NOTES Activity Coefficients of
[CO(NH~)~(NO~)~][CO(NH~)~(NO~)~] i n Divalent Metal Perchlorate and Other Salt Media by Zofia LibuS Department of Physical Chemistry, Technical University of Gdahsk, Gdahsk, Poland (Received July 9,1969)
It has recently been shown that manganese(II), cobalt(II), nickel(II), and zinc(I1) perchlorates display practically the same concentration dependences of the osmotic and activity coefficients up to considerable concentrations.l This behavior was ascribed to the existence of the corresponding metal cations exclusively in the form of octahedral hexaaquo complexes, possibly with second layers of hydrogen-bonded water molecules, whose interactions with the medium are essentially independent of the nature of the central metal atom. Slightly lower activity and osmotic coefficients of the Cu(C10~)2 and Mg(ClO& solutions were interpreted as arising from an outer-sphere association of the perchlorate anion with the hydrated cations in question. However, more intimate interactions consisting in the penetration of the anions into the first coordination spheres of the cations could not be excluded definitely in this case. It seemed interesting, therefore, to examine to what extent different divalent cations of the same group of metals may differ in their interaction with an anion which, on account of its bulkiness, may be expected to be unable to penetrate into the first coordination sphere. For this purpose the activity coefficients of the complex salt [Co(NH3)4(NO&] [Co(T\"3)2(NO2)4] in aqueous solutions of several divalent metal perchlorates of varying concentration were determined using the solubility met hod.
947 talline material was dried under vacuum at room temperature. It was analyzed for cobalt by repeated evaporation in sulfuric acid, followed by gently heating a n d weighing anhydrous COS04 thus obtained. Anal. Calcd for Co: 23.8%. Found: 23.7%. Procedures. The investigated solutions were saturated with the [Co(SH3)4(N02)2][Co(NH&(N02)4] complex salt at 18" following the procedure described by B r o n ~ t e d . ~The room temperature was always higher than 18", thus preventing- precipitation of the complex in further manipulations. The concentration of [CO(NH~)~(~\TO,)~] [ C O ( K H ~ ) ~ ( S Oin~ ) the ~ ] resulting solution was determined spectrophotometrically using the Unicam SP 500 spectrophotometer. The optical density of the solution under investigation was measured at 425 and 430 mp, while the reference cell always contained the corresponding solution without the complex salt. The necessary value of the molar extinction coefficient of [Co"(3) 4 (NO&] [ Co (;i\"3)2(NO,),] was found as the sum of the molar extinction coefficients of the [ C O ( N I - I ~ ) ~ ( N O ~ and ) ~ ]NH4[CoC~ (r\TH3)2(N02)4]complexes measured in their dilute aqueous solutions. The resulting value, the same within experimental error for both 425 and 430 mp, is 752. It was found to be the same in pure water and in different salt media of varying concentrations which were used in this work. In order to avoid errors arising from the aquation of the complex salt, all operations were performed in as short a time as possible, usually within 10 to 15 min from the beginning of the saturation operation. No detectable changes in the optical densities of the solutions took place in these time intervals.
Results and Discussion Table I lists the solubilities in mol/kg of solvent units of the [ C O ( N H ~ ) ~ ( N [CO(T\TH~)~(NO~)~] O~)~] complex salt in water solutions of >Ig(C104)2, Co(c104)2, Ni(C104)2, and Cu(C104)2 a t 18". As is seen, the soluExperimental Section bility in pure water found in different series of experiments was slightly different, probably as a result of Materials. Solutions of Mg(C104)2, Co(C104)2, gradual dissolution of the smallest crystals in the Xi(ClO&, C U ( C ~ O ~NaC104, )~, and NaCl were prepared solubility column. and analyzed as described in ref 1. The complex salt The mean ionic activity coefficient, y , of the complex [Co(NH3)4(N02)2] [Co(NH3),(N02)4]was prepared from 2 (SO2)4] was electrolyte [Co(NH3)4(K02)2][Co ("3) the [Co(NH3)4(NO2)2]Cl and N H ~ [ C O ( N H ~ ) Z ( N O Z ) ~ ] calculated from the equation complex compounds. The latter two had been obtained by the method of J@rgensen2 and purified s y = SOY0 by repeated recrystallizations from water. Equivalent amounts of dilute aqueous solutions of the above comwhere s and so are the solubilities in the presence of the pounds were mixed at room temperature. The precipicosolute and in pure water, respectively. The necestate was filtered off and washed with distilled water. sary value of yo, the activity coefficient of [Co(NH&In order to obtain as uniform crystals as possible, the (NOz)z][Co(NH3)2(NO&] in its saturated solution in material was placed in a big dish filled with water, where it remained for 2 weeks. Every 2 days, ( 1 ) Z. Libus' and T. Sadowska, J . Phys. Chem., 73,3229 (1969). the solution was removed from over the crystals and a (2) S. M.JZrgensen, Z . Anorg. Chem., 17,469, 477 (1898). new portion of distilled water was added. Conductivity (3) J. N. Bronsted and V. K. La Mer, J . Amer. Chem. SOC.,46,565 water was used in the two last operations. The crys(1924). Volume 74, Number 4
February 19, 2970
NOTES
948 Table I : Solubilities, 9, of [Co(NH3)4(NOz)~] [Co(NH.&(N02)4J in Aqueous Solutions of Mg(C10&, Co( C104)~, Cu(C104)2, and Ni(C104)z of Varying Molalities, rn, at 18’
0.0 0.0222 0.0558 0.0923 0.1901 0.3814 0.5815 0 * 7847 0.9996 1.3166 1 7014
3.130 3.841 4.226 4.527 5.249 6.371 7.621 8.986 10.53 13.09 17 54
0.0 0.0222 0.0532 0.0907 0.1845 0.3749 0.5691 0.7673 0.9763 1 ,2823 I . 6853
I
3.130 3.834 4.204 4.540 5.245 6.489 7.857 9.325 11.17 14.13 19.57
3.083 3.886 4.256 4.778 5.516 7.223 9.337 11.64 14.24 18.20 26.86
0.0 0.0275 0.0623 0.1185 0 2289 0.4768 0,7343 0.9897 1.2518 1.5411 1 ,9884 I
0.0 0.0251 0.0532 0.1068 0.2069 0.4106 0.6482 0.8734 1.0984 1.3503 1 7066 I
3.077 3.801 4.166 4.571 5.299 6.673 8.428 10.26 12.50 15.01 20.08
0.8
0.6
r-‘ 0.4 0.2 1
O
0.2
0.4
0.6
0.8
m.
1.0
1.2
1.4
1.6
1.8
0.50 CO(ClOq)2
0
Figure 1. The dependence of the activity coefficient, y, of [Co(NH3)a(N0&] [Co(NH3)z (NOZ)~] on the molal concentrations of divalent metal perchlorates in aqueous solution, at 18’.
pure water, was calculated from the limiting DebyeHuckel equation. Figure 1 shows plots of the activity coefficient of the complex electrolyte under investigation against the molal concentration of the cosolute, the latter being Mg(C104)2, Co(ClO&, h’i(C104)2, or Cu(C104)2. As is seen, the activity coefficient, y , in each case decreases monotonously with increasing concentration of the metal perchlorate in the whole concentration range investigated. While this decrease may be considered as a quite general effect of the coulombic interactions between the ions when lower concentrations are concerned, it seems to be accountable only in terms of ionic association for the region of higher concentrations. For our purposes, most essential seems to be the fact that the values of the activity coefficient of the [Co(KH& (K02)2][Co(n”&(K02)4] complex electrolyte are very nearly the same in equally concentrated solutions of the four metal perchlorates a t concentrations lower than approximately 0.2 m, and they remain approximately equal in the whole concentration range investigated, ie., up to 1.7 m. There seems to be little doubt t)hat, like the coincidence of the activity T h e Journal of Physical Chemistry
v
I + NL(CLO~)Z n
I
0.44
-
m ’ Nl (ClOJ
