1-Butyl-3-methylimidazolium hexafluorophosphate ([C4MIM] [PF6]) was purchased from Merck (Darmstadt, Germany).
Phosphate buffer (pH 7.0) used for pH adjustment, was prepared
with the addition of appropriate amount sodium dihydrogen phosphate and disodium hydrogen phosphate.
A certified reference
material CRM NIST 1515 Apple Leaves (National Institute for
Standards and Technology, Gaithersburg, MD, USA) was used.
Working standards of the corresponding metal ions were prepared by dilution of the stock standard solutions with distilled water.