A C T A U N I V E R S I T A T I S L O D Z I E N S I S F O L IA CH I M I C A 10, 1993 Wi t o l d C i e s i e l s k i * , W ł o d z i m i e r z J ę d r z e j e w s k i * C O U L O H E T R I C D E T E R M I N A T I O N OF D I T H I O P H O S P H A T E S W I T H T HE U S E O F T H E I N D U C E D I O D I N E - A Z I D E R E A C T I O N The c h a r a c t e r i s t i c s of p o t a s s i u m d i e t h y l - and d i b u t h y l - d i t h i o p h o s p h a t e s as the i n d u cto rs of i o d i n e - a z i d e r e a ctio n c a r r i e d out e l e c t r o l y t i c a l l y have bee n p r e s ent ed. The i n f l uen ce of the pH and c o n c e n t r a t i o n s of azide and iodide ions on the c ou rse of r e a c t i o n was e x a m ine d. The d e t e r m i n a t i o n s have bee n c a r r i e d out in the range of 1-20 nmol.
D i t h i o p h o s p h a t e s (DTP) are used as p e s t i c i d e s [l] , a d d i t i v e s to l u b r i c a n t s and a n t i c o r r o s i o n a g e nts [2] and also as f l o t a t i o n c o l l e c t o r s for s u l f i d e m i n e r a l s [3].
R0
R0 — P — S 0 M ® = DTP S
Due to such a b r o ad range of a p p l i c a t i o n of d i t h i o p h o s p h a t e s this group has b e c o m e an i m p o rta nt o b j e c t of ana l y t i c a l research. D i t h i o p h o s p h a t e s can be d e t e r m i n e d using t i t r a t i o n m e t h o d s ( i o d o m e t r i c [4] and a r g e n t o m e t r i c [5]) as well as s p e c t r o p h o t o m e t r i c [,1, 3, 4] and p o l a r o g r a p h i c m e t h o d s [ć] . A c o u l o m e t r i c m e t h o d of the d e t e r m i n a t i o n of d i t h i o p h o s p h a t e s based on the m e a s u r e m e n t of the e l e c t r i c c h a r g e n e c e s s a r y for
D e p a r t m e n t of I n s t r u m e n t a l Ana lysi s, U n i v e r s i t y of Łódź, L i n d l e y a 3, 90-131 Łódź, Poland.
the g e n e r a t i o n of the iodine taking part in the ind uced rea ctio n as been pre sent ed. The t e c h n i q u e of c o u l o m e t r i c t i t r a t i o n had p r e v i o u s l y been used for the d e t e r m i n a t i o n of a num ber of s u l phur c o m p o u n d s i n d ucin g the i o d i n e - a z i d e rea c t i o n [7-9].
E X P E R I M E N T A L
REA GENTS, S O L U T I O N S AND AP P A R A T U S Twice d i s t i l l e d water in glass app aratus.
S o d i u m azide, pure; s o l u t i o n c o n t a i n i n g 200 g NaNj per litre. P o t a s s i u m iodide, a n a l y t i c a l - r e a g e n t grade, 0.1 mol/1. H y d r o c h l o r i c acid, a n a l y t i c a l - r e a g e n t grade, 1 mol/1.
P o t a s s i u m d i e t h y l d i t h i o p h o s p h a t e (DEDTP, R = C 2H 5) and p o ta s s i u m d i b u t h y l d i t h i o p h o s p h a t e (DBDTP, R = nC^H^), s y n t h e s i z e d in the D e p a r t m e n t of I n s t r u m e n t a l A n a lysi s Ins titu te of C h e mistry, Adam M i c k i e w i c z U n i vers ity, Poznari, Poland; the exact c o n t e n t of d i t h i o p h o s p h a t e s in the r e a g e n t s was d e t e r m i n e d by p o t e n t i o m e t r i c t i t r a t i o n with s t a n d a r d sil ver ni t r a t e s o l u t i o n
[5].
S t a n d a r d s o l u t i o n s ( 1 * 1 0 "'5 m o l /1) of DEDTP and DBDTP, c o n t ai ning n a t r i u m h y d r o x i d e ( 1* 10” ^ m o l / 1), added in order to e l i m i n a t e hydrolysis; working sol utio ns (1*10 ^ or 5 * 1 0 -5 mol/1), p r e p a r e d by d i l u t i n g the s t a n d a r d s o l u t i o n with water dir e c t l y b e f o r e the m e a s u r e m e n t s were taken.
R e a c t i o n s o l u t i o n I containing s o d i u m azide (20 g per litre), p o t a s s i u m iodide O M O ’’5 m o l / 1), and the amo unt of h y d r o c h l o r i c acid to reach pH = 6.0.
R e a c t i o n s o l u t i o n II containing sodium azide (50 g per litre), p o t a s s i u m iodide (3* 10_3 m o l / 1), and the a mo unt of h y d r o c h l o r i c acid to reach pH = 6.0.
U n i v e r s a l c o u l o m e t r i c a n a l y s e r Rad elkis, type 0H-404.
E l e c t r o l y s i s cell with two p l a t i n u m e l e c t r o d e s of the area 2
5 cm each, w o r k i n g in g e n e r a t i n g cir c u i t and with a dou ble e l e c t r o d e type OH - 9 3 8 1 in an a m p e r o m e t r i c i n d i c a t o r - c i r c u i t with two p o l a r i z e d e l e c t r o d e s (me c h a n i c a l stirrer; a u t o m a t i c p i p e t t e s w it h c a p a c i t i e s of 10, 20, 50 and lOOpl).
THE C H A R A C T E R I S T I C S OF THE I O D I N E - A Z I O E R E A C T I O N IN D U C E D BY DEDTP AND DBDTP
In order to c h o o s e the o p t i m u m c o n d i t i o n s of i n d u c i n g of the i o d i n e - a z i d e r e a c t i o n w it h d i t h i o p h o s p h a t e s , the i n f l u e n c e of the pH of the sol ution, of the c o n c e n t r a t i o n of p o t a s s i u m iodide, of the c o n c e n t r a t i o n of s o d i u m azide, and of the cur r e n t on the c o u r s e of c o u l o m e t r i c t i t r a t i o n was i n v esti gated . S in ce the rate of the i o d i n e - a z i d e r e a c t i o n i nd uced by d i t h i o p h o s p h a t e s is r e l a t i v e l y low, in order to t it rate the e n t ire amo unt of the ind ucto r the delay time of the a n a l y s e r was set at 30 s. That made it p o s s i b l e to swi tch on the c u r r e n t in the g e n e r a t i n g cir c u i t again, when in 30 s as a r e s ult of the r e a c t i o n the i n d i c a t o r - c u r r e n t d e c r e a s e d b el ow the a s s u m e d value of 2 p A , at w h i ch the cu r r e n t in the g e n e r a t i n g c i r c u i t was s t o p p e d a u t o m a t i c a l l y .
The d e p e n d e n c e of the e l e c t r i c c h a r g e n e e d e d to g e n e r a t e the a m o unt of i od ine ne c e s s a r y for the t i t r a t i o n on the c o n d i t i o n s in w h i c h the e x p e r i m e n t was c o n d u c t e d is p r e s e n t e d on three c o n s e c u t i v e dia grams. The value AQ on the Y - a xis is the d i f f e r e n c e b e t w e e n the e l e c t r i c c h a r g e s use d in the p r e s e n c e and in the ab s e n c e of the ind u c t o r until the i n d i c a t o r - c u r r e n t r ea ches the value of 2 pA, at delay time 30 s.
Fi g u r e 1 p r e s e n t s the d e p e n d e n c e of the e l e c t r i c c h a rge on the pH of the sol ution. The p H - v a l u e 6.0 at w hi ch AQ is high has been c h o s e n as the o p t i m u m value. The use of s o l u t i o n s wh o se pH is lower is i n c o n v e n i e n t be c a u s e of the e m i s s i o n of the poi sono us, v o l a t i l e h y d r a z o i c acid.
Fi g ure 2 p r e s e n t s the i n f l uen ce of the c o n c e n t r a t i o n of so d i u m azide on the e l e c t r i c c h a n g e in the r e a c t i o n in d u c e d by DEDTP. For this com poun d, ch a n g e s of the g e n e r a t i n g c ur rent b e t w e e n 0.5 and 5 mA do not affect the e l e c t r i c change, which is p r o p o r t i o n a l to the amo unt of the ind ucto r w i t h i n the range of 1-20 nmol in the sample.
F i g ure 3 p r e s e n t s the i n f l u e n c e of the c o n c e n t r a t i o n of azide and of the cu r r e n t on the e l e c t r i c c h a n g e (AQ) in the r e a c t i o n ind uced by DBDTP. In the s o l u t i o n c o n t a i n i n g NaN^ at the c o n c e n t r a t i o n of 20 g per litre r e c t i l i n e a r d e p e n d e n c e of the e l e c t r i c c h a r g e on the a m o unt of the ind ucto r is o b s e r v e d
Fiq 1. R e l a t i o n b e t w e e n the e l e c t r i c c ha rge and the pH of so-3
lution (20 g NaN^ per litre, 3*10 m o l /1 KI)
1 - 10 nmol ( C „ H , 0 ) o P S S K , I = 1 m A ; 2 - 10 nmol ( C 4 H g 0 ) ? P S S K ,
1 5 L I = 5 mA
9 NaN3/l
Fig. 2. R e l a t i o n b e t w e e n the e l e c t r i c c ha rge and t h a c o n c e n t r a tion of NaN-j(10 nmol ( C 2H ^ 0 )2P S S K , pH = 6.0, 3*10 mol/1 KI)
Fig. 3 R e l a t i o n b e t w e e n the e l e c t r i c c ha rge and the a mo unt of (C4H 90 ) 2PSSK (pH = 6.0, 3 * 1 0 -3 mol/1 KI)
1 - g (N a N j ) = 50 g / 1 , Igen = 5 mA; 2 - gCNaN-j) = 50 g/1, Ig£jn = = 1 mA; 3 - f ( N a N 3 ) = 20 g/1, Igen = 5 mA; 4 - ¿«(NaNj) = 20 g/1,
^ e n = 1 mA; 5 ' £>(NaN3 ) = 5 g / 1 - Igen = 1 mA
only till the amo unt of 10 nmol in the sam ple (for 1 = 5 mA) is rea ched; this line may serve as the c a l i b r a t i o n line in the d e t e r m i n a t i o n of d i b u t h y l d i t h i o p h o s p h a t e in the range of 1-10 nmol. The d e t e r m i n a t i o n of DBDTP in the range of 1-20 nmol s h o u l d be c o n d u c e d in a s o l u t i o n c o n t a i n i n g s o d i u m azide at the c o n c e n t r a t i o n of 50 g per litre, at I = 5 mA.
The d e p e n d e n c e of the i n d i c a t o r - c u r r e n t on time for d i f f er ent c o n c e n t r a t i o n s of iodide in the t i t r a t i o n of DEDTP (Fig. 4) shows that the rate of the i nd uced r e a c t i o n d e c r e a s e s wit h i n c r e a s i n g c o n c e n t r a t i o n of i o d ide ions. At the same time, the g r o w t h of the c o n c e n t r a t i o n of KI c au ses the d e c r e a s e of the
Fig. 4. R e l a t i o n b e t w e e n indicator cu r r e n t and time for d i f f e r e n t c o n c e n t r a t i o n s of p o t a s s i u m iod ide (10 nmol ( C 2H 50 ) 2P S S K , pH =
= 6.0, 20 g per litre NaNj, Ig = 1 mA)
1 - 3 * 1 0 -3 mol/1 KIj 2 - m o "2 m o l /1 KI; 3 - 3* 10 -2 mol/1 KI
use of iodine. Si m i l a r r e l a t i o n s h i p s were o b t a i n e d for DBDTP. No m e a s u r e m e n t s were taken in s o l u t i o n s c o n t a i n i n g iod ide at c o n c e n t r a t i o n s b e l ow m o l / 1 , be c a u s e the n o n - i n d u c e d iod ine- - a z ide r e a c t i o n takes place.
M E A S U R E M E N T S
The d e t e r m i n a t i o n of d i e t h y l d i t h i o p h o s p h a t e was c o n d u c t e d in r e a c t i o n s o l u t i o n I at the g e n e r a t i n g cu r r e n t of 1 mA, and the d e t e r m i n a t i o n s of d i b u t h y l d i t h i o p h o s p h a t e in r e a c t i o n s o l ut ions I and II at the cur r e n t of 5 mA. In the c as e of the t i t r a t i o n with the cu r r e n t of 5 mA, in order to d e t e r m i n e the end point mor e p r e c ise ly, the a p p a r a t u s was p r o g r a m m e d to a u t o m a t i c a l l y lower the c u r r e n t in the g e n e r a t i n g c i r c u i t ten times at the m o m e n t when the i n d i c a t o r - c u r r e n t r e a c h e d the value of 1.8 pA.
T a b l e 1
R e s u l t s of the d e t e r m i n a t i o n of d i e t h y l d i t h i o p h o s p h a t e (DEDTP) and d i b u t h y l d i t h i o p h o s p h a t e (DBDTP)
Nr Co m p o u n d (nmol)T aken Found(nmol) Error (%) R.S.D. (%) Re a c t i o n s o l u t i o n 1 DEDTP 1.00 1.03 3.0 8.5 I 2 DEDTP 2.00 1.96 2.0 4.8 I 3 DEDTP 5.00 5.06 1.2 2.7 I 4 DEDTP 10.00 10.00 0.0 2.4 I 5 DEDTP 15.00 15.30 2.0 1.6 I 6 DEDTP 20.00 19.70 1.5 2.0 I 1 DBDTP 1.00 0.97 3.0 9.2 I 8 DBDTP 2.00 2.04 2.0 5.5 I 9 DBDTP 4.00 4.06 1.5 3.7 I 10 D BDTP 7.00 6.90 1.4 2.2 I 11 D BDTP 10.00 9.00 2.0 2.4 I 12 DBDTP 1.00 1.01 1.0 7.7 II 13 DBDTP 2.00 2.01 0.5 4.9 II 14 DBDTP 5.00 4.92 1.6 3.8 II 15 DBDTP 10.00 10.10 1.0 4.0 II 16 DBDTP 15.00 15.00 0.0 2.3 II 17 DBDTP 20.00 19.60 2.0 2.1 II II O • R.S.D. - r e l a t i v e s t a n d a r d dev iation.
A s a m ple of the ind u c t o r was i n t r o d u c e d into 20 ml of the re a c t i o n sol utio n, p l a c e d in the anode part of the e l e c t r o l y s i s cell- after s t a r t i n g the m e c h a n i c a l st i r r e r the d i r ect cu r r e n t of 1 or 5 mA was being p a s s e d th r o u g h the sol ution. The e l e c t r i c c h a r g e (Q) was n ot ed after the e nt ire a m o unt of the i n d ucto r had bee n t i t r a t e d and the a p p a r a t u s had been s w i t c h e d off a u t o m a t i cally, w h i ch h a p p e n e d after the cu r r e n t in the i n d i c a t o r ci r c u i t had r e a c h e d the value of 2 pA at the delay time of 30 s. Then
the c ha rge Qq was noted in a s o l u t i o n c o n t a i n i n g no inductor. The d e p e n d e n c e of A Q = Q - Qq on the amount of the i n d u cto rs s e r ves as c a l i b r a t i o n lines.
The r es ults of the d e t e r m i n a t i o n s have been p r e s e n t e d in T a b . 1.
The p r e s e n t e d c o u l o m e t r i c m et hod per m i t s the d e t e r m i n a t i o n of 1-20 nmols of d i e t h y l - and d i b u t h y l d i t h i o p h o s p h a t e as i n d u c t o r s of the i o d i n e - a z i d e reaction.
The i n d u c t i o n c o e f f i c i e n t , i n d i c a t i n g how many times the a m o unt of iodine used in the ind uced r e a ctio n is g r e a t e r than in the o r d i n a r y i o d o m e t r i c rea ction, in the c o n d i t i o n s used (pH = 6.0, 3*10 3 mol/1 KI) depens on the c o n c e n t r a t i o n of s o d i u m azide and r e a c h e s for DEDTP and DBDTP the v al ues 220 and 210 (20 g NaNj per litre) and 275 and 255 (50 g NaN^ per litre), r e s p e c t i v e l y .
The shape of the c ur ves r e p r e s e n t i n g the ele c t r i c c h a r g e as a fun c t i o n of the a mo unt of d i b u t h y l d i t h i o p h o s p h a t e (Fig. 3) can be e x p l a i n e d on the basis of the a s s u m e d m e c h a n i s m of the r e a c tion : R E S U L T S AND C O N C L U S I O N S (1) ( R 0 ) 2PSS' + I" = ( R 0 ) 2P = S + 21 I S I (2) ( R 0 ) 2 P S S ' + ( R 0 ) 2 P =
s
SI — — ( R 0 ) 2 P - S - S - P ( R 0 ) 2 + I (3)(R0)2P =
S+ N"—-(R0)2P = s + I"
(4) SI SN 3 ( R 0 ) 2P = S + I2N~ — - ( R 0 ) 2P = S + 3 N 2 + I* SNj SI( R 0 ) 2P = S + Ij + h2o
final p r o d u c t s not ind u c i n g the i o d i n e - a z i d e r e a c t i o n
I
SI
( R 0 ) 2P = S + 1 3 + H 20
1
The d e v i a t i o n from the r e c t i l i n e a r d e p e n d e n c e of the cha rge as a fun c t i o n of the c o n c e n t r a t i o n of ( C 4 H g0 ) 2PSSK can be e x p l a i n e d by the o c c u r r e n c e of r e a c t i o n 3; the h i g h e r the initial c o n c e n t r a t i o n of the inductor, the g r e a t e r part of it is t r a n s f o r m e d into the n o n - a c t i v e d i s u l p h i d e and in this way a c e r t a i n amo unt of DBDTP does not take part in f ur ther steps of the ind uced reaction. The inc reas e of the c o n c e n t r a t i o n of azide c au ses the inc r e a s e of the rate of r e a c t i o n 4 and, c o n s e q u e n t l y , the e l i m i n a t i o n of r e a c t i o n 3. The i n c r e a s e of the g e n e r a t i n g c u r r e n t c a u ses a qu i c k e r f o r m a t i o n of iodine and, in c o n s e q u e n c e , a quick t r a n s f o r m a t i o n of the e nt ire a mo unt of DBDTP into a c o m p o u n d with iodine ( r e a c t i o n 2). The ab s e n c e of d i t h i o p h o s p h a t e r en ders r e a c t i o n 3 i m p ossi ble. A s im ilar p h e n o m e n o n was o b s e r v e d in the c o u l o m e t r i c d e t e r m i n a t i o n of c y s t e i n e [8].
In the case of DEDTP no d e v i a t i o n from the r e c t i l i n e a r c h a r a c t e r of the d e p e n d e n c e of the e l e c t r i c c h a r g e on the a mo unt of the ind u c t o r is observed, w h i c h may be c o n n e c t e d with the r e l a t i v e small role of r e a c t i o n 3 in the m e c h a n i s m p r e s e n t e d above.
We are i n d e b t e d to Prof. Dr hab. J. S i e p a k and Dr K. Rem (Adam M i c k i e w i c z U n i v e r s i t y of Poznań, Pol and) for p r o v i d i n g us wit h p o t a s s i u m d i e t h y l d i t h i o p h o s p h a t e and d i b u t h y l d i t h i o p h o - s p h a t e .
A c k n o w l e d g e m e n t
R E F E R E N C E S
[l] G. Z w e i g, Tin:] A n a l y t i c a l M e t h o d s for P e s t i c i d e s Plant, G r o w t h R e g u l a t o r s and Ford A d d i tiv es, Vol. II, Acad. Press, New York 1964.
[2] E. E. R e i d, [in:] O r g a n i e C h e m i s t r y of B i v a l e n t Sulfur, Chem. Publ. Co., Vol. I, New York 1962.
[3] M. H . J 0 n e s , J . T . W 0 o d c 0 c k , Anal. Chem., 58, 1845 (1986). [4] H . B o d e , W . A r n s w a 1 d , Z. Anal. Chem., 1J95, 99 (1962). [5] S . V . L a r i 0 n o v , L . A . P o d o 1 s k a y a , Zh. Anal. K h i m . , 22, 1165 (1967). [6] R. F. M a k e n s, H. H. V a u g h a n , R. R. C h e 1 b e r g, A n a l . C h e m . , 27, 1062 (1955). [7] W. J ę d r z e j e w s k i , W. C i e s i e l s k i , Chem. Anal. (Warsaw), 22, 781 (1978). [8] W. J ę d r z e j e w s k i , W. C i e s i e l s k i , Chem. Anal. (Warsaw), 29, 85 (1984). [9] W. J ę d r z e j e w s k i , W. C i e s i e l s k i , M i k ro- chim. Acta (Wien), III, 177 (1984).
W it old C i e s iel ski, W ł o d z i m i e r z J ę d r z e j e w s k i K U L O M E T R Y C Z N E O Z N A C Z A N I E D I T I 0 F 0 S F 0 R A N Ó W Z W Y K O R Z Y S T A N I E M I N D U K O W A N E J R EA KCJI J 0 D 0 - A Z Y D K 0 W E J
P r z e d s t a w i o n o c h a r a k t e r y s t y k ę d i e t y l o - i d i b u t y l o d i t i o f o s f 0- ranów jako i n d u k t o r ó w rea kcji j o d o - a z y d k o w e j . Z ba dano w p ł yw pH oraz s t ę żeń azy dku i jodku na p r z e b i e g reakcji. O z n a c z e n i a kulo- m e t r y c z n e p r z e p r o w a d z o n o w z a k r e s i e 1-20 nmoli w próbce.
