Chemical Ozone Measurement ALFRED
EHMERT
Downloaded by UNIV OF GUELPH LIBRARY on June 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch018
Max Planck-Institut für Physik der Stratosphare, Weissenau
bei Ravensburg,
Germany,
A simple method for measuring ozone concentration semiautomatically is presented. The amount of sodium thiosulfate equivalent to the ozone present is measured by electrolysis with an iodinemeter. A l though smoke interferes, the true ozone content can be measured by difference at higher temperatures. The consistency of measurements with this method is demonstrated.
A w e l l k n o w n r e a c t i o n f o r m e a s u r i n g ozone is t h e o x i d a t i o n of p o t a s s i u m i o d i d e i n neutral solution : 0 + 2KOH + 1
2KI + 0 + H 0 3
2
2
(1)
2
I n t h e m e t h o d of C a u e r (2) t h e free i o d i n e , f o r m e d m o s t l y as I ~ , is b l o w n o u t w i t h t h e a i r w h i c h b r i n g s i n t h e ozone. T h e r e m a i n i n g p o t a s s i u m i o d i d e i s o x i d i z e d w i t h n i t r i c a c i d . T h i s m e t h o d is d i f f i c u l t , a n d t h e s o l u t i o n needs b u f f e r i n g . Furthermore, i t i s d i f f i c u l t t o m a k e t h e r e a c t i o n go t o c o m p l e t i o n , as t h e p o t a s s i u m i o d i d e c o n c e n t r a t i o n m u s t b e v e r y l o w i f s m a l l ozone c o n c e n t r a t i o n s a r e t o b e m e a s u r e d . B e s t results a r e o b t a i n e d b y u s i n g s o d i u m t h i o s u l f a t e i n a m o r e c o n c e n t r a t e d n e u t r a l s o l u t i o n of p o t a s s i u m i o d i d e . B y t h e q u i c k s e c o n d a r y r e a c t i o n 3
2 N a S 0 + L -> 2NaI + N a ^ O e 2
2
3
(2)
t h e a c t i v e free i o d i n e is i m m e d i a t e l y b r o u g h t i n t o t h e n o n a c t i v e i o n i c f o r m . T h e loss of s o d i u m t h i o s u l f a t e i s m e a s u r e d b y t i t r a t i o n o r c o u l o m e t r y . M e t h o d s f o r d e t e r m i n i n g ozone w e r e d e v e l o p e d b y R e g e n e r (10) a n d E h m e r t (7). R e c o r d i n g m e t h o d s w e r e c o n s t r u c t e d b y G l u c k a u f , H e a l , M a r t i n , a n d P a n e t h (9) a n d B o w e n a n d R e g e n e r (1). T h e y a d d e d fixed a m o u n t s of s o d i u m t h i o s u l f a t e a u t o m a t i c a l l y a n d r e c o r d e d t h e v o l u m e of a i r c o n t a i n i n g t h e e q u i v a l e n t a m o u n t of ozone. T h e a u t h o r d e v e l o p e d a s i m p l e m e t h o d f o r s e m i a u t o m a t i c a l l y m e a s u r i n g ozone c o n c e n t r a t i o n (5,7). T h e r e a c t i o n vessel ( F i g u r e 1 ) , r e q u i r e d s p e c i a l glasses s u c h as J e n a e r Gerâte-Glas w h i c h h a d n o influence o n t h e s o l u t i o n e v e n w h e n i t w a s i n t h e i o d i n e m e t e r f o r 2 d a y s . C a r e w a s used t o p r e v e n t l i g h t f r o m e n t e r i n g t h e s o l u t i o n . E x a c t l y 3 m l . of a n e u t r a l 2 % p o t a s s i u m i o d i d e s o l u t i o n (1 m l . of 0.0 L/V s o d i u m t h i o s u l f a t e i n 750 m l . of s o l u t i o n ) is p i p e t t e d i n t o b o t t l e 4. T h e b o t t l e i s a t t a c h e d t o t h e a p p a r a t u s . I f a i r i s t a k e n t h r o u g h j o i n t 3 b y a p u m p o r a c o n n e c t e d e v a c u a t e d vessel, t h e a i r e n t e r i n g a t t h e r i g h t side forces t h e s o l u t i o n t h r o u g h t h e s i n t e r e d - g l a s s filter a t t h e l o w e r e n d of t u b e 1 i n t o t h e b o t t l e . T h e a i r w h i c h f o l l o w s b u b b l e s t h r o u g h t h e s o l u t i o n . A t a r a t e of 2 l i t e r s p e r m i n u t e , f u l l r e a c t i o n o c c u r s a n d n o s o l u t i o n leaves b y j o i n t 3 w i t h t h e a i r . F r o m 5 t o 10 l i t e r s of a i r is u s e d . A f t e r t h e r e a c t i o n , b o t t l e 4 i s d e t a c h e d a n d 128
In OZONE CHEMISTRY AND TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.
129
Downloaded by UNIV OF GUELPH LIBRARY on June 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch018
EHMERT—CHEMICAL MEASUREMENT
l o w e r e d b y a b o u t 1 c m . , a n d t h e s o l u t i o n is b l o w n b a c k i n t o t h i s b o t t l e . A f t e r 2 m l . of w a t e r i s p o u r e d i n a t t h e u p p e r p o i n t , a l l of t h e s o l u t i o n i s c o l l e c t e d a g a i n i n b o t t l e 4. T h e r e s i d u a l c o n t e n t of s o d i u m t h i o s u l f a t e i n t h i s b o t t l e i s m e a s u r e d a n d c o m p a r e d w i t h t h a t of a b l a n k . T h i s measurement is simple a n d r a p i d w i t h a n iodinemeter [ C o l o r a G M B H (14a) L o r c h , G e r m a n y ] , w h o s e c i r c u i t scheme i s s h o w n i n F i g u r e 2. F o u r p l a t i n u m electrodes a r e b r o u g h t i n t o t h e s o l u t i o n i n b o t t l e 4, a n d t h e s o l u t i o n is m o v e d a g a i n s t these electrodes. T w o of t h e m a r e c o n n e c t e d t o a n e l e c t r i c p o t e n t i a l of 0.18 v o l t ( t h e v a l u e is n o t c r i t i c a l ) . A t t h i s v o l t a g e , p o l a r i z a t i o n p r e v e n t s e l e c t r o l y s i s . B y i t s d e p o l a r i z i n g a c t i o n , free i o d i n e causes a n e l e c t r i c c u r r e n t d i r e c t l y p r o p o r t i o n a l t o i t s c o n c e n t r a t i o n , b u t p o t a s s i u m i o d i d e p r o d u c e s n o c u r r e n t . A s l o n g as t h e r e is s o d i u m t h i o s u l f a t e i n t h e s o l u t i o n , n o e l e c t r i c c u r r e n t c a n f l o w . T h e s e n s i t i v i t y is h i g h e n o u g h t o a l l o w m e a s u r e m e n t w i t h s m a l l a m o u n t s of a i r .
FINE
Figure 2.
GROSS
Iodinemeter
In OZONE CHEMISTRY AND TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.
A D V A N C E S IN
130
CHEMISTRY SERIES
F r e e i o d i n e is p r o d u c e d i n t h e s o l u t i o n w h e n t h e p o t a s s i u m i o d i d e is e l e c t r o l y z e d b y t h e t w o o t h e r electrodes w i t h a s u i t a b l e c u r r e n t . W h e n a l l of t h e s o d i u m t h i o s u l f a t e has r e a c t e d w i t h i o d i n e , a c u r r e n t flows t h r o u g h t h e i n d i c a t i n g electrodes, w h i c h is linear w i t h time (Figure 3). F r o m F a r a d a y ' s c o n s t a n t , 1 γ of i o d i n e (1 γ = 1 0 ~ g r a m ) is o b t a i n e d f o r a n elec t r o l y s i s c u r r e n t of 38 /xa. i n 20 seconds. T h i s c o u l o m e t r i c m e t h o d does n o t n e e d c a l i b r a t e d s o l u t i o n s . T h e s e n s i t i v i t y is h i g h e n o u g h t o detect t h e e q u i v a l e n t w i t h a n a c c u r a c y t o 1 0 ~ g r a m of i o d i n e i n 10 m l . of s o l u t i o n . H i g h e r a c c u r a c y is p o s s i b l e . 6
7
Downloaded by UNIV OF GUELPH LIBRARY on June 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch018
M e a s u r e m e n t s a w a y f r o m i n d u s t r y a n d cities g a v e c o m p a r a b l e v a l u e s ( 5 ) , b u t l a r g e e r r o r s arose f r o m s m o k e a n d i n d u s t r i a l c o n t a m i n a t i o n . T h e s e components, b r o u g h t i n t o t h e s o l u t i o n w i t h t h e a i r , r e d u c e i o d i n e . A s t h e i r r e a c t i o n is n o t d i s t i n g u i s h a b l e f r o m t h e a c t i o n of s o d i u m t h i o s u l f a t e , t h e ozone c o n t e n t c a l c u l a t e d i s l o w . W i t h severe c o n t a m i n a t i o n b y s m o k e , " n e g a t i v e " ozone v a l u e s a r e f o u n d . A s these c o n t a m i n a n t s w i t h s t a n d a b r i e f h e a t i n g t o 150° C . w h i l e ozone u n d e r g o e s d e c a y , t h e t r u e ozone c o n t e n t c a n be m e a s u r e d b y difference. Such measurements, orig-
/
EXPOSED
SOLUTION
CONTROL
Ir—If—li—Χ—X—x—
NONEXPOSED SOLUTION
Λ Τ
Δ=2,4χΙ
lh
Λ
