Table 4.

Coded values of variables in the study

Variable	Nomenclature		Value
			(−1)	0	(1)
Solid–liquid ratio	X 1	(g/ml)	1:10	1:15	1:20
Ethanol concentration	X 2	(%)	65	75	85
Ultrasonic time	X 3	(min)	40	60	80
pH value	X 4		2	3	4