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Cite this: Environ. Sci.: Water Res.

Technol., 2016, 2, 693

Received 25th November 2015,Accepted 2nd April 2016

DOI: 10.1039/c5ew00276a

rsc.li/es-water

A new dipstick colorimetric sensor for detectionof arsenate in drinking water

Joyati Das and Priyabrata Sarkar*

The present work reports on a polymer hydrogel-based cost effective colorimetric dipstick sensor for

arsenicĲV). The method was based on the formation of a blue colored antimonyl–arseno–molybdate com-

plex in the presence of ammonium molybdate, potassium antimonyl tartrate and ascorbic acid. All these

reagents were encapsulated in a polymer hydrogel made of polyvinyl alcohol, acrylamide and glutaralde-

hyde. Plastic detector strips were dip coated with this hydrogel. Scanning electron microscopy (SEM)

depicted the highly porous structure of the polymer hydrogel permitting adsorption of AsĲV) confirmed by

energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and Fourier transform

infrared spectroscopy (FTIR) analysis. The advantages were low sample volume (10 ml), low detection limit

(10 μg L−1 by the naked eye), good stability (4 months), low cost (0.03$ per test for visual detection), and

high reproducibility. The sensor strips displayed selectivity in the presence of

694 | Environ. Sci.: Water Res. Technol., 2016, 2, 693–704 This journal is © The Royal Society of Chemistry 2016

Liu et al.17 developed an arsenic detection procedurethrough adsorption of DNA by iron oxide nanoparticles. Re-cently Siegfried et al.18 reported field testing of arsenic usinglyophilized bioreporter bacteria but preparation of the lyophi-lized bioreporter bacteria made this process tedious and timeconsuming.

The most well known analytical technique for field sam-ples employed for arsenic determination at the microlevel isthe spot test, the Gutzeit method.19 It involves the formationof a colored complex due to formation of arsine by reductionof arsenic under acidic conditions, either with mercuric bro-mide20 or with silver diethyldithiocarbamate.21 The advan-tage of this method is the elimination of expensive instru-ments and it can be employed in kit form for the analysis ofarsenic. Moreover, the method is sensitive, rapid and can beperformed easily.19 The major drawback is that the concen-trated hydrochloric acid used in this method is highly corro-sive and thereby deals with handling and transportationproblem.

The very well known Merck (Mumbai, India) kit (based onthe Gutzeit method; Merck Arsenic Test method: colorimetricwith test strips, 0.005–0.5 mg L−1 As3+/5+, prod.: 1.17927.0001,batch: HC80062, Merckoquant®) and Hach (Loveland, CO,USA) kit22 are widely used all over the world for arsenic detec-tion in groundwater and have a detection capability of 5–500μg L−1 of arsenic and both of them use solid sulfamic acid in-stead of concentrated hydrochloric acid.

Baghel et al.22 reported generation of arsine by reactingmagnesium turnings with oxalic acid and this arsine reducedauric chloride into metallic gold which appeared as a pink-violet color. However, the use of costly auric chloride showedthe major drawback of the process.

In the recent past, Das et al.23 developed two test kits fordetecting total arsenic in microgram levels in water by usingmercuric bromide and silver nitrate as the detector elements.Kearns et al.24 reported some improvement in the accuracyand precision of the Hach EZ test kit for quantifying inor-ganic arsenic concentration in drinking water by using digitalanalysis of the color developed on the detector strip. The dig-ital analysis would produce fewer false positive and false neg-ative results but running the test process for 24 h made theprocedure time consuming. Arsenator® Digital Arsenic TestKit commercialized by Wagtech, Palintest Ltd, United King-dom is also a very well known digitised arsenic detection kitavailable in the market but measurement is costly by this kit.Sharma et al.25 developed a disposable sensor for quantita-tive detection of arsenic using MATLAB software. But the low-est detection limit of this process was 18 μg L−1 and unfortu-nately it is greater than the maximum contamination level ofarsenic in groundwater (10 μg L−1) as guided by the WHO.Thus a significant opportunity is there for the developmentof a low-cost and environmental friendly simple colorimetricdetection of arsenic in ground water samples.

The kits available in the market measure total arsenic i.e.both AsĲV) and AsĲIII). The present study focuses on the detec-tion of only arsenate. Hydrogels are polymeric networks

consisting of crosslinked hydrophilic polymers that can ab-sorb and retain a large amount of water within them. Poly-acrylamide (PAM) has a typical three-dimensional (3D) net-work structure and it absorbs a large amount of water.PAM may be synthesized by a simultaneous crosslinkingpolymerization procedure using initiators like potassiumperoxydisulphate. PolyĲvinyl alcohol) (PVA) being an inexpen-sive biodegradable synthetic polymer, exhibits attractive fea-tures such as high transparency, very good flexibility, chemi-cal resistance, non toxicity and wide commercial availabilityand the presence of hydroxyl groups imparts the property ofhigh amount of water absorption. The properties of PVAhydrogels can be controlled by varying its crosslinking den-sity by agents like glutaraldehyde, acetaldehyde, formalde-hyde, etc.

Molybdenum forms the so called arsenomolybdate com-plex with arsenate to give an intense blue color under reduc-tive conditions. Since the development of this blue color isvery stable and corresponds to the concentration of the arse-nate ion, many researchers tried quantitative analysis of AsĲV)by spectrophotometry.20,26 Recently Okazaki et al.27 reporteda portable device equipped with two attachments of a CaCO3cartridge and a molybdenum loaded membrane holder to de-velop visual colorimetry for determination of trace arsenic ingroundwater. Although the minimum detection limit was 5μg L−1, use of a CaCO3 cartridge along with membrane filtersmade this process costly. Up to date, no report is available onthe use of a dipstick colorimetric procedure for AsĲV) and theperformance evaluation by digital analysis.

In the present study, we developed a colorimetric dip stripwhich turns blue in the presence of AsĲV) in water. The stripwas prepared by encapsulation of ammonium molybdate in aPAM–PVA hydrogel and used for the adsorption and detec-tion of AsĲV). The sensing was validated with results from theconventional spectrophotometric method and AAS at roomtemperature. The evaluation could be made by the naked eyeas well as digital and statistical analyses of the color of thedipstick strips in the presence of analyte i.e. AsĲV). The mini-mum detection level by visual observation (naked eye) is 10μg L−1 of AsĲV).

Materials and methodsChemicals and solutions

PAM was obtained from SRL, India. Ammonium molybdatetetrahydrate, potassium antimonyĲIII) tartrate hemihydrates,glutaraldehyde, and sulfuric acid were supplied by Merck,India. PVA and L-ascorbic acid were procured from LobaChemie, India. Working standard solutions of AsĲV) (10–250μg L−1) were prepared by stepwise dilution of stock solutionsof AsĲV) by dissolving sodium arsenate heptahydrate(Na2HAsO4·7H2O) (Merck) into 100 ml of water.

Preparation of polymer hydrogel

A 2.5% PVA (i.e. 0.5 g) and 15% PAM (i.e. 3 g) polymer mix-ture was first prepared by heating the polymer mixture in 20

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http://dx.doi.org/10.1039/c5ew00276a

Environ. Sci.: Water Res. Technol., 2016, 2, 693–704 | 695This journal is © The Royal Society of Chemistry 2016

ml of deionised (18.2 MΩ) water at 80 °C temperature untilthe solution became transparent.

Preparation of reagent

A reagent consisting of 2 ml of deionised water, 2 ml of 3%(24 mM) ammonium molybdate tetrahydrate, 1 ml of 0.56%(8 mM) potassium antimonyĲIII) tartrate hemihydrate and 5ml of 13.98% (2.5 M) sulfuric acid was prepared.

Preparation of detector strip

1 ml of this reagent was mixed with 400 μl of a PAM–PVApolymer hydrogel mixture and 20 μl of 12.5% glutaraldehyde(diluted from 25% stock) to make a stable reagent–polymermixture. Plastic detector strips were dip coated (HWTL 0.01Dip Coater, MTI corporation, USA) with this mixture anddried in an oven at 45 °C for 30 minutes.

Characterization

SEM and FTIR analysis. SEM analysis was performed byusing an EVO-18 Special Edition-CARL ZEISS (Germany) in-strument. The ammonium molybdate loaded PAM–PVA hy-drogel (MLPH) coated plastic strips were immersed in 10 mlof arsenic-free tap water and also in spiked water (with 250μg L−1 of arsenate) for 2 h to reach the maximum swellingequilibrium of the hydrogel. The swollen hydrogel sampleswere subsequently freeze-dried and lyophilized followedby SEM morphological investigation. The samples weremounted on a metal stub coated with platinum, the surfaceswere observed and photographed by SEM. The adsorption ofarsenic within the polymer hydrogel was also monitored byTEM (TEM model: JEOL JEM 2100 HR, Oxford Instruments,United Kingdom). FTIR analysis was conducted using aThermo Scientific Nicolet is10 (UK) instrument.

EDX analysis. An EDX analyser (Oxford Instruments, INCApenta FET X 3, United Kingdom) was used to obtain the com-position of molybdenum loaded PAM–PVA composite hydro-gel before and after adsorption of arsenate.

Effect of L-ascorbic acid (%) (reducing agent) concentration

The performance of the molybdenum blue-based polymer hy-drogel dipstick colorimetric system was optimized with re-spect to L-ascorbic acid (0.02%–0.15%) concentrations. It wasfound that treatment of 10 mg of L-ascorbic acid in 10 ml ofAsĲV) solution (0.1%) gave the best result (details arediscussed in the Results and discussion section).

Detection procedure of arsenicĲV)

10 ml of the sample [including blank tap water and AsĲV) con-taminated water] was poured into a 20 ml vial. It was treatedwith 10 mg of L-ascorbic acid. MLPH coated plastic stripswere dipped into vials containing different AsĲV) concentra-tions including blank [no AsĲV)] and kept at room tempera-ture for 20 minutes. The presence of arsenicĲV) in the samplesolution was detected by comparing the color with standard

chart already obtained with standard AsĲV) samples. Theintensity of the color was also measured by RGB analysis ofthe image of the color. The details of the procedure aredepicted in Fig. 1.

Field study

The dipstick sensor was also applied to test some field sam-ples and the comparison was performed with AAS as well asthe conventional spectrophotometric method.

Data analysis

All data were represented as mean ± SD (n = 3). The imagesof the dipsticks under different reaction conditions were cap-tured using a SONY Cybershot digital camera. For analyzingcolor values in the RGB (red, green, blue) system, digital im-ages were fitted into Adobe Photoshop 6 software, three rep-resentative areas on each test strip were analyzed and theRGB values were obtained. The effective intensity of the RGBvalues was subsequently calculated by subtracting the corre-sponding values obtained from blank i.e. water sample with-out any arsenate.

Proof of concept

Any color can be analyzed by its corresponding R, G and Bvalues. The effective intensity (AX) for any color valuesobtained from the color spots of the detector strips was cal-culated as follows:28

Ar = − log(Rs/Rb) (1)

Ag = −log(Gs/Gb) (2)

Ab = −log(Bs/Bb) (3)

where Rs, Gs, Bs and Rb, Gb, Bb are the average (n = 3) red,green and blue values obtained for the test sample and blank[no AsĲV)], respectively.

Interference study

The only interfering agent could be the phosphate ion. Am-monium molybdate forms a similar complex with phosphate,which under reductive conditions gives rise to molybdenumblue. The recommended maximum concentration of totalphosphate in drinking water is 0.1 mg L−1 i.e. 100 μg L−1

according to the US EPA. To check phosphate interference,we had prepared the phosphate standard chart and per-formed RGB analysis with that.

The maximum contamination limit of iron in drinking wa-ter is 0.3 mg L−1 according to the WHO. In the presence ofiron, there is a chance that precipitation of arsenic takesplace. We had spiked water samples with iron 10 timeshigher than the WHO's stipulated limit (3 mg L−1) and in thepresence of 10 μg L−1 and 250 μg L−1 of AsĲV) and applied ourkit in order to perform the iron interference study.

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Results and discussionSynthesis of polymer composite hydrogel

Hydrogels were prepared by chemical crosslinking of a PAM–PVA-based polymer composite. Acrylamide was converted topolyacrylamide (PAM) when the PAM–PVA composite polymerwas produced by heating and stirring in the presence of aninitiator and accelerating agent. When this PAM–PVA poly-mer composite was mixed with the reagent containing molyb-denum and glutaraldehyde, then acetal bridges could beformed between the pendant hydroxyl groups of the PVAchains and this might give rise to a PAM–PVA interpen-etrating network.29

SEM and TEM analyses

Fig. 3a shows the typical SEM image of MLPH before adsorp-tion of arsenate (soaked in only tap water). A porous struc-ture could be seen and the pores were connected to eachother. The surface was smooth due to the amorphous natureof the hydrogel and homogeneous entrapment of molybde-num within the polymer hydrogel. The functional groups cor-responding to PAM, PVA, molybdate and AsĲV) could furtherbe confirmed by FTIR and the presence of As peaks withinthe polymer hydrogel was confirmed by EDX analysis. The po-rous structures of MLPH suggested that it could have allowed

adsorption of arsenate from the aqueous solution since theeffective diffusion distance could be controlled by the averagedistance between neighbouring pores.30 After adsorption ofarsenate, the morphology of the hydrogel changed and thiscould be viewed in the SEM image in Fig. 3b. The surfaceturned rough due to the adsorption of arsenate by the com-posite hydrogel. The dark regions seen within the micro-graphs of TEM analysis (Fig. 3c) suggested that it might bedue to the adsorption of arsenic inside the polymer hydrogel.

FTIR analysis

The FTIR spectra of only PVA grafted PAM hydrogel, MLPHbefore and after adsorption of arsenate are presented inFig. 4a. The broad peaks at 3337.40 cm−1 (curve A), at 3225.25cm−1 (curve B) and at 3376.25 cm−1 (curve C) could be attrib-uted to overlapping of hydrogen bonded –O–H stretching vi-brations due to unreacted OH bonds in the PVA polymer and–N–H stretching vibrations corresponding to –NH bonds ofammonium molybdate tetrahydrate. The C–H stretching fromthe –CH2 could be observed at 2944.98 cm

−1 (curve A),2920.67 cm−1 (curve B) and 2943.25 cm−1 (curve C). The spe-cific absorption bands shown in curve A at 1659.26 cm−1,which could be due to the νCO group (amide band 1), at1603.65 cm−1 corresponding to the δNH group (amide band 2)

Fig. 1 Schematic diagram of the whole procedure.


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and at 1426.45 cm−1 corresponding to C–N stretching (amideband 3) of PAM were found in the synthesized copolymerdemonstrating the formation of a suitable PVA grafted PAMpolymer hydrogel. It might be interesting to note that thepeaks of amide band 1 and amide band 2 disappeared and

the peak of amide band 3 was shifted from 1426.45 cm−1 to1433.41 cm−1 in MLPH. This might be due to incorporationof molybdate within the polymer hydrogel (curve B). In curve B,the peak at 872.61 cm−1 appeared due to the asymmetric Mo–Obond vibration mode of molybdate which disappeared inFig. 4a curve C due to the formation of an arsenatomolybdatecomplex. The characteristic band at 832.71 cm−1 was observedin the spectrum of MLPH after adsorption of arsenate (curve C)and was due to As–O stretching vibrations. This peak was notpresent in curve A and curve B and hence this peak could ap-pear due to arsenate adsorption by the PVA grafted PAM strip.

EDX analysis

The MLPH (before and after adsorption of arsenate) wascharacterized by EDX. The corresponding EDX spectra ofMLPH before adsorption of AsĲV) (Fig. 3d) showed the peakscorresponding to the elements C, O and Mo, confirming theexistence of molybdenum on the surface of the composite.The presence of a peak corresponding to arsenic in the EDXspectra of arsenate adsorbed MLPH (Fig. 3e) suggested thatarsenic sorption took place in the polymer matrix.

Fig. 3 SEM images of (a) the porous structure of molybdenum loaded PAM–PVA copolymer hydrogel soaked in arsenate-free tap water; (b) molyb-denum loaded PAM–PVA copolymer hydrogel after adsorption of arsenate; (c) TEM image of molybdenum loaded PAM–PVA copolymer hydrogelafter adsorption of arsenate; EDX images of (d) molybdenum loaded PAM–PVA copolymer hydrogel; and (e) molybdenum loaded PAM–PVA copoly-mer hydrogel after adsorption of arsenicĲV).

Fig. 2 Possible structure of the arsenomolybdate complex andformation of an α-Keggin anion.


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Color development of MLPH

It is well known that molybdenum oxides form a heteropolyacidwith arsenate (i.e. antimonyl–arseno–molybdate complex) whichturns into molybdenum blue in the presence of L-ascorbic acid(i.e. under reductive conditions) (eqn (4) and (5)). PotassiumantimonyĲIII) tartrate hemihydrate speeds up the reaction pro-cess. The same reaction occurred within the hydrogel phaseafter adsorption of arsenate. The arsenate molybdate com-plexes prepared in this study could form a Keggin-like chemi-cal structure within the solution showed in Fig. 2.31

AsO43− + 12(NH4)2MoO4 + 24H

+ → (NH4)3AsO4 · 12Mo+6O3

+ 21NH4+ + 12H2O (4)

(NH4)3AsO4 · 12Mo+6O3 + (reducing agent) → (molybdenum

blue) + (weaker reducing agent) (5)

Detection of arsenate and preparation of standard color chart

Standard arsenicĲV) solutions of 10–250 μg L−1 together with ablank were measured using this method. The intensity of theblue color increased with the increase in concentration of arse-

nate in water. A standard color chart for arsenate was preparedbased on variations of the intensity of the blue color (Fig. 5a).The dipstick sensor was also tested with 1000 μg L−1 arsenite,but no blue color appeared. For the regions where water containssignificant quantities of arsenicĲIII) in addition to arsenicĲV),AsĲIII) could be oxidized to AsĲV) using some oxidizing agent e.g.KIO3.

20 The test applied on this oxidized sample would thusgive the total arsenic [AsĲIII) + AsĲV)] value. Thus the sensing kitis not strictly limited to only AsĲV) measurement but it indirectlymight give assessment of AsĲIII) by difference [total − AsĲV)].

Calibration curve

The proof of concept for this research work is presented inFig. 6a and b. The calibration curve was obtained by plotting10–250 μg L−1 of AsĲV) concentrations against their corre-sponding effective intensities (Fig. 6a and b). It could be con-cluded from Fig. 6a and b that more than one response couldbe used for determination of the arsenicĲV) concentration.Fig. 6a depicts that the B values were more sensitive (R2 =0.9857) in the range 10–100 μg L−1 of AsĲV) and Fig. 6b indi-cated good sensitivity of the R values (R2 = 0.941) in the 20–250 μg L−1 of AsĲV) concentration range.

Fig. 4 (a) FTIR spectra of (curve A) the PAM–PVA composite polymer crosslinked with glutaraldehyde, (curve B) molybdenum loaded PAM–PVAcomposite polymer hydrogel, (curve C) molybdenum loaded PAM–PVA composite polymer hydrogel after adsorption of arsenicĲV); (b) and (c)comparison of arsenicĲV) concentrations detected by the molybdenum blue dipstick sensor and analysis by AAS.


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Effect of L-ascorbic acid (%) (reducing agent) concentration

The effect of L-ascorbic acid concentration on the detectionof arsenate was investigated (Fig. 6c) using 300 μg L−1 AsĲV)solutions. From Fig. 6b, it could be seen that for higher con-centrations of AsĲV), the R values were more sensitive, thus ef-fective intensities of the R values were considered for thisanalysis. An increase in the effective intensity (Ar) (from0.02% L-ascorbic acid) was observed with increasing concen-trations of L-ascorbic acid, up to 0.1%, when the systemreached a plateau. Further addition of L-ascorbic acid (0.15%)to the system did not affect the signal response of the MLPHsensing strips. Based on the observed results, an L-ascorbicacid concentration of 0.1% (i.e. 10 mg in 10 ml of AsĲV) solu-tion) was selected as the optimum concentration for thisstudy.

Detection limit and detection time

The optimal complex formation time was found to be 20 mi-nutes at room temperature. To restrict the optimal complexformation time, a number of experiments were performed.For a particular composition of the film, the time varies.However, 20 minutes was the shortest period for the stipu-lated concentration of reagents. All other compositions gaveeither a higher time or leakage of color. The minimum detec-

tion limit of this process was 10 μg L−1 of arsenate present inwater (by visual observation). These strips could detect arse-nate in drinking water in the range of 10–250 μg L−1. TheRGB analysis showed that the method could give within 95%confidence limit when compared with the corresponding AASresults (Tables 2 and 3).

Interference study

We found that in the case of low as well as high concentra-tion range of phosphate, the green (G) values might predictphosphate very well. For 100 μg L−1 of phosphate, the G valuewas 0.066. We could only get the B and R value responses(which were important in the case of arsenate detection)when the phosphate concentration is above 100 μg L−1. Fromthis we can conclude that the arsenate detection method bythe present molybdenum blue dipstick sensor could sufferfrom phosphate interference when the phosphate concentra-tion was only above 100 μg L−1 (Fig. 5b). Thus the dipsticksensor for arsenate was free from phosphate interference upto 100 μg L−1.

If there are possibilities of high phosphate concentrationin water, a test could be conducted to confirm whether it ismore than 100 μg L−1. In such cases, 20 ml of water will betreated with 1 g of sulfamic acid and 100 mg of sodium

Fig. 5 (a) Standard color chart of arsenicĲV); (b) interference study.


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borohydride to reduce all arsenic to arsenic hydride, AsH3, avolatile gas, in about 10 minutes time.23 The water will haveno arsenic and a test with the dipstick sensor can be used onthis sample to check if the concentration of phosphate ishigher than 100 μg L−1. If this produces a color then RGB

analysis would give the intensity. The fresh sample will bechecked with the dipstick and if it generates more intensity,the difference (either B or R values) will be due to the pres-ence of arsenate in the sample. In our study we consideredphosphate (PO4

3−) not the phosphorous (P) interference.

Fig. 6 (a) Calibration graph for 10–100 μg L−1 of AsĲV); (b) calibration graph for 20–250 μg L−1 of AsĲV); (c) the effect of L-ascorbic acid (reducingagent) concentration (0.02–0.15%) on the detection of arsenate.

Table 1 Results of detection of arsenicĲV) in the field samples using test kits on the spot and comparison of results with the conventional method(performed by RGB analysis)

Test results

Sample no.

ArsenicĲV) foundby theconventionalmethod

ArsenicĲV) foundby the proposedmolybdenum bluedipstick method

Deviation betweenthe means (μg L−1)

%Variation

001(BARASAT Akrampur Pilot Plant latitude:22.2300°N longitude: 88.4500°E)

X ± SD (μg L−1) % RSD X ± SD (μg L−1) % RSD 2 0.44451.33 ± 1.53 0.336 453.33 ± 5.77a 1.27

02(BARASAT Nivedita Palli, latitude:22.2300°N longitude: 88.4500°E)

351 ± 1.0 0.284 353.33 ± 5.77a 1.63 2.33 0.66

03(BARASAT R. K. MissionBamunmura, latitude: 22.2300°N longitude: 88.4500°E)

191.67 ± 1.53 0.798 185 ± 5a 2.70 6.67 3.48

aHigher ranges detected via dilution of samples.


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In the case of the iron interference study, we found that inboth cases [3 mg L−1 of iron + 10 μg L−1 of AsĲV) and 3 mg L−1

of iron + 250 μg L−1 of AsĲV)] the color of the dipstick sensormatched the color of the strips dipped in iron-free 10 μg L−1

Table 3 ‘t’ test results with t-values from t-table considering 95% confidence limit

Sample name Calculated t-valuea Tabulated t-value (P = 0.05) for (n = 4)a

Sample 02 1.34 2.45Sample 03 1.97 2.45Sample 04 2.24 2.45Sample 06 2.04 2.45Sample 07 1.22 2.45Sample 08 2.31 2.45Sample 13 2.34 2.45

aIn all the cases the calculated t-values did not cross the tabulated t-value at 95% confidence level.

Table 4 Average, standard deviation (SD) and relative standard deviation (RSD) of B values obtained for different concentrations of AsĲV) on four differ-ent strips

Serialno.

Concentrations of AsĲV)solutions in μg L−1 Average

Standarddeviation

Relative standarddeviation (%)

The concentration values correspondingto the B values from the calibration curve in μg L−1

1 110 0.04245 0.0001 0.3611 109.6252 55 0.019 0.00082 4.2973 513 15 0.0043 0.000173 4.0754 14.1254 12 0.00305 0.000129 4.2328 11.125

Table 2 Results of detection of arsenicĲV) in the field samples using the proposed molybdenum blue dipstick method and comparison of the resultswith that of AAS (performed by RGB analysis)

Sampleno.

Place from where the arsenic contaminated samplewas collected

ArsenicĲV)concentrationsdetected by the presentmolybdenum bluedipstick method

ArsenicĲV)concentrationsdetected by AAS

% Deviationbetween the means

01 (tapwater)

Kolkata, West Bengal latitude: 22°34′203210.92″Nlongitude: 88°22′10.92″E

X ± SD (μg L−1) % RSD X ± SD (μg L−1) % RSD —


and 250 μg L−1 of arsenate solutions. Thus our kit is freefrom interference from iron at up to at least 10 times theWHO recommended limit.

Blank tap water was tested for the presence of iron usinga Merck iron test kit (114 759) and


(North), West Bengal. Additionally, the conventional spectro-photometric molybdenum blue method19 was used to esti-mate arsenicĲV) to compare the results. The standard devia-tion (SD) and relative standard deviation (RSD) for 3replicates for both the methods were calculated and reportedin Table 1. In all the trials, the % variations between themeans of the results of the proposed method and the conven-tional spectrophotometric molybdenum blue method wereless than 10% (Table 1).

The proposed method was also applied to groundwatersamples collected from Baruipur, South 24 Parganas, WestBengal, Ashokenagar, Kalyanghar, North 24 Parganas, WestBengal and Chakdah, South 24 Praganas, West Bengal,Kalyani, North 24 Parganas, West Bengal, Barasat, South 24Parganas, West Bengal, Nadia, West Bengal. For some of thefield samples, the measurements were done just after the col-lection and the others were acidified with 0.1% HCl20 andtested later. The AAS measurements of the same samples aresummarized in Table 2. Table 2 was constructed using RGBanalysis. The correlation coefficient between lower andhigher arsenic(v) concentrations measured by the presentmethod and AAS was found to be 0.9961 and 0.9981, respec-tively (Fig. 4b and c) which suggested a good agreement withthe AAS results. It might be noted that all the samples mightcontain only AsĲV) as there was little difference between theproposed method of AsĲV) estimation [valid for only AsĲV)] andthe AAS [valid for total arsenic i.e. AsĲIII) + AsĲV)] results. Incase arsenite [As(III)] is present in water sample, it can be oxi-dized to arsenate [As(V)]20 and measurement by dipstickwould give total arsenic [As(V) + As(III)] and could be com-pared with AAS results. The blank tap water was tested for ar-senic using the very well known commercially availableMerck kit (Merck Arsenic Test method: colorimetric with teststrips, 0.005–0.5 mg L−1 As3+/5+, prod.: 1.17927.0001, batch:HC80062, Merckoquant®) and also by AAS. No traces of arse-nic were found. All the field samples were tested for phos-phate concentration using the procedure described in theinterference study. Fortunately in all the cases, we found


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