Beck, Mihály Tibor

Görög, Sándor

English

Beck Mihály Tibor

Görög Sándor

University of Szeged

DETERMINATION OF COMPLEX STABILITY CONSTANT FROM CATALYTIC ACTIVITY I (Stability Constant of Iron(IH)-Triethylenetetramine Complex) By M. T. BECK and S. G Ö R Ö G Institute of Inorganic and Analytical Chemistry, The University, Szeged (Received March 11, 1958) The catalytic effect of Fe(I I I )TETA complex on the decomposition of H2Oa was inhibited by EDTA. From the extent of inhibit ion — from the init ial velocity as a function of concentration of EDTA — the stability constant of Fe(I I I )TETA could be calculated. The stability constant was also determined from the increasing of solubility of Fe(OH);> in the presence of TETA. The two values are in good agreement. Introduction In connection with our recent investigations [1] on the catalytic effect of iron(IlI)-triethylenetetramine (Fe(III)TETA) complex on the decomposition of hydrogen peroxide [2] it was necessary to know the stability constant of this complex. The experiments were carried out at relatively high pH thus the existence of the complex in a reliable concentration requires a very great stability constant for the complex. The generally applied pn metric method was not suitable for the determination of stability constant of this complex because these measurements have to be carried out at very low pn due to the very small solubility product of Fe(OH)3. Consequently, these measure-ments can not give reliable data for relations at about pa 10, where the kinetic experiments were carried out. Principle of method It was observed that the ethylenediaminetetraacetic acid (EDTA) inhibits the decomposition reaction owing to the following displacement reaction: Fe (III) TETA + EDTA ^ Fe (111) EDTA + TETA1 The Fe(III)EDTA complex has no catalytic activity: the velocity of decom-position of hydrogen peroxide in the presence of even a relatively great 1 For sake of simplicity the electric charge of complexes is not signed. 60 •M. T . B E C K A N D S. G Ö R Ö G amount of Fe(III)EDTA is equal to the velocity of non-catalytic reaction at the same pn- This phenomenon makes possible the determination of stability constant of Fe(IlI)TETA complex, since the velocity of decomposition reac-tion is a function of concentration of Fe(Ill)TETA. Thus the concentration of the complex can be calculated from the initial velocity. According to WANG the catalytic decomposition is bimolecular: — = £[Fe (Ill)TETA] [HA]. In our recent work it was shown that this expression is not correct : the bimolecular velocity constant increases with the concentration of Fe(IIl)TETA while the total concentration of TETA is constant. The variation of initial velocity constant with- the concentration of Fe(III)TETA complex is shown in Table 1. Table l [Fe (111) TETA]-10' k-10-3 mol. lit"1 sec-1' mol- 1 2,9 1,8 4,7 1,9 6,0 2,1 7,9 2,6 11,0 3,3 14,0 3,5-Using the data of Table 1 the calculation of concentration of Fe(III)TETA complex is possible on the basis of bimolecular velocity relationship. The equilibrium constant of the above displacement reaction is [Fe (III) EDTA] [TETA] [Fe(III)TETA][EDTA]' Further, the stability constants of the Fe(III)TETA and Fe(III)EDTA com-plexes are defined by the following expressions: [Fe(Hl)TETA] [Fe(lII)] [TETA] and [Fe (III) EDTA] s —[Fe(I I I )] [EDTA] ' Hence Knowing the stability constant of Fe(III)EDTA complex (/Q, the acidic dis-sociation constants of EDTA (K„) and TETA (/fj), as well as the concentra-tions of EDTA, TETA and Fe(lII)TETA, the stability constant of Fe(IIl)TETA C O M P L E X S T A B I L I T Y C O N S T A N T F R O M C A T A L Y T I C A C T I V I T Y 61-complex can be calculated. These concentrations can be calculated from the initial velocity of reaction, knowing the total concentrations. „ K* /r»[Fe(Hl)TETA]«£(cEDTA — [Fe(IH)EDTA]) 1 K [Fe(IlI)EDTA]«rcTETA where aT and a E are the fraction of TETA and EDTA in acid base system present as free TETA and EDTA4-. k I K I K I K I cct KlKlKlKl + K?KZKZ\Hi + A7Af[H f + Kl [H+] + [H+r C(E-K\ Ki: Kt K!% K f K i K i K ? -4, K?K£K£ [H+] + KFK? [H+J3+K? [ H + f + [ H 7 Another possibility for determination of stability constant of Fe(III)TETA complex is the measurement of increasing of solubility of the ferric iron in the presence of TETA at a given pn- The concentration of free Fe(IIl) is determined by the solubility product of ferric hydroxide: ' [Fe3+] = 1 , 1 [OH ]3 • The solubility of Fe(lll) in alcaline medium does increase in the presence of TETA, owing j o the formation of Fe(lIl)TETA complex. The concentration of the later is given by a kinetic measurement thus the stability constant can be calculated. Experimental Reagents. Eastman Kodak TETA technical quality was distilled over sodium at reduced pressure. All other reagents used were c. p. grade. Procedure. The reaction was followed by permanganometric determina-tion of non-decomposed hydrogen peroxide. Details of method are described in our paper [1]. Results and discussion The effect of EDTA on the velocity of decomposition was measured at two different concentrations of TETA at pH 10. Fig. 1 and Fig. 2 shows the results of kinetic experiments. The initial velocity (v0) was obtained from the kinetic curves by graphical differentiation. The actual concentrations of Fe(III)TETA were calculated from the initial velocity on the basis of bimo-lecular rate relationship using the data of Table 1. The concentration of Fe(III)EDTA is the difference between total Fe(III) and actual Fe(III)TETA concentrations. The concentration of TETA is practically constant owing to the great excess of it, the concentration of free EDTA was obtained as the difference of total EDTA and Fe (III) EDTA. The results of calculations are given in Table 2. and Table 3. The following numerical values of acidic dissociation constants of TETA and EDTA as well as the stability constant of Fe(UI)EDTA were used for calculations: 6 2 •M. T . B E C K A N D S. G Ö R Ö G Table 2 cFe(Hl)= 12,1-10~7; f T E T A = 5-10-s; f H A = 0,098; p H = 1 0 No. CEDTA'10 v0-W mol. l i t - 1 sec-1 [Fe(III)TETA]107 [Fe(III)EDTA]-107 [EDTA4_]-107 Kv io - 2 1 1 0 4,0 12,1 0 0 2 5 4,0 ~ 12,1 s 0 s 1,8 — 3 10 2,9 10,2 1,9 2,9 8,6 4 20 2,4 9,0 3,1 6,0 9,7 5 . 50 1,5 6,6 5,5 15,8 10,5 6 80 1,1 5,3 6,8 26,0 11,2 7 "100 0,5 2,8 9,3 32,2 5,4 8 200 — Ä 0 «12,1 «66,7 — average: 9,1 All concentrations are given in mol. lit-1 Fig. /. Effect of EDTA on the decomposition of H,02 catalysed by Fe(III)TETA. Data are given in Table 2 C O M P L E X S T A B I L I T Y C O N S T A N T F R O M C A T A L Y T I C A C T I V I T Y 63 -Table 3 cFe(III) =13,3-10- 7 ; CTETA = 1(T2; ^ = 0,105; pH= 10. No. CEDTA"10' mol. lit-1 sec""1 [Fe(Ill)TETA]-107 [Fe (III) EDTA] -107 [EDTA4"]-107 ATj • 10~21 1 0 4,9 13,3 0 0 2 10 4,9 «13,3 « 0 « 3,4 — 3 30 3,5 11,1 2,2 9,9 13,9 4 50 2,7 9,2 4,1 16,3 10,2 5 70 1,5 6,2 7,1 22,3 5,4 6 100 0,9 4,3 9,0 32,3 4,3 7 200 — « 0 «13,3 «66,3 — average: 8,5 All concentrations are given in mol. lit-1.' Fig. 2. Effect of EDTA on the decomposition of H202 catalysed by Fe(iII)TETA. Data are given in Table 3 6 4 •M. T . B E C K A N D S. G Ö R Ö G Acidic dissociation con-stants of TETA [3] : 8 I m in Fig 3. Data are given in Table 4 pKT 3,89 pKl 7,01 pKl 9,36 pKl 9,99 Acidic dissociation stants of EDTA [4] : pKf 1,99 PK£ 2,67 pKÏ 6,16 pKÏ 10,26 con-tion In the case of greatest TETA reaction was too great for exact calculation. The values of aT and a E at pH 10 calculated from the above constants are: aT = 0,454; ccE = 0,355. Stability constant of Fe(III)EDTA complex [5]: pK% = 25,1. The average value of sta-bility constant of Fe (111) TETA complex from the two series is 8,8-10». 2 Solubility measurements. Measured amount of TETA so-lution was addedto an -acidic Fe(C104)3 solution, the pu of the solution was adjusted to values of 10, the solution was vigorously shaken and then was filtered from the Fe(OH)3 pre-cipitate. The kinetic experiments were carried out with these solutions. Fig. 3 shows the results of these experiments, concentration the velocity, of decomposi-In these calculations two 2 It must be noted that the Fe(III)EDTA forms a mixed complex with hydrogen peroxide (RINGBOM A., S. SIITONEN and B. SAXGN: Anal. Chim. Acta 16, 541 (1957)). The influence of this fact was neglected because the error due to it may be only one order of magnitude, and the uncertainty of stability constant of Fe(III)EDTA complex is the same. C O M P L E X S T A B I L I T Y C O N S T A N T F R O M C A T A L Y T I C A C T I V I T Y 65 Table 4 %o2 = °.10; p H = I 0 No. cT E T A10° mol. lit 1 sec 1 [Fe(Ill)TETA].107 calculated from L = l ,MO~ 3 0 L = 4,0-10-38 1 2 3 10,8 5,4 2,7 8,3 3;7 26.3 12.4 1,0 0,9 26,8 25,3 All concen'rations. are given in mol. lit 1 different values of solubility product of iron(Ill) hydroxide were used [6], [7]. The data are given in Table 4. It must be noted that the concentrations are corrected considering the dilution of solution by adding hydrogen peroxide. It can be seen that the stability constant obtained from the inhibition experiments is in a very good agreement with the value from the solubility experiments. * * * The authors wish to express their gratitude to Prof. Dr. Z. G. SZABÓ for his kind interest and for the valuable discussions. , References [1] Beck, M. T., S. Görög: Acta Chim. Hung, (in press). [2] Wang, J. H.: J . Am . Chem. Soc. 77, 4715 (1955). Í3] Jonassen H. B., R. B. LeBlanc, A. W. Msibohm, R. M. Rogan: J . Am. Chem. Soc. 72, 2430 (1950). [4] Schwarzenbach G., H. Ackermann: Helv. Chim. Acta 30, 1798 (1947). [5] Schwarzenbach G., J. Heller: Helv. Ch im. Acta 34, 576 (1951). [6] Lange N. A.: Handbook of Chemistry (Sandusky, 1949), 7. Edit ion p. 1267. J7] Seel F.: Grundlagen der Analytischen Chemie und der Chemie in wassrigen Systemen (Weinheim/Bergstr. 1955), p. 326. "5 

Determination of complex stability constant from metalytic activity : I. Stability Constant of Iron(III)-Triethylenetetramine Complex

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Determination of complex stability constant from metalytic activity : I. Stability Constant of Iron(III)-Triethylenetetramine Complex

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