Hindawi Publishing CorporationJournal of Automated Methods and Management in ChemistryVolume 2011, Article ID 463286, 6 pagesdoi:10.1155/2011/463286
Research Article
An Environmental Friendly Procedure for PhotometricDetermination of Hypochlorite in Tap Water Employinga Miniaturized Multicommuted Flow Analysis Setup
Sivanildo S. Borges1 and Boaventura F. Reis2
1 Centro de Ciencias Exatas e Tecnologicas, Universidade Federal do Reconcavo da Bahia, Centro,44380-000 Cruz das Almas BA, Brazil
2 Centro de Energia Nuclear na Agricultura, Universidade de Sao Paulo, Avenida Centenario, 303 Sao Dimas,13400 970 Piracicaba SP, Brazil
Correspondence should be addressed to Boaventura F. Reis, [email protected]
Received 6 October 2010; Revised 7 March 2011; Accepted 9 March 2011
A photometric procedure for the determination of ClO− in tap water employing a miniaturized multicommuted flow analysis setupand an LED-based photometer is described. The analytical procedure was implemented using leucocrystal violet (LCV; 4,4′, 4′′-methylidynetris (N,N-dimethylaniline), C25H31N3) as a chromogenic reagent. Solenoid micropumps employed for solutionspropelling were assembled together with the photometer in order to compose a compact unit of small dimensions. After controlvariables optimization, the system was applied for the determination of ClO− in samples of tap water, and aiming accuracyassessment samples were also analyzed using an independent method. Applying the paired t-test between results obtained usingboth methods, no significant difference at the 95% confidence level was observed. Other useful features include low reagentconsumption, 2.4 μg of LCV per determination, a linear response ranging from 0.02 up to 2.0 mg L−1 ClO−, a relative standarddeviation of 1.0% (n = 11) for samples containing 0.2 mg L−1 ClO−, a detection limit of 6.0 μg L−1 ClO−, a sampling throughputof 84 determinations per hour, and a waste generation of 432 μL per determination.
1. Introduction
Since the use of chlorine for tap water treatment was adoptedin England in the 1880s [1], water chlorination has beenone of the most common disinfectant methods used bywaters suppliers [2]. Because chlorine is an efficient agentfor inactivating several types of microorganisms, it has beenpreferred as a disinfecting agent to assure the bacteriologicalquality of the drinking water [3]. The water bacterialcontamination can also occur in the distribution network;therefore, to prevent this occurrence, a free chlorine residualin excess of 0.2 mg L−1 must be maintained throughoutthe distribution lines [3, 4]. Therefore, the availabilityof sensitive and reliable analytical procedure for chlorinedetermination can be considered as an essential condition toassure the quality of tap water. Determination of free chloride
in tap water has been carried out by employing as detectiontechniques amperometry [5], spectrophotometry [6], ionselective electrode [7], chemiluminescence [8], and so forth.
In this work, we intend to develop a photometric proce-dure for the determination of residual chlorine in tap waterusing leuco crystal violet (LCV; 4,4′, 4′′-methylidynetris(N,N-dimethylaniline), C25H31N3) as chromogenic reagent.LCV is colorless in aqueous solution, but when oxidized atpH 4, it forms crystal violet dye (CV+), which absorbs elec-tromagnetic radiation with a maximum at 592 nm [9, 10].LCV has been employed in procedures for the determinationof hydrogen peroxide [11], iodine, iodide and hypoiodousacid [9, 12, 13], iridium [14], and oxidized manganese[10]. Although, LCV has been widely used in analyticalprocedures, it has not yet been used for hypochloritedetermination, which will be attempted in this investigation.
2 Journal of Automated Methods and Management in Chemistry
Nowadays, efforts have been made to environmentallyfriendly develop analytical procedures, and thus accomplish-ing the Green Analytical Chemistry (GAC) recommendation[15–17]. Reduction of reagent consumption and waste gen-eration are among the requisites necessary to attain the GACrecommendation [18, 19]. Multicommuted flow analysis(MCFA) [20–22] and multisyringe flow injection analysis(MSFIA) [18, 23] are efficient tools for handling low volumesof sample and reagent solutions, thus providing facilities tosave them. Solenoid minipumps have been employed to pro-pel solution in flow analysis systems replacing the peristalticpump, presenting as an advantage small dimension [24–26],a feature that has been exploited in order to downscale theflow system [27, 28]. This downscaling approach seems tobe a powerful tool to develop analytical procedure focusedto the GAC recommendation. In the research reported here,these features are exploited to design a miniaturized flowsystem setup based on the MCFA process for photometricdetermination of hypochlorite in tap water.
2. Experimental
2.1. Reagents Solutions. All chemicals were of analytical-grade reagent. Purified water with electric conductivity lowerthan 0.1 μS cm−1 was used throughout.
An acetate buffer solution (0.2 mol L−1) was preparedby dissolving 1.6408 g of sodium acetate in 20 mL of water.After dissolution, pH was adjusted to 4.0 using a 2.0 mol L−1
HCl solution. Afterwards, the volume was made up to100 mL with water. A 1.0 g L−1 leuco crystal violet (LCV;4,4′, 4′′-methylidynetris (N,N-dimethylaniline), C25H31N3)stock solution was prepared by dissolving 0.1 g of solidmaterial (Eastman Kodak) in 1 mL of phosphoric acid(Merck) 85% (v/v). After dissolution, the volume was madeup to 100 mL with water. A 0.1 g L−1 LCV working solutionwas prepared daily by adequate dilution of the stock solutionwith water.
A 1000 mg L−1 hypochlorite (ClO−) stock solution wasprepared by adequate dilution of a 10% (w/v) reagentsolution (Fluka) in 100 mL of a 0.01 mol L−1 NaOH solution.This solution was standardized using the iodometric titrationmethod [29]. Hypochlorite working solutions ranging from0.02 up to 2.0 mg L−1 were prepared daily by adequatedilution of the ClO− stock solution using a 10−4 mol L−1
NaOH solution.A set of tap water samples was collected from several
points of Piracicaba City. Prior to analysis, samples werealkalized by adding 1.0 mL of a 10−1 mol L−1 NaOH solutionto a volumetric flask, and the volume was made up to 100 mLwith sample.
2.2. Equipment. The apparatus included a microcomputerrunning a software written in QuicH BASIC 4.5, a digitalmultimeter with a serial interface (Minipa, ET-2231), threesolenoid minipumps (Bio-Chem Valve Inc., 090SP115-8), ahomemade interface to drive the solenoid pumps [30], whichwas coupled to the microcomputer through the paralleloutput port, a homemade regulated power supply (−12 V,+12 V) to feed the photometer, an orange high-bright LED
(λ = 590 nm), and a photodiode OPT301 (Burr Brawn). Ahomemade flow cell machined in acrylic, with an opticalpath length of 20 mm and an inner diameter of 1.0 mm,and a homemade bubble-removing microdevice machinedin acrylic, with an inner volume of 10 μL, was similar tothat used elsewhere [28]. All flow lines were of Tygon tubingwith 0.56 mm inner diameters. The system control and dataacquisition were performed by the microcomputer, runningsoftware written in Quick BASIC 4.5.
2.3. Analytical Procedure. The diagram of the proposed setupis shown in Figure 1. The network constituted by a transistor(Te), and resistors were used to control the intensity ofthe light beam emitted by the LED. The glass cylinders (g)conducted the light beams I1 and I2 from the LED up to theflow cell channel, and from there up to the photodetector(Det), respectively. When the software was run, the solenoidminipumps P1, P2, and P3 were switched in an alternatingOn/Off sequence, as shown in Table 1, so as to fill each flowline with the respective solution. Afterwards, mini-pump P2
was switched On/Off 40 times to wash the flow cell withcarrier solution. The calibration step included the reading ofboth the dark signal (Dk) and the reference signal (Ss). In thefirst case, the reading was done with LED switched Off while,in the second one, the LED emission was enabled by turningforward the variable resistor (10 kΩ) wired to the base of thetransistor (Tr). The emission intensity was adjusted to obtainan electric potential difference (Ss) of 2000 mV generatedby the photodetector (Det). The measurements Dk and Sswere converted to digital signal by the multimeter and sentto the microcomputer through the serial interface. Thesemeasurements were used for the absorbance calculation.
A micropumping On/Off switching event is named asa sampling cycle, thus the volumes of sample and reagentssolutions necessary to carry out the analysis were found byvarying the number of sampling cycles. As shown in Table 1(step d), micro-pumps P1 and P2 were switched simulta-neously, so that the coil (B1) was loaded with a mixtureof reagent and buffer solutions. Afterwards (step e), micro-pumps P2 and P3 were switched On/Off also simultaneously,so that slugs of the sample and a mixture of buffer andreagent solutions merged into the coil (B2). The reaction forforming the compound to be monitored proceeded while thesample zone was displaced towards the photodetector (Det).While steps (d) and (f) were performed, the microcomputercarried out data acquisition and saved this data as anASCII file, to allow further processing. The absorbance wascalculated using the following equation: Absorb =− log[(S0−Dk)/(Ss − Dk)], where S0 was the sample signal, with Ss andDk defined as in the previous paragraph.
While sample processing was occurring, a plot of theanalytical signal was displayed on the screen, in order to allowits visualization in real time. After the last event (step f), thesoftware returned to step (d), so as to start another analyticalrun.
To find the adequate volume of reagent solution, theassays were carried out using 20 sampling cycles for sampleand varying from 1 up to 5 the On/Off switching eventapplied to minipumps P1 and P2 (step e).
Journal of Automated Methods and Management in Chemistry 3
Te
W
R
S
bd
TmAp
h
Bf
fc
470
312
58
Int
Dm
10 k
10 k
12 V
−12 V
+12 V
P2
P3
B3
S0
I2I1
B2B1
P1
Mic
LED
Det20MΩ
g g
Figure 1: Diagram of the flow analysis module. S = sample or ClO− standard solution; R = 0.1 g L−1 LCV solution; Bf = acetate buffer,pH 4.0; Int = homemade power driver interface; Mic = microcomputer; Dm = digital multimeter; P1, P2, and P3 = solenoid minipumps;B1, B2, and B3 = flow lines, Tygon tubing 40, 15, and 25 cm long, respectively, and 0.56 mm inner diameter; bd = bubble removing device;Ap = acrylic plates, length, width and thickness of 40, 30, and 10 mm, respectively; h = holes, 2.0 mm diameter; Tm = Teflon membrane; fc =cutaway of the flow cell; W = waste; g = glass cylinders, 30 mm length and 1.0 mm diameter; LED = λmax of 590 nm, high bright (10,000 mcd);Te = transistor BC547; Det = photodetector OPT301; I1 and I2 = electromagnetic radiation beam from LED and to detector, respectively;S0 = signal output (mV).
The assays described above were carried out using a chro-mogenic reagent solution with a concentration of 0.1 g L−1
LCV and a 1.0 mg L−1 ClO− standard solution. To findthe appropriate reagent concentration, experiments wereperformed applying 12 and 8 sampling cycles for sample andreagent solution, respectively. Maintaining the hypochloriteconcentration at 1.0 mg L−1, the LCV concentration wasvaried from 0.02 up to 0.1 g L−1.
Aiming to prove the effectiveness of the proposed setup,a set of tap water samples was analyzed. To allow accuracyassessment, samples were also analyzed using a referenceprocedure [31]. In this procedure, hypochlorite in a pH 6.3medium reacts with the chromogenic reagent N,N′-diethyl-p-phenylenediamine (DPD), which is oxidized to a red-colored semiquinonoid cationic radical (DPD∗) that wasmonitored at 520 nm.
3. Results and Discussion
3.1. General Comments. According to Hatch [9] and Lambertet al. [13], the stoichiometric relationship of LCV oxidationby hypoiodous acid or iodine in acid medium results in
the generation of two electrons for each oxidized molecule,as indicated by the equations shown below:
HIO + LCV −→ I− + CV+ + H2O, (1)
I2 + LCV −→ 2I− + CV+ + H+, (2)
where LCV is leuco crystal violet and CV+ is the coloredcrystal violet specie.
Since LCV could be oxidized by hypochlorite, followinga reaction mechanism similar to that described for HIO, onewould expect that a relationship according to the equationshown below occurs:
HClO + LCV −→ Cl− + CV+ + H2O. (3)
Preliminary assays carried out using hypochlorite work-ing solutions showed that absorbance at 592 nm was directlyproportional to hypochlorite concentration, thus this featurewas employed to develop the analytical procedure.
Reduction of reagent consumption and waste generationare among the requisites necessary to attain the GACrecommendation [18, 19], and the volumes of solutions
4 Journal of Automated Methods and Management in Chemistry
Table 1: Minipumps switching pattern.
Step Event P1 P2 P3 Cycles Volume (μL)
(a) Filling flow lines On Off Off 30 360
(b) Washing flow cell Off On Off 40 240
(c) Photometer calibration Off Off Off 0 —
(d) DPD and acetate buffer insertion On On Off 4 48
(e) Sample insertion signal reading Off On On 12 144
(f) Signal reading Off On Off 40 240
The number 1 and 0 indicated that respective mini-pump is switched ON or OFF, respectively. Cycles indicate the selected number of times that each valvewas switched On/Off.
0.31
0.32
0.33
0.34
0.35
0.36
0.37
Volume (µL)
Abs
orba
nce
6 12 18 24 30
Figure 2: Effect of chromogenic reagent volume on the analyticalsignal.
delivered by the solenoid micropumps per stroke have beenused to calculate reagent consumption and waste generation[28]. Nominally, the solenoid micropumps used in thiswork should deliver a solution volume of 8 μL per stroke,nevertheless, in an earlier work, it was verified that volumedelivered per stroke was lower than the expected value [28].Thus, considering this fact, the volume of solution deliveredby stroke under working condition was determined, and wasdone by applying for each micropump 10 On/Off pulses.The water volume was collected into a vial and weighted todetermine the actual volume, which was found to be 6.0 μLper stroke.
3.2. Effect of the Reagents Concentration. The effect con-cerning the LCV concentration was investigated using a1.0 mg L−1 ClO− standard solution and applying 12 and 8sampling cycles for sample and reagent solution, respectively.The LCV concentration varied from 0.02 up to 0.10 g L−1.The absorbance increased up to a concentration of 0.08 g L−1
and showed a tendency to a constant value for higherconcentration. Thus, the 0.10 g L−1 solution was selected, soas to assure an excess of reagent throughout the sample zone.
3.3. Effect of the LCV Solution Volume. The assays com-mented on in previous sections were carried out by insertingaliquots of both LCV and buffer solution, each one with
0
volume (µL)
Abs
orba
nce
0 12 24 36 48 60 72 84 96 108
0.1
0.2
0.3
0.4
Sample
Figure 3: Effect of sample volume on the analytical signal. Standardsolution 1.0 mg L−1 ClO−; 24 μL of reagent and buffer solution.
a volume of 48 μL and an aliquot of sample solution with avolume of 72 μL. As is shown in Table 1 (step e), micropumpsP2 and P3 were also switched On/Off 12 times, therefore, thevolume of the sample zone was 144 μL. Aiming to minimizethe reagent consumption, a set of experiments was carriedout by varying the volume of the solution aliquot from 6up to 30 μL, which was done by varying the number ofsampling cycles (Table 1, step d) from 1 to 5. The curvein Figure 2 shows that for volumes higher than 18μL, thevariation of the analytical signal was not significant. Thus,the volume of 24 μL was selected as a compromise, in orderto save reagent solution while, on the other hand, assuring asufficient amount for allowing the reaction development tooccur.
3.4. Effect of Sample Volume. With the objective of verify theeffect caused by the sample volume on the analytical signal,assays were carried out by varying the volume of samplealiquot from 6 up to 102 μL, yielding the results shown inFigure 3. Analyzing this curve, we observe that for volumeshigher than 66 μL, the signal increment decreased step bystep. Thus, we would expect that signal magnitudes tendtoward a constant. When sample volume varied within therange of 18 to 60 μL, a linear relationship (R2 = 0.998) wasachieved. This behavior is quite different from that observedin the usual FIA system employing injection by loop. In order
Journal of Automated Methods and Management in Chemistry 5
to achieve a compromise between sensitivity and throughput,a sample volume of 72 μL was selected.
3.5. Performance of the Proposed System. Once the opera-tional conditions of the system were established, a set ofassays was carried out, in order to verify the overall perfor-mance of the system. The results achieved are compared withother procedures and are summarized in Table 2. As we cansee, the proposed procedure presented useful features: a widelinear response range, a low limit of detection, which wasestimated as three times standard deviation of blank dividedby the slope of the analytical curve, high throughput, lowvolume of waste generated, and low reagent consumption.Comparing these data with those presented in the referencedpapers [32–34], we observe that, excepting throughput (see[33]), the results are highly favorable for the proposedsystem.
An additional assay was carried out in order to verifywhether LCV also reacted with combined chlorine. Theassay was done using two sets of standard solutions. Thefirst one containing 1.0 mg L−1 ClO− plus 1.0 mg L−1 NH+
4
and 2.0 mg L−1 ClO− plus 2.0 mg L−1 NH+4 ; the second
one, prepared to contain equal ClO− concentration withoutammonium. In the first case, absorbances generated process-ing both solutions were ≈97% lower than those observedwhen standard solutions were prepared without ammonium.Therefore, we could consider these results as an indicationthat the LCV does not reacted with combined chloride.
3.6. Sample Analysis. A set of tap water samples wasanalyzed, employing the operational conditions shown inTable 1, yielding the results shown in Table 3. In order tomake accurate assessment, the samples were also analyzedusing a reference method [30]. Student’s paired t-testcalculated for the values obtained by both procedures forthe 95% confidence level was found to be 1.360. Since thetheoretical value is 2.447, there is no significant difference atthe 95% confidence level.
4. Conclusions
The performance of the proposed setup proved that mul-ticommuted flow injection analysis (MCFA), implemented
Table 3: Comparison of results obtained by proposed procedureand the reference method.
SampleHypochlorite (mg L−1)
Proposed DPD [31]
A 0.471 ± 0.008 0.462 ± 0.005
B 0.026 ± 0.001 0.026 ± 0.001
C 0.026 ± 0.00 0.027 ± 0.001
D 0.471 ± 0.004 0.428 ± 0.005
E 0.274 ± 0.006 0.270 ± 0.005
F 0.027 ± 0.001 0.026 ± 0.000
Results are average of the four consecutive sample analyses. No significantdifference at 95% confidence level, ttabled = 2.447, tcalculated = 1.360.
employing solenoid minipumps to propel solutions, affordedfacilities for downscaling the flow setup. Its reduced dimen-sion allowed a drastic reduction of reagent consumptionand waste generation, without any sacrificing of precision,accuracy, and throughput.
Although the LCV had been employed for many ana-lytical purposes, it had not yet been used for hypochloritedetermination. In this work, we proved that LCV can be usedas a chromogenic reagent for the photometric determinationof ClO− in tap water.
Acknowledgments
The authors acknowledge the CNPq, CAPES, FAPESP,PRONEX/FAPESB, and CNPq/INCTAA.
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