Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 44

− 1

− E

RT

αβ α

d
d

α
α

(2.31)

f () = A exp ,

α

T

a

where E ʱ is estimated by an isoconversional method, A ʱ is evaluated via the com-
pensation effect, and T ʱ and (d ʱ /d T ) ʱ are experimental values measured at the heat-
ing rate ʲ . Substitution of the values into Eq. 2.31 yields numerical values of f ( ʱ ),
which can further be matched to the theoretical f ( ʱ ) models (Fig. 1.4).
Figures 2.13 and 2.14 exemplify the application of the aforementioned method
in the case of thermal dehydration of nedocromil sodium trihydrate that occurs in
two well-separated steps [ 64 ]. For both steps, the E ʱ values estimated by an isocon-
versional method do not demonstrate practically any dependence of ʱ (Fig. 2.13 ).
The compensation effect (Eq. 2.29) has been used to estimate the A ʱ values, which
were substituted in Eq. 2.30 to evaluate the reaction model, g ( ʱ ) in numerical form
(points in Fig. 2.14 ). Comparison of the g ( ʱ ) values against the theoretical reaction
models suggests that the first step of dehydration follows the zero-order reaction
model, g ( ʱ ) = ʱ (R1).
However, for the second dehydration step, the g ( ʱ ) values fall between two dif-
fusion models D2 and D3 (Table 1.1). This is not very unusual considering the

Fig. 2.13 E ʱ values estimated by an isoconversional method for first ( diamonds ) and second ( tri-
angles ) steps of dehydration of nedocromil sodium trihydrate. (Reproduced from Zhu et al. [ 64 ]
with permission of Wiley)