Journal of
Electroanalytical
Chemistry
Journal of Electroanalytical Chemistry 577 (2005) 125–135
www.elsevier.com/locate/jelechem
Electrode and pH effects on electrochemical reactions during
ohmic heating
Chaminda P. Samaranayake, Sudhir K. Sastry
*
Department of Food, Agricultural and Biological Engineering, The Ohio State University, 590 Woody Hayes Drive, Columbus, OH 43210, USA
Received 23 August 2004; received in revised form 24 November 2004; accepted 26 November 2004
Abstract
Undesirable electrochemical phenomena at electrode|solution interfaces during ohmic heating can be avoided or effectively inhibited by choosing an appropriate electrode material. We attempted to understand the electrochemical behavior of four types of electrode materials: titanium, stainless steel, platinized-titanium, and graphite at pH 3.5, 5.0, and 6.5. The electrodes were examined
comparatively using a 60 Hz sinusoidal alternating current. Analyses of surface morphologies of the electrode surfaces, electrode
corrosion, hydrogen gas generation, and pH change of the heating media were performed. The results highlight the relatively inert
electrochemical behavior of platinized-titanium electrodes at all the pH values. A pilot scale study at 39.8 kW further demonstrates
the potential use of platinized-titanium electrodes for ohmic heating of foods with commonly available low-frequency alternating
currents.
2004 Elsevier B.V. All rights reserved.
Keywords: Ohmic heating; Electrochemical; Electrode; Corrosion; Electrolysis; pH
1. Introduction
In ohmic heating, the electrical energy provided to
the heating cell is ideally used only for heat generation;
and electrochemical reactions at electrode|solution interfaces are considered undesirable. Electrodes in ohmic
heating can be regarded as a ÔjunctionÕ between a solid-state conductor (i.e. current feeder) and a liquid-state
conductor (i.e. heating medium). They play a vital role
by conveying the current uniformly into the heating
medium. Various materials, so far, have been used as
electrodes in different ohmic heating studies and applications (Table 1).
At low-frequency (50–60 Hz) alternating currents,
corrosion of electrodes and apparent (partial) electrol*
Corresponding author. Tel.: +1 614 292 3508; fax: +1 614 292
9448.
E-mail addresses: samaranayake.3@osu.edu (C.P. Samaranayake),
sastry.2@osu.edu (S.K. Sastry).
0022-0728/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2004.11.026
ysis of the heating medium were noticed with most of
those electrodes. Tzedakis et al. [6] described some effects of these reactions on ohmic heating of foods. In
electrochemistry, it is generally known that both physical and chemical properties of electrodes (specifically,
the electrode surfaces) have an influence on electrochemical processes at the electrode|solution interfaces.
With some electrodes, a particular electrochemical
reaction may occur slowly or not at all; but with another type of electrode, the same reaction may be faster
under the same set of conditions. Such information
about electrodes under ohmic heating conditions is,
therefore, important to avoid or inhibit the electrochemical reactions by choosing appropriate electrode
materials.
The anodic and cathodic half-reactions of electrolysis
have strong pH dependences (see Eqs. (1)–(3)). Anodic
half-reaction:
2H2 OðliqÞ () O2ðgÞ þ 4Hþ
ðaqÞ þ 4e
ð1Þ
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C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
Table 1
Electrode materials used for ohmic heating studies and applications
Electrode material
Reference
Aluminum
Carbon (graphite)
Dimensionally stable anode (DSA)-type
Glassy carbon
Platinum
Platinized-titanium
Rhodium plated stainless steel
Stainless steel
Titanium
[1,2]
[3,4]
[5]
[5]
[6]
[6,7]
[8]
[5,9,10]
[5]
E/V = {1.23 0.015 log p (O2(g)) + 0.059
25 C.
Cathodic half-reactions:
pH},
at
2Hþ
ðaqÞ þ 2e () H2ðgÞ
ð2Þ
2H2 OðliqÞ þ 2e () H2ðgÞ þ 2OH
ðaqÞ
ð3Þ
E/V = {0.03 log p (H2(g)) 0.059 pH}, at 25 C.
The overall electrolysis reaction:
2H2 OðliqÞ () 2H2ðgÞ þ O2ðgÞ
ð4Þ
log Kf (H2O(liq)) = 41.546, at 25 C.
The pH also affects electrode corrosion. In particular, the above pH dependent cathodic half-reactions of
electrolysis (Eqs. (2) and (3)) can couple with the following (generalized) anodic half-reaction of corrosion of the
metallic electrode (M):
MðsÞ () Mnþ
ðaqÞ þ ne ; where n ¼ 1; 2; 3 . . . :
ð5Þ
Moreover, food formulations subjected to ohmic processing have various pH values. Therefore, the objective
of this study was to investigate comparatively the behavior of various electrode materials at different pH values.
We also examined the extents of reactions on a pilot
scale during an ohmic heating process.
2. Experimental
Fig. 1. The laboratory scale ohmic heater. (1) Hydrogen gas sensor. (2)
Teflon coated thermocouple. (3) Electrodes: slightly curved electrodes
(electrode gap: 9 cm). (4) Cooling water inlet. (5) Cooling water outlet.
(6) Removable lid: the electrodes, hydrogen sensor, and thermocouple,
are attached. It was tightly clamped to the cell body during ohmic
heating.
metal grade concentrated nitric acid (Fisher Scientific,
PA), AR grade dichloromethane (Mallinckrodt, KY),
concentrated HCl (Fisher Scientific, NJ), HPLC grade
acetonitrile (Mallinckrodt Baker Inc., NJ) and TBDMS
{N-Methyl-N-[(tert-butyldimethyl)silyl] trifluoroacetamide} (Regis Technologies, Inc., IL) were purchased
from the suppliers. Demineralized twice-distilled water
(Resistivity: 3 MXcm; pH: 5.5) was obtained from
the Reagent Laboratory Store, at The Ohio State
University.
2.3. Scanning electron microscopic (SEM) analysis
Surfaces of the electrodes before ohmic heating were
examined by a JEOL JSM-820 scanning electron
microscope.
2.1. Electrodes
2.4. Laboratory scale ohmic heating
Titanium, stainless steel (316), platinized-titanium,
and graphite were used as electrodes. All electrodes were
rectangular (7.5 cm · 5.2 cm) with slight curvature
(radius 4.5 cm) (see Fig. 1), and had the same geometric dimensions.
2.2. Chemicals
ACS grade sodium chloride (Fisher Scientific, NJ),
citric acid monohydrate (Aldrich, WI), and sodium
bicarbonate (Fisher Scientific, NJ), anhydrous sodium
sulfate (Cooper Natural Resources, Inc., TX), trace
2.4.1. Experimental setup
All the laboratory scale studies were performed using
a batch unstirred ohmic heater (Fig. 1). The ohmic heater was attached to the experimental setup as shown in
Fig. 2, and was powered by a low-frequency (60 Hz)
sinusoidal alternating current. An external cooling system was operated by a Haake F3 Fisons thermostatic
water bath having an inflow and outflow attached to
the ohmic heater. The cooling was required to perform
the experiments at relatively low temperatures and currents to avoid potential explosion hazards due to exces-
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
Thermocouple
127
Isolation
module
ohmic heater
Hydrogen
gas meter
0 – 250 ppm
V
60 Hz/ 0-110 V~
Public utility
Supply
A
Data
logger
Variac
V = voltage transducer
A = Current transducer
Microcomputer
Fig. 2. Schematic diagram of the laboratory scale experimental setup.
sive hydrogen accumulation (in the headspace) and possible arc formation.
2.4.2. Heating media
Each type of electrode was examined at an initial pH
of 3.5, 5.0, and 6.5 (at 25 C) using freshly prepared
aqueous heating media. The selected pH values represent the entire pH range of food formulations subjected
to ohmic processing. These desired pH values of the
heating media were achieved by either citric acid or sodium bicarbonate. The initial electrical conductivity of
the heating medium was adjusted by NaCl (0.1%, w/v),
and was kept constant at 2.35 mS cm1 (at 25 C) in this
comparative study to achieve prolonged heating exposure while controlling the temperature and current at
the intended levels. The above components used for
the pH and conductivity adjustments are also common
ingredients in food formulations.
2.4.3. Heating procedure
A volume of 250.00 (±0.12) ml was subjected to ohmic heating in each experimental run. The effective geometric surface area of each electrode involved in ohmic
heating was constant at 21.5 cm2 with that volume.
The ohmic heater was operated at a root-mean-square
(RMS) voltage of 110 V, which is the common singlephase public utility supply voltage in the US. The duration of heating was kept constant for the purpose of
comparison, and was limited to 307 s by the upper detection limit (250 ppm) of the hydrogen detector, and by
pronounced hydrogen generation with some electrode–
pH combinations. Since each electrode material had a
different electrical conductivity, and resulted in a different effective (capacitive) current, control of the temperature and current during ohmic heating required
different cooling water temperatures and/or input voltages, as described below.
The temperature of the cooling water was varied
from 13–17 C for the metallic electrodes to yield the
same time–temperature and time–current histories at
all the pH values during ohmic heating. However, since
the graphite electrodes exhibited much higher heating
rates and currents that could not be compensated by
the external cooling, those electrodes were examined at
a RMS voltage of 97 (±1) V, instead of the 110 V used
otherwise, and a cooling water temperature of 23
(±1) C to obtain the same time–temperature and
time–current histories as those of the metallic electrodes
(see Figs. 3 and 4). Since rates of electrochemical reactions depend upon the temperature and amount of current [11] passing through the ohmic cell, this precise
matching of time–temperature and time–current histories was considered necessary to eliminate the temperature and current as variables. The voltage reduction,
however, resulted in a reduced power input for the same
time–temperature and time–current histories. Therefore,
all data used for comparison were normalized on the basis of unit energy input.
The ohmic heating experiments were randomized
with respect to the electrode material at each pH.
Three replications were made per material at each
pH. The electrodes were thoroughly rinsed using
demineralized twice-distilled water before each run.
Adherent films formed on the titanium and stainless
128
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
ter (ICP–MS) [12]. The elemental carbon concentrations
were determined by a Perkin–Elmer Optima 3000 DV
inductively coupled plasma–optical emission spectrometer (ICP–OES) monitoring the emission spectra near
193.03 nm.
2.4.5. Hydrogen generation and pH measurements
A series U hydrogen detector (CEA Instruments,
Inc., NJ) was used to measure headspace hydrogen gas
generated during ohmic heating. The pH of the medium
before and after the ohmic heating treatment was measured by a Cole–Parmer 59003 Benchtop pH meter (resolution: 0.01 pH) at 25 C.
Fig. 3. Typical time vs. temperature curve for all the electrodes at all
the pH values during ohmic heating.
Fig. 4. Typical time vs. current curve for all the electrodes at all the pH
values during ohmic heating.
steel electrode surfaces during ohmic heating were removed by brushing and cleaning after three replicates
at each pH.
2.4.4. Analysis of electrode corrosion
Concentrations of Ti (from the titanium electrodes),
Fe (from the stainless steel electrodes), Pt (from the platinized-titanium electrodes), and elemental carbon (from
the graphite electrodes) migrating into the heating media
were taken as measures of electrode corrosion. In each
experimental run, once the ohmic heating was completed, a 25.00 (±0.03) ml sample was pipetted out, after
removing the electrodes and thoroughly mixing the
fluid. A 25.00 (±0.03) ml sample of the respective unheated heating medium was used as a method blank.
All the samples were collected into polypropylene sample bottles, and then stabilized by adding concentrated
nitric acid (5%, v/v). Quantitative analyses of the metal
ions were performed by a Perkin–Elmer Sciex ELAN
6100 DRC inductively coupled plasma–mass spectrome-
2.4.6. Analysis of migrated graphite corrosion products
An aqueous heating medium was prepared using only
NaCl, and without adding any citric acid or sodium
bicarbonate. The initial pH of the heating medium happened to be 5.52 at the same initial electrical conductivity. Ohmic heating was performed as previously
described, such that the time–temperature and time–current histories were as same as those of the metallic electrodes. Analysis of electrode corrosion, and
measurements of hydrogen generation and pH were also
carried out as described above. Identification of the
chemical nature of some elemental carbon species
migrating into the heating medium was attempted by
using gas chromatography–mass spectrometry (GC–
MS), and electrospray ionization–mass spectrometry
(ESI–MS), as described below.
2.4.6.1. GC–MS analysis. The pH measurements revealed the migration of soluble acidic organic compounds. These compounds were extracted by 2.5 ml of
dichloromethane after acidifying 250.00 (±0.12) ml of
the ohmically heated medium to a pH of 1 by adding
1:1 (v/v) HCl. The extraction involved 1 min of shaking
in a separating funnel, and a settling time of about 5
min. About 0.5 ml of dichloromethane extract could
be collected, and it was evaporated to dryness under reduced pressure at room temperature. The acidic compounds were derivatized by a 1:2 mixture of TBDMS/
acetonitrile and incubated at 60 C for 1 h, for the
GC–MS analysis. The unheated medium of the same
volume, subjected to the same extraction and derivatization procedures, was used as a method blank, and was
run under the same GC–MS conditions to identify the
migrated acidic organic compounds.
Gas chromatography was carried out in splitless
injection mode through a 95% dimethyl/5% diphenyl
polysiloxane column (30 m · 0.32 mm ID; 0.25 lm
film) using temperature ramps of 30 C/min from 40
to 180 C and 7 C/min from 180 to 350 C, with
He as the carrier at a flow rate of 2 ml/min. The injector and GC–MS interface temperatures were at 222
and 260 C, respectively. Mass spectra were acquired
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
in scan mode within the m/z range of 42–839, at a rate
of 1.4 scan/s. Structural identification of GC peaks
was performed by means of the NIST-98 library
search database.
2.4.6.2. ESI–MS analysis. The migration of polar organic compounds was further detected by using a Bruker
Esquire electrospray ionization mass spectrometer operated in positive ion mode. Heating medium samples before and after ohmic heating treatment were infused
directly into the electrospray source at 5–10 ll min1
using a capillary voltage of 3500 V, source temperature
250 C, and nitrogen as the drying gas. Data were acquired using Bruker Dolltonics DataAnalysis 2.0 software, in continuum mode until acceptable averaged
data were obtained.
2.5. Pilot scale study of electrode corrosion
An aqueous (tap water) solution having an initial pH
of 3.5 and an initial electrical conductivity of 1 S m1
was used as the heating medium. The pH and electrical
conductivity were adjusted by citric acid monohydrate
(0.03%, w/v) and sodium sulfate (0.8%, w/v), respectively. A pilot scale flow-through ohmic heater containing three cylindrical ohmic cells was used for this study.
Each cell consisted of two platinized-titanium electrodes; and each electrode had a surface area of 142.7
cm2.
Pre-heated (by steam) heating medium at 60 C was
pumped through the ohmic heater followed by cooling
in a scraped-surface heat exchanger. The heater was
operated at 39.8 kW using a 60 Hz sinusoidal alternating
current under steady state conditions with a continuous
liquid flow of 6.8 l min1. The inlet and outlet temperatures of the ohmic heater were 58 and 138 C,
respectively.
Approximately 1 l samples were collected from the
feed tank (initially containing 220 l) and after running
through the heat-hold-cool cycle of the ohmic processor
at steady state. From each of the 1 l samples, a 25.00
(±0.03) ml sample was pipetted out into a polypropylene
sample bottle, and then stabilized by adding concentrated nitric acid (5%, v/v) for chemical analysis. The
above sampling procedure was carried out twice for
each experimental run, and the whole experiment was
triplicated. Pt and Ti concentrations were determined
by a high resolution (double focusing sector based)
ThermoFinnigan Element 2 ICP–MS.
2.6. Data analysis
The total energy input, in the laboratory scale studies, was determined by integrating the power input
(Pinput = VI) vs. time curve for each experimental run.
129
The electrode corrosion normalized per unit energy
input was defined as the Ôcorrosion rateÕ. Two-factor
analysis of variance was used to determine whether
the type of electrode material and pH had significant
effects on corrosion rate. The changes in pH of the
heating media observed with some electrodes were analyzed by one-factor analysis of variance to determine
the effect of pH. The difference between the Pt and
Ti concentrations in the pilot scale study was evaluated
by a two-sample paired t-test. TukeyÕs specific comparison test determined which particular means were significantly different. The significance of differences was
defined as P 6 0.05. A statistical software package,
SPSS 11.5 for Windows (SPSS Inc., 2002), was used
for the statistical analyses.
3. Results and discussion
3.1. Electrode corrosion
An important consideration in ohmic heating is the
amount and the chemical nature of the corrosion products migrating into the food during the application of
electrical power. Table 2 shows a comparison of corrosion rates (ppb kJ1) of the electrode materials with respect to the migrations of their major (surface) elements
at different pH values. The analysis of variance suggests
that both type of electrode material and pH, as well as
their interactions, have significant effects on the corrosion rate. As can be seen from Table 2, the corrosion
rates of titanium and platinized-titanium electrodes are
not significantly different at any pH value, however
these are significantly lower (P 6 0.05) than the corrosion rates of stainless steel and graphite electrodes. On
the other hand, the corrosion rates of stainless steel
and graphite electrodes are not significantly different except at pH 3.5. Each of the electrode materials exhibits a
higher corrosion rate at pH 3.5 than at the other pH
values.
Table 2
Comparison of corrosion rates (in ppb kJ1) of the electrode materials
with respect to the migrations of their major (surface) elements at
different pH values
Electrode type
Corrosion rate* (ppb kJ1)
pH 3.5
Titanium (Ti)
Stainless steel (Fe)
Platinized-titanium (Pt)
Graphite (C)
a
0.26 (0.21)
14.20b (1.95)
0.25a (0.10)
26.6d (2.2)
pH 5.0
a
0.03 (0.01)
8.33c (0.30)
0.07a (0.04)
7.2c (0.0)
pH 6.5
0.05a (0.03)
11.43b,e (1.51)
0.05a (0.02)
8.4c,e (1.1)
a–e
Means with different superscript letters are significantly different
(P 6 0.05).
*
Numbers in parentheses indicate standard deviations of mean
(n = 3) corrosion rates.
130
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
trodes (Fig. 5(c)) would certainly enhance the microscopic surface area possessing a much higher double
layer capacitance. Clearly, the graphite electrodes had
a very large microscopic surface area because of the high
surface roughness (Fig. 5(d)) possessing the highest
capacitance out of the four electrodes. The highly efficient heat generation attributed by this huge capacitance
could not be controlled by the external cooling system,
and therefore necessitated a lower voltage, instead of
the 110 V used otherwise, in order to obtain the same
time–temperature and time–current histories as those
of the metallic electrodes. According to the SEM analysis, variation of the double layer capacitance of the electrodes can be represented as: titanium < stainless
steel platinized-titanium graphite.
Fig. 5. Typical SEM micrographs of the electrode materials:
(a) titanium, (b) stainless steel, (c) platinized-titanium, (d) graphite.
3.2. SEM analysis
Fig. 5 shows typical SEM micrographs (magnification: 2000·, accelerating voltage: 20 kV) of the four electrodes before the ohmic heating experiments. Although
the apparent geometric surface area of the electrodes involved in the ohmic heating was kept constant, surface
morphologies imply the existence of markedly different
surface areas on the micro scale. Since the capacitance
is directly proportional to the surface area, an electrode
having a larger microscopic surface area possesses a
higher electrical double layer capacitance per unit
apparent geometric surface area. The electrical double
layer of such an electrode is capable of holding more
charges before the double layer capacitor becomes ÔleakyÕ, inhibiting faradaic-type reactions at the electrode|solution interface. Moreover, the current flowing
through the interface then becomes mostly capacitive.
Therefore, the use of electrodes having a large microscopic surface area is beneficial in terms of inhibiting
the faradaic processes at the interfaces, as well as achieving more capacitive current for the heat generation.
Amatore et al. [5] have already introduced this concept,
suggesting the use of electrodes with large surface
roughness. However, the applications of highly porous
electrodes may be hindered because of the possibility
of harboring tiny food particles, microorganisms, or
macromolecular food components in the interstitial
spaces within the electrodes.
As can be seen from Fig. 5(a), the titanium electrodes
had a smooth surface indicating virtually no enhancement of surface area on the micro scale, and thereby a
poor double layer capacitance. The SEM micrograph
of the stainless steel electrodes (Fig. 5(b)) also indicates
no great enhancement of the microscopic surface area,
although it appears to have some cracks and valleys
on the surfaces. The tiny cracks, valleys, and bumps
spread over the surfaces of platinized-titanium elec-
3.3. Titanium electrodes
Titanium is considered to have high corrosion resistance and biocompatibility characteristics [13]. It is generally the material-of-choice for chloride environments.
Although the SEM micrograph (Fig. 5(a)) indicates
poor double layer capacitance, the corrosion rates of
titanium electrodes at all pH values were significantly
lower (P 6 0.05) than those of the stainless steel and
graphite electrodes. The reason could be the oxide layer
that covers active titanium metal, protecting it against
corrosion. Since titanium exhibits high affinity towards
oxygen [14], the protective oxide layer could be formed
by reacting with atmospheric oxygen even before using
the metal as electrodes, and also during the ohmic heating treatments. Tzedakis et al. [6] discussed the possibility of forming rutile (TiO2) due to electrochemical
processes during ohmic heating, partially passivating
the titanium electrodes.
Titanium forms various oxides having different colors, such as TiO2:anatase (yellowish), TiO2:rutile
(white), TiO1.9:oxygen deficit (bluish), and Ti3O5 (violet)
[14]. At all the pH values, we observed formations of
adherent surface films with a yellowish-brown color with
some blue and violet coloration, which therefore, imply
the electrochemical generation of oxygen (not measured)
during ohmic heating. There was no detectable pH
change of the heating medium at any pH value. The
hydrogen generation shown in Fig. 6, therefore, probably indicates the occurrence of an electrolysis reaction
(Eq. (4)) on the passivated titanium electrode surfaces.
It can be seen that the probable electrolysis becomes
more pronounced at pH 6.5 than at the other pH values.
Because of the high penetration ability of hydrogen into
titanium, the hydrogen generation may cause surface
embrittlement followed by surface disintegration [14]
contributing to electrode corrosion during ohmic heating. In particular, under more acidic conditions (i.e.
pH 3.5), TiO2 can undergo the following cathodic
half-reactions [14] causing migration of titanium ions
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
131
Table 3
pH changes of the heating media observed with stainless steel
electrodes at different pH values
DpH*
pH 3.5
pH 5.0
pH 6.5
+0.04a (0.01)
+0.18b (0.03)
+ 0.10c (0.01)
a–c
Means with different superscript letters are significantly different
(P 6 0.05).
*
Numbers in parentheses indicate standard deviations of mean
(n = 3) pH changes. Positive signs indicate increase of pH.
Fig. 6. Hydrogen generation with titanium electrodes during ohmic
heating.
into the heating medium resulting in a higher corrosion
rate:
2þ
TiO2ðsÞ þ 4Hþ
ðaqÞ þ 2e () TiðaqÞ þ 2H2 OðliqÞ
ðE ¼ 0:502 VÞ
ð6Þ
3þ
TiO2ðsÞ þ 4Hþ
ðaqÞ þ e () TiðaqÞ þ 2H2 OðliqÞ
ðE ¼ 0:666 VÞ
ð7Þ
3.4. Stainless steel electrodes
Stainless steel is an iron–chromium alloy containing
at least 11% chromium. The grade designated 316 belongs to the austenitic family of stainless steels, and contains chromium (17%), nickel (10%), and molybdenum
(2%) as major alloying elements [15]. In the food industry, stainless steels are in widespread use as food contact
surfaces. The stainless steel electrodes exhibited pronounced corrosion rates (Table 2), hydrogen generation
(Fig. 7), and also pH changes of the heating media
(Table 3) at all the pH values. In addition to the chemical reactivity of the alloying elements, the lack of double layer capacitance, as implied by the SEM analysis,
would be responsible for the pronounced electrochemical behavior. The observed adherent surface films
formed on the stainless steel electrodes during ohmic
heating were transparent with a light golden color and
some brown rust. The films, however, did not cover
the electrode surfaces uniformly, and showed several
cracks. Since there were pH changes in the heating media, the hydrogen generation shown in Fig. 7 does not solely represent the electrolysis reaction (Eq. (4)). Based on
the observations, assignment of predominant electrochemical reactions is attempted as follows.
Under more acidic conditions (i.e. at pH 3.5), the
cathodic half-reaction shown in Eq. (2) could couple
with the following anodic half-reaction (Eq. (8)) resulting in accelerated corrosion and hydrogen generation
(Eq. (9)) compared to those at the other pH values.
Anodic half-reaction:
MðsÞ () M2þ
ðaqÞ þ 2e ; ðwhere; M ¼ Fe; Cr; Ni; MoÞ
ð8Þ
Overall reaction:
2þ
MðsÞ þ 2Hþ
ðaqÞ () MðaqÞ þ H2ðgÞ
ð9Þ
When M = Fe, the values of the equilibrium constant
(log K = 14.86, at 25 C) and standard potential
(E = 0.44 V) also suggest the occurrence (even spontaneously) of the above overall reaction (Eq. (9)). The increase of pH could be attributed to the loss of Hþ
ðaqÞ ions
as the hydrogen gas, and FentonÕs reaction (Eq. (10)),
which liberates OH
ðaqÞ ions into the heating medium
[16].
Fe2þ þ H O () Fe3þ þ OH þ OH
ð10Þ
ðaqÞ
2
2
ðaqÞ
ðaqÞ
The required H2O2 for the above reaction can be generated by the following cathodic half-reaction (Eq. (11)):
O2ðgÞ þ 2Hþ
ðaqÞ þ 2e () H2 O2 ðE ¼ 0:68 VÞ
Fig. 7. Hydrogen generation with stainless steel electrodes during
ohmic heating.
ð11Þ
Under mildly acidic conditions, the significantly high
(P 6 0.05) pH changes associated with corrosion and
hydrogen generation can be explained by the following
overall reaction (Eq. (12)), which is the combination of
132
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
the cathodic and anodic half-reactions shown in Eqs. (3)
and (8), respectively:
MðsÞ þ 2H2 OðliqÞ () M2þ
ðaqÞ þ H2ðgÞ þ 2OHðaqÞ
ðM ¼ Fe; E ¼ 0:39 VÞ
ð12Þ
It may also be possible to have pH changes together
with generation of hydrogen and chlorine gases (Eq.
(14)) due to the cathodic half-reaction shown in Eq.
(3) and the following anodic half-reaction (Eq. (13)).
Anodic half-reaction:
2Cl
ðaqÞ () Cl2ðgÞ þ 2e ðE ¼ 1:36 VÞ
ð13Þ
Overall reaction:
2H2 OðliqÞ þ 2Cl
ðaqÞ () Cl2ðgÞ þ H2ðgÞ þ 2OHðaqÞ
ðE ¼ 2:19 VÞ
ð14Þ
Both overall reactions shown in Eqs. (12) and (14) however need to utilize electrical energy provided to the
heating cell.
3.5. Platinized-titanium electrodes
Platinization of titanium electrodes has been a popular choice because of the high cost of pure platinum electrodes for industrial processes [17,18]. On the other
hand, platinization is also an effective method of passivating titanium [14]. The platinized-titanium electrodes
exhibited significantly lower (P 6 0.05) corrosion rates
compared to those of the stainless steel and graphite
electrodes at all the pH values. Further, there were no
signs of hydrogen or any other gas evolution at the electrode|solution interfaces, and also no detectable pH
changes of the heating media under these experimental
conditions. The rich double layer capacitance, as indicated by the SEM analysis, would be the major reason
for this superior electrode performance. Tzedakis et al.
[6] have already demonstrated the superiority of platinized-titanium electrodes over platinum electrodes for
ohmic sterilization of food products with low-frequency
(50 Hz) alternating currents.
3.6. Graphite electrodes
Graphite, one of the allotropic forms of carbon, has
been used as an electrode material in electrochemical
applications for a long time. Although there are various
types of commercially available graphitic carbons, polycrystalline graphite (PCG) is the material most often referred to as ÔGraphiteÕ [19]. In spite of the very rich
double layer capacitance, as indicated by the SEM analysis, the corrosion rates of graphite electrodes at all the
pH values were significantly greater (P 6 0.05) than
those of titanium and platinized-titanium electrodes.
However, as in the case of platinized-titanium electrodes, there were no signs of gas evolution at the elec-
trode|solution interfaces, and also no detectable pH
changes of the heating media at any pH value tested.
The above electrochemical behavior can be explained
by means of the chemical structure of graphite, as
follows.
Graphite consists of sp2 hybridized carbon atoms arranged as parallel sheets of hexagonal rings. Since sp2
hybridized carbon is capable of forming covalent bonds
and has a propensity towards adsorption of a broad
range of substances, graphite electrode surfaces usually
contain various functional groups and oxides [19].
Therefore, the migration of surface functional groups
and oxides as organic compounds was anticipated during ohmic heating. In the analysis of these migratory
corrosion products (Section 2.4.6), the heating medium
exhibited a change of about 0.03 pH units indicating
the migration of acidic organic compounds. The observed corrosion rate was 6.1 ± 0.1 ppb kJ1. Fig. 8
shows the organic compounds identified by GC–MS
analysis.
The quasimolecular ion [M + H]+ peaks observed at
m/z 91 and 133 in the ESI–MS spectrum (Fig. 9) of the
heating medium after the ohmic heating treatment correspond well with the polar organic compounds identified by GC–MS. In addition, the peaks observed at m/z
109 and 123 may represent [M + H]+ ions for ortho- or
para-quinones (C6H4O2) and their corresponding methylated counterparts, respectively. Such types of quinones are reported to be present on most carbon
surfaces [19,20]. It is clearly seen that the organic compounds migrating into the heating medium always contain more than one carbon atom per molecule.
Therefore, migration of even a few molecules results
in intense corrosion rates if the corrosion is measured
as elemental carbon.
Since sp2 hybridized carbon has a high affinity towards oxygen [19], surface oxides and oxygen-containing functional groups could be formed by reacting
with atmospheric oxygen even before using the graphite
as electrodes, and also by to the electrochemical reactions during ohmic heating. Soffer and Folman [21] suggested the following anodic and cathodic half-reactions
related to the electrolysis of water creating oxides and
functional groups on graphite electrode surfaces.
OH
O
O
OH
OH
OH
(i)
(ii)
Fig. 8. Graphite corrosion products identified by GC–MS analysis: (i)
2-hydroxy, propanoic acid (i.e. lactic acid) (molar mass: 90); (ii) 2hydroxy, 4-methyl, pentanoic acid (molar mass: 132).
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
Int.
4
× 10
0.5
After
91.0
Int.
4
× 10
133
Before
123.0
149.0
2.5
0.0
100
200 m/z
75.1
2.0
149.0
109.0
1.5
133.0
1.0
0.5
0.0
20
60
40
100
80
120
140
160
180
m/z
Fig. 9. Positive ion ESI–MS spectra of the heating medium before and after ohmic heating.
Anodic half-reactions:
2Hþ
ðaqÞ
ð15Þ
C þ H2 OðliqÞ () C OH þ Hþ
ðaqÞ þ e
ð16Þ
C þ H2 OðliqÞ () C O þ
þ 2e
Cathodic half-reaction:
C þ H2 OðliqÞ þ e () C H þ OH
ðaqÞ
ð17Þ
where ÔCÕ represents sp2 hybridized carbon on the
graphite surface. During ohmic heating, once a set of
carbon atoms leaves the graphite surface as organic
compounds, a new set of sp2 hybridized carbon atoms
in the graphite structure is exposed to the heating medium, and keep undergoing the corrosion process. The
migration of compounds into the heating medium
could be due to thermal, electric field and pH (of the
heating medium) effects. The significantly high
(P 6 0.05) corrosion rate observed at pH 3.5 may be
due to acid catalyzed hydrolyses of ester and ether
linkages on the graphite electrode surfaces facilitating
the migration of functional groups as compounds than
at the other pH values.
The eqs. (15)–(17) basically indicate the adsorption
of electrolysis products on the electrodes causing oxidation and reduction of the surfaces, ultimately creating some functional groups. Soffer and Folman [21]
also reported the adsorption of chloride ions on the
graphite electrodes according to the following anodic
half-reaction:
C þ Cl
ðaqÞ () C Cl þ e
ð18Þ
Such adsorption processes, as well as the very rich double layer capacitance, apparently inhibited the gas evolution at the electrode|solution interfaces. Although there
were no detectable pH changes of the heating media in
the presence of citric acid and sodium bicarbonate, the
pH change of the electrolyte due to positive and negative
charging of the electrodes is considered to be a unique
property of high surface area graphite electrodes [21].
In general, graphite electrodes are also considered to
have a wider faradaic reaction-free potential window
compared to that of the metallic electrodes [19]. However, ordinary faradaic processes, such as generation
of hydrogen and oxygen due to the electrolysis of water
without adsorption on the surfaces, can also take place,
irrespective of the chemical nature of the surface groups
[21].
3.7. Pilot scale study of electrode corrosion
In the laboratory scale studies, platinized-titanium
exhibited the best electrode performance out of the four
electrode materials tested. Therefore, it was subjected to
further investigation on a pilot scale for electrode corrosion. Table 4 shows Pt and Ti concentrations of the
ohmically heated medium after running through the
heat exchanger when the ohmic heater was at the steady
state. The concentrations of Pt and Ti in the blank (i.e.
the heating medium in the feed tank before being subjected to ohmic heating) were too low for reliable measurements. Since platinized-titanium exhibited the
highest corrosion at pH 3.5 in the laboratory scale stud-
134
C.P. Samaranayake, S.K. Sastry / Journal of Electroanalytical Chemistry 577 (2005) 125–135
Table 4
Pt and Ti concentrations (in parts per trillions) of the ohmically heated
medium in the pilot scale study
Element
Concentration* (ppt)
Pt
Ti
61.6a (10.3)
69.2a (14.6)
a
Means with the same superscript letter are not significantly different (P > 0.05).
*
Numbers in parentheses indicate standard deviations of mean
(n = 6) concentrations.
Table 5
Comparison of estimated metal intakes via consumption of an 8 oz
ohmically heated meal with the published upper-level daily dietary
exposure limits for adult consumers
Element
Estimated intake
via 8 oz meal (lg)
Published upper-level
daily dietary exposure
limits (lg day1)
Pt
Ti
0.014
0.016
0.3a
600b
The estimation is based on unit conversions: 1 ppt = 1 picogram/g,
8 oz = 227 g, 1 pg = 1012 g = 106 lg.
a
Ysart et al. [22].
b
Reilly [23].
ies, the values shown in Table 4 would be the ÔworstcaseÕ concentrations.
Using these concentrations, we estimated intakes
of Pt and Ti with respect to a typical meal of 8 oz
(227 g) assuming the same conditions in food processing. The estimated values were then compared with recently published upper-level daily dietary exposure
limits for adult consumers (see Table 5). It can be seen
that the estimated metal intakes via consumption of an
ohmically heated meal of 8 oz are far below the published upper-level daily dietary exposure limits. Therefore, food processing by ohmic heating may be
performed on a pilot scale without significant electrode
corrosion using platinized-titanium electrodes and the
migrations of Pt and Ti may result in concentrations
that are far below the published dietary exposure limits.
4. Conclusion
Using the alternating frequency of 60 Hz, we demonstrated that the electrochemical behavior of an electrode
material is unique to the material itself. Although, in
general, a large microscopic surface area can suppress
electrochemical processes, the type and extent of electrochemical reactions are determined by the chemical nature of the electrode surface, as well as the pH of the
heating medium. All the electrode materials exhibited intense electrode corrosion at pH 3.5 compared to that of
the other pH values. Although the titanium electrodes
showed a relatively high corrosion resistance, apparent
electrolysis was seen at all the pH values during ohmic
heating. Stainless steel was found to be the most electrochemically active electrode material during ohmic heating at all the pH values. It was proven that the intense
corrosion of graphite electrodes was due to the migration of surface functional groups and oxides as organic
compounds during ohmic heating; and the pH of the
heating medium seemed to facilitate such migrations.
Because of the relatively inert electrochemical behavior,
platinized-titanium would be the electrode materialof-choice for ohmic heating at all the pH values. The
potential use of platinized-titanium electrodes for ohmic
heating operations was further demonstrated on a pilot
scale at 39.8 kW; and the concentrations of migrated Pt
and Ti were far below the published dietary exposure
limits.
Acknowledgments
Salaries and research support were provided by the
Ohio Agricultural Research and Development Center,
The Ohio State University. References to commercial
products and trade names are made with the understanding that no endorsement or discrimination by
The Ohio State University is implied.
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