Ion-Pair Extraction of Metalloporphyrins into Acetonitrile for Determination of Copper(II)

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Abstract
Equilibrium study of ion-pair extraction of a cationic water-soluble porphyrin [5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H2tmpyp4+] and its metalloporphyrins (MP) into the acetonitrile layer, separated by addition of sodium chloride (4.00 mol dm-3) to a 1:1 (v/v) acetonitrile−water mixed solvent, was carried out to develop a new and useful method for the determination of a subnanogram amount of copper(II). M denotes Zn2+, Cu2+, Co3+, Fe3+, and Mn3+, and P2- is porphyrinate ion. The extraction and dissociation constants of the ion-pair complexes, defined by Kex = [MP(ClO4)4]org[MP4+]aq-1[ClO4-]aq-4, Kdis,1 = [MP(ClO4)3+]org[ClO4-]org[MP(ClO4)4]org-1, and Kdis,2 = [MP(ClO4)22+]org[ClO4-]org[MP(ClO4)3+]org-1, were determined by taking into account the partition constant of sodium perchlorate (KD = 1.82 ± 0.01). The equilibrium constants were found to be KexKdis,1 = (7.2 ± 1.3) × 104, (6.4 ± 0.9) × 104, (1.35 ± 0.13) × 105, (4.8 ± 0.6) × 103, (1.23 ± 0.05) × 104, and (1.42 ± 0.07) × 103 at 25 °C for the free base porphyrin (H2tmpyp4+) and the metalloporphyrins of zinc(II), copper(II), cobalt(III), iron(III), and manganese(III), respectively. The Kdis,2 values were (2.9 ± 1.4) × 10-2, (3.1 ± 1.1) × 10-2, (8.0 ± 4.9) × 10-3, and (5.1 ± 2.2) × 10-2 for the free base porphyrins and the metalloporphyrins of zinc(II), copper(II), and cobalt(III), respectively. The results were developed for determination of a trace amount of copper(II) (3 × 10-8−4 × 10-6 mol dm-3) in natural water samples using H2tmpyp4+ with a molar absorptivity of 3.1 × 105 mol-1 dm3 cm-1 at a precision of 1.3% (RSD). The determination of copper(II) was not interfered by the presence of 10-4 mol dm-3 of Mn2+, Co2+, Ni2+, Hg2+, Cd2+, Ag+, Cr3+, V5+, Al3+, Mg2+, Ca2+, Br-, I-, SCN-, and S2O32- and 10-5 mol dm-3 of Fe3+, Zn2+, and Pd2+.