A C T A U S I V S R S I T ' A T I ' S L O D Z I E H S l S
____________ FOLIA CHIHICA 3 , 1984
J o a n n a Masłowska, J ó z e f Szmich
THE POTENTIOMETRIC STUDY OF C o ( I I ) COMPLEXES WITH MERCAPTOCAHBOXYLIC ACIDS*
The i n t e r a c t i o n between C o ( I I ) io n s and t h i o l a c t i c a c i d TLA o r t h i o g l y c o l i c a c i d TGA in aqueous 0.1 M NaClO* ha3 been s t u d i e d p o t e n t i o m e t r i c a l l y a t 2 0 °. The formy- t i o n o f complexes o f t h e ty p e Co(SuCOO) and Co(SRÇOO)î (whore R - CIi2 o r CHjCH) h a s been fo u n d . The s t a b i l i t y c o n s t a n t s o f t h e s e complexes have been d e te r m in e d as w e l l as t h e a c i d d i s s o c i a t i o n c o n s t a n t s o f TLA and TGA.
T h i o l a c t i c a c i d and t h i o g l y c o l i c a c i d , b e lo n g t o a p h y s i o l o g i c a l l y i m p o r ta n t c l a s s o f compounds. As t h e y behave as s t r o n g complexing a g e n t s , i t seemed t o be i n
t e r e s t i n g t o c o m p a rise t h e s t a b i l i t i e s o f C o ( I D com p l e x e s w i t h t h e s e a c i d s .
E x p e rim e n ta l
S t o c k s o l u t i o n were p r e p a r e d from r e a g e n t g ra d e c h e m i c a l s . Merck TGA and F lu k a TLA were p u r i f i e d by vacuum d i s t i l l a t i o n . A p p r o x im a te ly 0 .1 M s o l u t i o n s o f them were p r e p a r e d by d i l u t i n g a known w e ig h t o f t h e d i s t i l l e d a c i d s w i t h a i r - f r e e w a t e r . The s e s o l u t i o n s were s t a n d a r i s e d by a l k a l i m e t r i c t i t r a t i o n u s i n g p o t e n t i o m e t r i c a n d - p o i n t d e t e c t i o n . A c o b a l t o u s p e r c h l o r a t e s o l u t i o n , a p p r o x i m a t e ly 1 .0 0 M was p r e p a r e d by d i s s o l v i n g a weighed amount o f c o b a l t o u s c a r b o n a t e i n a known volume o f s t a n d a r d p e r c h l o r i c a c i d and d i l u t i n g ob t a i n e d s o l u t i o n w i t h a w a t e r . The s o l u t i o n was s t a n d a r i z e d t - i n g desodium d ih y d ro g e n e t y l e n d i a m i n e t e t r a a c e t a t e (EDTA).
A 0 .1 0 0 M p o ta s s iu m h y d r o x id e s o l u t i o n was p r e p a r e d from * P ra c a z o s t a ł a n a d e s ł a n a na I I I p o l s k o - r a d z i e c k i e sympozjum "Termodynamika i e le k t r o c h e m i a roztworów e l e k t r o l i t ó w " .
P o l i s h Chemical Reagents C o n c e n t r a te d V olum etric S o l u t i o n . The s o l u t i o n wan found t o c o n t a i n l e s s th a n 0»?.# c a r b o n a t e by po- t e n t i o r h e t r i c t i t r a t i o n .
The i o n i s a t i o n and s t a b i l i t y c o n s t a n t s were d e te r m in e d U3ing
th e pH—ti t r a t i o n method [ 1 ] . The pH o f all. s o l u t i o n s was mea s u r e d w i t h a P r e c i s i o n D i g i t a l R a d e lk is pH -m eter. A g l a s s e l e c t r o d e o f 0-14 pH ra n g e c a l i b r a t e d f r e q u e n t l y a g a i n s t b u f f e r s o f d i f f e r e n t p H -values was u sed i n c o n j u n c t i o n w i t h s a t u r a t e d c a l o mel e l e c t r o d e . P o t e n t i o m e t r i c t i t r a t i o n s were p e rf o r m e d u n d e r n i t r o g e n i n a w a t e r - j a c k e t e d b e a k e r , and t h e t e m p e r a t u r e o f t h e s o l u t i o n was k e p t c o n s t a n t 20 + 0.2°C d u r in g t h e t i t r a t i o n s and i t v t a c o n t r o l e d by an e l e c t r i c a l l y m a i n ta i n e d t h e r m o s t a t (UT-1 ty p o German). E f f e c t i v e s t i r r i n g was o b t a i n e d w i t h a m agnetic s t i r r e r . A ll th e p H - t i t r a t i o n s were c a r r i e d o u t i n medium com p o se d o f 0.1 M NaClO^, w i t h t h e s t a n d a r d 0 .1 0 0 M p o ta s s iu m hy d r o x i d e .
The i o n i s a t i o n c o n s t a n t s o f TGA and TLA were d e te r m in e d by t i t r a t i n g TOA. »nd TLA s o l u t i o n s . The t i t r a t i o n c u r v e s were drawn, from which t h e d i s s o c i a t i o n c o n s t a n t s were c a l c u l a t e d u s i n g t h e Bjarrum ’ s method.
The e x p e r i m e n t a l p r o c e d u r e ‘ u s e d t o d e te rm in e s t o i c h i o m e t r y o f t h e r e a c t i o n s between C o ( I I) and TGA o r TLA a s w e l l a s de t e r m i n i n g o f s t a b i l i t y c o n s t a n t s o f t h e complexes form ed, con s i s t e d i n p e rf o r m in g o f s e r i e s p H - t i t r a t i o n s TGA and TLA s o l u t i o n s , i n ab sen c e o r p r e s e n c e o f Co2 io n s a t v a r i o u s l i g a n d t o m e ta l r a t i o s .
The s t a b i l i t y c o n s t a n t s were c a l c u l a t e d u s i n g C a l v i n a nd r-i e l c h i o r ' s method [ 2 ] .
R e s u l t s and d i s c u s s i o n
Both TOA ( 1 ) and TLA ( I I ) -have two r e p l a c a b l e hydrogen i o n s a s s t r e s s e d in t h e fo rm u la e below!
H3 - CH0 - COOH HS - CH - COOH
The e q u i l i b r i a ^ In v o lv e d a r e s • O T [H A “ ]
' [ » j^ l
[ H+] I A2”] H2A « HA” + H*¡
where H2A r e p r e s e n t s t h e TGA o r TLA.
I t I s o b v io u s , t h a t K1 arid c o r r e s p o n d to d i s s o c i a t i o n o f c a r b o x y l i c and s u lp h y d r y l g ro u p , r e s p . KOH F i g . 1 . P o t e n t i o m e t r i c t i t r a t i o n o f 4 . 3 • 10*i* M TGA w it h * 0.1 M KOH F ig . 2 . P l o t o f n a s a f u n c t i o n o f pH f o r t i t r a t i o n o f 4 . 3 - 1 0~* M TGA w i t h 0 .1 M KOH
The a b s en c e o f i n f i e c i o n a f t e r add ing two s o l e s o f
( F i g . 1 ) , i n d i c a t e s tha-c th e p r o t o n o f s u l p h y d r y l ;;■-•our» i s . * : t i t r a t a b l e u n d e r t h e e x p e r i m e n t a l c o n d i t i o n s .
The c u rv e f o r t i t r a t i o n o f 4 . 5 • 10” " A ÏLA w i t h 0.1 !-a was a n a lo g o u s .
pK1 v a lu e s have been found (from F i g . 2) t o be 3 .2 9 and 3 .6 3 , pK’2 v a l jes a s 10.35 and 9.85 f o r T3A and TLA r e a p .
The s t o i c h i o m e t r y o f complex f o r m a ti o n s were d e te rm in e d from t i t r a t i o n c u rv e s i n t h e p r e s e n c e o f t h e Co2+ io n s ( F i g . 31 There i s no i n d i c a t i o n o f complex f o r m a ti o n d u r in g t h e t i t r a t i o n o f c a r b o x y l i e hydrogen o f TLA ov TGA ( t h e c u rv e s f o r TLA were a n a lo g o u s ) . S t r o n g complex f o r m a ti o n was e v i d e n t d u rin g t h e t i t r a t i o n o f t h e m ercap tan hydrogen.
The a d d i t i o n e q u im o la r o f Co2 * g r e a t l y a l t e r s t h e shape o f f r e e l i g a n d t i t r a t i o n c u rv e s as a r e s u l t o f complex f o r m a ti o n ( c u rv e 2 on F i g . 3 ) .
The r e a c t i o n betw een *a* i n t e r v a l o f 0-1 may be r e p r e s e n t ed by t h e e q u o tio n s
H2A + OH“ « HA" ♦ H-0 (1 ) Two i n f l e c t i o n s have a p e a r e d ' a t 2 pnd 3 moles of-KOH p e r mo
l e o f a c i d . The i n f l e c t i o n a t a - 2 s u g g e s t s t h e f o r m a ti o n o f CoA a c c o r d i n g t o e q u o t i o n :
Car~* + HgA * 2 0 i r - CoA + 2H20 ( 2 ) The i n f l e c t i o n a t a * 3 s u g g e s t s th e f o r m a ti o n o f Coa| “ . S in c e i t i s accompanied w i t h p r e c i p i t a t i o n o f h a l f o f t h e m etal ion I n t h e form o f Co(0H)2 , we can presume a d i s p r o p o r t i o n a t i o n o f CoA, i n a c co rd a n ce w i t h t h e f o ll o w i n g e q u o t i o n : •
I ' y ■ / - • ' • ; 1 \
2 Co A + 20H” » CoA2" + Co(0H)2 ( 3 )
Tne i n f l e c t i o n a t two moles o f KOH p e r mole o f l i g a n d when t.fj.s r.ol r r e t i o TLA and Co ( I I ) i s 2 : 1 ( c u r v e 3 on F i g . 3 )
2 _
corror.r.i.nas to t h e f o r m a ti o n o f CoA? r e p r e s e n t e d by t h e equo-Mole of KOH per mole of TLA
F ig . 3. P o t e n t i o m e t r i e t i t r a - t io n s o f 10-3 M TLA ( c u r v e 1 ) , 10-3 M TLA + 1 0 -3 M Co(CIO4)^ ( c u r v e 2), and 10* 3 M TLA + + 5 • 10-* M Co(C104)2 ( c u r ve 3)
Co2* ♦ 2H2A + 40H" . CoA2’ ♦ 4HgO ( 4 )
• )
./* The ^absence o f th e i n f l e c t i o n a t 1 .3 molea o f KOH p e r mole o f a c i d , p o i n ts out^,- t h a t 1 s 1 and 1 : 2 complexes a r e f o r c e d s im u lta n e o u s ly d u rin g th e t i t r a t i o n .
r The s to ic h io m e tr y o f complex^ f o r m a ti o n between TGA ana Cot I I ) was th e same.
In o r d e r to c a l c u l a t e th e fo rm a tio n c o n s ta n ts and o f com plexes CoA and Coa| *
M e lc h io r 's m ethod. The eva l u a t i o n o f n v a lu e s were made from d a ta o f p o t e n t i o - m e tr ic t i t r a t i o n s o f TGA ( o r TLA) w ith s ta n d a r d a l k a l i i n th e ab sen c e and p re s e n c e o f Co2* ( F ig . 4 ) . A t any pH, th e h o r iz o n t a l d i s t a n c e s betw een c u rv e s 1-2 m easures th e a d d i t i o n a l b a se consumed o r th e t o t a l number o f [a2“ 1 com plexed. A s e r i e s o f R and [ A2” ] v a lu e s a t v a r io u s pH were c a l c u l a t e d and fo rm a tio n cu r v e s w ere p l o t t e d . The v a l u e s o f lo g It, and lo g K2 were r e a d from g ra p h a t H
v a lu e s o f 0 . 5 and 1 .5 r e s p . and were r e f i n e d by r e l a x a t i o n ma~* th o d [ 3 ] . The c a l c u l a t e d v a l u e s o f l o g were 6 .2 4 and 6.11), v a lu e s o f l o g Kg were 7 . 4 0 and 6 .6 3 f o r TLA and TGA, r e s p e c t i v e l y .
R e fe re n c e s
[ 1 ] J . B J e r r u m , M etal ¿.mine f o r m a ti o n i n aqueous i o n s , Copenhagen 1941. [ 2 ] M. C a 1 v i n , N. 0 . M e 1 c h i o r , J . Am. . , , 3270 ( 1 9 4 8 ). r e s p . , we nave u s e d C a lv in and F i g . 4 . P o t e n t i o m e t r i e t i t r a t i o n o f 4 .3 • 10-* M TGA- ( c u r v e 1) and 4 .3 -10-* M TGA + IO-* M Co(C10^)2
[ 3 ] K. O. W a t k i n s , M„. M. J o n e 6 , J . I n o r g . N uclear Chem., 16, 187 (1 9 6 1 ).
D epartm ent o f Fundam ental Food C h em istry T e c h n ic a l U n iv e r s ity o f Łódź
J oa nn a M asłowska, J ó z e f Szmich
POTENCJOMETRYCZNE BADANIA KCWPLEKSÓW Co( I I ) Z KWASAMI MERKAPTOKARBOKSYLOWYMI
Badano p r z y u ż y c iu metod p o t e n c jo m e try c z n y ch pow staw anie kompleksów pomiędzy jonami C o (II) i kwasem tiomlekowym (1’LA) 0- r a z tio g lik o lo w y m (T G a).w r o z tw o r z e wodnym, 0,'1 M względem NaClO* w t e m p e r a t u r z e 20°C. Stw ie rd z o n o powstawanie kompleksów ty p u • Co(S.-;CCGJ i Co( SnCOO)|* ( g d z i e "R » CHj lu b CHjCil). Wyznaczono s t a ł e t r w a ł o ś c i tych kompleksów j a k ró w n ież s t a ł e d y s o c j a c j i TLA
i TOA.
Kwas tiomlekowy o r a z t i o g l i k o l o w y n a l e ż ą do g ru p y związków b ardzo ważnych z punk tu f i z j o l o g i c z n e g o . Poniew aż w ykazują one s i l n e w ł a s n o ś c i kom p leksu jące wydało s i e nam i n t e r e s u j ą c e po równanie t r w a ł o ś c i kompleksów t;ych kwasów z jonam i C o ( i l ) .
S a n n a M acaoBcK a, Kta&p HImhx
iI3T£HUKOr,ISTP5WBCKOE HCCJIE.^OBAHHE KOMTUIEKOOB C o ( I I ) C MEPKAflT OKAP BO H OBii !ffii KHGJ10TAMH
Up-A noMoajK n o T e łm a o M e ip a iie c K o r o MeT0,na HccJieAOBaiio o 6 p a 3 o « a n n e K o a m e x o o B C o ( I I ) o thomojiohhom (TLA) k THoraHKOJieBoit * (TOA) k h -
caóTasM b 80;ui0 M, O . l MojieaoM (NaC1 0 4 ) pacTBope, npw 20°C. Y c ia - hoi-.:e u o odpasosaHae KOMnaeKCOB THiia Co(SRCOO) u Co(SRC00)|"- (R *
