Composition of the aqueous phase:

The composition of the aqueous phase to a considerable extent affects the extraction of metal by carboxylic and sulphonic acids. As far as pH is concerned, there is no qualitative difference. Metals are preferentially extracted from aqueous solutions containing alkali. Sometimes, there is no extraction in its absence. An alternative way to increase the distribution is to use sodium salt of fatty acid instead of free acids.

Carboxylic acids react with a number of metal ions and the extraction can be made selective by adjusting the pH. Moreover, in the metal carboxylate extraction, the effect of electronegativity is so prominent that a cation exchange takes place.

M₁X_m(o) + M ^m+_2(a) ↔M₂X_m(o) + M ^m+_1(a)

The less electronegative element sodium is quantitatively exchanged in the organic phase by any more electronegative element say cobalt. Cobalt can be exchanged by Cu and Cu for Fe. Naphthenic acid extracts metal with increasing pH in the order Fe³⁺ < Cu²⁺ < In²⁺ < Co²⁺ < Fe²⁺ < Mn²⁺ over pH 1-10. Thus, a metal in the extraction series can be displaced from the organic phase by a metal to its left in that series.

2Fe ³⁺ + 3CuA_2(o)        ↔ 2FeA_3(o)  + 3Cu ²⁺

The use of carboxylic acid as metal extractants for commercial purposes is limited because of extractants loss, emulsion formation tendency and slow kinetics of the extraction.