ASSOCIATION OF SULFATE Iox
Jan. 5, 1956
0.0531 M solution of diethylaniline are presented in Table V and the average value of the quadrupole formation constant between diethylaniline and water, KEeW = [QD.w]/(CDCH,O),was found to be 2.65 f 0.10, (D = diethylaniline.)
WITH
TRIPOSITIVE COBALTAMMINE IONS
15
identical a t these two wave lengths. These results show that more than 80% of the pyridine is present in the ion-pair form. Adopting the mean value of KFy of 5.37 and Km = 7.9 X Kp = 9.4 x 10-7.
TABLE V
EFFECT OF WATERUPONDIETHYLANILINE (D) (C D ) ~ = 0.0531 M 2.00 cm. quartz cells Water, hl
0.274 0.551 1.09 2.08
Asan
1.39 0.960 0.580 0.380
KD.W.
[QD.w.]/CD
Q
0.705 1.47 3.09 5.24 Av. =
2.58 2.67 2.85 2.62 2.65 f 0.10
Pyridine solutions in acetic acid are also colorless and the spectrum of pyridine in acetic acid (curve I) and in excess perchloric acid (curve 2) are given in Fig. 6. The molar absorptivities in pure acetic acid were found to be 4.62 X lo3 a t 255.5 m p and 3.12 X lo3 at 261.0 mp, while in 0.1 fM perchloric acid these values a t the same wave lengths are 5.48 X lo3and 3.74 X lo3. The ratio of the molar absorptivities of pyridine in acetic acid a t 255.5 mp and 261.0 mp is 1.49, while this ratio is 1.47 in 0.1 M perchloric acid This suggests that the two spectra are identical and are caused by similar species. It is reasonable to assume that the base (Py) does not absorb a t either of these wave lengths and the observed absorption is caused by PyH+Acin acetic acid and by PyH+C104- in the presence of excess perchloric acid. Experiments using 1.01 and 2.03 X M pyridine in the presence and absence of excess perchloric acid yielded an average KPY of 5.52 f 0.04 at 255.5 mp and 5.21 0.09 a t 261.0 mp, assuming that the molar absorptivities of PyH fAc- and PyH +Clod- are
*
[CONTRIBUTION FROM
THE
Fig. 6.-Spectrum of pyridine: (CP,)~ = 2.03 X IO-' molar: I, in acetic acid; 11, in 0.05 M HClO4; 20 mm. cell used.
It was noted that when water was added to pyridine the effect on the spectrum was similar to that produced by perchloric acid, but no quantitative measurements were made to determine the species formed. I t may be stated that the "abnormal water effect'' is a general phenomenon and i t is not restricted to indicator bases. MINNEAPOLIS, MINN.
GEORGEHERBERT JONES LABORATORIES O F THE UNIVERSITY
OF
CHICAGO]
The Outer Sphere Association of Sulfate Ion with Tripositive Cobaltammine Ions BY FRANZ A. POSEY AND HENRYTAUBE RECEIVED SEPTEMBER 12, 1955 The equilibrium quotients, K, for the outer sphere association of cO(lW3)6++' and Co( KHa)jH20f++with SO,- have been measured a t several temperatures and ionic strengths by following the changes in optical density in the ultraviolet region of the absorption spectra of the cations as a function of sulfate ion concentration. Values of AFO, AH0 and A S 0 a t 25" are -4.53 kcal. mole-', 0.40 kcal. mole-', and 16.6 e.u. for the association of Co(NH3),+++ with SO4-. The equilibrium quotients calculated are independent of concentrations, of the wave length of light used, and vary with ionic strength in conformity with a Debye-Huckel equation. The quotient for the equilibrium ratio, (CO(NH~)~H~O+++.SO~-)/(CO(KH~)~SO~+), in 1 M NaCIOl a t 25' has been measured as 0.90 by following changes in the substitution equilibrium using light of wave length 560 mb. These experiments have also yielded a value for K in agreement with that measured using the instantaneous changes in the ultraviolet extinctions as SO1' is added to solutions of C O ( N H , ) ~ H ~ O + + + .
Linhard' has demonstrated that the presence in solution of certain anions can cause marked changes in the absorption spectra of tripositive Co(II1) and Cr(II1) complex ions in the wave length region of the strong ultraviolet band. I n an earlier paper2 we reported that this effect occurs when SO4= is added to a solution containing Co(NH3),H20+++. The changes in question occur immediately on mix(1) M.Linhard, Z. Elekfrochem., 50, 224 (1944). (2) H.Taube and F. A. Posey, THIS J O U R N A L , 75, 1463 (1953).
ing the reagents, in contrast to changes in the two bands in the wave length region of visible light, which for the ions in question take place quite slowly at room temperature, concomitant with substitution in the inner sphere of coordination of the central metallic ion. Therefore, if the changes of spectrum in the ultraviolet region are caused by the association of the cation with the anions, this interaction must be of such nature that the anion does not become equivalent to the six ligands in the
16
FRANZ A.
POSEY AND
HENRYTAUBE
VOl. 78
complex cation. It was the purpose of the work With the aquo ion, the additional equilibrium reported here to learn whether the rapid changes in CO(NHJsSO4' H20 = CO(NHs)5HzO+++.SOa- (2) spectrum can be interpreted by an equilibrium beis necessary to interpret some of the data. The tween the free ions and the "outer sphere" comequilibrium quotient for this reaction is represented plexes3 implied by the structural restriction menby K'. Both K and K' are concentration quotioned. We have chosen to work with sod=as the anion, tients. Data on the optical density in the ultraviolet reboth because of its charge and because i t is essentially transparent in the spectral region of interest, gion as a function of Sod- concentration a t constant and the cations, Co(NH3)s+++ and Co(NH3)~- ionic strength were treated according to the method If, in the equiH20+++. The existence of outer sphere complexes outlined by Newton and A r ~ a n d . ~ was strongly suggested if not proven by the previ- librium represented by equation 1, the free cation ous work,2 in which the slow spectrum changes in and the 1: 1 outer-sphere complex have extinction the visible region were studied. These data have coefficients independent of (SO4-), the relation now been amplified t o yield a value for the outer (3) sphere association quotient to compare with that obtained measuring the instantaneous changes in is obeyed for a series of experiments in which the extinction. Data with C O ( N H ~ ) ~ +are + +included activity coefficients of the ions involved in the to furnish some generalization of the results on equilibrium remained constant. Dois the optical outer sphere association. density of a solution containing no SO,=,D is that of a solution with Sod= present, D1 that for a soluExperimental tion in which M + + + is completely converted to A Beckman Model DU Quartz Spectrophotometer was used for spectral measurements. It was fitted with a cell the 1: 1 complex, and (SO4=)is the concentration of compartment, the temperature of which was controlled to free sod=a t equilibrium. Do is evaluated by direct within +0.lo by circulating fluid. Standard chemicals were measurement, D1 and K by plotting D against ( D of A. R. quality. The compounds Co( N H 3 ) 5 H ~ 0 ( C 1 0 a ) ~ D o ) / ( s o ~ - ) . I n most experiments, (sod=) was and Co(NHs)e(ClO& were used as sources for the complex cations and were purified by repeated crystallization. Solu- much in excess of (M+++)and the approximatevaltions of NaC104 and Mg(C104)Z were prepared by the reac- ues of D1 and K obtained by using stoichiometric tion of perchloric acid with the corresponding carbonate. so4- in the first plot of these data served to fix Measurements of extinction of solutions containing the Co- (SO4=) sufficiently well to make the second plot (111) complexes were made against blanks containing all the reagents except the Co(II1) salts. The blanks showed final. The optical densities of solutions in the study of significant extinction compared to water only when considerable NaC104 or Na2SOd was present, perhaps caused by the substitution equilibrium a t low (sod=)were residual Fe(II1) in the salts. measured, as in the earlier work a t high (SOa), a t Equilibrium with respect t o changes in the ultraviolet the wave length 560 mp. At this wave length, and extinction of Co( ",)(+I+ on adding Sod' was reached immediately on mixing and no further change was noted in fact for the whole region comprised of the two even after 24 hours had elapsed. With CO(N&)&O+++ absorption bands in the visible, the species Coand SO4-, after the immediate change on mixing, there is a (NHJ6H20+++ and C O ( N H ~ ) ~ H ~ O + + + . Shave O~' slow change caused by the replacement of complex-bound H 2 0 by ,504' which is detectable after several minutes a t the same extinction, but the spectra of these ions Then +. room temperature. For both systems, the ultraviolet meas- differ markedly from that of C O ( N H ~ ) ~ S O ~ urements were made immediately after temperature equi- for a series of experiments in which K' and K , as librium was established, ca. 10 min. after mixing. The well as the extinction coefficients of the colored spechanges in the visible spectrum a t 25' required the solutions cies, remain constant t o be stored for 6 to 8 weeks before equilibrium was reached.
+
There was no detectable change during this time in a companion solution containing Co( S H a ) 5 5 H ~ 0 + + + Na+, , H+, C104- but no SO,-. Equilibrium measurements a t 25' were confirmed by approaching the equilibrium distribution from the opposite side, that is, starting with a system containing initially Co( NHs)jS04+in excess of the equilibrium amount.
Definitions and Treatment of Data The outer sphere complex between the cation, MT++, and so4- is represented as M+++.S04=. D = log(lo/l) (measured against a blank omitting the Co(II1) salt) is referred to as the optical density. The units are 1. mole-' cm.-'. The equilibrium quotient, K , refers to the outer sphere association of C O ( N H ~ ) ~ +or + +Co(NH3)sHzO+++with Sod-. M + + ++ SO4'
M'++.S04'
(1)
(3) T h e t e r m "outer sphere association" is preferred b y t h e authors rather t h a n "ion-pair formation" since t h e latter t e r m is not defined with respect t o t h e constitution of t h e complex ion and offers no distinction between inner sphere and outer sphere forms of t h e associated species. T h e t e r m is also of value in describing complex-ion equilibria in exchange-labile systems for which t h e two forms are difficult t o characterize separately b u t in which there is a need for a term t o define t h e type of association in question.
D,' and DO represent the optical densities of solutions having all the Co(II1) in the sulfato and aquo forms, respectively, and D that of the solution a t equilibrium with respect to inner sphere substitution as well as outer sphere association. K' and K are evaluated by plotting (Dl' - D ) / ( D - DO) against l/(S04=),iterating if necessary to fix K ' , K and l/(S04-) to the accuracy the data justify. I n contrast to the situation which obtains when the labile equilibrium (1) is studied, the constancy of D1' and DO with electrolyte composition can be checked directly, nor is constancy of these values essential to the analysis of the system, because the quantities appropriate to any particular solvent composition can be measured directly. (4) T. W. Xewton and G. M. Arcand, THISJ O U K N A I . , 7 6 , 244:) (19.53).
Jan. 5, 1956
ASSOCIATION OF SULFATE IONWITH TRIPOSITIVE COBALTAMMINE IONS
Both types of experiments make use of data a t varying (Sod-), but with K' and K constant. The series used to evaluate the parameters D1,K' and K were performed a t constant ionic strength, replacing NaC104 by Na2S04. This procedure undoubtedly suffices for K', which describes an equilibrium between forms of the same net charge. The internal consistency of the data treated according to equations 3 and 4 is evidence that K also is sufficiently constant for the changes in composition a t constant ionic strength.
Results
17
of the type illustrated in Fig. 1. Thus the value of
K X a t 24.5' calculated for p = 0.0094 a t X 235, 240, 245, 250 and 255 mp axe 6.65 i 0.1, 6.8 f 0.1, 6.7 h 0.2, 6.6 f 0.2 and 6.8 f 0.5. TABLE I THEDISTRIBUTION OF Co( 111)BETWEEN THE OUTERSPHERE AT SULFATECOMPLEXAND Co( NH3)6i+ + AS CALCULATED VARIOUS WAVELENGTHS (Temperature = 24.5", 1.00 X lo-' MHClOd, 1.00 X M M(C1Odh).
(NarSOd X 104
Values of the ratio, (M + + + . SOc-)/(M *++), computed at various wave lengths 235 240 245 250 255 K X
