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Ein beitrag zur problem der membranelektrode. Die bestimmung des äquivalenzpunktes mit hilfe des membranpotentials. I. Fällungstitrationen von silbersalzen.
A 60
Journal of Electroanalytical Chemistry
VOL. 1
Acco r d i n g to these p r i n c i p l e s the following c a s e s may be distinguished: (.a) c u r r e n t z e r o , I . E . + R . E . ; ( b ) c u r r e n t constant, b 1 : 2 I . E . , b~ : I . E . + R . E . ; (c) v a r i a b l e c u r r e n t , c 1 : 2 I . E . , c 2 : I . E . + R . E . (a) C u r r e n t zero: this constitutes c l a s s i c a l potentiometry. The author mentions a number of r e c e n t developments in indicator and r e f e r e n c e e l e c t r o d e s , in methods for s i m p l e r and s h a r p e r e s t i m a t i o n s of the end-point and in automation. (b) C u r r e n t constant: b l : 2 I . E . ; b2: I . E . + R . E . : the c u r r e n t is g e n e r a l l y of the o r d e r of m i c r o a m p e r e s . If a f t e r each addition of a c e r t a i n amount of r e a g e n t a p o l a r o g r a m of the solution w e r e r e c o r d e d , then from these p o l a r o g r a m s one could read the difference in potential between the two e l e c t r o d e s (2 I . E . o r I. E. + R . E . ) fo r any given value of the c u r r e n t p a ssi n g through the t i t r a t i o n cell. The author has c a r r i e d this out by m e a n s of v o i t a m m e t r y for the t i t r at i o n of iron (ILI) with titan (III). (c) Variable current: e 1 : 2 I. E. : this method, called p o l a r o v o l t r y is a t r a n s i tion-type between p o t e n t i o m e t r y and a m p e r o m e t r y , both with 2 I . E . Graphical d e r i v a t i o n of the potentiogram is not possible h e r e . c2: I . E . + R. E. : sh o r t c i r cuited along a r e s i s t a n c e + g~lvanometer. This is a v e r y s i m p l e and cheap method. Here again it is not possible to give a g r a p h ic a l d e r i v a t i o n of the potentiogram. C . I . Moo. [196l AN AUTOMATIC T I T R I M E T E R FOR PLOTTING T R U E - S C A L E TITRATION CURVES. H. I r v i n g and L . D . Pettit. (Inorganic C h e m i s t r y L a b o r a t o r y , South P a r k s Road, Oxford, Gt. B r i t a i n . ) Analyst, 84 (1959) 641. A d e s c r i p t i o n of a modification to a Honeywell-Brown r e c o r d e r is r e p o r t e d , in o r d e r to obtain a p o t e n t i o m e t r i c ti tr a ti o n c u r v e in which the ordinate scal e (pH) always r e m a i n s d i r e c t l y proportional to the volume of titrant used, and also to r e a c h the best e q u il ib r iu m conditions .~vhen the rate of a d m i s s i o n of the t i t r a n t is varied during the titration (from 0.005 to 0.02 m l / m i n ) . The modification uses a single m o t o r , whose speed is v a r i e d by a V a r i a c t r a n s f o r m e r , so that the r e c o r d ing ch art and the titrant d e l i v e r i n g m i c r o - s y r i n g e may be d r i v e n simultaneously. All the apparatus d e s c r i b e d is for n o r m a l usage; m o r e o v e r a modification to the r e c o r d e r is r e p o r t e d f o r when it is used in conjunction with an E . I . L . Vibron e l e c t r o m e t e r , rood. 33B, and the p H - m e a s u r i n g attachment, rood. C33B, which allows d i r e c t readings to be'obtained from the ch ar t graduations to 0.01 unit of pH, and 0.003 unit of pH o r b e t t e r to be e s t i m a t e d . The p r e c i s i o n of the apparatus is tested by titration of 0.001 M EDTA solution and some f u r t h e r applications a r e outlined. [197]
G . d e An.
EIN BEITRAG ZUR PROBLEM DER MEMBRANELEKTRODE. DIE BESTIMMUNG DES )/.QUIVALENZPUNKTES MIT H I L F E DES MEMBRANPOTENTIALS. I. FJ~LLUNGSTITRATIONEN VON SILBERSALZEN. J. B e r s i e r , P. B e r s i e r , W. Trueb und F . H t i g l i . ( T e c h n i k u m , Winterthur, S e h w e i z . ) H e l v . C h i m . A c t a , 4 2 (1959) 55. Das feinporige M e m b r a n s y s t e m yon Aluminiumoxyddeckschichten wurde als M e m b r a n e l e k t r o d e bei F~tllingstitrationen yon S i l b e r s a l z e n verwendet. Die M e m brane wurden d u t c h anodische Oxydation yon R e i n s t a l u m i n i u m in 0.4 N O x a l s ~ u r e b~idern (Stromdichte ungePAhr 10 m A / c m 2) erhalten. Die Schichtdicke liegt zwiSchen 0.05 und 0.08 ram. Zu den p o t e n t i o m e t r i s c h e n Ti t r at i o n en wurden die Membrane in e i n e r geeigneten A p p a r a t u r als M e m b r a n e l e k t r o d e n e i n g e s e t z t . Die E l e k t r o l y t schliissel zu B e z u g s e l e k t r o d e n e r f o r d e r t e n b e s o n d e r e Sorgfalt. Das g e m e s s e n e Membranpotential ist gegen Rtihrung er~ pfindlich. Man rtthrt entweder m e c h a n i s c h oder d u t c h Luftstrom. Man m u s s darauf achten, dass die Veri~nderungen d e r Niveauunterschiede zwischen A u s s e n - und InnenlSsung sich auf + 0.5 cm beschr[inkten. Die Aufbewahrung d e r Membrane e r f o lg t in 0.1 N KCI-LSsungen. Zu den Potentialmessungen wurden K o m p e n s a t o r e n d e r F i r m a Metrohm (Herisau) verwendet. D e r g e s a m t e Messleehler betriig~ unge~ahr ± 3 inV. Es sind s~imtliche p o t e n t i o m e t r i s c h e T i t r a t i o n s k u r v e n d e r m i t M e m b r a n e l e k -
1960
A bs t r ac t s Section
A 61
troden ausgeKihrten ~allungstitration~n v o r g e l e g t and zwar die T i t r a t i o n s k u r v e n 2~von CI-, SCN-, B r - , J , C 2 0 4 , C r O 4 m i t Ag+ und yon Ag4" m i t CI-. Ja. Inc. [1981 STABILITY OF M E T A L COMPLEXES IN SOLUTION. C h e m . , 19 (1959) 156.
B.N. Mattoo.
Z.physik.
R e s u l t s a r e p r e s e n t e d of a p o t e n t i o m e t r i e study of ion a s s o c i a t i o n in f e r r i c sulphate and n i t rat e solutions at low total f e r r i c ion concentration which excludes the po s s i b i l i t y of any significant f o r m a t i o n of the polynuclear m e t a l - a n i o n c o m p l e x e s . Redox potential m e a s u r e m e n t s of Fe II/III solutions at low Fe III co n cen t r at i o n in the p r e s e n c e of sulphate and n i tr a te and using a Pt indicator e l e c t r o d e have been i n t e r p r e t e d in t e r m s of ion a s s o c i a t i o n in sulphate solutions which gives r i s e to c o m p l e x s p e c i e s Ye(SO4)+, Fe~S04)2, and at high SO42- concentration Ye(SO4)~-, and the single c o m p l e x Fe(NOs) in nitrate solutions. Competing e q u i l i b r i a , m a i n Iy h y d r o l y s i s , have been put f o r w a r d to account for K~tso: values at v ar i o u s I ' s interpolated (or extrapolated) f r o m the l i t e r a t u r e data a~d for p r e s e n t m e a s u r e ments at I = 0.275 (KHso~ = 0.034). R e s u l t s at v ar io u s I ' s have yielded the following values of the stepwise t h e r m o dynamic f o r m a t i o n constants at 25 ° using the extended Debye-iKickel equation for Fe(SO4)n3- 2n. PK1 = -4.04; pK 2 = -1.34; 5G 1 = -5.52 k C a l / m o l e ; 5G 2 = -1.83 k C a l / m o l e . The f o r m a t i o n of the r e l a t i v e l y unstable Ye(HSO4) 2+ co m p l ex has a l s o been accounted for tn the analysis of ~ e data by introducing one additional t e r m (K[ = f o r m a t i o n constant f o r Ye(HSO4)Z+); although a r e l i a b l e value for K{ cannot be given, it is definitely s m a l l enough to have no a p p r e c i a b l e influence on the r e s u l t s given for the sulphate c o m p l e x e s . Yor Fe(NO3) 2. the r e s u l t s are: pK 1 = -1.00; AG 1 = -1.36 k C a l / m o l e . The value of the f o r m a t io n quotient Ks, [Ye(SO4)33- ]/[/e(SO4)~][SO42-] has been assigned a p r e l i m i n a r y value of 0.5 at I = 3,0 (1 M Na2SO 4 solution). [199]
Di. Ci6.
E L E C T R O - M E T R I C STUDIES ON MERCUROUS TUNGSTATE. R . S . Saxena and C. M. Gupta. Z. physik. C h e m . , 19 (1959) 94. A r e v i e w of the e x i s t i n g l i t e r a t u r e shows that m e a g r e r e f e r e n c e s ar c available on the study of the r e a c t i o n between Na2WO 4 and ttg2(NO3) 2 in the p r e s e n c e of di ff eren t acid concentrations. In the absence of any detailed study on m e r c u r o u s tungstate~ it was c o n s i d e r e d worthwhile to investigate it c o m p r e h e n s i v e l y by e l e c t r o m e t r i c methods. In the paper r e s u l t s of c o n d u c t o m e t r i c and p o t e n t i o m e t r i c titrations have been included. The conduetivities w e r e m e a s u r e d by a Kohlrausch U n i v e r s a l Bridge, the a l t e r n a t i n g c u r r e n t was supplied by a low frequency o s c i l l a t o r (1000 c / s e e ) and the b a l a n c e - p o i n t was indicated by the sound m i n i m a in the headphones. C u r v e s w e r e plotted of the c o r r e c t e d conductance and the volume of the titrant. The b r e a k s in the c u r v e s c o r r e s p o n d e d to the formation of d i f f er en t compounds. Using d i f f eren t concentrations of r e a c t a n t s , p o t e n t i o m e t r i c titrations w e r e p e r f o r m e d both by d i r e c t a n d r e v e r s e methods. Hg2(NO3) 2 solution was added from the m i c r o b u r e t t e to Na2WO 4 solution in the electl~ode cell and vice v e r s a and the potential between the platinum indicator e l e c t r o d e and a S . C . E . m e a s u r e d and plotted against the volume of the titrant. A s h ar p deflection in the curve o c c u r s at the end-point which is m a r k e d by a pronounced m a x i m a in 5E/AC. The t i t r a t i o n s w e r e also c a r r i e d out in p r e s e n c e of 1070 and 20% alcohol. I n c r e a s i n g amounts of alcohol a r e found to have no effect on the end-point. Conductivity c u r v e s yielded one sharp b r e a k and p o t e n t i o m e t r i c titrations give one m a x i m a AE/SC c o r r e s p o n d ing to the f o rm at io n of a golden yellow compound 2 HgO. 3WO 3. Di. CiS.
Journal of Electroanalytical Chemistry
VOL. 1
Acco r d i n g to these p r i n c i p l e s the following c a s e s may be distinguished: (.a) c u r r e n t z e r o , I . E . + R . E . ; ( b ) c u r r e n t constant, b 1 : 2 I . E . , b~ : I . E . + R . E . ; (c) v a r i a b l e c u r r e n t , c 1 : 2 I . E . , c 2 : I . E . + R . E . (a) C u r r e n t zero: this constitutes c l a s s i c a l potentiometry. The author mentions a number of r e c e n t developments in indicator and r e f e r e n c e e l e c t r o d e s , in methods for s i m p l e r and s h a r p e r e s t i m a t i o n s of the end-point and in automation. (b) C u r r e n t constant: b l : 2 I . E . ; b2: I . E . + R . E . : the c u r r e n t is g e n e r a l l y of the o r d e r of m i c r o a m p e r e s . If a f t e r each addition of a c e r t a i n amount of r e a g e n t a p o l a r o g r a m of the solution w e r e r e c o r d e d , then from these p o l a r o g r a m s one could read the difference in potential between the two e l e c t r o d e s (2 I . E . o r I. E. + R . E . ) fo r any given value of the c u r r e n t p a ssi n g through the t i t r a t i o n cell. The author has c a r r i e d this out by m e a n s of v o i t a m m e t r y for the t i t r at i o n of iron (ILI) with titan (III). (c) Variable current: e 1 : 2 I. E. : this method, called p o l a r o v o l t r y is a t r a n s i tion-type between p o t e n t i o m e t r y and a m p e r o m e t r y , both with 2 I . E . Graphical d e r i v a t i o n of the potentiogram is not possible h e r e . c2: I . E . + R. E. : sh o r t c i r cuited along a r e s i s t a n c e + g~lvanometer. This is a v e r y s i m p l e and cheap method. Here again it is not possible to give a g r a p h ic a l d e r i v a t i o n of the potentiogram. C . I . Moo. [196l AN AUTOMATIC T I T R I M E T E R FOR PLOTTING T R U E - S C A L E TITRATION CURVES. H. I r v i n g and L . D . Pettit. (Inorganic C h e m i s t r y L a b o r a t o r y , South P a r k s Road, Oxford, Gt. B r i t a i n . ) Analyst, 84 (1959) 641. A d e s c r i p t i o n of a modification to a Honeywell-Brown r e c o r d e r is r e p o r t e d , in o r d e r to obtain a p o t e n t i o m e t r i c ti tr a ti o n c u r v e in which the ordinate scal e (pH) always r e m a i n s d i r e c t l y proportional to the volume of titrant used, and also to r e a c h the best e q u il ib r iu m conditions .~vhen the rate of a d m i s s i o n of the t i t r a n t is varied during the titration (from 0.005 to 0.02 m l / m i n ) . The modification uses a single m o t o r , whose speed is v a r i e d by a V a r i a c t r a n s f o r m e r , so that the r e c o r d ing ch art and the titrant d e l i v e r i n g m i c r o - s y r i n g e may be d r i v e n simultaneously. All the apparatus d e s c r i b e d is for n o r m a l usage; m o r e o v e r a modification to the r e c o r d e r is r e p o r t e d f o r when it is used in conjunction with an E . I . L . Vibron e l e c t r o m e t e r , rood. 33B, and the p H - m e a s u r i n g attachment, rood. C33B, which allows d i r e c t readings to be'obtained from the ch ar t graduations to 0.01 unit of pH, and 0.003 unit of pH o r b e t t e r to be e s t i m a t e d . The p r e c i s i o n of the apparatus is tested by titration of 0.001 M EDTA solution and some f u r t h e r applications a r e outlined. [197]
G . d e An.
EIN BEITRAG ZUR PROBLEM DER MEMBRANELEKTRODE. DIE BESTIMMUNG DES )/.QUIVALENZPUNKTES MIT H I L F E DES MEMBRANPOTENTIALS. I. FJ~LLUNGSTITRATIONEN VON SILBERSALZEN. J. B e r s i e r , P. B e r s i e r , W. Trueb und F . H t i g l i . ( T e c h n i k u m , Winterthur, S e h w e i z . ) H e l v . C h i m . A c t a , 4 2 (1959) 55. Das feinporige M e m b r a n s y s t e m yon Aluminiumoxyddeckschichten wurde als M e m b r a n e l e k t r o d e bei F~tllingstitrationen yon S i l b e r s a l z e n verwendet. Die M e m brane wurden d u t c h anodische Oxydation yon R e i n s t a l u m i n i u m in 0.4 N O x a l s ~ u r e b~idern (Stromdichte ungePAhr 10 m A / c m 2) erhalten. Die Schichtdicke liegt zwiSchen 0.05 und 0.08 ram. Zu den p o t e n t i o m e t r i s c h e n Ti t r at i o n en wurden die Membrane in e i n e r geeigneten A p p a r a t u r als M e m b r a n e l e k t r o d e n e i n g e s e t z t . Die E l e k t r o l y t schliissel zu B e z u g s e l e k t r o d e n e r f o r d e r t e n b e s o n d e r e Sorgfalt. Das g e m e s s e n e Membranpotential ist gegen Rtihrung er~ pfindlich. Man rtthrt entweder m e c h a n i s c h oder d u t c h Luftstrom. Man m u s s darauf achten, dass die Veri~nderungen d e r Niveauunterschiede zwischen A u s s e n - und InnenlSsung sich auf + 0.5 cm beschr[inkten. Die Aufbewahrung d e r Membrane e r f o lg t in 0.1 N KCI-LSsungen. Zu den Potentialmessungen wurden K o m p e n s a t o r e n d e r F i r m a Metrohm (Herisau) verwendet. D e r g e s a m t e Messleehler betriig~ unge~ahr ± 3 inV. Es sind s~imtliche p o t e n t i o m e t r i s c h e T i t r a t i o n s k u r v e n d e r m i t M e m b r a n e l e k -
1960
A bs t r ac t s Section
A 61
troden ausgeKihrten ~allungstitration~n v o r g e l e g t and zwar die T i t r a t i o n s k u r v e n 2~von CI-, SCN-, B r - , J , C 2 0 4 , C r O 4 m i t Ag+ und yon Ag4" m i t CI-. Ja. Inc. [1981 STABILITY OF M E T A L COMPLEXES IN SOLUTION. C h e m . , 19 (1959) 156.
B.N. Mattoo.
Z.physik.
R e s u l t s a r e p r e s e n t e d of a p o t e n t i o m e t r i e study of ion a s s o c i a t i o n in f e r r i c sulphate and n i t rat e solutions at low total f e r r i c ion concentration which excludes the po s s i b i l i t y of any significant f o r m a t i o n of the polynuclear m e t a l - a n i o n c o m p l e x e s . Redox potential m e a s u r e m e n t s of Fe II/III solutions at low Fe III co n cen t r at i o n in the p r e s e n c e of sulphate and n i tr a te and using a Pt indicator e l e c t r o d e have been i n t e r p r e t e d in t e r m s of ion a s s o c i a t i o n in sulphate solutions which gives r i s e to c o m p l e x s p e c i e s Ye(SO4)+, Fe~S04)2, and at high SO42- concentration Ye(SO4)~-, and the single c o m p l e x Fe(NOs) in nitrate solutions. Competing e q u i l i b r i a , m a i n Iy h y d r o l y s i s , have been put f o r w a r d to account for K~tso: values at v ar i o u s I ' s interpolated (or extrapolated) f r o m the l i t e r a t u r e data a~d for p r e s e n t m e a s u r e ments at I = 0.275 (KHso~ = 0.034). R e s u l t s at v ar io u s I ' s have yielded the following values of the stepwise t h e r m o dynamic f o r m a t i o n constants at 25 ° using the extended Debye-iKickel equation for Fe(SO4)n3- 2n. PK1 = -4.04; pK 2 = -1.34; 5G 1 = -5.52 k C a l / m o l e ; 5G 2 = -1.83 k C a l / m o l e . The f o r m a t i o n of the r e l a t i v e l y unstable Ye(HSO4) 2+ co m p l ex has a l s o been accounted for tn the analysis of ~ e data by introducing one additional t e r m (K[ = f o r m a t i o n constant f o r Ye(HSO4)Z+); although a r e l i a b l e value for K{ cannot be given, it is definitely s m a l l enough to have no a p p r e c i a b l e influence on the r e s u l t s given for the sulphate c o m p l e x e s . Yor Fe(NO3) 2. the r e s u l t s are: pK 1 = -1.00; AG 1 = -1.36 k C a l / m o l e . The value of the f o r m a t io n quotient Ks, [Ye(SO4)33- ]/[/e(SO4)~][SO42-] has been assigned a p r e l i m i n a r y value of 0.5 at I = 3,0 (1 M Na2SO 4 solution). [199]
Di. Ci6.
E L E C T R O - M E T R I C STUDIES ON MERCUROUS TUNGSTATE. R . S . Saxena and C. M. Gupta. Z. physik. C h e m . , 19 (1959) 94. A r e v i e w of the e x i s t i n g l i t e r a t u r e shows that m e a g r e r e f e r e n c e s ar c available on the study of the r e a c t i o n between Na2WO 4 and ttg2(NO3) 2 in the p r e s e n c e of di ff eren t acid concentrations. In the absence of any detailed study on m e r c u r o u s tungstate~ it was c o n s i d e r e d worthwhile to investigate it c o m p r e h e n s i v e l y by e l e c t r o m e t r i c methods. In the paper r e s u l t s of c o n d u c t o m e t r i c and p o t e n t i o m e t r i c titrations have been included. The conduetivities w e r e m e a s u r e d by a Kohlrausch U n i v e r s a l Bridge, the a l t e r n a t i n g c u r r e n t was supplied by a low frequency o s c i l l a t o r (1000 c / s e e ) and the b a l a n c e - p o i n t was indicated by the sound m i n i m a in the headphones. C u r v e s w e r e plotted of the c o r r e c t e d conductance and the volume of the titrant. The b r e a k s in the c u r v e s c o r r e s p o n d e d to the formation of d i f f er en t compounds. Using d i f f eren t concentrations of r e a c t a n t s , p o t e n t i o m e t r i c titrations w e r e p e r f o r m e d both by d i r e c t a n d r e v e r s e methods. Hg2(NO3) 2 solution was added from the m i c r o b u r e t t e to Na2WO 4 solution in the electl~ode cell and vice v e r s a and the potential between the platinum indicator e l e c t r o d e and a S . C . E . m e a s u r e d and plotted against the volume of the titrant. A s h ar p deflection in the curve o c c u r s at the end-point which is m a r k e d by a pronounced m a x i m a in 5E/AC. The t i t r a t i o n s w e r e also c a r r i e d out in p r e s e n c e of 1070 and 20% alcohol. I n c r e a s i n g amounts of alcohol a r e found to have no effect on the end-point. Conductivity c u r v e s yielded one sharp b r e a k and p o t e n t i o m e t r i c titrations give one m a x i m a AE/SC c o r r e s p o n d ing to the f o rm at io n of a golden yellow compound 2 HgO. 3WO 3. Di. CiS.