at pH 2.5, 3.0, 3.5 and without pH control were freshly prepared
in deionized water. The experiments were carried out
by shaking cotton yarn (0.5 g) with dye solution (50 mL), containing
the different salt concentrations, in a conical flask
(125 mL) at 30 C in a thermostatted shaker bath operated
at 75 rpm. The absorbance of lac dye solution for each dyeing
time was monitored by UVevisible absorption spectroscopy
(Cary 1E UVevisible spectrophotometer) until the absorbance
values at lmax 487 nm remained constant. The initial and equilibrium
dye concentrations were determined using a calibration
curve based on absorbance at lmax 487 nm versus dye concentration
in standard lac dye solutions. Eq. (1) was used to
calculate the amount of dye adsorbed at equilibrium (qe)
(mg/g cotton).