Chemical Hazards in the Workplace - ACS Publications - American

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Recent Developments in Electrochemical Solid Polymer Electrolyte Sensor Cells for Measuring Carbon Monoxide and Oxides of Nitrogen

Downloaded by NORTH CAROLINA STATE UNIV on January 7, 2013 | http://pubs.acs.org Publication Date: April 2, 1981 | doi: 10.1021/bk-1981-0149.ch035

A. B. LACONTI, M. E. NOLAN, J. A. KOSEK, and J. M. SEDLAK General Electric Company, Direct Energy Conversion Programs, 50 Fordham Road, Wilmington, MA 01887

Electrochemical gas detection instruments have been developed which use a hydrated solid polymer electrolyte sensor cell to measure the concentration of specific gases, such as CO, in ambient air. These instruments are a spin-off of GE aerospace fuel cell technology. Since no liquid electrolyte is used, time-related problems associated with liquid electrolytes such as corrosion or containment are avoided. This paper describes the technical characteristics of the hydrated SPE cell as well as recent developments made to further improve the performance and extend the scope of applications. These recent advances include development of NO and NO2 sensor cells, and cells in which the air sample is transported by diffusion rather than a pump mechanism. The intended use of the SPE detectors is by military, government and industrial personnel involved in air quality measurements. The commercial SPE CO dosimeter and direct-reading detection instruments are being widely used by steel mills, fire departments and various city, state and government regulatory agencies.

0097-6156/81/0149-0551$05.75/0 © 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Experimental S e n s o r C e l l O p e r a t i n g Mode. T h e s i m p l e s t method o f s e n s o r o p e r a t i o n i s a s a g a l v a n i c c e l l , w h e r e b y t h e s e n s o r a c t s as a f u e l c e l l a n d g e n e r a t e s a c u r r e n t p r o p o r t i o n a l t o t h e gas c o n c e n t r a t i o n t o b e d e t e c t e d (1_). H o w e v e r , when d e t e c t i n g c e r t a i n s p e c i e s i n a i r , i t i s d i f f i c u l t t o o b t a i n a c o u n t e r - r e f e r e n c e e l e c t r o d e i n an a c i d system t h a t w i l l m a i n t a i n t h e s e n s i n g e l e c t r o d e a t a p r e d e t e r m i n e d p o t e n t i a l o f a p p r o x i m a t e l y 1.0 V , t o m i n i m i z e i n t e r f e r e n c e . C o u n t e r - r e f e r e n c e e l e c t r o d e s s u c h a s P t / a i r (Og) o r n o b l e m e t a l / n o b l e m e t a l o x i d e s t r u c t u r e s h a v e r e s t p o t e n t i a l s i n t h e 1.0 t o 1.2 V r e g i o n . However, t h e y e x h i b i t v e r y low exchange c u r r e n t d e n s i t i e s (10-10 t o 1 0 - 1 jua/cm ) and a r e r e a d i l y p o l a r i z e d under small current drains. 2

2

I t has a l s o been demonstrated t h a t t h e s e n s i n g e l e c t r o d e can b e m a i n t a i n e d a t a p r e d e t e r m i n e d v o l t a g e o f 1.0 V b y e l e c t r i c a l l y b i a s i n g (constant voltage source) the sensing e l e c t r o d e v s . a s t a b l e c o u n t e r - r e f e r e n c e , s u c h a s P t / H 2 , H o r Pb02/PbS0i , H (1_, 2,3). With the former, the sensing electrode i s e l e c t r i c a l l y b i a s e d 1.0 V a b o v e t h e P t / H 2 , H p o t e n t i a l (0.0 V ) a n d w i t h t h e l a t t e r 0.7 V b e l o w t h e P b 0 / P b S 0 i , H+ p o t e n t i a l (1.7 V ) . Both o f t h e s e c o u n t e r - r e f e r e n c e e l e c t r o d e s e x h i b i t good r e v e r s i b i l i t y , b u t r e l i a b i l i t y and l i f e a r e n o t adequate. The m o s t c o n v e n i e n t a n d r e l i a b l e e l e c t r i c a l b i a s i n g method f o r u s e w i t h a h y d r a t e d S P E c e l l h a s b e e n shown t o b e a t h r e e e l e c trode p o t e n t i o s t a t i c c i r c u i t which maintains the sensing electrode a t a p r e d e t e r m i n e d p o t e n t i a l v s . a s t a b l e r e f e r e n c e (l>3.>ii). T h e m o s t r e v e r s i b l e r e f e r e n c e i s a P t / H 2 , H+, s t a t i c o r d y n a m i c . In p r a c t i c a l i n s t r u m e n t s , h o w e v e r , good a c c u r a c y a n d c o n v e n i e n c e a r e a c h i e v e d u s i n g a l a r g e s u r f a c e a r e a p l a t i n o i d m e t a l b l a c k / a i r (O2). A l l work r e p o r t e d i n t h i s s t u d y u t i l i z e d t h e a i r r e f e r e n c e w h i c h h a s a p o t e n t i a l o f a p p r o x i m a t e l y +1.05 V v s . a s t a n d a r d h y d r o g e n e l e c t r o d e (SHE). F o r convenience, a l l p o t e n t i a l s reported are v s . the SHE. +

t

+

+

2

+

Sensor C e l l C h a r a c t e r i s t i c s . T h e membrane a n d e l e c t r o d e a s s e m b l y u s e d i n t h e CO s e n s o r c e l l h a s t h r e e e l e c t r o d e s o f i d e n t i c a l composition: s e n s i n g , c o u n t e r and r e f e r e n c e . These e l e c t r o d e s a r e f a b r i c a t e d from a p l a t i n o i d b l a c k c a t a l y s t c o m p o s i t i o n b l e n d e d w i t h a Teflon binder. A t r a n s i t i o n m e t a l s c r e e n i s embedded i n t o t h e e l e c t r o d e s t o o b t a i n improved mechanical i n t e g r i t y and c u r r e n t c o l lection. T h e c o u n t e r e l e c t r o d e i s t h e same s i z e (2.k c m ) a n d c o n f i g u r a t i o n as t h e s e n s i n g e l e c t r o d e . The r e f e r e n c e e l e c t r o d e has a n a r e a o f a p p r o x i m a t e l y 0.32 c m . T h e e l e c t r o d e s a r e b o n d e d t o t h e membrane b y a p r o p r i e t a r y m e t h o d d e v e l o p e d b y G e n e r a l E l e c t r i c . T h e s p a t i a l c o n f i g u r a t i o n o f t h e r e s u l t a n t membrane a n d e l e c t r o d e assembly has been d e v e l o p e d t o a c h i e v e good v a p o r phase t r a n s p o r t and t o p r o v i d e a h i g h o u t p u t s i g n a l . 2

2


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T h e membrane a n d e l e c t r o d e a s s e m b l y u s e d f o r t h e d e t e c t i o n o f o x i d e s o f n i t r o g e n i s s i m i l a r t o t h a t u s e d f o r CO d e t e c t i o n , e x cept t h a t t h e s e n s i n g e l e c t r o d e i s f a b r i c a t e d from a g r a p h i t e T e f lon mix. A schematic o f the hydrated s o l i d polymer e l e c t r o l y t e sensor c e l l i s d e p i c t e d i n F i g u r e 1. T h e membrane a n d e l e c t r o d e a s s e m b l y i s housed i n Lexan p o l y c a r b o n a t e hardware. L e x a n was s e l e c t e d b e c a u s e o f i t s good p h y s i c a l p r o p e r t i e s ( t r a n s p a r e n c y , shock r e s i s t a n c e ) , and c h e m i c a l i n e r t n e s s / m i n i m a l e l u t i o n . The c a t a l y t i c s e n s i n g a n d r e f e r e n c e e l e c t r o d e s a r e p o s i t i o n e d o n one s i d e o f t h e c a t i o n e x c h a n g e membrane; a c a t a l y t i c c o u n t e r e l e c t r o d e i s p o s i t i o n e d o n t h e o t h e r s i d e o f t h e membrane o p p o s i t e t h e s e n s i n g e l e c trode. T h e c o u n t e r e l e c t r o d e compartment i s f l o o d e d w i t h w a t e r . E l e c t r o l y t i c c o n t a c t between t h e s e n s i n g e l e c t r o d e and t h e p l a t i noid m e t a l / a i r reference i s achieved through a hydrated s o l i d p o l ymer e l e c t r o l y t e membrane b r i d g e . The p e r f o r m a n c e c h a r a c t e r i s t i c s and o t h e r p r o p e r t i e s o f t h e c e l l a r e h i g h l y dependent on t h e morp h o l o g i c a l s t r u c t u r e o f t h e membrane a n d t h e m e t h o d o f h y d r a t i n g t h e membrane t o a c h i e v e a f i x e d w a t e r c o n t e n t . A l l work has been a c c o m p l i s h e d u s i n g p e r f l u o r o s u l f o n a t e i o n e x c h a n g e membranes m a n u f a c t u r e d b y E . I . d u P o n t a n d s o l d u n d e r t h e t r a d e name N a f i o n . N a f i o n i s a copolymer o f p o l y t e t r a f l u o r o e t h y l e n e (PTFE) and p o l y s u l f o n y l f l u o r i d e v i n y l e t h e r c o n t a i n i n g pendent s u l f o n i c a c i d groups. The s u l f o n i c a c i d groups a r e c h e m i c a l l y bound t o t h e p e r f l u o r o c a r b o n backbone.

1100 (CF CF ) 2

2

g

EW

1500 CFCF^

—

0

FC

0

EW

(CF CF )j= 2

2

0

CF

3

FC

CF

3

0 CF

2

CF

2

SO3H Two e x a m p l e s o f c o m p o s i t i o n a r e shown a b o v e . EW, o r e q u i v a l e n t w e i g h t , i s d e f i n e d a s t h e w e i g h t o f XR r e s i n w h i c h n e u t r a l i z e s one e q u i v a l e n t o f b a s e .


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Membranes a r e a l s o c h a r a c t e r i z e d b y t h e i r i o n e x c h a n g e c a p a c i t y ( I E C ) - m i l l i - e q u i v a l e n t s (meq) o f s u l f o n i c a c i d / d r y w e i g h t o f membrane. T h e r e l a t i o n s h i p b e t w e e n I E C a n d EW c a n b e e x p r e s s e d

I E C = ^

(1)


G e n e r a l l y , IEC i s determined by a c i d / b a s e and g r a v i m e t r i c w e i g h t a n a l y s i s .

t i t r a t i o n methods

Electrochemical Reaction/Transport. Electrochemical react i o n s o c c u r a t t h e e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e when g a s i s b r o u g h t t o t h e e l e c t r o d e s u r f a c e u s i n g a s m a l l pump. Gas d i f f u s e s through the electrode s t r u c t u r e to the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e , where i t i s e l e c t r o c h e m i c a l l y r e a c t e d . Some p a r a s i t i c c h e m i c a l r e a c t i o n s c a n a l s o o c c u r on t h e e l e c t r o c a t a l y t i c s u r f a c e between t h e r e a c t a n t gas and a i r . T o a c h i e v e maximum r e s p o n s e a n d r e p r o d u c i b i l i t y , t h e c h e m i c a l c o m b i n a t i o n must b e m i n i m i z e d a n d c o n t r o l l e d by p r o p e r s e l e c t i o n o f c a t a l y s t s e n s o r p o t e n t i a l and c e l l configuration. F o r CO, w a t e r i s r e q u i r e d t o complete t h e a n o d i c r e a c t i o n at the sensing electrode according to the following reaction: CO + H 0 2

= C0

2

+ 2H

+

+ 2e

(2)

A l s o , a p p r o x i m a t e l y 3.5 t o k m i l l i m o l e s o f H 0 a r e t r a n s p o r t e d w i t h e a c h meq o f H f r o m t h e s e n s i n g t o t h e c o u n t e r e l e c t r o d e s i d e . Water f o r b o t h t h e s e p r o c e s s e s i s s u p p l i e d by b a c k - d i f f u s i o n o f water from the c o u n t e r t o the s e n s i n g e l e c t r o d e s i d e . This i s r e a d i l y achieved with sensor c e l l s having a properly e q u i l i b r a t e d membrane a n d e l e c t r o d e a s s e m b l y . Back d i f f u s i o n o f water can be i n c r e a s e d b y u s i n g a membrane w h i c h i s t h i n n e r a n d / o r h a s h i g h e r water content. H y d r a t i o n o f t h e membrane a n d e l e c t r o d e a s s e m b l y , i n c l u d i n g t h e c h a n n e l between s e n s i n g and r e f e r e n c e e l e c t r o d e , c o n t r o l s o u t p u t and r e p r o d u c i b i l i t y . The SPE s e n s o r c e l l c o n f i g u r a t i o n i s shown i n F i g u r e 1. 2

+

T h e r e a c t a n t g a s must d i f f u s e t h r o u g h t h e e l e c t r o d e s t r u c t u r e w h i c h c o n t a i n s a i r (0 , N ) a n d a n y p r o d u c t s o f r e a c t i o n (C0 , N0 , NO, H 0 v a p o r , e t c . ) . Response c h a r a c t e r i s t i c s a r e dependent on electrode material, Teflon content, electrode porosity, thickness and d i f f u s i o n / r e a c t i o n k i n e t i c s o f t h e r e a c t a n t gas on t h e c a t a l y t i c surface. By o p t i m i z i n g c a t a l y t i c a c t i v i t y f o r a g i v e n r e a c t i o n and c o n t r o l l i n g t h e p o t e n t i o s t a t i c v o l t a g e on t h e s e n s i n g e l e c t r o d e , t h e c o n c e n t r a t i o n o f r e a c t a n t gas c a n b e m a i n t a i n e d a t e s s e n t i a l l y zero at the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e . A l l reactant species a r r i v i n g at the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e w i l l be r e a d i l y r e a c t e d . Under these c o n d i t i o n s , the r a t e o f d i f f u s i o n i s p r o p o r t i o n a l t o C , where 2

2

2

2

C = C

b

- C

e

(3)


2

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where C and C a r e t h e r e a c t a n t c o n c e n t r a t i o n s i n t h e b u l k gas phase and e l e c t r o d e / e l e c t r o l y t e s u r f a c e , r e s p e c t i v e l y . However, s i n c e C i s e s s e n t i a l l y e q u a l t o z e r o , t h e measured c u r r e n t i s p r o p o r t i o n a l t o t h e r e a c t a n t c o n c e n t r a t i o n i n t h e b u l k gas p h a s e . T h e maximum d i f f u s i o n c u r r e n t , i , c a n t h u s b e e x p r e s s e d a s D

e


e

where K i s a p r o p o r t i o n a l l y c o n s t a n t . A d e t a i l e d r e v i e w o f t h e o r i e s a n d m o d e l s f o r p o r o u s gas d i f f u s i o n c a t a l y t i c electrodes used i n f u e l c e l l - t y p e a p p l i c a t i o n s i s p r e s e n t e d b y A u s t i n (5). Two mechanisms o f mass t r a n s p o r t a r e possible. T h e r e may b e a t o t a l p r e s s u r e g r a d i e n t a c r o s s t h e e l e c t r o d e so t h a t f o r c e d flow ( p e r m e a b i l i t y ) o c c u r s . Alternately, t h e r e may b e o t h e r components ( N , 0 , w a t e r v a p o r , C 0 , e t c . ) that b u i l d up w i t h i n t h e e l e c t r o d e s t r u c t u r e t o g i v e a n a l m o s t c o n s t a n t total pressure. Mass t r a n s f e r i s t h e n a c c o m p l i s h e d b y d i f f u s i o n o f t h e r e a c t a n t gas t h r o u g h a s t a g n a n t f i l m o f gas i n t h e p o r e s . T h i s p r o c e s s i s much s l o w e r t h a n f o r c e d f l o w a n d l e a d s t o l a r g e p a r t i a l pressure gradients through the electrode s t r u c t u r e . It is t h i s m e c h a n i s m o f mass t r a n s f e r w h i c h l i k e l y p r e d o m i n a t e s . Mass t r a n s f e r r e l a t i o n s h i p s f o r p o r o u s gas d i f f u s i o n e l e c t r o d e s h a v e been s o l v e d u s i n g m o d i f i c a t i o n s o f F i c k s law. The gas d i f f u s i o n e l e c t r o d e s used i n t h e s e n s o r c e l l c o n t a i n a m i x t u r e o f l a r g e and fine pores. When t h e d i a m e t e r o f t h e p o r e s i n t h e e l e c t r o d e i s l e s s t h a n 0.1 m i c r o n , mass t r a n s f e r i s b y K n u d s d e n d i f f u s i o n . 2

2

2

!

S o l u t i o n s o f t h e c o m b i n e d e q u a t i o n s o f mass t r a n s f e r , k i n e t i c s and e l e c t r o c h e m i c a l t r a n s p o r t e x p r e s s e d i n terms o f t h e l i m i t i n g current, i g e n e r a l l y are o f the form L

nFDCfrA

(5)

where n F D A 6 thus a

= No. o f e l e c t r o n s / m o l e c u l e = Faraday s constant = D i f f u s i o n c o e f f i c i e n t of the reactant = Active electrode area = Diffusion layer thickness l i n e a r r e l a t i o n s h i p i s p r e d i c t e d where 1

gas

(6)

w h i c h i s i n agreement w i t h t h e e m p i r i c a l e q u a t i o n p r e v i o u s l y d i s cussed. F o r t h e c o n c e n t r a t i o n r a n g e s t u d i e d , g o o d l i n e a r i t y was o b s e r v e d when d e t e c t i n g C O , NO a n d N 0 in air. o

Potentiostatic Circuit. The e l e c t r i c a l c i r c u i t u s e d f o r b r e a d b o a r d t e s t i n g o f t h r e e - e l e c t r o d e s e n s o r c e l l s i s shown i n F i g u r e 2. A m p l i f i e r U1 s e n s e d t h e v o l t a g e b e t w e e n t h e r e f e r e n c e a n d


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2

Figure 1. Sensor cell assembly: 1, reservoir housing; 2, cap; 3, support plate; 4, M & E assembly; 5, base plate; 6, gasket; 7, contact pin; 8, thermistor; 9, nylon screw; 10, Teflon tape; 11, gasket; 12, gasket; 13, counter electrode; 14, sensor electrode; 15, reference electrode; 16, thermistor.

Figure 2.

Schematic of simple potentiostatic circuit for sensor tests


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s e n s i n g e l e c t r o d e s (common) a n d c o m p a r e d i t w i t h a p r e s e t r e f e r e n c e v o l t a g e a t R1. Any d i f f e r e n c e s i n s i g n a l were u s e d t o g e n e r a t e a c u r r e n t between t h e c o u n t e r and s e n s i n g e l e c t r o d e , w h i c h acted to e l i m i n a t e the d i f f e r e n c e voltage producing i t . The c u r r e n t r e q u i r e d c o u l d b e s e n s e d a c r o s s r e s i s t o r BM o r m e a s u r e d d i r e c t l y w i t h a n ammeter i n s e r i e s w i t h t h e c o u n t e r e l e c t r o d e . T h e c i r c u i t r y u s e d f o r t h e b r e a d b o a r d t e s t i n g o f NO a n d NOp s e n s o r c e l l s was v e r y s i m i l a r t o t h a t shown i n F i g u r e 2; o n l y t n e a p p l i e d p o t e n t i a l was c h a n g e d . A n a p p l i e d p o t e n t i a l o f +1.30 V v e r s u s t h e SHE r e f e r e n c e e l e c t r o d e was u s e d f o r NO o x i d a t i o n w h i l e a p o t e n t i a l o f 0.75 V v e r s u s t h e same r e f e r e n c e e l e c t r o d e was u s e d f o r N0 reduction. C u r r e n t m e a s u r e m e n t s w e r e a g a i n made b y m e a s u r i n g t h e v o l t a g e d r o p a c r o s s r e s i s t o r R-**. T h r e e e l e c t r o d e s y s tems w e r e u s e d f o r b o t h g a s e s . 2

Gases. M i x t u r e s o f c a r b o n monoxide i n a i r o r n i t r o g e n were o b t a i n e d f r o m e i t h e r M a t h e s o n , S c o t t o r A i r c o Gas P r o d u c t s . All g a s e s w e r e f a c t o r y - a n a l y z e d t o ± 5% a n d w e r e u s e d w i t h o u t f u r t h e r analysis. M i x t u r e s o f n i t r i c o x i d e i n n i t r o g e n were o b t a i n e d from M a t h e s o n Gas P r o d u c t s . H i g h c o n c e n t r a t i o n s (250 ppm), f a c t o r y a n a l y z e d t o ± 5/5, w e r e d i l u t e d w i t h a i r u s i n g s t a n d a r d gas m i x i n g t e c h niques. NO c o n c e n t r a t i o n s w e r e c a l c u l a t e d f r o m NO a n d a i r f l o w rates. N i t r o g e n d i o x i d e p e r m e a t i o n tubes were o b t a i n e d from M e t r o n i c s , Inc. T h e s e t u b e s w e r e f a c t o r y c a l i b r a t e d f o r u s e a t 30.0°C. The p e r m e a t i o n t u b e s were t h e r m o s t a t e d i n a w a t e r b a t h and a i r p a s s e d o v e r a t a known f l o w r a t e ( o f a t l e a s t 150 cc/min). Gas s a m p l e s w e r e drawn o f f a t 80 c c / m i n . Results

and D i s c u s s i o n

C u r r e n t - V o l t a g e C h a r a c t e r i s t i c s o f P l a t i n o i d and G r a p h i t e Electrodes i n A i r . The SPE s e n s o r c e l l s have been o p t i m i z e d f o r detection of c e r t a i n oxidizable or reducible species i n a i r . To achieve t h i s , the sensing electrode i s maintained at a voltage when t h e r e i s m i n i m a l i n t e r f e r e n c e b y t h e 0 i n the a i r . T h e two e l e c t r o d e s s t u d i e d i n d e t a i l have been p l a t i n o i d and g r a p h i t e s e n sing electrodes. 2

The a n o d i c and c a t h o d i c b e h a v i o r o f a p l a t i n o i d e l e c t r o d e s t r u c t u r e i s shown i n F i g u r e 3. A l l v o l t a g e measurements a r e r e f e r r e d t o t h e S H E . T h e r e s t p o t e n t i a l o f t h e P t / a i r (0 ) electrode i s a p p r o x i m a t e l y 1.05 V and b a c k g r o u n d c u r r e n t i s i n t h e o r d e r o f micro-amperes. The most u s e f u l range f o r d e t e c t i o n o f c e r t a i n o x i d i z a b l e s p e c i e s s u c h as CO, H , e t h y l e n e , a c e t y l e n e and e t h a n o l i s 1.05 t o 1.20 V. A b o v e 1.2 V the anodic background c u r r e n t r i s e s due t o f o r m a t i o n o f some o x y g e n . B e l o w 1.05 V a cathodic current due t o o x y g e n r e d u c t i o n i s o b s e r v e d . A l i m i t i n g current for oxyg e n r e d u c t i o n i s g e n e r a l l y o b s e r v e d i n t h e r a n g e Q.k t o 0.8 V (2). T h i s v o l t a g e r a n g e i s optimum f o r o x y g e n d e t e c t i o n u s i n g t h e s e 2

2



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Figure 3.

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CO sensor cell voltage-current relationships: (O) air, (£3) 40 ppm CO in air


35.

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Sensor

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559

sensing electrode structures. In t h i s paper, only the detection o f CO i n a i r o n p l a t i n o i d s t r u c t u r e s i s a d d r e s s e d . The c o r r e s p o n d i n g c u r v e f o r o x i d a t i o n o f CO i n a i r i s a l s o shown i n F i g u r e 3T h e c e l l r e s p o n s e i s e s s e n t i a l l y c o n s t a n t f o r t h e r a n g e 1.05 to 1.20 V. T h e optimum v o l t a g e s e l e c t e d f o r o x i d a t i o n o f CO i n a i r was 1.15 V . The a n o d i c and c a t h o d i c b e h a v i o r o f a g r a p h i t e e l e c t r o d e s t r u c t u r e i s shown i n F i g u r e k. With graphite electrode structures t h e u s e f u l r a n g e f o r d e t e c t i n g c e r t a i n o x i d i z a b l e (NO, H S ) o r r e d u c i b l e ( N 0 , CI2) s p e c i e s i s e x t e n d e d f r o m 0.7 t o 1.3 V before i n t e r f e r e n c e due t o a i r r e d u c t i o n o r o x y g e n a t i o n o f g r a p h i t e i s o b served, respectively. I n t h i s p a p e r , o n l y t h e d e t e c t i o n o f NO a n d NOo o n g r a p h i t e e l e c t r o d e s t r u c t u r e s i s a d d r e s s e d . T h e optimum v o l t a g e s s e l e c t e d f o r NO o x i d a t i o n a n d N 0 r e d u c t i o n i n a i r were 1.25 a n d 0.75 V respectively. 2


2

2

L i n e a r i t y o f Response and R e a c t i o n P r o d u c t s . The r e s p o n s e v s . c o n c e n t r a t i o n c u r v e s o b t a i n e d f o r C O , NO a n d N 0 gas s e n s o r c e l l s a r e d e p i c t e d i n F i g u r e s 5, 6 and 7 r e s p e c t i v e l y . In a l l instances g o o d l i n e a r i t y o v e r t h e r a n g e s t u d i e d was o b s e r v e d b e t w e e n c u r r e n t and p a r t i a l p r e s s u r e o f e a c h o f t h e above gases (as d e p i c t e d b y e q u a t i o n (k)). The p r o p o r t i o n a l i t y c o n s t a n t s , K , w i t h s t a n d a r d d e v i a t i o n f o r t h e gases a r e as f o l l o w s : 2

Gas

Range I n v e s t i g a t e d

CO

0 0 0

NO N0

2

to to to

(ppm)

K (jua/ppm)

1500 200 18

2.21 ± 0.05 2.60 ± 0.13 2.91 ± 0.0k

The o b s e r v e d s e n s o r c e l l r e s p o n s e l i n e a r i t y w i t h s t a n d a r d d e v i a t i o n makes a o n e - p o i n t c a l i b r a t i o n o f a d e t e c t o r p o s s i b l e f o r the c o n c e n t r a t i o n ranges i n d i c a t e d . When m e a s u r i n g CO t h e o n l y r e a c t i o n p r o d u c t f o u n d was C0 and mass b a l a n c e m e a s u r e m e n t s i n d i c a t e t h e r e a c t i o n d o e s p r o c e e d a c cording to equation (2). 2

E x p e r i m e n t s w e r e a l s o c o n d u c t e d t o m e a s u r e NO o x i d a t i o n p r o d u c t s and a i d i n i d e n t i f i c a t i o n o f t h e r e a c t i o n o c c u r r i n g a t t h e electrode surface. U s i n g an N 0 s e n s o r c e l l i n s e r i e s w i t h a n NO s e n s o r c e l l , 70-80 ppm N 0 w e r e d e t e c t e d i n t h e e x h a u s t o f a n NO s e n s o r c e l l w h i c h h a d b e e n e x p o s e d t o 2^0 ppm N O . Two NO s e n s o r c e l l s i n s e r i e s w e r e u s e d t o m e a s u r e n , t h e number o f e l e c t r o n s i n v o l v e d i n t h e o x i d a t i o n r e a c t i o n . The f o l l o w i n g r e l a t i o n s h i p was d e r i v e d f r o m F i c k s L a w , a n d was u s e d t o calculate n: 2

2

!

_ 2.2k n

~

x 107

FfC ( i

6

2

1 -

i°

l^T

2

(

. ( 7 )



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RESPONSE (/ua) Figure 4.

NO and N0 sensor cells voltage-current relationships: (O) air or 100 ppm CO in air, (A) 15 ppm NO in air, 10 ppm N0 in air 2

2



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Figure 5. Response vs. concentration for CO sensor cell: potentiostatic voltage, 1.15 V; gas flow, 60 cm /min; temperature, 25°C; TLV, 50 ppm; STEL, 400 ppm; OSHA PEL, 50 ppm. 3

500 1000 PPM CO IN AIR

6

1500

PPM NO IN AIR Figure 6.

Response vs. NO concentration: potentiostatic voltage, 1.3 V; gas flow, 60 cm /min; temperature, 25°C; TLV, 25 ppm; STEL, 35 ppm. 3


562

CHEMICAL

where F

HAZARDS

= the value o f Faraday s constant 1

IN T H E WORKPLACE

(96,500

coulombs/

mole) f

= t h e sensor

C

c e l l flow

rate

(liters/second)

= t h e NO c o n c e n t r a t i o n (ppm)

2.2k x 10?

= a c o n v e r s i o n f a c t o r t o c h a n g e f r o m ppm t o m o l a r volume

i^

= the current at the f i r s t

±2

- t h e c u r r e n t a t t h e second sensor


exhaust i°2

of the f i r s t

sensor c e l l

(amps)

d e l l measuring the

(amps)

= t h e c u r r e n t o b t a i n e d when f l o w i n g NO d i r e c t l y t h e s e c o n d s e n s o r c e l l (amps)

into

F l o w r a t e s w e r e v a r i e d f r o m 60 t o 5 c c / m i n . T h e NO c o n c e n t r a t i o n s w h i c h w e r e u s e d v a r i e d f r o m 2^3 ppm a t l o w f l o w r a t e s t o 1 1 . 9 ppm a t h i g h e r f l o w r a t e s , V a l u e s o f n a t h i g h e r (kO - 60 c c / m i n ) f l o w r a t e s w e r e f o u n d t o b e i n d e p e n d e n t o f NO c o n c e n t r a t i o n . D u e t o t h e l o w s e n s o r c e l l c u r r e n t s m e a s u r e d a t l o w f l o w r a t e s , NO c o n c e n t r a t i o n s were h e l d c o n s t a n t . D a t a from t h i s experiment i n d i c a t e t h e most p r o b a b l e v a l u e f o r n i s 2, l e a d i n g t o t h e f o l l o w i n g o x i d a t i o n r e a c t i o n : NO + H 0

^ N 0

2

The d e t e c t i o n o f N 0 this reaction.

2

2

+ 2 H + 2e" +

i n t h e exhaust

(8) o f a n NO s e n s o r

cell

verifies

Response Time. T h e r e s p o n s e - t i m e c u r v e f o r o x i d a t i o n o f CO w i t h a n SPE s e n s o r c e l l h a v i n g a p l a t i n o i d s e n s i n g e l e c t r o d e i s shown i n F i g u r e 8. S i m i l a r c u r v e s f o r t h e o x i d a t i o n o f NO a n d r e d u c t i o n o f N 0 w i t h a n SPE c e l l h a v i n g a g r a p h i t e s e n s i n g e l e c t r o d e a r e a l s o shown i n F i g u r e 8. A l l m e a s u r e m e n t s w e r e made a t 25°C a t gas f l o w o f 60 c c / m i n . The c u r r e n t - t i m e response c a n be estimated from t h e r e l a t i o n s h i p 2

1 - I / i = exp ( - t / r )

(9)

w h e r e i = maximum c u r r e n t I Y t

= current observed a t time, = time -

t , sec.

constant

time

T h e c o r r e s p o n d i n g p l o t s o f I n (1 - I / i ) v s . t show e x c e l l e n t l i n e a r i t y f o r t h e NO r e s p o n s e , b u t a l s o i n d i c a t e d e p a r t u r e s f r o m l i n e a r i t y f o r CO a n d N 0 a f t e r 1 0 - 1 2 s e c o n d s . The response time c h a r a c t e r i s t i c s o f these c e l l s a r e s e n s i t i v e t o c e l l geometry, 2



35.

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Figure 7. Response vs. N0 concentration: potentiostatic voltage, 0.75 V; gas flow, 80 cm /min; temperature, 25 °C; TLV, 5 ppm. 2

3

PPM NO IN AIR 2

300

V t—I—I—I—I 0

6

12

18

24

I

I

I

I

I

I

I

I

I

I

I

30

36

42

48

54

60

66

72

78

84

90

96

TIME (SECONDS) Figure 8. Response-time curves for ( ) oxidation of NO (1.3 V) and ( ) reduction of NO (0.75 V) on a graphite electrode and ( ) oxidation of CO (1.15 V) on a platinum electrode. Test gases: 50 ppm CO in air; 51.4 ppm NO in air; 7.4 ppm NO in air. x

g


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e l e c t r o d e c o m p o s i t i o n , and r e a c t a n t s p e c i e s . In general, response times a r e l e n g t h e n e d f o r s t r o n g l y a d s o r b e d r e a c t a n t s and f o r e l e c trodes with a large surface area. A c t i v e surface area for the p l a t i n o i d b l a c k and g r a p h i t e c a t a l y t i c e l e c t r o d e s i s a p p r o x i m a t e l y kO a n d 1 m e t e r / g , r e s p e c t i v e l y . Optimization of diffusion layer t h i c k n e s s (5), d e c r e a s e s i n e l e c t r o d e s u r f a c e a r e a , a n d i m p r o v e d w a t e r management a r e a l l u n d e r a c t i v e i n v e s t i g a t i o n as methods o f i n c r e a s i n g the speed o f sensor response to the s u b j e c t gases.


2

E f f e c t of Feed Flow. The e f f e c t o f f e e d f l o w on s e n s o r r e s p o n s e f o r t h e CO s e n s o r c e l l i s shown i n F i g u r e 9. Similar c u r v e s w e r e o b s e r v e d f o r NO a n d N 0 . G e n e r a l l y , t h e r e i s a marked i n c r e a s e i n s e n s o r c e l l r e s p o n s e w i t h i n c r e a s i n g f l o w (10 t o kO c c / m i n ) , f o l l o w e d b y a s l o w r i s i n g s l o p e a s f l o w e x c e e d s 50 c c / m i n . A f l o w o f 60 c c / m i n was s e l e c t e d f o r p r a c t i c a l u s e b a s e d o n t r a d e o f f s t u d i e s o f w a t e r management and f l o w d e p e n d e n c e . 2

S t u d i e s b y S e d l a k (6) h a v e shown a s i m i l a r r e s p o n s e - f l o w r e l a t i o n s h i p f o r l i q u i d e l e c t r o l y t e c e l l s which u t i l i z e a t e f l o n bonded d i f f u s i o n e l e c t r o d e . The e m p i r i c a l e q u a t i o n s and r e l a t i o n s h i p s d e r i v e d g e n e r a l l y a p p l y t o t h e SPE s e n s o r c e l l s . E f f e c t of Temperature. The r e s p o n s e l e v e l as a f u n c t i o n o f t e m p e r a t u r e (1 t o 4 0 ° C ) a t a f i x e d f e e d r a t e o f 60 c m 3 / m i n o f 100 ppm CO i n a i r i s shown i n F i g u r e 10. The d a t a f o l l o w an A r r h e n i u s r e l a t i o n s h i p of the type i

= K exp(-E/RT)

where E = a c t i v a t i o n e n e r g y ,

k

(10)

cal/mole

A s i m i l a r r e l a t i o n s h i p was f o u n d f o r t h e NO a n d N 0 s e n s o r c e l l s . T h e b a c k g r o u n d c u r r e n t o b s e r v e d o v e r t h e same t e m p e r a t u r e r a n g e w i t h z e r o a i r ( a i r c o n t a i n i n g none o f t h e t e s t gases) remained e s s e n t i a l l y c o n s t a n t 0 ± 2 jua. A l e a s t square f i t o f the temperat u r e d a t a f o r CO i n a i r (0 t o 1000 p p m ) , NO i n a i r (0 t o 200 ppm) and N 0 (0 to ^k ppm) y i e l d e d a c t i v a t i o n e n e r g i e s a s f o l l o w s : ~ 2

2

T e s t Gas

E, k

CO i n a i r NO i n a i r N0 The v a l u e s

2

in air

of E are i n d i c a t i v e of

cal/mole

7.7 7.1 7.6 a diffusion controlled pro-

cess. The compensated span r e s p o n s e o v e r t h e t e m p e r a t u r e range 1 t o i s shown i n F i g u r e 11. The c i r c u i t r y u s e d f o r compensation i s d e p i c t e d i n t h e same f i g u r e . T h e optimum p a r a l l e l r e s i s t o r t o a c h i e v e a c c u r a c y w i t h i n ± % o f t h e 25°C c a l i b r a t e d v a l u e i s 2.2K ohm.

k0°C


L A

CONTI

E T

A L .

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SPE Sensor

Cells


35.

Figure 10.

Effect of temperature on CO span response: potentiostatic voltage, 1.15 V; gas flow, 60 cm /min; 100 ppm CO in air. 3


565

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T H E

WORKPLACE


L i f e C h a r a c t e r i s t i c s of the Sensor C e l l s . T y p i c a l l i f e behavi o r o f t h e S P E CO s e n s o r c e l l s i s d e p i c t e d i n F i g u r e 12 f o r s e n s o r c e l l s w i t h r e s p o n s e s i n t h e r a n g e 1.8 t o 2.k j u a / p p m . The d a t a r e p o r t e d a r e f o r c e l l s t h a t had been c o n t i n u o u s l y p o t e n t i o s t a t e d and o p e r a t e d on t h e t e s t gas i n t e r m i t t i n g l y f o r a p p r o x i m a t e l y 8 h o u r s / day. S i m i l a r s t a b i l i t y has been o b s e r v e d f o r s e n s o r c e l l s d u r i n g continuous operation w i t h a t e s t gas. D u r i n g t h e f i r s t 15 t o 20 d a y s t h e r e i s a d e c r e a s e i n s i g n a l amounting t o 2 ? / d a y f o l l o w e d by a l e v e l i n g i n performance. In a p r a c t i c a l instrument, c a l i b r a t i o n i s o n l y r e q u i r e d d a i l y to weekly to achieve ± 1 0 ? accuracy during i n i t i a l l i f e operation. Subseq u e n t l y , weekly to monthly c a l i b r a t i o n s are r e q u i r e d . T y p i c a l SPE CO s e n s o r c e l l s o p e r a t e d i n c o m m e r c i a l i n s t r u m e n t s h a v e a n o u t p u t t h a t i s w i t h i n k0% o f t h e o r i g i n a l c a l i b r a t i o n s i g n a l a f t e r two t o three years of operation. L i f e t e s t i n g c o n d u c t e d o n NO S P E s e n s o r c e l l s showed a 1 ? / d a y i n c r e a s e i n s i g n a l o v e r t h e f i r s t 20 t o 30 d a y s f o l l o w e d b y a l e v e l i n g i n performance. T y p i c a l i n i t i a l s e n s o r r e s p o n s e f o r t h e NO s e n s o r c e l l i s 3 jua/ppm N O . With daily- c a l i b r a t i o n s , high accuracy l e v e l s can be m a i n t a i n e d throughout sensor l i f e . F o r example, a p r o t o t y p e NO d e t e c t i o n i n s t r u m e n t h a s o p e r a t e d f o r one y e a r w i t h t h e r e s p o n s e t o N O - i n - a i r w i t h i n 15? o f t h e o r i g i n a l c a l i b r a t i o n value. The N 0 sensor c e l l s demonstrate a decrease i n s i g n a l r a n g i n g from 0 t o 0 . 5 ? / d a y . T y p i c a l i n i t i a l s e n s o r r e s p o n s e i s 2.5 t o 3.0 jua/ppm N 0 . T h i s instrument a l s o demonstrates the c a p a b i l i t y to remain h i g h l y accurate w i t h infrequent c a l i b r a t i o n . Operation of a p r o t o t y p e NOp i n s t r u m e n t f o r a p p r o x i m a t e l y 200 d a y s h a s v e r i f i e d the need f o r minimal c a l i b r a t i o n e f f o r t . 2

2

Interferences. T a b l e I i s a l i s t o f gases which c o u l d p o t e n t i a l l y i n t e r f e r e w i t h a CO a n a l y s i s . Most o f t h e gases a r e r e moved b y u s e o f a P u r a f i l ( p o t a s s i u m p e r m a n g a n a t e o n a l u m i n a ) scrubber column. Gases n o t removed c o m p l e t e l y b y P u r a f i l a r e h y d r o g e n , e t h y l e n e , and a c e t y l e n e . I f t h e CO d e t e c t o r must b e u s e d i n an atmosphere w i t h h i g h c o n c e n t r a t i o n s o f t h e s e g a s e s , s p e c i a l p r e c a u t i o n s must be t a k e n t o i n s u r e t h e i r r e m o v a l . Column 3 o f Table I l i s t s the concentrations of the i n t e r f e r i n g species, after p a s s i n g through P u r a f i l , which w i l l produce a s i g n a l equivalent to 1 ppm C O . Table I I i s a l i s t o f gases which c o u l d p o t e n t i a l l y i n t e r f e r e w i t h a n NO a n a l y s i s a l o n g w i t h t h e c o n c e n t r a t i o n s o f t h e s e g a s e s w h i c h p r o d u c e a s i g n a l e q u i v a l e n t t o 1 ppm N O . O n l y H S had an e f f e c t o n s e n s o r c e l l p e r f o r m a n c e a n d was f o u n d t o d e c r e a s e t h e r e s p o n s e l e v e l b y 0 . 2 jua/ppm. H S, S0 and N 0 were e f f e c t i v e l y filt e r e d f r o m t h e g a s s t r e a m b y u s e o f t r i e t h a n o l a m i n e (TEA) a s shown i n Table II. To p r e v e n t TEA v a p o r s f r o m r e a c h i n g ( a n d t h u s p o i soning) t h e s e n s i n g e l e c t r o d e s u r f a c e , a s h o r t column o f a c a t i o n e x c h a n g e b e a d was p l a c e d a f t e r t h e f i l t e r . 2

2

2

2


35.

L A

CONTI

E T

A L .

Electrochemical

90

SPE Sensor

Cells

20 TEMPERATURE (°C)


"T

—s/V\A T

Figure 11.

= THERMISTOR

Temperature compensated output for CO sensor cells

2.40

40 50 CELL LIFE (DAYS) Figure 12.

CO sensor cell response as a function of time: potentiostatic voltage, 1.15 V; gasflow,60 cm /min; temperature, 25°C. 3


567

CHEMICAL

568

TABLE I INTERFERENCE L E V E L S , CO ANALYSIS

INTERFERENT* GAS H O

VAPOR


p

o

INTERFERENT CONC.

AIR

50

TO 100?

RH

16

TO

TLV

(ppm)

co

2

NO

N0 S0

2

2

H C H HCHO H S C H^ *KMnO^ ON ALUMINA 2

2

5000 25 5 5

1? 50 ppm 10 ppm 25 ppm 100 ppm 100 ppm 5 ppm 10 ppm 100 ppm

2

2

2

HpS SOp NOp C H 2

2

CO H S 2

S0 N0

2

2

WORKPLACE

ppm FOR

CONC.

OF CO

CO)

INTERFERENT RATIO (ppm GAS) (ppm CO)

0 0 0 0 2 16

2 10

50.0 6.0

0 0 18

5.h

(PURAFIL) F I L T E R USED I N A L L TESTS

TABLE I I INTERFERENCE L E V E L S , NO ANALYSIS

INTERFERENT GAS*

T H E

0 0 0

20?

3?

2

IN

(TLV = 50

EQUIVALENT

GAS

IN

HAZARDS

GAS

CONC.

(ppm)

(TLV = 25

EQUIVALENT CONC.

7.8

2.8

9.3 6.6 201.0 55.0 98.0

0.9 17.0 0.0 0.0 0.0

7.8

0.0 0.0 0.0

(TEA) (TEA) (TEA)

* A L L TESTS RUN WITHOUT

9.3 6.6

NO

(ppm)

A TEA F I L T E R EXCEPT WHERE

ppm FOR

NO)

INTERFERENT RATIO (ppm GAS) (ppm NO) 2.6 10.0 1/2.5

INDICATED


35.

LA

CONTI

E T

AL.

Electrochemical

SPE

Sensor

Cells

569

T a b l e I I I i s a l i s t o f gases w h i c h might i n t e r f e r e w i t h an NOp a n a l y s i s . Of a l l t h e gases t e s t e d , o n l y H S and S0 had a s i g n i f i c a n t r e s p o n s e ; o x i d a t i o n c u r r e n t s were o b s e r v e d f o r b o t h s p e cies. N e i t h e r gas g a v e a s i g n a l w h i c h was c o n s t a n t w i t h t i m e . A f t e r an i n i t i a l r e s p o n s e , t h e response t o b o t h gases s l o w l y s t a r t e d to d r i f t back to the background v a l u e . Exposure of the sensor c e l l t o t h e s e gases d i d n o t have any e f f e c t on t h e r e s p o n s e l e v e l t o N0 . B o t h g a s e s may b e f i l t e r e d o u t b y u s e o f H g C l immobilized o n a n i n e r t s u p p o r t as shown i n T a b l e I I I . 2

2

2


2

INTERFERENCE L E V E L S ,

N0

2

TABLE I I I ANALYSIS ( T L V = 5 ppm

INTERFERENT INTERFERENT G A S *

GAS CONC. (ppm)

H S

7.8 6.9 201.0 55.0 20.8 167.0

36 18 0 0 0 0

5.7 6.9

0 38

9

so

2

C H Cplfy NO CO 2

2

H S S0 2

2

EQUIVALENT NOp CONC. (ppm)

(0.1MHgCl ) (0.1MHgCl ) 2

2

* A L L TESTS RUN WITHOUT A

Hg C l

2

FOR

N0 ) 2

INTERFERENT RATIO (ppm INTERFERENT) (ppm N O J 1A.6 1/2.6

_ 1/1.6

F I L T E R EXCEPT WHERE INDICATED

Development o f a D i f f u s i o n Head S e n s o r C e l l . The u s e o f a i r s a m p l i n g pumps i n p o r t a b l e e l e c t r o c h e m i c a l gas d e t e c t i o n a p p a r a t u s i n t r o d u c e s p o t e n t i a l problems i n t o the i n s t r u m e n t . F i r s t , the sens o r c e l l r e s p o n s e i s dependent on gas f l o w r a t e . The sample f l o w r a t e , t h e r e f o r e , must b e a c c u r a t e l y c o n t r o l l e d t o o b t a i n r e p r o d u c i b l e r e s u l t s , o r t h e sample f l o w r a t e must be s e t h i g h enough t o i n s u r e a flow independent response. S e c o n d l y , f a i l u r e o f t h e pump i t s e l f c o u l d p r e v e n t a sample from r e a c h i n g t h e s e n s o r c e l l . T h i r d l y , t h e pumps a r e u s u a l l y one o f t h e l a r g e s t u s e r s o f c u r r e n t i n a p o r t a b l e i n s t r u m e n t and t h e r e b y l i m i t u s a b l e b a t t e r y l i f e . T h e n e e d f o r a n a i r s a m p l i n g pump c a n b e e l i m i n a t e d b y u s e o f a d i f f u s i o n tube having a set l e n g t h to diameter (L/d) r a t i o i n i t s g e o m e t r y f o r i n t r o d u c t i o n o f a gas s a m p l e . Proper s e l e c t i o n of the geometry and L / d r a t i o o f t h e d i f f u s i o n tube r e s u l t s i n an e l e c t r o c h e m i c a l c e l l w i t h a response which i s independent o f e x t e r n a l gas f l o w r a t e . A schematic o f a s o l i d polymer e l e c t r o l y t e d i f f u s i o n h e a d s e n s o r c e l l i s shown i n F i g u r e 13E q u a t i o n s were d e r i v e d t o p r e d i c t r e s p o n s e times and t h e pected steady s t a t e current f o r a pure d i f f u s i o n system.


ex-

570

CHEMICAL

For response

time,

the steady t

and t h e s t e a d y


D

IN

T H E WORKPLACE

state current occurs

= L AD

at

(11)

2

state current i s given by i

In these

HAZARDS

=

^ F A C ^ _

e

x

p

(

1

2

)

equations, = diffusion coefficient

(0.175 c m / s e c

C ^ = a m b i e n t CO c o n c e n t r a t i o n t

= time, tube)

L

= tube

A

= electrode

2

f o r CO)

(moles/cm3)

s e c (from time o f a d m i s s i o n o f sample

to base o f

length area

F i r s t , a n e s t i m a t e c a n b e made o f e x p e c t e d r e s p o n s e t i m e f r o m Equation (11). T u b e l e n g t h i s L = 10 cm a n d D = 0 . 1 7 5 c m / s e c . U s i n g t h e s e v a l u e s , t = 1^3 s e c o n d s was c o m p u t e d . Secondly, the s t e a d y s t a t e c u r r e n t was e s t i m a t e d f o r 1 ppm CO b y means o f E q u a t i o n (12). Note t h a t tube a r e a A = 2.85 c m , n F = 2 x 96,500 c o u l / m o l e a n d 1 ppm CO i s e q u i v a l e n t t o * k 0 8 x 1 0 " ^ moles/cm3. U s i n g t h e s e n u m b e r s , a s e n s o r r e s p o n s e o f 0 . 2 9 / i A / p p m CO was c a l culated. The s i m p l i f i e d d i f f u s i o n model p r o v i d e d p r e d i c t i o n s o f o p e r a t i n g c h a r a c t e r i s t i c s w h i c h were s u f f i c i e n t l y p r o m i s i n g t o p r o ceed w i t h an e x p e r i m e n t a l s t u d y . 2

2

1

A p r e - p r o t o t y p e pumpless CO d e t e c t o r f a b r i c a t e d f o r t e s t purposes i n c l u d e d an i n t e r f e r e n c e gas f i l t e r c a r t r i d g e w h i c h c o u l d be r e p l a c e d e a s i l y without disassembly o f t h e s e n s o r / d i f f u s i o n tube assembly. T h e s i g n a l r e s p o n s e i n t h i s a r r a n g e m e n t was 0 . 3 5 juA/ppm CO - i n g o o d a g r e e m e n t w i t h t h e o r e t i c a l p r e d i c t i o n s . Temperature c h a r a c t e r i s t i c s o f t h e d i f f u s i o n head u n i t were d e f i n e d o v e r ^ - 3 7 ° C The r e s u l t s a r e summarized i n T a b l e I V . Z e r o s t a b i l i t y was e x c e l l e n t over t h i s temperature range; the v a r i a b i l i t y i n zero r e a d i n g was o n l y t o t h e e q u i v a l e n t o f a few ppm C O . S i g n a l r e s p o n s e was e s s e n t i a l l y c o n s t a n t b e t w e e n 4 - 3 7 ° C a t 0.31 - 0.3 * M ^ / P P T h i s r e l a t i v e i n v a r i a n c e o f s i g n a l may b e a c o n s e q u e n c e o f a c o m pensation effect. T h a t i s , as t e m p e r a t u r e i n c r e a s e s , t h e t e s t gas becomes s l i g h t l y l e s s d e n s e o n a m o l e s C0/cm3 b a s i s , b u t t h e d i f fusion coefficient increases s l i g h t l y . f ,

! f

1

2 1

c

o

T h e e f f e c t o f a i r f l o w p a s t t h e d i f f u s i o n t u b e i n l e t was s t u d i e d t o s i m u l a t e ambient c o n v e c t i o n . The e f f e c t o f e x t e r n a l a i r f l o w s p e r p e n d i c u l a r t o t h e d i f f u s i o n t u b e f o r two d i f f e r e n t CO c o n c e n t r a t i o n s i s shown i n F i g u r e Ik. L i t t l e v a r i a t i o n was s e e n i n t h e r e s p o n s e o v e r t h e r a n g e 200 - 600 f t / m i n . Response times t o


LA

CONTI

E T A L .

Electrochemical

SPE Sensor

Cells

571

SPE SENSING ELECTRODE

\////////////A


DIFFUSION ' TUBE

Figure 13.

Figure 14.

Schematic of SPE diffusion head gas sensor

Response level vs. flow rate: (O) ~ 100 ppm CO in air; ({J) ~ 200 ppm CO in air


572

CHEMICAL

HAZARDS

IN T H E WORKPLACE


c a 90? o f s t e a d y s t a t e s i g n a l w e r e w i t h i n 2-3 m i n u t e s . This also i s i n r e a s o n a b l e conformance t o t h e o r e t i c a l p r e d i c t i o n s . R e c e n t work h a s e x t e n d e d t h e u s e o f a d i f f u s i o n b a s e d i n s t r u ment t o o n e i n w h i c h t h e d i f f u s i o n t u b e h a s b e e n r o t a t e d 90° w i t h r e s p e c t t o t h a t o f F i g u r e 13I n this configuration, there i s a p o s s i b i l i t y o f flow d i r e c t l y i n t o t h e d i f f u s i o n tube. T h i s tube h a s d i m e n s i o n s L = 1.6 cm a n d d = 0.87 cm. S u b s t i t u t i n g t h e s e v a l u e s i n t o E q u a t i o n (2) gives a p r e d i c t e d d i f f u s i o n c e l l response o f 1.5 jua/ppm C O . O b s e r v e d v a l u e s o f 1.1-1.2 jua/ppm CO w e r e a g a i n i n good agreement w i t h t h e o r e t i c a l p r e d i c t i o n s . TABLE I V TEMPERATURE DEPENDENCE, DIFFUSION HEAD SENSOR C E L L TEMPERATURE ° C

BACKGROUND ( u a )

23.3 k.O 2k.0 37.0 27.0 22.k 36.0 2k.5 3.9 23.8 TEST GAS

CO RESPONSE Qua)

1-3 1-3 3-k 2-k 2-3 0-2 2-3 1-3 0-2 2-3

137-138 133-13 * 131-132 1VI-1^2 131-132 127-128 1^5-1^6 132-13H 1VI-1H3 121-122 1

7 ppm CO I N A I R

P r o d u c t i o n Gas D e t e c t i o n I n s t r u m e n t s . A family of portable instruments has been developed f o r t h e d e t e c t i o n and m o n i t o r i n g o f CO l e v e l s i n a i r (1,8.). The instrument f a m i l y c o n s i s t s o f a d i r e c t r e a d i n g d e t e c t o r w i t h LCD d i s p l a y o f a c t u a l CO c o n c e n t r a t i o n a n d a p e r s o n a l CO d o s i m e t e r . B o t h t h e d e t e c t o r and t h e d o s i m e t e r measure t h e a c c u m u l a t e d CO e x p o s u r e o f p e r s o n n e l i n i n d u s t r i a l e n v i r o n m e n t s and p r o v i d e b o t h v i s i b l e a n d a u d i b l e alarms i f i n s t a n t a n e o u s l y u n s a f e l e v e l s o f CO a r e e n c o u n t e r e d . An accompanying support c o n s o l e i s u s e d f o r i n t e g r a t e d c u m u l a t i v e CO d o s a g e r e a d o u t a n d b a t t e r y charging.

Summary Gas d e t e c t i o n s e n s o r c e l l s y s t e m s h i g h l y s p e c i f i c f o r C O , NO a n d NO2 h a v e b e e n d e v e l o p e d . A l l sensor c e l l s u t i l i z e a s o l i d polymer e l e c t r o l y t e w i t h i n t e g r a l l y - b o n d e d f u e l c e l l e l e c t r o d e s . C u r r e n t - v o l t a g e c h a r a c t e r i s t i c s o f v a r i o u s c a t a l y s t m i x t u r e s were s t u d i e d t o d e t e r m i n e optimum s e n s o r c e l l o p e r a t i n g v o l t a g e s . Act i v a t i o n e n e r g i e s i n d i c a t i v e o f d i f f u s i o n - c o n t r o l l e d p r o c e s s e s were c a l c u l a t e d f o r CO a n d NO o x i d a t i o n a n d N0 reduction. Linear r e sponses o v e r wide c o n c e n t r a t i o n ranges were a l s o o b s e r v e d . Addit i o n a l l y , a CO s e n s o r c e l l s y s t e m u t i l i z i n g a d i f f u s i o n t u b e i n 2


35.

L A CONTI

E T A L .

Electrochemical

SPE Sensor

Cells

573

s t e a d o f a n a i r s a m p l i n g pump was c o n s t r u c t e d . Good a g r e e m e n t was o b t a i n e d between t h e o r e t i c a l and c a l c u l a t e d r e s p o n s e s . Acknowledgements


T h i s w o r k was p a r t i a l l y s u p p o r t e d b y t h e U . S . D e p a r t m e n t o f I n t e r i o r , Bureau o f Mines. The a u t h o r s would l i k e t o acknowledge t h e t e c h n i c a l g u i d a n c e o f D r s . Emery C h i l t o n a n d G e o r g e S c h n a k e n b e r g o f t h e B u r e a u o f M i n e s , whose many h e l p f u l s u g g e s t i o n s a n d comments c o n t r i b u t e d s i g n i f i c a n t l y t o t h e t e c h n o l o g y developments described i n this publication.

Literature Cited 1. LaConti, A.B.; Dantowitz, P.; Kegan, R.; Maget, H.J.R TIS Report No. 66, ASD 3, General Electric Company: Lynn, Mass., 1966 2. LaConti, A.B.; Maget, H.J.R. J. Electrochem. Soc., 1971, 118, p. 506 3. Dempsey, R.M.; LaConti, A.B.; Nolan, M.E. "Gas Detection Instrumentation Using Electrically-Biased Fuel Cell Sensor Technology", PB25H823, NTIS, Springfield, Va., 1978 4. Schnakenberg, G.H.; LaConti, A.B., "Improved CO Detection Systems", Proceedings, 17th International Conference, Bulgaria, 1977 5. Austin, L.G. "Fuel Cells", NASA SP-120 Office of Technology Utilization, NASA, Washington, D.C., 1967 6. Bay, H.W.; Blurton, K.F.; Sedlak, J.M.; Valentine, A.M.; Anal. Chem., 1974, 46, p. 1837 7. Jones, R.C.; LaConti, A.B.; Nuttall, L.J. "Carbon Monoxide Monitor Features Hydrated Solid Polymer Electrolyte Cell", Proceedings, American Industrial Hygiene Conference, New Orleans, Louisiana, May, 1977 8. Gruber, A.H.; Goldstein, A.G.; LaConti, A.B. "A New Family of Miniaturized Self-Contained CO Dosimeters and Direct Reading Detectors", Paper Presented at U.S. Environmental Agency Symposium on Personal Air Pollution Monitors, Chapel Hill, N.C., Jan., 1979 RECEIVED

October 6, 1980.


Chemical Hazards in the Workplace - ACS Publications - American

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