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From the intensities of powder diffraction lines the positions of the deuterons in ND 4 C1 have been determined at liquid nitrogen and room temperatures. The experimental results confirm at low temperature the space group TV and the tetrahedral structure of the ammonium ion with an N-D separation 1.03A. At room temperature the T* 1 space group persists. Neither free rotation nor disordered orientations of the ammonium ions have been observed. Details concerning deuteron vibrations are discussed. INTRODUCTIONA NOMALIES in the behavior of ammonium chloride /A were fi rs t observed by Simon 1 who detected a sharp transition at -30°C in the specific heat at constant pressure. X-ray investigations 2 showed that the nitrogen and chlorine atoms were arranged in a lattice of the CsCl type and that a change of 1 percent in the lattice constant accompanied the transition, but these investigations gave no information about the hydrogen atoms. Pauling 3 suggested that onset of free rotation of the ammonium ion might account for the change in specific heat. Quanitative details of the onset of rotation for dipole interactions were discussed by Fowler 4 and in a somewhat different form by Kirkwood. 5 Eucken 6 pointed out that the rotational heat required for the ammonium ion above the critical temperature was not in agreement with experiment. In the meantime, Frenkel 7 had suggested an order-disorder transition for the ammonium ion, detailed mathematical treatment being given by Nagamiya. 8 Concurrently experimental evidence in support of both the rotation and order-disorder theories accumulated. Hettich 9 observed the disappearance of the piezoelectric effect at the transition temperature and attributed it to rotation. Menzies and Mills 10 reported that below -30°C the Raman line of 183 cm" 1 was clearly visible, but that it was absent in the room temperature phase, while Krishnan 11 found that the line persisted in that phase but with much less intensity. Menzies and Mills, although favoring rotation, stated that a possible explanation for their results was onset of a simple torsional oscillation of the ammonium ion, while Couture and Mathieu 12 deduced rotation about a threefold axis from Raman polarization measure-. ments. Similar differences occurred among the infrared results, where Beck 13 observed rotational structure on at least one vibrational band, while Wagner and Hornig 14 found evidence for the order-disorder theory. Solid state experiments by Bridgman, 15 and Lawson, 16 and electron diffraction measurements by Laschkarew 17 did not lead to conclusive results.Since the difficulties arose from ignorance of the positions and behavior of the hydrogen atoms, it was decided to study ammonium chloride by neutron diffraction methods so that by comparison of experimental and calculated structure factors the correct distribution of the hydrogen atoms could be determined. Measurements by the powder method at liquid nitrogen and room temperature are reported in the present paper and it is shown that they establish the stru...

Substitution for a harmful chemical implies that the desired function is maintained without using the harmful chemical in question. Improvement can be achieved if a less harmful chemical can be used or the same function obtained by changing the process and eliminating the harmful chemical agent. In 1982, Denmark introduced an authority regulation requiring substitution if functional and economical alternatives exist. This paper summarizes the results of 162 examples of substitution investigated by the Danish Occupational Health Services. The identification and implementation of substitution alternatives is described as an iterative process with seven distinct steps. Several tools that are useful in evaluating alternatives are described, including Hansen solubility parameters and vapor hazard ratios. In addition to the technical issues surrounding substitution, this paper describes the social interactions necessary to include all affected individuals, along with those having the proper expertise in the decision-making process. The use of the described methods may result in a safer work-place by eliminating certain hazardous chemicals or practices that have historically been used in specific industries.

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