Table 3: Central composite experimental design, 4 variables and 3 levels.

ExperimentFlow rate HCl (mL min−1)Flow rate NaBH4 (mL min−1)HCl% (v/v)NaBH4 (% (m/v)) in NaOH 0.05%Absorbance Se(IV) 50 μg · L−1 ( ) dissolution

11.31.750.10.076
21.31.750.50.069
31.31.7300.10.081
41.31.7300.50.070
51.34.550.10.065
61.34.550.50.043
71.34.5300.10.066
81.34.5300.50.044
92.83.7150.20.167
105.91.750.10.222
115.91.750.50.228
125.91.7300.10.205
135.91.7300.50.200
145.94.550.10.162
155.94.550.50.112
165.94.5300.10.170
175.94.5300.50.115