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
treatment of pollution sources to meet environmental quality standards.
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

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                                   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

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PLOT OF CURRENT VS SWEEP RATE FOR
OXIDATION OF 5.7 X 10"3M TRIETHYLAMINE
ON Pt-F.LECTRODE.
             28

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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

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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

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                                   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

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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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