March, 1961
COBALTOUS ACETATE IN ACETICACID
to an aliphatic anionic wetting agent, the minimum surface tension obtainable with the synergistic combinations studied being 15.2 dynes/cm. (e) The lowest values represent the closest possible packing of the fluorinated alcohol when adsorbed a t the water-air interface under their equilibrium spreading pressures. (d) The transitory low values of the +'-alcohols observed in open vessels, especially when low concentrations are used, are caused
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by the evaporation of the fluorinated monolayer adsorbed a t the water-air interface. I n closed systems, the synergistic surface tensions are not transitory. (e) In cases where the anionic solubilizing agent is not sufficiently soluble in water, increasing the solvent power of the continuous phase by adding a mutual solvent makes it possible to solubilize the fluoroalcohols and so to obtain synergistic surface tension depressions.
SPECIES OF COBALT(I1) IN ACETIC ACID. PART I. COBALTOUS ACETATE IN THE PRESENCE OF WATER AND OF SODIUM ACETATE BY P. J. PROLL, L. H. SUTCLIFFE AND J. WALKLEY Department of Inorganic and Physical Chemistry, University of Liverpool, Liverpool, England Received August 29, 1960
The reversible variations of the visible spectrum of cobaltous acetate in anhydrous acetic acid with temperature and the effect of sodium acetate and of water have been studied over the temperature range 25 to 64'. From ion migration and spectrophotometric measurements the ionic species present under these conditions are postulated to be CoOAc +,Co(OAc)z, CO(OAC)~-, (~?(OAC)~Z-, the latter pair being favored by the addition of sodium acetate and increase of temperature while the former pair are favored by the addition of mater. Evidence is put forward for solvated Co(0Ac)z or Co(OAc)d2- being the reactive species in reactions in which cobaltous acetate is used as a catalyst. acetic anhydride to remove the water, this amount being Introduction estimated from freezing point measurements.6 After It has previoudy been reported that, several oxi- distillation, any excess water or acetic anhydride was dation-reduction reactions between metal acetates determined by a spectrophotometric method6 and removed. in anhydrous acetic acid occur a t measurable The acetic acid was then refluxed with commercial cobaltous acetate and fractionated with a column packed with Fenslre rates.' TWOof these, namely, the cerous acetate- helices. plumbic acetate2 and the cobaltous acetateCobaltous Acetate.-Solutions of known concentrations plumbic acetate:' reactions have been reported in were made by refluxing 99.95% pure cobalt sponge (Johndetail. During the investigation of the latter an son Matthey Ltd.) with acetic acid purified as described interesting postulation was necessary to explain above. Sodium Acetate.-A.R. quality sodium acetate was rethe kinetic scheme, namely that a dimeric form crystallized twice from anhydrous acetic acid and then of cobaltous acetate is present. In addition, the dried under a vacuum for 24 hours at 100'. Ion Migration Experiments.-The migration cell consisted observations were made that (a) the kinetics of the a W-shaped vessel having three compartments isolated reaction were affected by sodium acetate and by of from one another by sintered glass discs. The solution water, and (b) during the addition of sodium ace- under investigation was placed in the central compartment tate or on heating the anhydrous solutions the pink- and the solvent in the outer compartments. Five hundred red color turned to blue-the heating effect being v.d.c. was applied to platinum foil electrodes situated in the two outer limbs of the cell. Current was passed for reversible. 20 hours and then the polarity reversed, and current passed Cobaltous acetate has been used by many work- for a further 20 hours. The cobelt(I1) concentrations in ers in the field of autooxidation reactions as an im- the anode and cathode compartments were determined portant catalyst. However, little attention has spectrophotometrically. Spectrophotometry.-All measurements were made by been directed to the detailed role of this ~ a t a l y s t . ~means of a Unicam S.P. 500 spectrophotometer fitted with It was therefore decided to make a complete a thermostated cell compartment enabling temperatures of spectrophotometric investigation of cobaltous ace- all the solutions to be maintained to within +0.05". tate in acetic acid under as wide a variety of condiResults and Discussion tions as possible. This first paper of the series Ion Migration.-Ion migration experiments on deals with the solutions of the salt alone and in the presence of water and of sodium acetate. Future cobaltous acetate solutions3 led to the conclusion papers will deal .with the effect of lithium bromide that there are four likely species, namely, CoOAc+, CO(OAC)~-, CO(OAC)~~-. The first being and chloride to this system since cobaltous bromide CO(OAC)~, favored by the addition of water and the last two is an important autooxidation catalyst. by the addition of acetates or the application of Experimental heat. Acetic Acid.-Analytical reagent (A.R.) quality acetic Spectrophotometry.-The spectrum of cobaltous acid was purified by refluxing with finely divided A.R. chromium trioxide along with a calculated amount of A.R. acetate in anhydrous acetic acid is shown in Fig. 1, at five temperatures in the range 25 to 64". As can be seen it has a d c d transition band (526 (1) L. H. Sutoliffe and J . Walkley, Nature, 118, 999 (19%). (2) D. Benson and L. H. Sutcliffe, Trans. Faraday SOL, 66, 246 to 538 mp) in the visible region and a charge trans(1960). fer band (310 mp) in the ultraviolet region. The (3) D. Benson, P. J. Proll, L. H. Sutoliffe and J. Walkley, Disc. Faraday Soc., 19, GO (1980). (4) H.6, Blanahard. J. A m . Chem. SOC.,89, 2014 (1960).
(6) De Viwer, Rec. trow. c h i n . , 19, 101 (1893). (6) 6. Bruckenstein, J . Am. Chem. So&, 1 8 , 1921 (1956).
P. J. PROLL, L. H. SUTCLIFFE AND J. WALKLEY
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400 600 800 Wave length, mp. Fig. 1.-The absorption spectrum of cobaltous acetate in anhydrous acetic acid a t the temperatures: A, 25.0’; B, 38.0’; C, 47.8’; D, 58.7’; E, 64.0’.
Beer-Lambert law is obeyed over the whole spectrum to within ~ 1 over % the concentration range 5 X to 5 X M. It was noticed, however, that different samples of cobaltous acetate did not always have the same maximum to minimum ratio; this ratio being variable from 5:l to 10: 1 depending on the method of purification of the salt. Commercial CO(OAC)~ recrystallized at least three times had a large value of this ratio, as do crystals taken after refluxing cobalt metal sponge with acetic acid. These crystals are distinct from a powdery “sludge” which also appears in both cases. X-Ray photographs obtained by the Debye-Scherrer powder method show marked differences. The “sludge” photographs have the “crystal” photograph lines as well as some new ones. The line spacing does not appear to fit any regular shape and, in agreement with other workers’ results for C O ( O A C ) ~ ~ ~are H ~ consistent O, with a monoclinic structure.’ We have therefore assumed that the “sludge” contains a different form of cobaltous acetate which may be a dimer or higher polymer in support of the kinetic evidence obtained previo~sly.~It is interesting to note that variations in autooxidation rates of cumene depend on the mode of preparation of the cobaltous acetate catalyst used.4 We also concluded that the dimer (higher aggregates may also be present) is formed on heating, and is affected in the same way as the monomer by heating or by the addition of sodium (7) .J. N . van Siekerk an4 F. R. L. Schoening, Acta Cryat., 6, 813 (1983).
400 600 800 Wave length, mp. Fig. 2.-The absorption spectrum of cobaltous acetate in anhydrous acetic acid at 25’ with sodium acetate at the concentrations: A, 0.000 M; B, 0.03131 M ; C, 0.0625 M ; D, 0.125 M; E, 0.185 M ; F, 0.250 M ; G, 0.368 M ; H, 0.500 M . 200
acetate. The kinetic results suggest3 that the dimer does not predominate and is in rapid equilibrium with the monomer. Separate experiments show that although maximum to minimum ratio varies the value of the molar extinction coefficient a t the absorption maximum emax does not: the value Emin changes over the range 350 to 450 mp. The results that follow are for a cobaltous acetate solution having a maximum t o minimum ratio of -5 :1; solutions with a ratio of 1 O : l also showed the same effects, and give the same values of the extinction coefficient over all parts of the spectrum except in the vicinity of the minimum. The Effect of Sodium Acetate.-The effect of added sodium acetate on the spectrum a t 25” is shown in Fig. 2. In Fig. 3 a graph of €666mp us. sodium acetate concentration is shown at five temperatures between 25 and 64”. It can be seen from these two figures that there is a wave length shift from 526 to 565 mpl this latter wave length being the limit to which the absorption maximum moves. Thus the new species of cobaltous formed by the addition of sodium acetate has a maximum intensity of absorption at 565 mp. It may be noted here that heating the cobaltous acetate solution alone had the similar but less pronounced effect as the addition of sodium acetate, Le., the absorp-
COBALTOUS ACETATEIN ACETICACID
March, 1961
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0.2 0.4 0.6 Sodium acetate concn., M . Fig. 3.-The plot of €566 m p against the concentration of sodium acetate a t five temperatures: A, 25.0'; B, 38.0'; C, 47.8'; D, 58.7'; E, 64.0". 0
tion intensity was increased and the maximum was shifted to longer wave lengths. In both cases the following equilibria are likely to be operative I .
K2
+ Co(OAc),Ks Co(0Ac)s- + OAC- Jr CO(OAC)~'CO(OAC)~ OAC-
(2)
(3)
from which it can be shown that the molar extinction coefficient E a t any wave length is given by e = g
+ asK2[0A~-]+ E ~ K ~ K ~ [ O A C - ] *
where e2, E~ and E.$ are the extinction coefficients of CO(OAC)~, Co(0P~c)~-and CO(OAC),~-),respectively, at a given wave length. I n the case of the sodium acetate addition we have e =
E?
+ e3K2K~L/x[NaOAc]1/n + qKIKzK~[NaOAc]
where K A is the dissociation constant of sodium acetate in acetic acids which has a value (2.63 ==! 0.12) X lo-' a t 25'. I n the derivation of the above equation the plausible assumption is required that KPIC)Ac-](l K3)
