A plot of lnQt Qf versus time is expected to be linear
with a slope of k and an intercept of lnQ0 Qf if the reaction
is first order. Fig. 9 shows the plot of lnQt Qf as a function
of time for dyeing of cationised cotton fabrics with lac
dye using both ultrasonic and conventional heating methods.
As can be seen in this figure, the linear fitting of Eq. (5) holds
indeed and the values of dyeing rate constants could be obtained
as listed in Table 2.
The time of half dyeing t1/2, which is the time required for
the fabric to take up half of the amount of dye taken at equilibrium,
is estimated either from each isotherm directly
(Fig. 8) or from the following equation: