Milli q analytical reagent grade water purification system

All reagents were of analytical grade. Nitric acid (HNO₃, ultrapure grade, 60% w/w, Merck, Darmstadt, Germany) and supra-pure hydrogen peroxide (H₂O₂, supra-pure, 30% w/w, Supelco, Sigma Aldrich, Darmstadt, Germany) were used for the sample digestion. Ultra-pure deionized water (18 MΩcm⁻¹) from a Milli-Q analytical reagent-grade water purification system (Millipore, Darmstadt, Germany) was used for preparing all reagents and standard solutions. High-purity ICP Multielement Standard Solution XXI CertiPUR obtained from Merck (Darmstadt, Germany) was used for the calibration curve in the quantitative analysis, 10 mg/L Al, As, Cd, Co, Cr, Cu, Mg, Mn, Na, Ni, Pb, and Zn. For Hg, Mercury Standard for ICP Perkin Elmer 10 mg/L Hg in nitric acid (Atomic Spectroscopy Standard) was used. All plastic wares were cleaned by soaking for 24 h in 10% HNO₃, followed by washing with ultrapure water. Plastic containers, pipettes, and reagents in contact with the samples or standards were randomly checked for contamination. The gaseous argon used to form the plasma in the ICP-MS was of purity 6.0.

All chemicals used were reagent grade unless otherwise stated. Aqueous nickel solutions were prepared from a commercial standard solution of 1000 mg·L⁻¹ from Merck (Darmstadt, Germany). Nitric acid (65%), sodium chloride, sulfuric acid (95–97%), sodium hydroxide and toluene were obtained from Scharlab (Barcelona, Spain). Buffer solutions were prepared using N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES) from Biochemical (Barcelona, Spain). N,N-Dimethylformamide (DMF), ethanol (96%) and perchloric acid (60%) were obtained from Merck (Darmstadt, Germany). Hydrochloric acid was purchased from JT Baker (Phillipsburg, NJ, USA). Humic acid sodium salt was obtained from Aldrich (Steinheim, Germany). The active component of the liquid membrane, 1,2-cyclohexanedione bis-benzoylhydrazone (1,2-CHBBH), was synthesised as previously described [33 (link)]. All solutions were prepared using deionised water (18 MΩ·cm⁻¹) from a Milli-Q analytical reagent grade water purification system (Millipore, Bedford, MA, USA).
Solutions used as a source phase in optimization experiments consisted of 1 mg·L⁻¹ Ni(II) and 35 g·L⁻¹ NaCl. These solutions were adjusted to pH 8 using HEPES and sodium hydroxide. Appropriate amounts of sodium chloride and Humic acid sodium salt were added to source solutions, to investigate the effects of organic and inorganic ligands.