Abstract
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 a reagent blank. Beer's law holds over the concentration ranges of 3.5 – 95.0 μg, 3.0 – 0.105.0 μg, and 8.5 – 125.0 μg for zinc, cadmium, and lead, respectively, in 10 ml of the chloroform solution. The molar absorptivities are calculated to be Zn: 1.048 × 10 4 liters mol −1 cm −1; Cd: 1.054 × 10 4 liters mol −1 cm −1; and Pb: 1.014 × 10 4 liters mol −1 cm −1 with sensitivities in terms of Sandell's definition of 0.0062 μg Zn/cm 2, 0.010 μg Cd/cm 2, and 0.020 μg Pb/ cm 2, respectively. Ten replicate determinations of sample solutions containing 30 μg of zince, 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.
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