Analytica Chimica Acta 494 (2003) 225–233
Electrochemical determination of triclosan at a mercury electrode A. Safavi a,∗ , N. Maleki a,1 , H.R. Shahbaazi a,b a
b
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran Chemical and Manufacturing Pakshoo Co., No. 16, Saei Alley, North of Saei Park, Vali-Asr St., Tehran, Iran Received 4 November 2002; received in revised form 13 July 2003; accepted 21 July 2003
Abstract A voltammetric method for the determination of trace amounts of triclosan was developed. Differential pulse voltammetry of triclosan exhibits tensammetric peak at the adsorption and desorption potential of triclosan. In the potential range where adsorption occurs, the base current is depressed. The determination method is based on the adsorption of triclosan on hanging mercury drop electrode and desorption of triclosan at negative tensammetric peak. The system showed no positive tensammetric peak on hanging mercury drop electrode under the experimental conditions. The tensammetric peak potential of triclosan was between −1190 and −990 mV (versus Ag/AgCl), depending on pH, scan rate, accumulation potential, accumulation time and presence of organic materials. Variation of the scan rate between 80 and 500 mV s−1 caused a linear increase in the tensammetric peak height. The peak current was proportional to the concentration of triclosan over the range 2.5–60 g l−1 . Under optimum experimental conditions (pH = 7, accumulation potential of −450 mV and accumulation time of 90 s), the detection limit (DL) was obtained as 1.9 g l−1 and the relative standard deviation (R.S.D.) was lower than 3%. The method was applied to the determination of triclosan in samples of toothpaste (containing 0.3% triclosan) and wastewater. © 2003 Published by Elsevier B.V. Keywords: Triclosan; Electrochemical determination
1. Introduction Triclosan (2,4,4-trichloro-2-hydroxydiphenyl ether; TC) (Scheme 1), is a broad-spectrum antibacterial and antifungal agent, that is used in products such as antiseptic soaps, toothpastes, fabrics, and plastics [1]. The general methods for the determination of triclosan include gas chromatography, gas chromatography–mass spectrometry and high-pressure liquid chromatography [2–5]. These techniques require expensive instrumentation, which may not be available ∗ Corresponding author. Fax: +98-711-2286008/2280926. E-mail address:
[email protected] (A. Safavi). 1 Co-corresponding author.
0003-2670/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0003-2670(03)00920-6
in many laboratories. Also, analysis time is long, detection limit (DL) is sometimes poor and in some cases special pretreatment is required before analysis. Surface active substances adsorb onto the surface of a mercury drop electrode and change the capacitive (charging) current during differential pulse measurements. Peaks in voltammograms which are due to adsorption/desorption processes are referred to as tensammetric peaks. Tensammetry with adsorptive accumulation on a hanging mercury drop electrode is well established for the determination of trace concentrations of surfactants [6–10]. The electrochemical determination of triclosan on mercury electrode has not been investigated so far. To the best of our knowledge, only one report explained electrochemical behavior of triclosan at a
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2.3. Procedure
Scheme 1. Structure of TC (2,4,4 -trichloro-2 -hydroxy-diphenylether).
screen-printed carbon electrode and its determination at about 0.3% level [11]. The present work describes a very sensitive tensammetric method for the determination of triclosan by its accumulation on a hanging mercury drop electrode. The electrochemical behavior of triclosan in aqueous solutions at different pH values and different triclosan concentrations were studied.
2. Experimental 2.1. Reagents Triclosan was obtained from the Ciba Specialty Chemicals Inc. with a purity of minimum 99%. Stock solutions of triclosan were prepared by dissolving the solid in minimum amount of NaOH and diluting with water. Britton–Robinson (B–R) buffer was prepared by dissolving appropriate amounts of boric acid, orthophosphoric acid and glacial acetic acid in water and adjusting to the required pH value with sodium hydroxide solution. All chemicals were of analytical-reagent grade and used without further purification. Triply distilled deionized water was used throughout. 2.2. Instrumentation Voltammetric measurements were made with a Metrohm 694 (Herisau, Switzerland) VA Stand coupled with a Metrohm 693 VA Processor. Adsorptive and voltammetric experiments were carried out in a three-electrode arrangement with a Ag/AgCl, 3 M KCl reference electrode, a platinum wire counter electrode, and a multi-mode mercury drop electrode. The mercury electrode area was measured as 0.46 mm2 with a relative standard deviation (R.S.D.) of 3% for nine measurements. Dissolved oxygen was removed by bubbling argon in the solution. For optimization of pH, a Metrohm 691 pH meter was used.
2.3.1. General procedure All experiments were performed at room temperature. A 2 ml aliquot of buffer (B–R) solution (pH = 7) and appropriate volumes of sample solution were pipetted into a 10 ml volumetric flask, diluted to the mark and then transferred to the polarographic cell. The solution was purged with argon first for 10 min. and then for 30 s before each adsorptive stripping step. A 60 s preconcentration time was given to a new mercury drop at a potential of −450 mV whilst stirring. Subsequently, the stirrer was switched off for 5 s. Finally, a potential scan was carried out in a negative direction from −450 to −1350 mV in differential pulse, square wave or alternative current mode. Between experiments, the cell was treated with concentrated HNO3 and then washed with acetone and triply distilled water. 2.3.2. Procedure for analysis of real samples A suitable amount of a real sample was weighed and dissolved in water. The pH of solution was adjusted to >10 with dilute sodium hydroxide solution. Separation of other organic materials could be performed by two procedures. These involve either a one-step extraction with chloroform or a three-step extraction with hexane. The choice is dependent on the complexity of real samples. The one-step extraction is as follows. Extract 2 ml of sample solution with 1 ml of chloroform. After vigorous shaking, the solution was allowed to form the two phases. One milliliter of the aqueous solution was transferred to a 10 ml volumetric flask and 2 ml (B–R) buffer (pH = 7) was added and the solution was then diluted to the mark with water. Voltammetric determination was performed under the optimum conditions described in 2.3.1. in the presence of about 0.005 M EDTA. Exactly similar procedure was applied to the determination of triclosan in wastewater by taking an appropriate volume of wastewater sample. For obtaining higher accuracy and sensitivity and lower background, a three-step separation was performed as follows. An appropriate amount of sample was dissolved in water and the pH was adjusted to >12 with NaOH. A 10 ml clear solution was transferred in to a separating funnel. The separation was performed with 5 ml hexane. The basic aqueous phase was saved
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as analyte and the organic phase was discarded. Then the solution pH was adjusted to pH <1 with concentrated HCl. The acidic solution was extracted with hexane and the organic phase was saved as analyte and aqueous solution was discarded. In the final step the hexane phase (including triclosan) was back extracted with alkaline aqueous solution (pH > 12) and the aqueous solution was analyzed by the suggested electrochemical method. It should be noted that in order to avoid emulsion and to have two distinct phases, it is advisable to add sodium chloride (∼0.1 g) to the aqueous solution prior to the extraction in order to ease the separation step.
3. Results and discussion 3.1. Adsorptive and tensammetric characteristics of triclosan Fig. 1 shows the shape of the tensammetric curves obtained for increasing concentration of triclosan. The heights of the tensammetric peaks increase with increasing concentration of the triclosan. In all of the experiments, the base current is depressed. Thus, adsorption of triclosan on mercury electrode produced a lowering of charging current in a broad potential region. In general, tensammetric peaks can clearly be distinguished from Faradaic peaks, caused by
227
electron-transfer processes, since tensammetric waves are not observed by direct current polarographic techniques. Direct current tast polarography of various concentrations of triclosan has been studied on mercury dropping electrode and no wave was found. When the scan was preceded by deposition at −450 mV, the peak current was enhanced indicating that triclosan was adsorbed on the electrode. The adsorption peak increases with increasing deposition time up to several minutes, but eventually becomes constant as a result of saturation (see Section 3.2.2.2). Also the plot of the scan rate versus peak current of triclosan was linear (see Section 3.2.2.3), showing the adsorptive characteristics of the peak [12,13]. In most voltammetric experiments, differential pulse mode was used in the determination step. In this mode, the main portion of the capacitive contribution to the current is removed but, according to Myers and Osteryoung [14], the differential pulse voltammogram in the absence of the faradic reaction is essentially a differential capacity curve. 3.2. Effects of operational parameters 3.2.1. Effect of analytical parameters 3.2.1.1. Effect of variation of pH. The effect of pH on the stripping peak current was examined by differential pulse voltammetry and the results are shown in
500 (A)
Current (nA)
420 340 260 180 100
(H)
-450
-720
-990
-1260
-1530
-1800
Potential (mV)
Fig. 1. Voltammograms of the Britton–Robinson buffer solution, pH = 7 containing [triclosan] = A, blank; B, 117.21 mg l−1 ; C, 234.41 mg l−1 ; D, 351.62 mg l−1 ; E, 468.83 mg l−1 ; F, 586.03 mg l−1 ; G, 703.24 mg l−1 ; H, 820.45 mg l−1 ; Eacc = −450 mV; alternative current mode; ν = 80 mV s−1 ; stirring speed = 2000 rpm; tacc = 45 s; pulse height = 20 mV; modulation frequency = 60 Hz; phase angle = 90◦ .
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35
Current (nA)
30
25
20
15
10 2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
11.00
12.00
pH
Fig. 2. Effect of pH: [triclosan] = 50 g l−1 ; Eacc = −50 mV; stirring speed = 2000 rpm; tacc = 90 s; differential pulse mode; ν = 80 mV s−1 ; pulse height = 50 mV.
Fig. 2. It can be seen that the peak current reaches a maximum at pH range of 5.5–7.5. Therefore, pH = 7 was selected as the optimum pH, because at this pH, the sensitivity was highest, the peak was well defined and the base line was flat. 3.2.1.2. Effect of solution pH in the extraction step. The solution pH has influence on the distribution of triclosan between organic and aqueous phases. Therefore, the recovery and reproducibility of the method depend on the solution pH in the extraction step. The best pH values for transporting triclosan to aqueous phase were pH values >12. On the other hand, pH <1 was the best condition for transporting triclosan to the organic phase. 3.2.1.3. Effect of ionic strength. The effect of ionic strength of the sample solution on the stripping peak current was examined by varying the concentration of NaNO3 up to 1 M. The results showed that the change of ionic strength did not have any effect on peak current response. In the presence of chloride, the stripping peak for triclosan decreased slightly. The peak was diminished by 60% in the presence of 3 M chloride. To avoid chloride interference at high concentrations, the three-step extraction was employed instead of the one-step extraction.
3.2.2. Effect of instrumental parameters 3.2.2.1. Effect of variation of accumulation potential (Eacc ). The dependence of the tensammetric peak current on the preconcentration potential was examined over the range of 200 to −850 mV. An accumulation potential of −450 mV was used as the optimized potential. 3.2.2.2. Effect of variation of accumulation time (tacc ). Fig. 3 shows plots of cathodic peak current (ipc ) versus accumulation time for different concentrations of triclosan. At first, ipc increased with tacc , indicating that before adsorption equilibrium is reached, the longer the accumulation time, the more triclosan was adsorbed, and thus the larger was the peak current. However, after a specific period of accumulation time (about several minutes), the peak current leveled off, illustrating that adsorptive equilibrium of triclosan on the mercury electrode surface was achieved. The approximate accumulation times at adsorptive equilibrium were 190, 180, 170 and 150 s for triclosan concentrations of 13.25, 26.5, 39.75 and 50 g l−1 , respectively. The long accumulation time leads to improvement of the detection limit. However, the dynamic range becomes smaller due to saturation occurred at long deposition time. An accumulation
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229
35 a b
30
c
ipc (nA)
25
d
20 15 10 5 0 0
25
50
75
100
125
150
175
200
225
250
275
300
325
350
tacc. (s)
Fig. 3. Effect of accumulation time: (a) [triclosan] = 13.25 g l−1 ; pH = 7; Eacc = −450 mV; stirring speed = 2000 rpm; square wave mode; ν = 40 mV s−1 ; pulse height = 50 mV; modulation frequency = 60 Hz. (b) As (a) but [triclosan] = 26.5 g l−1 . (c) As (a) but [triclosan] = 39.75 g l−1 . (d) As (a) but [triclosan] = 50 g l−1 .
time of 90 s was selected to carry out the determination of triclosan. During this time (90 s), the adsorption of probable surfactants present in real samples is minimal (cf. Section 4.2). 3.2.2.3. Effect of variation of scan rate (ν). The effect of scan rate on the peak current was examined between 13 and 500 mV s−1 . The log of ipc increases with log ν, clearly demonstrating a linear range between 80 and 500 mV s−1 (slope = 1.08, r = 0.9985) and non-linearly thereafter. A scan rate of 80 mV s−1 which gave the best reproducibility was selected for further studies. 3.2.2.4. Effect of scanning wave forms and its parameters. The effect of applied waveform in determination step was examined. Linear sweep voltammetry (LSV) showed a very small tensammetric peak at high scan rates (∼400 mV s−1 ). The differential pulse modulation is used as the most common waveform for potential scan in stripping step. In voltammetric techniques, when the analyte is adsorbed on the electrode surface, the current is directly related to the scan rate [15]. Very high scan rates can be used but it is limited by reaction and instrumental limitation. On the other hand, the square wave voltammetry can be carried out at very high fre-
quencies for better signal to noise ratio. In this case, the peak current is enhanced and the peak potential shifted to negative potentials as a result of increasing the frequency of the square wave modulation. At higher frequencies, the peak became broadened and the background current cancelled out any analytical advantage of the greater peak heights. Therefore, the frequency selected for the method was 60 Hz. The other parameter that was investigated was pulse height. The parameter affects the peak height and peak potential. The peak current was increased with increasing the square wave pulse height and the differential pulse height. The optimum pulse height selected for square wave and differential pulse voltammetry were 50 and 100 mV, respectively. It can be noted that the maximum pulse height produced by the instrument are 50 and 100 mV for square wave and differential pulse voltammetry, respectively. 3.2.2.5. Effect of stirring speed. Fig. 4 shows the influence of stirring rate on voltammetric response and also on R.S.D. It can be found that peak current increased with stirring rate in the range of 800–3000 rpm, and levels off at higher scan rates. The R.S.D. value decreases rapidly from 800 to 1800 rpm and then rises after 2000 rpm. This is because at lower stirring rates the solution was stirred heterogeneously
8
8
7.5
7
7
6
6.5
5
6
4
5.5
Current
RSD%
A. Safavi et al. / Analytica Chimica Acta 494 (2003) 225–233
ipc (nA)
230
3
RSD% 5 500
1000
1500
2000
2500
3000
2 3500
rpm
Fig. 4. Effect of stirring speed on the peak current and its R.S.D. (%): [triclosan] = 18.75 g l−1 ; pH = 7; Eacc = −450 mV; tacc = 60 s; square wave mode; ν = 80 mV s−1 ; pulse height = 50 mV; modulation frequency = 60 Hz.
and at higher speed, the solution was stirred turbulently. A stirring rate of 2000 rpm which gave the best reproducibility was selected.
4. Analytical applications 4.1. Reproducibility, linear range and detection limit of the method The reproducibility of repeating the determination of low concentrations of triclosan was found under the optimum conditions. The R.S.D. for seven determinations were calculated as 2.37 and 2.77% for triclosan concentrations of 26.5 and 39.75 g l−1 , respectively. Typical adsorptive tensammetric wave and calibration graph are shown in Fig. 5. Under the optimized conditions the peak current of triclosan was found to be proportional to its concentration over the range 2.5–60 g l−1 and its equation was [current]nA = 0.8846 × [triclosan]ppb − 0.2199. The correlation coefficient (r) was 0.9985. The detection limit (YLOD = ¯ B + 3SB , where YLOD is the signal for detection X ¯ B is the mean of blank signal and SB is the limit, X standard deviation of blank signal) was obtained as 1.9 g l−1 [16]. The limit of detection can be lowered by extending the deposition time. However, the longer deposition time tends to decrease the linear dynamic
range as a result of electrode saturation. Also, by applying longer deposition time, organic surface active reagents can be adsorbed on the electrode surface and interfere with determination of triclosan. The recovery of the purposed method was more than 96% after extraction for five standard solutions. 4.2. Interferences Traditionally, the adsorption of organic compounds onto electrode surface can be regarded as a problem that limits adsorptive voltammetric measurements. The surface-active materials such as Triton X-305 could interfere by inhibiting adsorption of triclosan. Triton X-305 was added to the solution as a model surfactant; the sensitivity for triclosan was diminished by 21, 56, 75 and 100% at the ratio of 3, 6, 9 and 12 of Triton X-305/triclosan. This effect could be eliminated by the three-step extraction. After extraction, the recovery was more than 90% in the presence of Triton X-305 at the ratio of 600 of Triton X-305/triclosan. Also the effect of anionic surfactant (i.e. SDS) as the interfering substance was studied and the recovery was more than 75% at a concentration of 2% SDS. The reduction currents of some reducible elements present at high concentrations may affect the peak current. The possible interferences of several metal ions, such as, Na(I), K(I), Co(II), Zn(II), Cr(III),
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60
36
30
50
24
40 ipc (nA)
ipc (nA)
(P)
18
30
(A)
12
20
6
10
0 -750 (a)
0
-875 -1000 -1125 -1250 Epc (mV)
0
(b)
10 20 30 40 50 concentration of triclosan ( g/L)
60
Fig. 5. Typical voltammograms and calibration graph for determination of triclosan: (a) pH = 7; Eacc = −450 mV; stirring speed = 2000 rpm; tacc = 90 s; square wave mode; ν = 80 mV s−1 ; pulse height = 50 mV; modulation frequency = 60 Hz; [triclosan] = A, blank; B, 2.6 g l−1 ; C, 5.2 g l−1 ; D, 7.8 g l−1 ; E, 10.4 g l−1 ; F, 12.9 g l−1 ; G, 17.9 g l−1 ; H, 22.8 g l−1 ; I, 27.6 g l−1 ; J, 32.3 g l−1 ; K, 37.0 g l−1 ; L, 41.5 g l−1 ; M, 46.0 g l−1 ; N, 50.4 g l−1 ; O, 54.7 g l−1 ; P, 58.9 g l−1 . (b) Result of (a).
Fe(III), Ca(II), Mn(II), Al(III), Sn(II), Cu(II), Mg(II) and Pb(II) were investigated. Only zinc and cobalt were serious interferences at 1:1 and 5:1 (w/w ratios), respectively. The effect of these ions can be eliminated by the addition of EDTA. At low concentrations, EDTA is not interfering. However, at high concentrations of EDTA (=0.02 M), the peak current due to triclosan was decreased about 20%. It should be noted that the three-step extraction can eliminate the effect of all the interferences mentioned above; even in the presence of high concentrations of zinc, cobalt or EDTA. The effect of organic solvents on the peak current was examined and the results showed that the current decreased in the presence of acetone and ethanol and the peak potential was shifted to more negative potentials. The peak current for triclosan decreased 17 and 62% in the presence of 9 and 44% of ethanol/water. However, low concentrations of acetone and ethanol had no effect on the peak current. Hexane and chloroform that were used in extraction step had no effect on the signal.
4.3. Practical application and recovery In order to show the analytical applicability of the present method, the determination of triclosan was performed in real samples and complex matrices, i.e. toothpaste, wastewater, liquid soap and samples containing Triton X-100. The results showed that satisfactory recovery for triclosan could be obtained (Table 1) using the recommended procedure. For the determination of triclosan in wastewater, three samples from a stream of wastewater were tested for the level of triclosan using the recommended procedure in Section 2.3.2. Results of the determination are summarized in Table 1. As it can be seen, the detection limits obtained for determination of triclosan are not the same for different real samples. This is not only because of the difference in the sample matrices but also because of the use of different extraction procedures. In general, a three-step extraction preceded by a deposition time (between 60 and 90 s) yielded the best detection limits and the best reproducibility. The high detection limit for liquid soap,
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Table 1 Results of analysis of real samples Sample
DLa
nb
R.S.D. (%)
Addeda
Founda
Recovery (%)
Toothpaste (1)c
500
3
4
500 2500 5000
550 2400 4650
110 97 93
Toothpaste (2)d
30
4
3.5
– 2000 3000
2940 4800 5750
98 96 98
Toothpaste (3)d ,e
90
3
3
– 1000 2000
2900 3900 4800
97 98 96
Liquid Soapd Triton X-100 (2%)d
1000
2
7
2900 5000
2300 7200
80 85
300
3
5
400 600
340 460
85 78
Wastewater (1)
0.008
3
2.9
– 0.008 0.016
0.017 0.024 0.036
– 96 109
Wastewater (2)
0.008
4
3
– 0.009 0.015
0.015 0.026 0.027
– 108 90
Wastewater (3)
0.008
4
2.8
– 0.006 0.017
0.016 0.024 0.032
– 109 96
a
All concentrations are in ppm for real samples. Number of trials. c In the presence of EDTA. d Three-step extraction. e Without deposition time. b
is probably due to the fact that high concentration of surfactants in this type of sample causes retention of triclosan in aqueous solution (during the extraction steps) which in turn can affect the recovery and detection limit. To avoid background interferences, it is advisable to perform a three-step extraction with hexane. This procedure provides a better accuracy and sensitivity but is rather time consuming. The one-step extraction with chloroform is more convenient but less sensitive and is suitable for simple samples.
Acknowledgements The authors gratefully acknowledge the support of this work by Shiraz University Research Council.
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