Direct voltammetric determination of total iron with a goldmicroelectrode ensemble

Direct Voltammetric Determination of Total Iron with a GoldMicroelectrode Ensemble

E. A. Zakharova,*a E. E. Elesova,a G. N. Noskova,a M. Lu,b R. G. Compton*b

a 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: [email protected]

Received: July 31, 2012;&Accepted: August 28, 2012

AbstractThe 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 andsample 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 mgL�1 range was studied. A sensitive LOD (3s) value of 0.7 mgL�1 for total ironconcentration was calculated; total iron determination in different waters was shown.

Keywords: Iron, Gold microelectrode ensemble, Microelectrodes, Voltammetric determination

DOI: 10.1002/elan.201200417

1 Introduction

Iron is an abundant element in the environment [1] whichplays an important role in environmental, industrial,human and biological systems [2], and as such, is of keyinterest for analytical studies. Although iron exists inmany oxidation states ranging from �II to +VI [3], itmost commonly appears in its + II and + III oxidationstates in the environment. Many natural Fe(III) chelatorsexist in seawater and up to 99% of iron in waters is be-lieved to be found complexed to organic ligands and thusits concentration is low except when extremely stablecomplexes are formed [4]. Concentrations of iron rangingfrom 0.1 nM to 10 nM [5,6] have been suggested. Theconcentration of dissolved iron in freshwater is alsothought to be greater than that of seawater [7] due to theproximity to terrestrial and continental shelf sources.Soluble iron species in natural waters can vary from10�6 M for river water to 10�9 M for coastal sea water andto 10�11 M for ocean water [8]. The World Health Organi-zation (WHO) does not issue a health based guidelinevalue for iron [9]; however acceptably permissible valuesin drinking water likely range from 0.3–3 mg L�1, as levelsgreater than this causes water to become discolored andtaste metallic.

The abundance of iron and its variable oxidation statesin the environment calls for their accurate determinationand quantification. Many analytical methods have beendeveloped to detect and quantify iron; these include vol-tammetry [10], flow injection analysis [6], chemilumines-cence [11], spectrophotometry [12] and spectrofluores-

cence [13]. Spectrophotometric methods [6, 14, 15] havebeen widely used, however, these are relatively expensivefor use in ultra trace determination of metal ions and cansuffer from interferences. The size and weight of the in-struments also restricts their use to land based experi-ments.

Electrochemical methods such as stripping voltamme-try have been favoured due to their inexpensive, simpleand rapid nature [16,17]. Anodic and cathodic strippingvoltammetry, in particular, have shown many benefitssuch as good selectivity and highly sensitive determina-tion [18]. Sensitivity of direct voltammetric (VA) deter-mination of iron is limited by the value of the residual(capacitive) current and to date, publications report limitsin the 10�7–10�6 M range [19,20]. A significant increase insensitivity can be achieved by the use of adsorptive vol-tammetry on mercury electrodes [21], however, the useof toxic mercury is highly undesirable in routine analysisdue to the drawbacks of mercury toxicity [22,23]. As analternative, the use of graphite and other carbon electro-des for determination of iron at trace levels has been ex-plored involving a preliminary activation of the surfaceby thermal [24], electrochemical [25] and other methods[26]. Graphite electrodes for VA determination of iron inwater and various food products were used, where activa-tion of the surface results in modifying the electrode sur-face with functional groups (carboxyl, carbonyl, quinoid,phenol [27]), accelerating the reduction or oxidation ofiron ions.

Recently, ensembles of nano and microelectrodes wereused in electroanalysis [28]. Several advantages over

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macro-electrodes were seen, including higher sensitivity�achieved by a high signal to noise ratio, ability to work inenvironments with high resistance, compatibility witha two electrode cell configuration, amongst other bene-fits.

The use of carbon microelectrodes ensembles (C-MEE) modified with gold (Au-MEE) for iron determina-tion within the 0.002–0.02 mg L�1 range is reported hereand represents an approach not previously seen in the lit-erature. A number of factors were studied in order toobtain an optimized analytical signal for iron(0.003 mg L�1, 1/100 of the maximum permissible level)using gold ensemble modified carbon nanocompositeelectrodes (carbon black-polythene =CB/PE) (Au-MEE).We report a method for modifying CB/PE by gold, tech-niques to activate the electrode, the influence of back-ground nature and concentration, signal mode, varyingiron ion concentration, and influence of interfering for-eign ions on the Fe signal. As a result of optimizing theparameters, a simple and low cost method for trace irondetermination in aqueous solutions using long lived andsensitive Au-MEEs is proposed.

2 Experimental

2.1 Materials

All chemicals and reagents were of analytical grade quali-ty. Standard solutions (GOST, Russia) of 1 mg mL�1

Fe(III), Cr(VI), Cu, Zn, Ag, As(III), etc. were used. Solu-tions of lower concentration were prepared by dilutionwith doubly distilled water. The volume of the sample so-lution was 10.0 mL; the concentration of the analyte wasevaluated by addition of the standard solution (0.010–0.10 mL) to the sample. The composite material for man-ufacture of the working electrodes was industrially pro-duced (“Tomskneftekhim”, Tomsk) carbon concentrate oftechnical 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 N220with particle sizes, 24–33 nm, and 70% thermally stabi-lized polyethylene of high pressure.

Experiments were carried out at room temperature(22–23 8C) without removal of oxygen from the solutions.During the optimization stages, nitrogen gas was used topurge the solution for de-oxygenation purposes.

2.2 Apparatus

Voltammograms were obtained using an electrochemicalworkstation TA-4 (Tomanalyt, Tomsk) supplying threeelectrochemical-cells with three working electrodes. Thecalculated result was the mean of three simultaneous in-dependent measurements. A three-electrode system wasused, consisting of a vibrating composite CB/PE-electrode(3.9 mm in diameter) or the same modified with Au (Au/CB/PE) as working electrodes, and Ag/AgCl in 1 M KClas the reference and counter electrode. In some experi-

ments a graphite electrode impregnated with polyethy-lene and paraffin mixture was used.

Voltammograms were produced in linear sweep poten-tial form as well as cyclic mode (CV) in normal current-potential coordinates (I–E) and in the form of the firstderivative of dI/dE–E [29]. The analytical signal used forcalculation was the maximum peak in the dI/dE plot.

The Au/CB/PE surface was characterized by a ScanningElectron Microscope (SEM) Instrument (Oxford Lab);an ICP-MC (iCAP-6500 Duo, Thermo Scientific, UK)was used for comparative testing.

2.3 Electrode Preparation

2.3.1 Composite CB/PE Electrode

The fabrication of CB/PE electrodes by a semicommercial“casting” method was reported previously in 2011 [30].Briefly, composite granules were loaded in a casting ma-chine. After melting, the composite was heated to a tem-perature of 160 8C before the casting process. The body ofthe electrode was press-formed into low pressure polyeth-ylene (thickness of 0.55 mm, inside diameter of the work-ing part of the shell 3.9 mm with contact of stainless steelwire d=1 mm) before the molten composite was thensprayed into the electrode body under 65 kgcm�2 pres-sure. The mould was cooled before the newly made elec-trodes could be removed. This technology allowed about30 electrodes to be produced per hour.

The electrode surface was cut using a special cutter.The newly cut work surface (in the form of a disc, withdiameter, d=3.9 mm) did not require polishing as its sur-face presented a microelectrode ensemble of carbon-black particles (C-MEE) separated by insulating polyeth-ylene. The electrode resistance was in the order of 2–3 kW. When not in use, the composite electrode (C-MEE)was stored under a protective cap.

2.3.2 Preparation of a Gold Microelectrode Ensemble(Au-MEE)

The modified gold microelectrode ensemble was preparedby electrodeposition of gold on a freshly cut C-MEE sur-face. Electrolysis was carried out in a 500 mg L�1 solutionof AuCl4

� , at a potential of 0.0 V for a certain period oftime (10–300 s). An SEM-image of the gold particles canbe seen in Figure 1. Prior to deposition, the working elec-trodes were activated by electrochemical treatment (ET),which depends on the composition of the solution andelectrode material. The real (active) area of the Au-MEEsurface was determined coulometrically by taking into ac-count the charge of a gold oxide monolayer in 0.05 MH2SO4 (400 mC cm�2). For this, a cyclic voltammogramwas recorded in the range, 0 to 1.5 V, with scan rate of100 mV s�1 and the peak area of Au2O3 reduction wasmeasured [31]. Typically, the surface area of the goldmicro-ensembles was 0.01–0.02 cm2.

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2.4 Procedures

10 mL of background solution (or water sample) wasadded into three cells (a quartz cup with three electro-des). Electrochemical treatment of the working electrodeswas performed in the solutions as noted below. After thecleaning stage, using electrode vibration and equilibriumpotential (Eeq), direct current voltammograms of iron ionreduction (III!II) was obtained by linear change of thepotential from Eeq, with a scan rate of 20 mV s�1. The ana-lytical signal was the maximum peak of the first deriva-tive (dI/dE vs E). Quantification was fulfilled by the stan-dard addition method.

2.4.1 Water Analyses for Total Iron

Sample preparation: All forms of Fe in waters have to beconverted to Fe(III) and interfering organic substances(DOS) have to be destroyed. A general procedure was asfollows: the water sample (5–10 mL) was evaporated inquartz glasses with conc. HNO3 at 110–120 8C. The resi-due was heated in the oven at 450 8C for 20–30 min; ifblack residue was seen in the glasses, it was furtherheated in the oven for 10 min at 450 8C; this was then dis-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 ofthe solution could then be used in the cell with 0.005–0.01 M HCl as background electrolyte.

3 Results and Discussion

3.1 Optimizing the Voltammetric Signal of Fe

To choose the form of analytical iron signal for optimizedmeasurement, cyclic voltammograms were recorded atthe working electrode. Data were in the form of both cur-rent (I)–voltage (E) plots and also plots in derivative

mode (dI/dE vs. E). Figure 2 shows an example of theFe(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 ironsignal 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 isbetter resolved than the anodic peak. As such, all the an-alytical signals were subsequently produced from thecathodic 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 thereduction signal of Fe(III) to Fe(II) occurs only aftera sufficiently positive polarization of the electrode inHClO4, within a potential range, 1.4 to1.6 V, whereoxygen is produced from water. On a freshly cut C-MEE(as well as on freshly polished impregnated graphite elec-trode) in the presence of 0.01 M acids, the voltammetricsignal for the reduction of iron(III) was not detected upto a concentration of 0.3 mg L�1 without prior pretreat-ment. However, application of electrochemical treatment(ET), by applying a positive potential of 1.5 V for 15 s ina solution of 0.01 M HClO4, leads to the appearance ofa peak current for Fe(III) at a potential close to 0.5 V. Atlong ET times, the peak current first increases, but thenreaches a limit.

An impregnated graphite (IG) electrode, with surfacearea 0.2 cm2, was compared with a carbon composite elec-trode (C-MEE). After the same anodic surface activationto the IG- electrode, the cathodic signal of iron(III) dis-appeared after 1.5 to 2 hours. The C-MEE however, al-lowed iron determination without surface renewal for atleast three days, whilst maintaining high signal reproduci-bility. The calibration curve for iron at the C-MEE islinear within a limited concentration range (0.005–0.08 mgL�1), which is probably due to the small size ofthe electrode active surface. Further experiments showedthat modification of the C-MEE by gold can significantlyimprove the analytical performance of the electrode foriron determination by direct voltammetry.

3.3 Gold Microelectrode Ensemble (Au-MEE)

A series of experiments showed that Au-MEEs obtainedby electrolysis of an unstirred solution of 500 mgL�1 goldchloride (pH 2–3) at a potential of 0.0 V produced thebest metrological characteristics (sensitivity, accuracy, re-producibility) for iron determination. It should be notedthat a short-term electrochemical treatment of the elec-trode is necessary to obtain a good, stable signal forFe(III). Investigation of the Au-MEE surface with scan-ning electron microscopy (Figure 1) showed that the elec-trode consists of an ensemble of a large number of goldnano- or microelectrodes with particle size of 200–

Fig. 1. SEM-image of C-MEE after modification with goldnanoparticles.

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Voltammetric Determination of Total Iron with a Au-MEE


300 nm, separated by a nonconducting substrate made ofpolyethylene.

The height and shape of the iron reduction signals de-pended on the amount of deposited gold. The electrode-position time (tdep) was varied from 10 s to 300 s in500 mg L�1 AuCl4

� at Edep =0.0 V in an unstirred solution.The active surface area changed from 0.01 to 0.05 cm2.The height of the signal increased with tdep, but this alsocaused the residual current to increase. The accuracy ofiron(III) determination in known solutions of0.020 mg L�1 of Fe(III) in all cases was nearly 100%. Thesignal changes from wave form in coordinates I vs. E(one peak at t=10 s and 30 s in dI/dE vs. E coordinates),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) to25 days (tdep =40 s or more), depending on the depositionconditions.

As such, a compromise electrolysis time of 50 s for Audeposition was reached. As one can see from Figure 3,the form of the signal in the dI/dE vs. E coordinates isnot an ideal peak shape but nonetheless analyticallyuseful. Accumulation of gold on the C-MEE with tdep

from 10s to 90 s gave active surfaces on the Au-MEE

which did not exceed 30 % of the geometric area(0.12 cm2). The carbon black content in the compositeforms 30 % of the C-MEE, and since during electrolysis,gold is likely deposited on the most active particles onthe surface of the electrode, modification of the electrodedoes not result in a continuous film on its surface. Thecomposition of the electrode surface is such that it be-haves in effect as a gold electrode of equivalent geomet-ric area since the diffusion fields associated with the goldparticles will overlap on the timescale of the experiment.That said, the use of carbon/gold composites representsa lower cost option than bulk electrodes especially giventhe long lifetime of the electrodes after activation.

3.4 Different Factors Influencing the Iron Signal at a Au-MEE

3.4.1 Electrolytes

Due to the fact that iron(III) salts are known to hydro-lyze, 0.001–0.1 M solutions of the following acids H2SO4,HClO4, HNO3, HCl were studied as possible electrolytes.In all electrolytes, CV profiles of the cathodic current ob-served for the reduction of Fe(III) to Fe(II) was higher

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 mV�1. 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 mgL�1 Fe(III), (3) 0.04 mgL�1 Fe(III).




than the anodic current. As discussed above, the cathodicpeak in the dI/dE–E coordinate was chosen as the analyt-ical signal of iron.

The iron signal dependence on the concentrationpasses through a maximum, corresponding to the concen-tration of background solution in the 0.001 to 0.010 Mrange in all the investigated acids. Variation of the con-centration of the acids was studied to give the maximumpeak of iron. It was seen that the peak potential differ-ence in the all acids is greater than 0.056 V, which isa characteristic of reversible processes, so the limitingstage is not based on pure diffusion. Judging by the valueof DEp, the reduction for Fe(III) to Fe(II) at a Au-MEEoccurs fastest in hydrochloric acid.

The Fe signal was seen to be directly proportional to itsconcentration in the 0.005 to 0.2 mg L�1 range, and its de-pendence was investigated in a range of acidic media, theslope of which increases in the order: HClO4<H2SO4<HCl. This can be attributed to the order of increasingacid anion adsorption on the gold surface. Hydrochloricacid as a background electrolyte is favorable for the re-covery of iron(III), as chloride ions are adsorbed on goldmicroelectrodes in potentials more positive than 0.0 Vwhich may facilitate electron transfer by a bridging mech-anism [32]. An additional factor that contributed to thechoice of hydrochloric acid as the optimal electrolyte wasthat traces of chloride appear in solution from the refer-ence electrode salt bridge, such that Cl� ions as impurities

Fig. 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 mgL�1 AuCl4

� , linear potential scan from 0.7 to 0.2 V,scan rate=20 mVs�1 (1) background, (2,3) consecutive additions of 0.04 mgL�1 Fe(III).




may be present in the reagents used and in the analyzedsamples. Further from a test solution, we also noticedminimal interference from chromium was seen within theiron signal whilst using hydrochloric acid as the electro-lyte.

3.4.2 The Potential Scan Rate (v)

The effect of potential scan rate on the background andanalytical signal of iron in hydrochloric acid between 10to 80 mVs�1 was studied. A scan rate of 20 mVs�1 waschosen due to the best signal to noise ratio. We noted ear-lier that interestingly, a stable oxidation current is record-ed in the absence of iron using derivative voltammogramsfor 0.01 M HCl (and all used electrolytes) in the potentialrange, 0.40 to 0.50 V. This depended on the amount ofgold on the surface of the Au-MEE, solution composi-tion, electrical treatment and electrode potential scanrate. Using I vs. E coordinates, the oxidation peak (mag-nitude of 2–5 nA) is linearly dependent on dE/dt, whichis typical for the redox of surface confined species. It isinteresting to note that increasing the scan rate increasesthe peak, however the peak potentials do not depend onthe speed. This may indirectly indicate the reduction andoxidation of some surface compounds in the 0.40 to0.50 V range.

All the factors lead us to assume that after Au-MEEelectro-treatment (�1.0 to 0.70 V) and in the presence ofoxygen, premonolayer oxides of gold on the Au surfaceare formed, which reproduced stably at the gold activa-tion step. The existence of such nonstoichiometric oxidesof gold particles at the nanoscale, together with electroca-talytic activity has been well documented by Burke [32],whereby they can play the role of mediator in electrontransfer. Possibly the presence of such oxides contributestowards the reproducible and sensitive signals of iron(III)reduction, which conveniently appears within this poten-tial range. The analytical signal of iron is always mea-sured as the difference between the peak height of ironand the background current.

In the I vs. E coordinates, the peak current for Fe(III)grows as I=k v0.3–0.4. The exponent value of less 0.5 showsthat diffusion of Fe(III) to the electrode may not be fullylinear under our conditions on the Au-MEE. As the formof the Fe signal was less than ideal at higher u, we choseto use scan rate v=20 mV s�1for analytical measurements.

3.4.3 Electrochemical Treatment (ET) of Au Electrode

Activation of the Au electrode is an important step forobtaining stable and reproducible signals for Fe(III)!(II). This was performed by alternating the polarizationof Au-MEE in the negative and positive potentials ofiron where Ep =0.40 to 0.50 V. It was established that inhydrochloric acid, ET pulses of alternating potential from�1.0 to 0.70 V (for 1 s at each potential step, for 8 s) areeffective before registering each voltammogram. At morenegative potentials, ET (less than �1 V) at the surface of

the Au-MEE causes hydrogen to be released and a highcathodic current background leads to the distortion of theiron(III) signal. Using potentials more positive than0.70 V to conditions the Au-MEE, anodic dissolution ofgold could be seen. The optimized conditions of ET wasbased on: sensitivity of the iron, values of the backgroundpeak, reproducibility of the iron peak and accuracy ofiron determination in the test solutions. In our experi-ments, sensitivity was calculated as the ratio of the analyt-ical signal height to concentration (dI/dE to C).

After the gold surface is activated, desorption of pas-sive impurities occur during the electrochemical treat-ment. In our optimized conditions, reproducible signals ofiron were received and electrode worked well for 3 weekswithout any further treatment.

3.4.4 The Dependence of Iron(III) Peak (dI/dEmax) on theConcentration of Iron(III)

The chosen optimized conditions included using a 0.01 MHCl background electrolyte and v=20 mV s�1. Togetherwith the described method of electroactivation, the cali-bration dependence of the iron cathodic peaks measuredin the range 0.002 to 0.4 mg L�1 by scanning from 0.7 to0.2 V was obtained as shown in Figure 4.

The peak height, H, was found to increase linearly withthe concentration from 0.002 mg L�1 up to 0.4 mgL�1

Fe(III). The equation for linear regression was y=234.7 H+0.55, R2 =0.9994. The value of LOD (3s criteri-on) was calculated to be 0.7 mg L�1.

3.5 Interferences

It was found that the oxygen present in the solution is re-duced at Au-MEE at potentials more negative than 0.0 Vand does not interfere with the determination of iron.Moreover, the purging of an inert gas (nitrogen) for10 minutes through an electrochemical cell to removeoxygen, surprisingly, leads to less ideal peak shape ofiron.

The additions of 0.01 M potassium nitrate, sodium,magnesium, aluminum, 0.001 M alkali metal chloride,0.0001 M perchlorate, bromate, persulfate, and nitrite donot interfere with the iron peak in hydrochloric acid. Theaddition of potassium chloride at a concentration greaterthan 0.005 M and sodium chloride greater than 0.03 Mstrongly influenced the signal of iron. The backgroundsignal splits and shifts in the negative direction, making itdifficult to correctly measure the signal for iron. Thishighlights the importance of the surface gold oxide inFe(III) reduction.

The 10-fold excess of Cu, Zn, Bi, Sb(III), As(III),Se(IV) ions does not have an interfering effect. Silver wasadded in a 1 : 1 ratio without any signal distortion. Theeffect of chromate was studied in great detail and showedthat with the selected conditions for electrode activation,a 10-fold excess of chromate may be present with noeffect on the iron signal. The addition of Fe(II) causes an




increase in the cathodic signal for Fe(III). When thecathodic potential is set at 0.7 V, Fe(II) is oxidized toFe(III). Then following a potential sweep from 0.7 to0.2 V it is reduced with the Fe(III) diffusing to the elec-trode. Therefore concentration of iron determined by theproposed method refers to the total content of iron:Fe(II) and Fe(III).

3.6 Application to Fe(III) Determination in WaterAnalyses

The analysis of synthetic mixtures (three electrodes inthree cells) on the content of iron(III) spiked within therange 0.03–5.0 mg L�1 showed that recovery was from90%–120 % with Sr =12–15%. The analysis of the watersamples were proceeded analogously but instead ofadding aliquots of the standard solution, a volume of theprepared sample was added to a 0.01 M HCl backgroundelectrolyte. The method for sample preparation dependson the water quality. For cleaner water it was adequateenough to dilute the sample with twice distilled water.The results of analysis on tap water (Tomsk), industrial

water and natural water (Siberian region) samples areshown in Tables 1, 2 and 3. Quantification of the totaliron was proved using analysis of different aliquots of thesame sample (Table 1), calculating the recovery afterspiking the sample with known Fe quantities (Tables 1, 2)and finally comparing iron determination using two meth-ods: our proposed voltammetric analysis and ICP-MC(Table 3).

As one can see from Tables 1 and 2, different aliquotsof water samples gave the same amount of Fe, howeverafter spiking, this increased in accordance with the addedquantity.

As can see from Table 3, the 2 different analytical tech-niques gave very comparative results.

4 Conclusions

It has been shown that through optimized conditions ofpreparing and activating the working Au-microelectrodeensemble, it is possible to determine the total iron con-tent in the 0.002–0.4 mgL�1 range, giving a LOD (3s)

Fig. 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 s�1.

Table 1. Analysis of tap water for Fe by the proposed VA procedure with Au-MEE.

Sample Volume of the sample[a] aliquots (mL)

Found Fe (mg L�1)(n=3; P=0.95)

Smean (mg L�1)

Tap water 1.5 0.098�0.006 0.1031.0 0.104�0.0072.0 0.108�0.021

Tap water spiked with Fe(III) 0.30 mgL�1 0.2 0.406�0.022 0.4060.5 0.396�0.0321.0 0.415�0.018

[a] After sample preparation.




value of 0.7 mg L�1. Determination of such low concentra-tions of iron by direct cathodic voltammetry with currentsat nA level was possible due to the nature of the elec-trode and characteristic of the analyzer TA-4 whichallows currents down to 0.1 nA to be recorded without in-terference or noise. The cathodic current of Fe(III) reduc-tion in the I vs. E coordinates gave a waveform but noclear peak, however, a clear peak proportional to Fe con-centration could be seen in the dI/dE vs. E coordinates.

Studies have shown promising results of using Au-MEEto determine low concentrations of iron, for example, inultra-pure water. Other water types demand onlya simple sample preparation, after which the sample isdissolved 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 Vrange on Au-MEE electrodes. This eliminates the need todeoxygenate the solution using nitrogen, which cuts thecost and simplifies analysis. By carrying out the voltam-metric sweep in the cathodic direction together withsimple sample preparation, the effect of almost all othermetals and influence of organic ligands and DOS is elimi-nated. Compared to carbon (C-MEE) the gold electrode

is advantageous in being able to operate in a stable, re-producible and long-term manner on a single electro-chemical activation in the electrolyte solution.

Application of Au-MEE as a working electrode hasmany advantages in electrochemical analysis includingthe absence of toxic mercury and expensive Pt. By apply-ing an easy electrochemical treatment (ET) step, a stableelectrode lasting 3–4 weeks can be used to produce repro-ducible and sensitive results. As well as this, the ease andlow cost of operation due to simplicity of preparation andregeneration suggests that semi-industrial production ofAu-MEE electrodes is important and should be morewide-spread for successful electroanalysis of trace iron.

Acknowledgements

The authors would like to thank Dr Lei Xiao for provid-ing the SEM images and the Scientific Research Labora-tory of Hydrogeochemistry, Scientific Educational Centre“Water” of Tomsk Polytechnic University (TPU, Tomsk)which made the comparative analyses of waters featuredin Table 3.

Table 2. Recovery of Fe in industrial and natural waters (after sample preparation); n=6; P=0.95.

Sample Found Fe in samples(mg L�1)

Spiked Fe(III),(mg L�1)(addition)

Found Fe in spiked samples(mg L�1)

Found in spiked sample(mg L�1)

Recovery(%)

Waste water 73–1 2.37�0.27 3.60 5.82�0.45 3.45 95.8Waste water 73–2 2.18�0.24 2.80 4.61�0.39 2.43 86.8

3.60 5.77�0.48 3.59 99.7Waste water 73–3 1.71�0.13 4.80 5.85�0.61 4.14 86.3Swampy water 11 4.15�0.51 6.00 10.9�1.7 6.72 112Tap water 0.214�0.024 0.120 0.332�0.02 0.118 98.3

0.550 0.769�0.03 0.56 101

Table 3. Fe determination in the natural waters of the Siberian region by our proposed VA method and ICP-MS [a].

Sample Fe content (mg L�1) VA Fe content (mg L�1) ICP-MC

Well-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 9.655 572.38 2.52

Well water 5.8 5.7River waters, different samples 0.44 0.43

0.78 0.690.63 0.561.00 1.07

[a] Analyses were made in the Scientific Research Laboratory of Hydrogeochemistry in the Scientific Educational “Water” Centre ofTomsk Polytechnic University (TPU, Tomsk).




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Direct voltammetric determination of total iron with a goldmicroelectrode ensemble