The extension of the concept of isokinetic conditions into the dimension of pressure is discussed. The use of the enthalpy and entropy of activation at constant pressure and of the energy and entropy of activation at constant volume to characterize activated processes is discussed, and it is concluded that neither set can be rigorously justified as easier to understand, but that the constantvolume parameters are probably to be preferred most of the time. The effect of pressure up to 3 kbar on the rate of the acid-catalyzed hydrolysis of methyl acetate and ethylene oxide in acetone+ water mixtures has been measured. The constant-volume parameters of activation for the acidcatalyzed hydrolysis of methyl acetate vary with solvent in a less complicated way than the constantpressure parameters, the minima in the constant-pressure parameters not being present in the constant-volume parameters. The existence of the minima in the constant-pressure parameters is ascribed mainly to the large change of thermal expansivity when an organic liquid is added to water.The effect of solvent on the rate of hydrolysis of methyl acetate is dependent on the pressure, and reverses sign at about 2 kbar. The implications of this observation in the determination of reaction mechanisms is discussed.

No abstract

The reaction of carbon dioxide with water and hydroxide ions has been studied by measuring the rate of oxygen exchange between sodium carbonate and bicarbonate and oxygen-18 enriched water as a function of pH, ionic strength, and carbonate-bicarbonate concentration. I t has been shown that these reactions are catalyzed by carbonate and bicarbonate ions. Rate constants for the uncatalyzed and catalyzed reactions have been evaluated.

Oxygen-18 enriched oxygen and carbon dioxide-carbon monoxide atmospheres have been used in the determination of the oxygen self-diffusion coefficient in monoclinic zirconia at 990~ Oxygen exchange measurements were made on zirconia spheres in the average size ranges of 90 and 126~. The diffusion coefficient was found to be independent of both oxygen partial pressure over the range 1-18-19 atm and sphere size. The value of the diffusion constant has been used with previously determined electrical conductivity data to verify postulates of a defect model involving oxygen ion vacancies for monoclinic zirconia.The nature of the defect structure and the transport properties of oxygen in monoclinic zirconia have been the subjects of several recent investigations (1, 2). For example, Vest, Tallan, and Tripp (3) measured the electrical conductivity of zirconia at 990~ as a function of oxygen pressure from 1 to 10 -~ atm and found an electronic semiconductor, with the transition from n-type to p-type conductivity occurring at approximately 10 -16 atm. Their results,have been further discussed by KrSger (1) who advanced a Schottkytype model involving both isolated oxygen vacancies and zirconium vacancies singly associated with oxygen vacancies, and also by Douglass and Wagner (2) who proposed an anti-Frenkel type model involving oxygen vacancies and interstitials. It is predicted by both of these models that oxygen ionic conductivity may be considered to be predominant at moderately low oxygen pressures, between the extremes of n-and p-type electronic conductivity.Recently, Madeyski and Smeltzer (4, 5) have applied the gas-solid oxygen-18 exchange technique to determine the oxygen self-diffusion coefficient in monoclinic zirconia prepared as annealed spheres at temperatures in the range 808~176 and at oxygen pressures close to atmospheric. In this paper, these results have been extended at 990~ to oxygen partial pressures as low as 10 -19 atm, by the use of oxygen-18 enriched CO-CO~ atmospheres. ExperimentalIn Fig. 1, a schematic drawing of the gas preparation train and the reaction apparatus is given. The oxygen exchange apparatus is identical to that previously described (5).The preparation of microspheres of near-stoichiometric ZrO2 in the size ranges 75-105~ and 105-147~ by oxidation of 99.92 w/o (weight per cent) zirconium sheet (4), and of oxygen gas containing about 1.3 a/o (atomic per cent) 1sO has already been described (5). Carbon dioxide containing about 0.9 a/o isO was prepared by equilibrating normal carbon dioxide with oxygen-18 enriched water in the five-liter flask shown on the diagram, followed by removal of the water by trap-to-trap distillation between dry ice-acetone and liquid nitrogen traps. Carbon monoxide containing about 0.9 a/o 1sO was prepared by exposing either the oxygen or the CO2 prepared as above to an excess of activated charcoal for approximately 24 hr at 1080~ followed by transfer through three liquid nitrogen traps to a 5-liter storage flask with the aid of a Toepler pump. The equality of...

No abstract