Preconcentration with chromatomembrane cell and adsorptive polarographic determination of fluorine in air

Talanta 47 (1998) 25 – 32

Preconcentration with chromatomembrane cell and adsorptive polarographic determination of fluorine in air Yan-Fei Li, Hong Zhang, Fei Xiao, Zheng-Qi Zhang * College of Chemistry and Chemical Engineering, Hunan Uni6ersity, Changsha 410082, People’s Republic of China Received 13 September 1997; received in revised form 5 January 1998; accepted 7 January 1998

Abstract The present paper describes a procedure in which fluorine in the air was preconcentrated in a chromatomembrane cell and its content was determined by adsorptive polarography. In a pH 4.90 buffer solution the fluorine ion can form a ternary complex with La(III) and ALC. The complex can be adsorbed at the mercury electrode and yields a sensitive oscillopolarographic wave at − 0.67 V, which can be sensitized by Triton X-100. Over the range 3.0×10 − 8 –1.60×10 − 6 M, the peak currents are linearly proportional to the concentration of the fluoride. The detection limit is 1.0 ×10 − 8 M. First the fluorine in the air samples was preconcentrated in the chromatomembrane cells using 0.10 M NaOH solution, then its content was determined by complex-adsorptive polarography. © 1998 Elsevier Science B.V. All rights reserved.

1. Introduction Elemental fluorine is rarely found in industrial waste gases, but fluorine compounds are often, although in low concentration, constituents of industrial emissions. Fluorine occurs mostly as HF and sometimes as SiF4. Fluorine-containing gaseous pollutants or dust can cause chronic injuries, and cause disturbances in calcium metabolism. Fluorine compounds in the air may lead to appreciable damage to vegetation even when present in very low amounts. Therefore investigation of its quantitative procedure possesses ecological importance. The earlier procedures are based on the effect

* Corresponding author.

of the fluorine ion on color lakes of zirconium with alizarin. The metals are removed from the color lake to form the more stable metal fluoride. The lake is decolorized and the weaker color can be observed on a filter paper or measured with the aid of a spectrophotometer. Kaye [1] described a coulometric procedure for the determination of elementary fluorine. The potentiometry can be used for the determination of fluorides down to 2.0×10 − 6 M [2,3]. Belcher [4–7] recommended a spectrophotometric method for the determination of fluorine with a limit of detection of 5.0× 10 − 6 M, which was used to determine the fluoride in air [8]. An impinger containing 70 ml of 0.1 M NaOH solution and 5 ml of 3% H2O2 solution was used for the collection of fluoride in an air sample. However, Leithe [9] found that using the impinger for preconcentration of the fluorine in

0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(98)00050-2

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Fig. 1. Chromatomembrane cell: (1) case of the cell; (2) mass-exchange layer; (3) microporous membrane; (4) regulatory valve for absorbing solution; (5) air regulatory valve.

the air at the production sites of superphosphate and aluminum the absorption of fluorine was not complete. In 1994 Moskvin [11 – 17], recommended a chromatomembrane method, which is a new technique for separation of a substance. Using the chromatomembrane method the analyte is continuously extracted from a certain phase. The chromatomembrane method combines the main advantages of chromatographic (high efficiency of the mass-exchange process) and membrane (continuous regime) separation method [16]. In this paper, we describe a procedure which illustrates the fact that fluorine in the air is preconcentrated in the chromatomembrane cell and its contents were determined by adsorptive polarography.

2. Principle of chromatomembrane method The chromatomembrane method is one for mass exchange between a polar liquid phase and a non-polar liquid or gas phase within a chromatomembrane cell [11,12,17]. In a chromatomembrane cell Fig. 1 the mass-exchange process is carried out in the capillary medium of hydrophobic material with two preferential pore types differing in size, which is bounded on two sides by microporous hydrophobic membranes [13]. The mass-exchange layer was made of

porous polymer particles such as porous polytetrafluoroethylene(PTFE). The porousity between the particles form macropores with a radius of 0.50–3.0 mm [12]. The polar liquid, which does not wet the surface of the hydrophobic material, fills the macropores of the biporous matrix and moves within them. In the polymer particles there are numerous open micropores with a radius of 0.03–1.0 mm. The non-polar liquid or gas phase moves within the micropores, and contacts with the polar liquid within the macropores, thus the analyte transfer from the 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 the polar liquid phase also could not penetrate into the micropores because the capillary pressure of micropores can prevent this penetration. In order to prevent capillary effects, in design of a chromatomembrane cell the capillary pressure should exceed the value calculated from the known expression [13], Pc = (2s cos u)/Y

(1)

where Pc is the capillary pressure, s is the surface tension of the liquid phase, u is the contact angle between the liquid and the membrane material

Y.-F. Li et al. / Talanta 47 (1998) 25–32

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Fig. 2. Scheme of experimental device for absorption of fluorine in air: (1) buffer bottle; (2) rotameter; (3) chromatomembrane cell; (4) and (6) regulating valve for absorbing solution; (5) 0.10 M NaOH; (7) air regulating valve; (8) pump; (9) samples solution collector.

and Y is the pore radius. In the case of PTFE porous matrix with a contact angle of 108° the capillary effects become negligible within pores of 0.1 mm radius. So it can be guaranteed that the polar liquid phase moves within macropores and the non-polar liquid or gas phase moves within micropores. When we use gas to extract analyte from the polar liquid phase additional conditions have to be guaranteed [13], PL1 BPG2 +Pc

(2)

PG1 B PL2

where PL1 and PL2 are the liquid-phase pressures at the inlet and outlet of the mass-exchange space of the chromatomembrane cell, respectively, PG1 and PG2 are the gas-phase pressures at the inlet and the outlet of the mass-exchange space, respectively, and Pc is the capillary pressure in the micropores. Combining Eq. (2) and Eq. (3), and simply transformating, we obtain the following expression, PL1 − PL2 + PG1 − PG2 = D PL + D PG B Pc

Fig. 3. Polarograms of La(III)–ALC–F − –Triton X-100 system: (a) pH 4.90, 0.10 M (CH2)6N4, 0.10 M KNO3; (b) a + 1.25 × 10 − 5 M La3 + + 1.0× 10 − 5 M ALC; (c) b+ 5.0 × 10 − 7 M F − ; (d) c+ 0.0020% Triton X-100.

(3)

(4)

Fig. 4. The composition of the background solution of La(III) – ALC – F − – Triton X-100 system: pH 4.90, 0.10 M (CH2)6N4, 1.0×10 − 5 M ALC, 5.0 ×10 − 7 M F − (a) effect of the La(III) concentration: 0.01 M KNO3, 0.0010% Triton X-100; (b) effect of the KNO3 concentration: 1.25 ×10 − 5 M La3 + , 0.0010% Triton X-100; (c) effect of Triton X-100: 1.25 × 10 − 5 M La3 + , 0.10 M KNO3.

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Fig. 5. Cyclic voltammgrams of La(III)–ALC–F − – Triton X-100 system: pH 4.90, 0.10 M (CH2)6N4, 0.10 M KNO3, 1.25× 10 − 5 M La3 + , 1.0 × 10 − 5 M ALC, 1.0 × 10 − 6 M F − , 0.0020% Triton X-100, scan rate of 100 mV s − 1. (1) first scan; (2) second and repetitive scans.

Obviously, it is neccessary for the chromatomembrane process that the sum of pressure gradients for the liquid and gas phases should be less than the capillary pressure value. Thus the capillary pressure is the main physico-chemical parameter that defines the capabilities of the method [12].

3. Experimental

3.1. Apparatus 3.1.1. Chromatomembrane cell The experimental chromatomembrane cell Fig. 1 was made of polytetrafluoroethylene. The cell had membranes of 0.8 mm thickness with an average pore size of 0.5 mm. In the mass-exchange units the micropore sizes are as uniform as possible, e.g. 0.5 mm, and the average diameter of macropore size is 0.3 mm. The cells in this study allow variation of the aqueous phase flow within the 0–300 ml min − 1 range and of the air flow within the 0 – 1.0 dm3 min − 1 range. 3.1.2. Electroanalytical de6ice The single-sweep polarograms were recorded on a JP-1A oscillopolarograph (Chengdu Instrumental Factory). The polarographic cell has the three electrode system: a dropping mercury electrode(DME) as working electrode, a saturated calomel

electrode (SCE) reference electrode and a platitum wire 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 XJP-821 neopolarograph (Jiangsu Electroanalytical Instrumental Factory) in connection with a LZ3-100 X-Y recorder (Dahua Instrumental Factory) and a JM-01 (manual micro-metric screw delivery) hanging mercury drop electrode (HMDE) were used for cyclic voltammetry measurements. A PAR Model 273 Potentiostat/Galvanostat with a PAR Model 303 static mercury drop electrode, controlled by PAR Model 270 software, was used for normal pulse polarography, linear scan voltammetry and other electrochemical measurements. For pulse polarography the instrumental parameters were as follows: accumulation time, 120 s; accumulation potential, − 0.35 V; drop size, medium; pulse amplitude, 50 mV; pulse period, 2 s; equilibrium time, 15 s.

3.2. Reagents 3.2.1. Standard fluoride solution (1.0×10 − 3 M) A stock solution of sodium fluoride (GR, Changsha Chemicals) was prepared by dissolving 0.0208 g of NaF in 50 ml water, followed by dilution to 500 ml. 3.2.2. Lanthanum(III) solution (1.0×10 − 3 M) A suitable amount of lanthanum sesquioxide (99.999%, Hunan Institute for Rare Earth Element) was incandesced at 850°C for 2 h, and cooled to room temperature. 0.0408 g of the lanthanum sesquioxide was dissolved in 25 ml of 1.0 M HNO3 solution, and the solution was made up to 250 ml. 3.2.3. ALC solution (1.0×10 − 3 M) Alizarin complexone (0.0965 g) (AR, Beijing Chemicals) was dissolved in a suitable volume of 5 M NaOH solution, its pH value was adjusted to 4.90. Then the solution was diluted to 250 ml. 3.2.4. Hexamethylenamine buffer solution (1.0 M) Hexamethylenamine (70 g) (AR, Changsha Chemicals) and 50 g of KNO3 were dissolved in 450 ml water. Its pH value was adjusted to 4.90 with 5 M HNO3 solution, then diluted to 500 ml.

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3.2.5. Triton X-100 solution (0.020%) Triton X-100 (0.10 g) (GR, Beijing Chemicals) was dissolved in 250 ml water, and the solution was diluted to 500 ml.

was maintained over the cell throughout the analysis to prevent interference from oxygen. Record the derivative polarograms, starting the potential scan at − 0.35 V. The peak potential is − 0.67 V.

3.2.6. NaOH solution (0.10 M) Other reagents were of a suprapure or analytical-reagent grade. Water, redistilled in a fused-silica apparatus, was used throughout.

3.3.2.2. Analysis of air samples. The pH value of the collected solution was adjusted to 5–6 with dilute HNO3 solution. Analysis was performed as described above for pure sodium fluoride solution.

3.3. Procedures 3.3.1. Preconcentration of the fluoride in air Fig. 2 presents an experimental device for the preconcentration of the fluoride in air samples. Firstly all valves were closed, and the pump 8 was started. Then the valves 4 and 6 were regulated to control the flow rate of 0.05 ml min − 1 for absorbing solution (0.10 M NaOH), and then valve 7 was regulated to make the flow rate of air in 0.80 dm3 min − 1. About 3 ml of the solution was collected. 3.3.2. Adsorpti6e polarographic determination of F− 3.3.2.1. Polarography of pure sodium fluoride solution. 1.0 ml of the buffer solution, 1.25 ml of 1.0× 10 − 4 M La(III) solution, 1.00 ml of 1.0× 10 − 4 M ALC solution and 1.00 ml of the Triton X-100 solution were well mixed, and various amounts of standard F − solution was added. The mixture was diluted to 10 ml with water. After standing for 30 min, the solution was transfered to the polarographic cell, and purged with oxygen-free nitrogen for 10 min. A flow of nitrogen Table 1 Determination of fluorine in air samples Samples

F− found (mg m−3)

Main (mg m−3)

SD (%)

Laboratory 1 Laboratory 2 Atmosphere

0.67, 0.65, 0.68 0.45, 0.44, 0.47 0.083, 0.089, 0.080 2950, 3108, 3032

0.67 0.45 0.084

2.2 3.2 5.4

3030

2.6

Superphosphate plant

4. Results and discussion

4.1. The conditions of preconcentration 4.1.1. Choosing of absorbing solution The fluorine-containing compounds such as HF in air, are best collected in 0.01–1.0 M sodium hydroxide solution [18]. In this study a 0.10 M NaOH solution was used to absorb fluorine in air samples. 4.1.2. The flow rate of absorbing solution Using the chromatomembrane cell for the preconcentration of analyte in air samples the flow rate of the absorbing solution depends on the concentration of the analyte in the sample, the sensitivity of the analytical method and the solubility of the analyte. The hydrogen fluoride in air samples is easy to dissolve in dilute sodium hydroxide solution. If the concentration of fluoride in the sample is higher and the analytical method is more sensitive, the flow rate of the absorbing solution may be larger. In this work, the flow rate of 0.10 M NaOH solution was 0.050 ml min − 1. 4.1.3. The 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. However, the flow rate of the sample must be less than one which the chromatomembrane cell tolerates. In this study, the flow rate of the air samples was controlled at 0.80 dm3 min − 1.

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Fig. 6. The device for the comparison between chromatomembrane cell and impinger: (1) 2.0 M NaOH; (2) chromatomembrane cell 1; (3) collector for waste solution; (4) rotameter 1 for free-fluorine air; (5) thermostatical water bath; (6) HF solution (15%) (7) mix chamber; (8) rotameter 2 for sample air; (9) absorbing device (A or B); (10) pump, (11) 0.10 M NaOH, (12) collector for sample solutions, (13) 0.10 M NaOH.

4.2. Adsorpti6e polarographic determination of F− 4.2.1. Adsorpti6e polarography In a pH 4.3 – 6.0 buffer solution, the rare earth elements (RE) can form binary complexes with ALC [19,20], which possesses good electroanalytical characteristics and can be used for the determination of RE. Langmyhr [21] investigated in detail the composition of the ternary complex Ce(III)–ALC – F − with spectrophotometry, and found that the complex is a dipolymer. Li et al. [19,22] investigated the electroanalytical behaviors of the binary complex RE(III) – ALC and the ternary complex RE(III) – ALC – F − . The results showed that both the binary and the ternary complexes are adsorbed on the mercury electrode. We found that the addition of Triton X-100 into the system of La(III) – ALC – F − can increase the peak height of the ternary complex. The oscillopolarograms of La(III) – ALC – F − -Triton X100 system are shown in Fig. 3. In a pH 4.90 buffer solution, La(III) – ALC complex yields a sensitive oscillopolarographic wave p1 (Fig. 3b).

The ternary complex La(III)–ALC–F − yields a new sensitive polarographic wave p2 at −0.67 V (Fig. 3c), which can be sensitized by Triton X-100 (Fig. 3d). Its peak potential shifts in the negative direction with increasing pH value of test solution. The p2 can be used to determine the trace fluoride. Over the range 3.0× 10 − 8 –1.60× 10 − 6 M, the peak currents are linearly proportional to the concentration of the fluoride. The detection limit is 1.0× 10 − 8 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 15 repetitive experiments on a 5.0× 10 − 7 M F − solution. The relative standard deviation was 1.6%.

4.2.2. Effect of pH The pH value of the test solution affected the peak current and potential of p2. when the pH value was B 4.3 the La(III)–ALC–F − complex did not yield a polarographic wave, and for pH values \ 5.5, the p2 disappeared. In a pH 4.3–5.5 range the peak potential shifts in the negative

Y.-F. Li et al. / Talanta 47 (1998) 25–32

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Table 2 Comparison between chromatomembrane method and impinger Mixed gas samples

1 2 3

Temperature of water bath (°C)

25 30 40

direction with increasing pH value of the test solution. In a pH 4.90 solution, the peak potential was −0.67 V. Over the pH range 4.8 –5.2 the peak current of the p2 was the largest and most stable. A pH value of 4.90 was chosen for subsequent studies. The buffers such as HOAc –NaOAc and hexamethylenamine were tested, and the best results were obtained in hexamethylenamine buffer solution, so hexamethylenamine was used as the buffer.

4.2.3. The ratio between ALC and La(III) In the range of ratio between ALC and La(III) from 0.7 to 1.0 the peak current ip2 was the largest and most stable, so the ratio of 0.8 was chosen for subsequent studies. 4.2.4. Effect of La(III) concentration The La(III) concentration affected the peak current (Fig. 4a). When its concentration is B 8.0× 10 − 6 M the ip2 increases rapidly with increasing La(III) concentration, and when La(III) concentration is \ 1.0 ×10 − 5 M the ip2 remains constant. Accordingly, a concentration of 1.25× 10 − 5 M for La(III) was used throughout for maximum sensitivity. 4.2.5. Effect of KNO3 concentration Support electrolytes such as KCl, NaCl, KNO3 and NaNO3 were examined in 5.0× 10 − 7 M F − solution. The best results were obtained with KNO3 support electrolyte. The effect of its concentration on peak current is shown in Fig. 4b. Over the range 0.050 – 0.15 M the ip2 remains constant. A concentration of 0.10 M for KNO3 was used throughout.

Chromatomembrane method

Impinger

F− found (mg m−3) SD (%)

F− found (mg m−3) SD (%)

0.41 0.88 1.41

0.37 0.56 1.24

2.8 2.2 2.5

5.1 4.5 4.3

4.2.6. Effect of Triton X-100 concentration In order to choose the most suitable surfactant, various surfactants such as cetyl trimethyl ammonium bromide(CTMAB), sidium lauryl sulphate(SLS), p-octyl polyethylene glycol phenylether(OP), Tween-80 and Triton X-100 were examined. Triton X-100 showed the best sensitization, so we used it as a sensitizer. The influence of its concentration on the peak current is shown in Fig. 4c. The peak current increases rapidly with increasing Triton X-100 concentration from 0.0005 to 0.0015% and decreases greatly when the concentration of Triton X-100 \ 0.006%. A percentage of 0.0020% for the surfactant was chosen for subsequent studies. 4.2.7. Cyclic 6oltammetry Fig. 5 shows the cyclic voltammograms for the La(III)–ALC–F − –Triton X-100 system. The ternary complex La(III)–ALC–F − gives a cathodic peak at −0.67 V due to its reduction, and no peak was observed on the anodic branch, indicating that the reduction of the ternary complex is irreversible, and the binary complex La(III)–ALC gives both a cathodic peak on the cathodic branch and an anodic peak on the anodic branch, indicating that the reduction of the binary complex is reversible. In Fig. 5 subsequent repetitive scans yielded significantly smaller (but stable) cathodic peaks corresponding to the reduction of dissolved species. This behavior indicates that the adsorption of the binary and ternary complex on the mercury electrode is reactant adsorption [23], which agrees with normal pulse polarographic data.

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4.3. Analysis of air samples The single-sweep polarographic procedure proposed in this paper can be used to determine the fluorine in air samples. According to the procedure described in the section on the preconcentration of the fluoride in air, the fluorine in the samples was collected, and its content was determined by the procedure described in the section on analysis of air samples. The standard sodium fluoride solutions used for the preparation of calibration line were treated as described in the section on polarography of pure sodium fluoride. The regression equation of the calibration line has the form: Y=5.50X −0.094

(5)

where Y is the peak currrent in mA and X is the F − concentration in mg ml − 1. The correlation coefficient was 0.999. The results of the determination of the fluorine in the air samples are summarized in Table 1.

4.4. Comparison between chromatomembrane method and impinger The arrangement shown in Fig. 6 was used for the comparison between the chromatomembrane method and impinger [10]. The chromatomembrane cell 1 was used to prepare a fluorine-free air which is mixed with the gas containing HF (from the HF solution) in the mix chamber. At a certain temperature of water bath the content of HF in the gas may be maintained constant. The flow rate of air in the chromatomembrane cell 1 was constantly maintained at 0.80 dm3 min − 1. The mixed gas sample was absorbed with 0.10 M NaOH solution in the chromatomembrane cell 2

(Fig. 6A) or impinger (Fig. 6B). The results obtained are shown in Table 2. These results show that the chromatomembrane method is more effictive than the impinger for the preconcentration of the fluorine in the air samples.

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