fraction extracted, E

The fraction of the total quantity of a substance extracted (usually by the solvent) under specified conditions, i.e. $E A = Q A Q A '$ where $Q A$ is the mass of A extracted and $Q A '$ is the total mass of A present at the start.

Notes:

$E$ may be expressed as a percentage, $% E$ .
The term extractability is qualitative and should not be used as a synonym for fraction extracted.
If the aqueous phase is extracted with $n$ successive portions of solvent, the phase volume ratio (solvent/feed) being $r$ each time, the fraction extracted is given by:
$E n = 1 − ( r D + 1 ) − n$
If $n = r = 1$ and $E 1 = D 1 + D$ this expression is a concept of value in chromatography theory.
The fraction extracted is also known as the recovery factor, especially for a multistage process.

Source:

PAC, 1993, 65, 2373 (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2384