- Email: [email protected]
Preconcentration with membrane cell and adsorptive polarographic determination of formaldehyde in air
Talanta 57 (2002) 317– 322 www.elsevier.com/locate/talanta
Preconcentration with membrane cell and adsorptive polarographic determination of formaldehyde in air Zheng-Qi Zhang *, Hong Zhang, Guang-Fu He College of Chemistry and Chemical Engineering, Hunan Uni6ersity, Changsha 410082, People’s Republic of China Received 12 December 2001; received in revised form 31 December 2001; accepted 8 January 2002
Abstract The present paper describes a procedure that formaldehyde in air was preconcentrated in a membrane cell and its content was determined by adsorptive polarography. First the formaldehyde in air samples was preconcentrated in a membrane cell using water, then the formaldehyde reacted with 2,4-dinitrophenyl hydrazine to form 2,4-dinitrophenyl hydrazone, which can be adsorbed at the mercury electrode and yields a sensitive adsorptive polarographic wave. Over the range 6.0 ×10 − 10 –5.0×10 − 6 M, the peak currents are linearly proportional to the concentration of formaldehyde, the detection limit is 2.0 ×10 − 10 M. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Polarography; DNPH; Formaldehyde
1. Introduction Formaldehyde strongly irritates the mucous membranes; it is perceived in concentrations of 0.2 ppm and above. Formaldehyde is one of the most important industrial chemicals for the manufacture of synthetic resins, fertilizers, explosives and others. It possesses toxicity, and the animal experiments have shown formaldehyde to be a lung carcinogen [1]. Since the reactivity of formaldehyde is high and the number of known specific reactions is large, there exists a wide variety of methods for the determination of low formaldehyde concentrations [2]. Colorimetric procedures are most frequently used. In 1866 Rayner and Jephcott * Corresponding author.
reported a procedure for accurate determination of trace formaldehyde with Schiff reagent [3]. Chromotropic acid [4], acetylacetone, paminobenzenesulfonic acid and phenyl hydrazine were used for spectrophotometric determination of formaldehyde [5,6]. A gas chromatographic procedure for the determination of formaldehyde were based on prior derivatization procedure [7]. Adsorptive voltammetry and DPP are used to determine formaldehyde in air samples [8,9]. The application of membrane separations technique in analytical chemistry has been reported [10 –14]. This technique was used to collect the air pollutants [15 –20]. Using membrane cell, the analyte is continuously extracted from a certain phase. In the present paper, we design a membrane cell (Fig. 1), and use it to collect formaldehyde in air samples with water. In a dilute HCl
0039-9140/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 0 2 9 - 2
318
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
solution, the formaldehyde reacts with 2,4-dinitrophenyl hydrazine (DNPH) to form 2,4-dinitrophenyl hydrazone, which can yield an adsorptive polarographic wave on drop mercury electrode, so the formaldehyde can be determined by adsorptive polarography.
case of PTFE porous matrix the capillary effects become negligible within porous larger than 0.1 mm radius. So it can be guaranteed that the polar liquid phase moves within the macropores and the non-polar liquid or gas phase moves within the micropores.
2. Principle of operation of membrane cell
3. Experimental
Membrane cell is a device for mass-exchange between a polar liquid and a non-polar liquid or gas phase within a membrane cell. In membrane cell, the mass-exchange process is carried out in the capillary medium of hydrophobic material. The membrane cell used in this study are shown in Fig. 1. The micropores membrane 4 is a hydrophobic one. The mass-exchange layer 3 was made up of porous polymer particles such as porous polytetraflouro-ethylene(PTFE). The porosity between the particles forms macropores with a radius of 0.5 – 3.0 mm. In the polymer particles there are numerous open micropores with a radius of 0.03–1.0 mm. The macropores form channels for the polar liquid which do not wet the surface of the hydrophobic material, and the micropores form channels for the non-polar liquid or gas phase. When the non-polar liquid or gas moves within the micropores it can contact with the polar liquid within the macropores, thus the analyte transfer from original phase into the other. Throughout the mass-exchange space the gas or non-polar liquid phase does not enter into the macropores because the polar phase pressure is maintained higher than the pressure of the gas phase, and polar liquid phase also could not penetrate into micropores because the capillary pressure of micropores can prevent this penetration. The capillary pressure (Pc) depends on the surface tension of the liquid phase (|), the contact angle between the liquid and the membrane material (q) and the pore radius (k):
3.1. Apparatus
Pc = (2|cosq)/k.
3.1.1. Membrane cell The experimental membrane cell (Fig. 1) was made up of PTFE. The cell had a membrane of 0.8 mm thickness with an average pore size of about 0.7 mm. In the mass-exchange units (25× 25× 50 mm3) the micropore size is as uniform as possible, e.g. 0.7 mm, and the average diameter of macropore size is 0.5 mm. The cell in this study allow variation of the aqueous phase flow within the 0–500 ml min − 1 range and of the air flow within the 0–1.5 dm3 min − 1 range. 3.1.2. Electroanalytical de6ice The polarograms were recorded on a JP-1A oscillopolarograph (Chengdu Instrumenttal Factory). The polarographic cell has a three electrode system: a dropping mercury electrode (DME) as working electrode, a saturated calomel electrode (SCE) as reference electrode and platinum wire as auxiliary electrode. A drop time of 7 s was selected using a knocker,with a rest time of 5 s and a scan time of 2 s, the scan rate being 250 mV s − 1. An MP-2 stripping voltammetric analiz-
(1)
In order to prevent capillary effects, in design of membrane cell the Pc should exceed the value calculated from the above equation [21]. In the
Fig. 1. Membrane cell: (1) triton valve; (2) case of the cell; (3) mass-exchange layer; (4) micropores membrane.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
er(Shandong Instrumental Factory) with an Epson printer and a JM-01 (manual micro-metric screw delivery) hanging mercury drop electrode, controlled by micro-processor, and A PAR Model 273 potentiostat/Galvanostat with a PAR Model 303 static mercury drop electrode, controlled by PAR Model 270 software, were used for pulse polarography, linear scan voltammetry, cyclic voltammetry and other electrochemical measurements. For pulse polarography the instrumental parameters were as follows: accumulation potential, − 0.40 V; drop size, medium; pulse amplitude, 50 mV; pulse period, 2 s; equilibrium time, 15 s.
3.2. Reagents 3.2.1. Formaldehyde standard solutions A stock solution of formaldehyde was prepared by diluting 2.8 ml of 36– 38% formaldehyde solution to 1000 ml with water, which was standardized by iodometry. The solutions of lower concentration were prepared by serial dilution. 3.2.2. DNPH solution A stock solution of 2,4-dinitrophenyl hydrazine was prepared by dissolving 0.1000 g of DNPH (AR) in 24 ml of 6.0 M HCl solution, and diluted to 100 ml. 3.2.3. Tween-80 solution (0.10%) A stock solution of Tween-80 was prepared by dissolving 0.100 g of Tween-80 (AR) in 100 ml of water. Other reagents were of supra-pure or analytical-reagent grade. Water, redistilled in a fused-silica apparatus, was used throughout. 3.3. Procedures 3.3.1. Preconcentration of the formaldehyde in air The experimental device for preconcentration of formaldehyde in air samples is shown in Fig. 2, and the routines for preconcentration of formaldehyde in air were as follows: (I) First all valves were closed, and the pump 8 was started.
319
Fig. 2. Scheme of experimental device for absorption of aniline in air: (1) buffer bottle; (2) rotameter; (3) triton valve; (4) samples solution collector; (5) water; (6) membrane cell; (7) air regulating valve; (8) pump.
(II) Then the triton valve 3 was rotated to connect line 2 with line 3 to let absorbing solution to fill the membrane cell. When the solution outflows from the cell the triton valve 3 was rotated to cut off line 2 and 3. (III) The triton valve 3 was again rotated to connect line 1 with line 3, and valve 7 was regulated to make the flow rate of air in 0.5 dm3 min − 1. After about 5 min the valve 7 was closed, and the triton valve 3 was immediately rotated to connect line 2 with line 3 to collect about 1.0 ml of absorbing solution. Repeating above routine III, a 3–5 ml of the solution (total volume) was collected.
3.3.2. Adsorpti6e polarographic determination of formaldehyde 3.3.2.1. Polarography of pure formaldehyde solution. In a 10 ml beaker, 0.20 ml of 2,4-dinitrophenyl hydrazine solution and required amounts of formaldehyde standard solution were added and mixed well, then the mixture was stood for 15 min at 60 °C in a water bath. After cooling the mixture to room temperature in tap water, 2.00 ml of pH 5.7 buffer solution, 1.00 ml of 0.10 M NaCl solution and 0.10 ml of Tween-80 solution were added, and the mixture was diluted to 10 ml.
320
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
After transferring the solution to electrolytic cell, the derivative polarograms were recorded with potential scan rate of 250 mV s − 1, starting the potential scan at −0.40 V. The peak potential is − 0.78 V.
3.3.2.2. Analysis of air samples. Using the collected formaldehyde sample solutions in place of formaldehyde standard solution, analysis was performed as above described for pure formaldehyde solution.
4. Results and discussion
4.1. Conditions for preconcentration of formaldehyde 4.1.1. Selection of absorbing solution According to the literatures [7,8], the water was used to absorb formaldehyde in air samples in this study. 4.1.2. Flow rate of the air samples When the analyte is easy to dissolve in an absorbing solution, the larger the flow rate of the sample, the shorter the time of concentration can be. However, the flow rate of the sample must be less than one which the membrane cell tolerates. In order to absorb the formaldehyde in air samples completely, in this study, the flow rate of the air samples was controlled at 0.5 dm3 min − 1.
(NO2)2C6H5NHNCH2 + 2e+2H+ X (NO2)2C6H5NHNHCH3
(3)
When there is 0.0010% Tween-80 in the test solution, the peak current increases two-fold (Fig. 3c), which can be used to determine the trace formaldehyde. Over the range 6.0×10 − 10 –5.0× 10 − 6 M, the peak currents are linearly proportional to the concentration of formaldehyde. The detection limit is 2.0×10-10 M, which was taken as the concentration that gave a signal equal to three times the standard deviation of the blank signal, calculated from the calibration slope. The reproducibility was evaluated by 20 repetitive experiments on 6.0× 10 − 10 M formaldehyde solution. The relative standard deviation was 6.1%.
4.2.2. Effect of pH The dependence of the peak current on pH value is shown in Fig. 4a. When pH value was less than 5.5 the peak current increased with the increasing of pH value of test, and larger than 6.0, the peak current decreased. In the pH range from 5.5 to 6.0 the peak current is the largest. In this work the pH value was chosen to be 5.7. 4.2.3. Effect of NaCl concentration Support electrolytes such NaCl, KCl, NaNO3 and KNO3 were examined experimentally. The best results were obtained with NaCl support electrolyte. Over the range from 0.0050 to 0.045
4.2. Polarographic determination of formaldehyde 4.2.1. Adsorpti6e polarography In a dilute HCl solution, formaldehyde can react with 2,4-dinitrophenyl hydrazine to form 2,4-dinitrophenyl hydrazone, and the produce can be adsorbed on the mercury electrode and undergo electrochemical reaction to yield sensitive adsorptive polarographic peak (Fig. 3b) on the DME.
(2)
Fig. 3. Derivative polarograms: (a) pH 5.7; 0.010 M NaCl; 2.0 ×10 − 5 M DNPH; (b) a+5.0 × 10 − 7 M formaldehyde; (c) b +0.0010% Tween-80.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
321
70 °C the peak currents remained constant. So the derivation reaction carried out at 60 °C.
4.3.3. Effect of time At 60 °C the derivation reaction carried out rapidly, and was finished in 10 min. So after mixing formaldehyde and DNPH solution, the mixture was standed for 15 min at 60 °C. 4.4. Cyclic 6oltammetry
Fig. 4. Effect of pH and DNPH 5.0 × 10 − 7 M HCHO, 0.0010% Tween-80; (a) Effect of pH: 1.0 ×10 − 5 M DNPH; (b) Effect of DNPH concentration: pH 5.7.
M the peak current remains constant. A concentration of 0.010 M for NaCl was used in subsequent work.
4.2.4. Effect of surfactants Various cathodic, anodic and nonionic surfactants such as cetyltrimethylammonium bromide (ATMAB), sodium lauryl sulphate (SLS), p-octylpolyethyleneglycol phenyl ether (OP), Tween-80 and Triton X-100 were tested, and Tween-80 was found to be the best. The peak current increases rapidly with increasing Tween-80 concentration and the maximum value were attained from 0.00080 to 0.0050%. As Tween-80 concentration increases beyond 0.0050%, the peak current decreases. The suitable Tween-80 concentration for this study was 0.0010%.
Fig. 5 shows the cyclic voltammograms of formaldehyde system. The 2,4-dinitrophenyl hydrazone gives a cathodic peak at about − 0.78 V due to its reduction, and no peak was observed on the anodic branch, indicating that the reduction of 2,4-dinitrophenyl hydrazone is irreversible. In Fig. 5 subsequent repetitive scans yielded significantly smaller (but stable) cathodic peaks corresponding to the reaction of dissolved species. This behavior indicates that 2,4-dinitrophenyl hydrazone is adsorbed on the mercury electrode [22], which agrees with normal pulse polarographic data.
4.5. Analysis of samples The adsorptive polarographic procedure proposed in this paper can be used to determine the formaldehyde in air samples. According to the procedure described in the section on the preconcentration of formaldehyde in air, the formaldehyde in the air samples was collected, and its content was determined by the procedure described in the section on analysis of air samples.
4.3. Conditions of deri6ation reaction 4.3.1. Effect of DNPH concentration The effect of DNPH concentration on peak currents are shown in Fig. 4b. The suitable concentration of DNPH is from 1.5 to 2.5×10 − 5 M. So a DNPH concentration of 2.0×10 − 5 M was chosen for subsequent studies. 4.3.2. Effect of temperature The effect of temperature of derivation reaction on the peak current was examined experimentally. For derivation reaction of formaldehyde with DNPH, in the temperature range from 50 to
Fig. 5. Derivative cyclic voltammograms: pH 5.7; 1.0 ×10 − 6 M formaldehyde; 2.0 ×10 − 5 M DNPH; 0.010 M NaCl; 0.0010% Tween-80; scan rate of 100 mV s − 1; (a) First scan; (b) Second and repetitive scans.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
322
Table 1 Determination of formaldehyde in air samples Samplesa
Membrane cell
Impinger [23]
Formaldehyde found (mg m−3)
RSD (%)
Formaldehyde found (mg m−3)
RSD (%)
Atmosphere 1 2
0.013 0.022
6.5 7.8
0.0096 0.010
9.1 8.4
Chemical plant 1 (outdoor) 2 (workshop)
0.025 0.058
5.5 6.3
0.018 0.035
8.2 7.8
a
Main of three parallel determinations.
The standard formaldehyde solutions used for the preparation of calibration line were treated as described in the section on polarography of pure formaldehyde. The regression equation of the calibration line has the form: Y = 7.83X −0.03
(4)
where Y is the peak current in mA and X is the formaldehyde concentration in mg ml − 1. The correlation coefficient was 0.997. The results of determination of formaldehyde in the air samples were summarized in Table 1. The data sampled with impinger are also shown in Table 1. These data are smaller than those sampled with the membrane cell, which shows that the membrane cell is more effective than the impinger for the preconcentration of formaldehyde in air samples.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
References [1] Registry of Toxic Effects of Chemistry Substances. National Institute of Occupational Safety and Health, Washington, D.C. 1976. [2] Leithe W., The analysis of air pollutants, Translated by R. Kondor, Ann Arbor Science Publishers Ann Arbor, 1971, p.229 – 233. [3] A.C. Rayner, C.M. Jephcott, Anal. Chem. 33 (1961)
[19] [20] [21] [22] [23]
627 – 630. P.W. West, B.Z. Sen, Anal. Chem. 153 (1956) 177 – 185. V. Pratima, V.K. Gupta, Talanta 30 (1983) 443 – 445. M. Tanenbaun, Anal. Chem. 23 (1951) 354 – 356. C. Zhang, J. Yu, M. Zhai, Sepu 16 (1998) 363 – 364. Y. Li, W. Mou, Fenxi Huaxie 27 (1993) 804 – 807. Y. Yuan, P. Zang, Huanjing Huaxie 6 (4) (1987) 71 – 75. S. Motomizu, T Yoden, Anal. Chim. Acta 261 (1992) 461 – 469. T. Kuwaki, M. Akiba, M. Oshina, S. Motomizu, Bunseki Kagaku 36 (1987) T81 – T84. T. Aoki, S. Uemura, M. Munemiri, Anal. Chem. 55 (1983) 1620 – 1622. T. Kuwaki, K. Toei, M. Akiba, M. Oshna, S. Motomizu, Bunseki Kagaku 36 (1987) T132 – T135. H. Erxleben, L.N. Moskvin, T.G. Nikitina, J. Simon, Fresenius J. Anal. Chem. 36 (1998) 324 – 325. Z.-Q. Zhang, S.Z. Chen, Y.-F. Li, M.-F. Zhu, Anal. Chim. Acta 382 (1999) 283 – 289. Y.-F. Li, H. Zhang, F. Xiao, Z.-Q. Zhang, Talanta 47 (1998) 25 – 32. Li Y.-F., Application of Chromatomembrane method in Air Pollution Analysis, Thesis, Hunan University, 1998. Y.-F. Li, Z.-Q. Zhang, Fenxi Ceshi Xuebao 18 (1999) 73 – 77. Z.-Q. Zhang, H. Zhang, Q. Deng, Talanta 53 (2000) 517 – 523. Z.-Q. Zhang, Z.-X. Sun, Huaxie Zhuanganqi 20 (2000) 25 – 28. L.N. Moskvin, O.V. Rodinkov, J. Chromarogr. A725 (1996) 351 – 358. S. Deng, Fenxi Yiqi 1 (1984) 1 – 5. J.P. Lodge, Methods of Air Sampling and Analysis, third edn., Lowis Publishers, Michigan, 1989, p. 586.
Preconcentration with membrane cell and adsorptive polarographic determination of formaldehyde in air Zheng-Qi Zhang *, Hong Zhang, Guang-Fu He College of Chemistry and Chemical Engineering, Hunan Uni6ersity, Changsha 410082, People’s Republic of China Received 12 December 2001; received in revised form 31 December 2001; accepted 8 January 2002
Abstract The present paper describes a procedure that formaldehyde in air was preconcentrated in a membrane cell and its content was determined by adsorptive polarography. First the formaldehyde in air samples was preconcentrated in a membrane cell using water, then the formaldehyde reacted with 2,4-dinitrophenyl hydrazine to form 2,4-dinitrophenyl hydrazone, which can be adsorbed at the mercury electrode and yields a sensitive adsorptive polarographic wave. Over the range 6.0 ×10 − 10 –5.0×10 − 6 M, the peak currents are linearly proportional to the concentration of formaldehyde, the detection limit is 2.0 ×10 − 10 M. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Polarography; DNPH; Formaldehyde
1. Introduction Formaldehyde strongly irritates the mucous membranes; it is perceived in concentrations of 0.2 ppm and above. Formaldehyde is one of the most important industrial chemicals for the manufacture of synthetic resins, fertilizers, explosives and others. It possesses toxicity, and the animal experiments have shown formaldehyde to be a lung carcinogen [1]. Since the reactivity of formaldehyde is high and the number of known specific reactions is large, there exists a wide variety of methods for the determination of low formaldehyde concentrations [2]. Colorimetric procedures are most frequently used. In 1866 Rayner and Jephcott * Corresponding author.
reported a procedure for accurate determination of trace formaldehyde with Schiff reagent [3]. Chromotropic acid [4], acetylacetone, paminobenzenesulfonic acid and phenyl hydrazine were used for spectrophotometric determination of formaldehyde [5,6]. A gas chromatographic procedure for the determination of formaldehyde were based on prior derivatization procedure [7]. Adsorptive voltammetry and DPP are used to determine formaldehyde in air samples [8,9]. The application of membrane separations technique in analytical chemistry has been reported [10 –14]. This technique was used to collect the air pollutants [15 –20]. Using membrane cell, the analyte is continuously extracted from a certain phase. In the present paper, we design a membrane cell (Fig. 1), and use it to collect formaldehyde in air samples with water. In a dilute HCl
0039-9140/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 0 2 9 - 2
318
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
solution, the formaldehyde reacts with 2,4-dinitrophenyl hydrazine (DNPH) to form 2,4-dinitrophenyl hydrazone, which can yield an adsorptive polarographic wave on drop mercury electrode, so the formaldehyde can be determined by adsorptive polarography.
case of PTFE porous matrix the capillary effects become negligible within porous larger than 0.1 mm radius. So it can be guaranteed that the polar liquid phase moves within the macropores and the non-polar liquid or gas phase moves within the micropores.
2. Principle of operation of membrane cell
3. Experimental
Membrane cell is a device for mass-exchange between a polar liquid and a non-polar liquid or gas phase within a membrane cell. In membrane cell, the mass-exchange process is carried out in the capillary medium of hydrophobic material. The membrane cell used in this study are shown in Fig. 1. The micropores membrane 4 is a hydrophobic one. The mass-exchange layer 3 was made up of porous polymer particles such as porous polytetraflouro-ethylene(PTFE). The porosity between the particles forms macropores with a radius of 0.5 – 3.0 mm. In the polymer particles there are numerous open micropores with a radius of 0.03–1.0 mm. The macropores form channels for the polar liquid which do not wet the surface of the hydrophobic material, and the micropores form channels for the non-polar liquid or gas phase. When the non-polar liquid or gas moves within the micropores it can contact with the polar liquid within the macropores, thus the analyte transfer from original phase into the other. Throughout the mass-exchange space the gas or non-polar liquid phase does not enter into the macropores because the polar phase pressure is maintained higher than the pressure of the gas phase, and polar liquid phase also could not penetrate into micropores because the capillary pressure of micropores can prevent this penetration. The capillary pressure (Pc) depends on the surface tension of the liquid phase (|), the contact angle between the liquid and the membrane material (q) and the pore radius (k):
3.1. Apparatus
Pc = (2|cosq)/k.
3.1.1. Membrane cell The experimental membrane cell (Fig. 1) was made up of PTFE. The cell had a membrane of 0.8 mm thickness with an average pore size of about 0.7 mm. In the mass-exchange units (25× 25× 50 mm3) the micropore size is as uniform as possible, e.g. 0.7 mm, and the average diameter of macropore size is 0.5 mm. The cell in this study allow variation of the aqueous phase flow within the 0–500 ml min − 1 range and of the air flow within the 0–1.5 dm3 min − 1 range. 3.1.2. Electroanalytical de6ice The polarograms were recorded on a JP-1A oscillopolarograph (Chengdu Instrumenttal Factory). The polarographic cell has a three electrode system: a dropping mercury electrode (DME) as working electrode, a saturated calomel electrode (SCE) as reference electrode and platinum wire as auxiliary electrode. A drop time of 7 s was selected using a knocker,with a rest time of 5 s and a scan time of 2 s, the scan rate being 250 mV s − 1. An MP-2 stripping voltammetric analiz-
(1)
In order to prevent capillary effects, in design of membrane cell the Pc should exceed the value calculated from the above equation [21]. In the
Fig. 1. Membrane cell: (1) triton valve; (2) case of the cell; (3) mass-exchange layer; (4) micropores membrane.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
er(Shandong Instrumental Factory) with an Epson printer and a JM-01 (manual micro-metric screw delivery) hanging mercury drop electrode, controlled by micro-processor, and A PAR Model 273 potentiostat/Galvanostat with a PAR Model 303 static mercury drop electrode, controlled by PAR Model 270 software, were used for pulse polarography, linear scan voltammetry, cyclic voltammetry and other electrochemical measurements. For pulse polarography the instrumental parameters were as follows: accumulation potential, − 0.40 V; drop size, medium; pulse amplitude, 50 mV; pulse period, 2 s; equilibrium time, 15 s.
3.2. Reagents 3.2.1. Formaldehyde standard solutions A stock solution of formaldehyde was prepared by diluting 2.8 ml of 36– 38% formaldehyde solution to 1000 ml with water, which was standardized by iodometry. The solutions of lower concentration were prepared by serial dilution. 3.2.2. DNPH solution A stock solution of 2,4-dinitrophenyl hydrazine was prepared by dissolving 0.1000 g of DNPH (AR) in 24 ml of 6.0 M HCl solution, and diluted to 100 ml. 3.2.3. Tween-80 solution (0.10%) A stock solution of Tween-80 was prepared by dissolving 0.100 g of Tween-80 (AR) in 100 ml of water. Other reagents were of supra-pure or analytical-reagent grade. Water, redistilled in a fused-silica apparatus, was used throughout. 3.3. Procedures 3.3.1. Preconcentration of the formaldehyde in air The experimental device for preconcentration of formaldehyde in air samples is shown in Fig. 2, and the routines for preconcentration of formaldehyde in air were as follows: (I) First all valves were closed, and the pump 8 was started.
319
Fig. 2. Scheme of experimental device for absorption of aniline in air: (1) buffer bottle; (2) rotameter; (3) triton valve; (4) samples solution collector; (5) water; (6) membrane cell; (7) air regulating valve; (8) pump.
(II) Then the triton valve 3 was rotated to connect line 2 with line 3 to let absorbing solution to fill the membrane cell. When the solution outflows from the cell the triton valve 3 was rotated to cut off line 2 and 3. (III) The triton valve 3 was again rotated to connect line 1 with line 3, and valve 7 was regulated to make the flow rate of air in 0.5 dm3 min − 1. After about 5 min the valve 7 was closed, and the triton valve 3 was immediately rotated to connect line 2 with line 3 to collect about 1.0 ml of absorbing solution. Repeating above routine III, a 3–5 ml of the solution (total volume) was collected.
3.3.2. Adsorpti6e polarographic determination of formaldehyde 3.3.2.1. Polarography of pure formaldehyde solution. In a 10 ml beaker, 0.20 ml of 2,4-dinitrophenyl hydrazine solution and required amounts of formaldehyde standard solution were added and mixed well, then the mixture was stood for 15 min at 60 °C in a water bath. After cooling the mixture to room temperature in tap water, 2.00 ml of pH 5.7 buffer solution, 1.00 ml of 0.10 M NaCl solution and 0.10 ml of Tween-80 solution were added, and the mixture was diluted to 10 ml.
320
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
After transferring the solution to electrolytic cell, the derivative polarograms were recorded with potential scan rate of 250 mV s − 1, starting the potential scan at −0.40 V. The peak potential is − 0.78 V.
3.3.2.2. Analysis of air samples. Using the collected formaldehyde sample solutions in place of formaldehyde standard solution, analysis was performed as above described for pure formaldehyde solution.
4. Results and discussion
4.1. Conditions for preconcentration of formaldehyde 4.1.1. Selection of absorbing solution According to the literatures [7,8], the water was used to absorb formaldehyde in air samples in this study. 4.1.2. Flow rate of the air samples When the analyte is easy to dissolve in an absorbing solution, the larger the flow rate of the sample, the shorter the time of concentration can be. However, the flow rate of the sample must be less than one which the membrane cell tolerates. In order to absorb the formaldehyde in air samples completely, in this study, the flow rate of the air samples was controlled at 0.5 dm3 min − 1.
(NO2)2C6H5NHNCH2 + 2e+2H+ X (NO2)2C6H5NHNHCH3
(3)
When there is 0.0010% Tween-80 in the test solution, the peak current increases two-fold (Fig. 3c), which can be used to determine the trace formaldehyde. Over the range 6.0×10 − 10 –5.0× 10 − 6 M, the peak currents are linearly proportional to the concentration of formaldehyde. The detection limit is 2.0×10-10 M, which was taken as the concentration that gave a signal equal to three times the standard deviation of the blank signal, calculated from the calibration slope. The reproducibility was evaluated by 20 repetitive experiments on 6.0× 10 − 10 M formaldehyde solution. The relative standard deviation was 6.1%.
4.2.2. Effect of pH The dependence of the peak current on pH value is shown in Fig. 4a. When pH value was less than 5.5 the peak current increased with the increasing of pH value of test, and larger than 6.0, the peak current decreased. In the pH range from 5.5 to 6.0 the peak current is the largest. In this work the pH value was chosen to be 5.7. 4.2.3. Effect of NaCl concentration Support electrolytes such NaCl, KCl, NaNO3 and KNO3 were examined experimentally. The best results were obtained with NaCl support electrolyte. Over the range from 0.0050 to 0.045
4.2. Polarographic determination of formaldehyde 4.2.1. Adsorpti6e polarography In a dilute HCl solution, formaldehyde can react with 2,4-dinitrophenyl hydrazine to form 2,4-dinitrophenyl hydrazone, and the produce can be adsorbed on the mercury electrode and undergo electrochemical reaction to yield sensitive adsorptive polarographic peak (Fig. 3b) on the DME.
(2)
Fig. 3. Derivative polarograms: (a) pH 5.7; 0.010 M NaCl; 2.0 ×10 − 5 M DNPH; (b) a+5.0 × 10 − 7 M formaldehyde; (c) b +0.0010% Tween-80.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
321
70 °C the peak currents remained constant. So the derivation reaction carried out at 60 °C.
4.3.3. Effect of time At 60 °C the derivation reaction carried out rapidly, and was finished in 10 min. So after mixing formaldehyde and DNPH solution, the mixture was standed for 15 min at 60 °C. 4.4. Cyclic 6oltammetry
Fig. 4. Effect of pH and DNPH 5.0 × 10 − 7 M HCHO, 0.0010% Tween-80; (a) Effect of pH: 1.0 ×10 − 5 M DNPH; (b) Effect of DNPH concentration: pH 5.7.
M the peak current remains constant. A concentration of 0.010 M for NaCl was used in subsequent work.
4.2.4. Effect of surfactants Various cathodic, anodic and nonionic surfactants such as cetyltrimethylammonium bromide (ATMAB), sodium lauryl sulphate (SLS), p-octylpolyethyleneglycol phenyl ether (OP), Tween-80 and Triton X-100 were tested, and Tween-80 was found to be the best. The peak current increases rapidly with increasing Tween-80 concentration and the maximum value were attained from 0.00080 to 0.0050%. As Tween-80 concentration increases beyond 0.0050%, the peak current decreases. The suitable Tween-80 concentration for this study was 0.0010%.
Fig. 5 shows the cyclic voltammograms of formaldehyde system. The 2,4-dinitrophenyl hydrazone gives a cathodic peak at about − 0.78 V due to its reduction, and no peak was observed on the anodic branch, indicating that the reduction of 2,4-dinitrophenyl hydrazone is irreversible. In Fig. 5 subsequent repetitive scans yielded significantly smaller (but stable) cathodic peaks corresponding to the reaction of dissolved species. This behavior indicates that 2,4-dinitrophenyl hydrazone is adsorbed on the mercury electrode [22], which agrees with normal pulse polarographic data.
4.5. Analysis of samples The adsorptive polarographic procedure proposed in this paper can be used to determine the formaldehyde in air samples. According to the procedure described in the section on the preconcentration of formaldehyde in air, the formaldehyde in the air samples was collected, and its content was determined by the procedure described in the section on analysis of air samples.
4.3. Conditions of deri6ation reaction 4.3.1. Effect of DNPH concentration The effect of DNPH concentration on peak currents are shown in Fig. 4b. The suitable concentration of DNPH is from 1.5 to 2.5×10 − 5 M. So a DNPH concentration of 2.0×10 − 5 M was chosen for subsequent studies. 4.3.2. Effect of temperature The effect of temperature of derivation reaction on the peak current was examined experimentally. For derivation reaction of formaldehyde with DNPH, in the temperature range from 50 to
Fig. 5. Derivative cyclic voltammograms: pH 5.7; 1.0 ×10 − 6 M formaldehyde; 2.0 ×10 − 5 M DNPH; 0.010 M NaCl; 0.0010% Tween-80; scan rate of 100 mV s − 1; (a) First scan; (b) Second and repetitive scans.
Z.-Q. Zhang et al. / Talanta 57 (2002) 317–322
322
Table 1 Determination of formaldehyde in air samples Samplesa
Membrane cell
Impinger [23]
Formaldehyde found (mg m−3)
RSD (%)
Formaldehyde found (mg m−3)
RSD (%)
Atmosphere 1 2
0.013 0.022
6.5 7.8
0.0096 0.010
9.1 8.4
Chemical plant 1 (outdoor) 2 (workshop)
0.025 0.058
5.5 6.3
0.018 0.035
8.2 7.8
a
Main of three parallel determinations.
The standard formaldehyde solutions used for the preparation of calibration line were treated as described in the section on polarography of pure formaldehyde. The regression equation of the calibration line has the form: Y = 7.83X −0.03
(4)
where Y is the peak current in mA and X is the formaldehyde concentration in mg ml − 1. The correlation coefficient was 0.997. The results of determination of formaldehyde in the air samples were summarized in Table 1. The data sampled with impinger are also shown in Table 1. These data are smaller than those sampled with the membrane cell, which shows that the membrane cell is more effective than the impinger for the preconcentration of formaldehyde in air samples.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
References [1] Registry of Toxic Effects of Chemistry Substances. National Institute of Occupational Safety and Health, Washington, D.C. 1976. [2] Leithe W., The analysis of air pollutants, Translated by R. Kondor, Ann Arbor Science Publishers Ann Arbor, 1971, p.229 – 233. [3] A.C. Rayner, C.M. Jephcott, Anal. Chem. 33 (1961)
[19] [20] [21] [22] [23]
627 – 630. P.W. West, B.Z. Sen, Anal. Chem. 153 (1956) 177 – 185. V. Pratima, V.K. Gupta, Talanta 30 (1983) 443 – 445. M. Tanenbaun, Anal. Chem. 23 (1951) 354 – 356. C. Zhang, J. Yu, M. Zhai, Sepu 16 (1998) 363 – 364. Y. Li, W. Mou, Fenxi Huaxie 27 (1993) 804 – 807. Y. Yuan, P. Zang, Huanjing Huaxie 6 (4) (1987) 71 – 75. S. Motomizu, T Yoden, Anal. Chim. Acta 261 (1992) 461 – 469. T. Kuwaki, M. Akiba, M. Oshina, S. Motomizu, Bunseki Kagaku 36 (1987) T81 – T84. T. Aoki, S. Uemura, M. Munemiri, Anal. Chem. 55 (1983) 1620 – 1622. T. Kuwaki, K. Toei, M. Akiba, M. Oshna, S. Motomizu, Bunseki Kagaku 36 (1987) T132 – T135. H. Erxleben, L.N. Moskvin, T.G. Nikitina, J. Simon, Fresenius J. Anal. Chem. 36 (1998) 324 – 325. Z.-Q. Zhang, S.Z. Chen, Y.-F. Li, M.-F. Zhu, Anal. Chim. Acta 382 (1999) 283 – 289. Y.-F. Li, H. Zhang, F. Xiao, Z.-Q. Zhang, Talanta 47 (1998) 25 – 32. Li Y.-F., Application of Chromatomembrane method in Air Pollution Analysis, Thesis, Hunan University, 1998. Y.-F. Li, Z.-Q. Zhang, Fenxi Ceshi Xuebao 18 (1999) 73 – 77. Z.-Q. Zhang, H. Zhang, Q. Deng, Talanta 53 (2000) 517 – 523. Z.-Q. Zhang, Z.-X. Sun, Huaxie Zhuanganqi 20 (2000) 25 – 28. L.N. Moskvin, O.V. Rodinkov, J. Chromarogr. A725 (1996) 351 – 358. S. Deng, Fenxi Yiqi 1 (1984) 1 – 5. J.P. Lodge, Methods of Air Sampling and Analysis, third edn., Lowis Publishers, Michigan, 1989, p. 586.