lLCHEVA, Liliana

SZABÓ-ÁKOS, Zsuzsanna

English

Periodica Polytechnica (Budapest University of Technology and Economics)

STABILITY CONSTANT OF Cr(DMSO)!+ AND Fe(NC S)!-OUTER-SPHERE COMPLEX By L. ILCHEVA * and Zs. SZABO-A.KOS Department of General and Analytical Chemistry, Technical University, Budapest Presented by Prof. Dr. E. PUNGOR Received January 31, 1978 In the literature is no data available on Cr(DMSO)~+ and Fe(NCS)~­outer-sphere stability constant. This paper deals ,~ith its determination by applying conductivity measurements. Experimental Cr(DlVISOMN0 3)3 was obtained and purified according to the procedure given in (1,2). The solution of Fe(NCS)~- was received by mixing the solu-tions ofFe(NO a)3 and KSCN in ratio Fe : SCN = I : 6 (3). The purple precip-itate of Cr(DlVISO)oFe(NCS)o was received, separated from the solution and carefully washed as described in (4). 6.10-4M solubility of the outer-sphere complex Cr(DlVISO)oFe(NCS)o determined by atomic absorption was found. The solutions of different eoncentrations of Cr(DMSO)6Fe(NCS)6 were care-fully prepared by using conductivity water of low specific conductance (1.93 . 10-60hm-1 cm-1). The cell constant was 0.6185 as determined with standard potassium chloride solution. The measurements were carried out at 25 QC using water thermostate. Results and discussion The conductivity of Cr(DlVISO)oFe(NCS)o solutions measured at different equivalent concentrations are shown in columns I and 2 of Table 1. We tried to work at equivalent concentration smaller than 3.10-4, but probably due to the instability of Fe(NCS)~+ the conductivity of the solution increased very much. The mobility of the ions was determined by Onsager's equation: , _ , (') 1 b')c'/, _ } Ct/, /. - /'0 - a '0 T - '0 - s (I) * Department of Analytical Chemistry, Chemico-Technological Institute, Sofia, Bulgaria 346 L. ILCHEV A and ZS. SZAB6.AKOS Table 1 ye. 10' l IX I ·10' Ig·K' 1.73 484.0 0.8309 7.476 3.3891 1.64 489.9 0.8384 6.789 3.4076 1.55 497.1 0.8478 6.102 3.4229 1.50 500.0 0.8512 5.745 3.4376 1.40 510.0 0.8652 5.085 3.4407 1.34 512.0 0.8665 4.677 3.4701 1.28 518.0 0.8748 4.302 3.4762 1.20 526.0 0.8857 3.825 3.4825 1.09 533.5 0.8948 3.219 3.5170 0,95 548.9 0.9156 2.472 3.5262 0.84 556.0 0.9236 1.965 3.5785 0.77 565.7 0.9374 1.684 3.5519 where Ao is the zero concentration mobility; a' and b' at 25 QC for Z = 3 are 3.572 and 313.5, respectively; Z is charge carried by cation and anion; C is the concentration in gram equivalent per liter. Figure 1 shows the plot of A against Cl /2• The deviation between theoretical Onsager line (line 1) and the experimental one can be entirely attributed to the outer-sphere Cr(DMSO)~+ Fig. 1. Onsager plots of Cr(DMSO)6 Fe(NCS)s in aqueous solutions at 25 cC; (line 1) is a theoretical Onsager line STABILITY CONSTANT OF OUTER.SPHERE COMPLEX 347 and Fe(NCS)~- interaction. Its outer· sphere stability constant is expressed by the equation: K= [Cr (DMSO)6 Fe (NCS)6] [Cr (DMSO)~+] [Fe (NCS)~-] The fraction of free ions at equivalent concentration is obtained from: A 0;=-----Ao - s( 0;C)1/2 (2) (3) (4) Substituting the Onsager slope calculated from Eq. (1) into Eq. (4), 0; can be calculated by successive approximation. The results are given in the third column of the Table. The fourth column shows values of I = 3 Ca, whereas the last one gives Ig K', which is a value not yet corrected for non-ideal behaviour of the ions. (I is the calculated ionic concentration of the solution). The outer.sphere stability constant referred to pure water as standard state can be determined by a graphic method according to: 19 K' = 19 K - 9.162 I1/2 (5) If the ions of the solutions of different Cr(DMSO)6Fe(NCS)6 concentrations obey the Debye-Huckellimiting law the plot 19 K' against I1/2 will show a straight line 'with a slope 9.162. This plot is shown in Fig. 2, where the theoretical relationship is observed >vithin an experimental error up to I = 0.0006. The outer-sphere stability constant referred to pure water as standard state was found to be 19 K = 3.671 : 0.005. Almost the same value (ig K = 3.66) was obtained by- extrapolating the plot of 19 K' + 8 A Jl/2 ~ 1 + BaIl/2 3.7 IgK' 3.6 35 3.4 3j~-----~----__ ~ ________ ~ o 0.01 0.02 /1f2 0.03 Fig. 2. Extrapolation of Ig K' data according to Eq. (5) 348 L. ILCHEV A and ZS. SZABO.AKOS Fig. 3. Extrapolation of 19 K' data according to equation: 8 A F/" 0 I 19 K' + 1 + B a PI" = Ig K T 2 C I KO is the outer.sphere stability constant referring to pure water as standard state against I at 7.5 A. A and B are constants of the Debye-Hiickel theory whereas a is the mean effective diameter of the ions of the solutions. K' is the calculated stability constant not corrected for non-ideal beha"Viour. The theoretical stability constant of the outer-sphere complex Cr(DlVISO)GFe(NCS)6 in water can be calculated according to Bjerrum-Fuoss theory (5) 'vith the following considerations: Cr(DlVISO)~+ is not spherical and its effective ionic radius must be less than 7 A (6). The ionic radius of Fe(NCS)~- may be accepted as about 2.5 A (6). If 7.5 A is chosen as a contact distance of the outer-sphere complex Cr(DlVISO)5Fe(NCS)6 its calculated theoretical stability constant was 19 K = 3.7. In final conclusion, the main reason for the outer-sphere interaction bctween Cr(DlVISO)~+ and Fe(NCS)~- on the analogy of Co(en)~+ and Fe(CN)~- (7) as well as Co(NH3)~+ and CI- (8) appears to be the electrostatic attraction between the ions. Summary The stability constant of the outer-sphere complex Cr(DMSO)6Fe(NCS)6 has been determined in aq~eous solution at 25 QC by conductometry. According to the results 19 K = 3.671 ± 0.005. References 1. COTTOl'i" I. A.-FRANCIS, R: J. Amer. Chem. Soc. 82, 2986 (1960) 2. SZABO-_~KOS, Zs.-ORszlGH. L-BAZSA. Gy.: Magyar Kemiai Foly6irat 81, 53 (1975) 3. SZABO-_4KOS, Zs.-SnION, J.-BECK, M. T.: J. Thermal Anal. 8, 155 (1975) 4. SU.Bo-AKOS, ZS.-SIlIION, J.-BECK, M. T.: Hung. Sci. Instr. 1975/33, 25 5. BJERRmr, J.: Proc. 3. Symp. Coord. Chem., Akad. Kiad6 Bp. 1975 6. CONWAY. B. E.: Elektrochemische Tabellen. GOVI VerI. GmbH .. Pharm. VerI. Frankfurt-a. M:, 1957 . . 7. ILCHEVA, L.-BECK, M. T.: Magyar Kemiai Foly6irat 83, 524 (1977) 8. ILCHEVA, L.-BECK, 1\:1:. T.: Acta Chim. Acad. Sci. Hung. 97, 45 (1978) Dr. Liliana lLCHEYA Dr. Zsuzsanna SZABO-AKOS } H-lS21 Hungary 

STABILITY CONSTANT OF Cr(DMSO)!+ AND Fe(NCS)!-OUTER-SPHERE COMPLEX

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STABILITY CONSTANT OF Cr(DMSO)!+ AND Fe(NCS)!-OUTER-SPHERE COMPLEX

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Periodica Polytechnica (Budapest University of Technology and Economics)