABA. c 2+ = 1.65 × 10–6 M, 1.5 mL of 3.0%

Fig. 4. Effect of pH on absorbance of Cd(II)BPPABA. c 2+ = 1.65 × 10–6 M, 1.5 mL 3.0% triton X100. 1.5 mL

Cd

Triton X100, 3.0 mL of Na2B4O7–NaOH buffer solution, pH = 10.5.

Cd

0.2 g/L BPPABA DMF in 25 mL solution. ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ

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Table 2. Effect of foreign ions on the determination of the cadmium Tolerance limits, mg/L

Foreign ions –

2–

2–

+

K+, Na+, NO 3 , CO 3 , SO 4 , NH 4 3–

Sr2+, Ba2+, F–, PO 4 , CH3COO–

3000 1200

I–, Cl–, Ca2+, Mg2+

500

Mn2+, Zn2+, Pb2+

150 2–

Al3+, Fe3+, Br–, C 2 O 4

100

Ag+, Co2+, Sn2+

50

Cu2+, Ni2+, Hg2+

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