- Email: [email protected]
An environmentally friendly flow system for high-sensitivity spectrophotometric determination of free chlorine in natural waters
Microchemical Journal 96 (2010) 77–81
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
An environmentally friendly flow system for high-sensitivity spectrophotometric determination of free chlorine in natural waters Wanessa R. Melchert, Daniel R. Oliveira, Fábio R.P. Rocha ⁎ Instituto de Química, Universidade de São Paulo, 05513-970, PO Box 26077, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 20 November 2009 Received in revised form 3 February 2010 Accepted 4 February 2010 Available online 10 February 2010 Keywords: Green analytical chemistry Flow analysis Multicommutation Long pathlength spectrophotometry Free chlorine DPD
a b s t r a c t A green and highly sensitive analytical procedure was developed for the determination of free chlorine in natural waters, based on the reaction with N,N-diethyl-p-phenylenediamine (DPD). The flow system was designed with solenoid micro-pumps in order to improve mixing conditions by pulsed flows and to minimize reagent consumption as well as waste generation. A 100-cm optical path flow cell based on a liquid core waveguide was employed to increase sensitivity. A linear response was observed within the range 10.0 to 100.0 µg L− 1, with the detection limit, coefficient of variation and sampling rate estimated as 6.8 µg L− 1 (99.7% confidence level), 0.9% (n = 20) and 60 determinations per hour, respectively. The consumption of the most toxic reagent (DPD) was reduced 20,000-fold and 30-fold in comparison to the batch method and flow injection with continuous reagent addition, respectively. The results for natural and tap water samples agreed with those obtained by the reference batch spectrophotometric procedure at the 95% confidence level. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chlorine, hypochlorous acid and hypochlorite are strong oxidizing agents that are used in water treatment for disinfection (destruction of pathogenic microorganisms) [1]. The total concentration of these species is defined as free residual chlorine. The combined residual chlorine corresponds to chloramines (NH2Cl, NHCl2 and NCl3) formed in reaction with ammonia, which have a lower disinfectant effect [2,3]. The main disadvantage of chlorination is the formation of undesirable products with organic matter and/or bromide and iodide present in the water. The formation of by-products was first demonstrated in 1974, when chloroform was identified in tap water as a result of chlorination [4]. Excess of chlorine can lead to health problems such as stomach discomfort and irritation to eyes. In addition, chlorinated derivatives (e.g. trichloromethane, bromodichloromethane, dibromochloromethane and tribromomethane) are carcinogenic [5]. Measurements of residual chlorine in water are thus required to evaluate the efficiency of disinfection in the distribution system and the possibility of formation of organochlorine compounds. There is a recommended range (0.3–0.5 mg L− 1) that guarantees the efficiency of the process while minimizing the toxic effects of excess of chlorine [1]. The United States Environmental Protection Agency (EPA) established the maximum concentration of chlorine in drinking water as 4.0 mg L− 1, however the total trihalometh-
⁎ Corresponding author. Fax: + 55 11 3815 5579. E-mail address: [email protected] (F.R.P. Rocha). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.02.001
ane concentration should be lower than 80 µg L− 1 [6]. In Brazil, the maximum concentration of total chlorine (free and combined chlorine) in natural waters allowed by legislation is 10 µg L− 1 [7]. The methods recommended by the APHA for the determination of free and combined chlorine are based on titration with ammonium ferrous sulfate, using N,N-diethyl-p-phenylenediamine (DPD) as an indicator. In the absence of iodide ion, free chlorine reacts instantly with DPD to form a pink product. Additions of small amounts of iodide catalyze the reaction between monochloramine and DPD, while an excess of this ion also catalyzes the reaction with dichloroamines [8]. Flow injection spectrophotometric procedures have been proposed for determination of free chlorine in natural water samples or in bleaching products based on different chemistries [5,9], miniaturized systems [10] or multicommutation [11–13]. However, the detection limits for free chlorine were higher than those established by current legislation. One alternative to improve the sensitivity is the use of long pathlength spectrophotometry based on a liquid core waveguide (LCW). These cells are built with a material with a refractive index lower than that of the fluid inside it, this material usually being a polymer of the Teflon AF family [14]. Thus, radiation is constrained inside the waveguide due to the total internal reflection process. These devices have been exploited to increase the analytical path in spectrophotometry [15,16] and also to direct the emitted radiation to the detector system in luminescence measurements [14]. Flow systems are used for mechanization of analytical procedures in order to minimize intervention by the analyst, increasing the number of samples that can be processed per unit time and improving precision of the measurements. These systems also have great potential for the
78
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
development of clean analytical procedures, as waste generation is usually lower than in batch procedures [17]. This potential can be increased by exploiting the multicommutation approach [18,19], which allows the introduction of micro reagent amounts only when required in the analytical procedure, being an effective tool for the development of more environmentally friendly procedures. In addition, multicommuted systems designed with solenoid micro-pumps [20] also present advantages such as improved mixing imposed by pulsed flows, portability and low energy requirements, making them suitable for infield measurements. Differences in the reaction kinetics of free chlorine and monochloramine with DPD have been exploited for sequential determination in a flow analysis system [21]. For the determination of free chlorine, the DPD reagent and phosphate buffer pH 6.3 were introduced by confluence. For determination of total chlorine, potassium iodide was added after measurement of free chlorine to catalyze the reaction of chloramines with DPD. The chloramine concentration was calculated by difference considering the correction of the dilution caused by the catalyst solution. A linear response was obtained within the range 0.1 to 8.0 mg L− 1, with a detection limit of 0.07 mg L− 1. This procedure is an alternative for the determination of free and total chlorine, with the advantages that is able to analyze more samples in a short period of time and minimize interference from chloramines by kinetic discrimination. However, two detection systems were required and the detection limit was not suitable for the determination of total chlorine in natural waters at the limits established by environmental regulations. Another procedure for the spectrophotometric determination of free chlorine in the presence of other chlorinated species employed the acid yellow dye 17. Five minutes after mixing the dye with the analyte in borate buffer pH 9.0, measurements were performed at 395 nm [22]. A linear response was obtained up to 1 mg L− 1, with a detection limit estimated at 50 µg L− 1. The effect of concomitant species was evaluated and no interference from chloramines and chloride ions was observed. However, the dye reacts rapidly with chlorine dioxide, causing significant interference. In this work, an improved flow-based procedure for spectrophotometric determination of free chlorine in natural waters is described. Solenoid micro-pumps and long pathlength spectrophotometry were explored to reduce reagent consumption and increase sensitivity.
Fig. 1. Flow diagram of the system for free chlorine determination. P1–P4: solenoid micropumps; V1, V2: three-way solenoid valves; S: sample; C: carrier stream; R1 and R2: DPD and acetate buffer, respectively; B: 75 cm long polyethylene coil. D: long pathlength LCW cell coupled to the spectrophotometer (552 nm); x: Perspex joint point and W: waste vessel.
Japan, model U-3000) equipped with a 5-cm optical path glass cell due to the low free chlorine concentrations in the samples. 2.2. Reagents and solutions
2. Experimental
All solutions were prepared with analytical grade chemicals and distilled–deionized water. The reference solutions were prepared within the range 10.0–100.0 µg L− 1 hypochlorite in 1.0 × 10− 4 mol L− 1 NaOH by dilution of 24.20 g L− 1 commercial stock solution standardized by iodometric titration. The reagent R1 was an 8.4 × 10− 4 mol L− 1 DPD solution containing 2.0 × 10− 5 mol L− 1 EDTA and 0.01 mol L− 1 H2SO4. The reagent R2 was an 8.0 × 10− 2 mol L− 1 acetate buffer solution pH 4.0. Both reagents were prepared in water. Solutions of Mn2+, Cu2+, Fe3+, − Ca2+, Ba2+, SO2− 4 and NO3 were prepared by dissolving the appropriate salts in water. River and lake water samples were collected in unpolluted freshwater sources at Piracicaba city, São Paulo State, Brazil. Samples were filtered through a 0.45-μm cellulose acetate membrane and preserved at − 4 °C, being stabilized at the ambient temperature immediately before analysis. Tap water samples were collected at the campus of São Paulo University (São Paulo, Brazil).
2.1. Apparatus
2.3. Flow diagram
The flow system was constructed with four solenoid micro-pumps that dispense 12 (P1, P2 and P3) or 7 μL (P4) per pulse (Biochem Valve Inc., Boonton, NJ, USA; model 090SP), a pair of three-way solenoid valves (NResearch, West Caldwell, NJ, USA), 0.7 mm i.d. polyethylene tubes and a Perspex joint. A Pentium I microcomputer was used for system control and data acquisition. The solenoid micro-pumps and valves were controlled through a parallel port of the microcomputer by using a power drive based on a ULN2803 integrated circuit. Spectrophotometric measurements were carried out with a multichannel CCD spectrophotometer (Ocean Optics, Dunedin, FL, USA; model USB2000) with a tungsten-halogen light source (Ocean Optics, Dunedin, FL, USA; model LS-1). Optical fibers (100 or 600 μm) were used to transmit the radiation. A 100-cm optical path (250 μL internal volume, 0.55 mm i.d.) flow cell (Ocean Optics, Dunedin, FL, USA; model LPC-1) was also employed. The control software was developed in Visual Basic 6.0 (Microsoft, Redmond, WA, USA) and the software supplied by the manufacturer of the multi-channel spectrophotometer was used for data acquisition. The batch spectrophotometric procedure adopted as reference was performed as previously described [8]. However, measurements were carried out with an UV–vis spectrophotometer (HITACHI, Tokyo,
The flow manifold, shown in Fig. 1, was operated according to the switching course of the active devices described in Table 1. The binary sampling approach [18] was adopted for solution handling. Small aliquots of the solutions were sequentially inserted into the analytical path, generating a sampling profile that was repeated until the programmed
Table 1 Switching course of the solenoid micro-pumps and values for free chlorine determination with the flow system shown in Fig. 1. The status 1/0 indicates a pulse of current in the solenoid pump. 0 and 1 indicate that valves are switched off and on, respectively. Step
Description
P1
P2
P3
P4
V1
V2
Pulses or time
1 2 3 4 5
Sample insertion Reagent 1 insertion Reagent 2 insertion Stopped-flow Sample transportation and signal measurement Sample replacement
1/0 0 0 0 0
0 1/0 0 0 0
0 0 1/0 0 0
0 0 0 0 1/0
0 0 0 0 0
1 1 1 0 0
5a 2a 2a 20 s 130
1/0 0
0 0
0 0
0 1/0
1 1
0 0
100 50
6 a
5 sampling cycles.
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
number of sampling cycles was completed. The volume of each solution was defined by the programmed number of pulses of the corresponding micro-pump. The analytical cycle started by intercalating sample and reagent aliquots dispensed through the P1, P2 and P3 pumps (steps 1, 2 and 3). These solutions underwent fast mixing at the interfaces, thus establishing the first sampling cycle. The sequence was repeated five times to form the sample zone and then the flow was stopped for 20 s (step 4). The sample zone was directed to detection at 552 nm by actuation of P4 (step 5). The analytical signal was based on peak height and measurements were taken in triplicate. The analytical response was estimated as the difference between the analytical and blank signals. Valve V1 was used to replace solutions, avoiding passage through the analytical path and minimizing contamination risks. Before analyzing another sample, valve V1 was switched on while pump P1 was actuated (100 pulses) to fill the connection tube with the new sample. Pump P4 was then actuated (50 pulses) to remove the sample aliquot by the carrier through valve V1 (step 6). Valve V2 was used to avoid drawbacks caused by the hydrodynamic impedance of the long optical path flow cell in the sampling step [23]. 3. Results and discussion 3.1. General aspects and system optimization The determination of free chlorine was based on the reaction with DPD recommended by the APHA [8] and the product formed shows a maximum absorption at 552 nm. The flow system with solenoid micro-pumps (Fig. 1) was coupled to a 100-cm optical path flow cell in order to overcome the drawbacks of previous works, namely poor sensitivity, high reagent consumption and effluent generation. A problem observed by coupling the flow system with solenoid micro-pumps to the LCW cell was the significant diminution of the dispensed solution volumes due to the increase in system hydrodynamic impedance and the low torque of the propulsion devices [23]. In some circumstances, the dispensed volume was reduced by 4-fold in relation to the nominal volume. This drawback was circumvented by redirecting the flow during the sampling step by means of actuation of valve V2 (Fig. 1). Taking into account the analytical signal and the magnitude of the blank, the number of pulses of sample, reagent and carrier were optimized, as well as the reagent concentrations. The effects of the reactor coil length and the sample residence time were also evaluated. The effect of the volumetric fraction of sample and reagents was investigated by changing the number of pulses of the corresponding solenoid micro-pumps. This study was carried out using a 50 µg L− 1 hypochlorite solution, 8.4 × 10− 4 mol L− 1 DPD in 0.01 mol L− 1 H2SO4 and phosphate buffer 0.04 mol L− 1 (pH 6.3). The R1 solution was prepared in sulfuric acid medium to prevent degradation of the chromogenic reagent, which occurs rapidly in neutral or alkaline solutions. The oxidation of DPD significantly increased the blank signal due to radiation absorption by the reagent, which is a drawback for measurements with long optical path flow cells [15,24]. In the optimization of the number of pulses of R1, a gradual increase in both analytical and blank signals was observed with up to four pulses of the reagent, yielding an analytical response 40% higher in comparison to that obtained with one pulse of DPD reagent. On the other hand, the analytical signal did not significantly change for a number of pulses of R2 lower than four, but the blank signal decreased up to 7%, by changing from one to four pulses, due to the dilution effect. However, for six pulses of R2, the analytical signal was 21% lower due to dilution of the sample and DPD reagent. Thus, four pulses of R1 and R2 were selected for further studies. At this stage of optimization, the reagents were in large excess in relation to the analyte. Thus, by varying the number of sample pulses,
79
a gradual increase in the analytical signal was observed without affecting the blank, with up to 10 pulses of the hypochlorite solution. Large sample volumes diminished the signal due to dilution of the reagents. The best analytical response, taking into account the difference between the analytical and blank signals, and the magnitude of the blank signal, was obtained with four pulses of DPD solution and phosphate buffer (R1 and R2) and ten pulses of sample. Keeping the same volumetric fraction (2:2:5 for R1, R2 and sample, respectively), the effect of the number of sampling cycles was evaluated. The analytical response increased due to the lower dispersion of the sample zone, with up to five sampling cycles. In this condition, the total sample zone volume (540 µL) was higher than the analytical path volume, estimated as 400 µL. The effect of the DPD concentration was evaluated within the range 4.2 × 10− 4 to 3.4 × 10− 3 mol L− 1 and both the analytical and blank signals increased with the concentration of the chromogenic reagent. For example, with a 3.4 × 10− 3 mol L− 1 DPD solution, the signal was 30% higher in comparison to that obtained with a 4.2× 10− 4 mol L− 1 solution, and the blank signal was 16% higher. By considering these aspects, a concentration of 8.4× 10− 4 mol L− 1 DDP was selected. The effects of acidity and buffer concentration were evaluated using 50 µg L− 1 hypochlorite solution and 8.4 × 10− 4 mol L− 1 DPD in 0.01 mol L− 1 H2SO4. Taking into account its use in the reference method, a phosphate buffer was initially investigated [8]. For a buffer concentration of 0.2 mol L− 1, both analytical and blank signals increased with pH due to the oxidation of the DPD reagent, which is favored in an alkaline medium. The highest difference between the analytical and blank signals was observed at pH = 5.0, which was the lowest pH evaluated in view of the buffering capacity of the phosphate buffer, pKa2 (H3PO4) = 7.21. Evidence of a higher response at a lower pH was also observed when the effect of the buffer concentration was evaluated. The analytical signal decreased gradually at higher concentrations of the phosphate buffer (the analytical response was 50% higher when comparing the signals obtained with 0.1 and 0.5 mol L− 1 buffer concentrations). This effect is due to the change in acidity of the sample zone when solutions with low buffering capacity were used, as a result of the acid present in the solution R1. Thus, other buffer solutions were evaluated: 0.2 mol L− 1 solutions of citrate, acetate and hydrogen phthalate (pH 5.0) yielded similar results. The effect of acetate buffer acidity and concentration was then evaluated. It was observed that by increasing the pH to 5.5, the difference between the analytical and blank signals was about 60% lower in comparison to that obtained at pH 4.0. Lower pH values did not significantly affect the analytical response. The acetate buffer concentration was then varied within the range 0.02 to 0.2 mol L− 1 and the highest analytical response was obtained from 0.08 mol L− 1 concentration. It should be pointed out that previous studies on the determination of free chlorine by the DPD method generally used phosphate buffer at pH 6.3 without critically evaluating the effect of acidity on the chemical reaction [10,12,21]. The effects of sample residence time and reactor coil length were also evaluated and the values were selected by considering the analytical response and the magnitude of the blank signal. The analytical signal was increased by 30% by stopping the flow for 20 s, due to the higher residence time, with a negligible effect on sample dispersion. For free chlorine determination in natural waters this increase is significant, given the low concentration of this species in the samples and the low limits established by legislation. Longer stopping times did not affect the signal, indicating that the steady state condition was attained after a 20-s residence time. The reactor coil length affected the analytical and blank signals due to higher dispersion. However, the decrease in the blank value was more significant and a 75-cm reactor provided the highest analytical response. A summary of the ranges studied and the selected values is presented in Table 2.
80
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
Table 2 Ranges studied and optimized conditions for free chlorine determination. Parameter
Range studied
Selected value
Pulse of sample Pulse of R1 Pulse of R2 Sampling cycles [DPD] (mmol L− 1) [Acetate buffer] (mol L− 1) Buffer pH Reactor (cm) Stopping flow time (s)
1–12 1–6 1–6 1–8 0.42–3.36 0.02–0.2 3.75–5.75 20–100 0–50
10 4 4 5 0.84 0.08 4.00 75 20
3.2. Analytical features and application A linear response was observed within the range 10.0 to 100.0 µg L− 1 hypochlorite, described by equation A = (− 0.0329 ± 0.0031) + (0.0089± 0.0002)C (µg L− 1), r = 0.999 (Fig. 2). The detection limit (99.7% confidence level) and the coefficient of variation (n = 20) were estimated as 6.8 µg L− 1 and 0.9% respectively. The sampling rate was estimated as 60 determinations per hour. The effect of some species that are present in natural waters that 3+ − could interfere in the proposed procedure (SO2− , Ba2+, 4 , NO3 , Fe Mn2+ and Ca2+) was evaluated. The solutions were prepared in a known concentration of hypochlorite (50 µg L− 1) and the results were compared with those obtained with a solution in the absence of concomitants. In a preliminary study, it was observed that Fe3+ and Ca2+ interfered in the determination of free chlorine, requiring masking with EDTA (added to the R1 solution). However, the analytical signal diminished ca. 20 and 60% in the presence of 2.0 × 10− 4 mol L− 1 and 2.0 × 10− 2 mol L− 1 of this complexing agent, respectively. Given the effect of this reagent on sensitivity and the usual concentration of metal ions in natural water samples, the concentration of EDTA was fixed at 2.0 × 10− 5 mol L− 1, which did not affect sensitivity. In the presence of EDTA, a 10-fold excess of iron(III) and a 100-fold excess of the other evaluated species did not affect the analyte determination. Thus, the typical amounts of concomitants in natural waters did not interfere in the determination of hypochlorite by the proposed procedure.
Table 3 Determination of free chlorine in natural and tap water samples. Sample
1 2 3 4 5
ClO− concentration (µg L− 1) Proposed procedure
Reference procedure [8]
79.2 ± 0.2 37.4 ± 0.3 30.4 ± 0.2 44.7 ± 3.1 56.7 ± 0.2
79.1 ± 11.1 28.0 ± 4.9 29.8 ± 5.4 43.9 ± 11.6 58.8 ± 6.0
Natural water samples were analyzed by the proposed and reference [8] procedures and the results are presented in Table 3. The hypochlorite concentrations agreed with those obtained by the reference procedure at the 95% confidence level, according to a paired t-test. However, it should be noted that the determinations by the reference procedure were performed using a cell of 5-cm optical path, due to the low concentration of hypochlorite in the samples. The high deviations in the measurements by the reference procedure were due to the working range being close to the quantification limit. The analytical characteristics of the proposed procedure were better than those obtained in other flow procedures [5,9–13,21,25–28] with spectrophotometric detection (Table 4). In comparison to procedures based on the reaction with DPD [10–12,21], the proposed procedure achieved a lower detection limit and higher sampling rate. This difference is due to the use of low flow rates and long washing times (90 s) [21] in the previously proposed flow systems. In the procedures based on odianisidine [25], the detection limit is ca. 7 times higher than in the proposed procedure, affecting the working range (50 to 1300 µg L− 1). However, when using 4-aminoantipyrine and phenol [26] both the detection limit and the working range are comparable to the proposed system, but the sampling rate is 93% lower due to sample recirculation for analyte enrichment in the acceptor stream. The coefficient of variation of the proposed procedure (0.9%) was lower than that obtained in the procedures listed in Table 4, except in the single line procedure using 4-nitrophenylhydrazine. However, it is important to point out that this procedure lacked sensitivity, once ClO− solutions were 500 times more concentrated than that used in the proposed procedure. In relation to the procedure exploiting long pathlength spectrophotometry [13], which employs a different chemistry, the sampling rate and coefficient of variation were comparable, but the sensitivity was ca. 20-fold higher. The sensitivity of the proposed procedure and, consequently, the response range, are sufficient to comply with environmental legislation [7], in which the maximum allowable total chlorine (free chlorine + combined chlorine) concentration for fresh water is 10 µg L− 1. A comparison of reagent consumption in some procedures for free chlorine determination based on the DPD reaction is presented in Table 5. The consumption of the most toxic reagent (DPD) was 227-fold lower in the proposed method than in the system based on multicommutation with solenoid valves [11]. The consumption was reduced ca. 20,000-fold in relation to the batch method and 97, 79 and 53% in relation to a flow injection system with continuous reagent addition [21], miniaturization [10] and a falling drop [12] system, respectively. Thus, the proposed procedure is inherently greener than those previously reported and can have highly beneficial effects by reducing both costs and waste generation. In addition, other previous studies used highly toxic reagents [26]. 4. Conclusions
Fig. 2. Transient signals obtained for free chlorine reference solutions measured in triplicate. Numbers indicate free chlorine concentrations in µg L− 1. The inset shows the corresponding calibration graph.
By coupling a flow system designed with solenoid micro-pumps to a 100-cm optical path flow cell it was possible to determine free chlorine in samples of natural water supplies that were within the tolerable range in water bodies, as established by environmental legislation, without preconcentration. The proposed procedure is more environmentally
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
81
Table 4 Analytical features of spectrophotometric flow-based procedures for free chlorine determination. System
Reagent
LD (µg L− 1)
Linear range (µg L− 1)
Sampling rate (h− 1)
Coefficient of variation (%)
Ref.
FIA FIA FIA FIA + gas diffusion FIA + gas diffusion FIA miniaturized SIA SIA Multicommutation — solenoid valve Multicommutation — solenoid valve Multicommutation — solenoid valve + LCW Proposed procedure
DPD 3,3′-Dimethylnaphtidine 4-Nitrophenylhydrazine o-Dianisidine 4-Aminoantipyrine + phenol DPD o-Tolidine Tetramethylbenzidine DPD DPD o-Tolidine DPD
70 30 400 50 0.5 165 200 80 510 4500 – 6.8
100–8000 100–1000 1000–40,000 50–1300 5–250 125–1700 b 5000 90–1300 1400–9700 15,000–100,000 70–1000 10–100
– 150 110 38 4 – 11 60 45 20 45 58
3.0 1.2 0.6 1.5 3.1 – – – 1.4 2.5 1.0 0.9
[21] [27] [9] [25] [26] [10] [28] [5] [11] [12] [13] –
FIA = flow injection analysis; SIA = sequential injection analysis; LCW = liquid core waveguide.
Table 5 Reagent consumption in different procedures for free chlorine determination based on DPD method. Procedure
DPD amount (mg/determination)
Reference
Batch FIA with confluent streamsa FIA miniaturized Multicommutation with solenoid valves Flow system with falling drop Proposed procedure
500 0.8 0.083 5.9 0.055 0.026
[8] [21] [10] [11] [12] –
a
Estimate by considering a sampling rate of 100 measurements per hour.
friendly given the drastic reduction in reagent consumption (micrograms per determination). This analytical methodology employs the same reagents used in procedures carried out by control agencies, which will facilitate the implementation of the developed procedure for monitoring this species in natural waters. Acknowledgements The authors acknowledge fellowships and financial support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). D.L. Rocha is thanked for critical comments. References [1] http://www.who.int, accessed in 11/19/2009. [2] B. Saad, W.T. Wai, M.S. Jab, W.S.W. Ngah, M.I. Saleh, J.M. Slater, Development of flow injection spectrophotometric methods for the determination of free available chlorine and total available chlorine: comparative study, Anal. Chim. Acta 537 (2005) 197–206. [3] Z. Qiang, C.D. Adams, Determination of monochloramine formation rate constants with stopped-flow spectrophotometry, Environ. Sci. Technol. 38 (2004) 1435–1444. [4] J.C. Kotz, P.M. Treichel Junior, P.A. Harman, Chemistry & Chemical Reactivity, fifth ed Thomson, United States, 2003. [5] R.B.R. Mesquita, M.L.F.O.B. Noronha, A.I.L. Pereira, A.C.F. Santos, A.F. Torres, V. Cerdà, A.O.S.S. Rangel, Use of tetramethylbenzidine for the spectrophotometric sequential injection determination of free chlorine in waters, Talanta 72 (2007) 1186–1191. [6] http://www.epa.gov, accessed in 07/16/2008. [7] http://www.mma.gov.br/port/conama/, accessed in 02/27/2009. [8] A.D. Eaton, L.S. Clesceri, A.E. Greenberg, Standard Methods for the Examination of Water and Wastewater, 19th ed., American Public Health Association, Washington, DC, 1995.
[9] K.K. Verma, A. Jam, A. Townshend, Determination of free and combined residual chlorine by flow-injection spectrophotometry, Anal. Chim. Acta 261 (1992) 233–240. [10] K. Carlsson, L. Moberg, B. Karlberg, The miniaturisation of the standard method based on the N, N′-diethyl-p-phenylenediamine (DPD) reagent for the determination of free or combined chlorine, Water Res. 33 (1999) 375–380. [11] F.H. Salami, V.G. Bonifácio, G.G. Oliveira, O. Fatibello-Filho, Spectrophotometric multicommutated flow system for the determination of hypochlorite in bleaching products, Anal. Lett. 41 (2008) 3187–3197. [12] S.S. Borges, B.F. Reis, An automatic falling drop system based on multicommutation process for photometric chlorine determination in bleach, Anal. Chim. Acta 600 (2007) 66–71. [13] F.H. Salami, V.G. Bonifácio, O. Fatibello-Filho, L.H. Marcolino Junior, Determinação espectrofotométrica em fluxo de cloro em água usando célula de longo caminho óptico e multicomutação, Quim. Nova 32 (2009) 112–115. [14] T. Dallas, P.K. Dasgupta, Light at the end of the tunnel: recent analytical applications of liquid-core waveguides, Trends Anal. Chem. 23 (2004) 385–392. [15] K.O. Lupetti, F.R.P. Rocha, O. Fatibello-Filho, An improved flow system for phenols determination exploiting multicommutation and long pathlength spectrophotometry, Talanta 62 (2004) 463–467. [16] R.D. Waterbury, W. Yao, R.H. Byrne, Long pathlength absorbance spectroscopy: trace analysis of Fe(II) using a 4.5 m liquid core waveguide, Anal. Chim. Acta 357 (1997) 99–102. [17] F.R.P. Rocha, J.A. Nóbrega, O. Fatibello Filho, Flow analysis strategies to greener analytical chemistry. An overview, Green Chem. 3 (2001) 216–220. [18] F.R.P. Rocha, B.F. Reis, E.A.G. Zagatto, J.L.F.C. Lima, R.A.S. Lapa, J.L.M. Santos, Multicommutation in flow analysis: concepts, applications and trends, Anal. Chim. Acta 468 (2002) 119–131. [19] F.R.P. Rocha, P.B. Martelli, B.F. Reis, A multicommutation-based flow system for multi-element analysis in pharmaceutical preparations, Talanta 55 (2001) 861. [20] W.R. Melchert, C.M.C. Infante, F.R.P. Rocha, Development and critical comparison of greener flow procedures for nitrite determination in natural waters, Microchem. J. 85 (2007) 209–213. [21] G. Gordon, D.L. Sweetin, K. Smith, G.E. Pacey, Improvements in theN, N-diethyl-pphenylenediamine method for the determination of free and combined residual chlorine through the use of FIA, Talanta 38 (1991) 145–149. [22] B. Chiswell, K.R. O´Halloran, Acid yellow 17 as a spectrophotometric reagent for the determination of low concentrations of residual free chlorine, Anal. Chim. Acta 249 (1991) 519–524. [23] W.R. Melchert, F.R.P. Rocha, A greener and highly sensitive flow-based procedure for carbaryl determination exploiting long pathlength spectrophotometry and photochemical waste degradation, Talanta 81 (2010) 327–333. [24] C.M.C. Infante, F.R.P. Rocha, A critical evaluation of a long pathlength cell for flowbased spectrophotometric measurements, Microchem. J. 90 (2008) 19–25. [25] M.C. Icardo, J.V.G. Mateo, J.M. Calatayud, Selective chlorine determination by gas diffusion in a tandem flow assembly and spectrophotometric detection with odianisidine, Anal. Chim. Acta 443 (2001) 153–163. [26] G. Jin, J. Yang, J.F. Li, Determination of chlorine dioxide using capillary on-line concentration coupled with flow injection analysis, Microchim. Acta 148 (2004) 171–175. [27] E. Pobozy, K. Pyrzynska, B. Szostek, M. Trojanowicz, Flow-injection spectrophotometric determination of free residual chlorine in waters with3, 3′-dimethylnaphtidine, Microchem. J. 51 (1995) 379–386. [28] J.G. March, M. Gual, B.M. Simonet, Determination of residual chlorine in greywater using o-tolidine, Talanta 58 (2002) 995–1001.
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
An environmentally friendly flow system for high-sensitivity spectrophotometric determination of free chlorine in natural waters Wanessa R. Melchert, Daniel R. Oliveira, Fábio R.P. Rocha ⁎ Instituto de Química, Universidade de São Paulo, 05513-970, PO Box 26077, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 20 November 2009 Received in revised form 3 February 2010 Accepted 4 February 2010 Available online 10 February 2010 Keywords: Green analytical chemistry Flow analysis Multicommutation Long pathlength spectrophotometry Free chlorine DPD
a b s t r a c t A green and highly sensitive analytical procedure was developed for the determination of free chlorine in natural waters, based on the reaction with N,N-diethyl-p-phenylenediamine (DPD). The flow system was designed with solenoid micro-pumps in order to improve mixing conditions by pulsed flows and to minimize reagent consumption as well as waste generation. A 100-cm optical path flow cell based on a liquid core waveguide was employed to increase sensitivity. A linear response was observed within the range 10.0 to 100.0 µg L− 1, with the detection limit, coefficient of variation and sampling rate estimated as 6.8 µg L− 1 (99.7% confidence level), 0.9% (n = 20) and 60 determinations per hour, respectively. The consumption of the most toxic reagent (DPD) was reduced 20,000-fold and 30-fold in comparison to the batch method and flow injection with continuous reagent addition, respectively. The results for natural and tap water samples agreed with those obtained by the reference batch spectrophotometric procedure at the 95% confidence level. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Chlorine, hypochlorous acid and hypochlorite are strong oxidizing agents that are used in water treatment for disinfection (destruction of pathogenic microorganisms) [1]. The total concentration of these species is defined as free residual chlorine. The combined residual chlorine corresponds to chloramines (NH2Cl, NHCl2 and NCl3) formed in reaction with ammonia, which have a lower disinfectant effect [2,3]. The main disadvantage of chlorination is the formation of undesirable products with organic matter and/or bromide and iodide present in the water. The formation of by-products was first demonstrated in 1974, when chloroform was identified in tap water as a result of chlorination [4]. Excess of chlorine can lead to health problems such as stomach discomfort and irritation to eyes. In addition, chlorinated derivatives (e.g. trichloromethane, bromodichloromethane, dibromochloromethane and tribromomethane) are carcinogenic [5]. Measurements of residual chlorine in water are thus required to evaluate the efficiency of disinfection in the distribution system and the possibility of formation of organochlorine compounds. There is a recommended range (0.3–0.5 mg L− 1) that guarantees the efficiency of the process while minimizing the toxic effects of excess of chlorine [1]. The United States Environmental Protection Agency (EPA) established the maximum concentration of chlorine in drinking water as 4.0 mg L− 1, however the total trihalometh-
⁎ Corresponding author. Fax: + 55 11 3815 5579. E-mail address: [email protected] (F.R.P. Rocha). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.02.001
ane concentration should be lower than 80 µg L− 1 [6]. In Brazil, the maximum concentration of total chlorine (free and combined chlorine) in natural waters allowed by legislation is 10 µg L− 1 [7]. The methods recommended by the APHA for the determination of free and combined chlorine are based on titration with ammonium ferrous sulfate, using N,N-diethyl-p-phenylenediamine (DPD) as an indicator. In the absence of iodide ion, free chlorine reacts instantly with DPD to form a pink product. Additions of small amounts of iodide catalyze the reaction between monochloramine and DPD, while an excess of this ion also catalyzes the reaction with dichloroamines [8]. Flow injection spectrophotometric procedures have been proposed for determination of free chlorine in natural water samples or in bleaching products based on different chemistries [5,9], miniaturized systems [10] or multicommutation [11–13]. However, the detection limits for free chlorine were higher than those established by current legislation. One alternative to improve the sensitivity is the use of long pathlength spectrophotometry based on a liquid core waveguide (LCW). These cells are built with a material with a refractive index lower than that of the fluid inside it, this material usually being a polymer of the Teflon AF family [14]. Thus, radiation is constrained inside the waveguide due to the total internal reflection process. These devices have been exploited to increase the analytical path in spectrophotometry [15,16] and also to direct the emitted radiation to the detector system in luminescence measurements [14]. Flow systems are used for mechanization of analytical procedures in order to minimize intervention by the analyst, increasing the number of samples that can be processed per unit time and improving precision of the measurements. These systems also have great potential for the
78
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
development of clean analytical procedures, as waste generation is usually lower than in batch procedures [17]. This potential can be increased by exploiting the multicommutation approach [18,19], which allows the introduction of micro reagent amounts only when required in the analytical procedure, being an effective tool for the development of more environmentally friendly procedures. In addition, multicommuted systems designed with solenoid micro-pumps [20] also present advantages such as improved mixing imposed by pulsed flows, portability and low energy requirements, making them suitable for infield measurements. Differences in the reaction kinetics of free chlorine and monochloramine with DPD have been exploited for sequential determination in a flow analysis system [21]. For the determination of free chlorine, the DPD reagent and phosphate buffer pH 6.3 were introduced by confluence. For determination of total chlorine, potassium iodide was added after measurement of free chlorine to catalyze the reaction of chloramines with DPD. The chloramine concentration was calculated by difference considering the correction of the dilution caused by the catalyst solution. A linear response was obtained within the range 0.1 to 8.0 mg L− 1, with a detection limit of 0.07 mg L− 1. This procedure is an alternative for the determination of free and total chlorine, with the advantages that is able to analyze more samples in a short period of time and minimize interference from chloramines by kinetic discrimination. However, two detection systems were required and the detection limit was not suitable for the determination of total chlorine in natural waters at the limits established by environmental regulations. Another procedure for the spectrophotometric determination of free chlorine in the presence of other chlorinated species employed the acid yellow dye 17. Five minutes after mixing the dye with the analyte in borate buffer pH 9.0, measurements were performed at 395 nm [22]. A linear response was obtained up to 1 mg L− 1, with a detection limit estimated at 50 µg L− 1. The effect of concomitant species was evaluated and no interference from chloramines and chloride ions was observed. However, the dye reacts rapidly with chlorine dioxide, causing significant interference. In this work, an improved flow-based procedure for spectrophotometric determination of free chlorine in natural waters is described. Solenoid micro-pumps and long pathlength spectrophotometry were explored to reduce reagent consumption and increase sensitivity.
Fig. 1. Flow diagram of the system for free chlorine determination. P1–P4: solenoid micropumps; V1, V2: three-way solenoid valves; S: sample; C: carrier stream; R1 and R2: DPD and acetate buffer, respectively; B: 75 cm long polyethylene coil. D: long pathlength LCW cell coupled to the spectrophotometer (552 nm); x: Perspex joint point and W: waste vessel.
Japan, model U-3000) equipped with a 5-cm optical path glass cell due to the low free chlorine concentrations in the samples. 2.2. Reagents and solutions
2. Experimental
All solutions were prepared with analytical grade chemicals and distilled–deionized water. The reference solutions were prepared within the range 10.0–100.0 µg L− 1 hypochlorite in 1.0 × 10− 4 mol L− 1 NaOH by dilution of 24.20 g L− 1 commercial stock solution standardized by iodometric titration. The reagent R1 was an 8.4 × 10− 4 mol L− 1 DPD solution containing 2.0 × 10− 5 mol L− 1 EDTA and 0.01 mol L− 1 H2SO4. The reagent R2 was an 8.0 × 10− 2 mol L− 1 acetate buffer solution pH 4.0. Both reagents were prepared in water. Solutions of Mn2+, Cu2+, Fe3+, − Ca2+, Ba2+, SO2− 4 and NO3 were prepared by dissolving the appropriate salts in water. River and lake water samples were collected in unpolluted freshwater sources at Piracicaba city, São Paulo State, Brazil. Samples were filtered through a 0.45-μm cellulose acetate membrane and preserved at − 4 °C, being stabilized at the ambient temperature immediately before analysis. Tap water samples were collected at the campus of São Paulo University (São Paulo, Brazil).
2.1. Apparatus
2.3. Flow diagram
The flow system was constructed with four solenoid micro-pumps that dispense 12 (P1, P2 and P3) or 7 μL (P4) per pulse (Biochem Valve Inc., Boonton, NJ, USA; model 090SP), a pair of three-way solenoid valves (NResearch, West Caldwell, NJ, USA), 0.7 mm i.d. polyethylene tubes and a Perspex joint. A Pentium I microcomputer was used for system control and data acquisition. The solenoid micro-pumps and valves were controlled through a parallel port of the microcomputer by using a power drive based on a ULN2803 integrated circuit. Spectrophotometric measurements were carried out with a multichannel CCD spectrophotometer (Ocean Optics, Dunedin, FL, USA; model USB2000) with a tungsten-halogen light source (Ocean Optics, Dunedin, FL, USA; model LS-1). Optical fibers (100 or 600 μm) were used to transmit the radiation. A 100-cm optical path (250 μL internal volume, 0.55 mm i.d.) flow cell (Ocean Optics, Dunedin, FL, USA; model LPC-1) was also employed. The control software was developed in Visual Basic 6.0 (Microsoft, Redmond, WA, USA) and the software supplied by the manufacturer of the multi-channel spectrophotometer was used for data acquisition. The batch spectrophotometric procedure adopted as reference was performed as previously described [8]. However, measurements were carried out with an UV–vis spectrophotometer (HITACHI, Tokyo,
The flow manifold, shown in Fig. 1, was operated according to the switching course of the active devices described in Table 1. The binary sampling approach [18] was adopted for solution handling. Small aliquots of the solutions were sequentially inserted into the analytical path, generating a sampling profile that was repeated until the programmed
Table 1 Switching course of the solenoid micro-pumps and values for free chlorine determination with the flow system shown in Fig. 1. The status 1/0 indicates a pulse of current in the solenoid pump. 0 and 1 indicate that valves are switched off and on, respectively. Step
Description
P1
P2
P3
P4
V1
V2
Pulses or time
1 2 3 4 5
Sample insertion Reagent 1 insertion Reagent 2 insertion Stopped-flow Sample transportation and signal measurement Sample replacement
1/0 0 0 0 0
0 1/0 0 0 0
0 0 1/0 0 0
0 0 0 0 1/0
0 0 0 0 0
1 1 1 0 0
5a 2a 2a 20 s 130
1/0 0
0 0
0 0
0 1/0
1 1
0 0
100 50
6 a
5 sampling cycles.
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
number of sampling cycles was completed. The volume of each solution was defined by the programmed number of pulses of the corresponding micro-pump. The analytical cycle started by intercalating sample and reagent aliquots dispensed through the P1, P2 and P3 pumps (steps 1, 2 and 3). These solutions underwent fast mixing at the interfaces, thus establishing the first sampling cycle. The sequence was repeated five times to form the sample zone and then the flow was stopped for 20 s (step 4). The sample zone was directed to detection at 552 nm by actuation of P4 (step 5). The analytical signal was based on peak height and measurements were taken in triplicate. The analytical response was estimated as the difference between the analytical and blank signals. Valve V1 was used to replace solutions, avoiding passage through the analytical path and minimizing contamination risks. Before analyzing another sample, valve V1 was switched on while pump P1 was actuated (100 pulses) to fill the connection tube with the new sample. Pump P4 was then actuated (50 pulses) to remove the sample aliquot by the carrier through valve V1 (step 6). Valve V2 was used to avoid drawbacks caused by the hydrodynamic impedance of the long optical path flow cell in the sampling step [23]. 3. Results and discussion 3.1. General aspects and system optimization The determination of free chlorine was based on the reaction with DPD recommended by the APHA [8] and the product formed shows a maximum absorption at 552 nm. The flow system with solenoid micro-pumps (Fig. 1) was coupled to a 100-cm optical path flow cell in order to overcome the drawbacks of previous works, namely poor sensitivity, high reagent consumption and effluent generation. A problem observed by coupling the flow system with solenoid micro-pumps to the LCW cell was the significant diminution of the dispensed solution volumes due to the increase in system hydrodynamic impedance and the low torque of the propulsion devices [23]. In some circumstances, the dispensed volume was reduced by 4-fold in relation to the nominal volume. This drawback was circumvented by redirecting the flow during the sampling step by means of actuation of valve V2 (Fig. 1). Taking into account the analytical signal and the magnitude of the blank, the number of pulses of sample, reagent and carrier were optimized, as well as the reagent concentrations. The effects of the reactor coil length and the sample residence time were also evaluated. The effect of the volumetric fraction of sample and reagents was investigated by changing the number of pulses of the corresponding solenoid micro-pumps. This study was carried out using a 50 µg L− 1 hypochlorite solution, 8.4 × 10− 4 mol L− 1 DPD in 0.01 mol L− 1 H2SO4 and phosphate buffer 0.04 mol L− 1 (pH 6.3). The R1 solution was prepared in sulfuric acid medium to prevent degradation of the chromogenic reagent, which occurs rapidly in neutral or alkaline solutions. The oxidation of DPD significantly increased the blank signal due to radiation absorption by the reagent, which is a drawback for measurements with long optical path flow cells [15,24]. In the optimization of the number of pulses of R1, a gradual increase in both analytical and blank signals was observed with up to four pulses of the reagent, yielding an analytical response 40% higher in comparison to that obtained with one pulse of DPD reagent. On the other hand, the analytical signal did not significantly change for a number of pulses of R2 lower than four, but the blank signal decreased up to 7%, by changing from one to four pulses, due to the dilution effect. However, for six pulses of R2, the analytical signal was 21% lower due to dilution of the sample and DPD reagent. Thus, four pulses of R1 and R2 were selected for further studies. At this stage of optimization, the reagents were in large excess in relation to the analyte. Thus, by varying the number of sample pulses,
79
a gradual increase in the analytical signal was observed without affecting the blank, with up to 10 pulses of the hypochlorite solution. Large sample volumes diminished the signal due to dilution of the reagents. The best analytical response, taking into account the difference between the analytical and blank signals, and the magnitude of the blank signal, was obtained with four pulses of DPD solution and phosphate buffer (R1 and R2) and ten pulses of sample. Keeping the same volumetric fraction (2:2:5 for R1, R2 and sample, respectively), the effect of the number of sampling cycles was evaluated. The analytical response increased due to the lower dispersion of the sample zone, with up to five sampling cycles. In this condition, the total sample zone volume (540 µL) was higher than the analytical path volume, estimated as 400 µL. The effect of the DPD concentration was evaluated within the range 4.2 × 10− 4 to 3.4 × 10− 3 mol L− 1 and both the analytical and blank signals increased with the concentration of the chromogenic reagent. For example, with a 3.4 × 10− 3 mol L− 1 DPD solution, the signal was 30% higher in comparison to that obtained with a 4.2× 10− 4 mol L− 1 solution, and the blank signal was 16% higher. By considering these aspects, a concentration of 8.4× 10− 4 mol L− 1 DDP was selected. The effects of acidity and buffer concentration were evaluated using 50 µg L− 1 hypochlorite solution and 8.4 × 10− 4 mol L− 1 DPD in 0.01 mol L− 1 H2SO4. Taking into account its use in the reference method, a phosphate buffer was initially investigated [8]. For a buffer concentration of 0.2 mol L− 1, both analytical and blank signals increased with pH due to the oxidation of the DPD reagent, which is favored in an alkaline medium. The highest difference between the analytical and blank signals was observed at pH = 5.0, which was the lowest pH evaluated in view of the buffering capacity of the phosphate buffer, pKa2 (H3PO4) = 7.21. Evidence of a higher response at a lower pH was also observed when the effect of the buffer concentration was evaluated. The analytical signal decreased gradually at higher concentrations of the phosphate buffer (the analytical response was 50% higher when comparing the signals obtained with 0.1 and 0.5 mol L− 1 buffer concentrations). This effect is due to the change in acidity of the sample zone when solutions with low buffering capacity were used, as a result of the acid present in the solution R1. Thus, other buffer solutions were evaluated: 0.2 mol L− 1 solutions of citrate, acetate and hydrogen phthalate (pH 5.0) yielded similar results. The effect of acetate buffer acidity and concentration was then evaluated. It was observed that by increasing the pH to 5.5, the difference between the analytical and blank signals was about 60% lower in comparison to that obtained at pH 4.0. Lower pH values did not significantly affect the analytical response. The acetate buffer concentration was then varied within the range 0.02 to 0.2 mol L− 1 and the highest analytical response was obtained from 0.08 mol L− 1 concentration. It should be pointed out that previous studies on the determination of free chlorine by the DPD method generally used phosphate buffer at pH 6.3 without critically evaluating the effect of acidity on the chemical reaction [10,12,21]. The effects of sample residence time and reactor coil length were also evaluated and the values were selected by considering the analytical response and the magnitude of the blank signal. The analytical signal was increased by 30% by stopping the flow for 20 s, due to the higher residence time, with a negligible effect on sample dispersion. For free chlorine determination in natural waters this increase is significant, given the low concentration of this species in the samples and the low limits established by legislation. Longer stopping times did not affect the signal, indicating that the steady state condition was attained after a 20-s residence time. The reactor coil length affected the analytical and blank signals due to higher dispersion. However, the decrease in the blank value was more significant and a 75-cm reactor provided the highest analytical response. A summary of the ranges studied and the selected values is presented in Table 2.
80
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
Table 2 Ranges studied and optimized conditions for free chlorine determination. Parameter
Range studied
Selected value
Pulse of sample Pulse of R1 Pulse of R2 Sampling cycles [DPD] (mmol L− 1) [Acetate buffer] (mol L− 1) Buffer pH Reactor (cm) Stopping flow time (s)
1–12 1–6 1–6 1–8 0.42–3.36 0.02–0.2 3.75–5.75 20–100 0–50
10 4 4 5 0.84 0.08 4.00 75 20
3.2. Analytical features and application A linear response was observed within the range 10.0 to 100.0 µg L− 1 hypochlorite, described by equation A = (− 0.0329 ± 0.0031) + (0.0089± 0.0002)C (µg L− 1), r = 0.999 (Fig. 2). The detection limit (99.7% confidence level) and the coefficient of variation (n = 20) were estimated as 6.8 µg L− 1 and 0.9% respectively. The sampling rate was estimated as 60 determinations per hour. The effect of some species that are present in natural waters that 3+ − could interfere in the proposed procedure (SO2− , Ba2+, 4 , NO3 , Fe Mn2+ and Ca2+) was evaluated. The solutions were prepared in a known concentration of hypochlorite (50 µg L− 1) and the results were compared with those obtained with a solution in the absence of concomitants. In a preliminary study, it was observed that Fe3+ and Ca2+ interfered in the determination of free chlorine, requiring masking with EDTA (added to the R1 solution). However, the analytical signal diminished ca. 20 and 60% in the presence of 2.0 × 10− 4 mol L− 1 and 2.0 × 10− 2 mol L− 1 of this complexing agent, respectively. Given the effect of this reagent on sensitivity and the usual concentration of metal ions in natural water samples, the concentration of EDTA was fixed at 2.0 × 10− 5 mol L− 1, which did not affect sensitivity. In the presence of EDTA, a 10-fold excess of iron(III) and a 100-fold excess of the other evaluated species did not affect the analyte determination. Thus, the typical amounts of concomitants in natural waters did not interfere in the determination of hypochlorite by the proposed procedure.
Table 3 Determination of free chlorine in natural and tap water samples. Sample
1 2 3 4 5
ClO− concentration (µg L− 1) Proposed procedure
Reference procedure [8]
79.2 ± 0.2 37.4 ± 0.3 30.4 ± 0.2 44.7 ± 3.1 56.7 ± 0.2
79.1 ± 11.1 28.0 ± 4.9 29.8 ± 5.4 43.9 ± 11.6 58.8 ± 6.0
Natural water samples were analyzed by the proposed and reference [8] procedures and the results are presented in Table 3. The hypochlorite concentrations agreed with those obtained by the reference procedure at the 95% confidence level, according to a paired t-test. However, it should be noted that the determinations by the reference procedure were performed using a cell of 5-cm optical path, due to the low concentration of hypochlorite in the samples. The high deviations in the measurements by the reference procedure were due to the working range being close to the quantification limit. The analytical characteristics of the proposed procedure were better than those obtained in other flow procedures [5,9–13,21,25–28] with spectrophotometric detection (Table 4). In comparison to procedures based on the reaction with DPD [10–12,21], the proposed procedure achieved a lower detection limit and higher sampling rate. This difference is due to the use of low flow rates and long washing times (90 s) [21] in the previously proposed flow systems. In the procedures based on odianisidine [25], the detection limit is ca. 7 times higher than in the proposed procedure, affecting the working range (50 to 1300 µg L− 1). However, when using 4-aminoantipyrine and phenol [26] both the detection limit and the working range are comparable to the proposed system, but the sampling rate is 93% lower due to sample recirculation for analyte enrichment in the acceptor stream. The coefficient of variation of the proposed procedure (0.9%) was lower than that obtained in the procedures listed in Table 4, except in the single line procedure using 4-nitrophenylhydrazine. However, it is important to point out that this procedure lacked sensitivity, once ClO− solutions were 500 times more concentrated than that used in the proposed procedure. In relation to the procedure exploiting long pathlength spectrophotometry [13], which employs a different chemistry, the sampling rate and coefficient of variation were comparable, but the sensitivity was ca. 20-fold higher. The sensitivity of the proposed procedure and, consequently, the response range, are sufficient to comply with environmental legislation [7], in which the maximum allowable total chlorine (free chlorine + combined chlorine) concentration for fresh water is 10 µg L− 1. A comparison of reagent consumption in some procedures for free chlorine determination based on the DPD reaction is presented in Table 5. The consumption of the most toxic reagent (DPD) was 227-fold lower in the proposed method than in the system based on multicommutation with solenoid valves [11]. The consumption was reduced ca. 20,000-fold in relation to the batch method and 97, 79 and 53% in relation to a flow injection system with continuous reagent addition [21], miniaturization [10] and a falling drop [12] system, respectively. Thus, the proposed procedure is inherently greener than those previously reported and can have highly beneficial effects by reducing both costs and waste generation. In addition, other previous studies used highly toxic reagents [26]. 4. Conclusions
Fig. 2. Transient signals obtained for free chlorine reference solutions measured in triplicate. Numbers indicate free chlorine concentrations in µg L− 1. The inset shows the corresponding calibration graph.
By coupling a flow system designed with solenoid micro-pumps to a 100-cm optical path flow cell it was possible to determine free chlorine in samples of natural water supplies that were within the tolerable range in water bodies, as established by environmental legislation, without preconcentration. The proposed procedure is more environmentally
W.R. Melchert et al. / Microchemical Journal 96 (2010) 77–81
81
Table 4 Analytical features of spectrophotometric flow-based procedures for free chlorine determination. System
Reagent
LD (µg L− 1)
Linear range (µg L− 1)
Sampling rate (h− 1)
Coefficient of variation (%)
Ref.
FIA FIA FIA FIA + gas diffusion FIA + gas diffusion FIA miniaturized SIA SIA Multicommutation — solenoid valve Multicommutation — solenoid valve Multicommutation — solenoid valve + LCW Proposed procedure
DPD 3,3′-Dimethylnaphtidine 4-Nitrophenylhydrazine o-Dianisidine 4-Aminoantipyrine + phenol DPD o-Tolidine Tetramethylbenzidine DPD DPD o-Tolidine DPD
70 30 400 50 0.5 165 200 80 510 4500 – 6.8
100–8000 100–1000 1000–40,000 50–1300 5–250 125–1700 b 5000 90–1300 1400–9700 15,000–100,000 70–1000 10–100
– 150 110 38 4 – 11 60 45 20 45 58
3.0 1.2 0.6 1.5 3.1 – – – 1.4 2.5 1.0 0.9
[21] [27] [9] [25] [26] [10] [28] [5] [11] [12] [13] –
FIA = flow injection analysis; SIA = sequential injection analysis; LCW = liquid core waveguide.
Table 5 Reagent consumption in different procedures for free chlorine determination based on DPD method. Procedure
DPD amount (mg/determination)
Reference
Batch FIA with confluent streamsa FIA miniaturized Multicommutation with solenoid valves Flow system with falling drop Proposed procedure
500 0.8 0.083 5.9 0.055 0.026
[8] [21] [10] [11] [12] –
a
Estimate by considering a sampling rate of 100 measurements per hour.
friendly given the drastic reduction in reagent consumption (micrograms per determination). This analytical methodology employs the same reagents used in procedures carried out by control agencies, which will facilitate the implementation of the developed procedure for monitoring this species in natural waters. Acknowledgements The authors acknowledge fellowships and financial support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). D.L. Rocha is thanked for critical comments. References [1] http://www.who.int, accessed in 11/19/2009. [2] B. Saad, W.T. Wai, M.S. Jab, W.S.W. Ngah, M.I. Saleh, J.M. Slater, Development of flow injection spectrophotometric methods for the determination of free available chlorine and total available chlorine: comparative study, Anal. Chim. Acta 537 (2005) 197–206. [3] Z. Qiang, C.D. Adams, Determination of monochloramine formation rate constants with stopped-flow spectrophotometry, Environ. Sci. Technol. 38 (2004) 1435–1444. [4] J.C. Kotz, P.M. Treichel Junior, P.A. Harman, Chemistry & Chemical Reactivity, fifth ed Thomson, United States, 2003. [5] R.B.R. Mesquita, M.L.F.O.B. Noronha, A.I.L. Pereira, A.C.F. Santos, A.F. Torres, V. Cerdà, A.O.S.S. Rangel, Use of tetramethylbenzidine for the spectrophotometric sequential injection determination of free chlorine in waters, Talanta 72 (2007) 1186–1191. [6] http://www.epa.gov, accessed in 07/16/2008. [7] http://www.mma.gov.br/port/conama/, accessed in 02/27/2009. [8] A.D. Eaton, L.S. Clesceri, A.E. Greenberg, Standard Methods for the Examination of Water and Wastewater, 19th ed., American Public Health Association, Washington, DC, 1995.
[9] K.K. Verma, A. Jam, A. Townshend, Determination of free and combined residual chlorine by flow-injection spectrophotometry, Anal. Chim. Acta 261 (1992) 233–240. [10] K. Carlsson, L. Moberg, B. Karlberg, The miniaturisation of the standard method based on the N, N′-diethyl-p-phenylenediamine (DPD) reagent for the determination of free or combined chlorine, Water Res. 33 (1999) 375–380. [11] F.H. Salami, V.G. Bonifácio, G.G. Oliveira, O. Fatibello-Filho, Spectrophotometric multicommutated flow system for the determination of hypochlorite in bleaching products, Anal. Lett. 41 (2008) 3187–3197. [12] S.S. Borges, B.F. Reis, An automatic falling drop system based on multicommutation process for photometric chlorine determination in bleach, Anal. Chim. Acta 600 (2007) 66–71. [13] F.H. Salami, V.G. Bonifácio, O. Fatibello-Filho, L.H. Marcolino Junior, Determinação espectrofotométrica em fluxo de cloro em água usando célula de longo caminho óptico e multicomutação, Quim. Nova 32 (2009) 112–115. [14] T. Dallas, P.K. Dasgupta, Light at the end of the tunnel: recent analytical applications of liquid-core waveguides, Trends Anal. Chem. 23 (2004) 385–392. [15] K.O. Lupetti, F.R.P. Rocha, O. Fatibello-Filho, An improved flow system for phenols determination exploiting multicommutation and long pathlength spectrophotometry, Talanta 62 (2004) 463–467. [16] R.D. Waterbury, W. Yao, R.H. Byrne, Long pathlength absorbance spectroscopy: trace analysis of Fe(II) using a 4.5 m liquid core waveguide, Anal. Chim. Acta 357 (1997) 99–102. [17] F.R.P. Rocha, J.A. Nóbrega, O. Fatibello Filho, Flow analysis strategies to greener analytical chemistry. An overview, Green Chem. 3 (2001) 216–220. [18] F.R.P. Rocha, B.F. Reis, E.A.G. Zagatto, J.L.F.C. Lima, R.A.S. Lapa, J.L.M. Santos, Multicommutation in flow analysis: concepts, applications and trends, Anal. Chim. Acta 468 (2002) 119–131. [19] F.R.P. Rocha, P.B. Martelli, B.F. Reis, A multicommutation-based flow system for multi-element analysis in pharmaceutical preparations, Talanta 55 (2001) 861. [20] W.R. Melchert, C.M.C. Infante, F.R.P. Rocha, Development and critical comparison of greener flow procedures for nitrite determination in natural waters, Microchem. J. 85 (2007) 209–213. [21] G. Gordon, D.L. Sweetin, K. Smith, G.E. Pacey, Improvements in theN, N-diethyl-pphenylenediamine method for the determination of free and combined residual chlorine through the use of FIA, Talanta 38 (1991) 145–149. [22] B. Chiswell, K.R. O´Halloran, Acid yellow 17 as a spectrophotometric reagent for the determination of low concentrations of residual free chlorine, Anal. Chim. Acta 249 (1991) 519–524. [23] W.R. Melchert, F.R.P. Rocha, A greener and highly sensitive flow-based procedure for carbaryl determination exploiting long pathlength spectrophotometry and photochemical waste degradation, Talanta 81 (2010) 327–333. [24] C.M.C. Infante, F.R.P. Rocha, A critical evaluation of a long pathlength cell for flowbased spectrophotometric measurements, Microchem. J. 90 (2008) 19–25. [25] M.C. Icardo, J.V.G. Mateo, J.M. Calatayud, Selective chlorine determination by gas diffusion in a tandem flow assembly and spectrophotometric detection with odianisidine, Anal. Chim. Acta 443 (2001) 153–163. [26] G. Jin, J. Yang, J.F. Li, Determination of chlorine dioxide using capillary on-line concentration coupled with flow injection analysis, Microchim. Acta 148 (2004) 171–175. [27] E. Pobozy, K. Pyrzynska, B. Szostek, M. Trojanowicz, Flow-injection spectrophotometric determination of free residual chlorine in waters with3, 3′-dimethylnaphtidine, Microchem. J. 51 (1995) 379–386. [28] J.G. March, M. Gual, B.M. Simonet, Determination of residual chlorine in greywater using o-tolidine, Talanta 58 (2002) 995–1001.