Deposition and Stripping Properties of Mercury on Iridium Electrodes

T h e r e d u c t i o n of m e r c u r y on a f resh i r i d i um sur face is r eve r s ib l e w i th a ha l f -wave p o t e n t i a l of +162 m V (vs. SCE). A Hg(O) m o n o l a y e r is f o r m e d by u n d e r p o t e n t i a l d e p o s i t i o n (UPD) at + 600 m V d u r i n g t h e in i t ia l s cans of a r epe t i t i ve cyclic v o l t a m m o g r a m . Th i s m o n o l a y e r acts as b u l k Hg(O) d u r i n g f u r t h e r cycles, a n d t he r e d u c t i o n ha l f -wave po t en t i a l t h e n shi f t s t o w a r d s +410 mV. A n o d i z a t i o n of t he e l ec t rode at a p o t e n t i a l > + 1000 m V r e t u r n s t he r e d u c t i o n wave to its o r ig ina l posit ion . T h e p e a k for t h e o x i d a t i o n of b u l k Hg(O) f r o m t h e su r f ace o c c u r s a t +430 m V f o l l o w e d b y a p e a k for t h e u n d e r p o t e n t i a l s t r i p p i n g (UPS) of t he Hg(O) m o n o l a y e r at + 650 inV. The q u a n t i t y of Hg(O) for t h e m o n o l a y e r c o r r e s p o n d s to a 40% cove rage of t he Ir surface. T he u n d e r p o t e n t i a l sh i f t for t he b u l k a n d m o n o l a y e r s t r i p p i n g of Hg(O) f rom I r was f o u n d to b e 210 inV. No i n d i ca t i ons of a Hg-so lub le c o m p o u n d f o r m a t i o n b e t w e e n Hg a n d Ir we re found . T h e a p p l i c a t i o n o f m e r c u r y f i lm e l e c t r o d e s to s p e c i a t i o n o f t r a c e e l e m e n t s in n a t u r a l w a t e r s is p r e s e n t l y l i m i t e d e i t h e r d u e to t h e b a d s t a b i l i t y or r e p r o d u c i b i l i t y of t h e Hg f i lm in v a r y i n g e x p e r i m e n t a l c o n d i t i o n s (e.g., H g f i lms o n g l a s s y c a r b o n ) or to t h e d i s s o l u t i o n o f t h e s u b s t r a t e in H g a n d t h e f o r m a t i o n of i n t e r m e t a l l i c c o m p o u n d s w i t h t h e t e s t m e t a l i on (e.g., f i lms o n Pt) . A d e t a i l e d s t u d y of m a n y s u b s t r a t e s (1) s h o w e d t h a t t h e s e p r o b l e m s c o u l d b e o v e r c o m e b y u s i n g I r as a s u b s t r a t e . U n f o r t u n a t e l y , a l t h o u g h t h e e l e c t r o c h e m i c a l b e h a v i o r of m e r c u r y o n s u b s t r a t e s s u c h as p l a t i n u m , s i lve r , gold , n i c k e l , a n d c a r b o n is we l l d o c u m e n t e d in t h e l i t e r a t u r e , t h e s a m e c a n n o t b e s a id for i r i d i u m . D u r i n g t h e p a s t 30 years , o n l y a few s t u d i e s h a v e d e a l t w i t h m e r c u r y depos i t i o n o n i r i d i u m , a n d t h e s e w e r e m a i n l y c o n c e r n e d w i t h t h e b e h a v i o r of m e r c u r y as a p o i s o n for h y d r o g e n a n d o x y g e n a d s o r p t i o n o n t h e s u r f a c e o f t h e e l e c t r o d e (2-4). We r e p o r t h e r e t h e r e s u l t s of a s t u d y of t h e d e p o s i t i o n a n d s t r i p p i n g p r o p e r t i e s of m e r c u r y a t a n i r i d i u m r o t a t i n g d i s k e l e c t r o d e ( I r -RDE) . T h e a i m of t h i s w o r k w a s to q u a n t i f y t h e p a r a m e t e r s r e q u i r e d to o p t i m i z e t h e f o r m a t i o n a n d s t a b i l i t y of a t r u e m e r c u r y f i lm o n a n i r i d i u m s u b s t r a t e . T h i s s t u d y w as t h e r e f o r e d i r e c t e d to t h e d e t e r m i n a t i o n of (i) t h e r e d u c t i o n c h a r a c t e r i s t i c s of Hg( I I ) o n Ir , (ii) t h e o x i d a t i o n c h a r a c t e r i s t i c s of Hg(O) f r o m Ir, a n d (iii) t h e i n t e r a c t i o n s of t h e I r s u r f a c e w i t h H g in t e r m s of i t s s o l u b i l i t y , t h e u n d e r p o t e n t i a l of d e p o s i t i o n / s t r i p p i n g , a n d w o r k f u n c t i o n d i f f e r ences . Experimental Ins t rumenta t ion . -The e l e c t r o c h e m i c a l s y s t e m h a s b e e n p r e v i o u s l y d e s c r i b e d (1). I t c o n s i s t s of a m i c r o c o m p u t e r c o n t r o i l e d p o t e n t i o s t a t ( M o t o r o l a E X O R s e t + T a c u s s e l PRG5) , a c u s t o m b u i l t m e d i u m e x c h a n g e / f l o w t h r o u g h P l e x i g l a s cell, a n d a n i n v e r t e d p o l a r i z i n g m i c r o s c o p e (Lei tz E P I V E R T ) a l l o w i n g in-situ o b s e r v a t i o n a n d p h o t o g r a p h y . T h e w o r k i n g e l e c t r o d e a s s e m b l y w a s a T a c u s s e l EDI -55442 r o t a t i n g d i s k e l e c t r o d e (RDE) w i t h a 1 c m T e f l o n t ip i n to w h i c h a 2 m m d i a m • 10 m m l o n g I r cyl ind e r ( H e r a e u s G m h H ) w as p r e s s f i t ted. T h e e l e c t r o d e was r o t a t e d a t 1500 r p m d u r i n g t h e e x p e r i m e n t s . T h e c o u n t e r e l e c t r o d e w a s a P t rod. T h e r e f e r e n c e e l e c t r o d e , to w h i c h a l l p o t e n t i a l s a re r e f e r e n c e d , w a s a S C E (238 m V vs. N H E ) f i l led w i t h a s a t u r a t e d NaC1 s o l u t i o n a n d e q u i p p e d w i t h a 0.1M NaNO3 b r idge . Electrode pre trea tment . -The e l e c t r o d e w as i n i t i a l l y p o l i s h e d w i t h d i a m o n d s p r a y p a s t e s o f d e c r e a s i n g s ize, t h e s m a l l e s t o f t h e m b e i n g 1 ~ m u n t i l a m i r r o r l i k e su r f ace c o u l d b e s e e n u n d e r t h e m i c r o s c o p e a t 250 t i m e s . T h e e l e c t r o d e was t h e n w a s h e d w i t h d e i o n i z e d w a t e r a n d c a t h o d i z e d a t 2 0 0 0 m V for 5 r a i n in d e g a s s e d 1.0M HNO3. T h e e l e c t r o d e w a s k e p t in t h i s s o l u t i o n u n t i l t r a n s f e r r e d to a t e s t so lu t ion . *Electrochemical Society Active Member. 1Present address: Depar tment of Chemistry, State Universi ty of New York at Buffalo, Buffalo, New York 14214. Reagents . -Unless o t h e r w i s e i n d i c a t e d , al l c h e m i c a l s u s e d w e r e a n a l y t i c a l r e a g e n t g r a d e (Merck) . T h e Hg( I I ) u s e d for f i lm f o r m a t i o n , was p r e p a r e d b y d i s s o l u t i o n of t r i p l y d i s t i l l e d m e r c u r y in n i t r i c ac id , f o l l o w e d b y d i lu t i o n w i t h 0.1M HC104 to g ive 10-3M Hg(II) , u n l e s s o the r w i se i n d i c a t e d . All s o l u t i o n s w e r e p r e p a r e d w i t h 18 mol/gt w a t e r f r o m a Mi l l i po re b r a n d ion e x c h a n g e sy s t em. H i g h p u r i t y (99.95%) n i t r o g e n gas (Ca rbagas ) was u s e d to p u r g e o x y g e n f r o m s o l u t i o n s a n d to m a i n t a i n a n i t ro g e n b l a n k e t d u r i n g e x p e r i m e n t a t i o n . Results and Discussion General considerations.--One of t h e s i m p l e r a n d m o r e d i r e c t m e t h o d s for i n v e s t i g a t i n g t h e i n t e r a c t i o n s o f Hg a n d t h e Ir s u r f a c e is to e m p l o y cyc l ic v o l t a m m e t r y , u s i n g a Hg(I I ) s o l u t i o n a n d a " d e f i n e d " i r i d i u m sur face . S u c h a