For sequential extraction you take in consideration two things:

1) The choice of solvents: look for solvents (4 to 5 solvents) with different polarity index (don't choose solvents with close polarity index). Example: Hexane(apolar)/Chloroforme(4.1)/ethanol (5.2)/Water (9.0)

2) You can use the same plant material with all solvents. You start with the apolar and you finish with the more polar (for the cited example: hexane>Chloroforme>ethanol>water). But I prefer to realize extraction with each solvent using new plant material with respect of the same principle of polarity.

The attached file contains the polarity index of solvents

For sequential extraction you take in consideration two things:

1) The choice of solvents: look for solvents (4 to 5 solvents) with different polarity index (don't choose solvents with close polarity index). Example: Hexane(apolar)/Chloroforme(4.1)/ethanol (5.2)/Water (9.0)

2) You can use the same plant material with all solvents. You start with the apolar and you finish with the more polar (for the cited example: hexane>Chloroforme>ethanol>water). But I prefer to realize extraction with each solvent using new plant material with respect of the same principle of polarity.

The attached file contains the polarity index of solvents

To evaluate the antimicrobial activity of plants which method is best for sequential extraction (Soxhlet unit or Centrifugation)? please provide protocol for both the methods.

For sequential extraction of plants, the best is Soxhlet because your plant material is not present with your solvent, so you can change the solvent without changing the plant material. (many factors can affect the extraction efficiency: the plant material, solvents, temperature, ...).

The centrifugation -especially for plants extracts- is used as complementary to other technique for maximum extract and you can use the plant material for next extraction with next solvent. The centrifugation can be used -for example- if you realize the heat reflux extraction or other method in which your solvent is present with the plant material.

The attached figure represents the soxhlet work principle.

helpful discussion

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Noureddine Boulenouar Dear Sir, I would like to request further clarification of what you said:

"But I prefer to realize extraction with each solvent using new plant material with respect of the same principle of polarity."

Do you mean that you prefer using a fresh plant material fo another/different solvent and not the one already extracted with a less polar solvent?