Potassium ferrocyanide k4

Potassium ferricyanide K 3 [Fe(CN) 6 ], potassium ferrocyanide K 4 [Fe(CN) 6 ], 2-(Nmorpholino)-ethanesulfonic acid (MES), potassium dihydrogen phosphate, sodium monophosphate, methanol, perchloric acid, hydrochloride acid, N-hydroxysulfosuccinimide (sulfo-NHS), N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) and sodium nitrite were purchased from Sigma (St. Louis, MO, USA). 4-aminobenzoic acid (ABA) and DL-lysine monohydrochloride were provided by Acros (Geel, Belgium). 4carboxybenzo-18-crown-6 with a purity of 99% and 4-carboxybenzo-15-crown-5 with purity greater than 98% were provided by Acros and Sigma respectively. All other reagents used were from Merck (Darmstadt, Germany) and Fluka (Buchs, Switzerland). All reagents were of analytical grade. Pb(II), Cd(II) and Cu(II) stock solutions 10 -2 mol L -1 were prepared from Pb(NO 3 ) 2 •4H 2 O, Cd(NO 3 ) 2 •4H 2 O and Cu(NO 3 ) 2 •3H 2 O respectively and standardized complexometrically [18] . 0.1 mol L -1 acetic acid / acetate buffer solution (pH 4.5) was used for pH control. Ultrapure water from MilliQ System (Millipore, Billerica, MA, USA) was used in all experiments.