tion to 1/02, weighted mean values of 0.0185 f 0.0004 and 0.0115 f 0.0008 were calculated for the Pu(IT') hydrolysis quotients a t 25" in the H20 and D:O solvents, respectively. The uncertainties of the weighted means correspond to 2a = 2(1/ Ztul)'/?, where tu1 is the weighting factor. Thus, a K E / K Dratio of 1.6 is obtained a t 25" from these spectrciphotometric measurements. This result is similar to observations6in the cases of Kp(IV) and U(IV) for which K H / K Dratios &-ere found to be 1.6 and 1.2, respectively. This result is not in agreement with the conclusion reached in the kinetic studies of the Pu(T'1)-Pu(II1) reaction.' In this latter n.ork. the results seemed to indicate a K H , / K Dratio less than unity. This discrepancy is unresolved. Iiraus and Nelson4 give spectrophotometric data ohtained at 4700 A. from which it is possible to calculate 2 value for K in perchlorate solutions of ionit: strength 0.5 a t 25" with the criterion that the sum of the squares of the difference, (tcalcdEexptl), is made a minimum. Whereas these authors found an average value of 0.0249 for K by a trial and error procedure, with the use of their data and the above criterion together with the computer program wed in this study, a value of 0.019 f 0.002 was obtained. The parameters €1 and ez whicih together with the above value of K produced this minimum least squares sum had values of 54.7 f 0.3 and 9.9 f 3 J1-l em.-', respectively. From the temperature dependence of the Pu(IT) hrdrolysis quotient as given in Table I, a value of 8.5 f 0.9" kcal./mole is computed for AH for reaction 1 in H20. This result is in agreement with the value of 7.3 f 0.5 kcal./mole obtained for AH from electromotive force measurements qf the acidity dependence of the Pu(I11)Pu(IT') c ~ u p l e . A ~ somewhat higher AH is found for the hydrolysis reaction in D20, namely, 19.4 & 1.3" kcal./niole. X wider temperature difference over which K mas evaluated would have been desirable. Attempts were made to evaluate K in H20and in D,O a t a temperature of 2.4"; holvever, it was not possible to obtain a sufficient change in thc values of the observed molar extinction coefficients to provide meaningful data. At temperatures greater than 25O, the disproportionation of Pu(1T') makeq the extrapolation to zero time more difficult. In the evaluation of the hydrolysis quotient of Pu(1T') from measurements of the formal potentials of the PLI(III)-Pu(IV) couple, a value of 0.054 was obi nined for K a t 2.3" in perchlorate solutions of ionic strength t w 0 . 3 It is not clear why such discordmt values of the hydrolysis quotient have been obtained hy the spectrophotometric and potentiometric methods. Acknowledgments.-The authors wish to express their appreciation to Drs. J. F. Lemons and C. E. Holley, Jr., for helpful discussions and to R. H. r\Ioore irid R . E. J70gel for their assistance in the computer programming and in the statistical treatment of' the data. (11) Th, uncertainties g l \ e n f o r the ralues of A H were computed by t h e rne.liod of propagation of errors u1th t h e use of t h e e x p r e w o n , 1 I(1/T1 - 1 / T 1 ) 2 [ U Z K I / K I z U21~2/K22])1/2. zkUAh
+{
+
THERMOCHEMISTRY OF DIMETHOXYBORASE BY W.J. COOPER . ~ S DJ. F. MAPI Callerg Chemrcal Company, Callery, Pennsylaania Received ?Jotember 90, 1059
Dimethoxyborane was first prepared by Burg and Schlesinger' from the reaction of diborane with methyl alcohol. They found the compound to be unstable above Oo, disproportionating according to the equation 6HB(OCH3)?
BzHs
+ 4B(OCH3)3
(1)
Nore recently Cchida,? have reported the kinetics of the vapor phase disproportionation in which the reaction was found to be second order with respect to dimethoxyborane and surface dependent. A determination of the heat of formation of dimethoxyhorane would not only enable one to calculate the heat of the disproportionation reaction but also afford a means of estimating the strength of the B-H bond in a somewhat different manner than from the heat of formation of BH3.3 Preliminary investigation of the heat of combustion of dimethoxyborane showed that the combustion was insufficiently complete for an accurate determination. Since the hydrolysis was reported to proceed readily,' the heat of hydrolysis was studied and found to be a satisfactory measurement for heat of formation determination. Experimental Sample Preparation and Purity.-The dimethoxyborane was prepared by reaction of diborane and methyl alcohol and was purified by distillation in a lowtemperature Podbielniak column. The purity, as determined by hydrolyzable hydrogen, was found to be greater than 98y0. As an additional check on the purity, the 0" vapor pressure (275 mm.) was measured prior to filling the hydrolysis sample bulbs. The sample bulbs of thin-walled Pyrex were filled by vacuum condensation and sealed while frozen a t liquid nitrogen temperature. To minimize disproportionation, the sample bulbs were stored in liquid nitrogen until they vere weighed prior t o introduction into the calorimeter. Calorimeter.-The calorimeter was of a design similar to that described by Van Brtsdalen and B n d e r ~ o n . ~The calorimeter vessel was a 250-ml. silvered dewar flask. The calorimeter heater, approximately 25 ohms, was of No. 32 constantan wound non-inductively on the stirrer well. This heater was used both for electrical calibration and for adjustment of the calorimeter temperature. The platinum resistance thermometer had a resistance a t 0' of 25.506 ohms and was calibrated by the National Bureau of Standards. The calorimeter was immersed in a water-bath maintained a t 25 i 0.05'. An outlet tube connected to a gas measuring buret provided for collection and measurement of the evolved hydrogen. Calibration and Hydrolysis Reaction .--A series of electrical calibrations of the calorimeter system was made, as sho%nin Table I. In addition, a calibration was run in conjunction with each hydrolysis experiment. S o significant differences in energy equivalents nerr observed hetwecn initial and final states. The amount of dimethoxyborane which reacted was determined from the weight of sample, with completeness of (1) A. B. B u I g a n d H. I. Schlesinger, J . A m . Ciiem. Soc., 66, 4023 (1933). (2) H. S. Uchida, H.B. Xreider, -1. 1Iurcliisiin and $1. F. Xlasi, THISJOURXAL, 63, 1414 (1959) (3) R. E. hlcCoy and S . H. Bauer, J . Am. Chtrn. S O C . , 78, 2001 (1950). (4) E. R. Van Art?dalPn and K. P. Anrlrrson.
