NOTES
Dee., 1959 the basis of additional theoretical and experimental information. Acknowledgment.-The authors wish to thank Professor B. de B. Darwent for helpful discussions and comments.
0.12
2083
1
THE HYDRODYNAMIC VOLUME OF T H E HETEROPOLY HEXAMOLYBDOCOBALTATE(II1) ANION FROM VISCOSITY MEASUREMENTS BY MICHAELT . POPE’AND LOUISC. W. BAKER^ Contribution from the Department o f Chemistry, Boston University, Boston 16, Massachusetts Received July 6 , 1969
I n a recent paper, Kurucsev, Sargeson and West3 demonstrated that the Einstein viscosity equation is valid for various large spherical inorganic ions. They were able to show that the 12-tungstosilicate ion has a hydrodynamic radius of 5.6 A., in good agreement with that calculated from the molar volume4 and consistent with the crystallographic unit cell dimensions of H4 [SiWl204~] 5H20.6p6 Application of this method to the investigation of other heteropoly anions is limited by the solubilities of the salts available. In general, for most heteropoly anions, those salts which are readily purifiable are only soluble to an extent of up to about 20 g. per 100 ml. Therefore even saturated solutions usually have low molarity (less than 0.1 M ) because of the high molecular weights of the solutes. The viscosities of such solutions are not sufficiently greater than that of water to make possible a series of reasonably accurate measurements. The free acids of these anions are generally much more soluble, but in many cases are unstable, especially in concentrated (>0.2 M ) solutions. In the course of an investigation of the solution chemistry of the isomorphous 6-molybdo anions of trivalent Co, Cr, Fe and Al, it was found that the free 6-molybdocobaltic acid can be concentrated without decomposition. We have discussed the chemistry of these anions in a previous paper’ and recently have shown that they are of high thermodynamic stability and are monomeric in solution.8
-
The 6-molybdocobaltic acid was prepared by passing an aqueous solution of the purified ammonium salt through an Amberlite IR-120 (Rohm and Haas Co.) ion-exchange column in the hydrogen cycle.71~The effluent acid was concentrated at just below room temperature by placing it in a cellophane sac and blowing a current of warm air over the outside of the sac. The molarity of the resulting solution was determined by analysis of aliquots. A portion of the (1) Monsanto Research Fellow at Boston University. 342) Addressee for reprints. 13) T. Kuiucsev, A. M. Sargeson and B. 0. Wcst, TIILB JOURNAL,
61, 1567 (1957). (4) M. C. Baker, P. A. Lyons and S. J. Singer, J . A m . Chem. Sac., 77, 2011 (1955). (5) R. Signer and H. Gross, Helu. Chim. Acta, 11, 1076 (1934). (6) 0. Kraus, 2. Krist., 100, 394 (1939). (7) L. C. W. Baker, G. Foster, W. Tan, F. Scholniok and T. P. McCutcheon, J . A m . Chem. Sac., 77, 2136 (1955). ( 8 ) G. A. Tsigdinos, M. T. Pope and L. C. W. Baker, Abstracts of papers presented before the Division of Inorganic Chemistry. American Chemical Society National Meeting, Boston, Mass., April, 1959. (9) L. C. W. Baker, B. Loev and T. P. McCutcheon, J . A m . Chem. S o c . , 7 2 , 2374 (1950).
0.04 0.08 0.12 0.16 0.20 0.24 Molarity (c). Fig. 1.-Viscosity values for aqueous solutions of HalCoMoeOzl] a t 25”. The line shown was drawn from the origin by the method of least squares. effluent acid, concentrated a t 40’ by aspirating dry air through the solution for 48 hours, gave the eame viscosity results as the acid concentrated by the cellophane sac method. Viscosity measurements were made in an Ostwald viscometer at 25’.
The results of the viscosity determinations are recorded in Fig. 1. They can be expressed by the Jones-Dole lo equation 7 -
+
lo = A ~ EB~ TO
where A = 0 and B = 0.46, over the concentration range 0.035 to 0.23 M . The acid has the formula HB[COMOBO~~], and its three dissociation constants all lie close together, having pK’s within the range 2-3.1tR Assuming that the pK’s merely lie close to this range and assuming Gurney’s” value of +0.07 for the Bcoefficient of the solvated proton, the B-coefficient of the heteropoly particles was calculated to lie between +0.40 and +0.46. Whence, according to the Einstein equation, the apparent hydrodynamic volume of the [cohh6021]-3 ion falls between 265 and 300 A.3, within the concentration region investigated. This corresponds to ail hypothetical sphere with radius between 4.0 and 4.2 A. Since the structure of this series of isomorphous 6-molybdo anions has not yet been determined, there are no independent data for direct comparison with the above results. However, there is some evideiicel2 that these anions may have very similar structure t o that of a 6-heteropoly tungstonickelate of similar formula and known crystal structure.12 That tungsto complex is essentially isostructural with the 6-molybdotellurate(VI) anion, the structure of which was suggested by Andemonla and (10) G. Jones and M. Dole, ibid., 5 1 , 2950 (1929). (11) R. W. Gurney, “Ionic Processes in Solution,” McGraw-Hill Book Co., Inc., New York, N. Y.,1953, p, 159. (12) U. C. Agarwala, Doctoral Dissertation, Boston University.
1Y59.
(13) J. 6. Anderson, Nature, 140, 850 (1937).
2084
NOTES
proved by Evans.14-16 I n the crystals, these similar ions are both thick hexagonal discs, approximating oblate spheroids. From the crystallographic data, the effective minor radius (half-thickness) of each may be estimated as 2.7-3.0 A. and the effective major radius averages about 6.0 A. The volume of each of these two disc anions is about 600 A.s. In arriving at these estimates, the possible closeness of approach of water molecules to the irregular surface of the polyanion was considered, While the nickelate and tellurate complexes each contain twenty-four oxygen atoms, the 6-molybdocobaltate(II1) anion and its Cr, Fe and A1 isomorphs might contain as few as twenty-one oxygen atoms apiece (their proportion of “constitutional water’’ being uncertain at present) .7p8,12 The discrepancy between the effective hydrodynamic volume determined above and the probable range of the total crystallographic volume of the anion is easily understood in view of the size and probable disc shape of the anion. The size is below that for which good applicability of the Einstein equation should be assumed for polyions. Factors involving the fit of solvent molecules to the irregular surface of the solute particle are proportionately more important the smaller a polyanion becomes. These 6-molybdo ions are presumably far from spherical; and, if they have the probable disc-like structure, it is likely that their planes would become oriented parallel to the direction of liquid flow. In view of the dimensions cited, it is most reasonable that such anions would have an apparent hydrodynamic volume close to that observed. The results also indicate that thevdegree of solvation of the anion is negligible. This is consistent with the low charge density on the surface of heteropoly anions and with the negligible solvation observed for 12-tungstosilicate ion.a An attempt to measure the viscosity of solutions of the free acid of 6-molybdoiodate(VII), which is presumably isomorphous with the 6-molybdotellurate(VI),IBwas unsuccessful owing, apparently, to decomposition of the acid in concentrated solution. For the same reason, analogous measurements could not be made upon the heteropoly 6-molybdo anions of Cr(III), Fe(II1) or Al(II1). Acknowledgments.-This research was supported in part by a grant from Monsanto Chemical Company and in part by the U. S. Atomic Energy Commission, through Contract AT(30-1)-1853. (14) H. T. Evans, Jr., J . A m . Chem. Soc., 70, 1291 (1948). (15) H. T. Evans. Jr., Abstracts of papers presented before the American Crystallographic Association, Cambridge, Mass., 1954. (16) H. J. Emelha and J. S. Anderson, “Modern Aspects of Inorganic Chemistry,” Second Edition, D. Van Nostrand Co., Inc., New York, N. Y., 1952, pp. 225-226.
T H E PYROLYSIS OF ACETYLACETONE BY ROBERTG. CHARLES, W. M. HICKAM AND JOANVON HOENE We8tinghouse Research Laboratories, Pittsburgh 86, Pennsylvania Received J u l y 16, 1069
I n previous work concerning the pyrolysis of the metal acetylacetonates (I), acetylacetone itself was observed in several instances as one of the major
Vol. 63
degradation products. These results suggested that acetylacetone may be more heat stable than
the metal chelates derived from it. The present work on the pyrolysis of acetylacetone permits a more direct comparison of the heat stabilities and also permits a comparison of the degradation products from acetylacetone with those of the metal acetylacetonates. There appears to have been no previous comparison of the pyrolysis of a chelating agent with the pyrolyses of the metal chelates derived from it. Table I gives the gaseous products obtained by heating acetylacetone in sealed evacuated glass tubes for 4 hours a t the temperatures indicated. At the lowest temperature employed, 266”, little decompositionhas occurred. This is in contrast to the Cu(II), Ni(II), Co(II), Co(III), Al(III), Cr(III), Fe(II1) and Mn(II1) acetylacetonates, all of which decompose to a significant extent a t this temperature.’ The introduction of a metal atom into the acetylacetone molecule results, therefore, in a decrease of thermal stability, at least for these metals.
S
a
TABLEI THERMAL DEGRADATION OF ACETYLACETONE Samples heated 4.0 hours at the temperature indicated Components of gas
Acetylacetone Acetic acid Acetone
coz co
CHI
Moles gaa per mole of aoetylacetone taken 266’ 346’ 462’
0.99 -01 .00
.00
.00 .00
0.78 .08 .12
.Ol
.00
.00
0.00 .08 .88 .34 .39 -28
The principal gaseous products of pyrolysis for acetylacetone given in Table I are also major decomposition products for the metal acetylacetonates.’ This fact suggests that at least one posslble route for the decomposition of the metal acetylacetonates involves first the formation of acetylacetone followed by decomposition of the latter to the products given in Table I. That acetylacetone is actually found as nearly the sole decomposition product, a t the lower temperatures, for the Mn(II1) and Cu(I1) chelates supports this view.’ The fact that only small amounts of acetylacetone were found among the decomposition products of the other metal acetylacetonates studied may indicate that, in these instances, the decomposition of the initially formed acetylacetone is catalyzed by the
, ,
c
I
l
(1) J. von Hoene, R. 0.Charles and W. M. Hickam, THISJOURNAL, 62, 1098 (1958).
(2) On the basis of volatile decomposition products, sodium aoetylacetonate appears to be more stable than acetylacetone. The decomposition of the former compound may, however, involve non-volatile pyrolysis products which oannot be detected by the ma88 spectrometer (ref. 1).
I
