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Potassium nitrosodisulfonate as a reagent for oxidimetric titrations of inorganic cations in basic solutions
452
A n a l y t i c a C h i m i c a A cta. 74 (1975) 4 5 2 - 4 5 6 (~) Elsevier Scientific I't,blishing C o m p a n y , A m s t e r d a m - Printed ira T h e N e t h e r l a n d s
SHORT COMMUNICATION P o t a s s i u m n i t r o s o d i s u l f o n a t e as a r e a g e n t for o x i d i m e t r i e c a t i o n s in basic s o l u t i o n s
titrations of inorganic
MARIO COSPITO CA M E N . S. P i e r o a Grado. Pisu ( l t o l y ) and G I O R G I O R A S P I Istitttto di Chinlica Generale. U:liversitft di .~'iena. 5 3 1 0 0 - S i e n a ( I t a l y ) and At'c:denlitt Nttt,ale, L i v o r n o ( I t a l y ) ( R e c e i v e d 1st J u l y 1974)
P o t a s s i u m nitrosodisulfonate or F r e m y ' s r e a g e n t t is widely used as a selective oxidizing a g e n t for organic c o m l , o u n d s . A c o m p r e h e n s i v e a n d detailed review o f such a p p l i c a t i o n s in the field of o r g a n i c c h e m i s t r y has been recently published by Z i m m e r et al. z. T h e present w o r k evaluates the analytical a p p l i c a t i o n of p o t a s s i u m nitrosodisulfonate as a r e a g e n t for the o x i d i m e t r i c d e t e r m i n a t i o n o f s o m e inorganic c a t i o n s in slightly a l k a l i n e solutions in the presence of suitable c o m p l e x i n g agents. P r e l i m i n a r y e x p e r i m e n t s indicate that it is possible to titrate iron(II), c o b a l t ( l I ) , m a n g a n e s e ( I I ) , tin(II), c e r i u m ( l l I ) a n d u r a n i u m ( I V ) , with p o t e n t i o m e t r i c or bia m p e r o m e t r i c m e t h o d s for e n d - p o i n t detection. E x p e r i m e n tal Instrumentation. 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 were m a d e with an O r i o n Research Ionalyzer, model 801, a n d s m o o t h p l a t i n u m wire a n d s a t u r a t e d calomel electrodes. T w o identical platinum spheres o f surface area of a b o u t 0.05 c m 2, across which a c o n s t a n t voltage of 50 m V was applied, were used as indicating system for the b i a m p e r o m e t r i c end-points. All titrations were done at r o o m t e m p e r a t u r e ( 2 2 + 2 ° C ) . Potassium nitrosodisulJbmtte. P o t a s s i u m n i t r o s o d i s u l f o n a t e was p r e p a r e d by e l e c t r o c h e m i c a l o x i d a t i o n of basic solutions o f p o t a s s i u m h y d r o x y l a m i n e disulfonate at a p l a t i n u m a n o d e . This is a well k n o w n m e t h o d a'4, but some simplifications are r e p o r t e d below. T h e p r o c e d u r e w a s as follows. P r e p a r e p o t a s s i u m h y d r o x y l a m i n e disulfonate b y bubbling sulfur dioxide gas t h r o u g h a m i x t u r e of p o t a s s i u m nitrite a n d p o t a s s i u m acetate ( m o l a r r a t i o 1 : 1.2) in ice water, following the m e t h o d of Rollefson a n d O l d e r s h a w s. Filter the precipitate a n d wash m a n y times with ice w a t e r a n d then with purified m e t h a n o l a n d ether. This material can be stored indefinitely a t -- 5°C in a desiccator. P r e p a r e solutions of p o t a s s i u m n i t r o s o d i s u l f o n a t e by dissolving a l i q u o t s o f p o t a s s i u m h y d r o x y l a m i n e disulfonate in c o n c e n t r a t e d borate buffer solutions, p H 9.8,
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453
a n d placing the solutions obtained in a c o u l o m e t r i e cell; as g e n e r a t o r a n o d e u s e a p l a t i n u m foil of surface area a b o u t 40 c m z, a n d as c a t h o d e use a platinum wire, separated from the solution to be oxidized by i m m e r s i o n in a glass tube closed by a sintered glass disk a n d filled to the level of the a n o l y t e with the s a m e b o r a t e buffer. Efficient stirring is provided by a m a g n e t i c stirrer. T a k e aliquots c o n t a i n i n g a b o u t 3 g o f potassium h y d r o x y l a m i n e disulfonate (m.w. 269.3), and a p p l y a c u r r e n t intensity of 20 m A across the electrodes. If a total c u r r e n t efficiency of >~95~0 is assumed, a time of electrolysis o f a b o u t 6 h suffices to obtain 0.2 1 o f ca. 0.02 M potassium nitrosodisulfonate solution. T h e solutions o b t a i n e d by this m e t h o d always c o n t a i n e d unoxidized potassium h y d r o x y l a m i n e disulfonate. T h e titre of these solutions was c h e c k e d p o t e n t i o m e t r i c ally with a s t a n d a r d 0.02 N solution of vanadyl sulfate in the same buffer solutions 6 or by m e a s u r e m e n t of a b s o r b a n c e at 545 n m as suggested by G e h l e n a n d C e r m a k 7. T h e stability o f solutions o f potassium nitrosodisuifonate in c o n c e n t r a t e d b o r a t e buffer solution is satisfactory if they are stored at 0°C. T h e n o r m a l i t y o f a typical stock solution over a period o f 3 weeks is given in Table I. TABLE
1
STABILITY OF STOCK SOLUTIONS OF BUFFER SOLUTION, p H 9.8. A T O"C
Days of storage Normality
0 1.775. I 0 - z
POTASSIUM
7 !,763.10- 2
NITROSODISULFONATE
14 1.747.10 - 2
IN BORATE
2I 1.730. I 0 - 2
F o r a c c u r a t e work, daily standardization with vanadyl sulfate is recommended. O t h e r r e a , q e n t s . Borate buffer solutions were ca. 0.5 M in boric acid a n d 0.4 M in s o d i u m hydroxide. C a r b o n a t e - h y d r o g e n carbomite buffer solutions were c a . 1 M in the respective p o t a s s i u m salts. T a r t r a t e solutions (2 M) were prepared f r o m Seignette's salt and 1 M c y a n i d e solutions f r o m potassium cyanide. I r o n ( I I ) solutions were p r e p a r e d by dissolution o f M o h r ' s salt in 0.025 M sulfuric acid a n d s t a n d a r d i z e d with potassium p e r m a n g a n a t e . T i n ( l l ) solutions were p r e p a r e d by dissolution of the chloride d i h y d r a t e in 1 M h y d r o c h l o r i c acid and standardized with c e r i u m ( I V ) sulfate, with i r o n ( I l l ) as a n intermediate. C e r i u m ( I l l ) solutions were p r e p a r e d by dissolution of the sulfate m o n o h y d r a t e in 0.025 M sulfuric acid and s t a n d a r d i z e d by m e a n s o f the silvercatalyzed peroxydisulfate oxidation to cerium(IV). U r a n i u m ( I V ) solutions were obtained by electrochemical reduction o n a m e r c u r y pool of u r a n i u m ( V I ) sulfate solution in 0.05 M sulfuric acid, a n d s t a n d a r d i z e d with p o t a s s i u m p e r m a n g a n a t e . C o b a l t ( l I ) a n d m a n g a n e s e ( l l ) solutions were o b t a i n e d by dissolution o f the respective sulfate salts, after ignition at 450°C, in 0.025 M sulfuric acid. t
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All c h e m i c a l s used were o f reagent grade.
Determhlation o f il'on( l i ) , tin( l l ) and uraniton( IV). Place 20-25 ml o f b o r a t e buffer a n d 2 ml of 2 M t a r t r a t e solution in the titration cell. D e a e r a t e c o m pletely by bubbling p u r e nitrogen. Add the solution o f the cation to be d e t e r m i n e d , previously c o m p l e t e l y deaerated. Titrate with a s t a n d a r d solution o f p o t a s s i u m nitrosodisulfonate, d e t e c t i n g the e n d - p o i n t by the p o t e n t i o m e t r i c or b i a m p e r o m e t r i e method. Determination o f c o b a l t ( l l ) . Place 20-25 ml of b o r a t e buffer a n d 2 ml of 1 M p o t a s s i u m c y a n i d e s o l u t i o n in the titration cell. D e a e r a t e t h o r o u g h l y a n d proceed as described above, titrating with a d e a e r a t e d s t a n d a r d solution o f p o t a s s i u m nitrosodisulfonate. Because o f the r e a d y o x i d a t i o n of the c y a n i d e c o m p l e x o f c o b a l t ( l I ) by dissolved oxygen, c o b a l t ( l l ) might be m o r e c o n v e n i e n t l y d e t e r m i n e d by a n indirect m e t h o d as follo~s: place an excess of a s t a n d a r d solution o f F r e m y ' s reagent together with b o r a t e buffer a n d c y a n i d e solutions in the titration cell, d e a e r a t e t h o r o u g h l y , add the solution of c o b a l t ( l I ) a n d titrate the excess o f p o t a s s i u m n i t r o s o d i s u l f o n a t e with s t a n d a r d vanadyl sulfate. Determination oJ'manganese(ll). Place 20-25 ml of 1 M s o d i u m h y d r o x i d e a n d 2 ml of 2 M t a r t r a t e solution in the titration cell a n d p r o c e e d as described for iron(lI), etc. Determination o f cerium(Ill). Place 20-25 ml o f c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer s o l u t i o n in the titration cell a n d p r o c e e d as described for iron(ll), etc. Results at~d discussiotl P o t a s s i u m n i t r o s o d i s u l f o n a t e in solution can be r e p r e s e n t e d b y the structure 2 K + ( S O ~ - ) 2 N O . B e c a u s e o f its radical nature, this is a r a t h e r unstable c o m p o u n d which is generally very sensitive t o w a r d s reduction. Alkaline solutions in the p H r a n g e 8-12 are the m o s t stable but they also u n d e r g o d e c o m p o s i t i o n s'9. The stability o f solutions in c a r b o n a t e - h y d r o g e n c a r b o n a t e 3 or in a m m o n i a - a m m o n i u m chloride buffers is n o t great. C o n c e n t r a t e d b o r a t e buffer solutions o f F r e m y ' s radical s h o w a d e q u a t e stability in the p H r a n g e 9.6-10.0, especially w h e n stored at 0°C. W h e n p o t a s s i u m n i t r o s o d i s u l f o n a t e is used as an o x i d a n t for the cations used, it is r e d u c e d to p o t a s s i u m h y d r o x y l a m i n e disulfonate, as c h e c k e d by volta m m e t r i c investigation. The potential of the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate c o u p l e in the p H r a n g e 9.6-10.0 is 0.24 V (vs. SCE)~°; F r e m y ' s reagent in alkaline solution is thus c o m p a r a b l e in oxidizing p o w e r to p o t a s s i u m hexacyanoferrate(lll). Typical results o f titrations are presented in Table II. T a r t r a t e ions w e r e very effective as a c o m p l e x i n g a g e n t to keep the cations cited in a n alkaline solution. C y a n i d e was chosen as c o m p l e x i n g agent for c o b a l t ( l l ) instead o f ttlrtrate b e c a u s e the ' c o b a l t ( I l l ) t a r t r a t e - - c o b a l t ( l l ) tartr~lte c o u p l e has a potential very close to t h a t o f the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate couple in borate buffer m e d i u m , so that the p o t e n t i o m e t r i c a n d b i a m p e r o m e t r i c e n d - p o i n t s were p o o r l y defined. C e r i u m ( I l l ) w a s oxidized very s m o o t h l y by p o t a s s i u m n i t r o s o d i s u l f o n a t e in
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T A B L E II TITRATIONS
WITH POTASSIUM NITROSODISULF'ONATE
Cation
Product
M e d i u m '~
Taken (retool)
Fouml b (retool)
Fe(II)
Fe(lll)
B,T
Co(ll) a
Co(Ill)
B.CN
Co(ll)"
Co(Ill)
B,CN
0.0540 0.0540 0.1080 0.1080 0.0520 0. ! 040 0.0520
0.0539 0.0538 0.1080 0.1074 0.0512 0.1028 0.0523
0.0520
0.0520
0. 1040 0.1040 0.0480 0.0480 0.0672 0.0672 0.0960 0.0960 0.0190 0.0190 0.0390 0.0390 0.0432 0.0432 0.0648 0.0648 0.1080 0. 1080 0.0196 0.0196 0.0294 0,0294 0.0490 0.0490
0.1038 0,1041 0.0485 0.0482 0.0665 0.0670 0.0950 0.0952 0.0191 0.0188 0,0389 0.0390 0.0432 0,0430 0.0652 0,0650 0,1080 O. 1079 0.0197 0.0195 0,0298 0.0296 0.0491 0.0490
Mn(ll)
Mn(llI)
OH.T
Sn(ll)
Sn(IV)
B,T
Ce(lll)
Ce(IV)
CH
U(IV)
U(VI)
B,T
F r e m y ' s reagent (eqtdv.) per mole reductant
0.998 0.996 1.000 0.994 0.985 • 0.988 1.006 1.000 0.998 1.001 1.010 1.004 (I.990 0.997 0.990 O.992 2.010 • 1.978 1.994 2.OO0 1.000 0.996 1.006 1.003 1.000 0.999 2.010 1.990 2.028 2,014 2.004 2.000
Error (9~,)
Eml-poin t tletectJotl tnethod ¢
-0.2 - 0.4 0.0 --0.5 -- 1.6 -- I. 1 + 0.6 0.0 --0,2 + 0. ! + 1.0 + 0.4 -- !.0 --0.3 -- 1.0 -0.8 +0.5 -- I.O -- 0.3 0.O 0.0 --0.5 + 0.6 -4-0.3 0.0 -- 0. I +0.5 --0.5 + 1.3 +0.7 + 0.2 0.0
P BA P BA P P P BA P BA P BA P BA P BA P BA P BA P BA P BA P BA P ' BA P BA P BA
a M e d i u m : B, b o r a t e buffer; C H , c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer; O H , s o d i u m h y d r o x i d e ; T, t a r t r a t e : C N c y a n i d e . Average of three determinations. ¢ E n d - p o i n t d e t e c t i o n : P, p o t e n t i o m e t r i c ; BA. b i a m p c r o m e t r i c . a Direct method. Indirect m e t h o d .
b o r a t e buffer a n d in presence o f tartrate ions, b u t the resulting cerium(IV) o x i d i z e d t a r t r a t e ; the c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer p r o v i d e d a suitable m e d i u m for this titration. Titration o f m a n g a n e s e ( l I ) in b o r a t e buffer g a v e a p o o r l y defined p o t e n t i o metric c u r v e a n d the b i a m p e r o m e t r i c m e t h o d also did n o t s h o w the e q u i v a l e n c e p o i n t correctly. A .1 M s o d i u m h y d r o x i d e . m e d i u m w a s c h o s e n , a l t h o u g h the use o f p o t a s s i u m n i t r o s o d i s u l f o n a t e at so high a p H is n o t strictly correct o w i n g to a
456
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lack of stability 8. However, because of the r a p i d reaction rate, the results were good. T h e e n d - p o i n t s were o b t a i n e d f r o m p o t e n t i o m e t r i c curves a n d / o r b i a m p e r o m e t r i c plots. A few minutes sufficed for a t t a i n m e n t o f equilibrium n e a r the equivalence point. T h e potential b r e a k s o b s e r v e d for an a d d i t i o n o f 0.1 ml o f 0.02 N titrant solution a r o u n d the e q u i v a l e n c e p o i n t were a b o u t 650 mV for iron, m a n g a n e s e a n d tin, 400 mV for c o b a l t a n d u r a n i u m , a n d 200 m V for cerium. T h e reversibility of the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate system p r o v e d useful in the b i a m p e r o m e t r i c m e t h o d . T h e g r a p h s o b t a i n e d were V - s h a p e d for titrations of iron, m a n g a n e s e a n d cerium, reverse L - s h a p e d for titrations of tin a n d u r a n i u m , a n d L-shaped for t h e indirect titration o f cobalt, w h e r e the r e a c t i o n b e t w e e n n i t r o s o d i s u l f o n a t e a n d v a n a d y l ions was involved. In direct titrations of cobalt in the presence o f c y a n i d e , the b i a m p e r o m e t r i c m e t h o d was unsatisfactory. We a r e grateful to the Consiglio N a z i o n a l e delle Ricerche of Italy for financial s u p p o r t ( c o n t r a c t no. CT73.01082.03) to one o f us (G.R.). REFERENCES 1 2 3 4 5 6 7 8 9 10
E. Fremy, Aml. Chim. Phys., 15 (1845) 408. H. Zimmer, D. C. Lankin and S. W. Horgan. Chem. Rev., 71 (1971) 229. W. P. Wchrli and F. L. Pigott. Inorg. Chem., 9 (1970)2614. W. R. T. Cottrell and J. Farrar, J. Chem. Sot,. ,4. (1970) 1418. G. K. Rollefson and C. E. Oldershaw, J. Amer. Chem. Sot., 54 (1932) 977. G. Raspi, A. C i n q u a n t i n i and M. Cospito. unpublished data. H. Gehlen a n d J. Cermak, Z. Anorg. AIlfl. Chem., 275 (1954) 113. J. H. Murib and D. M. Rittcr, d. Amvr. Chem. Soc., 74 (1952)3394. B. J. Wilson and D. M. Ritter, lnorfl. Chem., 2 (1963) 974. S. Aoyagui a n d F. Kato, J. Electroanal. Chem. Interfacial Eiectrochem., 38 (1972) 243.
A n a l y t i c a C h i m i c a A cta. 74 (1975) 4 5 2 - 4 5 6 (~) Elsevier Scientific I't,blishing C o m p a n y , A m s t e r d a m - Printed ira T h e N e t h e r l a n d s
SHORT COMMUNICATION P o t a s s i u m n i t r o s o d i s u l f o n a t e as a r e a g e n t for o x i d i m e t r i e c a t i o n s in basic s o l u t i o n s
titrations of inorganic
MARIO COSPITO CA M E N . S. P i e r o a Grado. Pisu ( l t o l y ) and G I O R G I O R A S P I Istitttto di Chinlica Generale. U:liversitft di .~'iena. 5 3 1 0 0 - S i e n a ( I t a l y ) and At'c:denlitt Nttt,ale, L i v o r n o ( I t a l y ) ( R e c e i v e d 1st J u l y 1974)
P o t a s s i u m nitrosodisulfonate or F r e m y ' s r e a g e n t t is widely used as a selective oxidizing a g e n t for organic c o m l , o u n d s . A c o m p r e h e n s i v e a n d detailed review o f such a p p l i c a t i o n s in the field of o r g a n i c c h e m i s t r y has been recently published by Z i m m e r et al. z. T h e present w o r k evaluates the analytical a p p l i c a t i o n of p o t a s s i u m nitrosodisulfonate as a r e a g e n t for the o x i d i m e t r i c d e t e r m i n a t i o n o f s o m e inorganic c a t i o n s in slightly a l k a l i n e solutions in the presence of suitable c o m p l e x i n g agents. P r e l i m i n a r y e x p e r i m e n t s indicate that it is possible to titrate iron(II), c o b a l t ( l I ) , m a n g a n e s e ( I I ) , tin(II), c e r i u m ( l l I ) a n d u r a n i u m ( I V ) , with p o t e n t i o m e t r i c or bia m p e r o m e t r i c m e t h o d s for e n d - p o i n t detection. E x p e r i m e n tal Instrumentation. 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 were m a d e with an O r i o n Research Ionalyzer, model 801, a n d s m o o t h p l a t i n u m wire a n d s a t u r a t e d calomel electrodes. T w o identical platinum spheres o f surface area of a b o u t 0.05 c m 2, across which a c o n s t a n t voltage of 50 m V was applied, were used as indicating system for the b i a m p e r o m e t r i c end-points. All titrations were done at r o o m t e m p e r a t u r e ( 2 2 + 2 ° C ) . Potassium nitrosodisulJbmtte. P o t a s s i u m n i t r o s o d i s u l f o n a t e was p r e p a r e d by e l e c t r o c h e m i c a l o x i d a t i o n of basic solutions o f p o t a s s i u m h y d r o x y l a m i n e disulfonate at a p l a t i n u m a n o d e . This is a well k n o w n m e t h o d a'4, but some simplifications are r e p o r t e d below. T h e p r o c e d u r e w a s as follows. P r e p a r e p o t a s s i u m h y d r o x y l a m i n e disulfonate b y bubbling sulfur dioxide gas t h r o u g h a m i x t u r e of p o t a s s i u m nitrite a n d p o t a s s i u m acetate ( m o l a r r a t i o 1 : 1.2) in ice water, following the m e t h o d of Rollefson a n d O l d e r s h a w s. Filter the precipitate a n d wash m a n y times with ice w a t e r a n d then with purified m e t h a n o l a n d ether. This material can be stored indefinitely a t -- 5°C in a desiccator. P r e p a r e solutions of p o t a s s i u m n i t r o s o d i s u l f o n a t e by dissolving a l i q u o t s o f p o t a s s i u m h y d r o x y l a m i n e disulfonate in c o n c e n t r a t e d borate buffer solutions, p H 9.8,
SHORT
COMMUNICATION
453
a n d placing the solutions obtained in a c o u l o m e t r i e cell; as g e n e r a t o r a n o d e u s e a p l a t i n u m foil of surface area a b o u t 40 c m z, a n d as c a t h o d e use a platinum wire, separated from the solution to be oxidized by i m m e r s i o n in a glass tube closed by a sintered glass disk a n d filled to the level of the a n o l y t e with the s a m e b o r a t e buffer. Efficient stirring is provided by a m a g n e t i c stirrer. T a k e aliquots c o n t a i n i n g a b o u t 3 g o f potassium h y d r o x y l a m i n e disulfonate (m.w. 269.3), and a p p l y a c u r r e n t intensity of 20 m A across the electrodes. If a total c u r r e n t efficiency of >~95~0 is assumed, a time of electrolysis o f a b o u t 6 h suffices to obtain 0.2 1 o f ca. 0.02 M potassium nitrosodisulfonate solution. T h e solutions o b t a i n e d by this m e t h o d always c o n t a i n e d unoxidized potassium h y d r o x y l a m i n e disulfonate. T h e titre of these solutions was c h e c k e d p o t e n t i o m e t r i c ally with a s t a n d a r d 0.02 N solution of vanadyl sulfate in the same buffer solutions 6 or by m e a s u r e m e n t of a b s o r b a n c e at 545 n m as suggested by G e h l e n a n d C e r m a k 7. T h e stability o f solutions o f potassium nitrosodisuifonate in c o n c e n t r a t e d b o r a t e buffer solution is satisfactory if they are stored at 0°C. T h e n o r m a l i t y o f a typical stock solution over a period o f 3 weeks is given in Table I. TABLE
1
STABILITY OF STOCK SOLUTIONS OF BUFFER SOLUTION, p H 9.8. A T O"C
Days of storage Normality
0 1.775. I 0 - z
POTASSIUM
7 !,763.10- 2
NITROSODISULFONATE
14 1.747.10 - 2
IN BORATE
2I 1.730. I 0 - 2
F o r a c c u r a t e work, daily standardization with vanadyl sulfate is recommended. O t h e r r e a , q e n t s . Borate buffer solutions were ca. 0.5 M in boric acid a n d 0.4 M in s o d i u m hydroxide. C a r b o n a t e - h y d r o g e n carbomite buffer solutions were c a . 1 M in the respective p o t a s s i u m salts. T a r t r a t e solutions (2 M) were prepared f r o m Seignette's salt and 1 M c y a n i d e solutions f r o m potassium cyanide. I r o n ( I I ) solutions were p r e p a r e d by dissolution o f M o h r ' s salt in 0.025 M sulfuric acid a n d s t a n d a r d i z e d with potassium p e r m a n g a n a t e . T i n ( l l ) solutions were p r e p a r e d by dissolution of the chloride d i h y d r a t e in 1 M h y d r o c h l o r i c acid and standardized with c e r i u m ( I V ) sulfate, with i r o n ( I l l ) as a n intermediate. C e r i u m ( I l l ) solutions were p r e p a r e d by dissolution of the sulfate m o n o h y d r a t e in 0.025 M sulfuric acid and s t a n d a r d i z e d by m e a n s o f the silvercatalyzed peroxydisulfate oxidation to cerium(IV). U r a n i u m ( I V ) solutions were obtained by electrochemical reduction o n a m e r c u r y pool of u r a n i u m ( V I ) sulfate solution in 0.05 M sulfuric acid, a n d s t a n d a r d i z e d with p o t a s s i u m p e r m a n g a n a t e . C o b a l t ( l I ) a n d m a n g a n e s e ( l l ) solutions were o b t a i n e d by dissolution o f the respective sulfate salts, after ignition at 450°C, in 0.025 M sulfuric acid. t
-..j
454
SHORT COM M UNICATION
All c h e m i c a l s used were o f reagent grade.
Determhlation o f il'on( l i ) , tin( l l ) and uraniton( IV). Place 20-25 ml o f b o r a t e buffer a n d 2 ml of 2 M t a r t r a t e solution in the titration cell. D e a e r a t e c o m pletely by bubbling p u r e nitrogen. Add the solution o f the cation to be d e t e r m i n e d , previously c o m p l e t e l y deaerated. Titrate with a s t a n d a r d solution o f p o t a s s i u m nitrosodisulfonate, d e t e c t i n g the e n d - p o i n t by the p o t e n t i o m e t r i c or b i a m p e r o m e t r i e method. Determination o f c o b a l t ( l l ) . Place 20-25 ml of b o r a t e buffer a n d 2 ml of 1 M p o t a s s i u m c y a n i d e s o l u t i o n in the titration cell. D e a e r a t e t h o r o u g h l y a n d proceed as described above, titrating with a d e a e r a t e d s t a n d a r d solution o f p o t a s s i u m nitrosodisulfonate. Because o f the r e a d y o x i d a t i o n of the c y a n i d e c o m p l e x o f c o b a l t ( l I ) by dissolved oxygen, c o b a l t ( l l ) might be m o r e c o n v e n i e n t l y d e t e r m i n e d by a n indirect m e t h o d as follo~s: place an excess of a s t a n d a r d solution o f F r e m y ' s reagent together with b o r a t e buffer a n d c y a n i d e solutions in the titration cell, d e a e r a t e t h o r o u g h l y , add the solution of c o b a l t ( l I ) a n d titrate the excess o f p o t a s s i u m n i t r o s o d i s u l f o n a t e with s t a n d a r d vanadyl sulfate. Determination oJ'manganese(ll). Place 20-25 ml of 1 M s o d i u m h y d r o x i d e a n d 2 ml of 2 M t a r t r a t e solution in the titration cell a n d p r o c e e d as described for iron(lI), etc. Determination o f cerium(Ill). Place 20-25 ml o f c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer s o l u t i o n in the titration cell a n d p r o c e e d as described for iron(ll), etc. Results at~d discussiotl P o t a s s i u m n i t r o s o d i s u l f o n a t e in solution can be r e p r e s e n t e d b y the structure 2 K + ( S O ~ - ) 2 N O . B e c a u s e o f its radical nature, this is a r a t h e r unstable c o m p o u n d which is generally very sensitive t o w a r d s reduction. Alkaline solutions in the p H r a n g e 8-12 are the m o s t stable but they also u n d e r g o d e c o m p o s i t i o n s'9. The stability o f solutions in c a r b o n a t e - h y d r o g e n c a r b o n a t e 3 or in a m m o n i a - a m m o n i u m chloride buffers is n o t great. C o n c e n t r a t e d b o r a t e buffer solutions o f F r e m y ' s radical s h o w a d e q u a t e stability in the p H r a n g e 9.6-10.0, especially w h e n stored at 0°C. W h e n p o t a s s i u m n i t r o s o d i s u l f o n a t e is used as an o x i d a n t for the cations used, it is r e d u c e d to p o t a s s i u m h y d r o x y l a m i n e disulfonate, as c h e c k e d by volta m m e t r i c investigation. The potential of the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate c o u p l e in the p H r a n g e 9.6-10.0 is 0.24 V (vs. SCE)~°; F r e m y ' s reagent in alkaline solution is thus c o m p a r a b l e in oxidizing p o w e r to p o t a s s i u m hexacyanoferrate(lll). Typical results o f titrations are presented in Table II. T a r t r a t e ions w e r e very effective as a c o m p l e x i n g a g e n t to keep the cations cited in a n alkaline solution. C y a n i d e was chosen as c o m p l e x i n g agent for c o b a l t ( l l ) instead o f ttlrtrate b e c a u s e the ' c o b a l t ( I l l ) t a r t r a t e - - c o b a l t ( l l ) tartr~lte c o u p l e has a potential very close to t h a t o f the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate couple in borate buffer m e d i u m , so that the p o t e n t i o m e t r i c a n d b i a m p e r o m e t r i c e n d - p o i n t s were p o o r l y defined. C e r i u m ( I l l ) w a s oxidized very s m o o t h l y by p o t a s s i u m n i t r o s o d i s u l f o n a t e in
SHORT
455
COMM UNIC:ATION
T A B L E II TITRATIONS
WITH POTASSIUM NITROSODISULF'ONATE
Cation
Product
M e d i u m '~
Taken (retool)
Fouml b (retool)
Fe(II)
Fe(lll)
B,T
Co(ll) a
Co(Ill)
B.CN
Co(ll)"
Co(Ill)
B,CN
0.0540 0.0540 0.1080 0.1080 0.0520 0. ! 040 0.0520
0.0539 0.0538 0.1080 0.1074 0.0512 0.1028 0.0523
0.0520
0.0520
0. 1040 0.1040 0.0480 0.0480 0.0672 0.0672 0.0960 0.0960 0.0190 0.0190 0.0390 0.0390 0.0432 0.0432 0.0648 0.0648 0.1080 0. 1080 0.0196 0.0196 0.0294 0,0294 0.0490 0.0490
0.1038 0,1041 0.0485 0.0482 0.0665 0.0670 0.0950 0.0952 0.0191 0.0188 0,0389 0.0390 0.0432 0,0430 0.0652 0,0650 0,1080 O. 1079 0.0197 0.0195 0,0298 0.0296 0.0491 0.0490
Mn(ll)
Mn(llI)
OH.T
Sn(ll)
Sn(IV)
B,T
Ce(lll)
Ce(IV)
CH
U(IV)
U(VI)
B,T
F r e m y ' s reagent (eqtdv.) per mole reductant
0.998 0.996 1.000 0.994 0.985 • 0.988 1.006 1.000 0.998 1.001 1.010 1.004 (I.990 0.997 0.990 O.992 2.010 • 1.978 1.994 2.OO0 1.000 0.996 1.006 1.003 1.000 0.999 2.010 1.990 2.028 2,014 2.004 2.000
Error (9~,)
Eml-poin t tletectJotl tnethod ¢
-0.2 - 0.4 0.0 --0.5 -- 1.6 -- I. 1 + 0.6 0.0 --0,2 + 0. ! + 1.0 + 0.4 -- !.0 --0.3 -- 1.0 -0.8 +0.5 -- I.O -- 0.3 0.O 0.0 --0.5 + 0.6 -4-0.3 0.0 -- 0. I +0.5 --0.5 + 1.3 +0.7 + 0.2 0.0
P BA P BA P P P BA P BA P BA P BA P BA P BA P BA P BA P BA P BA P ' BA P BA P BA
a M e d i u m : B, b o r a t e buffer; C H , c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer; O H , s o d i u m h y d r o x i d e ; T, t a r t r a t e : C N c y a n i d e . Average of three determinations. ¢ E n d - p o i n t d e t e c t i o n : P, p o t e n t i o m e t r i c ; BA. b i a m p c r o m e t r i c . a Direct method. Indirect m e t h o d .
b o r a t e buffer a n d in presence o f tartrate ions, b u t the resulting cerium(IV) o x i d i z e d t a r t r a t e ; the c a r b o n a t e - h y d r o g e n c a r b o n a t e buffer p r o v i d e d a suitable m e d i u m for this titration. Titration o f m a n g a n e s e ( l I ) in b o r a t e buffer g a v e a p o o r l y defined p o t e n t i o metric c u r v e a n d the b i a m p e r o m e t r i c m e t h o d also did n o t s h o w the e q u i v a l e n c e p o i n t correctly. A .1 M s o d i u m h y d r o x i d e . m e d i u m w a s c h o s e n , a l t h o u g h the use o f p o t a s s i u m n i t r o s o d i s u l f o n a t e at so high a p H is n o t strictly correct o w i n g to a
456
SHORT COMMUNICATION
lack of stability 8. However, because of the r a p i d reaction rate, the results were good. T h e e n d - p o i n t s were o b t a i n e d f r o m p o t e n t i o m e t r i c curves a n d / o r b i a m p e r o m e t r i c plots. A few minutes sufficed for a t t a i n m e n t o f equilibrium n e a r the equivalence point. T h e potential b r e a k s o b s e r v e d for an a d d i t i o n o f 0.1 ml o f 0.02 N titrant solution a r o u n d the e q u i v a l e n c e p o i n t were a b o u t 650 mV for iron, m a n g a n e s e a n d tin, 400 mV for c o b a l t a n d u r a n i u m , a n d 200 m V for cerium. T h e reversibility of the n i t r o s o d i s u l f o n a t e - h y d r o x y l a m i n e disulfonate system p r o v e d useful in the b i a m p e r o m e t r i c m e t h o d . T h e g r a p h s o b t a i n e d were V - s h a p e d for titrations of iron, m a n g a n e s e a n d cerium, reverse L - s h a p e d for titrations of tin a n d u r a n i u m , a n d L-shaped for t h e indirect titration o f cobalt, w h e r e the r e a c t i o n b e t w e e n n i t r o s o d i s u l f o n a t e a n d v a n a d y l ions was involved. In direct titrations of cobalt in the presence o f c y a n i d e , the b i a m p e r o m e t r i c m e t h o d was unsatisfactory. We a r e grateful to the Consiglio N a z i o n a l e delle Ricerche of Italy for financial s u p p o r t ( c o n t r a c t no. CT73.01082.03) to one o f us (G.R.). REFERENCES 1 2 3 4 5 6 7 8 9 10
E. Fremy, Aml. Chim. Phys., 15 (1845) 408. H. Zimmer, D. C. Lankin and S. W. Horgan. Chem. Rev., 71 (1971) 229. W. P. Wchrli and F. L. Pigott. Inorg. Chem., 9 (1970)2614. W. R. T. Cottrell and J. Farrar, J. Chem. Sot,. ,4. (1970) 1418. G. K. Rollefson and C. E. Oldershaw, J. Amer. Chem. Sot., 54 (1932) 977. G. Raspi, A. C i n q u a n t i n i and M. Cospito. unpublished data. H. Gehlen a n d J. Cermak, Z. Anorg. AIlfl. Chem., 275 (1954) 113. J. H. Murib and D. M. Rittcr, d. Amvr. Chem. Soc., 74 (1952)3394. B. J. Wilson and D. M. Ritter, lnorfl. Chem., 2 (1963) 974. S. Aoyagui a n d F. Kato, J. Electroanal. Chem. Interfacial Eiectrochem., 38 (1972) 243.