Spectrophotometric determination of zinc, cadmium, and lead after extraction of their morpholine-4-carbodithioates into molten naphthalene and replacement by copper

Zinc, cadmium, and lead react quantitatively in the pH ranges of 3.9–9.2, 3.5–11.2, and 5.5–10.5, respectively, to form water insoluble and thermally stable complexes which are easily extracted into molten naphthalene. The solid naphthalene containing the colorless complex is dissolved in chloroform and then replaced by copper to develop a yellow color in the chloroform layer. The absorbance in each case is measured at 435 nm against reagent blank. Beer's law holds over the concentration ranges of 3.5–95.0, 3.0–105.0, and 8.5–125. 0 μg for zinc, cadmium, and lead, respectively, into 10 ml of the chloroform solution. The molar absorptivities are calculated to be Zn, 1.048 × 10⁴ liters mol⁻¹ cm⁻¹; Cd, 1.054 × 10⁴ liters mol⁻¹ cm⁻¹, and Pb, 1.014 × 10⁴ liters mol⁻¹ cm⁻¹ with sensitivities in terms of Sandell's definition of 0.0062 μg Zn/cm², 0.010 μg Cd/cm², and 0.020 μg Pb/cm², respectively. Ten replicate determinations of sample solutions containing 30 μg of zinc, 18.7 μg of cadmium, and 42.5 μg of lead give mean absorbances 0.480, 0.175, and 0.208 with standard deviations of 0.0017, 0.0013, and 0.0015 or relative standard deviations of 0.35, 0.74, and 0.72%, respectively. The interference of various ions has been studied and the method has been applied to the determination of cadmium in various synthetic mixtures and zinc and lead in some standard reference materials.