Development of a chemically modified electrode based on carbon paste and functionalized silica gel for preconcentration and voltammetric determination of mercury(II)

Development of a chemically modified electrode based on carbon paste and functionalized silica gel for preconcentration and voltammetric determination of mercury(II)
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  Analytrca Chunlca Acta, 271 1993) 143-148 Elsewer Science Pubhshers B V , Amsterdam 143 Development of a chemically modified electrode based on carbon paste and functionalized silica gel for preconcentration and voltammetric determination of mercury(H) L M Aletxo, M de Fiittma B Souza, 0 E S Godmho, G. de Ohveu-a Neto and Y Gush&em Instrtuto de Quikca, Urucamp, C P 6154,13081 Campmas, SP Brazd) Jo& C Morelra Instttuto de Quiimca, Unesp, C P 174, 14800 Araraquara, SP Brad (Received 10th Aprd 1992, revised manuscript recewed 3rd August 1992) Ahstraet A mercury-sensltlve chermcally modlfied electrode (CME) based on modtfied sd~ca gel-contauung carbon paste was developed The functional group attached to the slhca gel surface was 3-(2-th~obenxunldazolyl)propyl, whrch IS able to complex mercury ions This electrode was apphed to the detenmnatlon of mercudI1) Ions m aqueous solution The mercury was chenucally preconcentrated on the CME prior to voltammetnc determmahon by amxhc stnppmg m the dlfferentlal-pulse mode A cahbratlon graph covermg the concentration range from 0 08 to 2 mg 1-l was constructed The preclslon for SIX detenmnatlons of 0 122 and 0 312 mg I-’ Hg(II) was 3 2 and 2.9% (relatwe standard dewatlon), respectwely The detection hmit for a 5-mm preconcentratlon penod was 0 013 mg I-’ A study for foreign ions was also made Keywords Voltammetry, Carbon paste electrodes, Chemically moddied electrodes, Functlonahzed slhca gel, Mer- cury, Preconcentrafion Chemrcally modified electrodes (CMEs) have been developed m recent years for use m voltam- metxx measurements [l-S] One interest IS the use of these electrodes as chemical sensors for analytical determmatlons Several materials have been employed as surface modifiers urlth the ob- jective of preconcentratmg the analyte at the electrode surface via a selective reaction with the surface modifier The surface-bound analyte IS then determined by a conventIona voltammetrlc method [2-81 Correspondence to LM Aleno, Instltuto de Quimlca, Uru- camp, C P 6154, 13081 Campmas, SP (Brazil) The aim of this study was to develop a carbon paste CME for Hg(II) determmatlon Voltam- metric approaches to mercury determmatlon usu- ally mvolve the use of solid graphite electrodes, mcludmg stnppmg analysis [9] Gold electrodes are also used, m either a stationary or a flow system, for envlromnental samples [lo-131 The determmatlon of Hg(I1) urlth chemically modified solid electrodes has been reported [14-171 The use of carbon paste electrodes wth hgands mcor- porated m the carbon paste has also been at- tempted [l&19], with the purpose of nnprovmg the selectlvlty of the voltammetrlc determmatlon In this work, functlonahzed slhca gel was em- ooO3-2670/93/ 06 00 0 1993 - Elsevler Science Pubhshers B V All nghts reserved  144 ployed as preconcentratlon agent m a carbon paste CME Investigations on the preparation and uses of slhca gel wrth organic functional groups covalently bonded on its surface for pre- concentration of ions have been reported by oth- ers [20-231 2-Mercaptobenzlmldazole, which has already been employed for the determmatron of Hg(I1) [24], was chosen as the functlonahzatlon agent It was incorporated m the slhca gel surface by reaction with (3-chloropropyl)sica gel, result- mg m the product 3-(2-thlobenzlmldazolyl)propyl sdlca gel [25], which was employed as the elec- trode surface modifier This work mvolved the construction and voltammetrlc study of a chemically modified car- bon paste electrode (CMCPE) with functlonal- lzed slhca gel m presence of Hg(I1) ions One advantage of 2-mercaptobenzlmldazole 1s the good selectlvlty that it presents as a complexmg agent for Hg(I1) ions [25] The chemical selectm- lty of this functlonal group and the selectivity of voltammetry were combmed for preconcentratlon and determmatlon EXPERIMENTAL Apparatus and reagents Voltammetnc measurements were made with a Metrohm E 506 Polarecord and a Metrohm E 505 stand The voltammetrlc cell was a three- electrode system, conslstmg of the carbon paste electrode modified with functlonahzed s&a gel as the workmg electrode, an Ag/AgCl, saturated KC1 as the reference electrode and a platinum wire as the awuhary electrode Reagents The graphite used was UCP-1-200-U (Ultra Carbon), passed through a stamless-steel sieve of 250 mesh The modrfled slhca gel functlonahzed with 3-(2-thlobenzlrmdazolyl)propyl groups was prepared according to the literature [251 Electrode construction The CMCPEs were prepared with a mtiure of graphite, 37% NUJO~ nd silica gel functlonahzed with 3-(2-thlobenzlmldazolyl)propyl m propor- 5 In L M Aleuo et al /Anal Chun Acta 271 1993) 143-148 zyxwvu - Copper Wire - Platinum Wire - Carbon Paste + Functlonallzod Slllca Gel 3mm Inner) FIN 1 Design of the workmg electrode tlons of 5, 10 and 20%, respectively After the mtiure had been homogemzed, it was used to fill a glass tube of 1 d 3 mm up to a height of 2 5 cm The extremity of the tube was cut at an angle zyxwv  Fig 1) Electrical contact was made with a platinum wire, with one end placed m the carbon paste of the tube and the other attached to the copper wire of the electric cable The external surface of the electrode was smoothed on soft paper A new surface can be produced by scraping out the old surface and replacing the carbon paste Electrode condttwnuzg An electrode wrth a fresh surface was precon- ditioned by mnnersmg it m a stlrred 15 mg 1-l mercury solution at pH 3 00, adjusted with HNO,, for 10 mm wth open arcmt The speed of stirring was 500 rpm The dlfferentlal-pulse voltammo- gram was recorded over the range - 0 2 to + 0 6 V m 0 01 M KNO, solution In fact It was ob- served that this solution was also about 0 001 M m KC1 owmg to diffusion from the reference electrode The surface of electrode was then cleaned for 3 mm m 0 01 M KCN solution with open clrcult The cycle was repeated three more  L M AIeuro et aL Anal Chum Acta 271 1993) 143-148 145 tunes and m the final step the preconcentratlon tune was 5 mm Used surfaces also need four precondrtlomng cycles of about 5 mm Determinatwn zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA f mercury Preconcentratwn step The CMCPE was em- mersed m 25 ml of stirred 10-6-10-7 M mercury solution for a given period, with the pH value m the range 13-5 5 adjusted with HNO,, m a polyethylene beaker, and the operation was made with open circuit The electrode was then re- moved from the preconcentratlon cell and dried with absorbent paper Electrochemical measurement After the pre- concentration step, the electrode was nnmersed m the electrochemical cell contammg 0 01 M potassmm nitrate solution The differential-pulse voltammogram was then recorded over the range - 0 2 to + 0 6 V The supporting electrolyte was changed after each measurement and the elec- trode was cleaned with KCN as m the condltlon- mg step RESULTS AND DISCUSSION The properties of the material used m the carbon paste electrode were studied by Morelra et al [251 The slhca gel used was slhca modified with 3-chloropropyl groups, and rt can react with 2-mercaptobenzunldazole through the N or S atom Then results favoured the S atom, m ac- cord with the reaction lCH3l3 Cl +2 l+<D H The collection of mercury at the surface elec- trode 1s effected by the coordmatlon of metal through the sulphur atom of the product of the above reaction, 3-(2-thlobenzlmldazolyljpropyl silica gel, because protonatlon of the nitrogen a t z t 2 0 I I I I I to6 04 to2 00 -02 zyxwvutsrq E V Fig 2 Repetltwe cychc voltammogram of a 0 4 mg 1-l Ha(II) solutton The workmg electrode was a CMCPE urlth 10% of modlfied &a gel The reference elecrode was Ag/AgCl, saturated KCl, and the auxdlary electrode was a platinum wire Potential range, - 0 2 to +0 6 V, scan rate, 10 mV s-l atom of the -N=C-S- group take place m acidic media [25] Several characterlstlcs of the CMCPE with functlonahzed slhca gel for mercury concentra- tion measurements were exammed Figure 2 shows a repetitive cychc voltammogram of mer- cury solution m the potential range -0 2 to +0 6 V vs Ag/AgCl, saturated KCl, where It can be seen that the anodlc peak at 288 mV 1s well defined and does not change during the cycles The cathodic peak, around 0 00 V, 1s irregular and changes its form m each cycle The electrode process 1s then irreversible and the anodlc peak was chosen for the peak current measurement of mercury After the preconcentratlon step made with open arcult, the electrode was placed m the voltammetrlc cell with the supportmg electrolyte and a potential of - 0 2 V was applied at the begmmng of the scan The electrochemical reac- tion at the electrode can be expressed as  L M Alem et al /And Chun Acta 27X 1993) 143-148 ” I 0 i ; f 4 d B PH Ftg 3 Effect of the pH of the preconcentratlon solution of Hg(I1) on the anodlc peak current of Hg(II) Mode, dlfferen- teal-pulse voltammetry, preconcentratlon time, 5 mm, AE= - 50 mV, scan rate, 8 mV s -’ Hg(II) concentration, A = 0 2, 04,0=08mgI-’ When the potential scan reaches the value of +288 mV, an anodlc peak IS produced owmg the oxldatlon of mercury Hg” * Hg2++ 2e- The scan rate of 8 mV s-l used m the dtier- entlal-pulse mode showed no necessrty for a fared time at -0 2 V for the reduction of mercury and the scan started lmmedlately after the connectlon of the electrodes Figure 3 shows the relatlonshlps between the measured current and pH at mercury concentra- tions of 0 2, 04 and 0 8 mg 1-l The maxunum current was obtained at pH 4 0, but It was ob- served that at pH 3 0 the method shows a better tolerance to interfering ions The effect of the paste composition for elec- trodes containing 5, 10 and 20% of functlonahzed slhca 1s shown m Table 1 The results for a 0 4 mg l- ’ mercury01) solution Indicated that the paste containing 10% of modlfled slhca produced the highest current The cleaning solution was used 0 01 M KCN and was found to by very efficient EDTA, KC1 TABLE 1 Effect of carbon paste composltlon on the peak current of mercury(II) Concentration of modtied Current RSD (%) silica gel in carbon paste (%) (/LA) n=6) zyxwvutsrqp 50 120 46 100 172 36 200 136 40 and KSCN solutions were also tested, but were not efficient The mfluence of accumulation tune m the preconcentration step for mercury concentrations of008,02and04mg1-1 1sshownmFlg 4 The greatest increase m current occurred m the first 5 mm and this time was chosen for the preconcen- tration step Figure 5 shows a cahbratlon graph of current vs mercury concentration m the range 0 08-2 mg 1-l and Fig 6 shows some voltammograms of points on this graph The first four pomts on the curve (0 08-O 6 mg 1-l) obey the straight-line equation 1= 4 1oc - 0 073 I Tlme/mln Fig 4 Effect of the precxmcentratlon tnne on the anodlc peak current at various Hg(II) concentrations Mode, dtfferentlal- pulse voltammetry, A E = -50 mV, scan rate, 8 mV s-l Hg(II)concentratlon A =008, =02,0=04mgl-  L M Alerro et aL /Anal Chun Acta 271 1993) 143-W zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH 4- a c E 3- t kl ” 2- l- 0 I I , I 0 05 1 15 2 2 Concrntrotlon /mg/l Fig 5 Callbratlon graph of current versus mercury concentra- tion Mode, dfierentlal-pulse voltammetry, preconcentratlon time, 5 mm, AE = -50 mV, scan rate, 8 mV s-l 1 I 06 02 -0 2 E V) Fig 6 Dtierentlal-pulse voltammograms for dtierent Hp(II) wncentratlons after S-mm preconcentratlon (a) 0 2, (b) 0 4, (cl 0 6, (d) 0 8 mg I-’ AE = -50 mV, scan rate, 8 mV s-l 147 TABLE 2 Study of the effect of mterfenng Ions on the voltammetnc signal of mercury(I1) B Metal Ion (X) [Xl/D-k*+ Relatwe signal b PM111 100 097 z&I) 100 0 98 cd10 100 098 NI(II) 100 099 cd(II) 10 096 100 102 CUGI) 10 0 95 100 103 Ag(I) 10 104 100 137 a [Hg”] = 0 4 mg I-‘, accumulation time = 5 mm, pH = 3 0 b The relative signal IS the ratio of the peak current of Hg(II) m the presence to that m the absence of foreign Ions with a correlation coefficient of 0 998, where z IS the current m @A and C 1s the concentration of Hg(I1) m mg 1-l For points between 0 6 and 2 mg I-‘, the plot shows non-hnear behavlour, which reqmres the use of a calibration graph m this concentration range It was observed that the points m the non&near range obey a loganthmlc equation z=1021ogC+356 with a correlation coeffkxent of 0 999 The preczszon obtamed for s1x replicate deter- mmatlons of each of 0 122 and 0312 mg 1-l Hg(I1) m the linear range of the cahbratzon graph was 3 2 and 2 9% (relative standard deviation), respectively The detection hmlt evaluated at a szgnal-to-none ratio of 3 for 0 08 mg I-’ Hg(I1) was 13 pg 1-l The results of the study of the influence of foreign ions 1s presented m Table 2 Dlvalent ions showed virtually no interference m the concentra- tion range studied However, AgG) at a concen- tration ratlo of 10 1 showed slgmflcant mterfer- ence, as usually occurs m the voltammetrlc deter- mination of mercury in presence of sdver The results obtamed m this study show that the CMCPE with functlonahzed slhca gel has good potential for use m the voltammetrlc deter- mmatlon of mercury The hgand used, 3-(2~thlo- benzlmldozolyllpropyl, has a good selectlvlty for
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