A TG–DTA study of the sublimation of nicotinic acid

The Langmuir equation was used to construct vapor pressure curves for the sublimation phase of nicotinic acid. Three different approaches were used to calculate the pressure values. In the first, the ‘k’ term of the Langmuir equation was eliminated by comparing the sample with the reference compound, in the second, the actual values of the ‘k’ term were used and in the third approach, the ‘k’ term was kept constant. The vapor pressure curve constructed using the actual values of ‘k’ was in close agreement with the first. This indicates that the ‘k’ term of the Langmuir equation cannot be assumed to be constant and the systematic variation in the values of the ‘k’ term with temperature has to be taken into consideration.