Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry最新文献

The enthalpies of combustion of nicotinic acid and creatinine have been determined in an adiabatic rotating-bomb calorimeter. The enthalpies of formation have been obtained by combination of the experimental data with the accepted values for the enthalpies of formation of carbon dioxide and water. The results of other investigations on creatinine are discussed briefly. The resulting values and their estimated uncertainties are as follows: [Table: see text].

Simultaneous measurements of specific heat, electrical resistivity and hemispherical total emittance of hafnium containing 3.12 weight percent zirconium in the temperature range 1500 to 2400 K by a subsecond duration, pulse heating technique are described. The measurements indicate decreases in specific heat (by about 13%) and in electrical resistivity (by about 8%) as the result of the α → β transformation. Estimated inaccuracies of the measured properties are: 3 percent for specific heat, 1 percent for electrical resistivity and 5 percent for hemispherical total emittance.

Polarization effects on fluorescence measurements are a function of four independent variables. The first is F, the polarization ratio of the exciting light which reaches the sample. The second is r, the emission anisotropy of the sample, which is the polarization "response" of the sample to plane polarized exciting light. The third is G, the polarization ratio of the emission detection system, which is the ratio of the sensitivities of the detection system to vertically and horizontally polarized light. The fourth is α the viewing angle, which is the angle bet ween the direction of the propagation of the exciting light and the direction from which the emission is being detected. The intensity and the degree of polarization of the fluorescence emission that the sample exhibits are functions of F, r, and a, while the actual readings obtained with a typical spectrofluorimeter are functions of all four variables, F, r, α, and G. A theoretical analysis is made taking all these factors into account, and proper mathematical models are developed for the different modes of operation in which a fluorimeter can be used. These are verified experimentally with data obtained for a sample which has a high degree of emission anisotropy (Nile Blue A Perchlorate in glycerol). A recently designed goniospectrofluorimeter was used. Calibration procedures are developed and recommendations are made for modes of operation and fluorescence standards.

Over 4000 wave lengths are listed between 1900 and 9000 Å for Fe i, Fe ii, Ne i and Ne ii lines measured in a hollow cathode discharge tube with iron electrodes and a neon gas filling. Photoe-electric traces between 2400 and 5700 Å on a semiquantitative intensity scale are also included. For Fe i, energy values for 124 even and 240 odd levels have been computed. These have been used to calculate Ritz standards for most of the Fe i lines.

This equation of state was developed from PVT compressibility data on methane and ethane. The highly-constrained form originates on a given liquid-vapor coexistence boundary (described by equations for the vapor pressures and the orthobaric densities). It then requires only five least-squares coefficients, and ensures a qualitatively correct behavior of the P(ρ, T) surface and of its derivatives, especially about the critical point. This nonanalytic equation yields a maximum in the specific heats C _υ (ρ, T) a t the critical point.

Measurements of the temperature and energy of the α → β phase transformation, and the electrical resistivity near and at the transformation point of zirconium using a subsecond duration pulse heating technique are described. The results yield 1147 K for the transformation temperature and 3980 J · mol^-1 for the transformation energy. Electrical resistivity is found to decrease by 17 percent during the transformation. Estimated inaccuracies of the measured properties are: 10 K for the transformation temperature, 5 percent for the transformation energy, and 2 percent for the electrical resistivity.

Solubility isotherms of beta-tricalcium phosphate, β-Ca(PO₄)₂, prepared by heating mixtures of CaCO₃ and CaHPO₄ above 800 °C, were determined in the ternary system Ca(OH)₂-H₃PO₄-H₂O at 5, 15, 25, and 37 °C in the pH range 6.0-7.5 by equilibration with dilute H₃PO₄ solutions. The results indicate that β-Ca₃(PO₄)₂ has a negative thermal coefficient of solubility. The solubility product, K _s , was determined as a function of temperature by a generalized least-squares procedure; the resulting equation is $\text{log}{K}_s=-45723.26/T+287.4536-0.546763T;$ the values of K _s and its dispersion at 25 and 37 °C are 1.20(0.056), and 0.283(0.011) × 10^-29. Thermodynamic functions for the dissolution of the salt at the four experimental temperatures are reported. When treated as an adjustable constant, the Ca/P ratio in these β-Ca₃(PO₄)₂ solutions was found to have the value 1.514(0.010), confirming that the stoichiometry of the high temperature form of this salt is correctly indicated by the above formula. The computed isotherms at 25 °C for several calcium phosphates are compared; it is shown that β-Ca₃(PO₄)₂ is, next to Ca₅(PO₄)₃OH, the most stable at pH's above 6.36. The singular points of the β-Ca₃(PO₄)₂ isotherm with those of CaHPO₄ · 2H₂O and CaHPO₄ are at pH's 5.88 and 6.36, respectively.

Solubility of CaHPO₄ · 2H₂O was determined in the quaternary system Ca(OH)₂ - H₃PO₄ - NaCl - H₂O at 25 °C in the pH range 4.39-6.38; ionic strengths of the saturated solutions varied from 0.00485 to 0.545, Satisfactory constancy in the solubility product, ( $K_{\text{sp}}=\left[{\text{Ca}}^{2+}\right]\cdot \left[{\text{HPO}}_4^{2-}\right]\cdot {\gamma }_{{\text{Ca}}^{2+}}\cdot {\gamma }_{{\text{HPO}}_4^{2-}}=2.49\pm 0.05\times {10}^{-7}{\text{mol}}^2\times 1^{-2}$ ) was obtained when (i) the ion activity coefficients, γ _i , were calculated with the Debye-Hückel equation, $\text{log }{\gamma }_i=-A{Z}_i^2\sqrt{I}/\left(1+\text{B}{\alpha }_i\sqrt{I}\right)+0.0626I$ ; the value 0.0626 for the coefficient in the linear term was derived from the solubility data by utilizing a statistical procedure, and (ii) formation of an ion pair ${\text{NaHPO}}_4^{-}$ was taken into account; a statistically derived value for the stability constant of this ion pair is 7.0 ± 2.4 1 × mol^-1. The ion pair ${\text{NaHPO}}_4^{-}$ appears to have significant concentrations in physiological fluids.

The enthalpies of combustion of o-, m-, and p-chlorobenzoic acid have been determined in an adiabatic rotating-bomb calorimeter. The enthalpies of formation have been obtained by combination of the experimental data with the accepted values for the enthalpies of formation of water, carbon dioxide, and aqueous hydrochloric acid. The results of other investigators are discussed briefly. The resulting values and their estimated 95 percent confidence limits are as follows: o-Chlorobenzoic acidm-Chlorobenzoic acidp-Chlorobenzoic acid ΔHc° (25 °C)-3087.91 ± 0.69 kJ/mol-3068.05 ± 1.53 kJ/mol-3064.40 ± 0.66 kJ/molΔHf° (25 °C)-404.83 ± 0.74 kJ/mol-424.59 ± 1.55 kJ/mol-428.16 ± 0.72 kJ/mol. Where ΔHc° corresponds to the process: -404.61-424.37-429.94C₇H₅O₂Cl(c) + 7 O₂(g) + 198 H₂O(liq) → 7 CO₂(g) + [HC1 + 200 H₂O](liq).

A high-accuracy spectrophotometer, originally designed for work at visible wavelengths, was modified to permit measurements in the ultraviolet without degradation of its original performance. This was accomplished by equipping the spectrophotometer with a stable deuterium arc source, a highly efficient averaging sphere with fluorescent wavelength converter, a new grating, and achromatic sample-compartment optics. The modified spectrophotometer will be used for the development of new Standard Reference Materials, as well as for materials research, in the region between 200 and 300 nm.