D A Roman and L Rivera

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Mar ine Che m i s tr y , 38 (1992) 165-184 165Elsevier Science Publishers B.V., Amste rdam

T h e b e h a v i o u r o f a C u ( I I ) i o n s e l e ct iv e e le c t ro d ei n se aw a t er ; c o p p e r c o n s u m p t i o n c a p a c it y a n dc o p p e r d e t e r m i n a t io n sD . A . Rom~in and L. Rivera

Departamento d e Qulmica, F acultad de Ciencias Bdsicas, Universidad de Antofagasta, C ampusColoso, Casilla 170, Antofaga sta, C hile(Received 3 May 1990; revision accepted 31 January 1992)

ABSTRACTRom~in, D.A. and Rivera, L., 1992. The behaviour of a Cu(II ) ion selective electrode in seawater;copper consumption capacity and copper determinations.M ar. Chem., 38:165-184.

Determinations of copper consumption capacity (CuCs.C) and labile copper concentrations insurface coastal seawater, using a copper ion selective electrode (Cu-ISE) potentiometric method un-der predominantly diffusive conditions, are reported. For evaluation of the copper concentrations,the points of the endpoint contiguity zone of the CuCs.C titrat ion curve were treated by an ISE mul-tiple standard addition technique. The results were compared with those obtained by means of aChelex- 100 (calcic form) 'batch' procedure-potentiometric stripping analysis.The labile copper of the sample was determined at concentrations down to 10.70 nM with an aver-age RSD of 12%, independent of the Cu-ISE employed. For adjacent subsamples, the mean CuCs.Cvalues obtained for El Way seawater were equivalent to 81.05 and 48.00 nM copper, with an RSD of4 and 7%, and for Isla Santa Mafia seawater the value was equivalent to 70.27 nM copper, with anRSD ef 7%. The theoretical approach of the electrode diffusive mechanism proposed, which woulddepend, fundamentally, on the adsorptive, complexing and reducing properties of the dissolved or-ganic matter in the seawater sample, allows simultaneous analytical determination of CuCs.C andlabile copper concentration in seawater.

INTRODUCTIONIn copper(II) potentiometric measurements using a copper ion selectiveelectrode (Cu-ISE) with a membrane composed of pure copper sulphides andcopper(II) sulphide mixed with silver sulphide, generally the electrode be-haviour is non-Nernstian in real analytical samples (Lewenstam et al., 1985 ).

This has been observed in attempts to determine the copper concentration inseawater samples (Jasinsky et al., 1974; Oglesby et al., 1977). Thus, in spiteCorrespondence to: D.A. Rom~in, Departamento de Quimica, Facultad de Ciencias B~isicas,Universidad de Antofagasta, Campus Coloso, Casilla 170, Antofagasta, Chile.

0304-4203/92/$05 .00 1992 Elsevier Science PublishersB.V. All rights reserved.


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16 6 D.A. ROM,6aN AND L. RIVERA

o f a g r e e m e n t a m o n g t h e p o t e n t i a l s m e a s u r e d , t h e a n a l y t i c a l r e s ul ts o b t a i n e da r e n o t v e r y re l ia b l e , a n d a c c o r d i n g t o Z i r i n o a n d S e l i g m a n ( 19 81 ) s h o u l d b ec o n s i d e r e d a s a p p r o x i m a t e .T h e n o n - N e r n s t i a n r e s p o n s e o f t h e C u - I S E i n s e a w a t e r is d u e , f u n d a m e n -ta l ly , t o the p rese nce o f ch lo r ides (Og lesby e t a l ., 1977; W es ta l l e t a l ., 1979;Z i r i n o a n d S e l i g m a n , 1 98 1 ) , a n d i s m a n i f e s t e d a s t h e d e p e n d e n c e o f t h e m e a -s u r e d p o t e n t i a l o n t h e r a t e o f s ti r r in g o f t h e s a m p l e s o l u t i o n ( J a s i n s k y e t a l. ,1 97 4; Z i r i n o a n d S e l i g m a n , 1 9 81 ) . T h i s p r o b l e m h a s d i m i n i s h e d t h e i n t e r e s ti n th e s e d e v i c e s a s a n a l y t i c a l to o l s , p a r t i c u l a r l y i n c h l o r i d e - a n d b r o m i d e - e n -r i c h e d s a m p l e s , b u t a t t h e s a m e t i m e h a s s t i m u l a t e d s t u d i e s o n t h e C u - I S Er e s p o n s e m e c h a n i s m i n t h e p r e s e n c e o f i n t e r f e r e n c e ( L e w e n s t a m e t a l. , 1 98 7 ) ,a n d i n t h e p r a c t i c a l a n a l y t i c a l u t i l i z a t io n o f t h e s u p e r - N e r n s t i a n e l e c t r o d e s l o p ei n v o l v e d in a s e m i - e m p i r ic a l o r m o d i f i e d N e r n s t e q u a t i o n ( L e w e n s t a m e t a l.,1 9 8 5 ) .T h e h a l i d e i n t e r f e re n c e is m a n i f e s t e d t h r o u g h p a r a s i t ic r e a c t i o n s t h a t o c c u ro n t h e e l e c t r o d e s e n s o r s u rf a c e. I n e l e c t r o d e s w i t h p u r e C u2 S m e m b r a n e s t h em o s t i m p o r t a n t r e a c t i o n s h o u l d b e ( H e p e l , 1 98 2; L e w e n s t a m e t al ., 19 85 )C u2 S + C u 2+ + 2 n X - ~ C u S + 2 C uX ~ - n ( 1 )i f t h e i n f l u e n c e o f r e d o x p r o c e ss e s i s n o t c o n s i d e r e d . I n a n o t h e r a p p r o a c h , t h ep r e d o m i n a n t r e a c t i o n s s h o u l d b e r e d o x p ro c e s s e s w i t h p a r t i c i p a t i o n o f t h em a t e r i a l o f t h e m e m b r a n e e l e c t r o d e ( e.g . W e s ta l l e t a l. , 1 97 9; L e w e n s t a m e ta l . , 1985) :C u S + C u 2+ + 2 n X - ~ 2 C u C I ~ - n + S ( 2 )T h e p r e s e n c e o f e l e c t ro a c t i v e A gC 1 h a s a ls o b e e n c o n f i r m e d i n t h e e l e c t r o d em e m b r a n e s u r f a c e ( L e w e n s t a m e t a l., 1 9 8 5 ) . A c c o r d i n g t o W e s t a ll e t a l.( 1 9 7 9 ) , t h e n o n - N e r n s t i a n Ag2S/CuSm e m b r a n e e l e c t r o d e r e s po n s e i n c h l o-r i d e - c o n t a i n i n g m e d i a i s d u e t o a o n e - e l e c t r o n tr a n s f e r b e t w e e n C u a t t h ee l e c t r o d e s u r fa c e ( p r e s e n t a s a n i m p u r i t y ) a n d C u 2+ i n s o l u t i o n , f o r m i n g c u -p r o u s i o n . H o w e v e r , t h e s e th e o r e t i c a l c o n s i d e r a t i o n s a c c o u n t f o r a r a n g e o fc o n c e n t r a t i o n s t h a t a r e f a r g r e a t e r t h a n t h e r a n g e o f c o p p e r c o n c e n t r a t i o n sf o u n d i n s e a w a t e r , a n d a c c o u n t f o r n e i t h e r t h e n o n - N e r n s t i a n C u ( I I ) -I S E b e -h a v i o u r i n t h e t r a n s i ti o n z o n e b e t w e e n t h e f o r m a l r e sp o n s e d o m a i n s o f t h ee l e c t r o d e s ( n a m e l y , 2 9 a n d 5 9 m V p e r d e c a d e ) n o r t h e s u p e r - N e r n s t i a n r e -s p o ns e s ( o v e r t h e 5 9 m V p e r d e c a d e re s p o n se d o m a i n ) .M e a s u r e m e n t o f th e s o - c al le d c o p p e r c o m p l e x i n g c a p a c it y ( C u C C ) is hi g h lyc o n t r o v e r s i a l, b u t t h e p o t e n t i a l i m p o r t a n c e o f t h i s c o m p l e x i n g p a r a m e t e r i sa c k n o w l e d g e d i n t h e f i e ld o f d i s s o l v e d m e t a l s p e c i a t i o n ( H i r o s e a n d S u g i-m u r a , 1 98 5; B u c k l e y a n d V a n d e n B e r g , 1 98 6; H e r i n g e t a l. , 1 9 8 7 ) . C o p -p e r ( I I ) i s g e n e r a l ly c h o s e n f o r d e t e r m i n a t i o n o f s e a w a t e r c o m p l e x i n g o r t h em e t a l b i n d i n g c a p a c i t y , Lx, b e c a u s e C u ( I I ) f o r m s r e l a ti v e ly s t ab l e c o p p e rc o m p o u n d s w i t h t h e l i g a n d s p r e s e n t a n d b e c a u s e i t m a y b e d e t e r m i n e d b y


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Cu(II) -ION SELECTIVE ELECTRODE BEHAVIOUR 167n u m e r o u s a n a l y t i c a l m e t h o d s . H o w e v e r , t h e s e m e t h o d s a l l h a v e t h e o r e t i c a la n d p r a c t i c a l l i m i t a t i o n s (K ra m e r , 1 9 86 ; M o f fe t t a n d Z i k a , 1 98 7; H e r i n g e ta l . , 1987) .T h e s e a w a t e r C u C C is d e f i n e d as t h e a b i li ty o f th e s a m p l e t o r e m o v e t h ea d d e d c o p p e r f r o m t h e p o o l o f m e t a l i o n s , re p r e s e n ti n g , f u n d a m e n t a l l y , t h et o t a l o rg a n i c li g a n d c o n c e n t r a t i o n , LT. T h e re fo r e , th i s c o n c e p t mu s t b e c l e ar l yd i f f e r e n t i a te d f r o m t h e ' c o p p e r c o n s u m p t i o n c a p a c it y ' ( C u C s . C ) , t h a t is t os ay , t h e a m o u n t o f m e t a l c o n s u m e d d i r e c tl y b y t h e s a m p l e , e x p r e s se d in m o l e spe r l i tr e .C u ( I I ) - I S E p o t e n t i o m e t r i c t e c h n i q u e s ar e s u i ta b l e f o r d e t e r m i n i n g t h eC u C C a n d c o p p e r - b i n d i n g p r o p e r t ie s o f s a m p l e s o l u ti o n s p o o r i n c h l o ri d e s,s u c h a s f r e s h w a t e r s a n d r e c o n s t i t u t e d s o l u t i o n s c o n t a i n i n g i s o l a t e d n a t u r a lo rga n ic l i gands f rom so i l an d a lga l exu da tes (Buf f l e e t al ., 1977; B resn aha n e ta l. , 1978 ; Mc K nig h t an d M ore l , 1979; S t e ll a an d G anze r i l -Va len t in i , 1979;Su n d a a n d H a n s o n , 1 97 9; M c K n i g h t a n d M o re l , 1 98 0; Sa a r a n d W e b e r , 1 98 0;Buff l e e t a l. , 1980 ; G am ble e t al ., 1980; Caba n i ss an d S hum an , 1986; F i sha n d M o re l , 1 98 5; Sw e i le h e t al ., 1 9 8 7 ) . H o w e v e r , i t h a s b e e n r e p o r t e d t h a tt h e s e me t h o d s a r e n o t s u i t a b l e i n s e a w a t e r b e c a u s e o f a s t r o n g C u - ISE i n t e r -f e r e n c e f ro m t h e h i g h c h l o r i d e i o n c o n c e n t r a t i o n (B u f f l e e t a l. , 1 98 0; W a i t ean d M ore l , 1983 ; Lu nd , 1986; Caban i ss an d Sh um an , 1986; F i t ch e t a l. , 1986;S w e i l eh e t a l., 1 9 8 7 ) , a n d t h e i r r e l a t e d p r o b l e m s a s m e n t i o n e d a b o v e . N e v e r -t h e le s s , W e s t al l e t a l. ( 1 9 7 9 ) h a v e s h o w n t h a t i n u n s t i r r e d s e a w a t e r s a mp l e ss p i k e d w i th N T A , w h i c h h a v e b e e n a l l o w e d t o a p p r o a c h e q u i l i b r i u m i n t h er e g i o n n e x t t o t h e e l e c t ro d e s u rf a c e, i t i s f ea s i b le to m e a s u re t h e t o t a l c o p p e rc o n c e n t r a ti o n a n d t h e C u C C .I d ea ll y , t r ac e m e t a l s p e c i a t io n m e a s u r e m e n t s s h o u l d b e c o n d u c t e d i n s i tu ,t h e r e b y p r e v e n t i n g a n y a l t e ra t i o n o f th e d y n a m i c n a t u ra l s t at e o f t h e m e t a ls p e c ie s , a n d a n y r e a l i s ti c d i s c u s s i o n mu s t a l s o b e c o n c e rn e d w i t h t h e ro l e o fp a r t ic u l a te m a t t e r ( A n d r e a e , 1 9 8 6 ) . T h e i d e a l a n a ly t ic a l m e t h o d w o u l d d e-t e rm i n e s p e c i e s d i r e c tl y i n s i tu , n o n - i n v a s i v e l y a n d n o n -d e s t ru c t i v e l y , b u t t od a t e a l l m e t h o d s e m p l o y e d ar e i n v a s i v e , t o v a r y i n g e x t en t s , a n d f r e q u e n t lyt h e i r v a l i d it y is o n l y o p e r a ti o n a l . T h e t h e o r y b e h i n d t h e s e m e t h o d s i s p o o r lyu n d e r s t o o d (B e rn h a rd e t al. , 1 9 8 6 ) .T h e s t u d ie s o f C u -I S E r e s p o n s e i n s e a w a t e r h a v e b e e n l i m i t e d , f u n d a m e n -t al ly , t o t h e i n t e r f e r e n c e e f f e c t o f t h e c h l o r i d e i o n . O n t h e b a s i s o f p r e v i o u sda ta (W es ta l l e t a l. , 1979; Le w en s tam e t a l. , 1985 ) , t he fo l low ing in t e r fe renc em e c h a n i s m s h a v e b e e n p r o p o s e d f o r v a r i o u s ty p e s o f C u -I S E :Ag2 S + Cu 2+ + 4 n X - ~ 2 A g X ~ n + 2 C u X ~ - n + S ( 3 )C u S + C u 2+ + 2 n X - ~ 2 C u X ~ - ~ + S ( 4 )Cu2 S+ 2Cu 2+ + 4 n X - ~ 4 C u X ~ - ~ + S ( 5 )T h e s e r e a c t io n s e x p l a i n t h e i n t e r f e r e n c e p r o c e ss e s o n t h e m e m b r a n e s u rf ac e ,a n d t h e p o o r r e p ro d u c i b i l i t y a n d i n s t a b i li t y o f th e p o t e n t i a l r e s p o n s e , a s t h e


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168 D . A . R O M A N A N D L . R I V E R Ar es u lt o f th e f o r m a t i o n o f e l e m e n t a l s u lp h u r. T h e a m o r p h o u s s u l p h u r f o r m e dc o u l d m e c h a n i c a l l y b l o c k t h e e l e c t r o d e su r f ac e a n d m a y c a u s e t h e i rr e v e rs i -b i l it y o f re a c t i o n s ( 3 ) , ( 4 ) a n d ( 5 ) . I t b e c o m e s o b v i o u s t h a t d i r e c t d e t e r m i -n a t i o n o f c o p p e r i n t ti e p r e s e n c e o f h i g h c o n c e n t r a t i o n s o f c h l o r i d e o r b ro -m i d e i o n s is im p o s s i b l e i n s t ir r e d s o l u ti o n s . H o w e v e r , th e C u - I S E r e s p o n s ew i t h o u t s t i rr i n g a n d u n d e r t h e i n fl u e n c e o f s o m e r e d u c i n g a g e n t ( S m i t h a n dM a n a h a m , 1 97 3; W e s t a ll e t a l. , 19 79 ; L e w e n s t a m e t a l ., 1 9 85 ) m a y o v e r c o m ec e r t a i n i n t e r f e r e n c e e ff ec ts , a n d i m p r o v e t h e r e p r o d u c i b i l i t y a n d t h e s ta b il i-z a t i o n o f th e p o t e n t i a l m e a s u r e d . I n u n s t i r r e d c h l o r i d e - c o n t a i n i n g s o l u ti o n s ,d i f fu s i o n to a n d f r o m t h e e l e c t r o d e s p r e d o m i n a t e s . T h e r e f o r e , t h e e l e c t ro d em e c h a n i s m s w h i c h c o n t r o l t h e C u - I S E r e s p o n s e i n n a t u r a l s e a w a t e r , w h e r em a s s t r a n s p o r t o c c u r s f u n d a m e n t a l l y b y m e a n s o f d if f u si o n , a re n o t y e t f u ll yu n d e r s t o o d .S m i t h a n d M a n a h a n ( 1 9 7 3 ) h a v e re p o r t e d th a t th e a t t a i n m e n t o f s ta b lep o t e n t i a l s i n ta p - w a t e r a n d f r e s h w a t e r s a m p l e s r e q u i r e d t h e p r e s e n c e o f a m i l dr e d u c i n g a ge n t , a n d t h e y u s e d f o r m a l d e h y d e . F u r t h e r , Z i r i n o a n d S e l ig m a n( 1 98 1 ) s h o w e d a ' p o l a r o g r a p h i c ' b e h a v i o u r o f C u ( I I ) - I S E i n s e a w a t e r. T h ec o p p e r ( I ) e l e c t r o d e f u n c t i o n o f C u - I S E h a s a ls o b e e n d e m o n s t r a t e d ( N e s h -k o v a a n d S h e y t a n o v , 1 98 5 ) . O t h e r w i s e , t h e e f f e c t s o f d i s c r e t e o r g a n i c l ig a n d ss u c h a s E D T A ( e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d ) o n C u - I S E re s p o n s e in n o n -m a r i n e s a m p l e s , i n w h i c h t h e l i g a n d is i n ex c es s, h a v e s h o w n a b n o r m a l b e -h a v i o u r o f t h e e l e c t r o d e ( F i t c h e t a l. , 1 98 6 ) .

T h e s t at e o f k n o w l e d g e o f t h e C u - I S E b e h a v i o u r i n s e a w a t e r is l im i t e d ; t h ea p p l i c a t io n s u n d e r c o n d i t i o n s w i t h s t i r ri n g a r e h a r d l y r e li a b le a n d ' d o n o t r e p-r e s e n t t h e b e h a v i o u r o f c o p p e r i n s e a w a t e r. F o r i n s t a n c e , t h e r e su l ts o b t a i n e db y R o s e n a n d W i ll ia m s ( 1 9 7 8 ) , w h i c h d e m o n s t r a t e d t h a t m a r i n e f u lv ic a c i di s o l a t e d f r o m c o a s t a l s e a w a t e r d o e s n o t c o m p l e x C u 2+ i o n s i n s e a w a t e r , a r ei n c o n t r a d i c t i o n t o t h e a c t u a l s ta t e o f k n o w l e d g e . W e k n o w t h a t c o p p e r o r -g a n i c sp e c i e s e x i s t i n m a r i n e w a t e r s a n d t h a t t h e y c o r r e s p o n d t o a s ig n i f i c a n tf r a c ti o n o f t h e t o t a l d is s o l v e d c o p p e r c o n c e n t r a t i o n o f s e a w a t e r ( S u n d a a n dH a n s o n , 1 9 8 7 ) . S e l e c t iv e m o l e c u l a r c o m p o n e n t s o f t h e s e a w a t e r d i s s o l v e do r g a ni c m a t t e r ( D O M ) r e sp o n s ib l e fo r c o p p e r c o m p l e x a t i o n ha v e be e n o n lyp a r t ia l ly i d e n t i f i e d a n d c h a r a c t e r i z e d . S o m e l o w e r m o l e c u l a r w e i g h t ( m o l e c -u l a r w e i g h t l e s s t h a n 2 2 0 ) t r a c e c o m p o u n d s , s u c h a s f r e e a m i n o a c i d s a n dc a r b o h y d r a t e s , a l s o h a v e t h e r e q u i r e d o r g a n i c f u n c t i o n s t o c o m p l e x m e t a l( S i m o e s G o n c a l v e s a n d V a l e n t a , 1 98 2; S i m o e s G o n c a l v e s e t a l. , 19 83 ; H a a s ,1 9 8 6 ) , b u t t h e d e g r e e to w h i c h t h e y c o m p l e x m e t a l i o n s i n s e a w a t e r is n o tk n o w n . D i s s o l v e d m a c r o m o l e c u l a r h u m i c s u b s ta n c e s c o n s t i t u t e a s i g n if i ca n tf r a c ti o n o f t h e u n c h a r a c t e r i z e d D O M . T h e s e s u b s t a n c e s a r e r e l a ti v e ly h y d r o -p h o b i c a n d a r e g e n e r a ll y c o n s i d e r e d t h e m o s t i m p o r t a n t c o p p e r c o m p l e x i n gc o m p o u n d s o f c o a s t al s e a w a t e r ( R a s p o r e t al. , 1 9 8 4 ) . T o d a t e , h o w e v e r , t h ei n f l u e n c e o f h u m i c s u b s t a n c e s o n t h e I S E r e s p o n s e h a s n o t b e e n a d e q u a t e l ye x a m i n e d ( F i t c h e t a l. , 1 9 8 6 ).L e w e n s t a m e t al . ( 1 9 8 5 ) h a v e s h o w n t h a t in th e p r e s e n c e o f a n a d e q u a t e


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Cu(II) -ION SELECTIVE ELECTRODE BEHAVIOUR 169complexing and reducing reagent, which binds Cu (II), Cu (I) and Ag (I) morestrongly than chloride and bromide, the exchange reactions at the membraneCu-ISE-solution interphase are stabilized, permitting the analytical utiliza-tion of the super-Nernstian slope electrode in determining he copper concen-tration of the sample. However, faced with an eventual application in seawa-ter, it is important to determine whether the total concentration or that of aparticular copper species is being determined.We propose a Cu-ISE potentiometric research method, by means of which,in unstirred membrane-filteredsurface coastal seawater and at the natural pHof the samples, it is feasible to determine the CuCs.C and the labile dissolvedcopper concentration of the samples. For this last goal, the copper additionsat the CuCs.C titration endpoint contiguity zone were treated by an ISE mul-tiple standard addition technique. The copper concentrations thus obtainedwere compared with the results obtained by a Chelex-100 (calcic form ) 'batch'procedure-potentiometric stripping analysis (PSA). A description of thesuper-Nernstian electrode behaviour, considering the influence of seawatercomplexing organic substances on Cu-ISE response, and the practical utiliza-tion of the super-Nernstian slope electrode are also proposed and discussed.THEORY

The Cu-ISE behaviour in seawater under unstirred conditions would befundamentally determined by surface-active properties of the seawater DOM.The membrane electrode would behave in a similar way to certain seawatersuspended solids (Hunter, 1980; Plavsic et al., 1987; Zutic and Tomaic, 1989 )and its adsorptive interactions with DOM at the membrane electrode-sea-water interphase would occur in such a way that membrane solubilizationshould be disfavoured. For some time it has been acknowledged hat the char-acteristic surface charges exhibited by solids in seawater are due to adsorbedorganic constituents (Neihof and Loeb, 1974). In particular, dissolved poly-meric substances have the required properties for this interaction (Hunter,1980). Proteinaceous and humic substances derived from the degradation ofnatural products of marine origin are surface-active polymers carrying a netnegative charge at the pH of seawater (Neihof and Loeb, 1974). The organiccoating of electrodes made from other materials, such as platinum surfaces,immersed in natural seawater has also been shown (Kristoffersen et al., 1982 ).Recently, Hars~inyi et al. (1987) have shown a composition change of theCu (II)-ISE membrane in solutions containing chloride ions; this finding in-dicates chloride uptake into the membrane phase at low copper ion concen-tration if the solution is not stirred. Also, a few studies have considered theredox properties of fulvic and humic materials (Skogerboe and Wilson, 1981;Moffett and Zika, 1983; Millero, 1985; Moffett et al., 1985).In the electrode mechanism proposed below, the rate production of Cu (I)


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170 D.A. ROM,~N AND L. RIVERAc o u l d b e i n c r e a s e d b y o r g a n i c l ig a n d - m e t a l c h a r g e t r a n s f e r p r o c e ss e s a t th ee l e c t r o d e m e m b r a n e s u r f a c e , w h i c h c o u l d b e c a t a l y se d b y c h l o r i d e io n . C u ( I I )a n d C u ( I ) w o u l d b e s t a b i li z e d b y c o m p l e x a t i o n . A l s o , e f f ec t s o f t h e i o n i c s a lt so n t h e s t a b il i ty o f p o l~ c m e ri c s e a w a t e r D O M c o u l d b e i m p o r t a n t i n t h is a p -p r o a c h , s i m i l a r t o t h e i r e f fe c ts o n t h e c o n f o r m a t i o n a l s ta b il it y o f m a c r o m o -l e c u le s s u c h as p r o t e i n s ( D a m o d a r a n a n d K i n s e ll a , 1 98 1 ) .T h e b a s is o f a ll th e s o -c a ll ed s t r i p p i n g t e c h n i q u e s is t h e p r e c o n c e n t r a t i o n -m e a s u r e m e n t s e q u e n c e w i th a w o r k i n g e le c tr o de ; th u s a n u m b e r o f p a p e r sh a v e d e s c r i b e d v a r i a t i o n s o f t h is a p p r o a c h , i n w h i c h a n a l y te p r e c o n c e n t r a -t i o n i s c a r r i e d o u t a t c h e m i c a l l y m o d i f i e d e l e c t r o d e s w h o s e s u r f a c e s h a v e t h ea b i l i t y t o r e a c t w i t h a n d b i n d t h e a n a l y t e ( P r a b h u e t a l . , 1 9 8 7 ) . C o n s e -q u e n t l y , n o p o t e n t i a l n e e d s t o b e a p p l i e d . S o m e t h i n g s im i l a r c o u l d o c c u r w i t ht h e C u - I S E i m m e r s e d i n n a t u r a l s e a w a t e r, a s a r e su l t o f th e i n t e r a c t i o n b e -t w e e n t h e e le c tr o de m e m b r a n e a n d t h e D O M o f t he m e d i a .

I n th e b u l k o f t h e s e aw a t e r, D O M m u s t b e p r e s e n t f re e a n d a s m e t a l c o m -p l e x e s, i n c l u d i n g o f c o u r s e th e c o p p e r c o m p l e x e s , w h e r e a s i n t h e c o p p e r s e le c -t iv e m e m b r a n e s u r fa c e u n d e r u n s t i r re d m e a s u r e m e n t c o n d i t io n s , c o p p e r ( I )w o u l d b e f o r m e d a n d s h o u l d b e c o m p l e x e d i m m e d i a t e l y b y th e a d s o r b e d o r -g a n i c l i g a n d o n t h e e l e c t r o d e m e m b r a n e . C h l o r i d e i o n s s h o u l d c a t a l y s e t h ee l e c tr o n t r a n sf e r to w a r d s C u ( I I ) f r o m l ig a n d L a d s o r b e d o n t h e p o l a ri z e de l e c tr o d e m e m b r a n e , r e s u lt in g i n s u p e r - N e r n s t i a n e l e c t ro d e r e s p o n se s . S u c hc h l o r i d e e f fe c ts h a v e b e e n s h o w n i n v o l t a m m e t r i c m e a s u r e m e n t s o f C u ( I I )o r g a n i c i n t e r a c t i o n i n e s tu a r i n e w a t e r s ( N e l s o n ( 1 9 8 5 a , b ) a n d r e f e r e n c e s c i t e dh e r e i n ) , i n w h i c h C uC I~ - is i n v o l v e d a ls o . T h e f o l l o w i n g s i m p l i f i e d m o d e lr e a c ti o n s c o u l d a c c o u n t f o r th e m e c h a n i s m p r o p o s e d :C u S ( s ) + L ( a q ) -+ C u S - L ( s ) ( 6 )C u S - L ( s ) + C u 2+ ( a q ) ~ C u S - L - C u 2+ ( s) ( 7 )C u S - L - C u 2+ ( s ) + 2 C I - ( a q ) --, C u S - L - C u C 1 2 ( s ) ( 8 )S i m i l a r m o d e l r e a c t i o n s w o u l d a c c o u n t f o r t h e c o p p e r s e l e n i d e e l e c t r o d em e m b r a n e b e h a v io u r .

T h e p r e s e n c e o f a n o n - a d s o r p t i v e a n d n o n - r e d u c i n g s tr o n g c u p r i c c o m p l e x -i n g li g a n d m i g h t a f fe c t th i s m e c h a n i s m o n l y i f t h e l ig a n d i s ab l e t o o u t - c o m -p e t e c h l o r i d e i o n f o r c o n t r o l o f c o p p e r a t t h e e l e c t r o d e s u r f a c e .

F o r m a l l y , th e c o p p e r i o n s e le c ti v e e l e c t ro d e h a s t w o m o d e s o f r e sp o n s e : 2 9m V p e r d e c a d e i n th e a b s e n c e o f c h l o r id e , a n d 5 9 m V p e r d e c a d e i n m e d i a o fh i g h c h l o r i d e c o n c e n t r a t i o n ( W e s t a l l e t a l. , 1 9 7 9 ) . I f t h e e l e c t r o d e i s i n t h isl a st m o d e , i t i s i m p o s s i b l e to m e a s u r e c u p r i c i o n a c t iv i t y ; o n l y c o p p e r i n th ef o r m o f c u p r o u s i on s c a n be m e a s u r e d . C o n s e q u e n t l y , t h e n o n - N e r n s t i a n r e -s p o n s e s w o u l d c o m p r i s e t h e e l e c t ro d e r e s p o n s e i n t h e t r a n s i t io n z o n e b e t w e e nt h e t w o a b o v e - m e n t i o n e d d o m a i n s a n d t h o s e th a t o v e r c o m e t h e 5 9 m V p e rd e c a d e d o m a i n , w h i c h a re a l so c a ll e d s u p e r - N e r n s t i a n . .


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Cu (II)-ION SELECTIVE ELECTRODE BEHAVIOUR 171In the results given below, the Cu-ISE responses in coastal seawater aresuper-Nernstian. The above remarks pertain to unstirred seawater, which has

been allowed to approach equilibrium in the region next to the electrode sur-face. The Cu-ISE super-Nernstian response would depend fundamentally onthe 'primary quality' of the seawater DOM, which generally is in excess insurface coastal seawater with respect to the copper concentration of the me-dia. In this work we assumed that the copper reduction at the electrode sur-face affects the labile dissolved copper concentration only; thus theCuCl~- -L complex concentration equals the labile fraction of the metal in thesample. Otherwise, the endpoint of the seawater copper titration is inter-preted as the 'copper consumption capacity' of the sample, because other re-actions, such as displacement metal reactions, could be involved also. Forexample, calcium might already be bound to the ligand and be replaced bycopper. The adsorptive, complexing and chloride-catalysed reducing proper-ties of DOM in seawater would minimize or eventually eliminate the Cu-ISEmembrane corrosion caused by halides. Typical CuCs.C titration curves arepresented in Fig. 1. The copper additions considered at the endpoint titrationcontiguity zone, for the evaluation of the labile copper concentration by thestandard addition technique, are indicated.Before the titration equivalent point, copper speciation would be con-trolled in the bulk solution by DOM and in the interphase by eqns. ( 6 )- ( 9 ).After the titration equivalence point, although the concentration of cupric ionis high enough, the electrode process could be represented also by the reactionCuS-L-CuCI~- (s) + Cu E+ + 2C1- ~ CuS-L-CuCI~- (s) + CuC12 (9)

C u 1 5 0I 1 3 0IS 1 1 0E

v s 9 0SC 7 0E

5 0 0

E ( m Y )

I I , I I I I5 1 0 1 6 2 0 2 8 3 0 3 6V x E -3 ( m l )

Z - 1 08

6

4204 0

* A B + COrlon 9 4 - 2 9 ( A , E l W a y ; B , I s l a 8 t o .M a r ia ) a n d Ra d io m e t e r F - 1 1 1 2 ( C , E lW a y ) C u - 1 8 E were uoed.

Fig. 1. Seawater CuCs.C titrat ion and multiple standard addition dete rmina tion of copper.


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172 D.A. ROMANAND L. RIVERAB a s e d o n t h e d i f fu s i o n m o d e l a n d i t s a s s u m p t i o n s (L e w e n s t a m e t a l., 1 98 5 ) ,a n d c o n s i d e r i n g t h e e l e c t r o d e m e c h a n i s m o u t l i n e d i n t h i s w o r k , t h e f o l lo w i n g

w o r k i n g N e r n s t e q u a t i o n f o r th e p o t e n t i a l s o f t h e C u - I S E i n s e a w a t e r u n d e ru n s t i r r e d c o n d i t i o n s i s p ro p o s e d :Co.E = ' c o n s t a n t ' + S s N lo g A [ C uC s .C ] (~ /k )2 ( 1 0 )

T h i s e q u a t i o n w a s e m p l o y e d i n t h e l i n e a r i z a t io n o f th e e n d p o i n t c o n t i g u it yz o n e o f t h e C u C s . C t i tr a t i o n c u r v e . C c , is t h e u n k n o w n c o p p e r c o n c e n t r a t io n ;SsN th e e x p e r i m e n t a l s u p e r -N e rn s t i a n e l e c t ro d e s l o p e o b t a i n e d f ro m E v s. p C up l o t s ( p C u = - l o g [ C u 2+ ]aaded; s e e F ig . 2 ) ; A i s a n o p e ra t i o n a l p a r a m e t e r d e -f i n e d a s t h e i n v e r s e o f th e S s N / S N r a t io ( k ) , i n t h a t SN i s t h e 2 9 .5 m V p e rd e c a d e N e r n s t i a n m o d e e l e c t r o d e r e s p on s e ; th e ' c o n s t a n t ' t e r m r e p r e s en t s th ec o n d i t i o n a l p o t e n t i a l o f th e e l e c t r o d e s y s te m i n t h e e x p e r i m e n t a l c o n d i t i o n sd e s c r i b e d , r e l a t e d to t h e p o t e n t i a l o f t h e s e a w a t e r s a m p l e E .P u r s u i n g a r g u m e n t s s i m i l a r t o t h o s e o f S m i t h a n d M a n a h a n ( 1 97 3 ) , b u tc o n s i d e r i n g t h e a b o v e s u p e r -N e rn s t i a n fo c u s , t h e fo l l o w i n g m u l t i p l e s t a n d a rda d d i t i o n l i n e a r f u n c t i o n w a s a s s u m e d :10E n - E O / S S N - 1 = Z - 1 - A [ C u C s . C ] ( 1 / k ) 2 x C S X V S ( 11 )

Vo X Cc.T h i s e q u a t i o n i n d i c a t e s t h a t, i n th i s c a s e, t h e s e a w a t e r l a b il e c o p p e r c o n c e n -t r a ti o n , C cu , m a y b e p r e fe r a b ly d e t e r m i n e d f r o m t h e s l o p e o f a p l o t o f Z - 1

E vs 8 C E ( m Y )1 6 01 4 01 2 0l O 08 06 O4 o2 o

I !6 6 . 6 7 7 . 6 8p i C . ] ( u )

- ' * - A n B * COrion 04-2 g ( A E l W R y ; B , I o i a 8 l a .Maria) ~ RadkHItelor F-11'1~ (C, ElWay) C u-1 8E m u n d .

Fig. 2. Seawate r CuCs .C t i t ra t ion endp o in t con t igu i ty Cu-ISE po ten t ia l s . Di f fus ive cond i t ions .


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Cu (If)-ION SELECTIVE ELECTRODE BEHAVIOUR 17 3

v s. V s w h e r e s e v e ra l s t a n d a r d a d d i t i o n s a r e m a d e . E n a re t h e C u - I S E m e a -s u r e d p o t e n t i a l s i n t h e 110 s u b s a m p l e w h e n t h e c o p p e r s t a n d a r d a d d i t i o n v o l-u m e s ( V s ) o f o n e c o p p e r s ta n d a r d s o l u t io n ( C s ) h a v e b e e n m a d e . k w o u l dr e p r es e n t t h e C u ( I I ) - I S E e x p e r i m e n t a l N e r n s t i a n d e v i a t i o n , a n d w e s u g ge stt h a t k c o u l d b e in s o m e w a y a s s o c i a t e d w i t h th e p r i m a r y c h e m i c a l p r o p e r t i e so f th e s e a w a t e r D O M r e s p o n s i b l e f o r t h e e l e c t r o d e p r o c es s a ls o. A w o u l d b er e l a t e d to t h e d i f f u s i o n c o e f fi c i en t s o f C u ( I ) a n d C u ( I I ) c o m p l e x e s , D ' a n dD " r e s p e ct i v el y , w h i c h c a n n o t b e d e t e r m i n e d e x a c t l y b e c a u s e o f la c k o f v a l u e sf o r th e d i f f u s io n c o e f f i c i e n ts o f c o p p e r f re e i o n s i n s e a w a t e r m e d i a . I f t h eC u ( I I ) - D O M m e m b r a n e s u rf ac e f o r m e d c o m p l e x a n d t h e C u ( I I ) - D O M b u l ks o l u ti o n c o m p l e x c oe x is t a t th e C u - IS E m e m b r a n e - s e a w a t e r i n t e rp h a s e , t h e nt h e f l u xe s o f b o t h c o m p l e x e s s h o u l d b e e q u a l . T h e r e f o r e , w e c a n w r i te t h ef o l l o w i n g e q u a t i o n i f 1 : 1 s t o i c h i o m e t r i c r a t i o s a r e c o n s i d e r e d :D " ( [ C u ( I I ) L ] - [ C u ( I I ) L ] s ) = D ' ( [ C u ( I ) L ] - [ C u ( I ) L ] s ) ( 12 )I f a n a p p r o a c h s u c h a s t h a t o f L e w e n s t a m e t a l. ( 1 9 8 5 ) i s c o n s i d e r e d , A i sa p p r o x i m a t e d b y t h e e x p r e ss i onA=qO'/O' ( 1 3 )i n w h i c h q i s a f i tn e s s p a r a m e t e r t h a t s h o u l d r e f le c t t h e d e v i a t i o n o f D"/D'c a u s e d b y t h e D O M c o m p l e x a t i o n o f C u ( I ) a n d C u ( I I ) , r es p ec ti ve ly . T h isb e i n g t h e c a s e , i n t h e a b s e n c e o f c o m p l e x i n g l i g an d , t h e D"/D' r a t io i s 0 .45( L e w e n s t a m e t a l. , 1 98 5 ) ; t h e n q m a y b e e v a l u a t e d i f A h a s p r e v i o u s l y b e e nf o u n d .E X P E R I M E N T A L M E T H O D S

ApparatusP o t e nt ia l m e a s u r e m e n t s w e r e m a d e w i t h a R a d i o m e t e r ( C o p e n h a g e n ) p Hm e t e r 26 ( e x p a n d e d s c a l e) w i th a R a d i o m e t e r K -7 01 d o u b l e j u n c t i o n r ef er -

e n c e e l e c tr o d e . E q u i l i b r a t i o n p o t e n t i a l r e a d i n g s w e r e t a k e n i n u n s t i r r e d s o lu -t i o n s w h e n t h e r a t e o f p o t e n t i a l d r if t w a s le s s t h a n 1 m V h - 1. T w o c o p p e r i o ns e l e c t i v e e l e c t r o d e s w e r e u s e d : t h e R a d i o m e t e r S e l e c t r o d e ( C o p e n h a g e n )m o d e l F - 11 1 2 ( Cu ~ .a S e) a n d t h e O r i o n ( C a m b r i d g e , M A , U S A ) m o d e l 94 -2 9 A ( C u S / A g 2 S ) c u p r i c i o n s e l e c ti v e e l e c t ro d e . T h e w o r k i n g e le c t r o d e s w e r ep o l i s h e d b e f o r e e a c h C u C s . C t i t r a t io n r u n , a c c o r d i n g t o t h e i n s t r u c t i o n s s u p -p l i e d b y R a d i o m e t e r a n d O r i o n . A l l m e a s u r e m e n t s w e r e m a d e a t 2 5 _+ 0 .5 Ci n R a d i o m e t e r V - 5 3 3 a n d V - 5 3 5 g la ss c e ll s t h e r m o s t a t t e d b y m e a n s o f w a t e rc i rc u l a ti n g ( H a a k e ( B e r li n , F R G ) c i r c u la t o r m o d e l F K ) t h r o u g h th e c el lj a c k e t, a c c o m m o d a t e d o n a R a d i o m e t e r T T A - 8 0 t i tr a t io n a s se m b l y. T h e c u -p r i c ISE r esponds to l igh t and 02 (Wes ta l l e t a l . , 1979 ; Buf f l e e t a l . , 1980 ;Z i r i n o a n d S e l ig m a n , 19 81 ) , s o m e a s u r e m e n t s w e r e r u n u n d e r c o n s t a n t i ll u -


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174 D.A. ROMAN AND L. RIVERAm i n a t i o n ( a s m a l l a m o u n t o f m e t h y l e n e b lu e c h l o ri d e w a s a d d e d t o th e c i r-c u l a t in g w a t e r ) a n d O 2 -f re e c o n d i t i o n s . N2 f re e f r o m o x y g e n , p u r g e d t h r o u g ha 1 M N a O H s a t u r a t e d p y r o g a l lo l s o l u ti o n , w a s p a s s e d o v e r t h e s o l u t i o n a tl o w r at e s t o m i n i m i z e g a s fl o w i n d u c e d c i r c u la t i o n o f t h e s a m p l e s o l u t i o nd u r i n g a ll t h e m e a s u r e m e n t s u n t i l t h e e q u i l ib r a t i o n p o t e n t i a l w a s r e a ch e d .

A t t h e s t a rt o f e a c h s er ie s o f m e a s u r e m e n t s t h e I S E w a s p o l i s h e d a n d t h e nt r e a t e d a s f o ll o w s : i t w a s k e p t f o r 1 h i n a s ti r r e d 1 X 1 0 - 3 M E D T A s o l u t i o n ,p r e p a r e d i n K N O 3 s o l u t i o n w i t h a c o n d u c t i v i t y a t 2 5 C e q u i v a l e n t t o s ea w a -t e r ( i o n i c s tr e n g t h b u f f e r ) , f o r 0 .5 h i n a s ti r re d E D T A - c o p p e r s o l u t i o n(4X l0 -6 M C u 2+ a n d 6 X l0 -6 M E D T A ) , t h e n f o r 0 .5 h w i t h o u t s ti r ri n g i nt h is s a m e s o l u t io n , a n d f in a ll y w i t h d e i o n i z e d w a t e r . T h i s t r e a t m e n t w a s a d -e q u a t e f o r t h e O r i o n C u - I S E i n a ll m e a s u r e m e n t s o f e a ch o f th e C u C s . C t it ra -t io n s . H o w e v e r , f o r t h e R a d i o m e t e r C u - I S E i t w a s b e tt e r to e m p l o y a c o p p e r -E D T A s o l u t io n ( 6 1 0 - 6 M C u 2+ a n d 4 1 0 - 6 M E D T A ) , f o r th e c o p -p e r ( I I ) a d d i t i o n s a f t e r th e e n d p o i n t z o n e o f t h e C u C s . C t i t ra t io n . A f t e r e ac hm e a s u r e m e n t o f t h e s e ri es o f c o p p e r ( I I ) a d d i t i o n s ( f iv e to s e v e n p e r C u C s . Ct i tr a t i o n ) , t h e C u - I S E m e m b r a n e s w e r e c l e a n e d w i t h p i e ce s o f fe lt w h i c h h a dp r e v i o u s l y b e e n w a s h e d . W h e n n o t i n u s e t h e C u - I S E e l e c t ro d e s w e r e s to r e dd r y .

T h e o u t e r e l e c tr o d e c o m p a r t m e n t o f th e c a l om e l d o u b l e - j u n c t io n e le c tr o d ew a s f i ll e d w i t h K N O 3 i o n i c s t r e n g t h b u f f e r; s e a w a t e r w a s u s e d i n s t e a d o f t h eK N O 3 s o l u t i o n i n s o m e C u C s . C t i t r a t i o n e x p e r i m e n t s . S a t u r a t e d K C I a n dK N O 3 s o l u t i o n s w e r e p r e p a r e d f r o m K C1 a n d K N O 3 s o l u ti o n s t h a t h a d p r e -v i o u s ly b e e n p a s s e d t h r o u g h a b e d o f C h e l e x - 1 00 r e si n ( B i o - R a d , R i c h m o n d ,C A , U S A ) , p o t a s s i u m f o r m , t o r e m o v e tr a c e m e t a ls . W h e n n o t i n u s e t h ec a l o m e l d o u b l e - j u n c t i o n e l e c tr o d e w a s s t o r e d i n K N O 3 i o n i c s tr e n g t h b u f fe rs o l u t i o n .

F o r p o t e n t i o m e t r i c s t ri p p i n g a n a ly s is a R a d i o m e t e r I S S -8 2 0 a n a l y se r w a su s e d. R a d i o m e t e r T T A - 8 0 t i t r a ti o n u n i ts e q u i p p e d w i th R a d i o m e t e r V - 5 34a n d V - 5 4 0 g l a ss ce ll s w e r e e m p l o y e d a s e l e c t r o c h e m i c a l c e ll s. T h e w o r k i n ge l e c t ro d e w a s a R a d i o m e t e r F - 3 5 0 0 g l as sy c a r b o n e l e c t ro d e . A d o u b l e - j u n c -t i o n A g / A g C 1 e l e c t r o d e ( O r i o n 9 0 - 02 w i t h f il t e re d s e a w a t e r in t h e o u t e rc h a m b e r ) w a s u s e d a s a r e f e re n c e e le c t ro d e , a n d a p l a t i n u m f o il ( R a d i o m e t e rP - 1 3 1 2 ) w a s u s e d a s c o u n t e r e l e c t r o d e .

T h e c o p p e r a d d i t i o n s w e re m a d e w i t h a d ig i ta l B r a n d ( W e r t h e i ry - G l a -s h ti tt e, G e r m a n y ) T r a n s f e r p e t t o r m i c r o p i p e t t e .T h e s e a w a t e r a n d K N O 3 i o n i c s t r e n g th b u f f e r s o l u t io n w e r e c h e c k e d i n a

R a d i o m e t e r C D M 2 e c o n d u c t i m e t e r w it h o n e C D C 1 04 s ta n d a r d c ell a t 25 C .A l l l a b w a r e a n d p o l y e t h y l e n e b o t tl e s u s e d f o r h a n d l i n g t h e s e a w a t e r s a m -

p l es w e r e c l e a n e d w i t h E x t r a n M A 01 ( M e r c k ) d e t e r g e n t s o l u ti o n , r i n s e d w i t hd i s t i ll e d w a t e r , s o a k e d o v e r n i g h t in 6 M H C 1 , r i n s e d w i t h d i s t il l e d w a t e r ,s o a k e d o v e r n i g h t i n 0 .0 1 M E D T A , r i n s e d w i t h d is t il le d w a t e r a n d o r g a n icm a t t e r f re e d e i o n i z e d w a t e r .


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Cu(II)-ION SELECTIVE ELECTRODE BEHAVIOUR 175

Reagent and chemicalsI n C u - IS E m e a s u r e m e n t s , a ll C u ( I I ) a d d i t i o n s w e r e m a d e f r o m d i l u te d 0 .0 1M C u ( N O 3 ) E . X 3 H 2 0 ( M e r c k , D a r m s t a d t, F R G ) ( p .a . ) s to c k s o lu t io ns t a n d a r d i z e d a g a in s t E D T A . C h e m i c a l s u s e d i n P S A c o p p e r d e t e r m i n a t i o n s ,e x c e p t H g ( I I ) n i t r a t e ( p . a . ) , w e re o f Su p ra p u r g r a d e (M e rc k ) , a n d a ll C u ( I I )a d d i t i o n s w e r e m a d e f r o m c o n v e n i e n t l y d i l u t e d C u ( I I ) M e r c k T i tr is o l a m -p o u l e s . A l l o t h e r c h e m i c a l s w e re r e a g e n t g r a d e . D i s t i l le d w a t e r p a s s e d t h ro u g ha C o m i n g ( N e w Y o r k , U S A ) 3 5 08 h i g h - p u r it y d e m i n e r a l i z e r c a r tr id g e w a su s e d i n a ll t h e w o rk .

Seawater samplesSu r f a c e c o a s ta l s e a w a t e r s a m p l e s w e re c o l l e c t e d b y h a n d w i t h 2 1 p o l y e t h -y l e n e b o t t le s a t t w o l o c a t io n s , E 1 W a y a n d I s la Sa n t a M a r i a , o f f t h e c o a s t o fA n t o fa g a st a . T h e s a m p l e s w e r e i m m e d i a t e l y f il te r e d ( 0 .4 5 /~ m ) t h r o u g h a c id -w a s h e d S a r to r io u s ( G o t t i n g e n , G e r m a n y ) S M - 1 11 m e m b r a n e fi lt er s o f 4 7m m d i a m e t e r ( N u c l e o p o r e ( P l e a s e n t o n , C A , U S A ) p o l y c a r b o n a t e f i l t e r -h o l d e r ) . F o r e a c h 1 1 s u b s a m p l e a n e w f i l te r w a s u s e d . T h e f i r st 1 00 ml o ff i lt r a te w a s u s e d t o r i n s e t h e r e c e i v i n g f la s k a n d w a s t h e n d i s c a rd e d . T h e s u b -s a m p l e s w e r e i m m e d i a t e l y s u b j e c te d t o t h e a n a l yt ic a l p r o c e d u r e s.

ProceduresCuCs. C D titrationsT h i r t y m i l l il it r es o f f il te r e d s e a w a t e r w e r e d i s p e n s e d i n t o a t h e r m o s t a t t e dc e l l p l a c e d i n t h e T T A -8 0 t i t r a t i o n u n i t , w h i c h w a s s t i r r e d u n t i l a c o n s t a n tp o t e n t i a l v a r i a t io n w a s r e a c h e d ( t h is t o o k a p p r o x i m a t e l y 1 h ) , a n d o n e c o p-p e r ( I I ) d i lu t e d s t a n d a rd s o l u ti o n a d d i t i o n w a s m a d e ( 4 . 0 - 1 2 . 0 / A o f ( 1 . 5 -2 . 0 ) 1 0 -4 M c o p p e r s t a n d a rd s o l u t i o n ) w i t h c o n t i n u o u s s t ir r i n g fo r a p p ro x -

i ma t e l y 1 h u n t i l a c o n s t a n t p o t e n t i a l v a r i a t i o n w a s r e a c h e d . T h e s t i rr i n g w a st h e n s t o p p e d , l e a v in g t h e s a m p l e u n d i s t u r b e d u n t i l th e o p e r a t i o n a l e q u il ib r a -t i o n t i m e w a s r e a c h e d . E a c h m e a s u r e m e n t o f e v e r y C u C s . C t i t r a ti o n w a s m a d ei n a s i m i l a r m a n n e r , a d d i n g c o p p e r s o l u t i o n t o s e p a ra t e 3 0 m l a l iq u o t s o ff i lt e r e d s e a w a t e r s u b s a m p l e s ( t w o t i me s , t h r e e t i me s th e f i rs t c o p p e r a d d i t i o nv o l u m e , a n d s o o n ) . A s t h e e q u i l ib r a t i o n p o t e n t i a l t i m e s w e r e l on g , a n d t om i n i m i z e t h e e f fe c ts o f c o p p e r a d s o r p t i o n o n t h e c o n t a i n e r w a ll , t h e m e a s u r e -m e n t s w e r e n o t m a d e o n t h e s a m e 3 0 m l s e a w a te r a l iq u o t . F u r t h e r m o r e , o n l yd u p l ic a t e s o f C u C s . C t i tr a t i o n p e r s a m p l e w e r e c o n s id e r e d . T h e s a m p l e w a sm a i n t a i n e d a t 4 - 5 C d u r i n g t h e t i m e o f a n a ly s is . T h e C u C s . C t i tr a t i o n e n d -p o i n t v o l u m e s w e r e f o u n d b y t h e s e c o n d d e r iv a t iv e m e t h o d .Copper concentration by ISEU s i n g e q n . ( 11 ) , t h e l a b i le d i s s o l v e d c o p p e r c o n c e n t r a t i o n i n t h e s e a w a t e r


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176 D.A. ROMA N AND L. RIVERAw a s e v a l u a t e d b y a m u l t i p le s t a n d a r d a d d i t i o n p o t e n t i o m e t r i c t e c h n i q u e( S m i t h a n d M a n a h a n , 1 9 7 3 ). T h e C u C s . C ti t ra t io n e n d p o i n t v o l u m e co n ti -g u i ty z o n e p o t e n t ia l m e a s u r e m e n t s a n d t h e r e s p e c t iv e s u p e r - N e r n s t i a n s lo p ew e r e c o n s i d e r e d . T h e p o t e n t i a l o f t h e c o p p e r e l e c tr o d e c o m p a r e d w i th t h a t o ft h e r e f e r e n c e e l e c t r o d e o f t h e s a m p l e s o l u t i o n , E0 , w a s e v a l u a t e d b y e x t r a p o -

F I L ~ S IgA ~I~I~

~ c u " d e te m in e d ~ C ~ L ~ -I M (C a )~m

C a C s . C a n d C . Delia,minedCUl ~ t h e ~ ( I I ) -I S E m t l ~ lp rO lm e d n t h i s ~ k . (b )

C m t a k e n f o r t h eC Ur e s i n , Determined

Fig. 3. Experim ental approach. (a) represents the total dissolved copper conc entration. (b)represents the labile dissolvedcopper concen tration.


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Cu (II)-ION SELECTIVE ELECTRODE BEHAVIOUR 17 7lation from a least-squares adjusted plot of E vs. copper addition volumes ofa diluted copper standard solution for the appropriate seawater CuCs.C titra-tion endpoint volume contiguity zone.

Copper concentration by PSAAliquots (40 ml ) of filtered acidified seawater (0.015 M HNO3, Suprapurgrade) were employed to determine the total dissolved copper concentrationand the copper concentration of the metal not taken up by the resin in thesamples, following the experimental conditions of Jagner and Aren (1979)and Jagner et al. ( 1981 ). The plating times chosen were 3 h 4 min and themeasurements were made in separate aliquots, to minimize the effects of metaladsorption on the container walls. The copper concentrations were evaluatedby means of the normal standard additions equations (Bader, 1980).

Chelex- 1O0 (calcic o rm ) batch procedureThese experiments were performed in a manner similar to that of Figuraand McDuffie (1979), except that in this case 1 1 seawater subsamples and5.2 g of resin were employed. Ca-Chelex was prepared according to the pro-cedure of MacKey ( 1983 ), employing HNO3, HC1 and CaCIz, Suprapur grade(Merck). The Ca-Chelex resin was rinsed with deionized water until Ca (II)assay with calcein in the wash solution produced negative results.In Fig. 3 the experimental approach of this work is shown.RESULTS AND DISCUSSION

The CuCs.C and Ccu results for the surface coastal seawater under studyare shown in Tables 1 and 2, assuming a 1 : 1 copper ligand stoichiometricratio. These results represent the total copper consumption for the seawatersubsamples, and the dissolved labile copper concentrations in the media, re-spectively. The F test shows that there are no significant differences betweenthe results obtained by the Orion 94-29 A (CuS/Ag2S) and the RadiometerF-I 112 (CUl.aSe) Cu-ISE. Figure 2 shows linear plots for the copper addi-tions at the endpoint titration contiguity zone corresponding to the casesshown in Fig. 1. Table 3 shows the experimental SsN slope values and thecalculated parameters for and from eqn. (10), including the evaluation of qfrom eqn. (13) determined using the data reported in Table 2. It appears thatthe values of the q parameter obtained show that the DOM copper complex-ation in E1 Way and Isla Santa Maria seawater is determined by the organicligands inherent to each medium, and is independent of the Cu-ISE employed(see Table 3 ).The copper concentrations found in surface coastal seawater subsamplesdetermined by means of the Cu-ISE method developed in this paper wouldcorrespond to the labile copper fraction of total dissolved metal. In Table 4


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178 D.A. ROMAN AND L. RIVERAT A B L E 1C o p p e r ( I I ) c o n s u m p t i o n c a p a c i ty d e t e r m i n a t i o n r e s ul ts f o r s u r fa c e c o a s ta l s e a w a t er f r o m E l W a y a n dIs l a S a n t a M a r i aS e t s a m p l e s C u C s . C S D R S D

( n M ) ( % )1 ( 0 9 / 2 4 ) a 1 65 .32 ( 1 0 / 0 1 ) 7 8 .7 3.3 4.13 ( 1 0 / 0 3 ) 8 3 .44 ( 1 1 / 0 7 ) b 7 0 .85 ( 1 1 / 1 2 ) 5 0 .4 3 . 4 7 . 16 ( 1 1 / 1 4 ) 4 5 .67 ( 1 1 / 2 5 ) c 8 8.18 ( 0 1 / 1 1 ) 6 6 . 19 ( 0 1 / 1 3 ) 6 9 .2 4 . 8 6 .8

1 0 ( 0 1 / 1 5 ) 7 5 .5a A R a d i o m e t e r F - 1 1 1 2 C u - I S E w a s e m p l o y e d i n m e a s u r e m e n t s o n s u b s am p l e s 1 - 3 ( E l W a y ) .b A n O r i o n 9 4 - 2 9 C u - I S E w a s e m p l o y e d i n m e a s u r e m e n t s o n s u b s a m p l e s 4 - 6 ( E l W a y ) .C An O r i o n 9 4 - 29 C u - I S E w a s e m p l o y e d i n m e a s u r e m e n t s o n s u b s a m p l e s 7 - 1 0 ( I s la S a n t a M a r i a ) .N u m b e r s i n p a r e n t h e s e s i n d i c a t e t h e d a t e o n w h i c h t h e s a m p l e w a s r e m o v e d ; s a m p l e s w e r e a n al y s edi m m e d i a t e l y .

T A B L E 2P r i m a r y d a t a a n d c o p p e r d e t e r m i n a t i o n r e s u lt s fo r s u r fa c e c o as t a l se a w a t er , o b t a i n e d b y m e a n s o f C u -I S E s ta n d a rd a d d i t i o n p o t e n t i o m e t r i c m e t h o dS e t s a m p l e s E ( m V ) S u p e r - N e r n s t i a n k C cu

s u b s a m p l e s l o p e ( S sN ) (SsN/SN) (nM)1 (0 9/ 24 ) a 85 .6 66.1 2 .2 10.702 (1 0 / 01 ) 99 .5 82 .0 2 .8 8 .033 (1 0 / 03 ) 92 .0 80 .6 2 .7 8 .184 (1 1/ 07 ) b 118.3 67.1 2 .3 7 .555 (1 1 / 12 ) 97 .4 72 .5 2 .5 10 .076 (1 1 / 14 ) 97 .5 73 .0 2 .5 9 .927 ( 11/ 25 ) 175.0 132.6 4 .5 3 .938 (0 1/ 1 1) 134.0 147.7 5 .0 5 .989 (0 1/ 13 ) 116.5 114.6 3 .9 8 .50

10 (0 1/ 15 ) 128.1 132.5 4 .5 7 .40" A R a d i o m e t e r F - 1 11 2 C u - I S E w a s e m p l o y e d i n m e a s u r e m e n t s o n s a m p l e s 1 - 3 ( E l W a y ) .b a n O r i o n 9 4 - 29 C u - I S E w as e m p l o y e d in m e a s u r e m e n t s o n s a m p l e s 4 - 6 ( E l W a y ) .C An O r i o n 9 4 - 29 C u - I S E w a s e m p l o y e d i n m e a s u r e m e n t s o n s a m p l e s 7 - 1 0 ( I s la S a n ta M a r i a ) .N u m b e r s i n p a re n t h e s e s i n d i c a t e t h e d a t e o n w h i c h t h e s a m p l e w a s r e m o v e d ; s a m p l e s w e r e an a l y s edi m m e d i a t e l y .

a r e p r es e n t e d th e r e su l ts o f s u b j e c ti n g t h e s u b s a m p l e s to t h e C u - IS E m e t h o da n d a C h e l e x - 1 0 0 ( c a l c i c f o r m ) b a t c h p ro c e d u re . T h e m a j o r c o m p o s i t i o n s o fth e su b sam p le s ar e g ive n in Tab le 5 (R am ir e z , 1986 ; M i f io an d Torr es , 198 6 ) .


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Cu (II)-ION SELECTIVEELECTRODE BEHAVIOUR 179TABLE3Experimental a nd calcu lated parameters for and from eqn. (10)Samples Consta nt SsN A qa, b (mV)

1 -8 3. 2 66.1 1/2.2 1.012 - 104.9 82.0 1/2.8 0.793 -9 5. 9 80.6 1/2.7 0.824 -1 12 .9 67.1 1/2.3 0.975 - 106.3 72.5 1/2.5 0.896 -9 5. 5 73.0 1/2.5 0.897 - 176.9 132.6 1/4.5 0.498 - 171.5 147.7 1/5.0 0.449 -1 48 .7 114.6 1/3.9 0.5710 - 166.7 132.5 1/4.5 0.49

R a d i o m e t e r F-1112 Cu-ISE was employed in measureme nts on samples 1-3 (El Way). An Orion 94-29 Cu-ISE was employed in measur ement s on samples 4-10 (4-6, El Way; 7-10, Isla Santa Maria ).TABLE4Copper conc entra tion results for surface coastal seawater, de ter m inedby means of Cu-ISE potenti om-etry and pot entio metri c stripping analysisS et samples E1 Waya E1 Wayb Isla SantaMaria bCcu Cu-ISE (n M) 8.97 9.12 6.45

+ SD 1.04 0.82 0.99RSD (%) 12 9 15Ccu PSA (n M) 33.99 - 29.43batch _+ SD 4.72 - 6.77e ff luent RSD (%) 13.9 - 23.0Ccu (c) PSA (n M) 42.97 - 35.25+ SD 4.41 - 4.88

RSD (%) 10.3 - 13.8aA Rad iom ete r F-I 112 Cu-ISE was employed. (See Table 2. )bAn Orion 94-29 Cu-ISE was employed. (See Table 2. )CTotal dissolve d copper concentrat ion . (For further explanation , see the text .)

C o n s e q u e n t l y , a p p r o x i m a t e l y 2 0 % o f t h e t o t a l d i s s o l v e d c o p p e r c o n c e n t r a -t ion o f t h e su b samp le s i s p ar t o f t h e so - c a l l e d lab i l e me t a l f r ac t ion , a l t h ou g hi t is p o s s i b le t h a t s o m e c o p p e r s p e c i e s o f m e d i u m l a b i li t y ar e in c l u d e d a l s o .T h e e f f lu e n t s o f th e C h e l e x - 1 0 0 b a t c h p r o c e d u r e w e r e te s t e d u s i n g t h e C u - IS Et e c h n iq u e , w i t h n e g at ive r e su l ts , wh ic h f Qr th e r c on f i r m s t h e f in d in g s p r e v i -o u s ly m e n t i o n e d .A c c or d in g t o ou r r e su l t s , some b od ie s o f su r f ac e c oas t a l s e awat e r o f t h eA n t o f a g a s t a r e g io n s h o w a tr a n s i e n t e n r i c h m e n t i n c op p e r , p r e s u m a b l y a s ar e su l t o f u n t r e a t e d me t a l log e n ic was t e s f r om var iou s c op p e r in d u s t r y p lan t s .


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1 8 0T A B L E 5M a j o r c o m p o s i t i o n o f E1 W a y a n d I s la S a n t a M a r i a s e a w a t e r

D.A. ROM~.N AND L. RIVERA

E l W a y I s l a S a n t a M a r i aC h lo r in i ty ( % o) 19 .5 19 .5S a l in i t y ( % 0) 35 . 2 34 . 5A l k a l in i t y ( M 10 -3) 2 . 27 2 . 34pH 7 . 98 8 . 07C 1- ( g l - j ) 18 . 73 18 . 56S O ] - ( g l - l ) 2 . 7 4 2 . 6 5N a + ( g l - ~ ) 10 . 25 8 . 76K + ( g l - l ) 0 . 4 3 0 . 3 3C a 2+ ( g1 - 1 ) 0 . 44 0 . 37M g 2 + ( g l - j ) 1 .3 9 1 .3 3

A s w i ll b e s h o w n i n a l a te r p a p e r , i n s e a w a t e r su b s a m p l e s t a k e n a t t h e d u m ps it es , t h e C h e l e x - 1 0 0 b a t c h p r o c e d u r e - P S A s h o w s t h a t l es s t h a n 2 0 % o f t h et o ta l d i s s o l v e d c o p p e r c o n c e n t r a t i o n i s f o u n d i n a n o n - l a b il e m e t a l f o r m .R e g a r d le s s o f t h e m e t h o d e m p l o y e d t o d e t e r m i n e ' c o p p e r c o m p l e x i n g ca -p a c i t y ' , t h e r e s u l t s o b t a i n e d b y e a c h m e t h o d m u s t r e p r e s e n t t h e t o t a l l i -g a n d ( s ) c o n c e n t r a t i o n , L T, a v a il a b l e f o r t h e f o r m a t i o n o f s ta b le m e t a l s p e c i esi n t h e s e a w a t e r. A c c o r d i n g t o H i r o s e a n d S u g i m u r a ( 1 9 8 5 ) , t h e s e a w a t e r o r -g a n i c l i g a n d c o n c e n t r a t i o n o b t a i n e d b y t i tr a t i o n o f l i g a n d b y o n e m e t a l ( t h eM T m e t h o d ) s h o u l d b e h i g h e r t h a n t h e r e s ul ts s e c u r ed by m e a n s o f a m e t h o di n w h i c h t h e m e t a l s a r e t i t r a t e d b y l i g a n d s ( t h e L T m e t h o d ) . L T is , i n ge n e r a l,a d i ff ic u lt p a r a m e t e r t o m e a s u r e b y a n y m e t h o d , a n d t h e e x a m i n a t i o n o f th ee r r o r p r o p a g a t i o n e q u a t i o n l e a d s t o t h e o b s e r v a t i o n t h a t t h e l e a s t e rr o r r e su l tsf r o m m e t h o d s t h a t m o s t d i re c t l y m e a s u r e t h e C u C C ( F i s h a n d M o r e l , 1 98 5 ) .W e c o n s i d e r t h a t t h e d i r e c t m e t h o d s , a s d e v e l o p e d in t h is w o r k f o r m e a s u r i n gt h e ' c o p p e r c o m p l e x i n g c a p a c i ty ' o f s e a w a t e r, d e t e r m i n e t h e ' c o p p e r c o n -s u m p t i o n c a p a c i t y ' o f t h e s o l u b le f r a c ti o n o f s e aw a t e r . M a n y w o r k e r s h a v ed i s c o u r a g e d t h e us e o f t h e t e r m ' c o m p l e x a t i o n c a p a c i t y ' ( o f t e n u s e d s y n o n y -m o u s l y w i t h ' b i n d i n g c a p a c i t y ' ) b e c a u s e o f p o t e n t i a l a m b i g u i t i e s ( L u n d ,1 9 8 6 ) .

T h e C u C s . C , l ik e t h e C u C C ( N f i r n b e rg , 1 9 8 4 ) , c o u l d b e u s e d as a n o t h e rd i a g n o s t ic m e a s u r e o f c e r t a i n t ra c e m e t a ls i n n a t u r a l w a t e r . T h e C u C C is ac -k n o w l e d g e d n o w a s t h e t o ta l c o n c e n t r a t i o n o f d i s s o l v e d o r g a n ic l ig a n d , LT ,w h i c h is a s s o c ia t e d w i t h t h e f r a c t i o n o f t h e s e a w a t e r D O M c a p a b l e o f f o r m i n gs t ab l e m e t a l c o m p o u n d s w i t h t h e m e t a l c h o s e n t o ti t r a te t h e s a m p l e . I n m o r eg e n e r a l t e r m s , t h e C u C s . C m i g h t a l so b e c o n s i d e r e d a s a 's e a w a t e r m e t a l i n -t e r a c t i o n c a p a c i t y p a r a m e t e r ' , i n w h i c h t h e e v e n t u a l m e t a l d i s p l a c e m e n t re -a c t i o n s s h o u l d b e c o n s i d e r e d . T h u s , t h e C u C C m a y s i m p l y b e t h e p r i n c i p a lc o m p o n e n t o f a la r ge r C u C s . C . H o w e v e r , m o r e i n f o r m a t i o n is n e c es s a ry o n


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Cu( ll )-ION SELECTIVE ELECTRODE BEHAVIOUR 181the nature of the structural chemical bonding of the metal-DOM interac-tions, to clarify whether the coordination is simple or chelation is involved.

Our experiments indicate that Cu-ISE potentiometry, under conditions ofsuper-Nernstian electrode behaviour, can be useful for simultaneously meas-uring the CuCs.C and the labile Ccu in surface coastal seawater. However,these gains pertain to filtered, unstirred, and constant pH sample solutions,which have been allowed to approach equilibrium in the region next to theelectrode surface. Assays carried out in seawater subsamples contaminatedwith metallogenic wastes do not give the expected results. The principal prob-lem of the method proposed here is the long equilibration time and thereforethe potential loss of copper through surface adsorption, although naturalwaters enriched in humic materials are less exposed to loss of trace metalsduring storage (Heiden and Aikens, 1983 ).The electrode diffusive mechanism suggested would depend on the intrin-sic 'adsorptive, complexing and reducing properties' of the seawater DOM.Thus, only an exceptionally strong cupric complexing ligand, but with a poortendency to be adsorbed on the surface membrane electrode, would permitNernstian electrode behaviour. The data presented in this paper indicate thatthis is unlikely to occur in seawater.ACKNOWLEDGEMENT

This work was supported by FONDECYT/CONICYT Chile (grants 137/87 and 90/0600).

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18 2 D.A. ROM./~NAND L. RIVERAB u ff le , J ., D e l a d o e y , P ., Gr e t e r , F .L a n d H a e r d i , W . , 1 9 80 . S tu d y o f t h e c o mp le x f o r ma t io n o f

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184 D.A. ROMAN AND L. RIVERA( E d i to r s ) , C o m p le x a t io n o f T r a c e M e ta l s i n Na tu r a l W a te rs . M a r t i n u s N i jh o f f / D r . W . J u n k ,T h e Ha g u e , p p . 3 1 7 - 3 2 7 .

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