Table 2.

Actual investigated values of factors and their corresponding coded levels.

Variable	Code Values
	−1 (Minimum)	0 (Medium)	1 (Maximum)
X1, pH	4	6	8
X2, Chloride (ppm)	85.20	585.20	1085.20
X3, Sulfate (ppm)	70.04	370.04	670.04