Table 3.

Coded values (x) and uncoded values (X) of variables in the central composite design (CCD) experiments.

Variable (mmol/L)	Symbol		Coded level
	Uncoded	Coded	−1.682	−1	0	1	+1.682
Ca2+	X1	x1	1.59	5	10	15	18.41
Cu2+	X2	x2	0.65	1.0	1.5	2.0	2.34
Al3+	X3	x3	1.16	1.5	2.0	2.5	2.84