Abstract
A simple, sensitive and selective spectrophotometric method for the determination of zinc with 1-(2-pyridylazo)-2-naphthol, PAN, and surfactant was established. The sensitivity was found to be 0.0011 μg zinc/cm2 for the absorbance of 0.001 (molar extinction coefficient, 5.6×104) at 555 nm, which was superior to that of the dithizone or the zincon method currently used for the analysis of zinc in water.
Zinc reacts with PAN in an alkaline medium to form a water-insoluble and red-colored complex, which can be solubilized into water by a non-ionic surfactant such as Triton X-100. The absorbance of the solubilized complex is constant at pH 8.09.5. The calibration curve obeys Beer's law over the concentration range from 0 to 100 μg zinc/50 ml. The following ions interfere significantly with the determination of Zn; Mn(II), Fe(II, III), Co(II), Cu(II), Cd(II) and Ni(II). The interferences from Mn(II) and Fe(III) are eliminated in the presence of sodium citrate and sodium metaphosphate as masking agents and the other metals except Ni are effectively masked by the addition of N-(dithiocarboxy)-glycine prior to the color development. Though Ni(II) remains unmasked under the conditions described above, it is possible to correct the absorbance due to Ni-PAN complex by the use of EDTA. Adding EDTA, the Zn-PAN complex was rapidly and quantitatively decomposed, while that of nickel remains unchanged. The difference of the absorbance before and after adding EDTA is due to the Zn-PAN complex.
Thus the recommended procedure is as follows; (I). Transfer test solution containing less than 100 μg of zinc to a 50 ml volumetric flask. Add 1 ml of 5% sodium citrate solution, 1 ml of 5% sodium metaphosphate solution and about 10 mg of ammonium persulfate successively. Adjust the pH to 8.5 with an ammonium buffer solution, then add 2 ml of 20% Triton X-100 solution. Then, add about 50 mg of N-(dithiocarboxy)-glycine and finally 2 ml of 0.1 % alcoholic PAN solution. Dilute to the mark with water and shake the solution vigorously. (II). Add 5 ml of 0.05M EDTA solution to another solution treated as the above procedure (I) through the addition of PAN and dilute to the mark. Measure the absorbance of the solution (I) at 555 nm against the solution (II).
