17
This chapter provides an overview of the electrochemical methods and experimental conditions used to study the oxidation of hydrogen peroxide at platinum working electrodes in phosphate buffer solutions over a range of concentrations, temperatures and pH. The electrochemical techniques employed in this work were cyclic voltammetry (CV), chronoamperometry (CA) and staircase potentiometry (SCP).
A range of platinum electrodes were used; planar rotating disc; microelectrode; and a thiri layer flow cell with a planar electrode.
Details and comprehensive discussion of the results for CA experiments are given in Chapter 4, SCP experiments Chapters 5-8, whilst CA experiments on a thin layer flow cell attached to a sequential flow injection analysis (FIA) system are discussed in Chapters 7 and 8.
2.2 Instrumentation
2.2.1 Potentiostatic Equipment
The electrochemical cells used consisted of three electrodes, namely, the working electrode (WE), reference electrode
(RE)
and counter electrode (CE). The potential of the WE is controlled through the use of a potentiostat (or electrochemical analyzer) that controls the potential difference between the WE and RE by altering the current flowingthe WE and CE [27].
Two kinds of potentiostat were used during this work:
i) a BAS 100BIW Electrochemical Analyzer and accompanying BAS 100BIW Version 2.0 software (Bioanalytical System Inc., West Lafayette, Indiana, USA) was used for the experiments with rotating disc electrode and microelectrodes, and
ii) a BAS CV-27 Voltammograph (Bioanalytical System Inc., West Lafayette, Indiana, USA) was used for the experiments with thin layer flow cells attached to
a flow injection analysis system, and also for experiments carried out to evaluate the electrode surface roughness.
In this thesis, the convention that oxidation produces a positive current is adopted. This is in contrast to the convention used in the BAS equipment where reduction processes are reported as positive currents.
2.2.2 Flow Injection Analysis System
This system combines hydrodynamic control with the sensmg efficiency of electrochemistry by pumping liquid electrolyte through a thin layer cell.
The thin layer cell assembly with three electrodes located in a CC-5 cell compartment (Bioanalytical System Inc., West Lafayette, Indiana, USA) is shown in Fig. 2. 1 , where the WEs may be positioned in parallel or series. During this study, a single WE in the series position was subjected to a constant potential controlled by a BAS CV-27 Voltammograph.
The controlled flow of electrolyte was achieved using a Perstrop Analytical Environmental Instrument (perstrop Analytical Inc., Wilsonville, Oregon, USA). This was fitted with a 1 20 position sample carousel, sample valve and sample loops of various SIzes.
The operation of this FIA system was controlled by an accompanying software package (EnviroFlow 2. 1 , ALPKEM, Wilsonville, Oregon, USA) provided with the instrument. The analogue output from the CV-27 Voltammograph was connected to an AID converter fitted in place of the usual optical detector in the Perstrop Analytical device. This combination of electrochemical and analytical instrumentation converts the amperometric response to a signal proportional to the concentration of analyte entering the thin layer cell. The response for each H202-containing segment of solution entering the flow cell consisted of a well defined peak with little tailing.
Since the peak height was reported in arbitrary units by the EnviroFlow 2 . 1 software, it was necessary to relate this to the current response. For this purpose, a precision 0 to 1 000 m V variable DC power supply (Institute of Fundamental Sciences Electronics Workshop, Massey University) was used to calibrate the AID card.
Table 2. 1 lists the potential applied to the AID interface as a function of peak height reported by the ALPKEM software and Fig. 2.2 shows the graphical representation of
0 0
Cb)
BUSHING REFERENCE EI-t:Ci'ROOE AUXIUAAY E.LEC"T'BOOE INLET(a)
WORKING EL-c:...I"RODE PARAL.l...EL SERIES
I� �
(c)
1 9
Fig. 2. 1 The thin layer flow cell assembly. (a) the components, (b) the installed cell
Applied Potential Peak Height mV 1 012 counts 0 0.000 50 0.265 1 00 0. 540 1 50 0.855 200 1 . 1 1 5 250 1 .3 85 300 1 .685 3 50 1 . 995 400 2.295 450 2 . 595 500 2.9 1 5 550 3 .220 600 3 . 535 650 3 . 855 700 4. 1 70 750 4.480 800 4. 780 850 5 .085 900 5 .400 950 5 . 700 1 000 6. 000
Table 2 . 1 Peak height recorded by the FIA NO card listed as a function of applied
6
Cl) -§ 4
o o (\j ... o � - - .c C) 00) I .::s:.m 2
a... Fig. 2.2 2 1 o 250 500 750 1 000Applied Potential ! mV
Calibration of the FIA AID card. The slope of this calibration plot was used in combination with Eq. 2. 1 to convert peak hei
gh
t to current response for all thin layer flow cell experiments. Data are listed in Table 2. 1 .these data. The AID interface clearly demonstrates linear response over the range 0- 1000 mV with insignificant bias. The slope was found to be 6.00x l OI2 counts y-I and this was used together with the CV -27 current sensitivity setting to convert the peak height into a current response
peak height (counts) . . . 1
Current
(mA) =
12 I X senSItiVIty(mA
y- ) (2. 1) 6.00 x 10 (counts V- )2.3 Electrode Systems
The electrochemical cells utilised during this study consisted of three electrodes where the electrochemical reaction of interest takes place at the WE with the generation of the amperometric signal or faradaic current due to an electron transfer processes. The CE is driven by the potentiostatic circuit to balance the faradaic process at the WE but in the opposite direction (e.g. if reduction takes place at the WE, oxidation will occur at the CE). Processes at the CE are typically not of interest, and in most experiments the small currents observed dictate that the electrolytic products at the CE have no influence on the process at the WE.
The reference electrode is an invariant potentiometric probe to monitor the potential change in the WE relative to its own potential by circuit combination with the potentiostat.
2.3.1 Rotating Disc Electrode
A platinum rotating disc electrode (ROE) with geometric area of 0. 1 195 cm2 was used for the CV, CA and SCP experiments. For each experiment, the ROE was preconditioned prior to the electrochemical measurements by cycling the potential from
o to + 1 000 mV vs Ag/AgCI at 100 mV S-1 for 50 continual cycles, with termination at the anodic potential limit on the final cycle. This is believed to increase the activity of the electrode by coating its surface with a coherent film of platinum oxide [52], this pre treatment avoids the growth of a thick oxide layer which may decrease the activity of the electrode and alter its course by employing only an moderate potential excursion. All
such pre-treatment was carried out in the appropriate phosphate buffer electrolyte in the absence ofH202.
23
The RDE is vertically mounted on the shaft of a synchronous digitally controlled motor in the RDE- I device (Bioanalytical System Inc., West Lafayette, Indiana, USA) and rotated with constant angular velocity
(m)
in radS-1
about an axis perpendicular to the plane disc surface as shown in Fig. 2.3.The angular velocity is equal to 2nl, where
f
is the rotation rate in revolutions persecond. As a result of this motion, the fluid in an adjacent layer develops a radial velocity
that moves it a way from the disc centre, and the fluid is replenished by a flow normal to the surface. Hence, the rotated disc can be viewed as a pump that draws a fresh solution up from the bulk solution [95] .
A platinum coil with area of 1 .90 cm2 and an Ag/AgCI gel electrode (3M NaCl) with potential +197 mV vs the standard hydrogen electrode (SHE) (Bioanalytical System Inc., West Lafayette, Indiana, USA) were used as the CE and RE respectively, in the
electrochemical cell.
The potential of the RE was checked periodically against a saturated calomel electrode
(SCE) (Bioanalytical System Inc., West Lafayette, Indiana, USA) with potential
+244 mV vs SHE at 20 °C. There was an insignificant change in the electrode potential
over long periods.
The electrochemical measurements in the RDE system were made in 250 cm3 of phosphate buffer solution in a specially designed water-jacketed cell (IFS Glassblower Workshop, Massey University) maintained at the desired temperature with a circulating water bath (Colora, Messtecknik, GMBH, Germany) and monitored with a calibrated thermocouple placed within the cell at the same height as the WE.
2.3.1.1 Calibration of Rotation Rate
The rotation rate of the RDE was calibrated routinely to ensure correct performance. The calibration procedure was carried out independently by use of an optical digital tachometer (Extech Instruments, Taiwan) by attachment of a reflective strip to the body of the RDE WE.
The rotation speed was set at the RDE-I device through the RPM-adjust control over the range used in the course of this work, and compared with the rotation rate reported by the optical tachometer. A typical set of readings are given in Table 2.2, where good