The factors studied were the FeCl3 and DTAB concentrations. The
points of a central composite design are coded unities ffiffiffi 2
p
distant
from the central point (coded as zero point); hence, all points lie on
a circumference of radius ffiffiffi 2
p
. Table 1 shows the matrix of the central
composite design. The experiment corresponding to the central point
was carried out using five replicates. Fig. 1 illustrates the threedimensional
response surface graph obtained from the fit of the
experimental data.