Table 1. Levels for each factor in the experimental design.

Factor

Axial point
(-a)

Low
Level (-1)

Central level (0)

High
level (+1)

Axial point
(+a)

Time of exposure
(min.)

27

40

60

80

93

Acid concentration
(% vv)

0.50

0.75

1.13

1.50

1.75

Solid:liquid ratio
(g:ml)

1:3.3

1:3.0

1:2.5

1:2.0

1:1.7