Flow-injection amperometric detection of hydrazine by electrocalytic oxidation at a Prussian Blue film-modified electrode

Annlytrca Chmwa Acta, 257 (1992) 275-280

275

Elsewer Science Pubhshers B V , Amsterdam

Flow-injection amperometric detection of hydrazine by electrocatalytic oxidation at a Prussian Blue film-modified electrode Welymg Hou and Erkang Wang * Laboratory of Electroanalyttcal Chemzstty, Changchun Instrtute of Applied Chemistry, Cheese Academy of Scumces, Changchun, J&n 130022 (Chma)

(Received 22nd March 1991, reused manuscript recensed 13th September 1991)

Abstract A Prasslan Blue-modified glassy carbon electrode prepared by simple adsorption exhibited excellent electrocatalytic adivlty m the oxldatlon of hydrazme m ac&c mecha A film of the perfluorosulphomc acid polymer Nafion coated on top of the Prussian Blue-mochfied glassy carbon electrode can Improve the mechamcal stab&y of the Prussian Blue layer m the flow stream Hydrazme was detected by flow-mjection analysis at the modified electrode wth high senslt~lty The hnnt of detection was 0 6 ng Keywords AmperometIy, Cychc voltammetry, Flow system, Hydrazme, Pru~an

Because of the mdustnal and pharmacological nnportance of hydrazme, sensitive methods are needed for Its detection and determmatlon Voltammetrlc and amperometrlc detection methods for liquid chromatography (LC) are powerful techniques for monitoring easily oxldlzable species Unfortunately, hydrazme compounds with a large overpotential oxldlzed at ordinary carbon electrodes, are not suitable analytes for these methods One pronusmg approach for mmlmlzmg overvoltage effects 1s the use of electrocatalytlc chenucally modified electrodes (CMEs) Baldwin’s group has recently reported the enhancement of the amperometrlc detection of hydrazme compounds They used electrochemlcally treated glassy carbon electrodes for detectlon at about 0 2 V m LC [l] They also used carbon paste-modified electrodes contammg cobalt phthalocyanme (CoPC) for the detectlon of hydrazme at - 0 1 V m flow-inJection analysis (FIA) [21 Wang et al [31 used cobalt phthalocyanme-

Blue-modified electrode

cellulose acetate (CoPC-CA)-modlfled electrodes for the detection of hydrazme at about 0 6 V and other compounds m FIA A Prussian Blue (PB) [lron(III> hexacyanoferrate(H)] CME was studied by Neff, Itaya and co-workers [4-71 PB and analogous CMEs have been used for electrocatalytlcal studies The catalytic reduction of molecular oxygen [S], hydrogen peroxide [8], carbon monoxide [9,10], hexacyanoferrate(III), lron(III), aquoethylenedlammetetraacetatoruthemum(II1) and hexachlororrldate (IV) and the catalytic oxldatlon of ascorbic acid 1121 at PB-modified electrodes have been reported In addition, PB and analogous copper CMEs have been used for the detection of nonelectroactlve catlons m flow systems [13,14] The catalytic oxldatlon of reduced mcotmamlde aderune dmucleotlde at a hexacyanoferrate-modlfled mckel electrode and the reduction of Fe(II1) at such a CME have been reported [15,161 Cox and co-workers applied a ruthenium cyanide CME for

0003-2670/92/$05 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved

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the electrocatalytlc oxldatlon and determmatlon of A&II) [ 171and throcyanate Cl81 Recently, Cox and Gray applied a glassy carbon electrode coated with a ruthenium cyanide film for the detection of msuhn [19], cysteme and glutathlone 1201 m FIA Wang and Lu [213 also apphed a ruthenium cyanide film CME for the detection of hydrazme compounds at 088 V m FIA However, in the above studies, the PB and analogous CMEs were all modified by electrolysis with controlling current or potential, and the preparation procedures for these CMEs are tedious and substantial expenence with electrochemical techmques 1s needed In this work, it was found that a PB-modfiled glassy carbon electrode (PB-GCE) prepared by simple adsorption exhlblted excellent stability and electrocatalytlc activity m the oxldatlon of hydrazme m cyclic voltammetric experiments However, the PB-GCE was not stable m flowmg systems when the applied potential was more posrtlve than the 09 V Thomsen and Baldwin [141 reported that a film of the perfluorosulphonic acid polymer Naflon coated on top of the copper00 hexacyanoferrate (CuHCF) can lmprove the mechanical stability of the CuHCF layer In the present work, it was demonstrated that coating the PB-GCE with Naflon (NaflonPB-GCE) can unprove the stab&y and reproduclbdlty in flowmg streams The PB-GCE 1s very suitable for electrochemical detection m acrdlc solution Hydrazme was detected m flow systems with the Naflon-PB-GCE with high sensltlvlty The method is simple, effective and mexpensive

EXPERIMENTAL

Apparatus

A laboratory-budt cychc voltammetrrc system was used A three-electrode cell system with a glassy carbon (GC) electrode or PB-GC workmg electrode, a silver-silver chloride (saturated potassium chloride) reference electrode and a platmum auxdlary electrode was employed m the conventional electrochermcal expernnent A PAR Model 173 potentlostat equipped wth a Model

WANG

179 digital coulometer was used for the measurement of charge The flow-mJectlon system consisted of a Model 510 pump and a U6K injection valve The mJectlon volume was 20 ~1 A Model TLSA thm-layer electrochemical cell (Bloanalytlcal Systems) and a laboratory-built blpotentrostat were used for amperometrlc detection [22] A saturated calomel reference electrode (SCE) was employed The injector and detector were connected by a 44 cm length of 0 5 mm 1 d PTFE tubmg Reagents and procedure

A 5% solution of Nafion m ethanol was obtamed from Aldrich and dduted to 1% m ethanol-water (9 + 1, v/v) All chermcals were of analytical-reagent grade Hydrazme sulphate was obtained from BeiJmg Institute of Chemical Reagents All solutions were prepared with doubly dlsttlled water unless stated otherwlse Hydrazme sulphate solution for use m flow mJectlon was prepared with the mobile phase The mobile phase was 0 2 M KH,PO, (contalmng 50 PM EDTA) adjusted to pH 3 0 with H,PO, solution The flow-rate was 0 5 ml mm-’ 1 M KC1 solution was adjusted to pH 4 0 with HCl solution Elec&rodes

Glassy carbon electrodes were first polished wth 0 3-pm a-alumma m a water slurry using a pohshmg cloth The pohshed electrode was cleaned m an ultrasomc bath for 30 mm with distilled water that was changed several times This electrode 1s referred to as the fresh bare GCE The GCE was munersed m aqueous lron(III) hexacyanoferrate(II1) solution conslstmg of an equl-volume mixture of 0 1 M FeCl, 6H,O (10 mM HCl) and 0 1 M K,Fe(CN), (10 mM HCl), for 3 mm, and then nnsed with 10 mM HCl several times The resulting electrode wth an adsorbed film of PB is referred to as the PBGCE The electrode was cycled m 1 M KC1 (pH 3 0) between -0 1 and 12 V at a scan rate of 100 mV s-l until a steady current-voltage profile was obtamed, then cleaned m 10 mM HCl, allowed to an-dry and unmedlately coated with about a 5-~1 droplet of 1% Naflon solution m ethanol-water and allowed to an-dry This elec-

FLOW-INJECTION

AMPEROMETRY

277

OF HYDRAZINE

trode 1s referred to as the N&on-PB-GCE The PB-GCE and Naflon-PB-GCE were scanned m lMKCl(pH4.0)between -0land12Vata scan rate of 100 mV s-l for a few mmutes until a steady current-voltage profile was obtamed, then subjected to further expernnents The amount of PB adsorbed on the electrode was calculated by the total charge measured by Integrating the 0x1datlon currents of the adsorbent with potential scannmg from 0 2 to 0 6 V at a Scan rate of 5 mV s-l The coverage of the electrode was 17 mC LX-*, that is, 176 x 10e8 mol cm-* RESULTS AND DISCUSSION

Electrochemistry WIa conventwnd cell The cychc voltammograms (0%) m a conventlonal cell obtained for a PB-GCE (solid lines) and a Nafion-PB-GCE (dashed hne) immersed m 1 M KC1 (pH 4 0) solution are shown m Fig 1

The reduction and oxldatlon peaks of PB at about 0 3 and 0 95 V vs Ag/AgCl (saturated KCl) are due to the electron-transfer reactlons of the high-spin non(II1) ions Fe,[Fe(CN),],

+ 4K++ 4e-

= K,Fe,[Fe(CN)& and the low-spm iron Fe,[Fe(CN),],

ions

+ 3A-= 3e-+ Fe,[Fe(CN)&],

m the Pb film, respectively [7,23] The z-E curves for the electron-transfer reaction of high-spm iron(III) rons are SubJect to shght hysteresis effects and the voltammogram shows broad peaks and secondary current maxima at about 0 1 V However, after several redox cycles (n > 51, the hysteresis effects were decreased and the secondary current maxima almost dlsappeared Good voltammograms with sharp peaks and a steady response were obtamed It can also be seen from

10,50,100

E(V)

1.2 1.0 0.8 0.6 0.4 0.2 0 -0 2 Fig 1 Cychc voltammograms on a glassy carbon electrode () wth a PB film and (- - -1 wth a PB film coated wth Nafion m 1 M KCl (pH 4 0) solution Scan rate, 50 mV s- ‘, electrode dmmeter, 3 mm The numbers lo,50 and 100 represent the numbers of scanning cycles

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W HOUANDE

M KC1 (pH 4 0) show that no oxldatlon peak was obtamed with the bare GCE m the potential range - 0 2 to 12 V (curve A, c), whereas a defined oxldatlon peak Hrlth a peak potential of 0 95 V was observed vvlth the PB-GCE The catalytic currents observed were proportional to hydrazme concentration At the PB-GCE, it IS obvious that there 1s a large increase m current at 0 95 V (curves B, b and c) m the anodlc region correspondmg to the oxldatlon of Fe(CN)i- to Fe(CN)i- of the PB film itself and a decrease m or disappearance of the reductive current corre-

Fig 1 that the voltammogram at the PB-GCE after 100 cycles IS the same as that after 10 cycles This indicates that the GCE modified with PB by adsorption 1s stable Only the peak current was decreased at the PB-GCE coated with Naflon (dashed Ime) Figure 2 shows the cyclic voltammograms obtamed for (A) a bare GCE and (B) a PB-GCE (diameter 3 mm), both immersed m (a) 1 M KC1 (pH 4 0) blank solution and (b, c) m 1 M KCl (pH 4 0) solution containing hydrazme sulphate The cychc voltammograms of hydrazme sulphate m 1

1.2

1.0

0.8

0.6

0.4

WANG

0.2

0

-0.2

Fig 2 Cychc voltanunograms of hydrazme sulphate at (A) GCE and (B) PB-GCE m 1 M KCI (pH 4 0) solutlon Scan rate, 50 mV electrodediameter, 3 mm (a) Blank solution, (b) contammg 1 mM hydrazme sulphate solutlon, (cl contauung 2 mM hydrazme sulphate solution s- ‘,

279

FLOW INJECTION AMPEROMETRY OF HYDRAZINE

spondmg to the reduction of Fe(CN)z- to Fe(CN)i- of the PB film Itself when hydrazme sulphate was added 1111 M KC1 (pH 4 0) solution The catalytic oxldatlon currents observed were proportional to the concentration of hydrazme This suggests that the low-spm Iron ions m the PB film have catalytic actlvlty for the oxldatlon of hydrazme sulphate The overvoltage of hydrazme oxldatlon at the bare GCE IS decreased and the oxldatlon current of hydrazme 1s enhanced by use of the PB-GCE The PB film IS gradually dlsmtegrated by hydrolysis m solutions wth pH > 5 Therefore, the utility of PB-modlfied electrodes IS lumted m acidic solutions

.‘+-,,. /

0 6

Flow-mnjectwn amperometnc zinc

detectron of hydra-

To detect hydrazme at the PB-GCE, the experlment was performed m flow-mqectlon systems When the experiment was done with an electrode coated only wth the PB film (PB-GCE) at 0 8 V, the relatwe standard devlatlon (R S D > of the peak currents (I& with twenty repeated injections of 10 pg ml-’ hydrazme sulphate within 20 mm was 3 7% The response of the PB-GCE was stable at 0 8 V However, when the experunent was done with the PB-GCE at 10 V, the response decreased appreciably on repeated mJectlons of 10 pg ml-’ hydrazme sulphate The zp of the twentieth inJectIon (n = 20) was reduced to 50% of the untlal I,, (n = 1) This effect was attributed to the more unstable PB film and easier mechanical erosion of the film on exposure to the flowmg solution at a potential more peatlve than 0 9 V To overcome this problem, a film of the perfluorosulphoruc acid polymer Naflon was coated on top of the PB film The response urlth twenty repeated qectlons of 10 pg ml-’ hydrazme sulphate at the PB-GCE coated mth Naflon (Nafion-PB-GCE) at 10 V was stable, theRSD ofz,atlOand08Vwas24% This indicates that coating the PB-GCE with Naflon can improve the stability of the electrode m flow systems The stability of the response for seven measurements of a standard solution contammg 10 ,ug ml-l hydrazme sulphate during an unbroken 4-h period with a flow-rate of 0 5 ml mm-l was examined, and the R S D of zp was 7 9%

N/PB/CC

0.7

0 8

0.9

1 0

I 1

I z

Fig 3 Hydrodynanxc voltammograms of 10 pg ml-’ hydrazme sulphate at GCE and Ntion-PB-GCE Mobile phase. 0 2 M KH,PO, (contammg 50 PM EDTA), pH 3 0, flow-rate, 0 5 ml mm-’

Amperometrlc momtormg of hydrazme sulphate m flowmg streams may benefit from the electrocatalytlc response Hydrodynanuc voltammograms (HDVs) obtamed for hydrazme sulphate via flow-mJectlon amperometrlc detectlon with GCE and Nafion-BP-GCE are shown m Fig 3 They were obtamed by mjectmg 10 I.cg ml-’ hydrazme sulphate and varymg the potential between 0 6 and 12 V Clearly, the unmodlfled electrode (GCE) does not permit the convement determmatlon of hydrazme sulphate over the entire potential range The Nafion-PB-GCE, m contrast, exhibited peak-shaped HDVs wth a m-urn response at 10 V, and the response to hydrazme sulphate was greatly enhanced A potential of 10 V was selected for the detection of hydrazme sulphate at the Nafion-PB-GCE At a given potential, the Nafion-PB-GCE 1s more sensltlve than the GCE at 10 V The cahbratlon graph was linear for inJected amounts of hydrazme sulphate from 3 2 pg to 0 6 ng A 0 6-ng amount yielded a signal-to-noise ratlo of ca 3, and thus represented the detection limit for hydrazme sulphate The correlation coefficient was 0 9997, the slope of the cahbratlon graph was 865 pA ng-’ In conclusion, It has been demonstrated that a glassy carbon electrode with adsorbed Prusaan

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280

Blue can catalyse the electrooxldatlon of hydrazme, and coating the PB-GCE wtth Naflon can improve the stablhty of the electrode m flow systems The Nafion-PB-GCE has been applied to the detectlon of hydrazme m flow-inJection analysis wth high sensltlvlty The modrfied electrode IS easy to prepare, mexpenslve and stable The use of this modlfied electrode offers the posslblllty of extendmg voltammetnc techmques to hydrazme, which otherwlse 1s poorly suited to electroanalysis The support Foundation of Tech Research of Science and edged

of the National Natural Science Chma and the Basic and HlghDlvlslon of the State Commlsslon Technology 1s gratefully acknowl-

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