€544. A Convenient Solid for Calibration of the Gouy MagneticSusceptibility Apparatus. By B. N. FIGGIS and R. S. NYHOLM. THE measurement of magnetic susceptibilities by the Gouy method is a relative one, the apparatus being calibrated in terms of a substance of known susceptibility, for which water, nickel chloride solution, and powdered cupric sulphate pentahydrate or ferrous ammonium sulphate hexahydrate have been used. The low suceptibility of water is often inconvenient if small tubes are being calibrated. Nickel chloride solution requires accurate analysis before use and ferrous ammonium sulphate is often of questionable purity. This substance and copper sulphate do not pack well and several different values for the susceptibilities of both solids have been reported.The required properties for a calibrant are: (1) Readily available pure;(2) an accurately known and moderate susceptibility (xg = ;(3) stability in moist air;(4) xg must vary in a known and simple way, at least at room temperature; (5) easily and reproducibly packable into the Gouy tube. The complex mercury tetrathiocyanatocobaltate HgCo(CNS), offers some advantages and its susceptibility has therefore been accurately determined. at 20' 1 being used as reference, the gram susceptibility of the complex is 16.44 (-&O.OS) x at 20".As reported elsewhere,l it obeys the Curie-Weiss law, xg cc (T + lO)-l where T is expressed
The reactions of bromine atoms with alkanes and with methyl halides have been studied by competitive bromination in the gas phase and analysis of the products by gas chromatography. Absolute rate constants and Arrhenius parameters are given, based on those previously found for attack on methyl bromide. The activation energies for hydrogen abstraction are probably closely related to the strengths of the C-H bonds broken. The A factors predicted by transition-state theory agree well with those found. The deviations are similar to those obtained for chlorine-and fluorine-atom reactions.INVESTIGATORS have studied the hydrogen-abstraction reactions of bromine atoms both because of the intrinsic interest of the rate constants and the reaction systems and because measurement of the activation energies of the reactions could lead to more accurate values for bond-dissociation energies. The first rate constant of a reaction of an atom or radical with a molecule to be accurately measured was that of the bromine atom with hydrogen. Bodenstein and Lind obtained expressions for the rate of disappearance of bromine from mixtures with hydrogen by thermal processes. The value of the rate constant for reaction (1) derived from their work is shown in Table 1.
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