1979 Volume 1979 Issue 5 Pages 602-606
A highly sensitive spectrophotometric method for Zn2+ has been established by using α, β, γ, δ - tetrakis(1-methylpyridinium-4-yl)porphine (TMPyP, H2L4+). The complex, ZnL4+, is formed within 1 min at room temperature in the pH range of 9.2 to 10.8. The Soret band of ZnL4+ (at 437.2 nm, ε = 2.28 X 105 l. mol-1. cm-1) seriously overlaps with that of the reagent (H2L4+, at 422.8 nm). However, the reagent blank can be successfully decreased by converting H2L4+ to its lead complex (PbL4+, at 476.4 nm). Procedure: take a sample solution containing up to 6 μ g of Zn2+ into a 25 cm3 volumetric flask, add 1 cm3 of 10-4 mol. dm-3 TMPyP solution and 5 cm3 of borate-sodium hydroxide solution (pH 9.2-10.8). Allow to stand for 5 min. Add 2 cm3 of 10-4 mol dm-3 Pb2+ solution and dilute to the mark with water. After 2 min, measure the absorbance at 437 nm against water within 2 hr. Beer's law is obeyed over 0-6 μ g Zn2+/ 25 cm3 and the sensitivity for absorbance O.001 is as low as 3.66 x 10-4 μ g Zn2+ cm-2, which is about 4.4 times as sensitive as that of the dithizone method. The coefficient of variation is O.97% for 4.17 μ g of Zn2+(ten determinations). Many metal ions interfere with the determination. However, Zn2+ can be selectively separated from these ions by dithizonate extraction in the presence of masking agents (bis (2-hydroxyethyl) dithiocarbamate for Cd2+, citrate for Fe2+ and Fe3+ and thiocyanate for Cu2+ Ni2+ and Pb2+), followed by back-extraction with 0.1 mol. dm-8 HC1 solution. Results are quoted for the application to the determination of Zn2+in tap water.
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