Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

260 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSFor a first order reaction the above equation simplifies tok=C,dATldt + KATK(A - a) - C, A 7The original extends the discussion to more complex reactions and thedetermination of activation energies and heats of reaction. However, the equationswere developed for homogeneous reactions in solution and required twelveassumptions some of which are very difficult to satisfy when applied to DTA studiesof solid state reactions. These assumptions are (i) the heat transfer coefficientsand heat capacities of reactants and products are equal and constant, and (if)that the temperature is uniform throughout the sample and reference material.Freeman and Carroll352 and Wendlandt 35 have suggested simplifications inBorchardt and Daniels”” procedure.B~rchardt~~~ has also suggested a simplified procedure for determining initialreaction rates. If (dn/N,)/dt is the rate of reaction when the height of the DTApeak is AT, then-- -dnlNo-- ATdt AIt is assumed that the DTA peak can be approximated by a triangle of height AT,,,,and base b then4.5 E LECTRI C A L C 0 N D U CT I VITY ME AS URE MENTSThe methods for determination of the electrical conductivity of solids are discussedin Section 2.1.7. For most materials, a change in electrical conductivitycan be expected due to interaction of two solid phases. In a single phase transformation,the disorder in the solid must increase, causing an increase in conductivitywhich will be reversed when the new phase is formed. The change may be indicatedby a change in temperature coefficient or by a sharp discontinuity.Garn and Flaschen3” have used the method to detect a number of polymorphicphase transformations. Fujita and Damask3 56 studied the precipitation of carbonin iron which caused an increase in conductivity. The samples were carburisedat high temperature, quenched in brine at 0” C and stored under liquid nitrogen.All resistance measurements were made at liquid nitrogen temperature, and comparedwith a well aged iron/carbon specimen and a standard resistance. Jaky