Volume 17, Issue 10, 01 October 1949
Index of content:
The Normal Vibrations of Polyatomic Molecules as Calculated by Urey‐Bradley Field. III. A Table of Force Constants17(1949); http://dx.doi.org/10.1063/1.1747075View Description Hide Description
Assuming the Urey‐Bradley field for various chemical structures, stretching, bending and repulsive force constants were determined so as to give the best fit with the observed vibration frequencies. The magnitude of these force constants, especially of repulsive constants, was discussed.
The Kinetics of Membrane Processes. I. The Mechanism and the Kinetic Laws for Diffusion through Membranes17(1949); http://dx.doi.org/10.1063/1.1747076View Description Hide Description
The equations for diffusion in binary systems are extended to the case of diffusion through a membrane. Three elementary rate processes are considered at a solution‐membrane interface: (1) adsorption of the diffusing species, (2) desorption back into the solution, and (3) diffusion into the membrane, and an expression for the rate constant of the over‐all process of surface penetration is developed in terms of three specific rate constants. Various special cases are considered and discussed with reference to the experimental data. A general expression for the rate of diffusion of a species through a membrane under steady‐state conditions is derived, the rate being expressed as a function of activities. It is shown that the application of this expression to the case of a solvent passing through a membrane which is impermeable to the solute leads to the thermodynamically exact equation for the osmotic pressure. Expressions are given for the rate of flow of solvent and solute through a membrane as a function of the osmotic and hydrostatic pressures across the membrane.
The Kinetics of Membrane Processes. II. Theoretical Pressure‐Time Relationships for Permeable Membranes17(1949); http://dx.doi.org/10.1063/1.1747077View Description Hide Description
The rate equations developed in Part I for the flow of solvent and solute through a permeable membrane have been extended to the case in which the flow is measured by the rise and fall in a capillary inserted into the more concentrated solution. The differential equation obtained for this case has been integrated to give an explicit relationship between the height of rise in the capillary and the time. The equation is shown to give the height‐time relationship in the form found by experiment. It is shown that the osmotic pressure obtained with a semipermeable membrane is a special case of the more general theory of diffusion through membranes permeable to both solvent and solute. The possibility of positive and negative values of P, i.e., of initial rise or fall respectively in the capillary, is discussed, and the significance of these cases evaluated in terms of the molar volumes and permeation constants of solvent and solute.
The Kinetics of Membrane Processes. III. The Diffusion of Various Non‐Electrolytes through Collodion Membranes17(1949); http://dx.doi.org/10.1063/1.1747078View Description Hide Description
Measurements have been made of the rates of diffusion of various non‐electrolytes, and of water, through a collodion membrane separating a dilute solution from pure water. The determinations were made by measuring the rate of rise and fall in a capillary tube in the solution, and analyzing the results using the procedure described in a previous paper. The work was done with 0.25Nsolutions of sucrose, lactose, raffinose and mannitol, and was carried out at different temperatures so that energies and entropies of activation could be determined. The energy of activation for diffusion through the membranes was found in all cases to be slightly less than for free diffusion, a result that is shown to be consistent with a positive heat of adsorption of the solute and solvent at the membrane. Analogously the entropies of activation for membrane diffusion are somewhat less than for free diffusion, since the process of adsorption brings about a decrease in entropy. The rate of diffusion of water through a membrane is shown to depend upon the nature of the solute diffusing, a result which arises from the fact that diffusion is a mutual process. An experiment is described in which there is an initial net flow from the solution into the solvent.
On Permanent Charges in Solid Dielectrics. II. Surface Charges and Transient Currents in Carnauba Wax17(1949); http://dx.doi.org/10.1063/1.1747079View Description Hide Description
Permanent surfacecharges are produced in disks of carnauba wax by exposure to an external electric field. The nature of these charges is studied with a new method of analysis, which is based on the simultaneous measurement of the external current and of the induced charges on the electrodes of the capacitor containtaining the dielectric. The observed effects are explained by dielectric absorption and by transfer of charge from the electrode to the surface of the dielectric; this latter phenomenon is due to surface breakdown and continuous conduction currents. The results provide an adequate explanation for the electret.
17(1949); http://dx.doi.org/10.1063/1.1747080View Description Hide Description
With the object to obtain new data for the statistical analysis of hydrocarbon mixtures, measurements on diamagneticorganic compounds at room temperature have been performed. The precision was of the order of 1 per mil (10−3) of the diamagnetic effect. We checked the additivity rule and found it to be applicable within 5 per mil on the average. Mixtures were also investigated. A linear relation with the concentration was found to hold for hydrocarbon mixtures within the precision of measurement. For liquids containing electric dipole deviations up to 6 per mil were observed. We applied the results to the analysis of unknown mixtures.
17(1949); http://dx.doi.org/10.1063/1.1747081View Description Hide Description
The ultraviolet absorptionspectrum of decaborane in cyclohexane shows a broad absorption with a maximum molecular extinction coefficient of 3200±100A at wave‐length 2720±50A. The luminescence of decaborane dissolved in a glass at 77°K shows a continuous fluorescence extending from 3630±50A to about 6000A with a broad maximum at 4520A. No further luminescence was observed at wave‐lengths shorter than 9000A. The 0–0 transition is located at 3600±50A. The energy levels of a particle in a rectangular, hemispherical, and spherical box were used to test the applicability of a metallic model to decaborane. Agreement with the absorptionspectrum and the molecular dimensions was obtained.
Internal Rotation IV. The Optical Activity and Isomerization Energy of the Rotational Isomers of D‐Secondary Butyl Alcohol17(1949); http://dx.doi.org/10.1063/1.1747082View Description Hide Description
The specific rotation of secondary butyl alcohol was measured in dilute solutions of cyclohexane at intervals of 5° between 20°C and 70°C inclusive.
After corrections for solvent effect and change of refractive index with temperature had been made there remained a definite temperature dependence of the rotivity.
Interpretation of the results on the basis of three optically active rotational isomers (two of equal energy and the third less stable) gave the rotivity of the third isomer as −21° and the sum of the other two as +21°.
The heat of isomerization was found to be 803 cal. per mole.
The concentrations of the three rotational isomers at 20° are calculated as 42.3 percent, 42.3 percent, and 15.3 percent.
17(1949); http://dx.doi.org/10.1063/1.1747083View Description Hide Description
X‐ray diffraction methods have been developed for the study of materials at high temperatures under controlled atmospheres, and an investigation of the high temperature phases of the alkaline earth carbonates has been made. BaCO3 was found to change from orthorhombic (aragonite type) to hexagonal (calcite type) at 803°C, and to cubic (sodium chloride type) at 976°C. SrCO3 was found to change from orthorhombic (aragonite type) to hexagonal (calcite type) at 912°C. A few experiments with mixtures, which go through similar transitions, are described.
These transitions are related to the rotational activity of the carbonate ions and consideration of the nature of the transitions leads to a description of the probable rotational behavior of these ions in the various phases.
17(1949); http://dx.doi.org/10.1063/1.1747084View Description Hide Description
The 22−21, 30−31, 4−3−31, and the 50−51 rotational absorption transitions of HDO have been observed and identified on the basis of their Stark effect. By comparing the theoretical and experimental Stark shift, the dipole moment for all these transitions has been measured as 1.84±.01. The effective HOD angle is also determined as 104°±30′ and the parameter of asymmetry, k, as about −0.696. The line breadth Δν at one atmosphere is 0.38±0.04 cm−1. None of these quantities is in disagreement with independent determinations.
17(1949); http://dx.doi.org/10.1063/1.1747085View Description Hide Description
The emission lifetimes of the metastable triplet states (phosphorescent states) of a large variety of organic molecules have been measured. The lifetimes are in the range from 10−4 to about 10 seconds. It is shown that the transition probabilities corresponding to the shorter lifetimes are of the same magnitude as found in the light atoms of which the molecule is composed. The longer lifetimes, on the order of seconds, are found only among the aromatic compounds. A consideration of the perturbing singlet states in aromatic compounds shows that the matrix elements for the intercombination must be very much smaller than those responsible for the intercombination in the free carbon atoms. Direct evidence that the long‐lived states of the aromatic compounds are triplet states is obtained by showing that as the atomic number of chemically similar substituents is increased (e.g. substitute Br for Cl), the transition probability increases approximately in proportion to the increase in the square of the spin‐orbit interaction energy of the substituent atom. The luminescences are usually observable only when the substance is dissolved in a rigid solvent, and the effect of such a solvent on the lifetimes is discussed. Experimental data relating singlet‐triplet absorption strength to triplet state lifetime is presented.
17(1949); http://dx.doi.org/10.1063/1.1747086View Description Hide Description
An investigation is made of the thermodynamic stability of liquids under tension. Comparisons of the Helmholtz free energies show that small samples of liquid under tension are unstable with respect to two‐phase liquid‐vapor equilibrium at the same volume and temperature.
Defining the tensile strength as the negative pressure at which (∂P/∂V) T =0 for the liquid, it is possible to calculate ``maximum'' possible tensile strengths from equations of state. The van der Waals and Berthelot equations in reduced form can be adapted to such explicit calculation and give values much lower, and therefore much closer to experimental values, than previous calculations. For non‐associated liquids at 27°C, the van der Waals maximum tensile strengths are about 150 atmos. whereas the Berthelot values are about 750 atmos. The temperature dependence for the two equations is about 1 and 6 atmos. °C, respectively.
Comparisons with observation of the calculated reduced liquid volume, energy of vaporization, coefficient of thermal expansion, and bulk compressibility support the use of the van der Waals and Berthelot equations in computing liquid properties.
Infra‐Red Spectra of Propane, 1‐Deuteropropane, and 2‐Deuteropropane and Some Revisions in the Vibrational Assignments for Propane17(1949); http://dx.doi.org/10.1063/1.1747087View Description Hide Description
The infra‐red spectra of propane, 1‐deuteropropane, and 2‐deuteropropane are presented. The spectrum of propane does not have the bands at 720 and 1179 cm−1 reported by Wu and Barker.
The vibrational modes of the three molecules are described. The 748‐cm−1 band in the propanespectrum is unambiguously assigned to rocking of the CH2 group about an axis normal to the molecular axis and in the C3 plane; and the 1053‐cm−1 band is unambiguously assigned to rocking of the CH2 group about an axis normal to the C3 plane.
Unambiguous assignments for the remaining bands below 1350 cm−1 in the propanespectrum were not possible without an elaborate theoretical treatment beyond the scope of this paper.
17(1949); http://dx.doi.org/10.1063/1.1747088View Description Hide Description
A comprehensive investigation of the thermodynamic consistency of the large body of steam data above 150°C has been completed using the new thermodynamic scale information supplied by Professor J. A. Beattie to test the consistency of measuredvapor pressures, specific volumes, enthalpies at saturation and superheat and specific heats. As an aid in correlation and unification a new form of equation of state is employed which has some validity through the critical region. The results of the investigation indicate a satisfactory state of thermodynamic consistency over the liquid‐vapor range with respect to vapor pressures and saturation volumes deduced from the measuredvaporization quantities using the Beattie corrections to the international temperature scale. The consistency test includes also verification of the accuracy and usefulness of the new functional relation for the vapor phase, p=F(v, T) whose parameters were determined from the measured superheat p‐v‐T data. The superheat enthalpy data to 550°C at zero pressure is however not consistent with the present theoreticalinformation pertaining to Cp 0. The computed enthalpies of the vapor to 550°C and pressures of 300 kg/cm2 are satisfactory provided an empirical correction term is subtracted from the theoretical values of Cp 0. The accuracy of the Keenan‐Keyes steam tables values of specific volume, pressure, and enthalpy is confirmed.
17(1949); http://dx.doi.org/10.1063/1.1747089View Description Hide Description
Since the publication of Hittorf's famous paper in 1853 it has been assumed, in general, that macroscopic space charge does not exist in an electrolyte during electrolysis. An apparatus and procedures were developed which made possible reproducible data on potential distribution in an electrolyte. In a uniform column of electrolyte the potential rise was found to be a non‐linear function of the distance from the cathode, and it was shown that this non‐linearity could not have been due to concentration changes caused by the electrolysis, but must have been due to macroscopic space charges in the electrolyte. Calculations showed the space charge to be positive very close to the cathode, negative in the next quarter of the column, positive for the next nearly three‐quarters of the column and negative very close to the anode.
The eight electrolytes studied were 0.0024N solutions of CuSo4, CuCl2, ZnSO4, ZnCl2, Zn(C2H3O2)2, NiSO4, NiCl2 and Al2(SO4)3. The pure metal was used for the electrodes in each case. Each column of electrolyte was 8 cm×8 cm×40 cm long, and the constant potential difference was 8.000 volts. The largest observed space charge at the end of 15 minutes was in CuSO4 at 2 mm from the anode; it was an excess of 2×106Cu++ ions over SO4 −— ions per cm3, which is an excess of only 3 Cu++ ions for every 1012Cu++ ions originally present.
Although the space charges were all very small, they had very appreciable effects on the potential gradients, which also were calculated. The potential gradients in the different electrolytes were not constant, but had maximum deviations from constancy of from 5 to 20 percent in the explorable part of the column (to within 1 mm of each electrode). The average potential gradients in the explorable part were from 1.1 to 23 percent less than the 8/40 volt/cm. Thus Ohm's law was found not to hold even over the explorable part of the column.
17(1949); http://dx.doi.org/10.1063/1.1747090View Description Hide Description
Controversial evidence has appeared in the literature concerning the constancy of the lattice parameter of AgBr with varying exposure to radiation. This paper gives in detail experiments showing that the lattice parameter does not change appreciably with increasing exposure to x‐rays.
17(1949); http://dx.doi.org/10.1063/1.1747091View Description Hide Description
The decomposition of hydrogen peroxide vapor initiated by light of wave‐length 2537A was studied. The quantum yield of the reaction was found to be 1.7±0.4, essentially independent of pressure of peroxide and intensity of absorbed light. The reaction products were water and oxygen only. The rate of the reaction was found to be independent of temperature and was not affected by the addition of oxygen, nitrogen or water to the reaction mixture. The reaction scheme proposed isthis leads to a maximum quantum yield of 2.
17(1949); http://dx.doi.org/10.1063/1.1747092View Description Hide Description
It has been shown that hydrogen fluoride and fluorine gases do not undergo an exchange reaction at room temperature and that heating to 200°C for a matter of an hour or so in a brass vessel is necessary to accomplish exchange. Evidence is obtained that even in this case the observed exchange is heterogeneous and catalyzed by the metal fluorides on the wall of the vessel. Illumination by the light of a mercury arc did not accomplish the exchange under conditions such that an activation free energy of some 9 Kcal/mole for the reaction between a fluorine atom and the fluorine molecule or fluorine atom and HF molecule would have been necessary to prevent exchange.
A rapid exchange reaction in the dark at room temperature has been observed to occur between chlorine and HCl in the gaseous state. This is taken as evidence that the hydrogen halides exchange with the halogens through the formation of an intermediate complex of the general formula HX3, and some discussion of the improbability of the alternative atomic mechanism is given. The non‐existence of the HX3 mechanism in the case of the fluorine system is explained by the absence of electrons in the higher shells which are thought to be the main bonding forces involved in the stabilization of the HX3 complex in the case of chlorine, bromine, and iodine, and also by the polar character of the hydrogen‐fluorine bond in HF, increasing the probability of an alternative HX3 complex formation in which the hydrogen rests between the fluorine on the HF molecule and the fluorine molecule itself, thus keeping it clearly labeled and rendering interchange on dissociation of the complex unlikely.
17(1949); http://dx.doi.org/10.1063/1.1747093View Description Hide Description
Spectroscopic and electron diffraction data have been combined to make calculations of the free energy,entropy, and heat capacity of OF2 to the rigid rotator‐harmonic oscillator approximation between 298.16° and 1500°K. The spectrum of ClF is known well enough to calculate these properties to the next approximation from 298.16° to 2000°K. Thermal data have been combined with these results to give the complete thermodynamic properties of these two fluorine compounds. It is pointed out that the accepted dissociation energy of F2 is too high, and calculations show that it must be close to 1.5 ev per molecule.