EXPERIMENT

The heats of formation were determined using a high temperature reaction calorimeter with a typical accuracy of ±1kJ/mole. The measurements were made with the calorimeter set at 1373±2K, and using a protective argon atmosphere. The calorimeter was calibrated using pure copper. Samples were produced by mixing elemental powders in a mortar in the required molar ratio, and pressing them into a small pellet. Typical sample weight was about 100 mg. The nickel and iron powders used were reduced in hydrogen prior to preparation of the samples to remove oxygen and carbon which would be a source of errors.

The enthalpy of reaction is measured by two steps. ΔHReaction is obtained first by dropping the pellet into the calorimeter from room temperature. A minimum of six separate samples were measured. The pellets were subsequently removed and again dropped from room temperature into the calorimeter to obtain the heat content of the compound, ΔHHeat Content. The difference between the two measurements yields the heat of formation at 298K. The results are averages of the 6 individual measurements. With the standard deviations from the reaction and heat content experiments designated as δ1 and δ2 and from the calibration as δ3 the overall uncertainty in the measurements, δ, was determined from δ = (δ1^2 + δ2^2 + δ3^2)^1/2.

Material from the reacted compound is used to obtain an x-ray diffraction pattern to confirm that the reacted sample is the desired compound.