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Anion selectivity studies on liquid membrane electrodes
Analytica Chimica Acts, 65 (1973) 425-435
425
© Elsevier Scientific Publishing C o m p a n y , A m s t er d am - Pr i n t ed in T h e N e t h e r l a n d s
ANION SELECTIVITY STUDIES ON LIQUID MEMBRANE ELECTRODES
R O N A L D E. R E I N S F E L D E R a n d F R A N K L I N A. S C H U L T Z *
Department o f Chemistry, Florida Atlantic University, Boca Raton, Fla. 33432 (U.S.A.) (Received 28th N o v e m b e r 1972)
A n u m b e r o f ion-selective liquid m e m b r a n e .electrodes have b e e n described w h i c h r e s p o n d selectively to anions. These i n c l u d e electrodes for the m e a s u r e m e n t of i n o r g a n i c a n i o n s t-s, c a r b o x y l i c acid a n i o n s z'a'9, a m i n o acid a n i o n s 1° a n d anionic metal c h l o r i d e complexes 11. T h e liquid ion-exchange m e m b r a n e s o f these electrodes generally consist of a large extractable c a t i o n such as a t r a n s i t i o n m e t a l - p h e n a n t h r o line c o m p l e x or b u l k y q u a t e r n a r y a m m o n i u m i o n dissolved in a water-immiscible o r g a n i c solvent. A l t h o u g h a relatively large n u m b e r of usable electrodes h a v e been reported, a n d a c o m p r e h e n s i v e t h e o r e t i c a l t r e a t m e n t of liquid m e m b r a n e e l e c t r o d e b e h a v i o r has b e e n p r e s e n t e d I z. la, few experimental studies have b e e n m a d e o f the relationship b e t w e e n e l e c t r o d e selectivity a n d c o m p o s i t i o n o f the l i q u i d m e m b r a n e . Freiser e t al.lO. 14 h a v e found t h a t the selectivity of a m i n o acid a n i o n electrodes closely p a r a l l e l s the solvent extraction b e h a v i o r of the liquid phase as d e s c r i b e d b y the relative d i s t r i b u t i o n coefficients o f the a m i n o acid a n i o n - q u a t e r n a r y a m m o n i u m ion pairs. S c i b o n a e t al. 4" s have investigated the response o f t e t r a h e p t y l a m m o n i u m i o n - b e n z e n e electrodes to chloride, b r o m i d e a n d nitrate ion a n d have c o r r e l a t e d electrode selectivity to i o n - e x c h a n g e e q u i l i b r i u m constants, ion-pair f o r m a t i o n c o n s t a n t s a n d ionic mobilities. In this paper is r e p o r t e d a study o f electrode selectivity for the c o m m o n i n o r g a n i c a n i o n s C I - , B r - , I - , S C N - , N O 3 , C10~-, B F ? a n d P F ~ as a function of the i o n - e x c h a n g e molecule, its c o n c e n t r a t i o n in the m e m b r a n e p h a s e a n d the o r g a n i c solvent used to f o r m the liquid m e m b r a n e . T h e i o n - e x c h a n g e molecules investigated are the tris(1,10-phenanthroline) i r o n ( I I ) a n d tris(4,7-diphenyl-l,10p h e n a n t h r o l i n e ) i r o n ( I I ) c o m p l e x ions a n d t e t r a h e p t y l a m m o n i u m ion. The solvents investigated are n i t r o b e n z e n e , c h l o r o f o r m and n-amyl a l c o h o l . EXPERIMENTAL "
Reagents
All c h e m i c a l s used in this w o r k w e r e r e a g e n t - g r a d e materials w i t h the exception of s o d i u m f l u o r o b o r a t c . T e c h n i c a l - g r a d e s o d i u m f l u o r o b o r a t e ( M a t h e s o n , C o l e m a n a n d Bell) was recrystallized f r o m e t h a n o l - w a t e r . S t o c k s o l u t i o n s were p r e p a r e d f r o m these c h e m i c a l s with distilled water, a n d solutions c o n t a i n i n g 10- t_ * T o w h o m c o r r e s p o n d e n c e should be addressed.
426
R. E . R E I N S F E L D E R ,
F. A . S C H U L T Z
10 - 4 M o f the a p p r o p r i a t e a n i o n w e r e p r e p a r e d by serial dilution. O r g a n i c s o l v e n t s w e r e used w i t h o u t f u r t h e r purification.
Preparation o f liquid ion-exchange materials Tris (4,'7-diphenyl- 1 , 1 0 - p h e n a n t h r o l i n e ) iron (II) [ F e ( b p h e n ) a2 +] a n d tris ( 1,10p h e n a n t h r o l i n e ) i r o n ( I I ) [ F e ( p h e n ) 2+] liquid i o n - e x c h a n g e s o l u t i o n s were p r e p a r e d as n i t r a t e salts .by the following p r o c e d u r e . A p p r o x i m a t e l y 20 m m o l e s o f F e ( N H 4 ) z (SO4)z • 6 H 2 0 a n d a t h r e e f o l d excess o f the p h e n a n t h r o l i n e c o m p o u n d were d i s s o l v e d in 100 ml o f 0.1 M sulfuric acid. A q u e o u s h y d r o x y l a m i n e s o l u t i o n (1 rnl o f 10~/o) w a s a d d e d a n d the p H w a s a d j u s t e d to 5.0-5.5. T h e s o l u t i o n w a s t r a n s f e r r e d to a s e p a r a t o r y funnel, a n d a 200-fold a m o u n t o f solid s o d i u m n i t r a t e was dissolved in the solution. A 10.0-ml p o r t i o n o f the o r g a n i c solvent was a d d e d , a n d the c o m p l e x ion w a s e x t r a c t e d into t h e o r g a n i c phase. T h e o r g a n i c p h a s e was s e p a r a t e d a n d k e p t for use in the electrode. T h e c o n c e n t r a t i o n o f the c o m p l e x ion in the o r g a n i c s o l v e n t was determined spectrophotometrically. S o l u t i o n s o f t e t r a h e p t y l a m m o n i u m n i t r a t e in n i t r o b e n z e n e were p r e p a r e d by a p r o c e d u r e similar to that o f S c i b o n a et al. is Successive 10-20 ml p o r t i o n s of 7 M l i t h i u m nitrate w e r e e q u i l i b r a t e d with 10.0 ml o f a 1 . 0 . 1 0 - 2 M s o l u t i o n of t e t r a h e p t y l a m m o n i u m b r o m i d e in n i t r o b e n z e n e . T h e a q u e o u s solution was tested after e a c h e q u i l i b r a t i o n with silver nitrate s o l u t i o n until n o precipitate o f silver b r o m i d e c o u l d be d e t e c t e d . T h e o r g a n i c p h a s e was t h e n s e p a r a t e d a n d used in the electrode. Electrode assembly and potentiometric measurements T h e b o d y of a n O r i o n m o d e l 92-20 c a l c i u m ion e l e c t r o d e e q u i p p e d w i t h O r i o n series 92 m e m b r a n e s was u s e d .as the liquid m e m b r a n e electrode. T h e e l e c t r o d e was a s s e m b l e d as d e s c r i b e d in the m a n u f a c t u r e r ' s instructions, a n d the i n t e r n a l reference a n d liquid i o n - e x c h a n g e solutions w e r e injected into t h e a p p r o p r i a t e p o r t s in the e l e c t r o d e body. E a c h m e m b r a n e , was s o a k e d in i o n - e x c h a n g e solution for 1 h before use i n the e l e c t r o d e , a n d each a s s e m b l e d e l e c t r o d e w a s s o a k e d i n 10 - 2 M
n i t r a t e ion solution f o r 24-48 h before use. T h e internal reference solution was 1 0 - 2 M N a N O 3 a n d 10 - z M KC1.
151;1
..
insf F"NOT,
sl
ENO; I00
E?5O
El".~
o
I
0
I
20
I
40 TIME, rnln
I
60
I
80
F i g . l. Time-dependente l e c t r o d e b e h a v i o r o n t r a n s f e r b e t w e e n s o l u t i o n s . E l e c t r o d e : 1 . 5 . 1 0 - 3 M F e ( b p h e n ) a 2+ in n i t r o b e n z e n e .
10 - 2 M
nitrate and
10 - 2 M
iodide
SELECTIVITY
OF ANION-SELECTIVE
ELECTRODES
427 "
P o t e n t i o m e t r i e 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 B e c k m a n C e n t u r y SS p H meter. T h e reference e l e c t r o d e was a C o r n i n g m o d e l 467002 s a t u r a t e d c a l o m e l electrode. M e a s u r e m e n t s w e r e m a d e with t h e e l e c t r o d e i m m e r s e d to a depth o f 1 c m in the sample s o l u t i o n a n d with the s o l u t i o n stirred by m a g n e t i c stirring. E x c e p t for the m o s t dilute s o l u t i o n s little d i f f e r e n c e (usually less t h a n 2 m V ) was o b s e r v e d b e t w e e n r e a d i n g s in stirred a n d u n s t i r r e d solutions. P o t e n t i o m e t r i c m e a s u r e m e n t s w e r e m a d e using the s t a n d a r d scale setting of the m e t e r . M o s t readings were m a d e d i r e c t l y f r o m the face o f the p H meter, b u t in s o m e instances p o t e n t i a l - t i m e traces w e r e r e c o r d e d w i t h a B e c k m a n m o d e l 100500 recorder. E x c e p t in cases w h e r e the m e a s u r e d poteJatial exhibited e x t r e m e t i m e d e p e n d e n c y , t h e u n c e r t a i n t y o f these m e a s u r e m e n t s ' w a s -t-2 m V o r better. All experimental w o r k was carried o u t at a m b i e n t t e m p e r a t u r e , 22-1- 2 °. RESULTS
AND DISCUSSION
Time dependence o f e.m.f, measurements W h e n a n e l e c t r o d e is t r a n s f e r r e d between two solutions c o n t a i n i n g different a n i o n s a t i m e - d e p e n d e n t e.m.f, is observed. Typical b e h a v i o r is illustrated in Fig. 1 for a n e l e c t r o d e c o n t a i n i n g the nitrate salt o f F e ( b p h e n ) a2+ in nitrobenzene. T h e e q u i l i b r i u m potential o f the e l e c t r o d e in 10- 2 M nitrate is ~ o ~ - . I m m e d i a t e l y after i m m e r s i o n in i 0 - 2 M iodide, p o t e n t i a l E~~-st is observed. After ca. 30 min, e q u i l i b r i u m p o t e n t i a l E~i~- is r e a c h e d in iodide solution. W h e n the e l e c t r o d e is placed b a c k in 10-2 M nitrate, p o t e n t i a l s ENosi.,t a n d E~r~o~ .~, - are o b s e r v e d i n s t a n t a n e o u s l y a n d after 30 rain equilibration, respectively. E.m.f. differences E ] n- s t - - ~ S o a - a n d L-t.,t ~r~o; -- E'~ are i n s t a n t a n e o u s values, a n d the e.m.f, difference E~:---E~do; is a steady-state value. W i t h i n t h e a c c u r a c y o f the m e a s u r e m e n t s , it was f o u n d that E l n-st - - ~Soa-- = E[~ a.,NO tincta- ,. Ell n-st- E~IE ~ ' ~ o ~ " ENnOt- ; a n d E ~ g o ; - - - - - ~ ; , if sufficient time is allowed for equil i b r a t i o n o f the e l e c t r o d e in e a c h s o l u t i o n . This behavior was o b s e r v e d for all ions studied. T h e m a g n i t u d e o f the difference b e t w e e n i n s t a n t a n e o u s and steady-state values a n d the time r e q u i r e d to establish steady-state potentials increase d r a m a t i c ally as t h e difference b e t w e e n the selectivity coefficients of the two ions increases. F o r e x a m p l e , when a n i t r a t e - i o n e l e c t r o d e i s . p l a c e d in 1 0 - 2 M h e x a f l u o r o p h o s p h a t e solution, ~ia~ ' t~PF6---~PSF6" = 150 mV, a n d a p p r o x i m a t e l y f o u r d a y s are r e q u i r e d to reestablish the e q u i l i b r i u m potential in nitrate ion solution. T h e observed t i m e - d e p e n d e n t electrode b e h a v i o r was n o t studied f u r t h e r in this w o r k . Similar b e h a v i o r has been n o t e d previously for b o t h cation- a n d a n i o n sensitive liquid m e m b r a n e e l e c t r o d e s t 6 - x a w h e n a n electrode i s transferred b e t w e e n s o l u t i o n s o f different ions o r w h e n the c o n c e n t r a t i o n of o n e ion is c h a n g e d in the p r e s e n c e o f a n o t h e r . Slow i o n - e x c h a n g e p r o c e s s e s at the e l e c t r o d e , s o l u t i o n interface o r effects arising f r o m changes in c o n c e n t r a t i o n profiles w i t h i n the m e m b r a n e n o t a c c o u n t e d for by p r e s e n t t h e o r y t2" ~a m a y offer a n e x p l a n a t i o n .
Determination o f selectivity coefficients Selectivity coefficients were c a l c u l a t e d relative to n i t r a t e ion from the single ion c a l i b r a t i o n curves o b t a i n e d f r o m i n s t a n t a n e o u s and s t e a d y - s t a t e potential differences defined p r e v i o u s l y a n d s h o w n in Fig. 1. F o r i n s t a n t a n e o u s readings the e l e c t r o d e w a s e q u i l i b r a t e d in 10-2 M n i t r a t e solution before e a c h o f four m e a s u r e -
428
R. E. R E I N S F E L D E R ,
F. A. SCHULTZ
m e n t s in s o l u t i o n s c o n t a i n i n g f r o m 10-* to 10 -1 M of t h e s e c o n d ion. S t e a d y - s t a t e r e a d i n g s w e r e o b t a i n e d e i t h e r f r o m the s t e a d y - s t a t e potential differences in the s a m e e x p e r i m e n t s o r b y e q u i l i b r a t i n g the e l e c t r o d e in a 10-1 M s o l u t i o n o f the s e c o n d ion a n d t h e n m e a s u r i n g p o t e n t i a l s in 1 0 - 4 - 1 0 -x M solutions o f t h a t ion. E q u i v a l e n t results were o b t a i n e d by t h e two m e t h o d s o f steady-state m e a s u r e m e n t . P o t e n t i a l vs. log a n i o n activity plots w e r e p r e p a r e d b y m e a n s o f t h e expression log f x - = - - 0 . 5 0 9 #~/(1 + b # ) , w h e r e f x - is the activity coefficient a n d /~ the ionic strength, to c o n v e r t c o n c e n t r a t i o n s to activities. F o r all ions, these plots were linear b e t w e e n 10 - t M a n d 10 - 3 o r 10 - 4 M for b o t h i n s t a n t a n e o u s a n d steady-state m e a s u r e m e n t s . The slopes o f the plots, h o w e v e r , w e r e g e n e r a l l y s o m e w h a t less t h a n the N e r n s t i a n value o f -- 58.6 m V expected a t 22 °. R e p r e s e n t a t i v e slopes are p r e s e n t e d in T a b l e I. TABLE
I
SLOPES OF POTENTIAL vs. L O G STEADY-STATE MEASUREMENTS ELECTRODE*
Ion
ANION WITH
ACTIVITY PLOTS F e ( b p h e n ) 2+ L I Q U I D
FOR INSTANTANEOUS MEMBRANE
AND
Slope (m V )
el BrNO~" IBFZ SCN CIO~" PF~ ° 1.5" 10 - 4 M
Instantaneous
Steady state
---------
---------
40.0 47.2 51.3 55.2 50.3 57.7 59.3 59.0
Fe(bphen)
41.7 51.7 54.2 53.8 52.8 54.5 58.3 53.8
2+ in nitrobenzene.
Selectivity coefficients were d e t e r m i n e d b y m e t h o d I of S r i n i v a s a n a n d Rechnitz tT. In this p r o c e d u r e t h e potential o f t h e ion-selective e l e c t r o d e in a solution c o n t a i n i n g ion X - is c o m p a r e d to the p o t e n t i a l of t h e e l e c t r o d e in a solution c o n t a i n i n g n i t r a t e a t the s a m e activity. It is a s s u m e d t h a t the e l e c t r o d e r e s p o n s e o b e y s the e q u a t i o n E = E o - - 0 . 0 5 8 6 log [aND; + K x - m o 3 - ( a x - ) ]
(1)
In this w o r k t h e selectivity coefficient K x - r o D ; was c a l c u l a t e d f r o m two p o t e n t i o metric m e a s u r e m e n t s b y m e a n s o f eqn. (2): log Kx-/NOr = Enos (aNO~)-- Ex- ( a x - ) 0.0586
(2)
at activities ax - = aND3- c o r r e s p o n d i n g to 10-2 M c o n c e n t r a t i o n s in the two solutions.
Dependence o f electrode selectivity on liquid membrane composition T a b l e II presents i n s t a n t a n e o u s a n d steady-state selectivity coefficients for
SELECTIVITY FABLE
OF
ANION-SELECTIVE
429
ELECTRODES
II
INSTANTANEOUS IN N I T R O B E N Z E N E
AND
STEADY-STATE
SELECTIVITY
COEFFICIENTS
FOR
F e ( b p h e n ) a z÷ E L E C T R O D E S
Results given as log Kx-a~o;) ton
2.9" I 0 - s M F e ( b p h e n ) ~ +
1.5" 10 - 4 M F e ( b p h e n ) ] +
1.5" 10 - a M ".Fe(bphen)] +
Inst.
S t e a d y state
Inst.
Steady state
Inst.
Steady state
- - 1.38 -- 0.52 0.00 0.73 0.90 1.26 2.00 0.89
-- 1.704-0.02 - - 0.68 4- 0 . 0 2 0.00 0.95 ~ 0.02 1.01 4- 0 . 0 2 1.494-0.02 2 . 6 0 4- 0 . 0 1 3.65 4- 0 . 0 8
- - 1.20 -- 0.64 0.00 0.94 0.65 1.28 1.55 1.96
-- 1.604-0.02 - - 0 . 7 2 4- 0 . 0 2 0.00 1.09 4- 0 . 0 4 1.38 ± 0 . 0 6 1.544-0.01 2.81 4- 0.08 3 . 9 0 4- 0 . 0 4
- - 1.57 ' - - 0.68 0.00 0.41 0.41 " 0.76 0.87 1.04
CI" -- 1.604-0.02 [3~. . . . . 0 . 6 2 4- 0.06 NO a 0.00 [0 . 9 8 4- 0.04 BF~ 1.01 q- 0.02 ~CN1.404-0.04 CIO,~ 2 . 5 6 4- 0.03 PF~" 3.49:5:0.06
e l e c t r o d e s c o n t a i n i n g different c o n c e n t r a t i o n s o f F e ( b p h e n ) z + in n i t r o b e n z e n e as the l i q u i d m e m b r a n e . F o r the i n s t a n t a n e o u s m e a s u r e m e n t s the sequence of electrode selectivity is P F ~ > C I O ~ > S C N - > B F , ~ > I - > N O ~ > B r - > C I T h e overall r a n g e o f selectivity coefficients in this s e q u e n c e was c a . 105. Values of K Inst in Table II s h o w a slight b u t detectable inorease with increasing c o n c e n t r a t i o n of F e ( b p h e n ) az + for t h e m o r e selective ions, but c h a n g e s in c o n c e n t r a t i o n of the i o n - e x c h a n g e material d o n o t affect the selectivity o r d e r o f the e l e c t r o d e . With few e x c e p t i o n s t h e electrode selectivity sequence is the same for steadystate a n d i n s t a n t a n e o u s m e a s u r e m e n t s . T h e m o s t a p p a r e n t difference betwAeen the two sets o f m e a s u r e m e n t s is t h a t for ions for w h i c h the selectivity is better t h a n for nitrate, values o f K inst a r e m u c h g r e a t e r t h a n the c o r r e s p o n d i n g values of K 's. F o r c h l o r i d e a n d b r o m i d e ions, f o r w h i c h selectivity is lower, K l"s! is snmller t h a n K s'. These observa.tions are c o n s i s t e n t with the t i m e - d e p e n d e n t b e h a v i o r of the electrode s h o w n in Fig. 1. O w i n g to l o n g e q u i l i b r a t i o n times for s o m e ions it is difficult to o b t a i n r e p r o d u c i b l e values of steady-state selectivity coeffic.ients; therefore, p r i m a r i l y i n s t a n t a n e o u s values a r e used to c h a r a c t e r i z e e l e c t r o d e selectivity in this work. I n s t a n t a n e o u s selectivity coefficients o f electrodes c o n t a i n i n g F e ( p h e n ) az+ a n d t e t r a h e p t y l a m m o n i u m ( T H A +) ion in n i t r o b e n z e n e are given in Table III, The selectivity s e q u e n c e s a n d m a g n i t u d e s o f selectivity coefficients for-these two e l e c t r o d e s were similar to t h o s e o b s e r v e d for the F e ( b p h e n ) ] + electrode. V a l u e s o f K in"t f o r the F e ( b p h e n ) ] + a n d T H A + e l e c t r o d e s were virtually identical for all i o n s . M a g n i t u d e s o f K inst w e r e slightly smaller for the F e ( p h e n ) ] + electrode, indicating a s m a l l e r r a n g e o f selectivity coefficients for this exchanger. C h a n g e s i n selectivity o r d e r w e r e o b s e r v e d o n l y a m o n g iodide, t e t r a f l u o r o b o r a t e a n d t h i o c y a n a t e ions. F o r these ions the s e q u e n c e S C N - > B F ~ - > I , w a s o b s e r v e d w i t h the F e ( b p h e n ) az+ e l e c t r o d e , S C N " > I - > BF~" with the F e ( p h e n ) 2 + e l e c t r o d e a n d BF~- > S C N " > I with t h e T H A + electrode. T h e selectivity coefficients of these t h r e e ions differed by • n o m o r e t h a n a factor o f 5 u n d e r a n y c o n d i t i o n s . I n s t a n t a n e o u s selectivity coefficients o f electrodes c o n t a i n i n g F e ( b p h e n ) 2 + in c h l o r o f o r m a n d a m y l a l c o h o l are given i n T a b l e IY. T h e selectivity s e q u e n c e oh-
430
R.E. REINSFELDER,
F. A . S C H U L T Z
T A B L E III INSTANTANEOUS SELECTIVITY COEFFICIENTS OF AMMONIUM ION ELECTRODES IN NITROBENZENE
F c ( p h c n ) 2+
AND
TETRAHEPTYL--
( R e s u l t s g i v c n a s log K x - ~ o y ) Ion
5.10-'* M Fe(phen)~+:
CIBr NO~ IBFZ SCN ClOg PFg
--0.48 -- 0.21 0.00 1.18 0.68 1.34 2.28 3.20
1"10 - 2 M T H A + - - 1.444-0.11 -- 0.62 + 0.08 0.00 1.04+0.04 1.58 4- 0.09 1.48 +_0.06 2.864-0.11 4.114-0.11
T A B L E IV INSTANTANEOUS SELECTIVITY COEFFICIENTS CHLOROFORM AND AMYL ALCOHOL
OF Fe(bphen)~ + ELECTRODES
IN
( R e s u l t s g i v e n a s log K x - mo~ )
Ion
2.10 -a M Fe(bphen)] + in chloroform
2.10 -a M Fe(bphen)] + in a m y l alcohol
CIBrNO3 IBFg SCN CIO,~ PFg
- 1.34+0.06 --0.18+0.01 0.00 1.74 _+0.03 0.90 + 0.05 1.54 + 0.02 2.08 + 0.08 2.90 + 0.11
--0.60 --0.21 0.00 0.40 -- 0.07 0.72 0.68 0.82
served in c h l o r o f o r m w a s P F g > C l O g > I - > S C N - > B F 2 > N O ~ > B r - > C I - , w h i c h is similar to s e q u e n c e s o b s e r v e d with n i t r o b e n z e n e . Relative t o v a l u e s in nitrobenzene, selectivities for h e x a f l u o r o p h o s p h a t e a n d perchlorate were r e d u c e d by factors o f 10 a n d 5, respectively, and the selectivity for iodide w a s e n h a n c e d by a. factor o f 5. In a m y l a l c o h o l , selectivities for i o n s w i t h values a b o v e nitrate were all d i m i n i s h e d significantly a n d a very limited range o f selectivity coefficients w a s o b s e r v e d . T h e selectivity s e q u e n c e c o m m o n to the o t h e r s o l v e n t s w a s m o d i f i e d in a m y l a l c o h o l , wherein the s e q u e n c e o b s e r v e d w a s P F g > S C N - > CIO,~ > I - >
N O T >BF~" > B r - > C I - . F o r electrodes c o n t a i n i n g F e ( p h e n ) 2+ a n d T H A + in n i t r o b e n z e n e a n d F e ( b p h e n ) 2 + i n n i t r o b e n z e n e a n d c h l o r o f o r m , a c o m m o n s e q u e n c e o f a n i o n selectivity w i t h similar selectivity coefficients w a s o b s e r v e d : P F 6 > C l O g > S C N - , - ~ B F g --, I - > NO~" > B r - > C I - . A similar s e q u e n c e w a s o b s e r v e d for c o m m e r c i a l perchlorate a n d nitrate i o n electrodes, for w h i c h selectivity coefficients are listed in T a b l e V. T h e s e q u e n c e is valid for b o t h O r i o n electrodes, w h i c h are liquid m e m -
SELECTIVITY TABLE
OF
ANION-SELECTIVE
431
ELECTRODES
V
SELECTIVITY
COEFFICIENTS
OF
COMMERCIAL
ANION-SELECTIVE
ELECTRODES
(Results given as log Kx-~oj-) Ion
Perc.qlorate Orion
CIBr NO~" IBF,~ SCNCIO~" PF~
electrodes
92-81*
Beckman
-- 1.82 -- 0 . 4 3 0.00 0.90 --. 2.87 --
Nitrate 39616 b
-0.94 -- 0 . 4 0 0.00 0.86 1.52 1.60 2.25 2.90
electrodes
Orion 92-07 ~
Becknlan
--2.22 - 1.00 0.00 1.30 --3.00 --
--0.82 -- 0 . 3 4 0.00 0.78 0.96 1.26 1.65 2.10
396.I8 b
a Ref. 1, p p . 7 0 - 7 1 . Ref. 19.
b r a n e e l e c t r o d e s o f the t y p e u s e d in this w o r k , a n d B e c k m a n electrodes, w h i c h e m p l o y an i m m o b i l i z e d o r "solid" i o n - e x c h a n g e m e m b r a n e . It t h e r e f o r e a p p e a r s that for a n i o n - s e n s i t i v e e l e c t r o d e s b a s e d o n i o n - a s s o c i a t i o n e x t r a c t i o n s y s t e m s c h a n g e s in the i o n - e x c h a n g e material, c o n c e n t r a t i o n o f t h e ion exchanger, solvent or m a n n e r of m e m b r a n e c o n s t r u c t i o n h a v e little effect, o n t h e selectivity o f t h e electrode. T h e o n l y significant difference is o b s e r v e d w h e n a m y l a l c o h o l is used as solvent. In this case, however, t h e effect is to d i m i n i s h r a t h e r t h a n e n h a n c e the r a n g e o f e l e c t r o d e selectivity. T h e t h e o r y of l i q u i d m e m b r a n e e l e c t r o d e b e h a v i o r s 2. t a' defines three l i m i t i n g cases in w h i c h e l e c t r o d e selectivity m a y be r e l a t e d to the i o n - p a r t i t i o n i n g process. In t w o cases,.where the i o n - e x c h a n g e site a n d t h e c o u n t e r i o n a r e h i g h l y d i s s o c i a t e d in t h e m e m b r a n e p h a s e o r w h e r e the i o n - e x c h a n g e site a n d c o u n t e r ion are h i g h l y a s s o c i a t e d b u t t h e m o b i l i t y of t h e c o u n t e r i o n is m u c h g r e a t e r t h a n t h a t o f the site, t h e selectivity coefficient is given b y t h e expression: fix- k x K x , IV- -- fly_ kv.-
(3)
w h e r e fix- a n d fly- are t h e mobilities o f X - a n d Y - . in the m e m b r a n e p h a s e , a n d kx- a n d k v - are the single i o n - p a r t i t i o n coefficients b e t w e e n w a t e r a n d the s o l v e n t of the m e m b r a n e . T h e r a t i o k x - / k v - r e p r e s e n t s the e q u i l i b r i u m c o n s t a n t for the ionexchange process k x - Ik~¢ . •
X- (aqueous) +Y- imembrane) •
.
= X - ( m e m b r a n e ) + Y " (aqueous)
(4)
F o r a m e m b r a n e in w h i c h there is s t r o n g a s s o c i a t i o n a n d the sites are highly m o b i l e relative to the c o u n t e r ions, the selectivity coefficient is a p p r o x i m a t e d b y : K x - / v - ---- ffRX._K k y •
URY
.
(5)
432
R. E. R E I N S F E L D E R , F. A. S C H U L T Z
w h e r e fiRX a n d fi•v a r e the mobilities o f t h e ion-site pairs, a n d K~v is the e q u i l i b r i u m c o n s t a n t for the i o n - e x c h a n g e r e a c t i o n : X - ( a q u e o u s ) + R - Y ( m e m b r a n e ) ~:~ ~ ( m e m b r a n e )
+ Y- (aqueous)
(6)
If Kk-~ a n d K ~ are the dissociation c o n s t a n t s of the ion-site pairs in the m e m b r a n e , then k x - K~-v K k v = ky-KK-g
(7)
T h e e x t e n t o f ion a s s o c i a t i o n in o r g a n i c solutions c o r r e s p o n d i n g to the . c o m p o s i t i o n o f the liquid m e m b r a n e s u s e d in this w o r k is n o t k n o w n . C o n d u c t a n c e studies o f smaller q u a t e r n a r y a m m o n i u m salts in n i t r o b e n z e n e 2°-22 show t h a t ion-pair diss o c i a t i o n c o n s t a n t s a r e ca. 4- 10- 2 a n d d o n o t differ g r e a t l y a m o n g the anions s t u d i e d in this w o r k . The mobilities o f the a n i o n s a r e typically 2 to 3 times g r e a t e r t h a n the mobilities o f the q u a t e r n a r y a m m o n i u m ions. I o n a s s o c i a t i o n c o n s t a n t s o f Fe= (phen)a2 + a n d F e ( b p h e n ) ] + salts in n o n a q u e o u s solvents are n o t k n o w n , b u t it is e x p e c t e d t h a t mobiiities o f these c a t i o n s a r e m u c h less t h a n t h o s e o f the a n i o n s . It t h e r e f o r e seems r e a s o n a b l e to c o n c l u d e t h a t the selectivity coefficients o f b o t h the q u a t e r n a r y a m m o n i u m ion a n d i r o n - p h e n a n t h r o l i n e c o m p l e x electrodes c a n be a p p r o x i m a t e d b y t h e expression ( a s s u m i n g that fix- ----uv-) ,~ kx-' K x - /Y'~ -- k v -
(8)
E q u a t i o n (8) predicts that e l e c t r o d e selectivity is i n d e p e n d e n t o f the i o n - e x c h a n g e •m o l e c u l e a n d its c o n c e n t r a t i o n in the m e m b r a n e phase. T o a first a p p r o x i m a t i o n this is d e m o n s t r a t e d by the results o f the p r e s e n t study. E q u a t i o n s (4) a n d (8) p r e d i c t that e l e c t r o d e selectivity d e p e n d s p r i m a r i l y o n the relative strengths o f i o n - s o l v e n t ' i n t e r a c t i o n s in the t w o phases. T h e c h a n g e in s o l v a t i o n energy in eqn. (4) c a n be expressed in t e r m s o f the differences in enthalpie, s o f h y d r a t i o n in the a q u e o u s p h a s e a n d e n t h a l p i e s o f solvati0n in t h e m e m b r a n e p h a s e of i o n s X - a n d Y - : AHx° - n" - = (AH°=m, x - -- AH°.m. Y - ) .-- (AH.°, x - -- AH~, v - )
(9)
E n t h a l p i e s r a t h e r t h a n free energies o f solvation are c h o s e n b e c a u s e m o r e extensive values o f the f o r m e r q u a n t i t i e s a r e available for the p u r p o s e o f c o m p a r i s o n . I n the a p r o t i c solvents n i t r o b e n z e n e a n d c h l o r o f o r m , differences in s o l v a t i o n energies o f a n i o n s a r e e x p e c t e d to be smaller t h a n in water. T h e r e f o r e , regardless o f the s o l v e n t used to f o r m the m e m b r a n e , e l e c t r o d e selectivity is d e t e r m i n e d p r i m a r i l y by differences in h y d r a t i o n energies in the a q u e o u s phase. This is s h o w n in T a b l e VI w h e r e t h e enthalpies o f h y d r a t i o n a n d ionic radii o f i o n s studied in this w o r k a r e presented. T h e r e is g o o d c o r r e l a t i o n b e t w e e n e l e c t r o d e selectivity a n d i o n i c size a n d h y d r a t i o n enthalpy, T h e ions for w h i c h the e l e c t r o d e s h a v e h i g h e s t selectivity a r e the large h y d r o p h o b i c a n i o n s having t h e smallest n e g a t i v e h y d r a t i o n energies. In c o m p e t i t i o n with smaller, m o r e s t r o n g l y h y d r a t e d a n i o n s , these a n i o n s find t h e m selves preferentially d i s t r i b u t e d into the m e m b r a n e p h a s e of the electrode. S i m i l a r explanations, b a s e d o n relative a n i o n h y d r a t i o n energies, have b e e n a d v a n c e d to explain t h e solvent e x t r a c t i o n b e h a v i o r o f a n i o n s 2s' 29 a n d the selectivity o f a n i o n e x c h a n g e resins a°. It, therefore, a p p e a r s t h a t unless s o m e m e a n s o f p r o m o t i n g s t r o n g
SELECTIVITY
OF ANION-SELECTIVE
ELECTRODES
433
TABLE VI HYDRATION
Ion
ENTHALPIES
AND
IONIC
Reference
AH°;x -
(kcal mole-l)
CIBr-
NO~ IBF~ SCNCIOZ
RADII OF UNIVALENT
Radius (,4)
ANIONS
Reference .
--87.6 -- 79.8 -- 74.5 -- 69.7 -71.2
23 23 24 23 23
1.81 1.95 1.96 2.16 2.26
25 25 26 25 23
57.1
23
2.45 3.00
23 27
--
PF~
selective interactions in the m e m b r a n e phase can be found, the selectivity of liquid m e m b r a n e electrodes for c o m m o n inorganic anions Cannot be greatly altered from the orde r predicted by hydr a t i on •energies. Possible evidence of stronger inte/:actions in the m e m b r a n e phase exists in the case of the Fe(bphen)~ + - a m y l alcohol electrode, but the result is to decrease the range of electrode selectivity. Because amyl alcohol is an h y d r o x y l i c solvent, it i s expected to solvate anions to a degree more nearly equal that of water. As a result, differences in solvation energies between the aqueous a n d m e m b r a n e phases are minimized a n d more nearly equal selectivity coefficients are observed. The factors influencing electrode selectivity appear to be the same as those governing solvent extraction processes 14. Unfortunately there are few data on which quantitative comparisons o f selectivity a n d extraction behavior can be made. O n e instance in which a quantitative c o m p a r i s o n Can be made is illustrated in Fig. 2. The instantaneous selectivity coefficients o f a Fe(phen)~+'nitrobvnzene electrode a r e
3
/SCN- I Br:
-
"28
....
I 4
l 0
-4
I
I -8
-12
.
L~G~tr, kcol mole "1'
F i g . . 2 . . P I o t o f logK=x'=-t/N0~ vs. f r e e e n e r g y o f e x t r a c t i o n of F e ( p h e n ) ~ + salts i n t o n i t r o b e n z e n e a c c o r d i n g t o e q u i l i b r i u m 2 o f ref, 31. E l e c t r o d e : 5 . I 0 , 4 M F e ( p h e n ) ] + i n n i t r o b e n z e n e , ~ . " ~ :
434
R. E. R E I N S F E L D E R , F. A. S C H U L T Z
c o m p a r e d to the free e n e r g i e s of e x t r a c t i o n o f t h e c o r r e s p o n d i n g F e ( p h e n ) 2 + salts i n t o n i t r o b e n z e n e 31. G o o d c o r r e l a t i o n is o b t a i n e d for the five ions studied. O t h e r s o l v e n t e x t r a c t i o n s t u d i e s s h o w a n o r d e r o f a n i o n e x t r a c t a b i l i t y q u a l i t a t i v e l y similar o r identical to the o r d e r o f selectivity f o u n d in this w o r k . S u c h similar s e q u e n c e s a r e f o u n d in s y s t e m s e m p l o y i n g q u a t e r n a r y a m m o n i u m ions in t o l u e n e a n d d i c h l o r o m e t h a n e 32"33, t r i p h e n y l t e t r a z o l i u m salts in c h l o r o f o r m 34 a n d q u a t e r n a r y p h o s p h o n i u m , a r s o n i u m a n d s t i b o n i u m salts in c h l o r o f o r m , d i c h l o r o m e t h a n e , 1,2-dic h l o r o e t h a n e a n d 2 , 2 ' - d i c h l o r o d i e t h y l e t h e r 29. SUMMARY
Selectivity coefficients o f l i q u i d - m e m b r a n e e l e c t r o d e s for c o m m o n i n o r g a n i c a n i o n s w e r e m e a s u r e d in e l e c t r o d e s c o n t a i n i n g t r i s ( 1 , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) , t r i s ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) o r t e t r a h e p t y l a m m o n i u m i o n in nit r o b e n z e n e , a n d t r i s ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) i o n in n i t r o b e n z e n e , c h l o r o f o r m or n - a m y l a l c o h o l as the l i q u i d m e m b r a n e . W i t h t h e e x c e p t i o n o f t h e a m y l a l c o h o l electrode, selectivity coefficients w e r e relatively i n d e p e n d e n t o f m e m b r a n e c o m p o s i t i o n a n d f o l l o w e d a c o m m o n s e q u e n c e Of d e c r e a s i n g selectivity: PF~- > C I O ~ > S C N - ,~ I - -~ BF~" ~>N O a - > B r - > C I - . T h i s s e q u e n c e p a r a l l e l s t h e o r d e r of i n c r e a s i n g a n i o n h y d r a t i o n energy, s u g g e s t i n g t h a t a q u e o u s p h a s e s o l v a t i o n energies p l a y a p r e d o m i n a n t r o l e in d e t e r m i n i n g e l e c t r o d e selectivity for these i o n s . T i m e - d e p e n d e n t b e h a v i o r o f l i q u i d - m e m b r a n e e l e c t r o d e s o n transfer b e t w e e n s o l u , tions c o n t a i n i n g different ions also is d e s c r i b e d . I n s t a n t a n e o u s e.m.f, r e a d i n g s w e r e used t o d e t e r m i n e selectivity coefficients. RI~.SUMI~
. . . . . . . . I ) e s ' ~ e s u r e s d e coefficients de s~lectivit6 d ' ~ l e c t r o d e s it m e m b r a n e l i q u i d e o n t ~t6 effectu~es p o u r des a n i o n s i n o r g a n i q u e s c o u r a n t s , it l'aide d,61ectrodes c o n t e n a n t : tris-(1,10-p.h~nanthroline)fer_(I!) , t r i s - ( 4 , 7 , d i p h 6 n y l - ! , 1 0 - p h ~ n a n t h r o l i n e ) f e r ( I I ) o u ion t 6 t r a h e p t y l a m m o n i u m , d a n s le n i t r o b e n z ~ n e , et t r i s - ( 4 , 7 - d i p h ~ n y l - l , 1 0 - p h ~ n a n throline)fer(II~, d a n s le n i t r o b e n z ~ n e , le c h l o r o f o r m e , o u l ' a l c o o l - n - ~ m y l i q u e c o m m e m e m b r a n e liquide. A l ' e x c e p t i o n de l'~lectrode it a l c o o l a m y l i q u e , les coefficients de s61ectivit6 p r ~ s e n t e n t des v a l e u r s d ~ c r o i s s a n t e s , d a n s l'ordre: PF~->CIO~> S C N - ~. I - ~ BF~- > N O i " > B r - > C I - , l a i s s a n t s u p p o s e r q u e les ~nergies d e s o l v a t a t i o n j o u e n t ici u n r61e p r e d o m i n a n t . O n e x a m i n e 6 g a l e m e n t l'influence d u temps. L a d 6 t e r m i n a t i o n des coefficients de s61ectivit6 a ~t~ faite avec l e c t u r e f.e.m, i n s t a n t a n ~ e . ZUSAMMENFASSUNG
Selektivitiitskoeffizienten yon F ! f i s s i g - M e m b r a d 2 E l e k t r o d e n ftir d i e tiblichen a n o r g a n i s c h e n A n i o n e n w u r d e n mit. E l e k t r o d e n g e m e s s e n , die T r i s - ( 1 , 1 0 - p h e n a n t h r o l i n ) eisen(II)-, T r i s - ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n ) e i s e n ( I I ) - u n d T e t r a h e p t y l a m m o n i u m - I o n in N i t r o b e n z o l u n d T r i s - ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n ) e i s e n ( I I ) - I o n in N i t r o b e n z o l , C h l o r o f o r m o d e r n , A m y l a l k o h o l als Fltissig_-Membran e n t ' • lii~lten. 1VIit A u s n a h m e d e r A m y l a l k o h o l - E l e k t r o d - e w a r e n . die Selektivitiitskoeffizien-
SELECTIVITY OF ANION-SELECTIVE ELECTRODES
435
ten relativ unabh~ingig van der M e m b r a n z u s a m m e n s e t z u n g u n d folgten ¢iner norm a l c n Rcihenfolge klciner w c r d e n d e r Sclektivit~it: PF~" > C 1 0 ~ - > S C N "~,,~J BF~" > N O 3 > B r - > Cl -. Diese Reihenfolgc entspricht dcr Reihenfolgc z u n c h m c n d c r A n i o n e n - H y d r a t a t i o n s c n c r g i c u n d weist darauf hin, dass die Solvatationscnergien in der w~issrigen Phase cin¢ d o m i n i c r e n d c Rollc spielcn bci der B e s t i m m u n g der Elcktrodenselektivit~it far diesc Ionen. D a s Zeitverhalten der F l t i s s i g - M e m b r a n E l e k t r o d c n beim ~ b e r g a n g z w i s c h e n L 6 s u n g c n , die verschiedcne I o n e n enthaltcn, w i r d ebcnfalls beschriebcn. Die e.m.k.-Wertc w u r d e n m o m e n t a n aufgezeichnet, u m die Sclektivit~itskocffizientcn z u bcstimmen. REFERENCES 1 J. W. Ross, Jr., in R. A. Durst, Ion-Selective Electrodes, U.S. Nat. Bur. Stand. Spec, Publ. No. 314, W a s h i n g t o n , D.C., 1969, Chap. 2. 2 C. J. Coetzee and H. Freiser, Anal. Chem., 40 (1968) 2071. 3 C. J'. Coetzee a n d H. Freiser, Anal. Chem., 41 (1969) 1128. 4 P. R. Danesi, F. Salvemini, (3. Sciboha and B. Scuppa, J. Phys. Chem., 75 (1971) 554. 5 P. R. Danesi, G. Scibona a n d B. Scuppa, Anal. Chem., 43 (1971) 1892: 6 N. I s h i b a s h i a n d H. K o h a r a , Anal. Lett., 4 (1971) 785. 7 S. G. Back, Anal. Lett.~ 4 (1971) 793. 8 E. Paglia-Dubini, T. Mussini a n d R. (3alli, Z. Naturforsch. A, 26 (1971) 754. 9 W. M. H a y n e s a n d J. H. Wagenknecht, Anal. Lett., 4 (197i) 491. 10 M. M a t s u i and H. Freiser, Anal. Lett., 3 (1970) 161. 11 G. Seibona, L. Mantella a n d P. R. Danesi, Anal. Chem., 42 (1970) 844. 12 J . P. S a n d b l o m , (3. E i s e n m a n a n d J. L. Walker, Jr., J. Phys. Chem., 71 (1967) 3862. 13 G . : E i s e n m a n , in R. A. DurSt, Ion-Selective Electrodes, U.S. Nat. Bur. Stand. Spec. Publ. No. 314, W a s h i n g t o n , D.C., 1969, C h a p . 1. 14 H. J. James, (3. P. C a r m a c k and H. Freiser, Anal. Chem., 44 (1972) 853. 15 (3. Scibona, J. F. Byrum, K: K i m u r a a n d J. W. Irvine, Jr., in D. Dyrssen, J . O . Liljenzin and J. R y d berg, Solvent Extraction Chemistry, Wiley-Interscience, New York, 1967, pp. 398--407. 16 (3. A. Rechnitz and Z. F. Lin, Anal.• Chem., 40(1968) 696. 17 K. Srinivasan a n d G. A. Rechnitz, AnaLChem., 41 (1969) 1203. 18 J. Bagg a n d R. Vinen, Anal. Chem., 44 (1972) 1773. 19 F. A. Schultz, F l o r i d a Atlantic University, 1969, unpublished work. 20 C. R. W i t s c h o n k e a n d C. A. Kraus, J. Amer. Chem. Sac., 69 (1947) 2472. 21 E. Hirsch a n d R. M. Fuoss, 3. Amer: Chem. Sac., 82 (1960) 1018. 22 F. R . L o n g o , J. D. Kerstetter, T. F. K u m o s i n s k i a n d E. C. Evers, J. Phys. Chem., 7 0 ( 1 9 6 6 ) 4 3 i . 23 M. F. Halliwell and S. C. Nyburg, J. Chem. Sac., (1960) 4603. 24 (3. A. K r e s t o v and I. V . Egorova, lzv. Vyssh. Ucheb. Zaved., Khim. Teklmoi., 10 (1967) 750; Chem. Abstr., 69 (1968) 22691c. 25 L. Pauling, The Nature of the ChemicalBond, Cornell University Press, Ithaca, N.Y., 3rd Ed., 1960. 26 W. Masterton, D. Boloc0fsky a n d T. P. Leo,.J. Phys. Chem., 75 (1971) 2089. 27 R. D. W. Kemmitt, D. R. Russell a n d D. W. A. S h a r p , J. Chem. Sac., (1963) 4408. 28 R, M . D i a m o n d a n d D. (3. Tuck, Progr. lnoro. Chem., 2 (1960) 109. 29 N. A. (3ibsorl a n d D. C. W e a t h e r b u r n , Anal. Chim. Acta, 58 (1972) 159. 30 B. Chu, D. C. Whitney a n d R. M. D i a m o n d , d. lnoro. Nucl. Chem,, 24 (1962) 1405. 31 Y. Y a m a m o t o , T. T a r u m o t o and E. I w a m o t o , Anal. Lett., 2(1969) 1. 32 K. (3ustavii a n d (3. Schill, Acta Pharm. Suec., 3 (1966) 259. 33 I. M . Ivanov, L, M. (3indin a n d (3. N. Chichagova, lzu. Sib. Otd. Akad. Nauk SSSR, Set. Khim. Nauk, (1967) 100; Chem. Abstr., 69 (1968) 13377c. 34 J. Hala, Collect. Czech. Chem. Commun., 30 (1965) 1813.
425
© Elsevier Scientific Publishing C o m p a n y , A m s t er d am - Pr i n t ed in T h e N e t h e r l a n d s
ANION SELECTIVITY STUDIES ON LIQUID MEMBRANE ELECTRODES
R O N A L D E. R E I N S F E L D E R a n d F R A N K L I N A. S C H U L T Z *
Department o f Chemistry, Florida Atlantic University, Boca Raton, Fla. 33432 (U.S.A.) (Received 28th N o v e m b e r 1972)
A n u m b e r o f ion-selective liquid m e m b r a n e .electrodes have b e e n described w h i c h r e s p o n d selectively to anions. These i n c l u d e electrodes for the m e a s u r e m e n t of i n o r g a n i c a n i o n s t-s, c a r b o x y l i c acid a n i o n s z'a'9, a m i n o acid a n i o n s 1° a n d anionic metal c h l o r i d e complexes 11. T h e liquid ion-exchange m e m b r a n e s o f these electrodes generally consist of a large extractable c a t i o n such as a t r a n s i t i o n m e t a l - p h e n a n t h r o line c o m p l e x or b u l k y q u a t e r n a r y a m m o n i u m i o n dissolved in a water-immiscible o r g a n i c solvent. A l t h o u g h a relatively large n u m b e r of usable electrodes h a v e been reported, a n d a c o m p r e h e n s i v e t h e o r e t i c a l t r e a t m e n t of liquid m e m b r a n e e l e c t r o d e b e h a v i o r has b e e n p r e s e n t e d I z. la, few experimental studies have b e e n m a d e o f the relationship b e t w e e n e l e c t r o d e selectivity a n d c o m p o s i t i o n o f the l i q u i d m e m b r a n e . Freiser e t al.lO. 14 h a v e found t h a t the selectivity of a m i n o acid a n i o n electrodes closely p a r a l l e l s the solvent extraction b e h a v i o r of the liquid phase as d e s c r i b e d b y the relative d i s t r i b u t i o n coefficients o f the a m i n o acid a n i o n - q u a t e r n a r y a m m o n i u m ion pairs. S c i b o n a e t al. 4" s have investigated the response o f t e t r a h e p t y l a m m o n i u m i o n - b e n z e n e electrodes to chloride, b r o m i d e a n d nitrate ion a n d have c o r r e l a t e d electrode selectivity to i o n - e x c h a n g e e q u i l i b r i u m constants, ion-pair f o r m a t i o n c o n s t a n t s a n d ionic mobilities. In this paper is r e p o r t e d a study o f electrode selectivity for the c o m m o n i n o r g a n i c a n i o n s C I - , B r - , I - , S C N - , N O 3 , C10~-, B F ? a n d P F ~ as a function of the i o n - e x c h a n g e molecule, its c o n c e n t r a t i o n in the m e m b r a n e p h a s e a n d the o r g a n i c solvent used to f o r m the liquid m e m b r a n e . T h e i o n - e x c h a n g e molecules investigated are the tris(1,10-phenanthroline) i r o n ( I I ) a n d tris(4,7-diphenyl-l,10p h e n a n t h r o l i n e ) i r o n ( I I ) c o m p l e x ions a n d t e t r a h e p t y l a m m o n i u m ion. The solvents investigated are n i t r o b e n z e n e , c h l o r o f o r m and n-amyl a l c o h o l . EXPERIMENTAL "
Reagents
All c h e m i c a l s used in this w o r k w e r e r e a g e n t - g r a d e materials w i t h the exception of s o d i u m f l u o r o b o r a t c . T e c h n i c a l - g r a d e s o d i u m f l u o r o b o r a t e ( M a t h e s o n , C o l e m a n a n d Bell) was recrystallized f r o m e t h a n o l - w a t e r . S t o c k s o l u t i o n s were p r e p a r e d f r o m these c h e m i c a l s with distilled water, a n d solutions c o n t a i n i n g 10- t_ * T o w h o m c o r r e s p o n d e n c e should be addressed.
426
R. E . R E I N S F E L D E R ,
F. A . S C H U L T Z
10 - 4 M o f the a p p r o p r i a t e a n i o n w e r e p r e p a r e d by serial dilution. O r g a n i c s o l v e n t s w e r e used w i t h o u t f u r t h e r purification.
Preparation o f liquid ion-exchange materials Tris (4,'7-diphenyl- 1 , 1 0 - p h e n a n t h r o l i n e ) iron (II) [ F e ( b p h e n ) a2 +] a n d tris ( 1,10p h e n a n t h r o l i n e ) i r o n ( I I ) [ F e ( p h e n ) 2+] liquid i o n - e x c h a n g e s o l u t i o n s were p r e p a r e d as n i t r a t e salts .by the following p r o c e d u r e . A p p r o x i m a t e l y 20 m m o l e s o f F e ( N H 4 ) z (SO4)z • 6 H 2 0 a n d a t h r e e f o l d excess o f the p h e n a n t h r o l i n e c o m p o u n d were d i s s o l v e d in 100 ml o f 0.1 M sulfuric acid. A q u e o u s h y d r o x y l a m i n e s o l u t i o n (1 rnl o f 10~/o) w a s a d d e d a n d the p H w a s a d j u s t e d to 5.0-5.5. T h e s o l u t i o n w a s t r a n s f e r r e d to a s e p a r a t o r y funnel, a n d a 200-fold a m o u n t o f solid s o d i u m n i t r a t e was dissolved in the solution. A 10.0-ml p o r t i o n o f the o r g a n i c solvent was a d d e d , a n d the c o m p l e x ion w a s e x t r a c t e d into t h e o r g a n i c phase. T h e o r g a n i c p h a s e was s e p a r a t e d a n d k e p t for use in the electrode. T h e c o n c e n t r a t i o n o f the c o m p l e x ion in the o r g a n i c s o l v e n t was determined spectrophotometrically. S o l u t i o n s o f t e t r a h e p t y l a m m o n i u m n i t r a t e in n i t r o b e n z e n e were p r e p a r e d by a p r o c e d u r e similar to that o f S c i b o n a et al. is Successive 10-20 ml p o r t i o n s of 7 M l i t h i u m nitrate w e r e e q u i l i b r a t e d with 10.0 ml o f a 1 . 0 . 1 0 - 2 M s o l u t i o n of t e t r a h e p t y l a m m o n i u m b r o m i d e in n i t r o b e n z e n e . T h e a q u e o u s solution was tested after e a c h e q u i l i b r a t i o n with silver nitrate s o l u t i o n until n o precipitate o f silver b r o m i d e c o u l d be d e t e c t e d . T h e o r g a n i c p h a s e was t h e n s e p a r a t e d a n d used in the electrode. Electrode assembly and potentiometric measurements T h e b o d y of a n O r i o n m o d e l 92-20 c a l c i u m ion e l e c t r o d e e q u i p p e d w i t h O r i o n series 92 m e m b r a n e s was u s e d .as the liquid m e m b r a n e electrode. T h e e l e c t r o d e was a s s e m b l e d as d e s c r i b e d in the m a n u f a c t u r e r ' s instructions, a n d the i n t e r n a l reference a n d liquid i o n - e x c h a n g e solutions w e r e injected into t h e a p p r o p r i a t e p o r t s in the e l e c t r o d e body. E a c h m e m b r a n e , was s o a k e d in i o n - e x c h a n g e solution for 1 h before use i n the e l e c t r o d e , a n d each a s s e m b l e d e l e c t r o d e w a s s o a k e d i n 10 - 2 M
n i t r a t e ion solution f o r 24-48 h before use. T h e internal reference solution was 1 0 - 2 M N a N O 3 a n d 10 - z M KC1.
151;1
..
insf F"NOT,
sl
ENO; I00
E?5O
El".~
o
I
0
I
20
I
40 TIME, rnln
I
60
I
80
F i g . l. Time-dependente l e c t r o d e b e h a v i o r o n t r a n s f e r b e t w e e n s o l u t i o n s . E l e c t r o d e : 1 . 5 . 1 0 - 3 M F e ( b p h e n ) a 2+ in n i t r o b e n z e n e .
10 - 2 M
nitrate and
10 - 2 M
iodide
SELECTIVITY
OF ANION-SELECTIVE
ELECTRODES
427 "
P o t e n t i o m e t r i e 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 B e c k m a n C e n t u r y SS p H meter. T h e reference e l e c t r o d e was a C o r n i n g m o d e l 467002 s a t u r a t e d c a l o m e l electrode. M e a s u r e m e n t s w e r e m a d e with t h e e l e c t r o d e i m m e r s e d to a depth o f 1 c m in the sample s o l u t i o n a n d with the s o l u t i o n stirred by m a g n e t i c stirring. E x c e p t for the m o s t dilute s o l u t i o n s little d i f f e r e n c e (usually less t h a n 2 m V ) was o b s e r v e d b e t w e e n r e a d i n g s in stirred a n d u n s t i r r e d solutions. P o t e n t i o m e t r i c m e a s u r e m e n t s w e r e m a d e using the s t a n d a r d scale setting of the m e t e r . M o s t readings were m a d e d i r e c t l y f r o m the face o f the p H meter, b u t in s o m e instances p o t e n t i a l - t i m e traces w e r e r e c o r d e d w i t h a B e c k m a n m o d e l 100500 recorder. E x c e p t in cases w h e r e the m e a s u r e d poteJatial exhibited e x t r e m e t i m e d e p e n d e n c y , t h e u n c e r t a i n t y o f these m e a s u r e m e n t s ' w a s -t-2 m V o r better. All experimental w o r k was carried o u t at a m b i e n t t e m p e r a t u r e , 22-1- 2 °. RESULTS
AND DISCUSSION
Time dependence o f e.m.f, measurements W h e n a n e l e c t r o d e is t r a n s f e r r e d between two solutions c o n t a i n i n g different a n i o n s a t i m e - d e p e n d e n t e.m.f, is observed. Typical b e h a v i o r is illustrated in Fig. 1 for a n e l e c t r o d e c o n t a i n i n g the nitrate salt o f F e ( b p h e n ) a2+ in nitrobenzene. T h e e q u i l i b r i u m potential o f the e l e c t r o d e in 10- 2 M nitrate is ~ o ~ - . I m m e d i a t e l y after i m m e r s i o n in i 0 - 2 M iodide, p o t e n t i a l E~~-st is observed. After ca. 30 min, e q u i l i b r i u m p o t e n t i a l E~i~- is r e a c h e d in iodide solution. W h e n the e l e c t r o d e is placed b a c k in 10-2 M nitrate, p o t e n t i a l s ENosi.,t a n d E~r~o~ .~, - are o b s e r v e d i n s t a n t a n e o u s l y a n d after 30 rain equilibration, respectively. E.m.f. differences E ] n- s t - - ~ S o a - a n d L-t.,t ~r~o; -- E'~ are i n s t a n t a n e o u s values, a n d the e.m.f, difference E~:---E~do; is a steady-state value. W i t h i n t h e a c c u r a c y o f the m e a s u r e m e n t s , it was f o u n d that E l n-st - - ~Soa-- = E[~ a.,NO tincta- ,. Ell n-st- E~IE ~ ' ~ o ~ " ENnOt- ; a n d E ~ g o ; - - - - - ~ ; , if sufficient time is allowed for equil i b r a t i o n o f the e l e c t r o d e in e a c h s o l u t i o n . This behavior was o b s e r v e d for all ions studied. T h e m a g n i t u d e o f the difference b e t w e e n i n s t a n t a n e o u s and steady-state values a n d the time r e q u i r e d to establish steady-state potentials increase d r a m a t i c ally as t h e difference b e t w e e n the selectivity coefficients of the two ions increases. F o r e x a m p l e , when a n i t r a t e - i o n e l e c t r o d e i s . p l a c e d in 1 0 - 2 M h e x a f l u o r o p h o s p h a t e solution, ~ia~ ' t~PF6---~PSF6" = 150 mV, a n d a p p r o x i m a t e l y f o u r d a y s are r e q u i r e d to reestablish the e q u i l i b r i u m potential in nitrate ion solution. T h e observed t i m e - d e p e n d e n t electrode b e h a v i o r was n o t studied f u r t h e r in this w o r k . Similar b e h a v i o r has been n o t e d previously for b o t h cation- a n d a n i o n sensitive liquid m e m b r a n e e l e c t r o d e s t 6 - x a w h e n a n electrode i s transferred b e t w e e n s o l u t i o n s o f different ions o r w h e n the c o n c e n t r a t i o n of o n e ion is c h a n g e d in the p r e s e n c e o f a n o t h e r . Slow i o n - e x c h a n g e p r o c e s s e s at the e l e c t r o d e , s o l u t i o n interface o r effects arising f r o m changes in c o n c e n t r a t i o n profiles w i t h i n the m e m b r a n e n o t a c c o u n t e d for by p r e s e n t t h e o r y t2" ~a m a y offer a n e x p l a n a t i o n .
Determination o f selectivity coefficients Selectivity coefficients were c a l c u l a t e d relative to n i t r a t e ion from the single ion c a l i b r a t i o n curves o b t a i n e d f r o m i n s t a n t a n e o u s and s t e a d y - s t a t e potential differences defined p r e v i o u s l y a n d s h o w n in Fig. 1. F o r i n s t a n t a n e o u s readings the e l e c t r o d e w a s e q u i l i b r a t e d in 10-2 M n i t r a t e solution before e a c h o f four m e a s u r e -
428
R. E. R E I N S F E L D E R ,
F. A. SCHULTZ
m e n t s in s o l u t i o n s c o n t a i n i n g f r o m 10-* to 10 -1 M of t h e s e c o n d ion. S t e a d y - s t a t e r e a d i n g s w e r e o b t a i n e d e i t h e r f r o m the s t e a d y - s t a t e potential differences in the s a m e e x p e r i m e n t s o r b y e q u i l i b r a t i n g the e l e c t r o d e in a 10-1 M s o l u t i o n o f the s e c o n d ion a n d t h e n m e a s u r i n g p o t e n t i a l s in 1 0 - 4 - 1 0 -x M solutions o f t h a t ion. E q u i v a l e n t results were o b t a i n e d by t h e two m e t h o d s o f steady-state m e a s u r e m e n t . P o t e n t i a l vs. log a n i o n activity plots w e r e p r e p a r e d b y m e a n s o f t h e expression log f x - = - - 0 . 5 0 9 #~/(1 + b # ) , w h e r e f x - is the activity coefficient a n d /~ the ionic strength, to c o n v e r t c o n c e n t r a t i o n s to activities. F o r all ions, these plots were linear b e t w e e n 10 - t M a n d 10 - 3 o r 10 - 4 M for b o t h i n s t a n t a n e o u s a n d steady-state m e a s u r e m e n t s . The slopes o f the plots, h o w e v e r , w e r e g e n e r a l l y s o m e w h a t less t h a n the N e r n s t i a n value o f -- 58.6 m V expected a t 22 °. R e p r e s e n t a t i v e slopes are p r e s e n t e d in T a b l e I. TABLE
I
SLOPES OF POTENTIAL vs. L O G STEADY-STATE MEASUREMENTS ELECTRODE*
Ion
ANION WITH
ACTIVITY PLOTS F e ( b p h e n ) 2+ L I Q U I D
FOR INSTANTANEOUS MEMBRANE
AND
Slope (m V )
el BrNO~" IBFZ SCN CIO~" PF~ ° 1.5" 10 - 4 M
Instantaneous
Steady state
---------
---------
40.0 47.2 51.3 55.2 50.3 57.7 59.3 59.0
Fe(bphen)
41.7 51.7 54.2 53.8 52.8 54.5 58.3 53.8
2+ in nitrobenzene.
Selectivity coefficients were d e t e r m i n e d b y m e t h o d I of S r i n i v a s a n a n d Rechnitz tT. In this p r o c e d u r e t h e potential o f t h e ion-selective e l e c t r o d e in a solution c o n t a i n i n g ion X - is c o m p a r e d to the p o t e n t i a l of t h e e l e c t r o d e in a solution c o n t a i n i n g n i t r a t e a t the s a m e activity. It is a s s u m e d t h a t the e l e c t r o d e r e s p o n s e o b e y s the e q u a t i o n E = E o - - 0 . 0 5 8 6 log [aND; + K x - m o 3 - ( a x - ) ]
(1)
In this w o r k t h e selectivity coefficient K x - r o D ; was c a l c u l a t e d f r o m two p o t e n t i o metric m e a s u r e m e n t s b y m e a n s o f eqn. (2): log Kx-/NOr = Enos (aNO~)-- Ex- ( a x - ) 0.0586
(2)
at activities ax - = aND3- c o r r e s p o n d i n g to 10-2 M c o n c e n t r a t i o n s in the two solutions.
Dependence o f electrode selectivity on liquid membrane composition T a b l e II presents i n s t a n t a n e o u s a n d steady-state selectivity coefficients for
SELECTIVITY FABLE
OF
ANION-SELECTIVE
429
ELECTRODES
II
INSTANTANEOUS IN N I T R O B E N Z E N E
AND
STEADY-STATE
SELECTIVITY
COEFFICIENTS
FOR
F e ( b p h e n ) a z÷ E L E C T R O D E S
Results given as log Kx-a~o;) ton
2.9" I 0 - s M F e ( b p h e n ) ~ +
1.5" 10 - 4 M F e ( b p h e n ) ] +
1.5" 10 - a M ".Fe(bphen)] +
Inst.
S t e a d y state
Inst.
Steady state
Inst.
Steady state
- - 1.38 -- 0.52 0.00 0.73 0.90 1.26 2.00 0.89
-- 1.704-0.02 - - 0.68 4- 0 . 0 2 0.00 0.95 ~ 0.02 1.01 4- 0 . 0 2 1.494-0.02 2 . 6 0 4- 0 . 0 1 3.65 4- 0 . 0 8
- - 1.20 -- 0.64 0.00 0.94 0.65 1.28 1.55 1.96
-- 1.604-0.02 - - 0 . 7 2 4- 0 . 0 2 0.00 1.09 4- 0 . 0 4 1.38 ± 0 . 0 6 1.544-0.01 2.81 4- 0.08 3 . 9 0 4- 0 . 0 4
- - 1.57 ' - - 0.68 0.00 0.41 0.41 " 0.76 0.87 1.04
CI" -- 1.604-0.02 [3~. . . . . 0 . 6 2 4- 0.06 NO a 0.00 [0 . 9 8 4- 0.04 BF~ 1.01 q- 0.02 ~CN1.404-0.04 CIO,~ 2 . 5 6 4- 0.03 PF~" 3.49:5:0.06
e l e c t r o d e s c o n t a i n i n g different c o n c e n t r a t i o n s o f F e ( b p h e n ) z + in n i t r o b e n z e n e as the l i q u i d m e m b r a n e . F o r the i n s t a n t a n e o u s m e a s u r e m e n t s the sequence of electrode selectivity is P F ~ > C I O ~ > S C N - > B F , ~ > I - > N O ~ > B r - > C I T h e overall r a n g e o f selectivity coefficients in this s e q u e n c e was c a . 105. Values of K Inst in Table II s h o w a slight b u t detectable inorease with increasing c o n c e n t r a t i o n of F e ( b p h e n ) az + for t h e m o r e selective ions, but c h a n g e s in c o n c e n t r a t i o n of the i o n - e x c h a n g e material d o n o t affect the selectivity o r d e r o f the e l e c t r o d e . With few e x c e p t i o n s t h e electrode selectivity sequence is the same for steadystate a n d i n s t a n t a n e o u s m e a s u r e m e n t s . T h e m o s t a p p a r e n t difference betwAeen the two sets o f m e a s u r e m e n t s is t h a t for ions for w h i c h the selectivity is better t h a n for nitrate, values o f K inst a r e m u c h g r e a t e r t h a n the c o r r e s p o n d i n g values of K 's. F o r c h l o r i d e a n d b r o m i d e ions, f o r w h i c h selectivity is lower, K l"s! is snmller t h a n K s'. These observa.tions are c o n s i s t e n t with the t i m e - d e p e n d e n t b e h a v i o r of the electrode s h o w n in Fig. 1. O w i n g to l o n g e q u i l i b r a t i o n times for s o m e ions it is difficult to o b t a i n r e p r o d u c i b l e values of steady-state selectivity coeffic.ients; therefore, p r i m a r i l y i n s t a n t a n e o u s values a r e used to c h a r a c t e r i z e e l e c t r o d e selectivity in this work. I n s t a n t a n e o u s selectivity coefficients o f electrodes c o n t a i n i n g F e ( p h e n ) az+ a n d t e t r a h e p t y l a m m o n i u m ( T H A +) ion in n i t r o b e n z e n e are given in Table III, The selectivity s e q u e n c e s a n d m a g n i t u d e s o f selectivity coefficients for-these two e l e c t r o d e s were similar to t h o s e o b s e r v e d for the F e ( b p h e n ) ] + electrode. V a l u e s o f K in"t f o r the F e ( b p h e n ) ] + a n d T H A + e l e c t r o d e s were virtually identical for all i o n s . M a g n i t u d e s o f K inst w e r e slightly smaller for the F e ( p h e n ) ] + electrode, indicating a s m a l l e r r a n g e o f selectivity coefficients for this exchanger. C h a n g e s i n selectivity o r d e r w e r e o b s e r v e d o n l y a m o n g iodide, t e t r a f l u o r o b o r a t e a n d t h i o c y a n a t e ions. F o r these ions the s e q u e n c e S C N - > B F ~ - > I , w a s o b s e r v e d w i t h the F e ( b p h e n ) az+ e l e c t r o d e , S C N " > I - > BF~" with the F e ( p h e n ) 2 + e l e c t r o d e a n d BF~- > S C N " > I with t h e T H A + electrode. T h e selectivity coefficients of these t h r e e ions differed by • n o m o r e t h a n a factor o f 5 u n d e r a n y c o n d i t i o n s . I n s t a n t a n e o u s selectivity coefficients o f electrodes c o n t a i n i n g F e ( b p h e n ) 2 + in c h l o r o f o r m a n d a m y l a l c o h o l are given i n T a b l e IY. T h e selectivity s e q u e n c e oh-
430
R.E. REINSFELDER,
F. A . S C H U L T Z
T A B L E III INSTANTANEOUS SELECTIVITY COEFFICIENTS OF AMMONIUM ION ELECTRODES IN NITROBENZENE
F c ( p h c n ) 2+
AND
TETRAHEPTYL--
( R e s u l t s g i v c n a s log K x - ~ o y ) Ion
5.10-'* M Fe(phen)~+:
CIBr NO~ IBFZ SCN ClOg PFg
--0.48 -- 0.21 0.00 1.18 0.68 1.34 2.28 3.20
1"10 - 2 M T H A + - - 1.444-0.11 -- 0.62 + 0.08 0.00 1.04+0.04 1.58 4- 0.09 1.48 +_0.06 2.864-0.11 4.114-0.11
T A B L E IV INSTANTANEOUS SELECTIVITY COEFFICIENTS CHLOROFORM AND AMYL ALCOHOL
OF Fe(bphen)~ + ELECTRODES
IN
( R e s u l t s g i v e n a s log K x - mo~ )
Ion
2.10 -a M Fe(bphen)] + in chloroform
2.10 -a M Fe(bphen)] + in a m y l alcohol
CIBrNO3 IBFg SCN CIO,~ PFg
- 1.34+0.06 --0.18+0.01 0.00 1.74 _+0.03 0.90 + 0.05 1.54 + 0.02 2.08 + 0.08 2.90 + 0.11
--0.60 --0.21 0.00 0.40 -- 0.07 0.72 0.68 0.82
served in c h l o r o f o r m w a s P F g > C l O g > I - > S C N - > B F 2 > N O ~ > B r - > C I - , w h i c h is similar to s e q u e n c e s o b s e r v e d with n i t r o b e n z e n e . Relative t o v a l u e s in nitrobenzene, selectivities for h e x a f l u o r o p h o s p h a t e a n d perchlorate were r e d u c e d by factors o f 10 a n d 5, respectively, and the selectivity for iodide w a s e n h a n c e d by a. factor o f 5. In a m y l a l c o h o l , selectivities for i o n s w i t h values a b o v e nitrate were all d i m i n i s h e d significantly a n d a very limited range o f selectivity coefficients w a s o b s e r v e d . T h e selectivity s e q u e n c e c o m m o n to the o t h e r s o l v e n t s w a s m o d i f i e d in a m y l a l c o h o l , wherein the s e q u e n c e o b s e r v e d w a s P F g > S C N - > CIO,~ > I - >
N O T >BF~" > B r - > C I - . F o r electrodes c o n t a i n i n g F e ( p h e n ) 2+ a n d T H A + in n i t r o b e n z e n e a n d F e ( b p h e n ) 2 + i n n i t r o b e n z e n e a n d c h l o r o f o r m , a c o m m o n s e q u e n c e o f a n i o n selectivity w i t h similar selectivity coefficients w a s o b s e r v e d : P F 6 > C l O g > S C N - , - ~ B F g --, I - > NO~" > B r - > C I - . A similar s e q u e n c e w a s o b s e r v e d for c o m m e r c i a l perchlorate a n d nitrate i o n electrodes, for w h i c h selectivity coefficients are listed in T a b l e V. T h e s e q u e n c e is valid for b o t h O r i o n electrodes, w h i c h are liquid m e m -
SELECTIVITY TABLE
OF
ANION-SELECTIVE
431
ELECTRODES
V
SELECTIVITY
COEFFICIENTS
OF
COMMERCIAL
ANION-SELECTIVE
ELECTRODES
(Results given as log Kx-~oj-) Ion
Perc.qlorate Orion
CIBr NO~" IBF,~ SCNCIO~" PF~
electrodes
92-81*
Beckman
-- 1.82 -- 0 . 4 3 0.00 0.90 --. 2.87 --
Nitrate 39616 b
-0.94 -- 0 . 4 0 0.00 0.86 1.52 1.60 2.25 2.90
electrodes
Orion 92-07 ~
Becknlan
--2.22 - 1.00 0.00 1.30 --3.00 --
--0.82 -- 0 . 3 4 0.00 0.78 0.96 1.26 1.65 2.10
396.I8 b
a Ref. 1, p p . 7 0 - 7 1 . Ref. 19.
b r a n e e l e c t r o d e s o f the t y p e u s e d in this w o r k , a n d B e c k m a n electrodes, w h i c h e m p l o y an i m m o b i l i z e d o r "solid" i o n - e x c h a n g e m e m b r a n e . It t h e r e f o r e a p p e a r s that for a n i o n - s e n s i t i v e e l e c t r o d e s b a s e d o n i o n - a s s o c i a t i o n e x t r a c t i o n s y s t e m s c h a n g e s in the i o n - e x c h a n g e material, c o n c e n t r a t i o n o f t h e ion exchanger, solvent or m a n n e r of m e m b r a n e c o n s t r u c t i o n h a v e little effect, o n t h e selectivity o f t h e electrode. T h e o n l y significant difference is o b s e r v e d w h e n a m y l a l c o h o l is used as solvent. In this case, however, t h e effect is to d i m i n i s h r a t h e r t h a n e n h a n c e the r a n g e o f e l e c t r o d e selectivity. T h e t h e o r y of l i q u i d m e m b r a n e e l e c t r o d e b e h a v i o r s 2. t a' defines three l i m i t i n g cases in w h i c h e l e c t r o d e selectivity m a y be r e l a t e d to the i o n - p a r t i t i o n i n g process. In t w o cases,.where the i o n - e x c h a n g e site a n d t h e c o u n t e r i o n a r e h i g h l y d i s s o c i a t e d in t h e m e m b r a n e p h a s e o r w h e r e the i o n - e x c h a n g e site a n d c o u n t e r ion are h i g h l y a s s o c i a t e d b u t t h e m o b i l i t y of t h e c o u n t e r i o n is m u c h g r e a t e r t h a n t h a t o f the site, t h e selectivity coefficient is given b y t h e expression: fix- k x K x , IV- -- fly_ kv.-
(3)
w h e r e fix- a n d fly- are t h e mobilities o f X - a n d Y - . in the m e m b r a n e p h a s e , a n d kx- a n d k v - are the single i o n - p a r t i t i o n coefficients b e t w e e n w a t e r a n d the s o l v e n t of the m e m b r a n e . T h e r a t i o k x - / k v - r e p r e s e n t s the e q u i l i b r i u m c o n s t a n t for the ionexchange process k x - Ik~¢ . •
X- (aqueous) +Y- imembrane) •
.
= X - ( m e m b r a n e ) + Y " (aqueous)
(4)
F o r a m e m b r a n e in w h i c h there is s t r o n g a s s o c i a t i o n a n d the sites are highly m o b i l e relative to the c o u n t e r ions, the selectivity coefficient is a p p r o x i m a t e d b y : K x - / v - ---- ffRX._K k y •
URY
.
(5)
432
R. E. R E I N S F E L D E R , F. A. S C H U L T Z
w h e r e fiRX a n d fi•v a r e the mobilities o f t h e ion-site pairs, a n d K~v is the e q u i l i b r i u m c o n s t a n t for the i o n - e x c h a n g e r e a c t i o n : X - ( a q u e o u s ) + R - Y ( m e m b r a n e ) ~:~ ~ ( m e m b r a n e )
+ Y- (aqueous)
(6)
If Kk-~ a n d K ~ are the dissociation c o n s t a n t s of the ion-site pairs in the m e m b r a n e , then k x - K~-v K k v = ky-KK-g
(7)
T h e e x t e n t o f ion a s s o c i a t i o n in o r g a n i c solutions c o r r e s p o n d i n g to the . c o m p o s i t i o n o f the liquid m e m b r a n e s u s e d in this w o r k is n o t k n o w n . C o n d u c t a n c e studies o f smaller q u a t e r n a r y a m m o n i u m salts in n i t r o b e n z e n e 2°-22 show t h a t ion-pair diss o c i a t i o n c o n s t a n t s a r e ca. 4- 10- 2 a n d d o n o t differ g r e a t l y a m o n g the anions s t u d i e d in this w o r k . The mobilities o f the a n i o n s a r e typically 2 to 3 times g r e a t e r t h a n the mobilities o f the q u a t e r n a r y a m m o n i u m ions. I o n a s s o c i a t i o n c o n s t a n t s o f Fe= (phen)a2 + a n d F e ( b p h e n ) ] + salts in n o n a q u e o u s solvents are n o t k n o w n , b u t it is e x p e c t e d t h a t mobiiities o f these c a t i o n s a r e m u c h less t h a n t h o s e o f the a n i o n s . It t h e r e f o r e seems r e a s o n a b l e to c o n c l u d e t h a t the selectivity coefficients o f b o t h the q u a t e r n a r y a m m o n i u m ion a n d i r o n - p h e n a n t h r o l i n e c o m p l e x electrodes c a n be a p p r o x i m a t e d b y t h e expression ( a s s u m i n g that fix- ----uv-) ,~ kx-' K x - /Y'~ -- k v -
(8)
E q u a t i o n (8) predicts that e l e c t r o d e selectivity is i n d e p e n d e n t o f the i o n - e x c h a n g e •m o l e c u l e a n d its c o n c e n t r a t i o n in the m e m b r a n e phase. T o a first a p p r o x i m a t i o n this is d e m o n s t r a t e d by the results o f the p r e s e n t study. E q u a t i o n s (4) a n d (8) p r e d i c t that e l e c t r o d e selectivity d e p e n d s p r i m a r i l y o n the relative strengths o f i o n - s o l v e n t ' i n t e r a c t i o n s in the t w o phases. T h e c h a n g e in s o l v a t i o n energy in eqn. (4) c a n be expressed in t e r m s o f the differences in enthalpie, s o f h y d r a t i o n in the a q u e o u s p h a s e a n d e n t h a l p i e s o f solvati0n in t h e m e m b r a n e p h a s e of i o n s X - a n d Y - : AHx° - n" - = (AH°=m, x - -- AH°.m. Y - ) .-- (AH.°, x - -- AH~, v - )
(9)
E n t h a l p i e s r a t h e r t h a n free energies o f solvation are c h o s e n b e c a u s e m o r e extensive values o f the f o r m e r q u a n t i t i e s a r e available for the p u r p o s e o f c o m p a r i s o n . I n the a p r o t i c solvents n i t r o b e n z e n e a n d c h l o r o f o r m , differences in s o l v a t i o n energies o f a n i o n s a r e e x p e c t e d to be smaller t h a n in water. T h e r e f o r e , regardless o f the s o l v e n t used to f o r m the m e m b r a n e , e l e c t r o d e selectivity is d e t e r m i n e d p r i m a r i l y by differences in h y d r a t i o n energies in the a q u e o u s phase. This is s h o w n in T a b l e VI w h e r e t h e enthalpies o f h y d r a t i o n a n d ionic radii o f i o n s studied in this w o r k a r e presented. T h e r e is g o o d c o r r e l a t i o n b e t w e e n e l e c t r o d e selectivity a n d i o n i c size a n d h y d r a t i o n enthalpy, T h e ions for w h i c h the e l e c t r o d e s h a v e h i g h e s t selectivity a r e the large h y d r o p h o b i c a n i o n s having t h e smallest n e g a t i v e h y d r a t i o n energies. In c o m p e t i t i o n with smaller, m o r e s t r o n g l y h y d r a t e d a n i o n s , these a n i o n s find t h e m selves preferentially d i s t r i b u t e d into the m e m b r a n e p h a s e of the electrode. S i m i l a r explanations, b a s e d o n relative a n i o n h y d r a t i o n energies, have b e e n a d v a n c e d to explain t h e solvent e x t r a c t i o n b e h a v i o r o f a n i o n s 2s' 29 a n d the selectivity o f a n i o n e x c h a n g e resins a°. It, therefore, a p p e a r s t h a t unless s o m e m e a n s o f p r o m o t i n g s t r o n g
SELECTIVITY
OF ANION-SELECTIVE
ELECTRODES
433
TABLE VI HYDRATION
Ion
ENTHALPIES
AND
IONIC
Reference
AH°;x -
(kcal mole-l)
CIBr-
NO~ IBF~ SCNCIOZ
RADII OF UNIVALENT
Radius (,4)
ANIONS
Reference .
--87.6 -- 79.8 -- 74.5 -- 69.7 -71.2
23 23 24 23 23
1.81 1.95 1.96 2.16 2.26
25 25 26 25 23
57.1
23
2.45 3.00
23 27
--
PF~
selective interactions in the m e m b r a n e phase can be found, the selectivity of liquid m e m b r a n e electrodes for c o m m o n inorganic anions Cannot be greatly altered from the orde r predicted by hydr a t i on •energies. Possible evidence of stronger inte/:actions in the m e m b r a n e phase exists in the case of the Fe(bphen)~ + - a m y l alcohol electrode, but the result is to decrease the range of electrode selectivity. Because amyl alcohol is an h y d r o x y l i c solvent, it i s expected to solvate anions to a degree more nearly equal that of water. As a result, differences in solvation energies between the aqueous a n d m e m b r a n e phases are minimized a n d more nearly equal selectivity coefficients are observed. The factors influencing electrode selectivity appear to be the same as those governing solvent extraction processes 14. Unfortunately there are few data on which quantitative comparisons o f selectivity a n d extraction behavior can be made. O n e instance in which a quantitative c o m p a r i s o n Can be made is illustrated in Fig. 2. The instantaneous selectivity coefficients o f a Fe(phen)~+'nitrobvnzene electrode a r e
3
/SCN- I Br:
-
"28
....
I 4
l 0
-4
I
I -8
-12
.
L~G~tr, kcol mole "1'
F i g . . 2 . . P I o t o f logK=x'=-t/N0~ vs. f r e e e n e r g y o f e x t r a c t i o n of F e ( p h e n ) ~ + salts i n t o n i t r o b e n z e n e a c c o r d i n g t o e q u i l i b r i u m 2 o f ref, 31. E l e c t r o d e : 5 . I 0 , 4 M F e ( p h e n ) ] + i n n i t r o b e n z e n e , ~ . " ~ :
434
R. E. R E I N S F E L D E R , F. A. S C H U L T Z
c o m p a r e d to the free e n e r g i e s of e x t r a c t i o n o f t h e c o r r e s p o n d i n g F e ( p h e n ) 2 + salts i n t o n i t r o b e n z e n e 31. G o o d c o r r e l a t i o n is o b t a i n e d for the five ions studied. O t h e r s o l v e n t e x t r a c t i o n s t u d i e s s h o w a n o r d e r o f a n i o n e x t r a c t a b i l i t y q u a l i t a t i v e l y similar o r identical to the o r d e r o f selectivity f o u n d in this w o r k . S u c h similar s e q u e n c e s a r e f o u n d in s y s t e m s e m p l o y i n g q u a t e r n a r y a m m o n i u m ions in t o l u e n e a n d d i c h l o r o m e t h a n e 32"33, t r i p h e n y l t e t r a z o l i u m salts in c h l o r o f o r m 34 a n d q u a t e r n a r y p h o s p h o n i u m , a r s o n i u m a n d s t i b o n i u m salts in c h l o r o f o r m , d i c h l o r o m e t h a n e , 1,2-dic h l o r o e t h a n e a n d 2 , 2 ' - d i c h l o r o d i e t h y l e t h e r 29. SUMMARY
Selectivity coefficients o f l i q u i d - m e m b r a n e e l e c t r o d e s for c o m m o n i n o r g a n i c a n i o n s w e r e m e a s u r e d in e l e c t r o d e s c o n t a i n i n g t r i s ( 1 , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) , t r i s ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) o r t e t r a h e p t y l a m m o n i u m i o n in nit r o b e n z e n e , a n d t r i s ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n e ) i r o n ( I I ) i o n in n i t r o b e n z e n e , c h l o r o f o r m or n - a m y l a l c o h o l as the l i q u i d m e m b r a n e . W i t h t h e e x c e p t i o n o f t h e a m y l a l c o h o l electrode, selectivity coefficients w e r e relatively i n d e p e n d e n t o f m e m b r a n e c o m p o s i t i o n a n d f o l l o w e d a c o m m o n s e q u e n c e Of d e c r e a s i n g selectivity: PF~- > C I O ~ > S C N - ,~ I - -~ BF~" ~>N O a - > B r - > C I - . T h i s s e q u e n c e p a r a l l e l s t h e o r d e r of i n c r e a s i n g a n i o n h y d r a t i o n energy, s u g g e s t i n g t h a t a q u e o u s p h a s e s o l v a t i o n energies p l a y a p r e d o m i n a n t r o l e in d e t e r m i n i n g e l e c t r o d e selectivity for these i o n s . T i m e - d e p e n d e n t b e h a v i o r o f l i q u i d - m e m b r a n e e l e c t r o d e s o n transfer b e t w e e n s o l u , tions c o n t a i n i n g different ions also is d e s c r i b e d . I n s t a n t a n e o u s e.m.f, r e a d i n g s w e r e used t o d e t e r m i n e selectivity coefficients. RI~.SUMI~
. . . . . . . . I ) e s ' ~ e s u r e s d e coefficients de s~lectivit6 d ' ~ l e c t r o d e s it m e m b r a n e l i q u i d e o n t ~t6 effectu~es p o u r des a n i o n s i n o r g a n i q u e s c o u r a n t s , it l'aide d,61ectrodes c o n t e n a n t : tris-(1,10-p.h~nanthroline)fer_(I!) , t r i s - ( 4 , 7 , d i p h 6 n y l - ! , 1 0 - p h ~ n a n t h r o l i n e ) f e r ( I I ) o u ion t 6 t r a h e p t y l a m m o n i u m , d a n s le n i t r o b e n z ~ n e , et t r i s - ( 4 , 7 - d i p h ~ n y l - l , 1 0 - p h ~ n a n throline)fer(II~, d a n s le n i t r o b e n z ~ n e , le c h l o r o f o r m e , o u l ' a l c o o l - n - ~ m y l i q u e c o m m e m e m b r a n e liquide. A l ' e x c e p t i o n de l'~lectrode it a l c o o l a m y l i q u e , les coefficients de s61ectivit6 p r ~ s e n t e n t des v a l e u r s d ~ c r o i s s a n t e s , d a n s l'ordre: PF~->CIO~> S C N - ~. I - ~ BF~- > N O i " > B r - > C I - , l a i s s a n t s u p p o s e r q u e les ~nergies d e s o l v a t a t i o n j o u e n t ici u n r61e p r e d o m i n a n t . O n e x a m i n e 6 g a l e m e n t l'influence d u temps. L a d 6 t e r m i n a t i o n des coefficients de s61ectivit6 a ~t~ faite avec l e c t u r e f.e.m, i n s t a n t a n ~ e . ZUSAMMENFASSUNG
Selektivitiitskoeffizienten yon F ! f i s s i g - M e m b r a d 2 E l e k t r o d e n ftir d i e tiblichen a n o r g a n i s c h e n A n i o n e n w u r d e n mit. E l e k t r o d e n g e m e s s e n , die T r i s - ( 1 , 1 0 - p h e n a n t h r o l i n ) eisen(II)-, T r i s - ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n ) e i s e n ( I I ) - u n d T e t r a h e p t y l a m m o n i u m - I o n in N i t r o b e n z o l u n d T r i s - ( 4 , 7 - d i p h e n y l - l , 1 0 - p h e n a n t h r o l i n ) e i s e n ( I I ) - I o n in N i t r o b e n z o l , C h l o r o f o r m o d e r n , A m y l a l k o h o l als Fltissig_-Membran e n t ' • lii~lten. 1VIit A u s n a h m e d e r A m y l a l k o h o l - E l e k t r o d - e w a r e n . die Selektivitiitskoeffizien-
SELECTIVITY OF ANION-SELECTIVE ELECTRODES
435
ten relativ unabh~ingig van der M e m b r a n z u s a m m e n s e t z u n g u n d folgten ¢iner norm a l c n Rcihenfolge klciner w c r d e n d e r Sclektivit~it: PF~" > C 1 0 ~ - > S C N "~,,~J BF~" > N O 3 > B r - > Cl -. Diese Reihenfolgc entspricht dcr Reihenfolgc z u n c h m c n d c r A n i o n e n - H y d r a t a t i o n s c n c r g i c u n d weist darauf hin, dass die Solvatationscnergien in der w~issrigen Phase cin¢ d o m i n i c r e n d c Rollc spielcn bci der B e s t i m m u n g der Elcktrodenselektivit~it far diesc Ionen. D a s Zeitverhalten der F l t i s s i g - M e m b r a n E l e k t r o d c n beim ~ b e r g a n g z w i s c h e n L 6 s u n g c n , die verschiedcne I o n e n enthaltcn, w i r d ebcnfalls beschriebcn. Die e.m.k.-Wertc w u r d e n m o m e n t a n aufgezeichnet, u m die Sclektivit~itskocffizientcn z u bcstimmen. REFERENCES 1 J. W. Ross, Jr., in R. A. Durst, Ion-Selective Electrodes, U.S. Nat. Bur. Stand. Spec, Publ. No. 314, W a s h i n g t o n , D.C., 1969, Chap. 2. 2 C. J. Coetzee and H. Freiser, Anal. Chem., 40 (1968) 2071. 3 C. J'. Coetzee a n d H. Freiser, Anal. Chem., 41 (1969) 1128. 4 P. R. Danesi, F. Salvemini, (3. Sciboha and B. Scuppa, J. Phys. Chem., 75 (1971) 554. 5 P. R. Danesi, G. Scibona a n d B. Scuppa, Anal. Chem., 43 (1971) 1892: 6 N. I s h i b a s h i a n d H. K o h a r a , Anal. Lett., 4 (1971) 785. 7 S. G. Back, Anal. Lett.~ 4 (1971) 793. 8 E. Paglia-Dubini, T. Mussini a n d R. (3alli, Z. Naturforsch. A, 26 (1971) 754. 9 W. M. H a y n e s a n d J. H. Wagenknecht, Anal. Lett., 4 (197i) 491. 10 M. M a t s u i and H. Freiser, Anal. Lett., 3 (1970) 161. 11 G. Seibona, L. Mantella a n d P. R. Danesi, Anal. Chem., 42 (1970) 844. 12 J . P. S a n d b l o m , (3. E i s e n m a n a n d J. L. Walker, Jr., J. Phys. Chem., 71 (1967) 3862. 13 G . : E i s e n m a n , in R. A. DurSt, Ion-Selective Electrodes, U.S. Nat. Bur. Stand. Spec. Publ. No. 314, W a s h i n g t o n , D.C., 1969, C h a p . 1. 14 H. J. James, (3. P. C a r m a c k and H. Freiser, Anal. Chem., 44 (1972) 853. 15 (3. Scibona, J. F. Byrum, K: K i m u r a a n d J. W. Irvine, Jr., in D. Dyrssen, J . O . Liljenzin and J. R y d berg, Solvent Extraction Chemistry, Wiley-Interscience, New York, 1967, pp. 398--407. 16 (3. A. Rechnitz and Z. F. Lin, Anal.• Chem., 40(1968) 696. 17 K. Srinivasan a n d G. A. Rechnitz, AnaLChem., 41 (1969) 1203. 18 J. Bagg a n d R. Vinen, Anal. Chem., 44 (1972) 1773. 19 F. A. Schultz, F l o r i d a Atlantic University, 1969, unpublished work. 20 C. R. W i t s c h o n k e a n d C. A. Kraus, J. Amer. Chem. Sac., 69 (1947) 2472. 21 E. Hirsch a n d R. M. Fuoss, 3. Amer: Chem. Sac., 82 (1960) 1018. 22 F. R . L o n g o , J. D. Kerstetter, T. F. K u m o s i n s k i a n d E. C. Evers, J. Phys. Chem., 7 0 ( 1 9 6 6 ) 4 3 i . 23 M. F. Halliwell and S. C. Nyburg, J. Chem. Sac., (1960) 4603. 24 (3. A. K r e s t o v and I. V . Egorova, lzv. Vyssh. Ucheb. Zaved., Khim. Teklmoi., 10 (1967) 750; Chem. Abstr., 69 (1968) 22691c. 25 L. Pauling, The Nature of the ChemicalBond, Cornell University Press, Ithaca, N.Y., 3rd Ed., 1960. 26 W. Masterton, D. Boloc0fsky a n d T. P. Leo,.J. Phys. Chem., 75 (1971) 2089. 27 R. D. W. Kemmitt, D. R. Russell a n d D. W. A. S h a r p , J. Chem. Sac., (1963) 4408. 28 R, M . D i a m o n d a n d D. (3. Tuck, Progr. lnoro. Chem., 2 (1960) 109. 29 N. A. (3ibsorl a n d D. C. W e a t h e r b u r n , Anal. Chim. Acta, 58 (1972) 159. 30 B. Chu, D. C. Whitney a n d R. M. D i a m o n d , d. lnoro. Nucl. Chem,, 24 (1962) 1405. 31 Y. Y a m a m o t o , T. T a r u m o t o and E. I w a m o t o , Anal. Lett., 2(1969) 1. 32 K. (3ustavii a n d (3. Schill, Acta Pharm. Suec., 3 (1966) 259. 33 I. M . Ivanov, L, M. (3indin a n d (3. N. Chichagova, lzu. Sib. Otd. Akad. Nauk SSSR, Set. Khim. Nauk, (1967) 100; Chem. Abstr., 69 (1968) 13377c. 34 J. Hala, Collect. Czech. Chem. Commun., 30 (1965) 1813.