Electrocatalytic oxidation and flow amperometric detection of

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Electrocatalytic oxidation and flow amperometric detection of hydrazine on a dinuclear ruthenium phthalocyanine-modified electrode

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Mehrdad Ebadi 0

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Abstract: Electrocatalytic oxidation of hydrazine on a dinuclear ruthenium phthalocyanine ((RuPc)2) modified electrode was studied using cyclic voltammetry (CV) and rotating disc electrode (RDE) techniques. At pH = 13, a four-electron oxidation of hydrazine to N2 was observed. A suitable mechanism was proposed by analyzing the rate equation and the Tafel slope. The flow injection analysis was performed to characterize the (RuPc)2-modified electrode as an amperometric sensor for the detection of hydrazine. The electrode displays an excellent accuracy and precision in phosphate solution at pH 12 and 13. The linearity range was from 30 nM to 1 mM with a correlation coefficient of 0.9998. Key words: ruthenium phthalocyanine, electrocatalysis, surface-modified electrode, hydrazine, amperometric sensor. Résumé : On a étudié l’oxydation électrocatalytique de l’hydrazine sur une électrode modifiée de phtalocyanine de ruthénium dinucléaire (RuPc)2 en faisant appel aux techniques de voltampérométrie cyclique (VC) et d’électrode à disque tournant (EDT). À un pH = 13, on observe une oxydation à quatre électrons de l’hydrazine en N2. On propose un mécanisme approprié en faisant une analyse de l’équation de vitesse et de la pente de Tafel. On a réalisé une analyse de l’écoulement de l’injection pour caractériser l’électrode modifiée de (RuPc)2 comme détecteur ampérométrique pour la détection de l’hydrazine. L’électrode est à la fois exacte et précise dans une solution de phosphate à des pH de 12 et 13. La plage de linéarité va de 30 nM à 1 mM avec un coefficient de corrélation de 0,9998. Mots clés : phtalocyanine de ruthénium, électrocatalyse, électrode à surface modifiée, détecteur ampérométrique. [Traduit par la Rédaction]

Ebadi

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Introduction

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Recently (1) we reported the electrochemical properties of the dinuclear ruthenium phthalocyanine (RuPc)2 species and the electroreduction of O2 and H2O2 catalyzed by (RuPc)2modified graphite electrodes. The solution and surface cyclic voltammetry of (RuPc)2 displays a series of metal-localized processes. Previous studies of ruthenium phthalocyanine complexes have focused on their synthesis and chemical and physical properties (2–9). The exploration of ruthenium phthalocyanine as a catalyst has not been adequately investigated. Most of the work reported in the literature on catalysis by metallophthalocyanine species (MPcs) has been directed toward the study of cobalt and iron phthalocyanine. However, our work indicates that ruthenium phthalocyanine is as versatile a catalyst as CoPc and FePc (10). The modified (RuPc)2 electrode exhibits catalytic activity towards a wide variety of molecules, including hydrazine. There have been many published reports on the mechanism and kinetics of hydrazine oxidation. Oxidation of hydrazine catalyzed by platinum (11, 12), palladium (13, 14), nickel (15), gold (16), mercury (17), silver (18), vanadium (19), manganese (20, 21), iridium (22), rhodium (23), iron (24), cobalt (25–27), ruthenium (28), and graphite electrodes modified by metallophthalocyanines of VO, Fe, Zn, Co, Cu, Ni, and Mn (29, 30) have been reported. In aqueous

solutions, four-electron oxidation of hydrazine to N2 was reported, while a two-electron oxidation to N2H2 was observed in organic solvents (12, 31). The electrode materials and solvents have a profound influence on the oxidation mechanism. Fukumoto et al. studied the catalytic activity of metal electrodes in the anodic oxidation of hydrazine by means of the palladium membrane method (32). The experimental results revealed that the initial step in the mechanism occurred either through deprotonation of hydrazine followed by four-electron oxidation to N2 (mechanism I) or via formation of the N2H3 radical followed by three-electron oxidation to N2 (mechanism II). The oxidation of hydrazine on Pt, Ir, and Co proceeds via mechanism I, whereas on metals such as Pd, Au, and Ni oxidation occurs via mechanism II. Zagal and co-workers have reported the oxidation of hydrazine catalyzed by MPc, MTSPc, and MP compounds (M = Cu, Cr, Ni, Fe, Co, VO, Zn, or Mn; TSPc = tetrasulphophthalocyanine; P = porphyrin) adsorbed on graphite electrodes (25). They proposed that the initial step in the mechanism involves oxidation of the metal center, followed by catalytic oxidation of coordinated hydrazine. Many reports have become available on the detection of hydrazine using techniques such as amperometry (33–40), chemiluminescence (41–43), and spectrophotometry (44–48). In the spectrophotometry and chemiluminescence methods,

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Received 19 September 2002. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 28 February 2003.

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E. Mehrdad. Chemistry Department, York University, Toronto, ON M3J 1P3, Canada. (e-mail: [email protected]).

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Can. J. Chem. 81: 161–168 (2003)

I:\cjc\cjc8102\V03-012.vp Friday, February 28, 2003 10:07:18 AM

doi: 10.1139/V03-012

© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003

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hydrazine is analyzed through its ability to inhibit or promote a chemical reaction. An example is the kinetic– spectophotometric determination of hydrazine via inhibition of the reaction between bromate and hydrochloric acid (46). In the amperometric method, hydrazine is detected by oxidation on a working electrode. The oxidation potential of 1.0 V vs. AgCl/Ag is reported (37). To reduce the applied potential and increase the electrode sensitivity, many groups have reported the use of chemically modified electrodes (CME). The choice of the surface modifier is based on its catalytic activity and stability towards hydrazine. In this article, the catalytic activity of the (RuPc)2-modified electrodes toward the oxidation of hydrazine is described. The main products of the redox reaction are determined using electrochemical methods such as cyclic voltammetry (CV) and rotating disc electrode (RDE). A suitable mechanism is proposed by analyzing the Tafel slopes and the rate equations. The performance of an (RuPc)2-modified electrode for the detection of hydrazine in a flow-through system is reported.

Experimental Material Hydrazine sulfate (NH2NH2·H2SO4) (Analar), potassium hydrogen phosphate monobasic (KH2PO4) (Analar), and sodium hydroxide (NaOH) (Analar) were used without further purification. THF was distilled over sodium wire. The buffer solutions were prepared using 0.1 M KH2PO4. The desired pH was obtained by the addition of a 0.1 M phosphoric acid or a 0.1 M NaOH solution. Nitrogen gas was used to remove dissolved oxygen in the solutions prior to the cyclic voltammetric experiment (CV). The following parameters were used in all calculations: DN 2 H 4 = 1.4 × 10–5 cm2 s–1, and 0.01 cm2 s–1 for the kinematic viscosity of water (49).

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Apparatus and electrode preparation Electrochemical measurements were performed with an RDE3 Pine potentiostat and an X-Y recorder. A pyrolytic graphite disc, sealed to a copper shaft with polyolefin heatshrinkable tubing, was used as a working electrode in the surface CV experiments. The graphite electrode was mounted on the shaft with the basal plane of the graphite exposed (BPG). The surface of the electrode was polished with 0.05 µ m alumina powder, sonicated in doubly distilled water, and rinsed with acetone and water before each experiment. A saturated calomel electrode was used as a reference electrode. The chromatograms were obtained using an HPLC system consisting of a Waters 590 isocratic pump, a Waters 715 auto sampler, and an HP G1049 electrochemical cell. An AgCl/Ag electrode was used as a reference electrode and glassy carbon (GC) was used as a working electrode. The GC electrode was modified as described below. Exposing polished GC and BPG electrodes to the THF solution of (RuPc)2 (0.1 mM) modified the graphite electrodes. After 1 min of soaking, the electrode was removed, rinsed with doubly distilled water, and transferred to an electrochemical cell. The (RuPc)2 that was deposited on the electrode was determined to be between 3 to 5 layers thick,

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assuming that (RuPc)2 adsorbed flat onto the surface of BPG (1, 10).

Results and discussion Solution and surface CV of (RuPc)2 In our previous paper (1) we have reported the solution and surface CV of (RuPc)2 in organic and inorganic media. The solution CV of (RuPc)2 in THF displays four reversible redox waves at 0.98 V (wave I), 0.48 V (wave II), –0.13 V (wave III), and –0.55 V (wave IV) vs. SCE and an irreversible wave at –1.38 V (wave V) vs. SCE. The reversible waves are assigned to one-electron redox couples centered at metal centers, and the irreversible wave is assigned to a twoelectron redox process centered at the Pc rings (1, 10). [1]

Wave I: [(Ru(III)Pc(–2))2]2+ + e– ⇔ [(Ru(II, III)Pc(–2)) 2]+

[2]

Wave II: [(Ru(II, III)Pc(–2))2]+ + e– ⇔ (Ru(II)Pc(–2))2

[3]

Wave III: (Ru(II)Pc(–2))2 + e– ⇔ [(Ru(I, II)Pc(–2))2]–

[4]

Wave IV: [(Ru(I, II)Pc(–2))2]– + e– ⇔ [(Ru(I)Pc(–2))2]2–

[5]

Wave V: [(Ru(I)Pc(–2))2] 2– + 2e– ⇔ [(Ru(I)Pc(–3))2]4–

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The surface CV of (RuPc)2 in methylene dichloride displays four quasi-reversible surface waves at 0.85 V (wave A), 0.31 V (wave B), –0.45 V (wave C), and –0.94 V (wave D) vs. AgCl/Ag. They are assigned to the metal localized processes corresponding in assignment to those described above as I, II, III, and IV (1, 10). The surface CV of (RuPc)2 in aqueous medium displays pH-dependent surface waves (Fig. 1). At low pH, two reversible surface waves (waves C and D) are observed, while at high pH two additional waves (waves A and B), positive of wave C, are detected. Wave A is only observed at very high pH values (not shown) (1, 10). [6]

Wave D: [(Ru(I, II)Pc(–2))2H] + e– + H + ⇔ [(Ru(I)Pc(–2))22H]

[7]

Wave C: [(Ru(II)Pc(–2))2] + e– + H + ⇔ [(Ru(I, II)Pc(–2))2H]

[8]

Wave B: [OH(Ru(II, III)Pc(–2))2] + e– ⇔ [(Ru(II)Pc(–2)) 2] + OH–

[9]

Wave A: [(OHRu(III)Pc(–2))2] + e– ⇔ [OH(Ru(II, III)Pc(–2))2] + OH–

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A Pourbaix diagram of waves B, C, and D along with the hydrazine oxidation wave are shown in Fig. 2. Hydrazine electrocatalytic oxidation An (RuPc)2-modified electrode shows strong catalytic activity towards the oxidation of hydrazine at pH = 13, while a © 2003 NRC Canada


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Fig. 1. Cyclic voltammetry of surface bound (RuPc)2 on a BPG electrode in phosphate buffer solution; pH = 11.3 (——) and pH = 2.4 (—). -3.5E-05 -3.0E-05

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Current (A)

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wave D wave C

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bare BPG electrode shows no activity toward the oxidation of hydrazine (Fig. 3). A plot of the hydrazine oxidation current as a function of the square root of the scan rate yields a straight line (R = 0.999, n = 8), characteristic of a diffusioncontrolled process. The oxidation current shows a strong dependence on pH (Fig. 2). As the pH decreases, the oxidation peak potential shifts toward a more positive potential and at the same time the oxidation current decreases. At pH values lower than 9, the (RuPc)2-modified electrode loses its activity toward the oxidation of hydrazine. Similar behavior has been reported for hydrazine oxidation catalyzed by polymeric iron teraaminophthalocyanine (50). The oxidation current is at its highest intensity at pH = 13. As the pH is lowered, the current intensity gradually decreases until pH = 8, where the catalyst loses it activity (50). To calculate the number of electrons involved in the oxidation at pH = 13, a series of rotating disc electrode CVs was conducted at different spin velocities. In these experiments, the catalytic activity of the (RuPc)2-modified electrode was reduced by 10% after each cycle. The decrease in activity was due to gradual loss of (RuPc)2 from the surface of the electrode. By increasing the number of adsorbed (RuPc)2 layers, the loss of activity was significantly reduced. To eliminate the surface variation and the loss of electrode activity a new surface was used for each spin velocity. For a rotating-disc electrode, the mass transfer limiting current is described by (51) [10]

Id = 0.201zFAD2/3v–1/6ω1/2Co,

where A (cm2) is the area of the electrode, v (cm2 s–1) is the kinematic viscosity, ω (rpm) is the spin rate, Co (mole cm–3) is the bulk concentration of the redox active species, and z is the number of electrons involved in the redox reaction.

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Ip = 2.99 × 105 z(αnα)1/2 ACo D1/2 υ 1/2

[12]

Ep = k – (0.03/αnα) log(υ)

The value of αnα can then be calculated from the slope of Ep vs. log(υ). The slope of the line at high over-potential yields a value of αnα = 0.22 (R = 0.999, n = 8). By substituting this value into eq. [11] along with known values for the other parameters the number of electrons involved in the oxidation of hydrazine is calculated to be z = 4.2. The calculated number of electrons at pH = 13 (i.e., z = 4) indicates that the net oxidation of hydrazine occurs as follows (11–32) [13]

NH2NH2 + 4OH– → N2 + 4H2O + 4e–

Using the procedure described above, a value of z = 1 at pH = 9 was observed. A value of z = 0.7 has been reported for hydrazine oxidation in organic solvents (12, 31). The experimental data show that the formation in the solution of N2H5+, which is not easily oxidizable, resulted in the small z value. Based on the above argument we propose that at pH values lower than 9, the formation of N2H5+ in solution and (or) the incapability of the solution to deprotonate the coordinated N2H4 prevents the oxidation of hydrazine to nitrogen. Kinetics of N2H4 oxidation at pH = 13 The rate of hydrazine oxidation can be defined as [14]

I = zFAk [N2H4]p[OH–]q,

where k is the heterogeneous rate constant, z is the total number of electrons involved in the oxidation reaction, and p and q are the reaction orders of hydrazine and hydroxide ion, respectively. The reaction order of hydrazine is determined from a plot of the logarithm of the kinetic current vs. log [N2H4]. The slope of the line is close to unity (slope = 0.9, R = 0.99, n = 5), indicating first-order dependence. The reaction order of OH– is determined from a plot of the hydrazine oxidation peak potential as a function of pOH. The slope of the line (slope = 100 mV per pH) is close to the theoretical value of 59 q αnα–1 for q = 1 and αnα = 0.5, indicating that one OH– is involved in the rate-determining step of hydrazine oxidation (10, 51, 52). Substituting these values © 2003 NRC Canada


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where υ (V s–1) is the scan rate and αna is a parameter reflecting the irreversibility of the reaction. When A, Co, D, and αna are known, the value of z can be calculated from the slope of Ip vs. υ1/2. To determine the value of αnα, one can use the relation for the peak potential.

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For a first order reaction, the plot of 1/IRD vs. 1/ω1/2, known as the Koutecky–Levich plot, yields a straight line (R = 0.99, n = 6). The slopes of the plots obtained in the mass transfer region at pH = 13 (slope = 8.5 × 103) correspond well with a theoretical value for z = 4 (slope = 8.3 × 103). The linearity of the plot also indicates that hydrazine oxidation is first order with respect to dissolved N2H4. In a confirmatory experiment, the oxidation of N2H4 at pH = 13 was examined during CV at an (RuPc)2-modified electrode. For a totally irreversible redox reaction, the peak current in cyclic voltammetry is expressed as (51): [11]

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Fig. 2. A Pourbaix diagram indicating the pH dependence of waves B, C, and D and the hydrazine oxidation peak potential. 400

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Wave B (slope = –72 mV pH ) 200

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Hydrazine oxidation (slope = 100mV pH )

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Wave C (slope = 66. 8 mV pH–1 ) -400 -600 -800 -1000

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Wave D (slope = 66. 4 mV pH )

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Fig. 3. Cyclic voltammetry of N2H4 oxidation (2.0 × 10–3 M) on an (RuPc)2-modified and bare BPG electrode in 0.1 M NaOH (pH = 13); scan rate = 25 mV s–1. -5.0E-06

will require that the studies be performed under similar conditions. According to these results, the kinetic current of hydrazine oxidation can be written as: [15]

BPG

I = zFAk [N2H4][OH–]

Current (A)

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into eq. [14], one then derives k = 1.19 × 10–10 cm4 mole s–1 at E = –0.4 vs. SCE. We would not attempt to compare the observed k value to published data since the experimental conditions, such as cell design, applied potential, and concentration of electrolytes, will have a profound influence on the value of the rate constant. A quantitative comparison

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Mechanism of N2H4 oxidation at pH = 13 Oxidation of hydrazine at pH = 13 occurs in the positive waves C and D (Fig. 2). The CVs of (RuPc)2 obtained in the presence and the absence of N2H4 show that hydrazine oxidation occurs at the potential where wave B is observed, indicating that the active catalyst on the surface is [(Ru(III, II)Pc(–2))2]+. By transferring the (RuPc)2 surface into an N2H4-free solution following a hydrazine oxidation experiment, the initial surface CV of (RuPc)2 is observed, showing that the ruthenium dimer retains its structure during and after the oxidation process. Based on our CV data, the calculated electron stoichiometries, and previously published reports, we propose the following mechanism for the oxidation of hydrazine on an (RuPc)2-modified electrode. [16]

(Ru(II)Pc(–2))2 ⇔ [(Ru(III, II)Pc(–2))2]+ + e–

[17]

[(Ru(III, II)Pc(–2))2]+

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+ NH2NH2 ⇔ [N2H4(Ru(III,II)Pc(–2))2]+ © 2003 NRC Canada


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[19]

[N2H4(Ru(III,II)Pc(–2))2]+ + OH– ⇔ [N2H3(Ru(II)Pc(–2))2] + H2O [N2H3(Ru(II)Pc(–2))2] → intermediate product + e–

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The steps following eq. [19] are very fast and are not ratedetermining steps. These steps would lead to the formation of N2 as the final oxidation product. The rate equation for the proposed mechanism for hydrazine oxidation (eq. [16] to [19]) under steady-state conditions is as follows (10, 52):

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[20]

I =

zFA k2 S[NH2 NH2 ][OH– ] {((k2 [N2H4 ][OH– ]/ k1o) exp(–(1 – α1) f η1) + (k2 [OH– ] [N2H4 ]/ k4o) exp(−(1 − α 4) f η4) + k2 [N2H4 ]/ k3 + (k2 [N2H4 ]K3 / k4o) exp(−(1 − α 4) f η4) + [OH– ] + k− 2 / k3 + (k− 2 K3 / k4o) exp(−(1 − α 4) f η4) + (k− 2 K3 / k4o) exp(−(1 − α 4) f η4) exp(− f η1) + [OH– ]exp(− f η1) + k− 2 k3 exp(− f η1))}

where S (mole cm–2) = (Ru(II)Pc(–2))2 + [(Ru(III, II)Pc(–2))2]+ + [N2H4(Ru(III,II)Pc(–2))2]+ + [N2H3(Ru(II)Pc(–2))2]; [NH2NH2] and [OH–] are concentrations with units of mole cm–3; ηi is defined as (Eapplied – Eio′); k1, k–2, k–3, k–1, and k4 have units of s–1 while k2 and k3 have units of cm3 mole–1 sec–1; and K3 = k–3/k3.1 The analysis of the rate equation — to predict experimentally observed data such as the Tafel slope and the rate dependence — has been used as a diagnostic method, to examine the validity of the proposed mechanism (10, 52). The experimental Tafel slopes are obtained from the plot of Ep vs. log (υ), and the rate dependence of hydrazine oxidation towards [N2H4] and [OH–] is described above. When k3 0. When k4o > 0, eq. [22] simplifies to [23]

I =

zFA[NH2 NH2 ][OH− ]k2 S exp((1 − α 4) f η4) [k2 [N2H4 ][OH− ]/ k4o + k2 [N2H4 ]K3 / k4o + k− 2 K3 / k4o ]

Equation [23] predicts a Tafel slope of 60/(1 – α4) at high over-potential. The reaction orders in hydrazine and OH– are

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[24]

p = ∂ log i / ∂ log[N2H4 ] = 1 −

[k2 [N2H4 ][OH− ] + k2 [N2H4 ]K3 ] [k2 [N2H4 ][OH− ] + k2 [N2H4 ]K3 + k− 2 K3 ]

[25]

q = ∂ log i / ∂ log [OH− ] = 1 −

k2 [N2H4 ][OH− ] [k2 [N2H4 ][OH− ] + k2 [N2H4 ]K3 + k− 2 K3 ]

In a case where k–2 >> K3 and k2, both eqs. [24] and [25] predict a first-order dependence towards [N2H4] and [OH–]. The experimentally observed Tafel slope of 64 mV (R = 0.998, n = 6) at low over-potential (η1 > 0) is predicted by eq. [23] using α4 = 0.5. Equation [21] also predicts the correct rate dependence toward [N2H4] and 1

[OH–]. Equation [23] predicts the correct rate dependence under the condition mentioned above.

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Detection of hydrazine at (RuPc)2 under flow injection conditions Figure 4 illustrates the chromatograph of a hydrazine sample (0.1 mM) on an (RuPc)2-modified GC and a bare GC electrode. The intense LC signals are observed only on the

k–1 and k1 have the same magnitude and dimension (i.e., k–1/k1 = 1) (10, 52).

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Table 1. 75

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Detection limit (LOD) (µg L–1) 0.96 0.5 3b 2.56 0.1 19.84 3.2 320 3.2 2 0.04 3 0.05 99.2 2.72 40 100

Method Amperometric Amperometric Amperometric Amperometric Amperometric Amperometric Amperometric Amperometric Amperometric Chemiluminescence Chemiluminescence Chemiluminescence Spectrophotometric Spectrophotometric Spectrophotometric Spectrophotometric Spectrophotometric

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Range (µg L–1)

Correlation coefficient

0.96–32000 1–800 0.0128–0.128c 3.2–320 0.3–500

0.9992 0.992 0.9963

6.4–12 800 3200–19 200 5.76–19 200 5–40 000 0.1–100 10–10 000 0.12–60 150–992 9.6–1024 50–5000 150–1000

0.995 0.999 0.9998 0.998 0.998

0.994 0.999

Reference This worka 33 34 35 36 37 38 38 40 41 42 43 44 45 46 47 48

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a HPLC system; flow rate 0.5 mL min–1; mobile phase NaOH–KHCO3 buffer (pH = 12.0); injection volume 5 µL; potential applied = 0.0 V vs. AgCl/Ag. b Units are (mg L–1) for this entry only. c Units are (g L–1) for this entry only.

Fig. 4. Chromatogram of hydrazine on an (RuPc)2-modified GC and bare GC electrodes. HPLC parameters: flow rate = 0.5 mL min–1, mobile phase phosphate solution (pH = 12.0), injection volume 5 µL, potential applied = 0.0 V vs. AgCl/Ag. 10 9

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Signal intensity ×10 (A)

8 7 6 5 4 3 2 1 0 0

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modified electrode, demonstrating that hydrazine can only be detected by an (RuPc)2 GC electrode at 0.0 V vs. AgCl/Ag. The applied potential of 0.0 V vs. SCE was selected to eliminate interference arising from the reduction of any O2 dissolved in the mobile phase. The precision, accuracy, linearity, and detection limits of the hydrazine detection method, utilizing an (RuPc)2-modified GC electrode, are discussed below. The precision of the hydrazine detection method, using an (RuPc)2-modified electrode, was examined by multiple injections of 0.1 mM N2H4 in a mobile phase consisting of phosphate solution at pH = 12. After twenty successive injections (5 µL each) of the sample solution, a relative standard deviation of 2.7% (peak height) was observed, which

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indicates a good precision and stability of the surface. The precision of the method was further examined using sample solutions containing 0.1 mM to 0.01 M hydrazine. Only at hydrazine concentrations of not more than 1 mM a good precision was observed. To determine a desirable pH, the response factor of a 0.1 mM sample at 0.0 V vs. AgCl/Ag was examined at different pH. The best response factor was observed in the pH range of 12–13. At lower pH, the signal intensity would significantly decrease until pH = 9 where the surface would lose its catalytic activity. To determine the optimum injection volume, multiple injections of from 1 to 100 µL of a 0.1 mM N2H4 sample were performed, and the signal precision was examined. The best results were obtained for an injection volume of 5 µL. Based on the above results, the optimum method parameters are an injection volume of 5 mL, a hydrazine concentration