Monitoring of electrochemically inactive compounds by amperometric gas sensors

Sensors

and Actuators,

6 (1984)

269

269

288

MONITORING OF ELECTROCHEMICALLY BY AMPEROMETRIC GAS SENSORS J R STETTER,

Argonne

Natlonul

S ZAROMB

Laboratory,

(Received July 5, 1984,

and M W FINDLAY,

Argonne,

IL 60439

In revised form December

INACTIVE

COMPOUNDS

Jr

(US

A )

19.1984,

accepted January 11, 1985)

Abstract Electrochemical sensors have many useful apphcatrons m air and toxic gas monrtormg, but they are extremely hmlted m the gaseous species that they can detect To extend the apphcablhty of amperometrlc gas-sensing mstruments to electrochemlcally inactive compounds, gas samples were exposed to a heated platinum or gold filament before being introduced mto the sample chambers of several different amperometnc sensors The sensors were of the three-electrode type, having platmum-black reference and counter electrodes and sensing electrodes made of platmum black, powdered gold or vapor-deposlted platmum or gold on porous tetrafluoroethylene membranes All three electrodes were m contact with a 25 - 30 wt % sulfuric acid solution The responses of four different sensors to various compounds at 20 “C were measured at sensing electrode potentials rangmg from 0 9 V to 1 4 V uersus RHE (reversible hydrogen electrode m the same electrolyte), with and without a heated platinum filament at the sample inlet Of 10 compounds tested, only two elicited srgntilcant responses without the filament With the filament heated to about 700 “C, each of the tested compounds elicited a slgmflcant response under certain condltlons Moreover, the particular sensors and electrode potentials correspondmg to the strongest responses were different for each compound Quahtatlvely slmllar, but quantltatlvely more pronounced, responses were obtamed with the same filament heated to 800 “C or 1050 * 50 “C, or with a gold filament heated to 950 f 50 “C The responses were proportional to concentration m the 0 - 50 ppm range, and usually proportional m the 0 - 200 ppm range

1 Introduction Amperometnc gas-sensmg mstruments offer the advantages of relatively low cost, portability, ruggedness, real-time output, selectlvlty and sensltlvlty to such compounds as carbon monoxide [l], hydrogen sulfide [2 - 3 ] , nitrogen dioxide [ 2, 41, sulfur dloxlde [ 51 or the hydrazmes [6] However, Elsewer Sequola/Prmted

In The Netherlands

270

such instruments exhibit a weak or 1ns1gnlficantresponse to many species, such as benzene and most aromatic and ahphatlc hydrocarbons The obJect1ve of this work was to extend the apphcablllty of amperometrlc gas-sensing instruments to electrochemlcally inactive species and thereby lay the groundwork for a portable and selective mon1tonng instrument based on an array of amperometrlc gas sensors capable of detecting, identifying and quantify1ng many of the hazardous u constituents that may be encountered 1n chemical sp1lL and other emergency situations [ 71 To render an array of different amperometrlc gas sensors sensitive to electrochemlcally inactive compounds, we investigated the posslblllty of subJect1ng ar samples containing such compounds to partial oxldatlon or pyrolysis by exposing them to a heated noble metal filament before introducing them into an amperometic sensor Such partial oxidation products as carbon monoxide, phosgene, sulfur dioxide or nitrogen dioxide should then be detectable electrochem1cally A slmllar approach has been used to detect acrylonltr1le by preheating rt over a manganese oxide catalyst at 400 - 450 “C 181 or by pyrolyzing V1kane (sulfuryl fluonde) 1n a mull1te (3A1203* 2S102) housing at 1000 “C [9] The following sections deal with our experunental evaluation of this approach 2. Chemicals and mater&s Most of the amperometrlc sensors used 1n these experiments were constructed 1n our laboratory using sensmg electrodes made either of a metal catalyst vapor-deposited zn uacuo on a 21tex (porous tetrafluoroethylene) membrane or of a suspension of platinum black and polytetrafluoroethylene particles pressed onto a Zltex membrane and slntered for four mmutes at 310 - 315 “C The metals used for vapor deposltlon were 99 99% pure platmum, gold or aluminum obtained from the Special Mater&s D1vls1onof Argonne National Laboratory The platinum black was purchased from Engelhard Industries, East Newark, NJ, U S A The polytetxafluoroethylene particles were 0 05 - 0 5 micron 1n size, obtamed 1n the form of an aqueous dispersion (Teflon 30 TFE from E, I DuPont de Nemours & Company) The electrolyte for the cells was 25 - 30 wt % sulfuric acid 1n distilled water The Z1tex sheets used for the electrode substrate were either type G-110 (1 - 2 micron pores) or E606-223 (2 - 5 micron pores) supplied by Chemplast, Inc , of Wayne, NJ, U S A AlI liquids used 1n preparation of test vapors were of ACS grade or better For reference standards, certified gases contalnlng pure a= or known concentrations of carbon monoxide or hydrogen sulfide 1n pure a~ were purchased from Scott Specialty Gases, Plumsteadvllle, PA, USA 3 Equipment and methods A hot-wire flammable-gas sensor (Bacharach Instrument Co , Plttsburgh, PA, U S A , Model #800-Oil), having a 0 010 cm diameter platinum

271

Filling Port

28% HlSO,

\ FILAMENT

Y

, SECTION

L

/

Y ELECTROCHEMICAL

CELL

Fig 1 Conflguratlon of components m the lmtlal combined sensor experiments summarized m Figs 2 - 4 and Table 2

hot wue and electrochemical

wire about 1 8 cm long, was used as the heated platmum filament Gold filaments were prepared by replacmg the platinum wme m the Bacharach sensor with 0 008 cm diameter gold wires 8 - 12 cm long Power to the filament was supplied by an adJustable constant-voltage supply Each filament was mounted m a chamber so that the gas flow was directed along the length of the filament (Fig 1) to ensure maxnnum exposure of the gas sample to the heated wire The electrochemical cells used m the earhest expenments (Figs 2 - 4) included one commercial sensor (Energetlcs Science HzS Sensor #1234, which has a powdered gold sensing electrode) and three expenmental cells constructed at Argonne (ANL EXP III, ANL EXP IV, and ANL EXP V [cf Table 11) The sample flow rate past each sensing electrode was 10 ml/mm m these earliest experiments In subsequent experiments, the sample flow rate was mcreased to 20 ml/mm, the filament-cell conhguratlon was as shown m Fig 5 and a different set of cells was used (Raney Au, PR3 or PRS, V d Pt, PR2 or PR6, Pt black, PRl or PR5, and V d Au, PR4 or PR7 [cf Table 11) The counter and reference electrodes of all cells were platinum black on Zltex, and the electrolyte was 25 - 30 wt % sulfunc acid The three-electrode sensors were held at constant potential using an adJustable potentlostat clrcult [lo] The sensing electrode was held at a fixed potential versus the reference electrode, and the current between the counter and workmg electrodes was amphfled and measured The potential of the reference electrode m each of these sensors was measured relative to

272 TABLE

1

ExperImental Sensor label

sensor

data

Sensmg

electrode

Design

preparation

(Fig )

Used m experiments of Figures

1

2

4

ANL

EXP III

Vapor-depowted Zltex GllO

ANL

EXP IV

Platmum black-TFE 30 suspension, Bltered, and pressed onto Zltex E606-223, and smtered at 310 - 315 “C

1

2-4

ANL

EXP V

Vapor-deposited Zitex E606-223

A) on

1

2-4

Vapor-deposlted Au-Al alloy (8120 A) on Zltex E606 223, bathed for 1 hour m a stwred hot 10 M KOH solution to remove the alummum

5

6

7

V d Pt PR2 PRB

Vapor deposited Zltex E606-223

5

6

7

Pt black PRl PR5

Same as ANL

5

6-7

V d Au PR4 PR7

Vapor-deposlted E606-223

5

6-7

Raney PR3 PR8

Au

platinum

Pt-Au

(5100

alloy (5700

platinum

a) on

(5 100 a) on

EXP IV

gold (6030

A) on Zltex

an Hg/Hg,S04 reference and found to be approximately 1 1 V versus RHE (reversible hydrogen electrode m the same electrolyte) Test samples of the desrred gas and concentration were prepared from the liquid phase using an evaporation mamfold similar to one described previously [ 111 An a~ flow rate of 2 l/mm was mamtamed through the system The hquld was inJected into the heated aa streams from a lo-p1 syringe at a rate controlled by a Sage Model 355 syrmge pump Concentrations were calculated from the rate of mjectlon, den&y and molecular weight of the evaporated liquid by straghtforward apphcatlon of the perfect gas law

4. Experunental results In prehmmary experiments, a sample of 200 ppm of benzene m a~ was circulated at a rate of 10 ml/mm past a heated 0 01 cm diameter platmum filament and thence through the sampling chamber of sensor cell ANL EXP IV, which had a platinum-black sensing electrode The latter was potentiostatted at 1 2 V versus RHE m a 25 - 30 wt % sulfuric acid electrolyte The

273 2OOppm

Benzene

Cell= EXP IV (Pt Gas

flow-

black

working

102v

v‘,,= 1 3v

I

I

a 750v

1.

I

TIME

(mid

15

electrode)

10 mL/min

lb

;

0 513v

I A

Fig 2 Effect of mcreasmg filament temperature on the response of sensor EXP IV to benzene A filament voltage ( Vfll) of 1 3 V corresponds to an estimated filament tempera ture of about 700 “C (A = pure air, B = 200 ppm of benzene in an-, C = pure air (with a simultaneous increase 1n Vfll to 1 02 V),D = 200 ppm benzene,E = pure an-, F = 200 ppm benzene, G = filament off. and H = pure air) Note Time scale runs from right to left

through that electrode increased with the voltage across the filament (Fig 2) To estimate the filament temperature at a @ven apphed voltage, the literature values of the reslstlvlty ratios p(t)/p(20 “C) for platinum and gold were first plotted versus the temperature t (“C) The ratio of the measured resistances of the heated and unheated filament was then used to read the temperature directly from the plots The highest platinum filament temperature reached m the experiment of Fig 2 was estunated m this way to be about 700 “C The power requxed to mamtam the filament at that temperature was about 1 8 W Above that temperature, the first filament tested burned out Most of the mltlal tests were therefore performed using a filament temperature of about 700 “C Substltutron of the platinum-black sensing electrode by an electrode made of either pure platinum or a platinum and gold alloy vapor-deposited on porous polytetrafluoroethylene under otherwise sunllar condltlons (sensor cells ANL EXP III or V) yielded proportionately slmllar responses, whereas no slgnlflcant response was obtamed with a powdered gold sensmg electrode under the same condltlons (Fig 3) The responses to various test compounds at 20 “C of four different sensors, each havmg one of the four sensing electrodes of Fig 3, were measured at sensing electrode potentials rangmg from 0 9 V to 1 4 V uerssus RHE, with and without a heated platinum filament at the sample inlet The followmg 10 compounds were tested cyclohexane, chloroform, tetrachloroethylene, acetic acid, benzene, nltrobenzene, pyndme, benzyl chloride, ethyl acrylate and tetrahydrofuran Only the last two compounds elicited slgmhcant responses without the filament However, vc&h the filament heated to about 700 “C, each of the tested compounds yielded a sign&cant response under certain condltlons Moreover, the particular sensors and electrode potentials correspondmg to the strongest responses were different for each compound current

274

200ppm %enzene Filament voltage = 1 3V Gas flow = 1Occlmin EXP m

(Pt-black)

fil &on

fil boff 03VO

01 i

+fi

B~Qwor

its

Pt)

&3, H

0 ‘0

q.Ll

01 0

5

__

0 ES1 S N 1234

fil ion

I

5

I

0

TIME

(Au)

(mm)

Fig 3 Response of four different sensors to benzene, with and wlthout a hot (700 “C) platmum filament at the sample Inlet (sensmg electrode potential 1 2 V us RHE) Al, A3, B2, C2, D2, D4 = 200 ppm of benzene In an-, A2, A4, Bl, B3, Cl, C3, Dl, D3, D5 = pure a1r

The ordinates m Figs 4(a) and (b) are m units of (S/N)/ppm, where S/N 1s the ratio of the response signal S to the noise level N during the same test * These units permrt a comparison of the different cells under different experimental condltlons on the basis of their ability to detect the same concentration of the tested compound AlternatIvely, if the mmlmum detectable concentration (ppm),,, IS defined as that yielding a signal s mln --2N

(1)

then The dashed vertical lme m Fig 4(b) represents a break-off point beyond which the data are considered to be less reliable because of an excessively *The stgnal S IS the drfference between the sensmg electrode of a tested compound and m its absence (pure dir)

currents In the presence

E

00

1 1

Electrode

10

Sensing

12 (Volts

potential

Potential

13 RHE)

14 15

0

v

*

8

00

of four different

0

V

0

q

Of1

Ftlamcsnt -700%

on the response

vs

Pt-Au Alloy

Vapor-DeposIted Gold Powder

Plahum

Catalyst

Vapor-Deposited

Platmum Black

Senstng Electrode

Fig 4 Effect of sensmg electrode (a) and tetrachloroethylene (b)

(a)

3

: 320

25

4G

09

IO Senmg

11

12

13 vs

I

I

&tEI

1-4

I I

-..

t

I

15

I

to tetrahydroturan

(Volts

filament

Potentml

a heated

Electrode

cells with or wkhout

(b)

6

10

12

14

18

2 cn

276 TABLE

2

Effect of sensmg electrode potential and electro-catalyst 10 selected compounds, with and wlthout a hot platmum Poten teal

Sensmg electrode catalyst

Filament

Response Benzene

composltlon on the response filament m the sample Inlet

[(SIWlppm

to

J

Pyrrdme

Nltrobenzene

BenAy chloride

CO1 02

1 01

on

1 =a
on





uersus

RHE

(VI Pt black

10


off on

11

off

12

off

13

off

on on

Vapor deposlted

10

off

Pt 11

off on

12

off off on

Vapordeposited Pt-Au

10

off on

11

off

12

off

on on 13

off on

Powdered

Au

1 0

off on

11

off

12

off

on on 13

off on

aThe underscored values are those cusston of results) bQuestlonable results

1

1 1

b

b

b

&y

fia

Od

1
CO1 02

on 13

1 1

1 1 1 1 1 1 1

used m the ldentlflcatlon

1 1 1 1


scheme

1 1 1 1 1 1 1

1 1 1 1 1

of Table

1 ua


1 1 1 1 1 1 1 1


1 1 1 1 1 1 1 1

5 (cf

DIS-

slow response* of some cells to tetrachloroethylene at electrode potentials above about 1 3 - 1 4 V uersus RHE The dashed curves are regarded as less *The response time IS deftned here as the time required following sample for the measured slgnal to reach 90% of Its steady state value

exposure

to a test

277

Response I(SIN)lppm Tetra-

chloroethylene


1

Chloro form


04

0 16



-0

07

0 25 10 14





02 015



1 1 15 18
1

1 Ethyl acrylate

15 208 0 33 085 045 115 0 55 11

Acetw acid


02

Cyclohexane

Sensor number

07 07 20 10



1 1F 2 2F 3 3F

01 0 15 18 18

co1 co1

4 4F 5 5F 6 6F

Tetrahydro furan 11 208

02 03 13 12











7 7F







8 8F

0 13 10

12 07 =a 14

035 01 03

reliable than the sohd ones m mstances where the response fluctuates undely with changmg potential The results obtamed wth the IO tested compounds are @ven m Table 2 Because of occasionally slow responses at potentials of 0 9 V, 14 V, and

278 Sample Exhaust

7

Filament

-

Reaction

Chamber

(Ceramic

or

Stainless

Back

Steel

Cover

Lining)

c

Counter

Reference

Electrode

Electrode

i

Sensing

Exposure

Chamber

Electrode

Fig 5 Exploded view of sensor filament umt Sample IS drawn into filament reactlon chamber where partial pyrolysis or oxldatlon occurs Products then pass mto exposure Material polypropylene (filament chamber where electrochemical reaction occurs 2 5 cm X 2 5 cm X 3 8 cm Electrolyte houslng section has ceramic liner) Dlmenslons volume 2 7 ml Electrode area sensing electrode = 1 13 cmZ, counter electrode 2: 0 9 cm*, reference electrode ~0 25 cm*

1 5 V uersus RHE, only the range from 1 0 V to 1 3 V has been included in Table 2 In the experiments summarized m Table 2, the filament-sensor configuration used was that of Fig 1 (filament facing the sensing electrode) In these first experiments the primary emphasis was to ascertain that enhanced sensltlvlty can be obtamed through the use of a heated filament Placing the filament close to the sensmg electrode was expected to ensure such enhancement However, it was also possible that some of the heat radlatrng from the filament could cause sp~~ous results by rsusmg the temperature of the sensing electrode To ehmmate this posslblllty, all subsequent experiments used the conflguratlon shown m Fig 5 (filament shielded from the sensor by a baffle) This subsequent work focused on linearity tests and on the effects of different heated catalysts placed m the sample upstream of the amperometr~ sensors To test the proportionality of the signal response to concentration, measurements were performed at two or three different concentrations of the test compounds The platinum filament temperature m these tests was increased to about 800 “C, and the sample flow rate was increased to 20 ml/mm The results are summmzed m Table 3 To test the effectiveness of different catalysts, the followmg two noble metals were selected (1) a heated 0 010 cm diameter platinum filament and (2) a heated 0 008 cm diameter gold filament Four compounds were then

50 100 200

50 100 200

50 200

50 200

50 200

20 50

Tetrachloroethylene

Chloroform

Acetlc acid

Tetrahydrofuran

Cyclohexane

Ethyl acrylate

gold

value)

0 0030a

0 0050

0 003a

0 003a

0 0020 0 0035a

13v

Raney

Sensltlvlty (us RHE)

to reactant

(> 3 mm to 90% of the steady-state

50 200

Pyrldme

(ppm)

50 200

aSlow response

cell response

Concentration

Benzene

Compound

ProportIonaMy of electrochemical to about 800 “C

TABLE 3

0 0020 0 0023

09v

(PAlppm)

followmg

0 0016 0 0015

0 00040 0 00050

0 100 0 080

0 088 0 108a

0 0044 0 0040

0 0036 0 0037

13v

0 0030 0 0030

11v

0 010 0 010

0 21 0 20

0 010

0 010

0 020 0 020 0 015

1ov

Platinum

0 20 0 20

0 020 0 020

0 18 0 23

0 040 0 050

12v

black

electrocatalysts

of the sample to a platrnum

sensing electrode

exposure

Vapor-deposited gold

for the mdlcated

concentration

heated

0 0075 0 0080

0 0070 0 0055a

0011 0 010

0 0020 0 0018

0 011

0 013

0 0060 0 0070 0 0113a

0 0050 0 0055

0 0060 0 0063

13v

Vapor-deposited platinum

and potentials

filament

10

8

4

2

Fig 6 Response of platinum black sensmg electrodes at 1 2 V versus RHE to samples of 200 ppm benzene pre-exposed to a platmum or gold filament at various temperatures An apphed voltage (Vfll)of 2 0 V corresponds to about 1100 “C for the platmum frlament, whereas Vfll= 1 8 V corresponds to an estimated temperature of 1000 “C for the gold filament

selected from the 10 previously tested ones benzene, which had yielded a good response with a heated platinum filament m combmatlon with a platinum-black sensing electrode, tetrachloroethylene, which tended to yield a sluggish response with the same combmatlon, and mtrobenzene and benzyl chloride, neither of which had previously ehclted a reliable response These four compounds were passed through one of the two heated catalysts and then to the sensors maintained at various potentials The results are given m Table 4 On the basis of the curves obtained by plotting filament voltage Vfll uersus response to benzene (Fig 6), the optimum Vf,, for platmum was about 1 9 - 2 0 V, whereas the optimum Vf,,for gold was about 1 8 V Although the response was still increasing with increasing Vfll, the filaments became fragile and burned out easily when higher power was applied The responses of several different sensors to different concentrations of the tested compounds, with each of the two noble metal flalments heated

TABLE

4

Electrochemical sensor 50 “C or a gold filament

response followmg at 950 4 50 “C

catalysis

by a platmum

filament

at

1050

Sensmg electrode

Sensing electrode potential (V us RHE)

Heated catalyst

Sensltlvlty Benzene

Nitrobenzene

Benzyl chloride

Tetrachloroethylene

Pt black

1 2

None Pt Au

a

a

a

a

0 014 0 045 a

a a

a 0 015 a a

0 005 0 05d a *

11

10

Vapordeposited Pt

a 0 01

0 17b.d a a

0 02

0 16b a a a

a a a

a a a

11

None Pt Au

a a

a a

a

a

6 x 10-4b*c

5 x lO-5

3 x10-5 a

None Pt Au

a a a

a a a

a a

a 2 x lo-4b a a

2 x 10-h’

3 x lo-4a

None Pt Au

a a

a

a

3 x 10-s

a a a

5 x 10-4 0 013

a a

a a

a 7 5 x 10-S a

a

a

a

3 x lo-4b a d d

a

a

8 x 10-4b

a a a

a

a

a

a

15x10--Cb

35x10-4

a a a

a

a

a

5 x lo-4b.d 10x10-4b a a

1 2

1 2

11

10

dResponse pound bResponse CNon-llnear dNon lmear

3x10-2c 0 25

0 OIC a a

a a a

10

Au

0 02ac B

None Pt Au

11

Raney

None Pt A*

a a

0 02 0 25b*C B

I 2

IO

Vapordepostted Au

None Pt Au

(pA/ppm)

mdlstmgulshabfe

None Pt Au None Pt Au None Pt Au None Pi! Au None Pt Au from

a d 4 x 10-S a a

a a

4 x lo--4b*c

3 x 10-e

a

a

a a

d a

noise for samples

time > 3 mm (to 90% of steady-state above 50 ppm above 100 ppm

of 50

value)

5 x 10-S d a a a a a 200

1 6 x 10-jb a a 2 x x0-7

ppm of the tested

corn

1

282

283

I

I

I

284

to a temperature of 1000 + 100 “C (1050 * 50 “C for Pt and 950 f 50 “C for Au),* are summarized m Figs 7(a - e) and Table 4 5. Dlscusslon of results The last column of Table 2 contams ‘sensor numbers’ assigned to sensors m a possible instrument havmg characterlstlcs identical to the corresponding experimental sensors As shown m Table 5, different pmrs of the selected sensors can be used not only to detect but also to identify each of the 10 selected compounds on the basis of then different response ratios For instance, the (S/N)/ppm values for benzene are 0 9 with Sensor No 4F and 0 7 with Sensor No lF, and the ratio of the responses of these two sensors 1s different for benzene than for any of the other nme compounds On the other hand, the (S/N)/ppm values for nltrobenzene are 0 5 with Sensor No 4F and 0 25 with Sensor No 3F This 2 1 ratio 1s different from that for the other listed compounds, except tetrachloroethylene and tetrahydrofuran, which yield nearly the same ratio (1 8 1) (cf Table 2) However, it 1s easy to dlstmgulsh between nltrobenzene and tebrachloroethylene or tetrahydrofuran by means of Sensor Nos 7F and 3 or 1F and 6 Sensor Nos 1 and 1F correspond to the same sensor wrth the filament off or on (cf Table 2) The same applies to the other pairs (2/2F through 8/8F) Identlflcatlon of any of the 10 compounds may thus be possible with TABLE

5

Tentatively

selected

Compound

Benzene Pyrldme Nltrobenzene Benzyl chloride Tetrachloroethylene Chloroform Ethyl acrylate Acetic acid Tetrahydrofuran Cyclohexane

sensors for optlmal First sensor number

4F 6F 4F 1F 7F 5F 8 7F 1F 1F

detectlon

Second sensor number

(SIN)/ ppm

1F 4F 3F 2F 3 7F 1F 4 6 5F

07 15 0 25 G3 20 26 20 07 20 0 85

(SIN)/ ppm

09 30 05 04 40

10 25 10 20 10

and ldentlflcatlon

*Although the estimated temperature of temperature varlatlons durmg each comparative f10 “C

Lowest detectable concentration (ppm) WIthout ldentl fication

With Identlfication

22 07 4 5 05 02 08 2 1 2

3 13 8 7 1 08 1 3 1 23

each filament IS uncertam experiment are estimated

(+50 “C), the to be less than

an instrument comprlsmg eight sensors Moreover, since each of these eight sensors will yield two different readings - one with and one wlthout a heated filament - one can obtain a set of 16 values for each tested sample From the (S/N)/ppm values listed m Table 4, it 1s possible to estimate the lowest detectable concentration, (ppm),,,, according to eqn (2) These concentrations range from 0 2 ppm for chloroform to 5 ppm for benzyl chloride without ldentlflcatlon, or from 0 8 ppm for chloroform to 8 ppm for mtrobenzene If a sample contams a single compound that must be Identified (based on the S/N value of the least sensitive sensor used for ldentlflcatlon) As shown m Fig 6, the gold filament ehclts a much stronger response than platinum for comparable filament voltages, and hence for roughly comparable temperatures Lest the difference be attributed to the use of different platinum-black sensors (No PRI with the gold filament and No EXP IV with the platinum filament) a single point 1s plotted, indicated by III, which represents a measurement with a platinum filament and the same sensor (No PRl) as was used with the gold filament Because this point falls far below the curve shown for the platinum filament, there can be no question of the far greater effectiveness of the gold filament This greater effectiveness of the gold filament, m spite of the approximately 100 “C lower temperature, may be due to its four to five times larger surface area and/or to the catalytic properties of gold From the data of Table 4, It appears that use of a gold filament as a precatalyst yields the widest range of response, with all compounds respondmg to at least one sensor after exposure Selectlvlty can be improved for some compounds by comparing responses with gold and platinum filaments Table 3 shows that the sensltlvlty (1 e , response/concentration ratio) of four different sensing electrodes 1s substantially constant oter the range 0 - 50 ppm (usually 0 - 200 ppm) for eight listed compounds A similar proportlonahty of the response of the same (or of equivalent) sensors to the concentration of the four compounds of Table 4 preexposed to a heated platinum or gold catalyst 1s shown m Figs 7(a - e) As indicated by these Figures and by Table 4, the response 1s substantially linear m most cases The few significant devlatlons from lmemty are associated with tetrachloroethylene at concentrations m excess of 100 ppm and with benzene or mtrobenzene at concentrations m excess of 50 ppm, but only at certain sensing electrode potentials As IS also indicated m Table 4, a sluggsh response to benzene, benzyl chloride or tetrachloroethylene was observed occasionally Such slug@shness was associated m most instances with the use of a heated gold filament Preliminary results also indicate that, at low flow rates (10 ml/mm), the response time of the cells may be lengthened through exposure to fllamentcatalyzed reaction products However, this effect IS seen only with speclflc compounds and has proved to be transient m all cases to date We attribute these observations to an occasional gradual bulldup of certain chemical species The wider response range generated by the heated gold filament may

286

be due to the weaker catalytic actlvlty of gold as compared with that of platinum, resulting m higher concentrations of incomplete oxldatlon products being introduced mto the sensmg electrode chambers However, this weaker catalytic actlvlty may also result m a gradual buildup of certam electrochemlcally reactmg species, which would account for the occasionally observed slower response 6 Conclusions The followmg conclusions can be drawn from the foregoing results (1) All or most electrochemrcally Inactive, partly oxldlzable or pyrolyzable compounds can ehclt a response from an appropriate amperometnc sensor followmg pre-exposure to a suitable, heated, noble-metal filament (2) The filament composltlon and temperature have a maJor effect on the responses ehclted by var.~ous compounds (3) Pre-exposure to a heated gold filament ehclts responses from a larger number of compounds than pre-exposure to a platinum filament heated to a comparable or higher temperature (4) The responses usually increase with increasing filament temperature (5) For a @ven array of amperometnc sensors operated without and with a given filament at a given temperature, the ratios of elicited responses ~111usually be sufflclently distinct for each compound to be ldentlfiable (6) These responses were found to be approximately proportlonal to concentrations m the range 0 - 50 ppm and usually m the range 0 - 200 ppm of the tested compound (7) The mmlmum concentrations of the 10 tested compounds that are detectable with an array of the eight sensors of Table 5, operating wrth and without a heated platinum filament, range from 0 2 ppm for chloroform to 5 ppm for benzyl chlonde wrthout ldentlflcatron, or from 0 8 ppm for chloroform to 8 ppm for nltrobenzene with ldentlflcatlon of a smgle compound

7 Acknowledgments The authors gratefully acknowledge the support and funding of this work by the U S Coast Guard Thanks are also due to Dr Wllham R Penrose for making useful suggestions dunng the performance of this work and for streamlmmg the manuscript, and also to Dr Laszlo Redey for constructively revlewmg this paper References 1 I-I G Oswm and K F Blurton, US Patent 3 776 832 (1973) 2 J M Sedlak and K F Blurton, Talanta, 23 (1976) 445.23 (1976) 811 3 J R Stetter, J M Sedlak and K F Blurton, J Chromatog Scl , 15 (1977) 4 J M Sedlak and K F Blurton, J Electrochem SOC , 123 (1976) 1476

125

287 5 K F Blurton and J R Stetter, J Chromatog

, 155 (1978) 35 6 J R Stetter, K A Tellefsen, R A Saunders and J J DeCorpo, Talanta, 26 (1979) 799 7 J R Stetter, S Zaromb, W R Penrose, M W Fmdlay, Jr, T Otagawa and A J SIncall, Portable device for detectmg and ldentlfymg hazardous vapors, Proc 1984 Hazardous Material Spills Conf , Nashv&. TN, Aprtl 1984 8 0 Peterson and H D Schmidt, Ger Offenl 2812613 (9127179) 9 Interscan Corporation, Chatsworth, CA, data sheet on Momtormg Instrument for Vlkane Gas Fumigant 10 A J Bard and L R Faulkner, Electrochemical Methods - Fundamentals and Applt cations, Wiley, New York, 1980, Fig 13 4 5, p 718 11 J R Stetter and K Teilefsen, SC Tech Aerosp Rep {STAR), NASA CR 153048, July, 1977

Blographles

Solomon Zaromb obtained his Ph D degree m Chemistry from the Polytechnic Institute of New York m 1954 From 1953 to 1955 he studied protein-anion bmdmg m electrolytes by light scattenng as a Research Associate at the Massachusetts Institute of Technology Hu work on toxic gas sensing dates from 1968 - 1969, startmg with several grants from the National Institutes of Health for research on remote detection of gaseous m pollutants by laser spectroscopic techniques He co-mvented and co-developed a portable Hazardous Gas Monitor which received a 1984 Industrial Research IR-100 award He IS now an Electrochemlst with the Energy and Enwonmental Systems Dlmslon, Argonne National Laboratory, where he 1s prunarlly concerned with development of portable instruments for the detection, ldentlflcatlon and momtormg of hazardous constituents m m Joseph R Better received his Ph D degree m Physical Chemistry m 1975 from the State Unlverslty of New York at Buffalo He Joined Energetlcs Science, Elmsford (now Hawthorne), New York, as Senior Research Chemist m 1974 and served as Director of Chemical Research from 1977 to 1980 He has developed several electrochemical and semiconductor sensors that are now m commercial use, mcludmg a detector for hydrazmes m au Dr Stetter has received awards for his instrument development work, mcludmg a NASA Certlflcate of Recognltlon (1978) and two IR-100 awards (1977 and 1984) He IS now Sclentlst and Manager of the Geoscience and Envuonmental Instrument&on Section, Energy and Enwonmental Systems Dlmslon, and Group Leader of the Organic Analysis Group, Argonne National Laboratory HIS maJor research mterest IS the development of improved methods, sensors and instruments for pollutant analysis Mehun W Fmdlay, Jr, graduated with a B S m Chemistry from Lewis Unlverslty (Lockport, Ilhnols) m 1976 Followmg two years of graduate work m physical chemistry at Flonda State Umverslty, he Joined Escor

288

Envrronmental Consultants, Evanston, Illmols, as an AnalytIcal Chemist In 1980 he took a poatlon at Argonne Natlonal Laboratory as a Scientific Assistant with the Energy and Environmental Systems Dlvlslon He 1s now completing a M SC degree m Soti Chemistry at Northwestern Unlverslty, while workmg as an Environmental Chemist zn the Enwonmental Research Dlvlslon Hrs major interest 1s m the momtonng of enwonmental contamlnants and the effects of atmospheric pollutants on soil and soil chemistry