EPA-600/2-76-021
June 1976
Environmental Protection Technology Series
ELECTROCHEMICAL ANALYSIS OF
SULFIDIC AND AMINE ODORANTS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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iEPA-600/2-76-021
June 1976
ELECTROCHEMICAL ANALYSIS OF
SULFIDIC AND AMINE ODORANTS
•by
Jerry N. Nwankwo and Amos Turk
Department of Chemistry
The City College of New York
New York, N.Y. 10031
Grant No. 802396
Project Officer
John Nader
Emission Measurement and
Characterization Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
. OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for pub-
lication. Approval does not signify that the contents necessarily re-
flect the views and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute
endoresement or recommendation for use.
ii
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ABSTRACT
Oxidation of odorous vapors at the anode of an electro-
chemical cell was studied as a promising approach to achieving
instrumental analysis of odors.
The technique of linear potential sweep cyclic voltam-
metry was used to investigate the oxidizability of several
amines, sulfides, and their mixtures on platinum, gold, glassy-
carbon, carbon paste, and graphite electrodes. Results of the
analyses of individual amines showed that the ease of oxidation
on a platinum electrode in acetonitrile containing NaClO. as
supporting electrolyte was: tertiary > secondary > primary.
Results for runs conducted on mixtures containing combina-
tions of primary, secondary, and tertiary amines indicated that
separate current peaks were not obtained for the various amines.
Instead, a composite peak was obtained for any given mixture.
Results for sulfides using a platinum electrode i-ndicated that
allyl sulfide, n-butyl sulfide, and tert-butylsulfide could be
oxidized in acetonitrile solution with 0.1 M NaClO. as
supporting electrolyte. The order was: tert-butylsulfide >
n-butylsulfide > allylsulfide. When amines and sulfides were
mixed, a clear separation of the peaks was observed. The
results indicate that it is possible to characterize a mixture
of amines and sulfides by linear potential sweep cyclic
voltammetry. Oxidation of sulfides and amines on gold, glassy-
carbon, and graphite electrodes did not reveal any significant
advantage over a platinum electrode. However, a carbon-paste
electrode was superior to platinum in the oxidation of sulfides.
iii
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CONTENTS
Page
ABSTRACT ill
LIST OF FIGURES vi
INTRODUCTION . . 1
TECHNIQUES 2
Chronopotentiometry 2
Chronoaraperometry 3
Linear Potential Sweep Voltanunetry 4
Differential Pulse Voltanunetry 5
Chronocoulometry 5
PREVIOUS STUDIES 6
EXPERIMENTAL. 7
Reagents and Solutions 9
RESULTS AND DISCUSSION 10
Oxidation of Amines 10
Oxidation of Sulfides 17
Effects of Variation of Supporting Electrolyte . 25
An Approach to Quantitative Analysis 26
CONCLUSIONS 35
ACKNOWLEDGMENT. 35
REFERENCES 36
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FIGURES
No. Page
1 Flow Diagram for Current Measurement 8
2 Cyclic Voltammogram of 0-Dianisidine in 1M HjSO.;
Concentration, 5mM. Ft- electrode 12
3 Cyclic Voltammogram for 2mM n-Butylamine in Aceto-
nitrile; 0.1M in NaClO^. Sweep Sate 5.3 V/min.
Pt-electrode 12
4 Cyclic Voltammogram for 1.84 mM Solution of Benzyl-
amine in Acetonitrile, 0.1M in NaClO.. Sweep Rate
5.3 V/min. Pt-electrode 13
5 Cyclic Voltammogram for 1.95 mM Solution of Diethyl-
amine in Acetonitrile, O.lM in NaCIO.. Sweep rate
5.3 V/min; Pt-electrode 13
6 Cyclic Voltammogram for 1.43 mM Solution of Triethyl-
amine in Acetonitrile, 0.1M in NaClO.. Sweep rate
5.3 V/min; Pt-electrode 14
7 Voltammogram for a Mixture of n-Butylamine and
Diethylamine on Pt-electrode. Sweep Rate 5.3 V/min 14
8 Voltammogram for a Mixture of n-Butylamine and
Triethylamine on Pt-electrode. Sweep Rate 5.3 V/min 15
9 Voltammogram for a Mixture of n-Butylamine, Diethyl-
amine and Triethylamine on Pt-electrode. Sweep Rate
5.3 V/min 15
10 Voltammogram for a Mixture of n-Butylamine, Diethyl-
amine and Triethylamine on Glassy Carbon Electrode.
Sweep Rate 5.3 V/min 18
11 Cyclic Voltammogram for Allyl Sulfide on Pt-electrode.
Sweep rate 5.3 V/min 18
12 Cyclic Voltammogram for n-Butyl Sulfide on Pt-elec-
trode. Sweep rate 5.3 V/min 19
13 Cyclic Voltammogram for t-Butyl Sulfide on Pt-elec-
trode. Sweep Rate 5.3 V/min 19
14 Cyclic Voltammogram for Solvent-electrolyte System
on Pt-electrode. Sweep Rate 5.3 V/min 21
VI
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FIGURES
(Continued)
NO.
15 Voltammograra of n-Btitylsulfide and t^Butylsulfide
on Pt-electrode. Sweep Rate 5/3 V/min 21
16 Voltammogram of a Mixture of n-Butylsulfide, Tert-
butylsulfide and Triethylamine on Pt-electrode.
Sweep Rate 5.3 V/min 22
17 Cyclic Voltammogram of a Mixture Containing n-Butyl-
sulfide, t-Butylsulfide, n-Butylamine and Triethyl-
amine. Run Conducted on Pt-electrode at 5.3 V/min 22
18 Voltammogram of t-Butylsulfide and Triethylamine on
Pt-electrode. Sweep Rate 5.3 V/min. The Sensitivity
Here is Half That of Fig. 17 24
19 Voltammogram of Tert-Butylsulfide, Diethylamine and
Triethylamine on Pt-electrode. Sweep Rate 5.3 V/min 24
20 Plot of Current vs Sweep Rate for Oxidation of
5.7 x 10-3M Triethylamine on Pt-electrode 28
21 Plot of Current vs Concentration of Triethylamine
Using Pt-electrode. Sweep Rate 9.6 V/min 29
22 Voltammogram for the Oxidation of Allyl Sulfide on
Pt-electrode After 3 Consecutive Runs. Sweep Rate
5.3 V/min 32
1/2
23 Q vs t ' Plot for Various Concentrations of Triethyl-
amine; Potential Step from SOOmV to 1400mV 34
VII
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SYMBOLS
2
A Area (on )
a Transfer coefficient
"d3 Bulk concentration
COx Bulk concentration of the oxidized species
D Diffusion coefficient (on^/sec)
Dox Diffusion coefficient of the oxidized species
erfc(x) Error function complement of x
EW Working electrode potential
F Faraday's Constant (96,490 coulombs/equivalent)
i Current (amp or yamp)
id Diffusion current
ka, kc Rate constant for anodic and cathodic reaction
n Number of electrons exchanged per molecule of
the electroactive species
na Number of electrons exchanged in the rate-
controlling step
IT 3.1416
R Gas constant
Q Coulombs (amp-sec)
T Temperature in °K
t Time (sec)
t Drop time (sec)
T Chronopotentiometric transition time (sec)
Vlll
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INTRODUCTION
Many odorous emissions to the atmosphere that engender
community odor nuisances contain a large number of components.
Exhausts from rendering plants or diesel engines, for example,
are so complex that it has not been possible to obtain suffi-
cient analytical information to be able to reproduce their
odors synthetically, despite much effort. Sensory methods have
therefore been most frequently used to characterize such
odors . The disadvantages of sensory odor evaluation are
well known. The screening and training of judges, and the use
of an odor panel in the field or in the laboratory, are cumber-
some and time-consuming procedures, and they often yield widely
scattered results.
On the other hand, the history of "mechanical noses,"
devices whose responses to vapors purport to simulate human
olfaction, has been disappointing; nothing comparable to a
light-meter or a decibel meter has ever been developed. Since
the transducing mechanism in olfaction has yet to be worked out,
this circumstance is not surprising. However, any convenient
instrumental method of odor measurement that would offer
reasonable correlation with sensory properties would be of
great help in monitoring the dispersal of odors in the outdoor
atmosphere and in rationalizing the design of control systems.
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To this end, we have been interested for some time in
various measures of the specific oxidizability of odorous
effluents. Various ambient temperature oxidation systems have,
in fact, been used for odor control, such as ozonization,
chlorination, and permanganate oxidation. Experience with these
systems has shown that many of the most highly odorous compounds,
such as mercaptans and amines, are the most easily oxidizable.
The circumstances are complex, because various alternate reac-
tion pathways may be involved, but in most sequences the change
in odor with progressive degrees of oxidation is very drastic.
On various theoretical and practical grounds, we have
assumed that the most promising approach to achieving instru-
mental analysis of odors at this stage of our knowledge about
olfaction would be oxidation at the anode of an electrochemical
cell. A number of relatively new and highly discriminating
techniques ' have been developed in recent years for which
equipment components are commercially available. These methods
are summarized in the following sections:
TECHNIQUES
Chronopotentiometry
Chronopotentiometry is -a controlled current technique in
which the electrolysis of an analyte occurs at a stationary
electrode in a quiescent solution. The cell current is
suddenly stepped to some fixed value and the resulting working
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hundred mV/sec). Cyclic voltammetry is an extension of the
potential sweep technique in which the potential is swept back
and forth over the same region several times. The principle
governing this technique is explained later in this report.
Differential Pulse Voltammetry
This technique consists of superimposing a fixed-height
potential pulse at regular intervals on the slowly varying
potential associated with dc polarography. The current is
sampled twice during each operating interval and the difference
between these two samples is presented to the output of the
amplifying system in the form of a peak. The current difference
is proportional to concentration of the electroactive material.
Chronocoulometry
This is relatively a new technique and involves current-
time integration of the Cottrell equation for chronoamperometry
(Equation 3). A charge-time relationship results and is given
by
Q= 2 nFCox (Vy,)* .
where Q = charge in coulombs. Thus a diffusion-controlled
1/2
chronocoulometric response exhibits a Q vs t ' proportion-
• 1/2
ality. In the event of adsorption, the 0 vs t ' . plots should
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yield positive intercepts on the Q axis for potential steps
that are sufficiently large.
PREVIOUS STUDIES
Results of the examination of aliphatic amines have been
reported by Mann . Ke showed that the ease of oxidizability
is:
tertiary > secondary > primary
Other workers have also investigated the oxidation of
aliphatic ' ' and aromatic ' amines at stationary elec-
trodes by potential sweep cyclic voltammetry. Most of these
reports have been concerned with elucidation of the reaction
mechanism in non-aqueous solvents such as acetonitrile and
dimethyIsulfoxide and very few, if any, have dealt with the
analytical application of this technique to the determination
(9)
of amines and sulfides. Drushell and Miller have studied
anodic polarographic estimation of aliphatic sulfides in
petroleum, and oxidation of sulfur compounds, notably cysteine,
was investigated by Davis and Bianco . Also, Nicholson
has reported on the oxidation of aliphatic and aromatic sul-
fides. We have applied the techniques of linear potential
sweep cyclic voltammetry and chronocoulometry to the studies
of anodic oxidation of amines and sulfides.
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EXPERIMENTAL
Cyclic voltammograms were obtained by the use of a Tacussel
Potentiostat Type PRT 20-2. The current was determined by
measuring the voltage drop across a precision resistor in series
with the working electrode. Current-voltage curves were recorded
using a Tektronix 5103N storage oscilloscope with a Type 5B10N
plug-in time base. When the voltammogram was satisfactory, a
photograph was taken with a Type C-5 Polaroid camera. The
linear potential sweep was initiated by a triangular voltage
function generated by a Wavetek Type 134 function generator,
with a DC voltage offset capability. A flow diagram for
current measurement is shown in Fig. 1. The 50fi terminator on
the output of the function generator ensures a maximum of 10V
P-P amptitude for any given wave form.
All electrochemical experiments were performed in a water-
jacketed all-glass cell of about 125 ml capacity with a five-
hole Teflon cap. The working electrode was a Beckman #39273
platinum bottom electrode and the reference was a saturated
calomel electrode.
For chronocoulometric experiments, the Tacussel potentio-
stat was used to apply the potential step. Current was sampled
across a precision resistor connected to our own operational
amplifier system utilizing Philbrick P85AU amplifiers. The
iodine-iodide redox system was used to check the performance
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FUNCTION
00
50*
•>—
1
TACUSSEL
POTENTIOS1AT
SCOPE
POTENTIAL
JACKE7E&
CELL
IEI(TO SCOPE)!
I INTEGRATING SECTlOMl
(CHRONOCOULOMETRY)l
FIGURE 1. FLOW DIAGRAM FOR CURRENT MEASUREMENT.
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of the method. The measuring cirucit is shown in Fig. 1.
Switch S was thrown simultaneously with the intiation of the
potential step. The resistors labelled R could be varied to
achieve any desired amplification when solutions containing
low concentrations of electroactive materials were being analyzed,
Reagents and Solutions
Reagent grade chemicals were purified by distillation.
The solvent, acetonitrile, was purified by refluxing for 48 hrs
over P2(-)5 anc^ ^en distilling over K_CO_. The supporting elec-
trolytes, NaClO., Et.NClO. and Et.NBF. were purified by recrys-
tallization from suitable solvents.
Millimolar solutions of the amines and sulfides were pre-
pared by introducing calculated volumes into 25 ml of aceto-
nitrile containing 0.1 M concentrations of the appropriate
supporting electrolyte. More dilute solutions were prepared by
dilution.
The experimental procedure consisted of recording a voltam-
mogram of a blank containing no amine or sulfide, and then re-
cording another voltammogram in the presence of a calculated
amount of the odorant compound. The analyte solution was de-
oxygenated with purified nitrogen for about 10 minutes before
each measurement. Current integrals were similarly recorded
on a blank before the introduction of the analyte of interest.
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RESULTS AND DISCUSSION
The cyclic volt ammo-gram of a 5 x 10 M solution of
o-dianisidine in 1 M H^SO,, taken on a platinum electrode, is
shown in Fig. 2. The sweep rate was 2.5 V/min and the ratio
of anodic to cathodic current was unity, indicating a reversible
system. This agrees with published data and suggests that our
measuring system is functioning normally. The peak potential
was 0.61 V vs saturated calomel electrode (SCE). Chronocoulo-
metric analysis of the iodine-iodide system gave results that
(12)
agreed with those of Christie and his co-workers .
Oxidation of Amines
The peak potentials obtained for the amines investigated
are shown in Table I. The data were obtained using a sweep rate
Table I
Peak Potentials for some Odorous Amines at 5.3 V/min sweep rate
Concentration
Compound Peak Potential (Volts) (millimolar)
n-butylamine 1.24 2.0
benzylamine 1.44 1.84
diethylamine 1.02 1.95
triethylamine 0.93 1.43
of 5.3 V/min at the given concentrations. The indicating elec-
trode was an unshielded .Pt-electrode in acetonitrile, 0.1 M in
10
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NaClO*. Each measurement was made on a clean electrode surface,
The cyclic voltammograms for n-butylamine, benzylamine, diethyl-
amine, and triethylamine are shown in Figures 3, 4, 5, and 6
respectively. We observe from the Figures and Table I, that
the ease of oxidizability is:
triethylamine > diethylamine > n-butylamine > benzylamine
This order is in perfect agreement with the results of Mann '
Results for runs conducted with a mixture of (a) n-butyl-
amine + diethylamine, (b) n-butylamine + triethylamine, and
(c) n-butylamine + diethylamine + triethylamine are shown in
Figures 7, 8, and 9 respectively. What is very significant in
these voltammograms is that separate current-peaks are not
obtained for a mixture of these amines using linear potential
sweep voltammetry at 5.3 V/min. Instead, a composite peak is
obtained for any given mixture. For a mixture of primary,
secondary, and tertiary amines a single peak appears at 1.09 V
vs standard calomel electrode. This is about the average of
the individual peak potentials obtained separately for the
amines. The mixture of primary and secondary amines had a peak
at 1.29 V vs standard calomel electrode and a peak at 0.97 V
was obtained for the mixture of primary and tertiary amines.
Thus, linear potential sweep voltammetry under these conditions
appears to be incapable of differentiating among different
amines, probably because of the proximity of the individual
11
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o o
FIGURE 2. CYCLIC VOLTAMMOGRAM OF 0-DIANISIDINE
IN 1M H2S04; CONCENTRATION, 5mM
Pt-ELECTRODE
0-0
FIGURE 3. CYCLIC VOLTAMMOGRAM FOR 2mM
n-BUTYLAMINE IN ACETONITRILE,
0.1M IN NaCIO,. SWEEP RATE
5.3 V/min.
'4'
Pt-ELECTRODE
12
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FIGURE 4.
CYCLIC VOLTAMMOGRAM FOR 1.84 mM
SOLUTION OF BENZYLAMINE IN ACETONITRILE,
0.1M IN NaC10A. SWEEP RATE 5.3 V/min
'4*
Pt-ELECTRODE.
0-0
FIGURE 5. CYCLIC VOLTAMMOGRAM FOR 1.95 mM
SOLUTION OF DIETHYLAMINE IN
ACETONITRILE 0.1M IH:NaC104.
SWEEP RATE 5.3 V/min.-
Pt-ELECTRODE.
13
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SOyuA
o-o
FIGURE 6. CYCLIC VOLTAMMOGRAM FOR 1.43 mM
SOLUTION OF TRIETHYLAMINE IN
ACETONITRILE, O.lM IN NaCK>4 .
SWEEP RATE 5.3 V/min.;~Et-
ELECTRODE.
2-2 v
FIGURE-7. VOLTAMMOGRAM FOR A MIXTURE OF
: n-BUTYLAMINE AND DIETHYLAMINE
ON Pt-ELECTRODE. SWEEP RATE
5.3 V/min.
14
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2-ZV
O-O
FIGURE 8. VOLTAMMOGRAM FOR A MIXTURE OF
n-BUTYLAMINE AND TRIETHYLAMINE
ON Pt-ELECTRODE. SWEEP RATE
5.3 V/min.
o-o
FIGURE 9. VOLTAMMOGRAM FOR A MIXTURE OF
n-BUTYLAMINE, DIETHYLAMINE AND
TRIETHYLAMINE ON Pt-ELECTRODE.
SWEEP RATE 5.3 V/min.
15
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oxidation potentials. However, it may be possible to analyze
amines as a class and to differentiate them from other odorants
All the voltammograms for the amines indicate that the
oxidations are irreversible. Only anodic peaks were obtained,
in contrast to the reversible oxidation of o-dianisidine shown
in Fig. 2. The small cathodic peaks around'0.26 V vs standard
calomel electrode in Figs. 3, 4, and 5 could be due to impurity
in the amines or the supporting electrolyte. They are
certainly not due to reduction of the oxidation product of the
amines.
All the measurements were performed on platinum electrodes.
A gold electrode appeared to be oxidized at potentials above
0.90 V and was therefore considered to be unsuitable. No
clearly defined current peaks were observed on glassy carbon
or on wax-impregnated graphite electrodes. The voltammogram
obtained on glassy-carbon electrode for a mixture of n-butyl-
amine, diethylamine, and triethylamine is shown in Fig. 10.
This trace was obtained under the same experimental conditions
as Fig. 9. The composite peak now appears at 1.23 V vs
standard calomel electrode instead of 1.09 V vs standard
calomel electrode as obtained in Fig. 9. Runs conducted on a
carbon-paste electrode prepared according to the method of
Marcoux et al. showed well-defined voltammograms for amines.
This electrode is therefore as good as platinum for oxidation
of the chosen amines, and both are superior to glassy-carbon
or wax-impregnated graphite electrodes.
16
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Oxidation of Sulfides
Results for sulfides using a platinum electrode at a sweep
rate of 5.3 V/min indicated that allyl sulfide, n-butylsulfide,
and tert-butylsulfide could be oxidized on platinum in aceto-
nitrile with 0.1 M NaC104 as supporting electrolyte.
Table II shows the peak potentials for the sulfides at the
Table II
Peak Potentials for some Odorous Sulfides at 5.3 V/min sweep rate
Concentration
Compound Peak Potential (Volts) (millimolar)
Allyl sulfide 1.84 - 1.56
n-butylsulfide 1.78 1.15
t-butylsulfide 1.45 1.14
given concentrations. The voltammograms for the three sulfides
are shown in Figures 11, 12, and 13 respectively. We observe
that tert-butylsulfide is more readily oxidized than allyl
sulfide and n-butyl sulfide, since it has the lowest peak
potential. Comparison with triethylamine (Fig. 6), shows that
the amine is more readily oxidized. The peak potentials are
0.93V and 1.45 V (vs standard calomel electrode) for the amine
and sulfide respectively. The shoulder around 1.93 V in Fig. 13,
for example, may be due to oxidation of the perchlorate ion,
which is known to have a discharge potential of 2.10 V on a
Pt-electrode in acetonitrile. A blank run on the solvent-
17
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2-2v
o-o
FIGURE 10. VOLTAMMOGRAM FOR A MIXTURE OF
n-BUTYLAMINE, DIETHYLAMINE AND
TRIETHYLAMINE ON GLASSY CARBON
ELECTRODE. SWEEP RATE 5.3 V/min,
FIGURE 11. CYCLIC VOLTAMMOGRAM FOR ALLYL
SULFIDE ON Pt-ELECTRODE.
SWEEP RATE 5.3 V/min.
18
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2-?V
0-0
FIGURE 12.
CYCLIC VOTAMMOGRAM FOR n-BUTYL
SULFIDE ON Pt-ELECTORDE.
SWEEP RATE 5.3 V/min.
FIGURE 13.
CYCLIC VOLTAMMOGRAM FOR t-BUTYL
SULFIDE ON Pt-ELECTRODE.
SWEEP RATE 5.3 V/min.
19
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electrolyte system is shown in Figure 14. There is a shoulder
at around 1.94 V, confirming that the perchlorate ion is being
discharged at this potential. The oxidation of the solvent is
indicated at around 2.20 V. A small cathodic peak is seen
around 0.26 V and the appearance of this peak in previous runs
is attributable to an impurity in the supporting electrolyte.
Runs conducted on a mixture of n-butylsulfide and tert-
butylsulfide did not show clearly defined separate peaks. One
peak was obtained at a potential of 1.72 V, which is more
anodic than the potential at which tert-butylsulfide is oxidized,
This is shown in Fig. 15. The cathodic peak has been attributed
to an impurity in the supporting electrolyte.
Fig. 16 shows the voltammogram obtained for a mixture of
n-butylsulfide, tert-butylsulfide, and triethylamine on a
platinum electrode, at a sweep rate of 5.3 V/min. The voltammo-
gram of Figure 17 is for a mixture of n-butylsulfide and tert-
butylsulfide on one hand, and also n-butylamine and triethyl-
amine on the.other hand. What appears to be very significant is
that a clear separation of peaks is observed for the oxidation
of amines and sulfides respectively, when these are components
of a mixture. The amine peak appears around 0.95 V and the
sulfide peak at around 1.72 V. This gives a separation of
770 mV between the peaks.
It therefore appears possible to characterize a mixture
of amines and sulfides by linear potential sweep vcltammetry.
20
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0.0
FIGURE 14.
CYCLIC VOLTAMMOGRAM FOR SOLVENT-
ELECTROLYTE SYSTEM ON Pt-ELECTRODE.
SWEEP RATE 5.3 V/min.
SOuA
2-2 V
FIGURE 15.
VOLTAMMOGRAM OF n-BUTYLSULFIDE
AND t-BUTYLSULFIDE ON
Pt-ELECTRODE. SWEEP RATE
5.3 V/roin.
21
-------
0-0
FIGURE 16. VOLTAMMOGRAM OF A MIXTURE OF
n-BUTYLSULFIDE, TERT-BUTYLSULFIDE
AND TRIETHYLAMINE ON Pt-ELECTRODE
SWEEP RATE 5.3 V/min.
FIGURE 17.
CYCLIC VOLTAM4DGRAM OF A MIXTURE
CONTAINING n-BUTYLSULFIDE,
t-BUTYLSULFIDE, n-BUTYLAMINE AND
TRIETHYLAMINE. RUN CONDUCTED ON
Pt-ELECTRODE AT 5.3 V/min.
22
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Quantitative determination would thereafter be effected by
controlled potential voltammetry. This is a matter for future
work.
The amines are generally more readily oxidized than
the sulfides. Oxidation of a mixture of amines only would
be expected to occur around 1.0 V while a mixture containing
only sulfides is expected to oxidize above 1.72 V, at a given
sweep rate, say 5.3 V/min. The position of the peak could
be directly related to any specific amine or sulfide. This
would, of course, require calibration of the measurement
apparatus with known odorous compounds, in terms of peak
potentials. For example, in Fig. 18 we have the voltammogram
of a mixture of triethylamine and t-butylsulfide. Peak
potentials appear at 0.800 V and 1.43 V, and these closely
approximate to the values obtained when these compounds were
measured separately, as shown in Tables I and II. For a
mixture containing t-butylsulfide, diethylamine, and triethyl-
amine, there is a shift of the amine peak towards a more anodic
value of 0.86 V but the sulfide peak still appears at 1.43 V.
This is illustrated in Fig. 19.
Voltammograms obtained using a carbon-paste electrode,
prepared as previously indicated, showed that this electrode
is superior to the platinum electrode in the anodic oxidation
of sulfides. The anodic waves were better defined and are more
suited for quantitative analyses. Also, for the same geometric
23
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i oo/.A
0-0
FIGURE 18.
VOLTAMMOGRAM OF t-BUTYLSULFIDE
AND TRIETHYLAMINE ON Pt-ELECTRODE .
SWEEP RATE 5.3 V/min. THE SENSITIVITY
HERE IS HALF THAT OF FIGURE 17.
0-0
FIGURE 19.
VOLTAMMOGRAM OF TERT-BUTYLSULFIDE,
DIETHYLAMINE AND TRIETHYLAMINE ON
Pt-ELECTRODE. SWEEP RATE 5.3 V/min
24
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area of working electrodes, carbon-paste gives greater current
density for a fixed concentration of analyte. In the oxida-
tion of allyl sulfide, it was noticed that filming occurred on
a platinum electrode but was negligible on a carbon-paste
electrode. The evidence for filming was non-reproducibility of
current upon repetitive sampling on the same surface.
Effects of Variation of Supporting Electrolyte
Variation in supporting electrolyte causes appreciable
changes in the shape of the current-time curves during oxidation
of amines and sulfides. For example, oxidation of allyl sulfide
on carbon-paste in acetonitrile with 0.1 M Et.NBF. as supporting
electrolyte, did not give any well-defined peak. Results
obtained with other odorants, such as thiophenol, benzylmer-
captan, t-butyl sulfide, and some amines, showed that perchlorates
are generally preferred to the tetrafluoroborates as supporting -
electrolytes.
(14)
In fact, it has been postulated that the reaction
potential is subject to variations with change in electrolyte
because of alteration of the liquid junction potential involved
in the reference probe. In addition, variation in the support-
ing electrolyte may alter the double layer at the electrode-
solution interface enough to cause significant variation in
reaction rate, and hence in the shape of the current-time curve.
25
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An Approach to Quantitative Analysis
In linear sweep peak voltammetry, the current-potential
curve exhibits a maximum or peak and the current is given by
ip = kn2 AD? ,CbV2
where i = peak current, amperes
2
A = area of electrode, cm
V = rate of potential change, volts/sec
k = a constant, called the Randles-Sevcik constant
D = diffusion coefficient of the electroactive
2
species, cm /sec
C = bulk concentration of the electroactive material,
moles/ml
and n = number of electrons transferred in the reaction.
The value of i is therefore seen to be directly propor-
tional to concentration and the sweep rate. The curves may be
characterized by the peak potentials E , as we have shown above
in the oxidation of amines and sulfides, or by the half-wave
potential, E /0. The above equation strictly holds for a rapid
p/z
charge-transfer (reversible) process, but we have seen from the
voltammograms of the amines and sulfides that their reactions
on a platinum electrode are irreversible.
In chronocoulometric analysis, a charge-time relationship
V» I/O- ' '
obtains, and is given by Q = 2nFAC (Dt/ir) ' , where Q is the
26
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charge (coulombs), n is the number of electrons, F is the
Faraday (96,490 coulombs), and t is the time (sec). Thus a
diffusion-controlled chronocoulometric response exhibits a
1/2
Q vs t ' proportionality. It is also seen that Q is directly
proportional to concentration for a given t.
We have therefore examined the applicability of the cyclic
voltammetric and chronocoulometric equations to the quantitative
determination of amines and sulfides. The chronocoulometric
investigation has attempted to correlate total charge at a given
time with bulk concentration of amines and sulfides.
The dependence of peak current.on concentration and sweep
rate respectively, was examined using triethylamine. A plot
of current against the square root of the sweep rate was
linear, as shown in Fig. 20. The concentration of triethyl-
amine was 5.7 x 10~ M in acetonitrile solution, 0.1 M in
NaClO.. The currents were highly reproducible at a given sweep
rate.
Figure 21 shows a plot of current against concentration
at a sweep rate of 9.6 V/min. There appears to be a deviation
from linearity at concentrations greater than 5 millimolar.
This is probably due to uncompensated IR drop which increases
with increasing concentration. Because of this, the potential
of the working electrode is not exactly equal to the applied
potential, and the measured current is below the theoretical
27
-------
PLOT OF CURRENT VS SWEEP RATE FOR
OXIDATION OF 5.7 X 10"3M TRIETHYLAMINE
ON Pt-F.LECTRODE.
28
-------
10
<£>
O 1
FIGURE 21.
8
PLOT OF CURRENT VS
Pt-ELECTRODE . SWEEP RATE,
_ _+. CONCENTRATION,
CONCENTRATION OF TRIETHYLAMINE USING
9.6 V/min .
-------
value. The observed displacement of the peak potential towards
more anodic values with increasing concentration is consistent
with the above reasoning.
The range of current linearity at low concentrations
offers promise that potential sweep peak voltammetry may be
applied to quantitative analysis. Sensitivity could be improved
by increasing the sweep rate, with due electronic compensation
for the background current.
Identical results were obtained for n-butylamine and
diethylamine. When a mixture of amines was analyzed, the total
current obtained was equal to the sum of the currents obtained
when the amines were separately analyzed. These findings are
shown in Table III. The total current for a mixture of amines
Table III
Oxidation of Amines on Pt-electrode, Sweep Rate 9.6 V/min
Composition Concentration
of solution yl/50 ml CH-CN Current, yA
Diethylamine 10 80
Triethylamine 10 80
Diethylamine 40 305
Diethylamine + 10 of each amine 160
triethylamine
Diethylamine + 40 of diethylamine
triethylamine . + 10 of triethylamine
30
-------
is therefore directly proportional to their overall
concentration.
The current value obtained for a mixture of 40 yl of
diethylamine and 10 yl of triethylamine in 50 ml of solution
is less than the sum of the currents obtained when corresponding
concentrations of the amines were analyzed separately (see
Table III). This difference would be due to preferential
adsorption of one component or to uncompensated IR drop which
leads to a decrease in current with increasing concentration
(Fig. 21).
Analyses of sulfides indicated current reproducibility
for n-butylsulfide and t-butylsulfide but not for allyl
sulfide. Our observation was based on consecutive runs on the
same surface of a platinum electrode. No such decrease
occurred, however, when a carbon-paste electrode was used. It
would appear that adsorption of allyl sulfide, or one of its
oxidation products, is responsible for this pronounced decrease
in current when-platinum electrode is used. The magnitude of
this phenomenon is illustrated in Fig. 22, which is a voltam-
mogram of allyl sulfide taken after three consecutive runs on
a platinum electrode. The current has decreased from an
initial value of 210 yA (Fig. 11), to 110 yA. Peak (b) of the
latter figure was obtained on a second sweep of the 3rd run.
The cause of this and similar phenomena will be investigated,
since its elimination is essential to the design of an
31
-------
0-0
FIGURE 22.
VOLTAMMOGRAM FOR THE OXIDATION
OF ALLYL SULFIDE ON Pt-ELECTRODE
AFTER 3 CONSECUTIVE RUNS. SWEEP
RATE 5.3 V/min.
32
-------
instrument for quantitative determination of sulfides. Irre-
versible dimerization of allyl radical-ion to dlsulfides
(resulting from the one-electron oxidation of allyl sulfide)
might take place, giving rise to an insoluble film on the
electrode surface. Such dimerization does not occur with
n-butylsulfide or with t-butylsulfide, possibly because of
steric hindrance.
Oxidations of sulfides on gold and glassy-carbon electrodes
did not reveal any significant advantage over platinum elec-
trodes. The current peaks were not clearly defined.
Chronocoulometric runs with different concentrations of
triethylamine on a Pt electrode in acetonitrile, 0.1 K in
NaClO. as supporting electrolyte, are shown in Fig. 23. The
potential was stepped from 500 to 1400 mV. Although a linear
1/2
relationship exists between charge and t ' , we did not obtain
a direct proportionality between charge and concentration .at
a given time. For a time of 1 sec, the total charge was 200,
480, and 1040 yC for solutions containing 5, 10, and 20 yl of
triethylamine respectively, in 25 ml of acetonitrile.
The non-proportionality of charge to concentration could
be due to adsorption effects, which are apparent from Fig. 23.
1/2
In the event of adsorption, the Q vs t ' plots should yield
positive intercepts on the Q axis at zero time for potential
steps that are sufficiently large. Such intercepts were
obtained, after due compensation for double-layer capacitive
33
-------
1400 -
ioo -
CM
0-2 O3> 0-4 0-5 0-fe O-7 O-8 0'9 1-0
» TIME72 (Sec'/2)
.1/2
M
FIGURE 23. Q VS f"' ' PLOT FOR VARIOUS CONCENTRATIONS OF
TRIETHYLAMINE; POTENTIAL STEP FROM 500mV TO
1400mV.
34
-------
charge. The adsorption effects are maximal at high concen-
trations and will make the chronocoulometric response non-
diffusion controlled. Hence Q would be expected to increase
out of proportion to bulk concentration as observed.
CONCLUSIONS
We have attempted the qualitative and quantitative
characterization of amines, sulfides, and their mixtures. It
is significant that a clear separation of peaks is observed
for the oxidation of amines and sulfides respectively, when
these are components of a mixture, using the technique of linear
potential sweep cyclic voltammetry. An attempt to relate total
charge to bulk concentration of analyte using chronocoulometry
indicated that adsorption effects could be important at higher
concentrations.
The next phase of this investigation will involve
authentic samples from several industrial odor sources. We are
hopeful that a combined cyclic voltammetric and chronocoulo-
metric technique could be applied to the instrumental analysis
of odorous amines and sulfides.
ACKNOWLEDGMENT
A grant from the Mobil Foundation contributed to the
support of this work.
35
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REFERENCES
(1) A. Turk, J. T. Wittes, L. R. Reckner and R. E. Squires,
"Sensory evaluation of diesel exhaust odors," National
Air Pollution Administration Publication, AP-60, (1970).
(2) P. Delahay, New Instrumental Methods in Electrochemistry,
Wiley (Interscience), New York, 1954.
(3) C. K. Mann, "Cyclic Stationary electrode voltammetry of
some aliphatic amines," Anal. Chem. 36: 2424 (1964).
(4) K. K. Barnes and C. K. Mann, "Electrochemical oxidation
of primary aliphatic amines," J. Org. Chem. 32; 1474 (1967)
(5) R. F. Dappo and C. K. Mann, "Anodic oxidation of triethyl-
amine," Anal. Chem. 35j 677 (1963).
(6) C. D. Russel, "Reaction of triethylamine at platinum
anodes in acetonitrile solution; solvent background with
perchlorate supporting electrolyte," Anal. Chem. 35;
1291 (1963).
(7) F. T. Eggertsen and F. T. Weiss, "Effects of structure of
certain amine indicators on oxidation potential and color
intensity on oxidation," Anal. Chem. .28: 1008 (195.6).
(8) R. N. Adams, J. McCluure and J. B. Morris, "Chrono-
potentiometric studies at solid electrodes," Anal. Chem.
3£: 471 (1958).
( 9) H. V. J3rushell and J. F. Miller, "Anodic polarography of
sulfur compounds in petroleum and its fractions," Anal.
Chim. Acta 15_: 389 (1956) .
(10) D. G. Davis and E. Bianco, "An electrochemical study of
the oxidation of L-cysteine," J. Electroanal. Chem. 12;
254 (1966) .
(11) R. S. Nicholson, "Some examples of the numerical solution
of nonlinear integral equations," Anal. Chem. 37, 667
(1965) .
(12) J. H. Christie, G. Lauer and R. A. Osteryou.ng, "Measure-
ment of charge passed following application of a potential
step-application to the study of electrode reactions and
adsorption," J. Electroanal. Chem. 7_: 60 (1964).
(13) L. S. Marcoux, K. B. Prater, B. G. Prater and R. N. Adams,
"A nonaqueous carbon paste electrode," Anal. Chem. 37:
1446 (1965) .•'.'•
(14) P. T. Cottrell and C. K. Mann, "Electrochemical oxidation
of aliphatic sulfides under nonaqueous conditions,"
J. Electrochem. Soc. 116(11): 1499 (1969).
36
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-021
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
ELECTROCHEMICAL ANALYSIS OF SULFIDIC AND AMINE
ODORANTS
5. REPORT DATE
June 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Jerry N. Nwankwo and Amos Turk
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The City College of New York
Department of Chemistry
Convent Avenue at 138th Street
New York, New York 10031
10. PROGRAM ELEMENT NO.
1AA010 (26AAP-065)
11. CONTRACT/GRANT NO.
802396
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final Report (6/1/73-11/30/74)
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Oxidation of odorous vapors at the anode of an electrochemical cell was studied
as a promising approach to achieving instrumental analysis of odors.
The technique of linear potential sweep cyclic voltammetry was used to investi-
gate the oxidizability of several amines, sulfides, and their mixtures on platinum,
gold, glassy-carbon, carbon paste, and graphite electrodes. Results of the analyses
of individual amines showed that the ease of oxidation on a platinum electrolyte was;
tertiary > secondary > primary.
Results for runs conducted on mixtures containing combinations of primary,
secondary, and tertiary amines indicated that separate current peaks were not
obtained for the various amines. Instead, a composite peak was obtained for any
given mixture. Results for sulfides using a platinum electrode indicated that
allyl sulfide, n-butyl sulfide and tert-butylsulfide could be oxidized in aceto-
nitrile solution with 0.1 M NaC104 as supporting electrolyte. The order was:
tert-butysulfide > n-butylsulfide > allysulfide. When amines and sulfides were
mixed, a clear separation of the peaks was observed. The results indicate that it
is possible to characterize a mixture of amines and sulfides by linear potential
sweep cyclic voltammetry.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Odors
*Amines
*0rganic sulfides
*Measurement
^Electrolytic analysis
Electrolytic cell
Oxidation reduction reactions
06P
07C
14G
14B
07D
07B
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