research article electroanalytical methodology for the

Research ArticleElectroanalytical Methodology for the DirectDetermination of 24-Dichlorophenoxyacetic Acid in SoilSamples Using a Graphite-Polyurethane Electrode

Fernanda Ramos de Andrade Renata Alves de Toledo and Carlos Manoel Pedro Vaz

Brazilian Agricultural Research Corporation Embrapa Instrumentation Rua XV de Novembro 145213560-970 Sao Carlos SP Brazil

Correspondence should be addressed to Carlos Manoel Pedro Vaz carlosvazembrapabr

Received 2 November 2013 Accepted 6 February 2014 Published 16 March 2014

Academic Editor Davood Nematollahi

Copyright copy 2014 Fernanda Ramos de Andrade et al This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

An electroanalytical methodology was developed for the direct determination of the herbicide 24-dichlorophenoxyacetic acid(24-D) using a graphite-polyurethane composite electrode and square wave voltammetry (SWV) 24-D exhibited one reductionpeak with characteristics of an irreversible process at minus054V (versus AgAgCl) which is controlled by the diffusion of the reagenton the electrode surface After the experimental parameters optimization (pH 20 119891 = 50 sminus1 119886 = 050V and Δ119864

119894

= 003V)analytical curves were constructed in the range of 066mg Lminus1 to 262mg Lminus1 Detection (LD) and quantification (LQ) limits were176120583g Lminus1 and 586 120583g Lminus1 respectively The methodology was successfully applied to measure the percolation of the herbicide24-D in undisturbed soil columns of different granulometric compositions

1 Introduction

24-D (24-Dichlorophenoxyacetic acid) is one of the mostwidely used herbicides in the world and is characterizedas low-cost quite efficient even at low concentrations andmoderately hazardous (class II) according to the WorldHealthOrganization [1] Although this herbicide easilymovesthough the soil its leaching into the groundwater can beminimized due to the degradation by microorganisms andalso the absorption by plants Its primary route of degradationin the environment is by microorganisms which increaseswith temperature humidity pH and organic matter content[2 3] In Brazil the maximum acceptable concentration of24-D in water is 40 120583g Lminus1 according to resolution 357 of theBrazilian National Environment Council [4]

Several chromatographic [5ndash9] and immunoassay meth-ods [10ndash13] have been used to determine 24-D in biologi-cal and environmental matrices However chromatographicmethodologies are time-consuming due to the derivatiza-tion step and complex extraction procedures Immunoassay

methods use biological subtracts that may present poorstability and in some cases result in low repeatability of theanalytical signals for a long analysis interval [14]

Electroanalytical procedures have been successfullyapplied for the determination of inorganic and organiccompounds including pesticides in many environmentalmatrices due to their good sensitivity short time analysislow-cost and the possibility of sample analysis withoutextractions or pretreatments [15ndash17]

It is well known from the literature that 24-D exhibitspoor electroactivity on most common electrode materialsurfaces such as carbon platinum and mercury [2] For thisreason it is crucial to develop modified electrodes for thedetermination and evaluation of 24-D Under this circum-stance the electrochemical behavior for 24-D was investi-gated using a biosensor based on competitive immunoassay[18] an immunochemical biosensor based on a screen-printed amperometric transducer andmonoclonal antibodies[19] an electrochemical sensor based on alkaline phosphataseinhibition [20] a carbon paste electrode modified with

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2014 Article ID 308926 9 pageshttpdxdoiorg1011552014308926

2 International Journal of Electrochemistry

polyaniline [21] a glassy carbon electrode modified withcobalt chloride (III) protoporphyrin IX [22] a glassy carbonelectrode modified with 24-D-molecular imprinted film[23] and conducting polymers (polyaniline and polypyrrole)as sensor materials [24] However despite the intensive useof this herbicide and its potential contamination of waterresources there is little information about the applicationof electroanalytical techniques for the analysis of 24-D inenvironmental samples which motivates additional studiesin this area

Currently there has been an increasing interest in the useof composite electrodes for a great variety of applications [25]and the graphite-polyurethane composite electrode (GPU)developed by Mendes and coworkers [26] is an importantelectrodematerial for the determination ofmany compoundsof environmental significance such as organic pesticidesThis electrode has been successfully used for the determi-nation of dopamine [27] imipramine [28] hydroquinoneparacetamol and atenolol [29] rutin [30] and indole-3-aceticacid [31]

The aim of the present work is to develop for the firsttime an electroanalytical methodology to directly quantifythe herbicide 24-D in soil samples using the GPU electrodeand the square wave voltammetry technique (SWV) Theprocedure was used to evaluate the behavior of 24-D inundisturbed soil columns under field conditions in order todetermine the concentration and distribution of the applied24-D along the soil profile with time and also the residualamounts of herbicide remaining in the soil after a relativelylong period of time (100 days)

2 Experimental

21 Chemical and Reagents All reagents were of analyticalgrade (Sigma) and were used without prior purification Allsolutions were prepared with purified water from a Milli-Q system (Millipore) Britton-Robinson (BR) buffer solution(01mol Lminus1) was used as supporting electrolyte The pH ofthe solutions was adjusted with 10mol Lminus1 NaOH Stockstandard solution of 24-D herbicide (221mg Lminus1) was pre-pared in methanol kept under refrigeration and protectedfrom light

24-D commercial formulation (DMA 806 BR-DowAgroSciences Industrial LTDA) was obtained in the localmarket with a declared amount of 670 g Lminus1 A stock solutionof the commercial formulation (403mg Lminus1) was prepared inaqueous media kept under refrigeration and protected fromlight as well The 24-D commercial formulation was appliedto the soil surface using a watering container at themaximumamount specified by the manufacturer (503mg Lminus1) Fora uniform distribution of the herbicide both soils werecarefully irrigatedwith 6 liters ofwater just after the herbicideapplication

22 Instrumentation Voltammetricmeasurements were per-formed using a potentiostatgalvanostat AUTOLABPGSTAT30 in an electrochemical cell made from borosilicate glasswith a Teflon cover which contains orifices to insert the

working (GPU) reference (AgAgCl-KCl 30mol Lminus1) andauxiliary (platinum wire) electrodes and for nitrogen purg-ing

The GPU electrode was constructed by mixing graphiteand polyurethane resin at the ratio 60 40 (ww) [26] Thereis no special condition for electrode storage The easy andfast regeneration of the GPU electrode surface was achievedby polishing the electrode disk surface with a 2000 gritsand paper before each measurement to get a clean andreproducible electrode surface

23 Electroanalytical Methodology Analytical curves wereobtained through the external standard calibration proce-dure The sensitivity was checked by the limit of detection(DL = 3120590120579) and quantification (QL = 10120590120579) where 120590 is thestandard deviation of ten voltammograms registered for theblank and 120579 is the slope of the analytical curve [32] for anaverage of six curves

The precision was calculated by the relative standarddeviation (RSD) and checked by an experiment performedin one day (119899 = 10 intra-assay precisionrepeatability) aswell as by experiments in different days (119889 = 6 interassayprecisionintermediate precision) with a 033mg Lminus1 24-Dsolution

The accuracy was evaluated in terms of relative error(bias ) obtained from 24-D recovering experiments for twosoils (sandy and clayey) The experiments were performed(119899 = 3) by adding 100mL of water to 50 g of dried soil Themixture was fortified with a 033mg Lminus1 24-D solution (iespiked with 150 120583L from a 221mg Lminus1 24-D stock solution)agitated for 24 h in a horizontal shaker and centrifugedfor 20min at 13500 rpm (Sorvall SL-50T Rotor diameter= 222 cm) The pH of the supernatant was adjusted to 20with concentrated phosphoric acid and the concentrationof 24-D was determined with the previously developedelectroanalytical method by means of a standard additionprocedure where 50 120583L of a 221mg Lminus1 24-D stock solutionwas used to obtain concentrations of 088 143 and 197mgLminus1 of 24-D The 24-D estimated concentration was directlyobtained through extrapolation in the 119909-axis from the linearregression of the 119868

119901

versus 11986224-D curvesThe robustness of the methodology was checked during

the optimization of the electrolyte pH After selecting thebest pH value a small pH variation study was done nearthis region in order to measure the methodology capacity toremain unaffected by short-term pH fluctuations A statis-tical analysis was performed (119905-test with a 95 confidenceinterval) to estimate the effect of small pH variation on themethodology performance

24 Soil Columns Experiments The commercial herbicideformulation was applied to the surface of two types of soila sandy textured (AM1) and a medium clayey textured soil(AM2)

Soil samples were chemically (pH in CaCl2

organicmatter-OM and cation exchange capacity-CEC) and physi-cally (clay silt and sand percentages) characterized in the soillaboratory PIRASOLO Piracicaba Sao Paulo Brazil

International Journal of Electrochemistry 3

(a)

Assay tube

Vacuum pump

PVC tube

Soil

Porous ceramic chamberSoil

solution

(b)

Figure 1 (a) Undisturbed soil in cement cylindrical containers (b) Soil solution extraction system

Undisturbed soil cores were packed in cement cylindricalcontainers with a total volume of 10m3 capacity (Figure 1)After the herbicide application the soil containers were keptuncovered to sunlight and rainfall to provide the herbicidenatural transport and degradation along the soil profileWater percolation was monitored at depths of 25 50 and75 cm for three months Soil solutions were obtained usingporous cup extractors made of silicon carbide (SiC) whichwere installed in each cement container at the same afore-mentioned depths After 100 days from the beginning of theexperiment soil samples were collected with a Dutch-typeauger at the same sampling depths and in all the cementcontainers These soil samples were used to determine theresidual amounts of the herbicide remaining in both soilsSoil solutionswere also collected from the three depths beforeapplication of the herbicide Soil containers with no herbicideaddition (blank) were also prepared sampled and analyzedfor comparison Figure 1 presents the field experimental setupwith three soil solution extractors and the vacuumpumpusedto collect the soil solutions in three depths

The collected soil solutions were buffered and directlyanalyzed using the developed electroanalytical procedureAll the disturbed soil samples extracted (5 g) at the end ofthe experiment were treated with 10mL of acidified waterpH 10 (H

2

SO4

90mol Lminus1) and 15mL of dichloromethaneThe herbicide was extracted by an ultrasound system for 1 hThe extracts were combined and filtered through quantitativepaper The final extract was reduced in a rotary evaporatorup to 10mL To certify that all the dichloromethane wasevaporated a flux of N

2

was applied over the extract for2minThefinal extract was then diluted in 20mLof BR buffersolution and analyzed with the developed electroanalyticalprocedure

3 Results and Discussion

The voltammetric response of 24-D (105mg Lminus1) was ini-tially evaluated and compared using different carbonworkingelectrodes such as glassy carbon (GC) carbon paste (CP)electrode and GPU Figure 2 illustrates the square wave

0

5

10

15

20

25

(d) (c)

(f)

(e)

(b)

(a)

00 03 06 09 12

minusI

(120583A

)

minusE (V versus EAgAgCl )

Figure 2 SWV curves for 24-D (105mg Lminus1) in 01mol Lminus1 BRbuffer solution (pH 20) obtained using different working electrodes(a) blank in glassy carbon (b) 24-D in glassy carbon (c) blank incarbon paste (d) 24-D in carbon paste (e) blank in GPU and (f)24-D in GPU Parameters 119891 = 50 sminus1 119886 = 050V Δ119864

119894

= 003V 119905acc= 10 s and 119864acc = 00V

voltammograms of each working electrode in 01mol Lminus1 BRbuffer solution (pH 20)

24-D presents a very well-defined reduction peak on theGPU electrode at minus054V (versus AgAgCl) whereas on theGC and on the CP electrodes the reduction process exhibitsbroadened and poor voltammetric profilesThe best responseof theGPUelectrode suggests a favorable interaction betweenthe polyurethane and the herbicide that results in a moreeffective electron transfer process

31 Cyclic Voltammetric Studies 24-D voltammetric curveswere registered in the range of 10ndash500mV sminus1 (Figure 3) toinvestigate the degree of reversibility of the electrode processand also to determine the predominate type of mass trans-port The irreversibility nature of the reaction is suggested by


0

20

40

60

0

4

8

12

minusI

(120583A

)

minusI p

(120583A

)

0 5 10 15 20 25

minusE E A )

12 (mV sminus1)12

Figure 3 CV curves for 24-D (105mg Lminus1) in 01mol Lminus1 BR buffersolution (pH 20) Scan rate (a) 10 (b) 20 (c) 50 (d) 100 (e) 200 and(f) 500mV sminus1 Inset 119868

119901

versus V12 graph (1199032 = 0995)

the absence of anodic peak in the reverse scan and confirmedby the linear displacement of the peak potential (119864

119901

)with theincrease of the scan rate [33] From the linearity of 119868

119901

versusV12 graph (inset of Figure 3 119868

119901

(120583A) = 099 + 044V12 (mVsminus1)12 1199032 = 0995) it is possible to conclude that the rate-limiting step of the 24-D electrochemical reaction at GPUelectrode is diffusion-controlled [34] A plot of log 119868

119901

versuslog V gave a straight line (log 119868

119901

(120583A) = minus011 + 042 log V(mV sminus1) 1199032 = 0995) with a slope of 042 very close to thetheoretical value of 050 which is expected for a diffusion-controlled process [34]

32 Electroanalytical Determination of 24-D SWVwas cho-sen from other electroanalytical techniques due to its highsensitivity and fast voltammograms acquisition In order toobtain the best analytical signal for 24-D some experimentswere performed to optimize the electrolyte pH and the acqui-sition parameters of the SWV technique namely frequency(119891) pulse amplitude (119886) scan increment (Δ119864

119894

) accumulationtime (119905acc) and accumulation potential (119864acc)

The effect of the pH was studied in the range of 20 to80 The cathodic peak potential shifted linearly (dEdpH =minus0064VpH) to more negative potentials for pH rangingfrom 20 to 35 indicating that H+ is involved in the electrodereaction The best analytical signal was obtained in pH 20which was then selected for the analytical applications

The slope of minus0064mVpH is close to the Nerstianminus0059mVpH and consequently indicates that equal num-bers of electrons and protons were involved in the elec-trochemical reduction of 24-D From the literature it issuggested that 4 electrons and 4 protons participated in thereduction of the acid carboxylic group to the correspondentalcohol [35]

The frequency was optimized in the range of 10 sminus1 to100 sminus1 and for analytical purposes a frequency value of50 sminus1 was selected The pulse amplitude (010 to 050V) andthe scan increment (001 to 005V) were also varied andthe best values for these parameters were 050V and 003Vrespectively

Although the reaction is controlled by diffusion adsorp-tion plays an important role in the electroanalytical signal of24-D on the GPU electrode For this reason the accumu-lation potential and time were also studied It was observedthat accumulation times higher than 10 s did not significantlyincrease the analytical signal due to the saturation of theGPU electrode surface The accumulation potential (119864acc)also affects the analytical signal with the highest peak currentobtained for 119864acc = minus040V This behavior suggests that onlythe reagent adsorbs on the electrode surface since the elec-trode reaction occurs at minus054VTherefore the accumulationpotential and time were selected as minus040V and 10 sec for thesubsequent analytical studies

After optimizing the voltammetric acquisition parame-ters analytical curves were obtained in the range of 066to 262mg Lminus1 in BR buffer solution (pH 20) The linearregression equation for the analytical curves was 119868

119875

(120583A)= (086 plusmn 009) + (447 plusmn 005) 11986224-D (mg Lminus1) with adetermination coefficient 1199032 = 0999 A detection limit (DL)of 176 120583g Lminus1 and a quantification limit (QL) of 586120583g Lminus1were obtained according to the procedure described inSection 23The precisionwas checked in terms of intra-assayprecision (repeatability) and interassay precision (intermedi-ate precision)The RSD values for these statistical parameterswere 25 and 39 respectively

Average recovered values of 24-D (concentration level033mg Lminus1) in both soils were 936 plusmn 12 for the sandysoil (AM1) and 946 plusmn 42 for the clayey soil (AM2) Theaccuracy was evaluated in terms of the relative errors (bias) which describes the deviation from the expected valuesin fortified samples The obtained bias values were 68 forsoil AM1 and 58 for soil AM2 which are within acceptablelimits The robustness checked by small variations on theelectrolyte pH (ie from 18 to 22) was satisfactory sincethere is a 95 probability that the true population parameter(peak current) is between 509 120583A and 698 120583A

In order to check the applicability of the developedelectroanalytical procedure for soil solutions withoutprevious treatment analytical curves for 24-D were alsoobtained in the same concentration range as previouslydiscussed for the supporting electrolyte in clayey and sandysoil solutions Figure 4 shows square wave voltammogramsfor varying concentrations of 24-D in the clayey soil solutionThe insert shows the analytical curves for both soils and forBR buffer solution The obtained linear regression equationswere 119868

119875

(120583A) = (085 plusmn 012) + (457 plusmn 007) times 11986224-D (mgLminus1) 1199032 = 0999 for the sandy soil and 119868

119875

(120583A) = (101 plusmn 010)+ (399 plusmn 005) times 11986224-D (mg Lminus1) 1199032 = 0999 for the clayeysoil Results show no significant differences in the analyticalsensitivity of the method when comparing electrolyte(485 plusmn 013 120583AmgLminus1) with sandy (457 plusmn 007 120583AmgLminus1)and clayey soil solutions (399 plusmn 005 120583AmgLminus1)


Table 1 Comparison between the proposed electroanalytical methodology and other ones already published in the literature for the detectionof 24-D

Electrode Technique pH Linear range(mg Lminus1)

LD(120583g Lminus1) Sample Recovery References

Graphite-polyurethanecomposite electrode SWV 2 066ndash262 176 Soil 936 plusmn 12 (sandy soil)

946 plusmn 42 (clayey soil) This work

Static mercury drop electrode DPV 23 0075ndash175 50 Water and soil

70ndash85 plusmn 7 (soil)90ndash95 plusmn 2 (tap water)80ndash90 plusmn 5 (well water)90 plusmn 6 (river water)

[2]

Silica-gel modified carbonpaste electrode Amperometry 5 0ndash221 995 Water mdash [35]

Molecularly imprintedpolypyrrole membrane CV 686 022ndash221 1835 Water 92ndash108 [36]

Table 2 Some physicochemical parameters of the soil studied

Sample pH CaCl2Organic matter

(g dmminus3)Cation exchangeable capacity

(mmolc dmminus3) Clay Silt Sand

AM1 0ndash25 cm 51 12 44 62 08 93AM1 25ndash50 cm 53 3 29 71 09 91AM1 50ndash75 cm 52 6 37 61 19 92AM2 0ndash25 cm 51 21 74 369 101 53AM2 25ndash50 cm 52 15 67 428 122 45AM2 50ndash75 cm 54 8 58 399 121 48

0

10

20

30

40

50

0

3

6

9

12

15

minusI

(120583A

)

minusI p

(120583A

)

minusE E A )

C Lminus1)

Figure 4 SWV curves for different 24-D concentrations in clayeysoil solution Concentration (a) blank (b) 066 (c) 099 (d) 132 (e)164 (f) 197 (g) 230 and (h) 262mg Lminus1 Parameters pH 20 119891 =50 sminus1 119886 = 050V Δ119864

119894

= 003V 119864acc = minus040 V and 119905acc = 10 s Insetanalytical curves in electrolyte (◼) sandy (e) and clayey (998795) soilsolutions

Thus the direct analysis of 24-D in soil solutions canbe performed without significant interferences of dissolvedsolutes and colloids present in the soil solution

Table 1 shows the comparison between the proposedelectroanalytical methodology (GPUSWV) and other onesalready published in the literature for the determination of

24-D The detection limit of the GPUSWV methodologyis lower than the others previously reported [2 35 36]Moreover high degrees of recovery were obtained in soilsamples with standard deviations lower than 5 suggestingthe feasibility of the proposed methodology In general GPUelectrode is an environmental friendly option comparingto the use of mercury electrodes and it is simple fastregeneration and good stability open the possibility of its usefor the analysis of other environmental contaminants

33 Percolation of 24-D in Soil Columns After evaluationof the applicability of the GPU electrode for determining24-D in soil solutions without previous treatment the elec-troanalytical procedure was also applied to study transportand percolation of 24-D in two soils (sandy and a clayeytextured) which were previously submitted to commercialherbicide solution treatment according to the experimentalprocedure described in Section 24

Table 2 presents some physicochemical properties of thesoils evaluated Soil AM1 is a sandy soil andAM2 is amediumtexture soil with 40 clay (designated here as clayey soil)ThepH values of the two soils are similar (pH around 5) and didnot change significantly along the soil profiles The amountof organic matter (OM) and cation exchange capacity (CEC)contents are higher for the clayey soil and decreased withdepths as expected (higher concentration of roots and moredecomposed materials close to the soil surface)

Table 3 shows the measured concentrations of 24-D inthe extracted soil solutions using the developed electro-analytical methodology at different depths for soils AM1


00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)

Figure 5 SWV curves for the 24-D herbicide in the two soil solution samples extracted at a 25 cm depth (a) AM1 and (b) AM2 Parameters119891 = 50 sminus1 119886 = 050V Δ119864

119894

= 003 V 119905acc = 10 s and 119864acc = minus040V Inset decay of concentration with time Samples (a) sample withoutherbicide (b) 24 hours (c) 48 hours (d) 72 hours (e) 7 days (f) 30 days and (g) 60 days for AM1 and (a) sample without herbicide (b) 24hours (c) 48 hours (d) 72 hours (e) 7 days (f) 30 days and (g) 85 days for AM2

Table 3 Concentration of 24-D in the soil solutions extracted with the porous cup extractor system in three depths for soils AM1 and AM2

SampleConcentration of 24-D (mg Lminus1)

AM1 AM225 cm 50 cm 75 cm 25 cm 50 cm 75 cm

SHa NDc ND ND ND ND NDBRb ND SAd ND SA SA SA24 h 107 plusmn 001 SA ND 152 plusmn 003 ND 073 plusmn 002

48 h 067 plusmn 007 SA SA 102 plusmn 007 119 plusmn 003 085 plusmn 006

72 h 086 plusmn 008 SA 062 plusmn 003 062 plusmn 002 SA 055 plusmn 007

7 days ND SA SA ND 017 plusmn 001 017 plusmn 003

14 days ND 019 plusmn 002 ND ND ND ND30 days ND 017 plusmn 005 ND ND ND ND52 days ND ND ND SA ND ND60 days ND ND ND ND ND ND71 days ND ND ND SA 016 plusmn 003 ND73 days ND SA ND ND ND ND76 days ND ND ND ND ND ND85 days ND ND ND ND SA NDaSample without herbicide (SH) bblank sample (BR) cconcentration below the quantification limit of the technique (ND) dno samplemdashvolume extracted wasnot enough for the analysis (SA)

and AM2 as a function of time In both soils 24-Dwas detected in the deeper layer (75 cm depth) 24 hoursafter its application on the soil surface The highest 24-Dconcentration values were detected in the first three daysafter its application After one week practically no 24-Dwas detected in the extracted soil solutions for both soilsaccording to the measured concentrations and their standarddeviations Figure 5 presents square wave voltammograms ofthe 24-D herbicide measured in the AM1 (Figure 5(a)) andAM2 (Figure 5(b)) extracted soil solutions at 25 cm depth

The inserted graphics show the concentration decay as a func-tion of time

24-D concentrations measured in both soils were verysimilar in terms of spatial dissipation There were no signif-icant differences in the herbicide spatial distribution for thesandy and clayey soils as measured in the extracted soil solu-tionsThis was probably caused by the anionic behavior of the24-D exhibiting low sorption on clay minerals (negativelycharged) and a negative correlation with pH and to someextent a positive correlation with OMThese results (Table 3)


Table 4 Concentration of 24-D in the soil solutions extractedfrom the soil samples collected with an auger after 100 days of theherbicide application

Depth C24-D (mgKgminus1)

AM10ndash25 cm NDa

25ndash50 cm 076 plusmn 004


AM20ndash25 cm 058 plusmn 003

25ndash50 cm NDa

50ndash75 cm NDa

aConcentration below the quantification limit of the technique (ND)

show a very low persistence of 24-D in the studied soils(around 7ndash14 days) which is in accordance with the reportedhalf-life for field dissipation of about 10 days reported for the24-D in soils [37]

Herbicides derived from weak acids such as the 24-D(pKa 28) lose protons and are predominantly in the anionicform in soils (pH 5 to 8) and since soil pH is higher thanthe herbicide pKa the ionized species are predominantHowever a reduction in herbicide sorption due to thenegative nature of soil net charge is expected [38 39] Forthis reason 24-D sorption in soils is generally lower than thatof cationic or basic herbicides and therefore such herbicideis more susceptible to photodegradation and microbiologicaldegradation [40]

After 100 days from the beginning of the experimentsoil samples were collected for residual analysis of 24-Din the soils profile During this period of time the totalprecipitation was 236mm and the average temperature andrelative humidity were 245∘C and 80 respectively

The concentrations of 24-D determined at depths 0ndash25 cm 25ndash50 cm and 50ndash75 cm are shown in Table 4 Asignificant difference in the herbicide concentration alongthe profile and between the two soils can be verified Theaverage concentrations obtained in the soil profile (0 to75 cm of depth) were 069mg Lminus1 and 038mg Lminus1 for soilsAM1 (sandy) and AM2 (clayey) respectively These resultsindicate a higher accumulation of 24-D in the sandy soil 100days after the application on the soil surfaces This behaviorcan be attributed to the higher amount of organic mattercontent in the clayey soil (AM2) compared to the sandy soil(AM1) which provides a favorable environment for herbicidemicrobiological degradation Differences in concentrationalong the soil profile between soils can be explained by highermacroporosity and hydraulic conductivity of the sandy soil(AM1) when compared to the clayey soil AM2

4 Conclusions

The developed electroanalytical procedure using the GPUelectrode allowed the direct determination of the herbicide24-D in soil solutions without previous extraction orpretreatment The obtained limit of detection was adequatein order to study the percolation and sorption behavior of

24-D in soils under field conditions The effect of matricesor soil solution interferences on the electroanalyticalresponses of the two soils was minimal as verified in theanalytical curves providing consistentmeasurements in bothporous cup extracted soil solution and solutions extractedfrom soil samples collected with an augerTheGPU electrodehas proved to be an alternative for the direct electroanalyticaldetermination of 24-D in soil samples to the conventionallyused mercury electrode

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors gratefully acknowledge support from the Brazil-ian Agricultural Research Corporation (EMBRAPA) underGrant no 010510010202 and the Brazilian National Coun-cil for the Scientific and Technological Development (CNPq)under Grant no 3010572009-5 Special thanks are due toRene de Oste for his technical support in the field experi-ments

References

[1] ldquoThe WHO recommended classification of pesticides byhazardrdquo 2013 httpwwwwhointipcspublicationspesticideshazard rev 3pdf

[2] N Maleki A Safavi and H R Shahbaazi ldquoElectrochemicaldetermination of 24-D at a mercury electroderdquo AnalyticaChimica Acta vol 530 no 1 pp 69ndash74 2005

[3] J Gaultier A Farenhorst J Cathcart and T Goddard ldquoDegra-dation of [carboxyl-14C] 24-D and [ring-U-14C] 24-D in 114agricultural soils as affected by soil organic carbon contentrdquo SoilBiology and Biochemistry vol 40 no 1 pp 217ndash227 2008

[4] Database of Brazilian Environmental Ministry National Envi-ronmental Concil 2013 httpwwwmmagovbrportconamaresres05res35705pdf

[5] W-H Ding C-H Liu and S-P Yeh ldquoAnalysis of chlorophe-noxy acid herbicides in water by large-volume on-line derivati-zation and gas chromatography-mass spectrometryrdquo Journal ofChromatography A vol 896 no 1-2 pp 111ndash116 2000

[6] E A Hogendoorn R Huls E Dijkman and R HoogerbruggeldquoMicrowave assisted solvent extraction and coupled-columnreversed-phase liquid chromatography with UV detectionuse of an analytical restricted-access-medium column for theefficient multi-residue analysis of acidic pesticides in soilsrdquoJournal of Chromatography A vol 938 no 1-2 pp 23ndash33 2001

[7] N Rosales-Conrado M E Leon-Gonzalez L V Perez-Arribasand L M Polo-Dıez ldquoDetermination of chlorophenoxy acidherbicides and their esters in soil by capillary high performanceliquid chromatography with ultraviolet detection using largevolume injection and temperature gradientrdquo Analytica ChimicaActa vol 470 no 2 pp 147ndash154 2002


[8] O P de Amarante Jr N M Brito T C R Dos SantosG S Nunes and M L Ribeiro ldquoDetermination of 24-dichlorophenoxyacetic acid and its major transformation prod-uct in soil samples by liquid chromatographic analysisrdquoTalantavol 60 no 1 pp 115ndash121 2003

[9] A B Vega A G Frenich and J L M Vidal ldquoMonitoring ofpesticides in agricultural water and soil samples fromAndalusiaby liquid chromatography coupled to mass spectrometryrdquoAnalytica Chimica Acta vol 538 no 1-2 pp 117ndash127 2005

[10] B B Dzantiev A V Zherdev M F Yulaev R A Sitdikov N MDmitrieva and I Y Moreva ldquoElectrochemical immunosensorsfor determination of the pesticides 24-dichlorophenoxyaceticand 245-trichlorophenoxyacetic acidsrdquo Biosensors and Bioelec-tronics vol 11 no 1-2 pp 179ndash185 1996

[11] E P Medyantseva M G Vertlib M P Kutyreva E IKhaldeeva G K Budnikov and S A Eremin ldquoThe specificimmunochemical detection of 24-dichlorophenoxyacetic acidand 245-trichlorophenoxyacetic acid pesticides by ampero-metric cholinesterase biosensorsrdquo Analytica Chimica Acta vol347 no 1-2 pp 71ndash78 1997

[12] M Dequaire C Degrand and B Limoges ldquoAn immunomag-netic electrochemical sensor based on a perfluorosulfonate-coated screen-printed electrode for the determination of 24-dichlorophenoxyacetic acidrdquo Analytical Chemistry vol 71 no13 pp 2571ndash2577 1999

[13] J C Chuang J M van Emon J Durnford and KThomas ldquoDevelopment and evaluation of an enzyme-linkedimmunosorbent assay (ELISA) method for the measurementof 24-dichlorophenoxyacetic acid in human urinerdquo Talantavol 67 no 3 pp 658ndash666 2005

[14] O P de Amarante Jr T C R dos Santos G S Nunes andM LRibeiro ldquoConcise revision of methods for determination of theherbicide dichlorophenoxyacetic acid (24-D)rdquo Quimica Novavol 26 no 2 pp 223ndash229 2003

[15] R Inam E Z Gulerman and T Sarigul ldquoDetermination oftriflumizole by differential pulse polarography in formulationsoil and natural water samplesrdquo Analytica Chimica Acta vol579 no 1 pp 117ndash123 2006

[16] H Mercan and R Inam ldquoDetermination of the fungicide ani-lazine in soil and river water by differential pulse polarographyrdquoCleanmdashSoil Air Water vol 36 no 10-11 pp 913ndash919 2008

[17] T Thriveni J R Kumar J Y Lee and N Y Sreedhar ldquoStudyof the voltammetric behaviour of the ethalfluralin and methal-propalin and its determination in environmental matrices athanging mercury drop electroderdquo Environmental Monitoringand Assessment vol 151 no 1ndash4 pp 9ndash18 2009

[18] C G Bauer A V Eremenko E Foster-Ehrentreich et alldquoZeptomole-detecting biosensor for alkaline phosphatase in anelectrochemical immunoassay for 24-dichlorophenoxyaceticacidrdquo International Journal of Computer Vision vol 18 no 3pp 2453ndash2458 1996

[19] T Kalab and P Skladal ldquoDisposable multichannel immun-osensors for 24-dichlorophenoxyacetic acid using acetyl-cholinesterase as an enzyme labelrdquo Electroanalysis vol 9 no 4pp 293ndash297 1997

[20] F Mazzei F Botre S Montilla R Pilloton E Podesta and CBotre ldquoAlkaline phosphatase inhibition based electrochemicalsensors for the detection of pesticidesrdquo Journal of Electroanalyt-ical Chemistry vol 574 no 1 pp 95ndash100 2004

[21] F R Simoes L H C Mattoso and C M P Vaz ldquoModifiedcarbon paste-polyaniline electrodes for the electrochemicaldetermination of the herbicide 24-Drdquo Sensor Letters vol 2 no3-4 pp 221ndash225 2004

[22] S Chaiyasith T Tangkuaram and P Chaiyasith ldquoElectrocat-alytical of chlorophenoxycarboxylic acids at a protoporphyrinIX cobalt(III) chloride modified glassy carbon electroderdquo Jour-nal of Electroanalytical Chemistry vol 581 no 1 pp 104ndash1102005

[23] Z Wang J Kang X Liu and Y Ma ldquoVoltammetric responseof 24-D-molecularly imprinted film modified glassy carbonelectrodesrdquo Indian Journal of Chemistry A vol 45 no 8 pp1848ndash1851 2006

[24] F R Simoes L H Capparelli Mattoso and C M P Vaz ldquoCon-ducting polymers as sensor materials for the electrochemicaldetection of pesticidesrdquo Sensor Letters vol 4 no 3 pp 319ndash3242006

[25] T Navratil and J Barek ldquoAnalytical applications of compositesolid electrodesrdquo Critical Reviews in Analytical Chemistry vol39 no 3 pp 131ndash147 2009

[26] R K Mendes S Claro-Neto and E T G Cavalheiro ldquoEvalua-tion of a new rigid carbon-castor oil polyurethane composite asan electrode materialrdquo Talanta vol 57 no 5 pp 909ndash917 2002

[27] R A de Toledo M C Santos E T G Cavalheiro and L HMazo ldquoDetermination of dopamine in synthetic cerebrospinalfluid by SWV with a graphite-polyurethane composite elec-troderdquo Analytical and Bioanalytical Chemistry vol 381 no 6pp 1161ndash1166 2005

[28] R A de ToledoM C Santos KMHonorio A B F da Silva ET G Cavalheiro and L HMazo ldquoUse of graphite polyurethanecomposite electrode for imipramine oxidationmdashmechanismproposal and electroanalytical determinationrdquo Analytical Let-ters vol 39 no 3 pp 507ndash520 2006

[29] P Cervini L A Ramos andE TGCavalheiro ldquoDeterminationof atenolol at a graphite-polyurethane composite electroderdquoTalanta vol 72 no 1 pp 206ndash209 2007

[30] A R Malagutti V G Zuin E T G Cavalheiro and L HMazoldquoDetermination of rutin in green tea infusions using square-wave voltammetry with a rigid carbon-polyurethane compositeelectroderdquo Electroanalysis vol 18 no 10 pp 1028ndash1034 2006

[31] R A de Toledo and C M P Vaz ldquoUse of a graphite-polyurethane composite electrode for electroanalytical deter-mination of indole-3-acetic acid in soil samplesrdquoMicrochemicalJournal vol 86 no 2 pp 161ndash165 2007

[32] J C Miller and J N Miller Statistics for Analytical ChemistryEllis Horwood PTR Prentice-Hall New York NY USA 1993

[33] A Arranz I Dolara S F de Betono J M Moreda A Cidand J F Arranz ldquoElectroanalytical study and square wavevoltammetric techniques for the determination of 120573-blockertimolol at the mercury electroderdquo Analytica Chimica Acta vol389 no 1ndash3 pp 225ndash232 1999

[34] D K Gosser Jr Cyclic Voltammetry Simulation and Analysis ofReaction Mechanisms VCH Publishers New York NY USA1994

[35] A G S Prado H T Barcelos A O Moura A R Nunes andE S Gil ldquoDichlorophenoxyacetic acid anchored on silica-gelmodified carbon paste for the determination of pesticide 24-Drdquo International Journal of Electrochemical Science vol 7 pp8929ndash8939 2012


[36] C Xie S Gao Q Guo and K Xu ldquoElectrochemical sensor for24-dichlorophenoxy acetic acid using molecularly imprintedpolypyrrole membrane as recognition elementrdquo MicrochimicaActa vol 169 no 1 pp 145ndash152 2010

[37] R D Wauchope T M Buttler A G Hornsby P W MAugustijn-Beckers and J P Burt ldquoThe SCSARSCES pesti-cide properties database for environmental decision-makingrdquoReviews of Environmental Contamination and Toxicology vol123 pp 1ndash155 1992

[38] S BWeed and J BWeber Pesticides in Soil andWater edited byW D Guenzi Soil Science Society of America Madison WisUSA 1986

[39] R S Oliveira Jr W C Koskinen and F A Ferreira ldquoSorptionand leaching potential of herbicides on Brazilian soilsrdquo WeedResearch vol 41 no 2 pp 97ndash110 2001

[40] W G Johnson T L Lavy and E E Gbur ldquoSorption mobilityand degradation of triclopyr and 24-D on four soilsrdquo WeedScience vol 43 no 4 pp 678ndash684 1995

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


polyaniline [21] a glassy carbon electrode modified withcobalt chloride (III) protoporphyrin IX [22] a glassy carbonelectrode modified with 24-D-molecular imprinted film[23] and conducting polymers (polyaniline and polypyrrole)as sensor materials [24] However despite the intensive useof this herbicide and its potential contamination of waterresources there is little information about the applicationof electroanalytical techniques for the analysis of 24-D inenvironmental samples which motivates additional studiesin this area

Currently there has been an increasing interest in the useof composite electrodes for a great variety of applications [25]and the graphite-polyurethane composite electrode (GPU)developed by Mendes and coworkers [26] is an importantelectrodematerial for the determination ofmany compoundsof environmental significance such as organic pesticidesThis electrode has been successfully used for the determi-nation of dopamine [27] imipramine [28] hydroquinoneparacetamol and atenolol [29] rutin [30] and indole-3-aceticacid [31]

The aim of the present work is to develop for the firsttime an electroanalytical methodology to directly quantifythe herbicide 24-D in soil samples using the GPU electrodeand the square wave voltammetry technique (SWV) Theprocedure was used to evaluate the behavior of 24-D inundisturbed soil columns under field conditions in order todetermine the concentration and distribution of the applied24-D along the soil profile with time and also the residualamounts of herbicide remaining in the soil after a relativelylong period of time (100 days)

2 Experimental

21 Chemical and Reagents All reagents were of analyticalgrade (Sigma) and were used without prior purification Allsolutions were prepared with purified water from a Milli-Q system (Millipore) Britton-Robinson (BR) buffer solution(01mol Lminus1) was used as supporting electrolyte The pH ofthe solutions was adjusted with 10mol Lminus1 NaOH Stockstandard solution of 24-D herbicide (221mg Lminus1) was pre-pared in methanol kept under refrigeration and protectedfrom light

24-D commercial formulation (DMA 806 BR-DowAgroSciences Industrial LTDA) was obtained in the localmarket with a declared amount of 670 g Lminus1 A stock solutionof the commercial formulation (403mg Lminus1) was prepared inaqueous media kept under refrigeration and protected fromlight as well The 24-D commercial formulation was appliedto the soil surface using a watering container at themaximumamount specified by the manufacturer (503mg Lminus1) Fora uniform distribution of the herbicide both soils werecarefully irrigatedwith 6 liters ofwater just after the herbicideapplication

22 Instrumentation Voltammetricmeasurements were per-formed using a potentiostatgalvanostat AUTOLABPGSTAT30 in an electrochemical cell made from borosilicate glasswith a Teflon cover which contains orifices to insert the

working (GPU) reference (AgAgCl-KCl 30mol Lminus1) andauxiliary (platinum wire) electrodes and for nitrogen purg-ing

The GPU electrode was constructed by mixing graphiteand polyurethane resin at the ratio 60 40 (ww) [26] Thereis no special condition for electrode storage The easy andfast regeneration of the GPU electrode surface was achievedby polishing the electrode disk surface with a 2000 gritsand paper before each measurement to get a clean andreproducible electrode surface

23 Electroanalytical Methodology Analytical curves wereobtained through the external standard calibration proce-dure The sensitivity was checked by the limit of detection(DL = 3120590120579) and quantification (QL = 10120590120579) where 120590 is thestandard deviation of ten voltammograms registered for theblank and 120579 is the slope of the analytical curve [32] for anaverage of six curves

The precision was calculated by the relative standarddeviation (RSD) and checked by an experiment performedin one day (119899 = 10 intra-assay precisionrepeatability) aswell as by experiments in different days (119889 = 6 interassayprecisionintermediate precision) with a 033mg Lminus1 24-Dsolution

The accuracy was evaluated in terms of relative error(bias ) obtained from 24-D recovering experiments for twosoils (sandy and clayey) The experiments were performed(119899 = 3) by adding 100mL of water to 50 g of dried soil Themixture was fortified with a 033mg Lminus1 24-D solution (iespiked with 150 120583L from a 221mg Lminus1 24-D stock solution)agitated for 24 h in a horizontal shaker and centrifugedfor 20min at 13500 rpm (Sorvall SL-50T Rotor diameter= 222 cm) The pH of the supernatant was adjusted to 20with concentrated phosphoric acid and the concentrationof 24-D was determined with the previously developedelectroanalytical method by means of a standard additionprocedure where 50 120583L of a 221mg Lminus1 24-D stock solutionwas used to obtain concentrations of 088 143 and 197mgLminus1 of 24-D The 24-D estimated concentration was directlyobtained through extrapolation in the 119909-axis from the linearregression of the 119868

119901

versus 11986224-D curvesThe robustness of the methodology was checked during

the optimization of the electrolyte pH After selecting thebest pH value a small pH variation study was done nearthis region in order to measure the methodology capacity toremain unaffected by short-term pH fluctuations A statis-tical analysis was performed (119905-test with a 95 confidenceinterval) to estimate the effect of small pH variation on themethodology performance

24 Soil Columns Experiments The commercial herbicideformulation was applied to the surface of two types of soila sandy textured (AM1) and a medium clayey textured soil(AM2)

Soil samples were chemically (pH in CaCl2

organicmatter-OM and cation exchange capacity-CEC) and physi-cally (clay silt and sand percentages) characterized in the soillaboratory PIRASOLO Piracicaba Sao Paulo Brazil


(a)

Assay tube

Vacuum pump

PVC tube

Soil


solution

(b)




2

SO4


2




0

5

10

15

20

25

(d) (c)

(f)

(e)

(b)

(a)

00 03 06 09 12

minusI

(120583A

)



119894

= 003V 119905acc= 10 s and 119864acc = 00V





0

20

40

60

0

4

8

12

minusI

(120583A

)

minusI p

(120583A

)

0 5 10 15 20 25

minusE E A )

12 (mV sminus1)12


119901

versus V12 graph (1199032 = 0995)


119901


119901


119901


119901


119901



119894







119875




119875


119875










[2]








0

10

20

30

40

50

0

3

6

9

12

15

minusI

(120583A

)

minusI p

(120583A

)

minusE E A )

C Lminus1)


119894









00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)


119894
















AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


(a)

Assay tube

Vacuum pump

PVC tube

Soil


solution

(b)




2

SO4


2




0

5

10

15

20

25

(d) (c)

(f)

(e)

(b)

(a)

00 03 06 09 12

minusI

(120583A

)



119894

= 003V 119905acc= 10 s and 119864acc = 00V





0

20

40

60

0

4

8

12

minusI

(120583A

)

minusI p

(120583A

)

0 5 10 15 20 25

minusE E A )

12 (mV sminus1)12


119901

versus V12 graph (1199032 = 0995)


119901


119901


119901


119901


119901



119894







119875




119875


119875










[2]








0

10

20

30

40

50

0

3

6

9

12

15

minusI

(120583A

)

minusI p

(120583A

)

minusE E A )

C Lminus1)


119894









00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)


119894
















AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


0

20

40

60

0

4

8

12

minusI

(120583A

)

minusI p

(120583A

)

0 5 10 15 20 25

minusE E A )

12 (mV sminus1)12


119901

versus V12 graph (1199032 = 0995)


119901


119901


119901


119901


119901



119894







119875




119875


119875










[2]








0

10

20

30

40

50

0

3

6

9

12

15

minusI

(120583A

)

minusI p

(120583A

)

minusE E A )

C Lminus1)


119894









00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)


119894
















AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









[2]








0

10

20

30

40

50

0

3

6

9

12

15

minusI

(120583A

)

minusI p

(120583A

)

minusE E A )

C Lminus1)


119894









00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)


119894
















AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


00 03 06 09 12

0

10

20

30

0 20 40 60 80

00

03

06

09

12

(g)

(d)(c)

(f)(e)

(b)

(a)

t (days)

minusI

(120583A

)


C(m

g Lminus1)

(a)

0 20 40 60 80 100

00

05

10

15

(f)(e)

(d)

(g)

(c) (b)

(a)

00 03 06 09 12

t (days)

0

10

20

30

minusI

(120583A

)


C(m

g Lminus1)

(b)


119894
















AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




AM10ndash25 cm NDa




25ndash50 cm NDa

50ndash75 cm NDa






4 Conclusions





Acknowledgments


References




















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of













































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

research article electroanalytical methodology for the

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