Presence of Masking (Sequestering ) Agents:

Masking agents also known as sequestering agents are themselves complex formers not necessarily forming chelates. They are mainly used to prevent particular metals from taking part in their usual reactions and therefore, the interference of the undesirable elements is removed without the actual separation step.

The masked metal forms a water soluble complex most often negatively charged. In solvent extraction, the masking agents are used to prevent certain metal ions from forming extractable complexes and thus, they increase the selectivity. The use of masking agents like cyanide, tartarate, citrate, fluoride and EDTA is restricted largely to metal chelate extraction systems. In highly acidic solutions encountered in many extraction systems, most of the masking agents do not function effectively.

The masking agent forms sufficiently strong complexes with the interfering metals to prevent their extraction either altogether or at least until the pH is much higher than the value needed for the quantitative extraction of the metal ion of interest. It should be known that very often the desired metal of interest also forms a complex with the masking agent and a much higher pH is needed for its extraction. In some cases, the rate of formation of chelates has been slowed down by the presence of masking agents like EDTA; thereby, reducing the rate of extraction.

After having known some of the important variables of the aqueous phase, we now discuss two important variables of the organic phase.