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Box–Behnken central composite design for the continuous variables.
| Run | Factors | ||
|---|---|---|---|
| Water/benzoic acid ratio | Ethyl alcohol/water ratio | pH | |
| 1 | 4 : 1 | 3 : 1 | 10 |
| 2 | 5 : 1 | 2 : 2 | 10 |
| 3 | 4 : 1 | 1 : 1 | 10 |
| 4 | 3 : 1 | 2 : 1 | 10 |
| 5 | 5 : 1 | 3 : 1 | 7.5 |
| 6 | 4 : 1 | 2 : 1 | 7.5 |
| 7 | 4 : 1 | 2 : 1 | 7.5 |
| 8 | 4 : 1 | 2 : 1 | 7.5 |
| 9 | 3 : 1 | 1 : 1 | 7.5 |
| 10 | 5 : 1 | 1 : 1 | 7.5 |
| 11 | 3 : 1 | 3 : 1 | 7.5 |
| 12 | 5 : 1 | 2 : 1 | 5 |
| 13 | 4 : 1 | 1 : 1 | 5 |
| 14 | 4 : 1 | 3 : 1 | 5 |
| 15 | 3 : 1 | 2 : 1 | 5 |
