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Complex formation of iron(III) with diglycolic and iminodiacetic acids
J inorg,nucl.Chem., 1972,Vol. 34, pp. 987-997. PergamonPress. Printed in Great Britain
COMPLEX FORMATION OF IRON(Ill) WITH DIGLYCOLIC AND IMINODIACETIC ACIDS* ALDO
NAPOLI
Istituto di C h i m i c a Analitica, Universiffa di R o m a , R o m a , Italia
(Received 18 May 1971 ) A b s t r a c t - - C o m p l e x formation in a q u e o u s solution b e t w e e n iron(Ill) and diglycolic (HzDig) and iminodiacetic acid (H21m) has been studied at 25°C in a m e d i u m of constant ionic strength 0.5 M NaCIO4, by a spectrophotometric method. At - l o g [H +] values less than 2 and with e x c e s s of ligand (CL/CM up to 500) the formation of m o n o n u c l e a r c o m p l e x e s with a metal-ligand molar ratio of 1 : 1 has been observed. T h e formation c o n s t a n t s and the related equilibria are: Fe 3÷ + D i g z ,~ F e D i g ÷ Fe :~÷+ Im 2- ~ F e l m +
log fll = 5.04 log/31 = 10.72.
INTRODUCTION
THIS paper reports a spectrophotometric investigation of the complex formation between iron(Ill) and diglycolic (oxydiacetic) and iminodiacetic acids (H~Dig and H~Im respectively) at 25°C in a medium of constant ionic strength 0-5M NaCIO4. Diglycolic acid is known to form complexes with various bivalent metal ions [1-4], rare earth ions [5] and the uranyl ion [6]; similarly iminodiacetic acid [7-16], and some complexes with aluminium ions are also known[17]. The protonation constants of the two species of diglycolic acid are reported in the literature[I-3, 5, 18, 19], but have been remeasured under the present experimental conditions by potentiometric methods [20]. *Work carried out with the C N R aid. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
R. M. T i c h a n e and W. Bennett, J. Am. chem. Soc. 79, 1293 (1957). M. Y a s u d a , K. Y a m a s a k i and H. Ohtaki, Bull. chem. Soc.Japan 33 1067 (1960). E. Campi, G. Ostacoli, N. Cibrario a n d G. Saini, Gazz. chim. Ital. 91, 361 ( 1961 ). D. H. Klein and G. J. Kessels, J. inorg, nucl. Chem. 26, 1325 (1964). 1. G renthe and 1. T o b i a s s o n , A cta chem. Scand. 17, 2101 (1963). S. R a m a m o o r t h y a n d M. Santappa, Bull. chem. Soc. Japan 42, 411 (1969). G. S c h w a r z e n b a c h , E. K a m p i t s c h and R. Steiner, Heir. ehim. Acta 28, 1133 (1945). S . J . R . C h a b e r e k and A. E. Martell,J. Am. chem. Soc. 74, 5052 (1952). K. Suzuki and K. Y a m a s a k i , Naturwissenschaften 44, 396 (1957). K . W . Bernauer, D. Walz a n d S. FaUab, Heir. chim. Acta 41, 2094 (I 958). L . C . T h o m p s o n , Inorg. chem. 1 , 4 9 0 (1962). K. S. Rajan and A. E. Martell,J. inorg, nucl. Chem. 26.789 (1964). J . J . R . F r a u s t o da Silva, Rev. Port. quire. 7, 163 (1965). S. Misumi and M. Aihara, Bull. chem. Soc. Japan 39, 2677 (1966). T. T. Lai and T s u n g - Y u C h e n , J. inorg, nucl. Chem. 29, 2975 (1967). R. N a s a n e n , P. Tilus and S. Ojapera, Suom. kemistilehti 43B, 355 (1970). A. Liberti and A. N a p o l i , J . inorg, nucl. Chem. 33, 89 (1971). W. Ostwald, Z. phys. Chem. 3, 186(1889). J. Kapetanidis and D. Monnier, Heir. chim. A cttl 42, 1725 (1959). A. Napoli, in press. 987
988
A. N A P O L I
The existence of the protonated form of iminodiacetic acid: + / NH2
CH2COOH
\ CH2COOH
allows this acid to be considered as triprotic. Almost all authors [7, 8, 11-13, 14, 22] report only two protonation constants, for H i m - and H2Im species and ignore the existence of the protonated form H3Im ÷, although considered it for analogous acids[21,22]. Some workers observed this species in aqueous solution[23-25] and recently in our Institute all three protonation constants were determined at 25°C in 0.5M NaCIO, by potentiometric measurements [ 17]. EXPERIMENTAL Materials Diglycolic acid Fluka and iminodiacetic acid BDH. The acids were purified by crystallization from water. Solutions have been made by weighing the acids, dried at I 15°C, and have been titrated with sodium hydroxide by the potentiometric method. Iron(Ill) perchlorate. Iron(II1) perchlorate stock solution was prepared from Fe(NOa)3. nH20 Merck. The salt was dissolved with excess concentrated perchloric acid and nitric acid and a part of the excess HCIO4 were expelled by heating with an infrared lamp. In the final solution no nitrate, chloride, or sulphate could be detected. The iron(Ill) contents were determined by complexometric titration [26]. Sodium perchlorate. Stock solution was prepared by neutralising HCIO4 with sodium carbonate Merck to the extent of 98 per cent and boiling off CO2. Neutralization was complete with NaOH. The salt concentration was determined by evaporating a known amount of solution in an electric oven at 125°C; the sample was dried and weighed as NaCIO4. Instruments Beckman model D U and DK2A spectrophotometers with I cm quartz cells were used for the absorbance measurements. E.M.F. measurements were made with a Radiometer valve potentiometer priM 4. A Thalamid glass electrode was calibrated in concentration units by using a cell arrangement similar to that described by Forsling et al.[27]. The cell used was:
Ag, AgCl(s)l [Na +] = 0.50M, [CI-] = 0.01 M, [CIO4-] = 0"49MI[Na+I = [CIO,-] = 0.50M. The Ag, AgCI electrode was prepared as recommended by Brown[28]. Under our experimental conditions, since [OH-] was always negligible with respect to [H+], the fem of the cell can be expressed as [29]: E = E°+ 59.16 log [H +] +J[H+].
(1)
By using the values of E ° and J, determined before and after each measurement, from the value of E, [H +] can be calculated from Equation (1). 21. 22. 23. 24. 25. 26. 27. 28. 29.
A. Furlani, M. Maltese, E. Mantovani and C. Maremmani, Gazz. chim. ltal. 97, 1423 (1964). G. Anderegg, Heir. chim. Acta 50, 2333 (1967). K. Nakamoto, Y. Morimoto and A. E. Martell,J. am. chem. Soc. 84, 2081 (1962). D. Chapman, D. R. Lloyd and R. H. Prince, J. chem. Soc. 3645 (1963). M. A. Storik and V. N. Kumok, Zh. obshch. Khim. 37, 1722 (I 967). G. Chariot, Les mdthodes de la Chimie A nalitique p. 970, Masson, Paris (I 961). W. Forsling, S. Hietanen and L. G. SilMn,Acta chem. scand. 6, 901 (1952). A. S. Brown,J.Am. chem. Soc. 56, 646 (1934). G. Biedermann and L. G. Sill6n,Arkiv. kemi. 5, 425 (1953).
Complex formation of iron(I 11)
989
Method of calculation Because of the acid character of iron(Ill), hydrolytic reactions take place even at low pH, which lead to precipitation of the hydroxide. At pH > 2 hydrolysis of metal ion is already appreciable: furthermore, even in the presence of excess ligand the absorbance values vary with the time. Therefore investigation has been made at values of pH not greater than 2 and with excess ligand. The data for the evaluation of equilibria and for calculation of constants were obtained from the "straight line method"[30, 31], from the absorbance curves of solutions of iron complexes with respect to pH (with constant ligand excess) and from the A = A(log CD plot for constant values of [H+]. The existence of polynuclear complexes in the system was examined by means of the pH/absorbance curves for solutions containing various small concentrations of the metal ion and a constant excess of ligand. If the formation reaction is considered (omitting charges for clarity): F e + n H ~ L ~ FeH,L. + xH + with: k:
[Fe H rL.] [H +]-~ [Fe][HqL]"
in the presence of a large excess of ligand the following relationship is valid: Y = log A/(Ao--A) + n log y = x log [H +] + tog k + n log C/~
(2~
where A0 is the limiting value of the absorbance, Ct, is the total concentration of the ligand and y is a correction term for the dissociation of the ligand: q
C1.
q
E,[H~L] 0
3, = [HqL]
~,#,t,[H+] ' 0
[HqLI
/3qL[H'I q
(/3~1,are the over all protonation constants of the acid H qL). If the value of n is known, a plot of Y vs. - l o g [H +] gives a straight line, the slope of which corresponds to the hydrogen ions split off in complex formation. If the value ofx is known and Y is plotted vs. --log CL at constant [H +] a straight line is obtained with slope ofn. In both cases the intercept on the abscissa allows one to calculate the value ofk. RESULTS T h e u.v. s p e c t r u m o f i r o n ( I l l ) c h a n g e s f o r p H v a l u e s g r e a t e r t h a n 1 if an e x c e s s o f i m i n o d i a c e t i c o r d i g l y c o l i c a c i d s is p r e s e n t . N e i t h e r r e a g e n t a b s o r b s in t h e s p e c t r a l r a n g e e x a m i n e d . A b s o r p t i o n s p e c t r a o f s o l u t i o n s c o n t a i n i n g iron(Ill) and the ligand were recorded using a solution of iron(Ill) as a blank; s u c h s p e c t r a s h o w a b a n d w i t h a m a x i m u m n e a r 2 8 0 n m ( F i g . i ). A s this m a x i m u m is n o t c h a n g e d b y v a r y i n g h y d r o g e n i o n o r m e t a l - l i g a n d c o n c e n t r a t i o n r a t i o o n l y o n e c o m p l e x c a n b e p r e d o m i n a n t in s o l u t i o n . T h e w o r k i n g w a v e l e n g t h w a s f i x e d at 2 8 0 n m a n d t h e a b s o r b a n c e v a l u e s h a v e b e e n c o r r e c t e d f o r t h e m e t a l i o n c o n t r i b u t i o n b y s u c c e s s i v e a p p r o x i m a t i o n s . T h e r e s u l t s o b t a i n e d a r e in a g r e e m e n t w i t h t h o s e o b t a i n e d o p e r a t i n g at o t h e r w a v e l e n g t h s .
T h e iron( l l l )-iminodiac etic acid s y s t e m I n F i g . 2 t h e v a l u e s A/CM vs. - - l o g [ H +] a r e r e p o r t e d , t h e l i g a n d c o n c e n t r a t i o n 30. E. Asmus, Z. analyt. Chem. 178, 104 (1960). 31. K.S. Klausen and F. J. Langmyhr, Anal. chim. acta 28, 501 (1963).
990
A. NAPOLI O- 500 2
CM=I × 10-4 M CL
=5x I0 -z M
I
0-300
0.100
I
260
300
340
)% nm
Fig. 1. Absorption spectra of solutions containing iron(Ill) and the ligand recorded using a solution of iron(Ill) as a blank. - l o g [ H ÷] = 2.0; C u r v e 1: iminodiacetic acid; Curve 2: diglycolic acid.
being constant, while the iron(III) concentration was varied in the range 5 × 10 -52 × 10 -4 M. A s all the points fall on the same curve, polynuclear complexes can be considered negligible. The "straight line method" was applied at - l o g [H ÷] = 2.0 by varying the ligand concentration at a fixed concentration of metal ion. The results, plotted in Fig. 3, show that a straight line is obtained for n = 1. This means that in solutions a mononuclear complex with metal-ligand molar ratio of 1:1 is predominant. The interc~ e 5 × IO-5M o I x 10-4M
e2 × 10-4M 040C
CL.: 0 0 2 M o X
0'20C
I
0.000
2 -log
[H']
Fig. 2. lron(l ll)-iminodiacetic acid system, h = 280 nm.
Complex formation of iron(l 11)
///
991
"=2
5
i/A
-0"5
Fig. 3. lron(lll)-iminodiacetic acid system. "Straight line method". X = 2 8 0 n m : - l o g [H +] = 2.0;COL = 0-10M; V = 50ml.
cept on the y axis, q, is related to the equilibrium constant by the relationship: ! [~1
=
--
qV/Co,
where V is the final volume of the solutions and CoL the total concentration of the stock solution of the ligand. U n d e r the experimental conditions o f - log[H +] = 2.0, V = 50 ml, Cot. = 0-10M: q - ( - 0 , 8 5 + 0 . 1 0 ) ml -~ fl'l = (4.25 +_0.5) × 102 M -1 log fl'l = 2.63 +_ 0.05. F r o m the analysis of the function A = A(Iog [H+]) it is possible to calculate the n u m b e r of hydrogen ions split off on complex formation and the corresponding equilibrium constant. Fe+H:~Im.
" FeH3_flm+xH +
k = [FeH3-xlm][H+]x [Fel[H.~lm] T h e results are reported in T a b l e 1, while the values of log A[(Ao--A)+log'y vs. --log [H +] are plotted in Fig. 4. T h e experimental points fall on a straight line,
992
A. N A P O L I Table
1. l r o n ( l l l ) - i m i n o d i a c e t i c
system
- l o g [ H +]
A
log3,
logA.yl(,'lo--.4)
1"30 1"50 1-58 1"68 1"80 1'94
0"052 0"138 0"184 0"250 0'307 0"355
0"13 0"20 0"25 0"30 0"36 0"46
--0'72 --0'11 0'14 0"46 0"79 1"20
h = 2 8 0 n m ; CL = 0"02M; C M = I ' 0 X 10-4 M;Ao = 0"420.
with a slope of 3 and with an intercept on the x axis of 1.54. Therefore: log k = 3 log [H ÷] --log
CL = - - 4 " 6 2 + 1.70 = --2.92-----0.06.
T h e values of log A/(Ao--A)+ log 3' vs. log CL are reported in Table 2 and in Fig. 5 for - l o g [ H +] = 2.0. F r o m the intercept on the x axis [3.13], it can be deduced that: log k = - 6 . 0 0 + 3.13 = -2.87___ 0.06
agreeing with the preceding result.
1.0
0
+
2
o~
-10g [H+]
-l.O
Fig. 4. lron(I I l)-iminodiacetic acid system, h = 280 nm.
C o m p l e x f o r m a t i o n of i r o n ( l 1 I)
993
T a b l e 2. l r o n ( l l l ) - i m i n o d i a c e t i c acid system -- log Ct.
A
2.70 2.40 2.22 2-10 2-00 1.70
0-194 0.263 0-296 0.320 0.341 0.377
log A . y / ( A , , - A
)
0.44 0.74 0.89 1-02 1.15 1.45
X = 280 nm; -- log [H +] = 2.0: C~I = 1.0 × 10-4M: A,, = 0.420: log'), = 0.51.
The iron (lll)-diglycolic acid system Equilibria involving polynuclear complexes were eliminated in solutions with ligand excess, since curves A/CM=f(Iog [H+]) overlap for various C,~t values in the range 5 x 10-5-2 × 10-4 M(Fig. 6). The "straight line m e t h o d " was applied at - l o g [H +] --- 2.0 by varying the concentration of the ligand with a constant concentration of iron(III). The experimental details are given in Fig. 7, from which it is seen that a straight line is obtained for n -- 1. The intercept on the y axis (0.43 m1-1) gives the logarithm of the conditional formation constant: log 13'1 = 2.33 _+0.05.
10
+
F
I
2
3
,,
-IC
Fig. 5. I r o n ( l l l ) - i m i n o d i a c e t i c a c i d s y s t e m . ,k = 280 n m ; -
JINC Vol. 34 No. 3 - G
log [H +] = 2-0.
994
A. N A P O L I
c~ e 5 x IO-SM o I x 10-4M e 2 x 10-4M
CL
=0"05 M
0-500
0.40C cD
X
0.30C
020C
0.100
0.000 2
-log
[H÷1
Fig. 6. l r o n ( l l l ) - d i g l y c o l i c acid system, h = 280 nm.
IO
/'=/ 2
/=2 0"5
0"5
i/A
ot Fig. 7. Iron(11 l ) - d i g l y c o l i c acid system. "Straight line method". X = 280 nm; - l o g [H +] = 2.0; CoL = 0.10M; V = 50ml.
Complex formation of iron(l 1I)
995
A study of the effect of hydrogen ion concentration on the absorbance values makes it possible to define the mechanism of the reaction more precisely and to calculate the equilibrium constant. On the assumption that the composition of the complex is 1 : 1 the equation of the complex forming reaction can be expressed as follows (omitting charges, except for the hydrogen ions) Fe+HzDig.
" FeH2_xDig+xH +
with: k = [FeH"-xDig][H+]x [Fe][H2Dig] T h e results are reported in Table 3 and plotted in Fig. 8 from which x is 2 and the logarithm of the equilibrium constant is given by: log k = 2 log [H +] - l o g CL = -- 2.88 + 1"30 = -- 1.58 -----0.06. The values of log A/(Ao - A ) + log y vs. - log CL for -- log [H +] = 2-0 are reported in Table 4 and plotted in Fig. 9. F r o m the intercept on the abscissa axis (2.44) log k can be calculated: as: log k = 2 log [H +] - l o g CL = - 4 - 0 0 + 2 . 4 4 = - 1.56___0.06 in good agreement with preceding data.
DISCUSSION
At - l o g [H +] ~< 2 and with ligand excess (CL/CM up to 500) iron(Ill) forms a mononuclear complex with iminodiacetic acid or diglycolic acid, with a molar ratio 1 : 1 and with the simplified formulae F e l m ÷ and F e D i g ÷, respectively. Table 5 summarises the values of the calculated formation constants. The equilibrium constant k is related to fl~ by the relationship: log k = log fl~ + x log [H +] + log y. Table 3. iron(l I I)-diglycolic acid system - l o g [ H +]
A
log~/
IogA.y/(Ao--A)
1.13 1.23 1.30 1.40 1.50 1.65
0.095 0.122 0.173 0.255 0.313 0..90
0.01 0.01 0.01 0.02 0.02 0.03
--0.67 --0.54 --0-33 --0.05 0-14 0.42
h =
280 nm; CL = 0 . 0 5 M : C . M:A,, = 0.560.
=
1.0×
10 4
996
A. N A P O L 1
I •0
O
+
0 o
-I .O
Fig. 8. lron(lll)-diglycolic acid system, h = 280 nm.
F o r iminodiacetic acid, x = 3 and log y = 0.07 (at - log [H +] = 2.0): log k = 2.33 - 4 . 0 0 + 0 . 0 7 = - 1.60. F o r diglycolic acid, x = 2 and log y = 0.51 (at - log [H +] = 2.0): log
k =
2.63
4.00 + 0.51 = - 2.86.
-
T h e s e values agree very well with those obtained with the other methods. The stability constant/3 can be calculated from the relationship:
/3 =/3', aLa~, where a/~ and o~M are the side reaction coefficients of the ligand and of the metal ion. By using the protonation constants of acids and hydrolysis constants of Table 4. lron(IIl)-diglycolic acid system -- log CL
A
logA . y/(A0--A)
2.70 2'40 2'22 2'10 2.00 1.70
0.169 0'274 0.323 0.352 0.387 0.460
--0.28 0'07 0.22 0.32 0'45 0.78
h = 280 nm; -- log [H +] = 2-0; Cu =I.0xl0-4M; A o = 0 . 5 6 0 ; log y = 0.07.
Complex formation of iron(l I I)
997
I.c
+
-i o
Fig. 9, lron(ll l)-diglycolicacid system. X= 280 n m : - log [H ÷] = 2'0. Table 5. Constants of iron(l 11) complexes I minodiacetic acid Diglycolic acid 3.76 log fl~l,= 9" 17 6-56 Iogfl2t, = 11.73 Iogfl~l,= 13.49 log/3~ log k* logfll logk*
Method
"Straight line" (-log [H +] = 2.0) 2.63 A = A (log [H+])cL.cu A = A(log CL)H, Cu
2.33 -2.87 -2.92
-
1-56
-
1.58
*k= [Felm+][H+]3 [Fe'][H31m+]" [FeDig+][H+] 2 *k [Fe']lHeDig] " i r o n (111) 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 u s e d [ 1 7 , 20, 32] f o r i m i n o d i a c e t i c log/3 = 10.72 a n d f o r d i g l y c o l i c a c i d log fl = 5-04 can b e c a l c u l a t e d . T h e diffe r e n c e in s t a b i l i t y is d u e to t h e d e c r e a s e in d o n o r a b i l i t y w h e n an e t h e r o x y g e n a t o m is s u b s t i t u t e d for a n i t r o g e n a t o m . F r o m a c o m p a r i s o n o f t h e s t a b i l i t y o f the iminodiacetic acid-iron(Ill) complex and similar complexes of other aminoc a r b o x y l i c a c i d s with i r o n ( I l l ) t h e f o l l o w i n g o r d e r o f s t a b i l i t y is o b t a i n e d : EDTA
> NTA > H2Im > Glycine
a c c o r d i n g to o t h e r m e t a l i o n s c o m p l e x e s . S u c h d e c r e a s i n g in s t a b i l i t y c a n b e p r e d i c t e d on the b a s i s o f the n u m b e r o f c h e l a t e r i n g s p r e s e n t in t h e c o m p l e x [3 3]. 32. A. S. Wilson and H. Taube, J. Am. chem. Soc. 74, 3509 (l 952). 33. H. M. N. H. Irving and K. Sharpe, J. inorg, nucl. Chem. 33, 233 ( 1971 ).
COMPLEX FORMATION OF IRON(Ill) WITH DIGLYCOLIC AND IMINODIACETIC ACIDS* ALDO
NAPOLI
Istituto di C h i m i c a Analitica, Universiffa di R o m a , R o m a , Italia
(Received 18 May 1971 ) A b s t r a c t - - C o m p l e x formation in a q u e o u s solution b e t w e e n iron(Ill) and diglycolic (HzDig) and iminodiacetic acid (H21m) has been studied at 25°C in a m e d i u m of constant ionic strength 0.5 M NaCIO4, by a spectrophotometric method. At - l o g [H +] values less than 2 and with e x c e s s of ligand (CL/CM up to 500) the formation of m o n o n u c l e a r c o m p l e x e s with a metal-ligand molar ratio of 1 : 1 has been observed. T h e formation c o n s t a n t s and the related equilibria are: Fe 3÷ + D i g z ,~ F e D i g ÷ Fe :~÷+ Im 2- ~ F e l m +
log fll = 5.04 log/31 = 10.72.
INTRODUCTION
THIS paper reports a spectrophotometric investigation of the complex formation between iron(Ill) and diglycolic (oxydiacetic) and iminodiacetic acids (H~Dig and H~Im respectively) at 25°C in a medium of constant ionic strength 0-5M NaCIO4. Diglycolic acid is known to form complexes with various bivalent metal ions [1-4], rare earth ions [5] and the uranyl ion [6]; similarly iminodiacetic acid [7-16], and some complexes with aluminium ions are also known[17]. The protonation constants of the two species of diglycolic acid are reported in the literature[I-3, 5, 18, 19], but have been remeasured under the present experimental conditions by potentiometric methods [20]. *Work carried out with the C N R aid. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
R. M. T i c h a n e and W. Bennett, J. Am. chem. Soc. 79, 1293 (1957). M. Y a s u d a , K. Y a m a s a k i and H. Ohtaki, Bull. chem. Soc.Japan 33 1067 (1960). E. Campi, G. Ostacoli, N. Cibrario a n d G. Saini, Gazz. chim. Ital. 91, 361 ( 1961 ). D. H. Klein and G. J. Kessels, J. inorg, nucl. Chem. 26, 1325 (1964). 1. G renthe and 1. T o b i a s s o n , A cta chem. Scand. 17, 2101 (1963). S. R a m a m o o r t h y a n d M. Santappa, Bull. chem. Soc. Japan 42, 411 (1969). G. S c h w a r z e n b a c h , E. K a m p i t s c h and R. Steiner, Heir. ehim. Acta 28, 1133 (1945). S . J . R . C h a b e r e k and A. E. Martell,J. Am. chem. Soc. 74, 5052 (1952). K. Suzuki and K. Y a m a s a k i , Naturwissenschaften 44, 396 (1957). K . W . Bernauer, D. Walz a n d S. FaUab, Heir. chim. Acta 41, 2094 (I 958). L . C . T h o m p s o n , Inorg. chem. 1 , 4 9 0 (1962). K. S. Rajan and A. E. Martell,J. inorg, nucl. Chem. 26.789 (1964). J . J . R . F r a u s t o da Silva, Rev. Port. quire. 7, 163 (1965). S. Misumi and M. Aihara, Bull. chem. Soc. Japan 39, 2677 (1966). T. T. Lai and T s u n g - Y u C h e n , J. inorg, nucl. Chem. 29, 2975 (1967). R. N a s a n e n , P. Tilus and S. Ojapera, Suom. kemistilehti 43B, 355 (1970). A. Liberti and A. N a p o l i , J . inorg, nucl. Chem. 33, 89 (1971). W. Ostwald, Z. phys. Chem. 3, 186(1889). J. Kapetanidis and D. Monnier, Heir. chim. A cttl 42, 1725 (1959). A. Napoli, in press. 987
988
A. N A P O L I
The existence of the protonated form of iminodiacetic acid: + / NH2
CH2COOH
\ CH2COOH
allows this acid to be considered as triprotic. Almost all authors [7, 8, 11-13, 14, 22] report only two protonation constants, for H i m - and H2Im species and ignore the existence of the protonated form H3Im ÷, although considered it for analogous acids[21,22]. Some workers observed this species in aqueous solution[23-25] and recently in our Institute all three protonation constants were determined at 25°C in 0.5M NaCIO, by potentiometric measurements [ 17]. EXPERIMENTAL Materials Diglycolic acid Fluka and iminodiacetic acid BDH. The acids were purified by crystallization from water. Solutions have been made by weighing the acids, dried at I 15°C, and have been titrated with sodium hydroxide by the potentiometric method. Iron(Ill) perchlorate. Iron(II1) perchlorate stock solution was prepared from Fe(NOa)3. nH20 Merck. The salt was dissolved with excess concentrated perchloric acid and nitric acid and a part of the excess HCIO4 were expelled by heating with an infrared lamp. In the final solution no nitrate, chloride, or sulphate could be detected. The iron(Ill) contents were determined by complexometric titration [26]. Sodium perchlorate. Stock solution was prepared by neutralising HCIO4 with sodium carbonate Merck to the extent of 98 per cent and boiling off CO2. Neutralization was complete with NaOH. The salt concentration was determined by evaporating a known amount of solution in an electric oven at 125°C; the sample was dried and weighed as NaCIO4. Instruments Beckman model D U and DK2A spectrophotometers with I cm quartz cells were used for the absorbance measurements. E.M.F. measurements were made with a Radiometer valve potentiometer priM 4. A Thalamid glass electrode was calibrated in concentration units by using a cell arrangement similar to that described by Forsling et al.[27]. The cell used was:
Ag, AgCl(s)l [Na +] = 0.50M, [CI-] = 0.01 M, [CIO4-] = 0"49MI[Na+I = [CIO,-] = 0.50M. The Ag, AgCI electrode was prepared as recommended by Brown[28]. Under our experimental conditions, since [OH-] was always negligible with respect to [H+], the fem of the cell can be expressed as [29]: E = E°+ 59.16 log [H +] +J[H+].
(1)
By using the values of E ° and J, determined before and after each measurement, from the value of E, [H +] can be calculated from Equation (1). 21. 22. 23. 24. 25. 26. 27. 28. 29.
A. Furlani, M. Maltese, E. Mantovani and C. Maremmani, Gazz. chim. ltal. 97, 1423 (1964). G. Anderegg, Heir. chim. Acta 50, 2333 (1967). K. Nakamoto, Y. Morimoto and A. E. Martell,J. am. chem. Soc. 84, 2081 (1962). D. Chapman, D. R. Lloyd and R. H. Prince, J. chem. Soc. 3645 (1963). M. A. Storik and V. N. Kumok, Zh. obshch. Khim. 37, 1722 (I 967). G. Chariot, Les mdthodes de la Chimie A nalitique p. 970, Masson, Paris (I 961). W. Forsling, S. Hietanen and L. G. SilMn,Acta chem. scand. 6, 901 (1952). A. S. Brown,J.Am. chem. Soc. 56, 646 (1934). G. Biedermann and L. G. Sill6n,Arkiv. kemi. 5, 425 (1953).
Complex formation of iron(I 11)
989
Method of calculation Because of the acid character of iron(Ill), hydrolytic reactions take place even at low pH, which lead to precipitation of the hydroxide. At pH > 2 hydrolysis of metal ion is already appreciable: furthermore, even in the presence of excess ligand the absorbance values vary with the time. Therefore investigation has been made at values of pH not greater than 2 and with excess ligand. The data for the evaluation of equilibria and for calculation of constants were obtained from the "straight line method"[30, 31], from the absorbance curves of solutions of iron complexes with respect to pH (with constant ligand excess) and from the A = A(log CD plot for constant values of [H+]. The existence of polynuclear complexes in the system was examined by means of the pH/absorbance curves for solutions containing various small concentrations of the metal ion and a constant excess of ligand. If the formation reaction is considered (omitting charges for clarity): F e + n H ~ L ~ FeH,L. + xH + with: k:
[Fe H rL.] [H +]-~ [Fe][HqL]"
in the presence of a large excess of ligand the following relationship is valid: Y = log A/(Ao--A) + n log y = x log [H +] + tog k + n log C/~
(2~
where A0 is the limiting value of the absorbance, Ct, is the total concentration of the ligand and y is a correction term for the dissociation of the ligand: q
C1.
q
E,[H~L] 0
3, = [HqL]
~,#,t,[H+] ' 0
[HqLI
/3qL[H'I q
(/3~1,are the over all protonation constants of the acid H qL). If the value of n is known, a plot of Y vs. - l o g [H +] gives a straight line, the slope of which corresponds to the hydrogen ions split off in complex formation. If the value ofx is known and Y is plotted vs. --log CL at constant [H +] a straight line is obtained with slope ofn. In both cases the intercept on the abscissa allows one to calculate the value ofk. RESULTS T h e u.v. s p e c t r u m o f i r o n ( I l l ) c h a n g e s f o r p H v a l u e s g r e a t e r t h a n 1 if an e x c e s s o f i m i n o d i a c e t i c o r d i g l y c o l i c a c i d s is p r e s e n t . N e i t h e r r e a g e n t a b s o r b s in t h e s p e c t r a l r a n g e e x a m i n e d . A b s o r p t i o n s p e c t r a o f s o l u t i o n s c o n t a i n i n g iron(Ill) and the ligand were recorded using a solution of iron(Ill) as a blank; s u c h s p e c t r a s h o w a b a n d w i t h a m a x i m u m n e a r 2 8 0 n m ( F i g . i ). A s this m a x i m u m is n o t c h a n g e d b y v a r y i n g h y d r o g e n i o n o r m e t a l - l i g a n d c o n c e n t r a t i o n r a t i o o n l y o n e c o m p l e x c a n b e p r e d o m i n a n t in s o l u t i o n . T h e w o r k i n g w a v e l e n g t h w a s f i x e d at 2 8 0 n m a n d t h e a b s o r b a n c e v a l u e s h a v e b e e n c o r r e c t e d f o r t h e m e t a l i o n c o n t r i b u t i o n b y s u c c e s s i v e a p p r o x i m a t i o n s . T h e r e s u l t s o b t a i n e d a r e in a g r e e m e n t w i t h t h o s e o b t a i n e d o p e r a t i n g at o t h e r w a v e l e n g t h s .
T h e iron( l l l )-iminodiac etic acid s y s t e m I n F i g . 2 t h e v a l u e s A/CM vs. - - l o g [ H +] a r e r e p o r t e d , t h e l i g a n d c o n c e n t r a t i o n 30. E. Asmus, Z. analyt. Chem. 178, 104 (1960). 31. K.S. Klausen and F. J. Langmyhr, Anal. chim. acta 28, 501 (1963).
990
A. NAPOLI O- 500 2
CM=I × 10-4 M CL
=5x I0 -z M
I
0-300
0.100
I
260
300
340
)% nm
Fig. 1. Absorption spectra of solutions containing iron(Ill) and the ligand recorded using a solution of iron(Ill) as a blank. - l o g [ H ÷] = 2.0; C u r v e 1: iminodiacetic acid; Curve 2: diglycolic acid.
being constant, while the iron(III) concentration was varied in the range 5 × 10 -52 × 10 -4 M. A s all the points fall on the same curve, polynuclear complexes can be considered negligible. The "straight line method" was applied at - l o g [H ÷] = 2.0 by varying the ligand concentration at a fixed concentration of metal ion. The results, plotted in Fig. 3, show that a straight line is obtained for n = 1. This means that in solutions a mononuclear complex with metal-ligand molar ratio of 1:1 is predominant. The interc~ e 5 × IO-5M o I x 10-4M
e2 × 10-4M 040C
CL.: 0 0 2 M o X
0'20C
I
0.000
2 -log
[H']
Fig. 2. lron(l ll)-iminodiacetic acid system, h = 280 nm.
Complex formation of iron(l 11)
///
991
"=2
5
i/A
-0"5
Fig. 3. lron(lll)-iminodiacetic acid system. "Straight line method". X = 2 8 0 n m : - l o g [H +] = 2.0;COL = 0-10M; V = 50ml.
cept on the y axis, q, is related to the equilibrium constant by the relationship: ! [~1
=
--
qV/Co,
where V is the final volume of the solutions and CoL the total concentration of the stock solution of the ligand. U n d e r the experimental conditions o f - log[H +] = 2.0, V = 50 ml, Cot. = 0-10M: q - ( - 0 , 8 5 + 0 . 1 0 ) ml -~ fl'l = (4.25 +_0.5) × 102 M -1 log fl'l = 2.63 +_ 0.05. F r o m the analysis of the function A = A(Iog [H+]) it is possible to calculate the n u m b e r of hydrogen ions split off on complex formation and the corresponding equilibrium constant. Fe+H:~Im.
" FeH3_flm+xH +
k = [FeH3-xlm][H+]x [Fel[H.~lm] T h e results are reported in T a b l e 1, while the values of log A[(Ao--A)+log'y vs. --log [H +] are plotted in Fig. 4. T h e experimental points fall on a straight line,
992
A. N A P O L I Table
1. l r o n ( l l l ) - i m i n o d i a c e t i c
system
- l o g [ H +]
A
log3,
logA.yl(,'lo--.4)
1"30 1"50 1-58 1"68 1"80 1'94
0"052 0"138 0"184 0"250 0'307 0"355
0"13 0"20 0"25 0"30 0"36 0"46
--0'72 --0'11 0'14 0"46 0"79 1"20
h = 2 8 0 n m ; CL = 0"02M; C M = I ' 0 X 10-4 M;Ao = 0"420.
with a slope of 3 and with an intercept on the x axis of 1.54. Therefore: log k = 3 log [H ÷] --log
CL = - - 4 " 6 2 + 1.70 = --2.92-----0.06.
T h e values of log A/(Ao--A)+ log 3' vs. log CL are reported in Table 2 and in Fig. 5 for - l o g [ H +] = 2.0. F r o m the intercept on the x axis [3.13], it can be deduced that: log k = - 6 . 0 0 + 3.13 = -2.87___ 0.06
agreeing with the preceding result.
1.0
0
+
2
o~
-10g [H+]
-l.O
Fig. 4. lron(I I l)-iminodiacetic acid system, h = 280 nm.
C o m p l e x f o r m a t i o n of i r o n ( l 1 I)
993
T a b l e 2. l r o n ( l l l ) - i m i n o d i a c e t i c acid system -- log Ct.
A
2.70 2.40 2.22 2-10 2-00 1.70
0-194 0.263 0-296 0.320 0.341 0.377
log A . y / ( A , , - A
)
0.44 0.74 0.89 1-02 1.15 1.45
X = 280 nm; -- log [H +] = 2.0: C~I = 1.0 × 10-4M: A,, = 0.420: log'), = 0.51.
The iron (lll)-diglycolic acid system Equilibria involving polynuclear complexes were eliminated in solutions with ligand excess, since curves A/CM=f(Iog [H+]) overlap for various C,~t values in the range 5 x 10-5-2 × 10-4 M(Fig. 6). The "straight line m e t h o d " was applied at - l o g [H +] --- 2.0 by varying the concentration of the ligand with a constant concentration of iron(III). The experimental details are given in Fig. 7, from which it is seen that a straight line is obtained for n -- 1. The intercept on the y axis (0.43 m1-1) gives the logarithm of the conditional formation constant: log 13'1 = 2.33 _+0.05.
10
+
F
I
2
3
,,
-IC
Fig. 5. I r o n ( l l l ) - i m i n o d i a c e t i c a c i d s y s t e m . ,k = 280 n m ; -
JINC Vol. 34 No. 3 - G
log [H +] = 2-0.
994
A. N A P O L I
c~ e 5 x IO-SM o I x 10-4M e 2 x 10-4M
CL
=0"05 M
0-500
0.40C cD
X
0.30C
020C
0.100
0.000 2
-log
[H÷1
Fig. 6. l r o n ( l l l ) - d i g l y c o l i c acid system, h = 280 nm.
IO
/'=/ 2
/=2 0"5
0"5
i/A
ot Fig. 7. Iron(11 l ) - d i g l y c o l i c acid system. "Straight line method". X = 280 nm; - l o g [H +] = 2.0; CoL = 0.10M; V = 50ml.
Complex formation of iron(l 1I)
995
A study of the effect of hydrogen ion concentration on the absorbance values makes it possible to define the mechanism of the reaction more precisely and to calculate the equilibrium constant. On the assumption that the composition of the complex is 1 : 1 the equation of the complex forming reaction can be expressed as follows (omitting charges, except for the hydrogen ions) Fe+HzDig.
" FeH2_xDig+xH +
with: k = [FeH"-xDig][H+]x [Fe][H2Dig] T h e results are reported in Table 3 and plotted in Fig. 8 from which x is 2 and the logarithm of the equilibrium constant is given by: log k = 2 log [H +] - l o g CL = -- 2.88 + 1"30 = -- 1.58 -----0.06. The values of log A/(Ao - A ) + log y vs. - log CL for -- log [H +] = 2-0 are reported in Table 4 and plotted in Fig. 9. F r o m the intercept on the abscissa axis (2.44) log k can be calculated: as: log k = 2 log [H +] - l o g CL = - 4 - 0 0 + 2 . 4 4 = - 1.56___0.06 in good agreement with preceding data.
DISCUSSION
At - l o g [H +] ~< 2 and with ligand excess (CL/CM up to 500) iron(Ill) forms a mononuclear complex with iminodiacetic acid or diglycolic acid, with a molar ratio 1 : 1 and with the simplified formulae F e l m ÷ and F e D i g ÷, respectively. Table 5 summarises the values of the calculated formation constants. The equilibrium constant k is related to fl~ by the relationship: log k = log fl~ + x log [H +] + log y. Table 3. iron(l I I)-diglycolic acid system - l o g [ H +]
A
log~/
IogA.y/(Ao--A)
1.13 1.23 1.30 1.40 1.50 1.65
0.095 0.122 0.173 0.255 0.313 0..90
0.01 0.01 0.01 0.02 0.02 0.03
--0.67 --0.54 --0-33 --0.05 0-14 0.42
h =
280 nm; CL = 0 . 0 5 M : C . M:A,, = 0.560.
=
1.0×
10 4
996
A. N A P O L 1
I •0
O
+
0 o
-I .O
Fig. 8. lron(lll)-diglycolic acid system, h = 280 nm.
F o r iminodiacetic acid, x = 3 and log y = 0.07 (at - log [H +] = 2.0): log k = 2.33 - 4 . 0 0 + 0 . 0 7 = - 1.60. F o r diglycolic acid, x = 2 and log y = 0.51 (at - log [H +] = 2.0): log
k =
2.63
4.00 + 0.51 = - 2.86.
-
T h e s e values agree very well with those obtained with the other methods. The stability constant/3 can be calculated from the relationship:
/3 =/3', aLa~, where a/~ and o~M are the side reaction coefficients of the ligand and of the metal ion. By using the protonation constants of acids and hydrolysis constants of Table 4. lron(IIl)-diglycolic acid system -- log CL
A
logA . y/(A0--A)
2.70 2'40 2'22 2'10 2.00 1.70
0.169 0'274 0.323 0.352 0.387 0.460
--0.28 0'07 0.22 0.32 0'45 0.78
h = 280 nm; -- log [H +] = 2-0; Cu =I.0xl0-4M; A o = 0 . 5 6 0 ; log y = 0.07.
Complex formation of iron(l I I)
997
I.c
+
-i o
Fig. 9, lron(ll l)-diglycolicacid system. X= 280 n m : - log [H ÷] = 2'0. Table 5. Constants of iron(l 11) complexes I minodiacetic acid Diglycolic acid 3.76 log fl~l,= 9" 17 6-56 Iogfl2t, = 11.73 Iogfl~l,= 13.49 log/3~ log k* logfll logk*
Method
"Straight line" (-log [H +] = 2.0) 2.63 A = A (log [H+])cL.cu A = A(log CL)H, Cu
2.33 -2.87 -2.92
-
1-56
-
1.58
*k= [Felm+][H+]3 [Fe'][H31m+]" [FeDig+][H+] 2 *k [Fe']lHeDig] " i r o n (111) 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 u s e d [ 1 7 , 20, 32] f o r i m i n o d i a c e t i c log/3 = 10.72 a n d f o r d i g l y c o l i c a c i d log fl = 5-04 can b e c a l c u l a t e d . T h e diffe r e n c e in s t a b i l i t y is d u e to t h e d e c r e a s e in d o n o r a b i l i t y w h e n an e t h e r o x y g e n a t o m is s u b s t i t u t e d for a n i t r o g e n a t o m . F r o m a c o m p a r i s o n o f t h e s t a b i l i t y o f the iminodiacetic acid-iron(Ill) complex and similar complexes of other aminoc a r b o x y l i c a c i d s with i r o n ( I l l ) t h e f o l l o w i n g o r d e r o f s t a b i l i t y is o b t a i n e d : EDTA
> NTA > H2Im > Glycine
a c c o r d i n g to o t h e r m e t a l i o n s c o m p l e x e s . S u c h d e c r e a s i n g in s t a b i l i t y c a n b e p r e d i c t e d on the b a s i s o f the n u m b e r o f c h e l a t e r i n g s p r e s e n t in t h e c o m p l e x [3 3]. 32. A. S. Wilson and H. Taube, J. Am. chem. Soc. 74, 3509 (l 952). 33. H. M. N. H. Irving and K. Sharpe, J. inorg, nucl. Chem. 33, 233 ( 1971 ).