Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

236 EXPERIMENT A L METHOD s FOR HE T E R o GE N E o u s RE A c T I o N sthe proportion ofpara-hydrogen increases, until it reaches 100 % at liquid hydrogentemperature. The preparation of hydrogen rich in para-hydrogen is accomplishedby contact with a paramagnetic catalyst such as charcoal at liquid hydrogen orliquid nitrogen temperature2”. Once prepared, the enriched “para-hydrogen” canbe stored for several months at room temperature in the presence of glass, mercuryor tap grease. An apparatus for the continuous production of para-hydrogen hasbeen described by Farkas and Sachsse’ 30. The early work on preparation propertiesand reactions of hydrogen modifications has been summarised by Farkas”’. Manykinetic studies have been made by Eley and co-workersJ7* 38*232i233. A fl owsystem used to measure the para-hydrogen conversion over nickel oxides is shownin Fig. 30234. Hydrogen is purified, dried and split into reference and samplestreams. The latter stream is then passed over an excess of nickel oxide/aluminacatalyst at liquid nitrogen temperature which enriches the para-hydrogen content to50%. After being partially converted to normal hydrogen by the catalyst, thestream then passes through the thermal conductivity cell for comparison with thereference stream.Analysis of hydrogen mixtures with a thermal conductivity cell is well established.The most accurate measurements are obtained by use of a thermal conductivitygauge with the walls immersed in liquid nitrogen and the wire heated to 160” K.This is the temperature when the difference in the rotational specific heats of orthoandpara-hydrogen is a maximum’35. Various modifications of thermal conductivitygauges have been made to improve their convenience in use236. A roomtemperature flow analyser based on a thermal conductivity cell has been developedby Weitzel and White’” which is claimed to be as sensitive as low temperatureunits. Bridge current and temperature must be controlled very carefully, but theunit is relatively insensitive to changes in pressure and flow rate.10 G r . ~ . ~ C kJI tbdk whfi NRC blockFig. 30.Diagram of flow system for the measurement of the para-hydrogen conversioncatalysts. From ref. 234.over