- Email: [email protected]
An automatic falling drop system based on multicommutation process for photometric chlorine determination in bleach
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
An automatic falling drop system based on multicommutation process for photometric chlorine determination in bleach Sivanildo da Silva Borges, Boaventura F. Reis ∗ ˜ Paulo, Avenida Centenario ´ Centro de Energia Nuclear na Agricultura, Universidade de Sao 303, ˜ Paulo, Brazil Piracicaba, CEP-13400970, Sao
a r t i c l e
i n f o
Article history:
a b s t r a c t In this work an automatic photometric procedure for the determination of chlorine in
Received 31 August 2006
bleach samples employing N,N -diethyl-p-phenylenediamine (DPD) as chromogenic reagent
Received in revised form
is described. The procedure was based on a falling drop system where the analyte (Cl2 ) was
15 November 2006
collected by a DPD solution drop (50 L) after its delivery from the sample bulk that was
Accepted 17 November 2006
previously acidified. The flow system was designed based on the multicommutation pro-
Published on line 25 November 2006
cess assembling a set of three-way solenoid valves, which under microcomputer control afforded facilities to handle sample and reagent solution in order to control analyte deliv-
Keywords:
ering and solution drop generation. The analyte volatilization was improved by coupling
Multicommutation
online a little heating device. The detection system comprised a green LED (515 nm) and a
Flow analysis
phototransistor. Aiming to prove the usefulness of the proposed procedure a set of bleach
Spectrophotometry
samples was analyzed. Comparing the results with those obtained with reference method
Chlorine
no significant difference at 95% confidence level was observed. Other profitable features
Falling drop system
such as a linear response ranging from 15 up to 100 mg L−1 Cl2 (R = 0.999); a detection limit
Breach
of 4.5 mg L−1 Cl2 estimated based on the 3 criterion; a relative standard deviation of 2.5% (n = 10) using a typical bleach sample containing 25.0 mg L−1 Cl2 ; a consumption of 55 g of DPD per determination; and a analytical frequency of 20 determinations per hour were also achieved. © 2006 Published by Elsevier B.V.
1.
Introduction
Chlorine (Cl2 ) is widely used in the form of hypochlorite as disinfecting agent in water supplying systems and in domestic environments as well as a bleaching agent. Products based on hypochlorite available in the market are produced by different manufacturers, thus chlorine determination could be performed to assure the quality. In this sense, the availability of the reliable analytical procedure presenting as inherent features robustness, appropriated sensitivity, low cost for its
∗
Corresponding author. Tel.: +55 19 3429 4639; fax: +55 19 3429 4610. E-mail address: [email protected] (B.F. Reis). 0003-2670/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.aca.2006.11.050
implementation, and generation of low waste volume should be useful. Analytical procedures for hypochlorite determination in different sample matrices based on chemiluminescence [1–3], potentiometry [4], voltammetry [5,6], spectrophotometry [7–12], and indirect titration [13] have been proposed. Nevertheless, most procedures do not attain the requirements described above. Nowadays, attention has been paid to environment friendly methods. This requirement could be accomplished using less
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
aggressive reagent and/or employing analytical procedure with ability to generate reduced volume of waste. The last one requirement has been achieved using procedure based on sequential injection analysis [14,15], or multicommutation process [16–19]. The falling drop approach [20] is also a tool that could provide facilities to develop analytical procedures with ability to generate low volume of waste, thus accomplishing the green chemistry requirement [21]. Reduction of waste generation employing procedures based on sequential injection or multicommutation approach is possible depending on the system design [14,22], while for procedure based on the falling drop approach low waste volume generation is an inherent feature [20]. In this work we intend to develop an automatic flow analysis procedure based on the falling drop process for photometric determination of Cl2 in bleach samples using N,N diethyl-p-phenylenediamine (DPD) as chromogenic reagent, which is oxidized by free chlorine generating a pink compound that is monitored at 515 nm [23]. The procedure will be implemented employing a flow injection analysis system based on the multicommutation process [24–26]. The photometric detection will be performed employing a LED based homemade photometer and endeavor will be directed to obtain a compact and robust setup.
2.
Experimental
2.1.
Reagents and samples
All chemicals used were of analytical grade. Purified water (electric conductivity less than 0.1 S cm−1 ) was used throughout. A 1000 mg L−1 Cl2 stock solution was prepared from commercial hypochlorite solution and after dilution it was standardized according to the procedure established in the Standard Methods [23]. The working solutions ranging from 20.0 up to 100.0 mg L−1 Cl2 were prepared everyday by appro-
67
priated dilution from the stock solution with a 0.01 M NaOH solution. A 0.11% (w/v) N,N -diethyl-p-phenylenediamine (Merck, Germany) solution was prepared dissolving 0.0275 g of solid in 25 mL of H2 SO4 solution (0.005 M). This solution could be used during a week. A phosphate buffer solution (pH 6.3) was prepared by dissolving 2.4 g of Na2 HPO4 and 4.6 g of KH2 PO4 in 100 mL of water. A 1.0% (w/v) ascorbic acid solution was prepared everyday dissolving 1.0 g of solid in 100 mL of water. A 1.0 M HCl solution that was used as carrier solution was prepared by appropriated dilution with water from the concentrated stock solution (12 M). Samples of bleach were purchased from the local market and analyzed after appropriated dilution.
2.2.
Apparatus
The equipment setup comprised a Pentium III microcomputer furnished with an electronic interface card (PCL-711S, Advantech); an IPC-4 Ismatec peristaltic pump with Tygon pumping tubes; a homemade LED based photometer [27]; a homemade device for drop generation; transmission lines of polyethylene tubing (0.8 mm i.d.); an aquarium pump; five three-way solenoid valves; and a homemade electronic interface to provide the potential difference and current intensity required to drive the solenoid valves [28]; two optic-fiber cables, 20 cm long, 1.0 mm diameter; a homemade online warming device (200 L inner volume) [29]. The microcomputer was furnished with a software wrote in Quick BASIC 4.5 that was designed with facilities to control the flow system and to perform data acquisition.
2.3.
Signal detection and drop generation modules
The photometric system tailored to detect the compound formed into the chromogenic solution drop while it grew was assembled according to the electronic diagram shown in Fig. 1. The electric potential difference presented at the output of the operational amplifier (AO2, pin 6) was a function of the radi-
Fig. 1 – Diagram of detection system and signal conditioning. Tr = Transistor BC547; LED = green LED, 515 nm; Til78 = phototransistor; OA1 and OA2 = operational amplifiers; C1, C2, C3 and C4 = 1 F tantalum capacitors; S0 = output signal to PCL711S analog input.
68
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
Fig. 2 – Diagram of the flow system and of the drop formation unit. PC = Computer; DS = drop formation unit; R1 = chromogenic reagent, DPD 0.11% (w/v); R2 = buffer phosphate solution, pH 6.3: (a) tube barb fitting; (b) sliding acrylic cylinder; (c) cylinder holder; (d) screw to hold the sliding acrylic cylinder; (e) drop head, Tygon tubing piece 10 mm long and 2.0 mm i.d.; (f) reagent solution drop; (g) Green LED, 515 nm; (h) drop head height; (i) phototransistor; (j) detection system holder; (k) carrier solution and/or sample waste; (l) light beam; (m) drop formation unit; (W) waste collector containing 100 mL of a 1% ascorbic solution; f1 and f2 = optic-fibers; ASi and AS0 = inlet and outlet of the air stream. It was located at 2 mm bellow of drop-head.
ation beam intensity arriving at the phototransistor, which was read by the microcomputer through the PCL711S interface card. The phototransistor (Til78) and green LED ( = 515 nm) were assembled in the drop generating unit shown in Fig. 2. As we can see two optic-fiber cables (f1 , f2 ) were installed in the drop generation unit to conduct the light beam (l). The optic-fiber cables were assembled to allow that the drop of the reagent solution grew between them without contact. Under this condition, the radiation beam penetrated into the reagent drop where light absorption could occur. The signal variation presented at the operational amplifier output was a function of the radiation beam attenuation, which could be caused by the absorption of the chemical compound present in the drop. When the software was started the microcomputer requested the performing of the calibration step, which was done maintaining the LED off and adjusting the output signal (S0 ) to 0 V (dark measurement) turning the variable resistor (20 k) wired to the operational amplifier OA2 (pin 3). Afterwards, a drop of the reagent solution was generated and the LED was enabled to emit radiation by turning the variable resistor wired to the base of the transistor (Tr). The radiation beam intensity was increased up to the output signal (S0 ) attained a value around 800 mV. The reagent drop solution generation unit depicted in Fig. 2 comprised a cylinder of acrylic (11.0 mm diameter) with a central hole (1.0 mm) perforated at the longitudinal axis; an acrylic block with a central lead hole (11.1 mm diameter), which was used to hold the acrylic cylinder (b) in the vertical position permitting also its vertical sliding. The drop head (e) was positioned between the optic-fibers (f1 , f2 ) and its appropri-
ated height position (h) was found sliding the acrylic cylinder (b). Both drop generation unit and photometer modules were accommodated into a metallic box (15 cm × 18 cm × 30 cm).
2.4.
Procedure
The flow system was designed based on the multicommutation process [24–26] and its diagram is depicted in Fig. 3. In this configuration, all valves are switched OFF and the solutions are pumped to back their storing vessels. When the software was run the microcomputer requested the performing of the photometer calibration step that was carried out as it was described above. The flow lines cleaning step was performed maintaining all valve switched ON for a time interval of 90 s. Afterwards, the valve V2 was maintained switched ON during a time interval of 15 s to displace to waste (W) the sample residue remaining into the heating device (HD). While this step was in course the aquarium pump (AP) and the valve V5 were maintained switched ON in order to purge the remaining residual Cl2 into the drop generation unit (DS). The step for drop head cleaning was performed maintaining valves V3 and V4 switched ON during a time interval of 10 s. While this step was done the pumping flow rate was 50-fold in order to save time. After this step, the microcomputer established again the preset pumping flow rate and started the monitoring of the next drop growing. When the drop detaching from the drop head occurred, the photodetector generated a sudden signal variation that was sensed by microcomputer, which was instructed by the software to switch OFF valves V3 and V4 . Afterwards, the system was ready to begin the next analytical run.
69
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
tion was pumped to back its storing vessel (Rcs ). Valve V5 was switched ON to direct the remaining Cl2 from the drop formation unit towards the collecting vessel (SH). After a delay time of 30 s, the microcomputer started the routine to detect the drop falling in order to record the whole drop signal profile. Aiming to improve the analytical throughput the drop monitoring was stopped after an interval time of 100 s and valve V5 was maintained switched ON for 30 s to direct the remaining Cl2 into the drop formation unit towards the collecting vessel (SH). Considering that system parameters such as sample volume, drop head height (h), flow rates, and temperature could affect the analytical signal, a set of experiments was carried out in order to establish the appropriated operational conditions. After the instrument calibration, all steps of the analytical run summarized in Table 1 were performed automatically by the microcomputer. Intending to prove the usefulness of the proposed analytical system a set of bleach samples were analyzed employing the best operational conditions summarized in Table 1. Aiming to accuracy assessment the samples were also analyzed using the reference procedure (DPD) presented at the Standard Methods [23].
Fig. 3 – The flow system diagram. V1 , V2 , V3 , V4 and V5 = Three-way solenoid valves; R1 = chromogenic reagent, DPD; R2 = buffer phosphate solution, pH 6.3; Cs = carrier solution, HCl 1.0 M; S = sample; RR1 , RR2 , RCS and RS = solutions (R1 , R2 , Cs, S) circulation towards their storing vessels; PP = peristaltic pump; AP = aquarium pump to circulate the air stream; HD = heating device, SH = Cl2 collecting vessel containing NaOH 1.0 M; W = waste collector.
The sampling step consisted of 50 sampling cycles, which was performed switching ON sequentially valves V1 and V2 for a time interval of 0.5 s. Under this condition, the warming device (HD) was filled with the mix comprising sample (S) and carrier solution (Cs). Next valve V1 was turned OFF and carrier solution stream displaced sample zone through the heating device (HD) towards the drop formation unit (DS). In the next step, valves V3 and V4 were switched ON in order to displace both chromogenic reagent and buffer solutions towards the drop head (e). While this step was in course the aquarium pump (AP) was switched ON in order to maintain the air circulation into a closed circuit to improve the analyte delivering from the sample bulk. After 100 s delay time valve V2 and the warming device (HD) were switched OFF, thus carrier solu-
3.
Results and discussions
3.1.
Effect of the drop head position
As it is shown in Fig. 2 the drop solution was formed in the end of the drop head (e) in order to intercept the radiation beam while it grew. In earlier work [30] it was observed that the distance (h) between the drop head extremity and radiation beam could affect signal measurement. Intending to find the appropriated position experiments were carried out displacing down the cylinder (b). The assays were carried out without using a standard solution, therefore the radiation beam crossed the drop solution without undergoing absorption. Considering signal magnitude and precision of measurements as the main parameters, better results were achieved when the drop head (e) extremity was positioned at 9.0 mm distal from the bottom of the cylinder holder (c).
Table 1 – Sequence of the steps to carry out the analytical run Step 1 2 3 4 5 6 7 8 9 10 a
V1
V2
V3
V4
V5
AP
HD
Time (s)
0 0 1 0 0 0 1 0 0 0
0 0 0 1 0 0 0 1 1 0
1 0 0 0 1 1 0 0 1 0
1 0 0 0 1 1 0 0 1 0
0 0 0 1 0 0 0 0 0 1
0 0 0 1 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 0
90 – 20 15 10 – 1 0.5 100 30
These steps were repeated 50 times.
Description Form the drop Calibrate the photometer Fill the sample line Clean the analytical pathway Clean the drop head Detect the drop falling Samplinga Insert carrier solutiona Drop monitoring Purge the residual Cl2
70
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
Fig. 4 – Signal records performed during a drop growing step.
3.2.
Chlorine detection
As it is shown in Fig. 2 the drop generation and detection module were assembled to allow that when drop solution intercepted the radiation beam, which underwent strong dispersion, reduced the intensity of the radiation reaching the phototransistor. As we can see in Fig. 4, the increasing started after a time interval of 70 s, which attained maximum value when the time interval elapsed was about 90 s, and afterwards it decreased up to the drop detaching from the drop head. To allow the photometer calibration the drop growing was halted by switching OFF valves V1 and V2 after a delay time of 90 s. The drop was formed using the chromogenic reagent solution and the output signal was adjusted to 800 mV. When Cl2 was delivered from sample bulk it was absorbed by the drop solution forming the pink compound that absorbed radiation at 515 nm. In this sense, the radiation beam leaving the drop was less intense than that coming from the LED. The phototransistor sensed this variation and as a consequence the electric potential difference monitored at the operational amplifier output (S0 , Fig. 1) was lower than 800 mV. The records of Fig. 5 show that the signal decreased analyte concentration in the drop. From these results the absorbance was derived employing the equation: Abs = −[log (Vs − Vd )/(V0 − Vd )], where V0 (800 mV) and Vs and were signals monitored without and with analyte in the drop, respectively, and Vd (0 mV) was dark measurement (see Section 2.3). A linear relationship (R = 0.999) was found, therefore indicating that this feature could be exploited to develop the analytical procedure.
3.3.
Fig. 5 – Records of the analytical signals concerning to Cl2 concentrations. T = 30 ◦ C; h = 32.0 mm; sample volume = 417 L.
cycles were varied from 10 to 50, thus volume of sample zone varied from 83.4 to 417 L yielding the results shown in Fig. 6. As we can see the signal varied within the range of 750–650 mV, thus presenting a ratio of 3.34 (L mV−1 ). A linear relationship was found (R = 0.998) between sample volume and the variation of the electric potential difference. This result is quite different as compared with the response observed employing usual flow system, where after increasing sample volume a linear relationship was not achieved.
3.4.
Effect of temperature
The focus of the procedure was the determination of volatile species (Cl2 ) in liquid samples. In this sense, we expect that temperature could affect the volatilization rate of the analyte, thus influencing the sensitivity of the analytical procedure. Aiming to ascertain this supposition a set of experiments were performed varying the temperature of the warming device from 25 up to 50 ◦ C. Better results were achieved maintain-
Effect of sample volume
Generally in flow injection systems the analytical signal increases with the injected sample volume. Since in the proposed system the volume influence on signal could be not predictable, experiments were carried out to ascertain its effect. The time intervals settled to insert sample and carrier solutions were maintained at 0.5 s and the number of sampling
Fig. 6 – Effect of the sample volume on the analytical signal magnitude. Cl2 = 20 mg L−1 ; T = 25 ◦ C; h = 32.0 mm.
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
Table 2 – Comparative results using a reference method Sample A B C D E
Proposed method (g L−1 ) 19.1 24.9 22.2 20.8 21.5
± ± ± ± ±
0.2 0.6 0.4 0.9 0.2
Reference method [25] (g L−1 ) 18.9 24.4 21.9 21.1 20.6
± ± ± ± ±
0.1 0.2 0.5 0.6 0.3
Results are average of the three consecutive sample analysis. Applying the t-test for 95% confidence it was found 1.633 and the theoretical value is 2.776.
ing the temperature at 30 ◦ C. When temperature higher than this value was employed the precision of the measurements was lessened. Under this condition, it was observed that little water drops was formed in the cross surface of the optical fiber, thus causing the observed lessening effect.
3.5.
Sample analyses
Intending to prove the usefulness of the proposed system, a set of bleach samples were analyzed employing the optimal operational conditions summarized in Table 1 yielding the results that are shown in Table 2. Aiming the accuracy assessment the bleach samples were analyzed using a reference method [23]. Applying the paired t-test no significant difference at the 95% confidence level was observed. Other profitable features such as a linear response ranging from 15 up to 100 mg L−1 Cl2 (R = 0.999); a detection limit of 4.5 mg L−1 Cl2 ; a relative standard deviation 2.5% (n = 10) processing a typical bleach sample containing 25.0 mg L−1 Cl2 ; a reagent consumption of 55 g DPD per determination; a waste generation of 3.8 mL per determination; and an analytical frequency of 20 determinations per hour were also achieved.
4.
Conclusions
The linear response as function of the sample volume could be exploited to provide the software with adapting resource to improve sensitivity by enlarging the number of sampling cycles. The analytical module comprised some electronic devices controlled by microcomputer; nevertheless it can be easily assembled using non-expensive devices. The software designed to control the analytical system and to perform data acquisition allows easy operation of the system. The results obtained analyzing bleach samples proved that the proposed procedure is reliable. After working continuously for 8 h no significant variation of linear response range and precision of measurements were observed, thus we could conclude that the long-term stability of the proposed system is very good.
71
Coupling the drop generation and the detection units and implementing the analytical procedure based on multicommuted process permitted that a compact and downsized analytical instrument was obtained with an ability to process low volume of chromogenic reagent.
references
´ L.F. Capitan-Vallvey, ´ [1] J. Ballesta Claver, M.C. Valencia Miron, Anal. Chim. Acta 522 (2004) 267. [2] D. Gonzalez-Robledo, M. Silva, D. Perez-Bendito, Anal. Chim. Acta 228 (1990) 123. [3] J.B. Claver, M.C.V. Miron, Anal. Chim. Acta 522 (2004) 267. [4] A. Brown, A. Bergren, Am. Chem. Soc. 219 (2000) 537. [5] A. Bergren, J.H. Brown, Am. Chem. Soc. 221 (2001) 779. [6] F. Kodera, M. Umeda, A. Yamada, Bunseki Kagaku 53 (2004) 905. [7] B. Narayana, K. Vipin, M. Mathew, N.V. Sreekumar, Indian J. Chem. Sect. A Inorg. Bio-Inorg. Phys. Theor. Anal. Chem. 43 (2004) 573. [8] P. Gengan, S.B. Jonnalagadda, Bull. Chem. Soc. Ethiop. 19 (2005) 1. [9] L.A. Ramos, K.R. Prieto, E.T.G. Cavalheiro, C.C.S. Cavalheiro, J. Chem. Educ. 82 (2005) 1815. [10] B. Narayana, M. Mathew, K. Vipin, N.V. Sreekumar, T. Cherian, J. Anal. Chem. 60 (2005) 706. [11] K. Carlsson, L. Moberg, B. Karlberg, Water Res. 33 (1999) 375. [12] L. Moberg, B. Karlberg, Anal. Chim. Acta 407 (2000) 127. [13] L. Erdey, K. Vigh, Talanta 10 (1963) 439. [14] E.M.G. Santos, C.M.C.M. Couto, A.N. Araujo, M.C.B.S.M. Montenegro, B.F. Reis, J. Braz. Chem. Soc. 15 (2004) 701. [15] A.M. Pimenta, A.N. Araujo, M.C.B.S.M. Montenegro, C. Pasquini, J.J.R. Rohwedder, I.M. Raimundo, J. Pharm. Biomed. Anal. 36 (2004) 49. [16] E. Rodenas-Torralba, B.F. Reis, A. Morales-Rubio, M. Delaguardia, Talanta 66 (2005) 591. ´ [17] F.R.P. Rocha, J.A. Nobrega, O. Fatibello-Filho, Green Chem. 3 (2001) 216. [18] P.D. Zanavaras, C.K. Zacharis, G.A. Theodoridis, E.A. Kalaitzantonakis, A.N. Voulgaropoulos, Anal. Chim. Acta 547 (2005) 98. [19] A.L.D. Comitre, B.F. Reis, Talanta 65 (2005) 846. [20] H. Liu, P.K. Dasgupta, Anal. Chem. 67 (1995) 4221. [21] H.H. Liu, P.K. Dasgupta, Anal. Chem. 68 (1996) 1817. [22] H.H. Liu, P.K. Dasgupta, Trends Anal. Chem. 15 (1996) 468. [23] APHA, AWWA and WPCF, Standard Methods for the Examination of Water and Wastewater, 16th ed., Washington, 1985, pp. 309–314. ´ E.A.G. Zagatto, J.L.F.C. Lima, R.A. Lapa, [24] B.F. Reis, M.F. Gine, Anal. Chim. Acta 293 (1994) 129. [25] F.R.P. Rocha, P.B. Martelli, B.F. Reis, Anal. Chim. Acta 438 (2001) 11. [26] F.R.P. Rocha, B.F. Reis, Anal. Chim. Acta 409 (2000) 227. [27] M.A. Feres, B.F. Reis, Talanta 68 (2005) 422. [28] A.F. Lavorante, A. Morales-Rubio, M. de la Guardia, B.F. Reis, Anal. Bioanal. Chem. 381 (2005) 1305. [29] M. Smiderle, M.C. Tomazzini, B.F. Reis, Quim. Nova 27 (2004) 50. [30] S.S. Borges, R.M. Frizzarin, B.F. Reis, Anal. Bioanal. Chem. 385 (2006) 197.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/aca
An automatic falling drop system based on multicommutation process for photometric chlorine determination in bleach Sivanildo da Silva Borges, Boaventura F. Reis ∗ ˜ Paulo, Avenida Centenario ´ Centro de Energia Nuclear na Agricultura, Universidade de Sao 303, ˜ Paulo, Brazil Piracicaba, CEP-13400970, Sao
a r t i c l e
i n f o
Article history:
a b s t r a c t In this work an automatic photometric procedure for the determination of chlorine in
Received 31 August 2006
bleach samples employing N,N -diethyl-p-phenylenediamine (DPD) as chromogenic reagent
Received in revised form
is described. The procedure was based on a falling drop system where the analyte (Cl2 ) was
15 November 2006
collected by a DPD solution drop (50 L) after its delivery from the sample bulk that was
Accepted 17 November 2006
previously acidified. The flow system was designed based on the multicommutation pro-
Published on line 25 November 2006
cess assembling a set of three-way solenoid valves, which under microcomputer control afforded facilities to handle sample and reagent solution in order to control analyte deliv-
Keywords:
ering and solution drop generation. The analyte volatilization was improved by coupling
Multicommutation
online a little heating device. The detection system comprised a green LED (515 nm) and a
Flow analysis
phototransistor. Aiming to prove the usefulness of the proposed procedure a set of bleach
Spectrophotometry
samples was analyzed. Comparing the results with those obtained with reference method
Chlorine
no significant difference at 95% confidence level was observed. Other profitable features
Falling drop system
such as a linear response ranging from 15 up to 100 mg L−1 Cl2 (R = 0.999); a detection limit
Breach
of 4.5 mg L−1 Cl2 estimated based on the 3 criterion; a relative standard deviation of 2.5% (n = 10) using a typical bleach sample containing 25.0 mg L−1 Cl2 ; a consumption of 55 g of DPD per determination; and a analytical frequency of 20 determinations per hour were also achieved. © 2006 Published by Elsevier B.V.
1.
Introduction
Chlorine (Cl2 ) is widely used in the form of hypochlorite as disinfecting agent in water supplying systems and in domestic environments as well as a bleaching agent. Products based on hypochlorite available in the market are produced by different manufacturers, thus chlorine determination could be performed to assure the quality. In this sense, the availability of the reliable analytical procedure presenting as inherent features robustness, appropriated sensitivity, low cost for its
∗
Corresponding author. Tel.: +55 19 3429 4639; fax: +55 19 3429 4610. E-mail address: [email protected] (B.F. Reis). 0003-2670/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.aca.2006.11.050
implementation, and generation of low waste volume should be useful. Analytical procedures for hypochlorite determination in different sample matrices based on chemiluminescence [1–3], potentiometry [4], voltammetry [5,6], spectrophotometry [7–12], and indirect titration [13] have been proposed. Nevertheless, most procedures do not attain the requirements described above. Nowadays, attention has been paid to environment friendly methods. This requirement could be accomplished using less
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
aggressive reagent and/or employing analytical procedure with ability to generate reduced volume of waste. The last one requirement has been achieved using procedure based on sequential injection analysis [14,15], or multicommutation process [16–19]. The falling drop approach [20] is also a tool that could provide facilities to develop analytical procedures with ability to generate low volume of waste, thus accomplishing the green chemistry requirement [21]. Reduction of waste generation employing procedures based on sequential injection or multicommutation approach is possible depending on the system design [14,22], while for procedure based on the falling drop approach low waste volume generation is an inherent feature [20]. In this work we intend to develop an automatic flow analysis procedure based on the falling drop process for photometric determination of Cl2 in bleach samples using N,N diethyl-p-phenylenediamine (DPD) as chromogenic reagent, which is oxidized by free chlorine generating a pink compound that is monitored at 515 nm [23]. The procedure will be implemented employing a flow injection analysis system based on the multicommutation process [24–26]. The photometric detection will be performed employing a LED based homemade photometer and endeavor will be directed to obtain a compact and robust setup.
2.
Experimental
2.1.
Reagents and samples
All chemicals used were of analytical grade. Purified water (electric conductivity less than 0.1 S cm−1 ) was used throughout. A 1000 mg L−1 Cl2 stock solution was prepared from commercial hypochlorite solution and after dilution it was standardized according to the procedure established in the Standard Methods [23]. The working solutions ranging from 20.0 up to 100.0 mg L−1 Cl2 were prepared everyday by appro-
67
priated dilution from the stock solution with a 0.01 M NaOH solution. A 0.11% (w/v) N,N -diethyl-p-phenylenediamine (Merck, Germany) solution was prepared dissolving 0.0275 g of solid in 25 mL of H2 SO4 solution (0.005 M). This solution could be used during a week. A phosphate buffer solution (pH 6.3) was prepared by dissolving 2.4 g of Na2 HPO4 and 4.6 g of KH2 PO4 in 100 mL of water. A 1.0% (w/v) ascorbic acid solution was prepared everyday dissolving 1.0 g of solid in 100 mL of water. A 1.0 M HCl solution that was used as carrier solution was prepared by appropriated dilution with water from the concentrated stock solution (12 M). Samples of bleach were purchased from the local market and analyzed after appropriated dilution.
2.2.
Apparatus
The equipment setup comprised a Pentium III microcomputer furnished with an electronic interface card (PCL-711S, Advantech); an IPC-4 Ismatec peristaltic pump with Tygon pumping tubes; a homemade LED based photometer [27]; a homemade device for drop generation; transmission lines of polyethylene tubing (0.8 mm i.d.); an aquarium pump; five three-way solenoid valves; and a homemade electronic interface to provide the potential difference and current intensity required to drive the solenoid valves [28]; two optic-fiber cables, 20 cm long, 1.0 mm diameter; a homemade online warming device (200 L inner volume) [29]. The microcomputer was furnished with a software wrote in Quick BASIC 4.5 that was designed with facilities to control the flow system and to perform data acquisition.
2.3.
Signal detection and drop generation modules
The photometric system tailored to detect the compound formed into the chromogenic solution drop while it grew was assembled according to the electronic diagram shown in Fig. 1. The electric potential difference presented at the output of the operational amplifier (AO2, pin 6) was a function of the radi-
Fig. 1 – Diagram of detection system and signal conditioning. Tr = Transistor BC547; LED = green LED, 515 nm; Til78 = phototransistor; OA1 and OA2 = operational amplifiers; C1, C2, C3 and C4 = 1 F tantalum capacitors; S0 = output signal to PCL711S analog input.
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Fig. 2 – Diagram of the flow system and of the drop formation unit. PC = Computer; DS = drop formation unit; R1 = chromogenic reagent, DPD 0.11% (w/v); R2 = buffer phosphate solution, pH 6.3: (a) tube barb fitting; (b) sliding acrylic cylinder; (c) cylinder holder; (d) screw to hold the sliding acrylic cylinder; (e) drop head, Tygon tubing piece 10 mm long and 2.0 mm i.d.; (f) reagent solution drop; (g) Green LED, 515 nm; (h) drop head height; (i) phototransistor; (j) detection system holder; (k) carrier solution and/or sample waste; (l) light beam; (m) drop formation unit; (W) waste collector containing 100 mL of a 1% ascorbic solution; f1 and f2 = optic-fibers; ASi and AS0 = inlet and outlet of the air stream. It was located at 2 mm bellow of drop-head.
ation beam intensity arriving at the phototransistor, which was read by the microcomputer through the PCL711S interface card. The phototransistor (Til78) and green LED ( = 515 nm) were assembled in the drop generating unit shown in Fig. 2. As we can see two optic-fiber cables (f1 , f2 ) were installed in the drop generation unit to conduct the light beam (l). The optic-fiber cables were assembled to allow that the drop of the reagent solution grew between them without contact. Under this condition, the radiation beam penetrated into the reagent drop where light absorption could occur. The signal variation presented at the operational amplifier output was a function of the radiation beam attenuation, which could be caused by the absorption of the chemical compound present in the drop. When the software was started the microcomputer requested the performing of the calibration step, which was done maintaining the LED off and adjusting the output signal (S0 ) to 0 V (dark measurement) turning the variable resistor (20 k) wired to the operational amplifier OA2 (pin 3). Afterwards, a drop of the reagent solution was generated and the LED was enabled to emit radiation by turning the variable resistor wired to the base of the transistor (Tr). The radiation beam intensity was increased up to the output signal (S0 ) attained a value around 800 mV. The reagent drop solution generation unit depicted in Fig. 2 comprised a cylinder of acrylic (11.0 mm diameter) with a central hole (1.0 mm) perforated at the longitudinal axis; an acrylic block with a central lead hole (11.1 mm diameter), which was used to hold the acrylic cylinder (b) in the vertical position permitting also its vertical sliding. The drop head (e) was positioned between the optic-fibers (f1 , f2 ) and its appropri-
ated height position (h) was found sliding the acrylic cylinder (b). Both drop generation unit and photometer modules were accommodated into a metallic box (15 cm × 18 cm × 30 cm).
2.4.
Procedure
The flow system was designed based on the multicommutation process [24–26] and its diagram is depicted in Fig. 3. In this configuration, all valves are switched OFF and the solutions are pumped to back their storing vessels. When the software was run the microcomputer requested the performing of the photometer calibration step that was carried out as it was described above. The flow lines cleaning step was performed maintaining all valve switched ON for a time interval of 90 s. Afterwards, the valve V2 was maintained switched ON during a time interval of 15 s to displace to waste (W) the sample residue remaining into the heating device (HD). While this step was in course the aquarium pump (AP) and the valve V5 were maintained switched ON in order to purge the remaining residual Cl2 into the drop generation unit (DS). The step for drop head cleaning was performed maintaining valves V3 and V4 switched ON during a time interval of 10 s. While this step was done the pumping flow rate was 50-fold in order to save time. After this step, the microcomputer established again the preset pumping flow rate and started the monitoring of the next drop growing. When the drop detaching from the drop head occurred, the photodetector generated a sudden signal variation that was sensed by microcomputer, which was instructed by the software to switch OFF valves V3 and V4 . Afterwards, the system was ready to begin the next analytical run.
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a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
tion was pumped to back its storing vessel (Rcs ). Valve V5 was switched ON to direct the remaining Cl2 from the drop formation unit towards the collecting vessel (SH). After a delay time of 30 s, the microcomputer started the routine to detect the drop falling in order to record the whole drop signal profile. Aiming to improve the analytical throughput the drop monitoring was stopped after an interval time of 100 s and valve V5 was maintained switched ON for 30 s to direct the remaining Cl2 into the drop formation unit towards the collecting vessel (SH). Considering that system parameters such as sample volume, drop head height (h), flow rates, and temperature could affect the analytical signal, a set of experiments was carried out in order to establish the appropriated operational conditions. After the instrument calibration, all steps of the analytical run summarized in Table 1 were performed automatically by the microcomputer. Intending to prove the usefulness of the proposed analytical system a set of bleach samples were analyzed employing the best operational conditions summarized in Table 1. Aiming to accuracy assessment the samples were also analyzed using the reference procedure (DPD) presented at the Standard Methods [23].
Fig. 3 – The flow system diagram. V1 , V2 , V3 , V4 and V5 = Three-way solenoid valves; R1 = chromogenic reagent, DPD; R2 = buffer phosphate solution, pH 6.3; Cs = carrier solution, HCl 1.0 M; S = sample; RR1 , RR2 , RCS and RS = solutions (R1 , R2 , Cs, S) circulation towards their storing vessels; PP = peristaltic pump; AP = aquarium pump to circulate the air stream; HD = heating device, SH = Cl2 collecting vessel containing NaOH 1.0 M; W = waste collector.
The sampling step consisted of 50 sampling cycles, which was performed switching ON sequentially valves V1 and V2 for a time interval of 0.5 s. Under this condition, the warming device (HD) was filled with the mix comprising sample (S) and carrier solution (Cs). Next valve V1 was turned OFF and carrier solution stream displaced sample zone through the heating device (HD) towards the drop formation unit (DS). In the next step, valves V3 and V4 were switched ON in order to displace both chromogenic reagent and buffer solutions towards the drop head (e). While this step was in course the aquarium pump (AP) was switched ON in order to maintain the air circulation into a closed circuit to improve the analyte delivering from the sample bulk. After 100 s delay time valve V2 and the warming device (HD) were switched OFF, thus carrier solu-
3.
Results and discussions
3.1.
Effect of the drop head position
As it is shown in Fig. 2 the drop solution was formed in the end of the drop head (e) in order to intercept the radiation beam while it grew. In earlier work [30] it was observed that the distance (h) between the drop head extremity and radiation beam could affect signal measurement. Intending to find the appropriated position experiments were carried out displacing down the cylinder (b). The assays were carried out without using a standard solution, therefore the radiation beam crossed the drop solution without undergoing absorption. Considering signal magnitude and precision of measurements as the main parameters, better results were achieved when the drop head (e) extremity was positioned at 9.0 mm distal from the bottom of the cylinder holder (c).
Table 1 – Sequence of the steps to carry out the analytical run Step 1 2 3 4 5 6 7 8 9 10 a
V1
V2
V3
V4
V5
AP
HD
Time (s)
0 0 1 0 0 0 1 0 0 0
0 0 0 1 0 0 0 1 1 0
1 0 0 0 1 1 0 0 1 0
1 0 0 0 1 1 0 0 1 0
0 0 0 1 0 0 0 0 0 1
0 0 0 1 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 0
90 – 20 15 10 – 1 0.5 100 30
These steps were repeated 50 times.
Description Form the drop Calibrate the photometer Fill the sample line Clean the analytical pathway Clean the drop head Detect the drop falling Samplinga Insert carrier solutiona Drop monitoring Purge the residual Cl2
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Fig. 4 – Signal records performed during a drop growing step.
3.2.
Chlorine detection
As it is shown in Fig. 2 the drop generation and detection module were assembled to allow that when drop solution intercepted the radiation beam, which underwent strong dispersion, reduced the intensity of the radiation reaching the phototransistor. As we can see in Fig. 4, the increasing started after a time interval of 70 s, which attained maximum value when the time interval elapsed was about 90 s, and afterwards it decreased up to the drop detaching from the drop head. To allow the photometer calibration the drop growing was halted by switching OFF valves V1 and V2 after a delay time of 90 s. The drop was formed using the chromogenic reagent solution and the output signal was adjusted to 800 mV. When Cl2 was delivered from sample bulk it was absorbed by the drop solution forming the pink compound that absorbed radiation at 515 nm. In this sense, the radiation beam leaving the drop was less intense than that coming from the LED. The phototransistor sensed this variation and as a consequence the electric potential difference monitored at the operational amplifier output (S0 , Fig. 1) was lower than 800 mV. The records of Fig. 5 show that the signal decreased analyte concentration in the drop. From these results the absorbance was derived employing the equation: Abs = −[log (Vs − Vd )/(V0 − Vd )], where V0 (800 mV) and Vs and were signals monitored without and with analyte in the drop, respectively, and Vd (0 mV) was dark measurement (see Section 2.3). A linear relationship (R = 0.999) was found, therefore indicating that this feature could be exploited to develop the analytical procedure.
3.3.
Fig. 5 – Records of the analytical signals concerning to Cl2 concentrations. T = 30 ◦ C; h = 32.0 mm; sample volume = 417 L.
cycles were varied from 10 to 50, thus volume of sample zone varied from 83.4 to 417 L yielding the results shown in Fig. 6. As we can see the signal varied within the range of 750–650 mV, thus presenting a ratio of 3.34 (L mV−1 ). A linear relationship was found (R = 0.998) between sample volume and the variation of the electric potential difference. This result is quite different as compared with the response observed employing usual flow system, where after increasing sample volume a linear relationship was not achieved.
3.4.
Effect of temperature
The focus of the procedure was the determination of volatile species (Cl2 ) in liquid samples. In this sense, we expect that temperature could affect the volatilization rate of the analyte, thus influencing the sensitivity of the analytical procedure. Aiming to ascertain this supposition a set of experiments were performed varying the temperature of the warming device from 25 up to 50 ◦ C. Better results were achieved maintain-
Effect of sample volume
Generally in flow injection systems the analytical signal increases with the injected sample volume. Since in the proposed system the volume influence on signal could be not predictable, experiments were carried out to ascertain its effect. The time intervals settled to insert sample and carrier solutions were maintained at 0.5 s and the number of sampling
Fig. 6 – Effect of the sample volume on the analytical signal magnitude. Cl2 = 20 mg L−1 ; T = 25 ◦ C; h = 32.0 mm.
a n a l y t i c a c h i m i c a a c t a 6 0 0 ( 2 0 0 7 ) 66–71
Table 2 – Comparative results using a reference method Sample A B C D E
Proposed method (g L−1 ) 19.1 24.9 22.2 20.8 21.5
± ± ± ± ±
0.2 0.6 0.4 0.9 0.2
Reference method [25] (g L−1 ) 18.9 24.4 21.9 21.1 20.6
± ± ± ± ±
0.1 0.2 0.5 0.6 0.3
Results are average of the three consecutive sample analysis. Applying the t-test for 95% confidence it was found 1.633 and the theoretical value is 2.776.
ing the temperature at 30 ◦ C. When temperature higher than this value was employed the precision of the measurements was lessened. Under this condition, it was observed that little water drops was formed in the cross surface of the optical fiber, thus causing the observed lessening effect.
3.5.
Sample analyses
Intending to prove the usefulness of the proposed system, a set of bleach samples were analyzed employing the optimal operational conditions summarized in Table 1 yielding the results that are shown in Table 2. Aiming the accuracy assessment the bleach samples were analyzed using a reference method [23]. Applying the paired t-test no significant difference at the 95% confidence level was observed. Other profitable features such as a linear response ranging from 15 up to 100 mg L−1 Cl2 (R = 0.999); a detection limit of 4.5 mg L−1 Cl2 ; a relative standard deviation 2.5% (n = 10) processing a typical bleach sample containing 25.0 mg L−1 Cl2 ; a reagent consumption of 55 g DPD per determination; a waste generation of 3.8 mL per determination; and an analytical frequency of 20 determinations per hour were also achieved.
4.
Conclusions
The linear response as function of the sample volume could be exploited to provide the software with adapting resource to improve sensitivity by enlarging the number of sampling cycles. The analytical module comprised some electronic devices controlled by microcomputer; nevertheless it can be easily assembled using non-expensive devices. The software designed to control the analytical system and to perform data acquisition allows easy operation of the system. The results obtained analyzing bleach samples proved that the proposed procedure is reliable. After working continuously for 8 h no significant variation of linear response range and precision of measurements were observed, thus we could conclude that the long-term stability of the proposed system is very good.
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Coupling the drop generation and the detection units and implementing the analytical procedure based on multicommuted process permitted that a compact and downsized analytical instrument was obtained with an ability to process low volume of chromogenic reagent.
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