Table 6.

Code limit for variables used in the experiment design

Factor	Unit	Symbol	Levels
			− 2	− 1	0	+ 1	+ 2
NaNo3	(g/L)	X1	0.6	1.075	1.55	2.025	2.5
NaHCO3	(g/L)	X2	0	4.2	8.4	12.6	16.8
BFCE	(%)	X3	0.25	0.375	0.5	0.625	0.75
pH	–	X4	8	8.75	9.5	10.25	11