Direct voltammetric determination of total iron with a goldmicroelectrode ensemble
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    Direct voltammetric determination of total iron with a goldmicroelectrode ensemble Direct voltammetric determination of total iron with a goldmicroelectrode ensemble Document Transcript

    • Full PaperDirect Voltammetric Determination of Total Iron with a GoldMicroelectrode EnsembleE. A. Zakharova,*a E. E. Elesova,a G. N. Noskova,a M. Lu,b R. G. Compton*ba Physical and Analytical Chemistry Department, Tomsk Polytechnic University, Tomsk, Russiab Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford Chemistry, South Parks Road, Oxford OX1 3QZ, UK*e-mail: Richard.compton@chem.ox.ac.ukReceived: July 31, 2012; &Accepted: August 28, 2012 Abstract The behavior of the Fe(II)/(III) redox system at a Au microelectrode ensemble (Au-MEE) based on a solid compo- site by means of direct and cyclic voltammetric analysis (VA) is reported. With a simple electrode activation and sample preparation, the influence of dissolved organic substances was eliminated, providing highly sensitive results. The analytical signal was based on the maximum cathodic current (I) of the first derivative (dI/dE), and iron deter- mination within the 0.002–0.04 mg LÀ1 range was studied. A sensitive LOD (3s) value of 0.7 mg LÀ1 for total iron concentration was calculated; total iron determination in different waters was shown. Keywords: Iron, Gold microelectrode ensemble, Microelectrodes, Voltammetric determination DOI: 10.1002/elan.2012004171 Introduction cence [13]. Spectrophotometric methods [6, 14, 15] have been widely used, however, these are relatively expensiveIron is an abundant element in the environment [1] which for use in ultra trace determination of metal ions and canplays an important role in environmental, industrial, suffer from interferences. The size and weight of the in-human and biological systems [2], and as such, is of key struments also restricts their use to land based experi-interest for analytical studies. Although iron exists in ments.many oxidation states ranging from ÀII to + VI [3], it Electrochemical methods such as stripping voltamme-most commonly appears in its + II and + III oxidation try have been favoured due to their inexpensive, simplestates in the environment. Many natural Fe(III) chelators and rapid nature [16, 17]. Anodic and cathodic strippingexist in seawater and up to 99 % of iron in waters is be- voltammetry, in particular, have shown many benefitslieved to be found complexed to organic ligands and thus such as good selectivity and highly sensitive determina-its concentration is low except when extremely stable tion [18]. Sensitivity of direct voltammetric (VA) deter-complexes are formed [4]. Concentrations of iron ranging mination of iron is limited by the value of the residualfrom 0.1 nM to 10 nM [5, 6] have been suggested. The (capacitive) current and to date, publications report limitsconcentration of dissolved iron in freshwater is also in the 10À7–10À6 M range [19, 20]. A significant increase inthought to be greater than that of seawater [7] due to the sensitivity can be achieved by the use of adsorptive vol-proximity to terrestrial and continental shelf sources. tammetry on mercury electrodes [21], however, the useSoluble iron species in natural waters can vary from of toxic mercury is highly undesirable in routine analysis10À6 M for river water to 10À9 M for coastal sea water and due to the drawbacks of mercury toxicity [22, 23]. As anto 10À11 M for ocean water [8]. The World Health Organi- alternative, the use of graphite and other carbon electro-zation (WHO) does not issue a health based guideline des for determination of iron at trace levels has been ex-value for iron [9]; however acceptably permissible values plored involving a preliminary activation of the surfacein drinking water likely range from 0.3–3 mg LÀ1, as levels by thermal [24], electrochemical [25] and other methodsgreater than this causes water to become discolored and [26]. Graphite electrodes for VA determination of iron intaste metallic. water and various food products were used, where activa- The abundance of iron and its variable oxidation states tion of the surface results in modifying the electrode sur-in the environment calls for their accurate determination face with functional groups (carboxyl, carbonyl, quinoid,and quantification. Many analytical methods have been phenol [27]), accelerating the reduction or oxidation ofdeveloped to detect and quantify iron; these include vol- iron ions.tammetry [10], flow injection analysis [6], chemilumines- Recently, ensembles of nano and microelectrodes werecence [11], spectrophotometry [12] and spectrofluores- used in electroanalysis [28]. Several advantages overElectroanalysis 2012, 24, No. 11, 2061 – 2069  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2061
    • Full Paper E. A. Zakharova et al.macro-electrodes were seen, including higher sensitivityÀ ments a graphite electrode impregnated with polyethy-achieved by a high signal to noise ratio, ability to work in lene and paraffin mixture was used.environments with high resistance, compatibility with Voltammograms were produced in linear sweep poten-a two electrode cell configuration, amongst other bene- tial form as well as cyclic mode (CV) in normal current-fits. potential coordinates (I–E) and in the form of the first The use of carbon microelectrodes ensembles (C- derivative of dI/dE–E [29]. The analytical signal used forMEE) modified with gold (Au-MEE) for iron determina- calculation was the maximum peak in the dI/dE plot.tion within the 0.002–0.02 mg LÀ1 range is reported here The Au/CB/PE surface was characterized by a Scanningand represents an approach not previously seen in the lit- Electron Microscope (SEM) Instrument (Oxford Lab);erature. A number of factors were studied in order to an ICP-MC (iCAP-6500 Duo, Thermo Scientific, UK)obtain an optimized analytical signal for iron was used for comparative testing.(0.003 mg LÀ1, 1/100 of the maximum permissible level)using gold ensemble modified carbon nanocompositeelectrodes (carbon black-polythene = CB/PE) (Au-MEE). 2.3 Electrode PreparationWe report a method for modifying CB/PE by gold, tech-niques to activate the electrode, the influence of back- 2.3.1 Composite CB/PE Electrodeground nature and concentration, signal mode, varying The fabrication of CB/PE electrodes by a semicommercialiron ion concentration, and influence of interfering for- “casting” method was reported previously in 2011 [30].eign ions on the Fe signal. As a result of optimizing the Briefly, composite granules were loaded in a casting ma-parameters, a simple and low cost method for trace iron chine. After melting, the composite was heated to a tem-determination in aqueous solutions using long lived and perature of 160 8C before the casting process. The body ofsensitive Au-MEEs is proposed. the electrode was press-formed into low pressure polyeth- ylene (thickness of 0.55 mm, inside diameter of the work-2 Experimental ing part of the shell 3.9 mm with contact of stainless steel wire d = 1 mm) before the molten composite was then2.1 Materials sprayed into the electrode body under 65 kg cmÀ2 pres- sure. The mould was cooled before the newly made elec-All chemicals and reagents were of analytical grade quali- trodes could be removed. This technology allowed aboutty. Standard solutions (GOST, Russia) of 1 mg mLÀ1 30 electrodes to be produced per hour.Fe(III), Cr(VI), Cu, Zn, Ag, As(III), etc. were used. Solu- The electrode surface was cut using a special cutter.tions of lower concentration were prepared by dilution The newly cut work surface (in the form of a disc, withwith doubly distilled water. The volume of the sample so- diameter, d = 3.9 mm) did not require polishing as its sur-lution was 10.0 mL; the concentration of the analyte was face presented a microelectrode ensemble of carbon-evaluated by addition of the standard solution (0.010– black particles (C-MEE) separated by insulating polyeth-0.10 mL) to the sample. The composite material for man- ylene. The electrode resistance was in the order of 2–ufacture of the working electrodes was industrially pro- 3 kW. When not in use, the composite electrode (C-MEE)duced (“Tomskneftekhim”, Tomsk) carbon concentrate of was stored under a protective cap.technical polyethylene (master batch) in the form ofblack color granules from 1.4 to 5.0 mm in size. The con-centrate consists of 30 % technical carbon black N220 2.3.2 Preparation of a Gold Microelectrode Ensemblewith particle sizes, 24–33 nm, and 70 % thermally stabi- (Au-MEE)lized polyethylene of high pressure. Experiments were carried out at room temperature The modified gold microelectrode ensemble was prepared(22–23 8C) without removal of oxygen from the solutions. by electrodeposition of gold on a freshly cut C-MEE sur-During the optimization stages, nitrogen gas was used to face. Electrolysis was carried out in a 500 mg LÀ1 solutionpurge the solution for de-oxygenation purposes. of AuCl4À , at a potential of 0.0 V for a certain period of time (10–300 s). An SEM-image of the gold particles can be seen in Figure 1. Prior to deposition, the working elec-2.2 Apparatus trodes were activated by electrochemical treatment (ET),Voltammograms were obtained using an electrochemical which depends on the composition of the solution andworkstation TA-4 (Tomanalyt, Tomsk) supplying three electrode material. The real (active) area of the Au-MEEelectrochemical-cells with three working electrodes. The surface was determined coulometrically by taking into ac-calculated result was the mean of three simultaneous in- count the charge of a gold oxide monolayer in 0.05 Mdependent measurements. A three-electrode system was H2SO4 (400 mC cmÀ2). For this, a cyclic voltammogramused, consisting of a vibrating composite CB/PE-electrode was recorded in the range, 0 to 1.5 V, with scan rate of(3.9 mm in diameter) or the same modified with Au (Au/ 100 mV sÀ1 and the peak area of Au2O3 reduction wasCB/PE) as working electrodes, and Ag/AgCl in 1 M KCl measured [31]. Typically, the surface area of the goldas the reference and counter electrode. In some experi- micro-ensembles was 0.01–0.02 cm2.2062 www.electroanalysis.wiley-vch.de  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 11, 2061 – 2069
    • Voltammetric Determination of Total Iron with a Au-MEE mode (dI/dE vs. E). Figure 2 shows an example of the Fe(II)/(III) system at a Au-MEE electrode in HCl. From Figure 2, one can see that the derivative curves (B) have a better form for measurement of the iron signal than the usual I vs. E curves (A) since the capaci- tive current could be subtracted by this procedure. Also, Fe(III) additions gave proportional growth to both catho- dic and anodic peaks; however the cathodic peak shape is better resolved than the anodic peak. As such, all the an- alytical signals were subsequently produced from the cathodic reduction of Fe(III) and were measured in deriv- ative mode: dI/dE vs. E. 3.2 Carbon-Black Microelectrode Ensemble (C-MEE) Direct voltammetry of iron at C-MEEs showed that theFig. 1. SEM-image of C-MEE after modification with gold reduction signal of Fe(III) to Fe(II) occurs only afternanoparticles. a sufficiently positive polarization of the electrode in HClO4, within a potential range, 1.4 to1.6 V, where oxygen is produced from water. On a freshly cut C-MEE2.4 Procedures (as well as on freshly polished impregnated graphite elec-10 mL of background solution (or water sample) was trode) in the presence of 0.01 M acids, the voltammetricadded into three cells (a quartz cup with three electro- signal for the reduction of iron(III) was not detected updes). Electrochemical treatment of the working electrodes to a concentration of 0.3 mg LÀ1 without prior pretreat-was performed in the solutions as noted below. After the ment. However, application of electrochemical treatmentcleaning stage, using electrode vibration and equilibrium (ET), by applying a positive potential of 1.5 V for 15 s inpotential (Eeq), direct current voltammograms of iron ion a solution of 0.01 M HClO4, leads to the appearance ofreduction (III!II) was obtained by linear change of the a peak current for Fe(III) at a potential close to 0.5 V. Atpotential from Eeq, with a scan rate of 20 mV sÀ1. The ana- long ET times, the peak current first increases, but thenlytical signal was the maximum peak of the first deriva- reaches a limit.tive (dI/dE vs E). Quantification was fulfilled by the stan- An impregnated graphite (IG) electrode, with surfacedard addition method. area 0.2 cm2, was compared with a carbon composite elec- trode (C-MEE). After the same anodic surface activation to the IG- electrode, the cathodic signal of iron(III) dis-2.4.1 Water Analyses for Total Iron appeared after 1.5 to 2 hours. The C-MEE however, al- lowed iron determination without surface renewal for atSample preparation: All forms of Fe in waters have to be least three days, whilst maintaining high signal reproduci-converted to Fe(III) and interfering organic substances bility. The calibration curve for iron at the C-MEE is(DOS) have to be destroyed. A general procedure was as linear within a limited concentration range (0.005–follows: the water sample (5–10 mL) was evaporated in 0.08 mg LÀ1), which is probably due to the small size ofquartz glasses with conc. HNO3 at 110–120 8C. The resi- the electrode active surface. Further experiments showeddue was heated in the oven at 450 8C for 20–30 min; if that modification of the C-MEE by gold can significantlyblack residue was seen in the glasses, it was further improve the analytical performance of the electrode forheated in the oven for 10 min at 450 8C; this was then dis- iron determination by direct voltammetry.solved in 1 mL HCl (1 : 1) and evaporated at 100–120 8C.Finally, the resultant residue was dissolved in 0.2 mL HCl(conc.) and 6.8 mL twice distilled water. An aliquot of 3.3 Gold Microelectrode Ensemble (Au-MEE)the solution could then be used in the cell with 0.005– A series of experiments showed that Au-MEEs obtained0.01 M HCl as background electrolyte. by electrolysis of an unstirred solution of 500 mg LÀ1 gold chloride (pH 2–3) at a potential of 0.0 V produced the best metrological characteristics (sensitivity, accuracy, re-3 Results and Discussion producibility) for iron determination. It should be noted that a short-term electrochemical treatment of the elec-3.1 Optimizing the Voltammetric Signal of Fe trode is necessary to obtain a good, stable signal forTo choose the form of analytical iron signal for optimized Fe(III). Investigation of the Au-MEE surface with scan-measurement, cyclic voltammograms were recorded at ning electron microscopy (Figure 1) showed that the elec-the working electrode. Data were in the form of both cur- trode consists of an ensemble of a large number of goldrent (I)–voltage (E) plots and also plots in derivative nano- or microelectrodes with particle size of 200–Electroanalysis 2012, 24, No. 11, 2061 – 2069  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 2063
    • Full Paper E. A. Zakharova et al.Fig. 2. Cyclic voltammograms (CV) of reduction (from 0.7–0.2 V) and oxidation (0.2–0.7 V) of Fe(II/III) system in 0.005 M HCl withAu-MEE. Scan rate: 20 mVÀ1. Left hand side: normal CV (I vs. E), right hand side: derivative CV (dI/dE vs. E) in the same 0.005 MHCl solution. (1) Background, (2) 0.02 mg LÀ1 Fe(III), (3) 0.04 mg LÀ1 Fe(III).300 nm, separated by a nonconducting substrate made of which did not exceed 30 % of the geometric areapolyethylene. (0.12 cm2). The carbon black content in the composite The height and shape of the iron reduction signals de- forms 30 % of the C-MEE, and since during electrolysis,pended on the amount of deposited gold. The electrode- gold is likely deposited on the most active particles onposition time (tdep) was varied from 10 s to 300 s in the surface of the electrode, modification of the electrode500 mg LÀ1 AuCl4À at Edep = 0.0 V in an unstirred solution. does not result in a continuous film on its surface. TheThe active surface area changed from 0.01 to 0.05 cm2. composition of the electrode surface is such that it be-The height of the signal increased with tdep, but this also haves in effect as a gold electrode of equivalent geomet-caused the residual current to increase. The accuracy of ric area since the diffusion fields associated with the goldiron(III) determination in known solutions of particles will overlap on the timescale of the experiment.0.020 mg LÀ1 of Fe(III) in all cases was nearly 100 %. The That said, the use of carbon/gold composites representssignal changes from wave form in coordinates I vs. E a lower cost option than bulk electrodes especially given(one peak at t = 10 s and 30 s in dI/dE vs. E coordinates), the long lifetime of the electrodes after activation.to a peak form in the I vs. E coordinates (double peak att = 90 s and 300 s in dI/dE vs. E coordinates). The lifetimeof the electrode changed from 12 days (tdep = 10 s) to 3.4 Different Factors Influencing the Iron Signal at a Au-25 days (tdep = 40 s or more), depending on the deposition MEEconditions. 3.4.1 Electrolytes As such, a compromise electrolysis time of 50 s for Audeposition was reached. As one can see from Figure 3, Due to the fact that iron(III) salts are known to hydro-the form of the signal in the dI/dE vs. E coordinates is lyze, 0.001–0.1 M solutions of the following acids H2SO4,not an ideal peak shape but nonetheless analytically HClO4, HNO3, HCl were studied as possible electrolytes.useful. Accumulation of gold on the C-MEE with tdep In all electrolytes, CV profiles of the cathodic current ob-from 10s to 90 s gave active surfaces on the Au-MEE served for the reduction of Fe(III) to Fe(II) was higher2064 www.electroanalysis.wiley-vch.de  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 11, 2061 – 2069
    • Voltammetric Determination of Total Iron with a Au-MEEFig. 3. Comparison of I vs. E (1’, 2’, 3’) and dI/dE vs. E plots (1, 2, 3) for Fe(III) reduction at a Au-MEE. Conditions for gold elec-trolysis on the composite C-MEE electrode were optimized: tdep = 50 s in 500 mg LÀ1 AuCl4À , linear potential scan from 0.7 to 0.2 V,scan rate = 20 mV sÀ1 (1) background, (2,3) consecutive additions of 0.04 mg LÀ1 Fe(III).than the anodic current. As discussed above, the cathodic The Fe signal was seen to be directly proportional to itspeak in the dI/dE–E coordinate was chosen as the analyt- concentration in the 0.005 to 0.2 mg LÀ1 range, and its de-ical signal of iron. pendence was investigated in a range of acidic media, the The iron signal dependence on the concentration slope of which increases in the order: HClO4 < H2SO4 <passes through a maximum, corresponding to the concen- HCl. This can be attributed to the order of increasingtration of background solution in the 0.001 to 0.010 M acid anion adsorption on the gold surface. Hydrochloricrange in all the investigated acids. Variation of the con- acid as a background electrolyte is favorable for the re-centration of the acids was studied to give the maximum covery of iron(III), as chloride ions are adsorbed on goldpeak of iron. It was seen that the peak potential differ- microelectrodes in potentials more positive than 0.0 Vence in the all acids is greater than 0.056 V, which is which may facilitate electron transfer by a bridging mech-a characteristic of reversible processes, so the limiting anism [32]. An additional factor that contributed to thestage is not based on pure diffusion. Judging by the value choice of hydrochloric acid as the optimal electrolyte wasof DEp, the reduction for Fe(III) to Fe(II) at a Au-MEE that traces of chloride appear in solution from the refer-occurs fastest in hydrochloric acid. ence electrode salt bridge, such that ClÀ ions as impuritiesElectroanalysis 2012, 24, No. 11, 2061 – 2069  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 2065
    • Full Paper E. A. Zakharova et al.may be present in the reagents used and in the analyzed the Au-MEE causes hydrogen to be released and a highsamples. Further from a test solution, we also noticed cathodic current background leads to the distortion of theminimal interference from chromium was seen within the iron(III) signal. Using potentials more positive thaniron signal whilst using hydrochloric acid as the electro- 0.70 V to conditions the Au-MEE, anodic dissolution oflyte. gold could be seen. The optimized conditions of ET was based on: sensitivity of the iron, values of the background peak, reproducibility of the iron peak and accuracy of3.4.2 The Potential Scan Rate (v) iron determination in the test solutions. In our experi-The effect of potential scan rate on the background and ments, sensitivity was calculated as the ratio of the analyt-analytical signal of iron in hydrochloric acid between 10 ical signal height to concentration (dI/dE to C).to 80 mV sÀ1 was studied. A scan rate of 20 mV sÀ1 was After the gold surface is activated, desorption of pas-chosen due to the best signal to noise ratio. We noted ear- sive impurities occur during the electrochemical treat-lier that interestingly, a stable oxidation current is record- ment. In our optimized conditions, reproducible signals ofed in the absence of iron using derivative voltammograms iron were received and electrode worked well for 3 weeksfor 0.01 M HCl (and all used electrolytes) in the potential without any further treatment.range, 0.40 to 0.50 V. This depended on the amount ofgold on the surface of the Au-MEE, solution composi- 3.4.4 The Dependence of Iron(III) Peak (dI/dEmax) on thetion, electrical treatment and electrode potential scan Concentration of Iron(III)rate. Using I vs. E coordinates, the oxidation peak (mag-nitude of 2–5 nA) is linearly dependent on dE/dt, which The chosen optimized conditions included using a 0.01 Mis typical for the redox of surface confined species. It is HCl background electrolyte and v = 20 mV sÀ1. Togetherinteresting to note that increasing the scan rate increases with the described method of electroactivation, the cali-the peak, however the peak potentials do not depend on bration dependence of the iron cathodic peaks measuredthe speed. This may indirectly indicate the reduction and in the range 0.002 to 0.4 mg LÀ1 by scanning from 0.7 tooxidation of some surface compounds in the 0.40 to 0.2 V was obtained as shown in Figure 4.0.50 V range. The peak height, H, was found to increase linearly with All the factors lead us to assume that after Au-MEE the concentration from 0.002 mg LÀ1 up to 0.4 mg LÀ1electro-treatment (À1.0 to 0.70 V) and in the presence of Fe(III). The equation for linear regression was y =oxygen, premonolayer oxides of gold on the Au surface 234.7 H + 0.55, R2 = 0.9994. The value of LOD (3s criteri-are formed, which reproduced stably at the gold activa- on) was calculated to be 0.7 mg LÀ1.tion step. The existence of such nonstoichiometric oxidesof gold particles at the nanoscale, together with electroca- 3.5 Interferencestalytic activity has been well documented by Burke [32],whereby they can play the role of mediator in electron It was found that the oxygen present in the solution is re-transfer. Possibly the presence of such oxides contributes duced at Au-MEE at potentials more negative than 0.0 Vtowards the reproducible and sensitive signals of iron(III) and does not interfere with the determination of iron.reduction, which conveniently appears within this poten- Moreover, the purging of an inert gas (nitrogen) fortial range. The analytical signal of iron is always mea- 10 minutes through an electrochemical cell to removesured as the difference between the peak height of iron oxygen, surprisingly, leads to less ideal peak shape ofand the background current. iron. In the I vs. E coordinates, the peak current for Fe(III) The additions of 0.01 M potassium nitrate, sodium,grows as I = k v0.3–0.4. The exponent value of less 0.5 shows magnesium, aluminum, 0.001 M alkali metal chloride,that diffusion of Fe(III) to the electrode may not be fully 0.0001 M perchlorate, bromate, persulfate, and nitrite dolinear under our conditions on the Au-MEE. As the form not interfere with the iron peak in hydrochloric acid. Theof the Fe signal was less than ideal at higher u, we chose addition of potassium chloride at a concentration greaterto use scan rate v = 20 mV sÀ1 for analytical measurements. than 0.005 M and sodium chloride greater than 0.03 M strongly influenced the signal of iron. The background signal splits and shifts in the negative direction, making it3.4.3 Electrochemical Treatment (ET) of Au Electrode difficult to correctly measure the signal for iron. ThisActivation of the Au electrode is an important step for highlights the importance of the surface gold oxide inobtaining stable and reproducible signals for Fe(III)! Fe(III) reduction.(II). This was performed by alternating the polarization The 10-fold excess of Cu, Zn, Bi, Sb(III), As(III),of Au-MEE in the negative and positive potentials of Se(IV) ions does not have an interfering effect. Silver wasiron where Ep = 0.40 to 0.50 V. It was established that in added in a 1 : 1 ratio without any signal distortion. Thehydrochloric acid, ET pulses of alternating potential from effect of chromate was studied in great detail and showedÀ1.0 to 0.70 V (for 1 s at each potential step, for 8 s) are that with the selected conditions for electrode activation,effective before registering each voltammogram. At more a 10-fold excess of chromate may be present with nonegative potentials, ET (less than À1 V) at the surface of effect on the iron signal. The addition of Fe(II) causes an2066 www.electroanalysis.wiley-vch.de  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 11, 2061 – 2069
    • Voltammetric Determination of Total Iron with a Au-MEEFig. 4. Plot of maximum (dI/dE) vs. C for Fe(III) in 0.01 M HCl at a Au-MEE electrode with a scan rate of 20 mV sÀ1.increase in the cathodic signal for Fe(III). When the water and natural water (Siberian region) samples arecathodic potential is set at 0.7 V, Fe(II) is oxidized to shown in Tables 1, 2 and 3. Quantification of the totalFe(III). Then following a potential sweep from 0.7 to iron was proved using analysis of different aliquots of the0.2 V it is reduced with the Fe(III) diffusing to the elec- same sample (Table 1), calculating the recovery aftertrode. Therefore concentration of iron determined by the spiking the sample with known Fe quantities (Tables 1, 2)proposed method refers to the total content of iron: and finally comparing iron determination using two meth-Fe(II) and Fe(III). ods: our proposed voltammetric analysis and ICP-MC (Table 3). As one can see from Tables 1 and 2, different aliquots3.6 Application to Fe(III) Determination in Water of water samples gave the same amount of Fe, however Analyses after spiking, this increased in accordance with the addedThe analysis of synthetic mixtures (three electrodes in quantity.three cells) on the content of iron(III) spiked within the As can see from Table 3, the 2 different analytical tech-range 0.03–5.0 mg LÀ1 showed that recovery was from niques gave very comparative results.90 %–120 % with Sr = 12–15 %. The analysis of the watersamples were proceeded analogously but instead ofadding aliquots of the standard solution, a volume of the 4 Conclusionsprepared sample was added to a 0.01 M HCl backgroundelectrolyte. The method for sample preparation depends It has been shown that through optimized conditions ofon the water quality. For cleaner water it was adequate preparing and activating the working Au-microelectrodeenough to dilute the sample with twice distilled water. ensemble, it is possible to determine the total iron con-The results of analysis on tap water (Tomsk), industrial tent in the 0.002–0.4 mg LÀ1 range, giving a LOD (3s)Table 1. Analysis of tap water for Fe by the proposed VA procedure with Au-MEE.Sample Volume of the sample Found Fe (mg LÀ1) Smean (mg LÀ1) [a] aliquots (mL) (n = 3; P = 0.95)Tap water 1.5 0.098 Æ 0.006 0.103 1.0 0.104 Æ 0.007 2.0 0.108 Æ 0.021Tap water spiked with Fe(III) 0.30 mg LÀ1 0.2 0.406 Æ 0.022 0.406 0.5 0.396 Æ 0.032 1.0 0.415 Æ 0.018[a] After sample preparation.Electroanalysis 2012, 24, No. 11, 2061 – 2069  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 2067
    • Full Paper E. A. Zakharova et al.Table 2. Recovery of Fe in industrial and natural waters (after sample preparation); n = 6; P = 0.95.Sample Found Fe in samples Spiked Fe(III), Found Fe in spiked samples Found in spiked sample Recovery (mg LÀ1) (mg LÀ1) (mg LÀ1) (mg LÀ1) (%) (addition)Waste water 73–1 2.37 Æ 0.27 3.60 5.82 Æ 0.45 3.45 95.8Waste water 73–2 2.18 Æ 0.24 2.80 4.61 Æ 0.39 2.43 86.8 3.60 5.77 Æ 0.48 3.59 99.7Waste water 73–3 1.71 Æ 0.13 4.80 5.85 Æ 0.61 4.14 86.3Swampy water 11 4.15 Æ 0.51 6.00 10.9 Æ 1.7 6.72 112Tap water 0.214 Æ 0.024 0.120 0.332 Æ 0.02 0.118 98.3 0.550 0.769 Æ 0.03 0.56 101Table 3. Fe determination in the natural waters of the Siberian region by our proposed VA method and ICP-MS [a].Sample Fe content (mg LÀ1) VA Fe content (mg LÀ1) ICP-MCWell-water, 2500–2504 m of depth 128 121Pore water 189 197Petroleum well water 171 177Petroleum well water 107 120Stratal water, different regions 1.81 1.71 9.7 9.6 55 57 2.38 2.52Well water 5.8 5.7River waters, different samples 0.44 0.43 0.78 0.69 0.63 0.56 1.00 1.07[a] Analyses were made in the Scientific Research Laboratory of Hydrogeochemistry in the Scientific Educational “Water” Centre ofTomsk Polytechnic University (TPU, Tomsk).value of 0.7 mg LÀ1. Determination of such low concentra- is advantageous in being able to operate in a stable, re-tions of iron by direct cathodic voltammetry with currents producible and long-term manner on a single electro-at nA level was possible due to the nature of the elec- chemical activation in the electrolyte solution.trode and characteristic of the analyzer TA-4 which Application of Au-MEE as a working electrode hasallows currents down to 0.1 nA to be recorded without in- many advantages in electrochemical analysis includingterference or noise. The cathodic current of Fe(III) reduc- the absence of toxic mercury and expensive Pt. By apply-tion in the I vs. E coordinates gave a waveform but no ing an easy electrochemical treatment (ET) step, a stableclear peak, however, a clear peak proportional to Fe con- electrode lasting 3–4 weeks can be used to produce repro-centration could be seen in the dI/dE vs. E coordinates. ducible and sensitive results. As well as this, the ease and Studies have shown promising results of using Au-MEE low cost of operation due to simplicity of preparation andto determine low concentrations of iron, for example, in regeneration suggests that semi-industrial production ofultra-pure water. Other water types demand only Au-MEE electrodes is important and should be morea simple sample preparation, after which the sample is wide-spread for successful electroanalysis of trace iron.dissolved in a HCl background. Using 0.01 M HCl avoidsinterference from trace impurities and gave the most sen-sitive Fe(III) determination compared to other acids. Theanalysis is carried out without removal of dissolvedoxygen, as oxygen does not reduce within the 0.2–0.7 V Acknowledgementsrange on Au-MEE electrodes. This eliminates the need todeoxygenate the solution using nitrogen, which cuts the The authors would like to thank Dr Lei Xiao for provid-cost and simplifies analysis. By carrying out the voltam- ing the SEM images and the Scientific Research Labora-metric sweep in the cathodic direction together with tory of Hydrogeochemistry, Scientific Educational Centresimple sample preparation, the effect of almost all other “Water” of Tomsk Polytechnic University (TPU, Tomsk)metals and influence of organic ligands and DOS is elimi- which made the comparative analyses of waters featurednated. Compared to carbon (C-MEE) the gold electrode in Table 3.2068 www.electroanalysis.wiley-vch.de  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 11, 2061 – 2069
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