EXTRACTION-SPECTROPHOTOMETRIC DETERMINATI ON OF TRACE NICKEL WITH 5-NITRO-2-PYRIDINECARBALDEHYDE 5-NITRO-2-PYRIDYLHYDRAZONE

5-Nitro-2-pyridinecarbaldehyde 5-nitro-2-pyridylhydrazone (5-NPA-5-NPH ), in which an electron-withdrawing nitro group was introduced to tile 5-position of the pyridine ring in both the hydrazine and aldehyde mo iety of 2-pyridinecarbaldehyde 2-pyridylhydrazone (PAPH), was found to be most sensitive for the determination of metal ions. In particular, 5-NPA-5-NPH forms a stable 1 : 2 (metal:ligand) complex with nickel ( II), which is quantitatively extracted from aqueous solutions of pH 5. 5 similar to 10.0 into benzene, the extract having an absorption maxim um at 561 nm. The calibration graph prepared by the proposed procedure was linear over the range 0.02 similar to 0.5 mu g cm(-1) of nickel a nd passed through the origin. The apparent molar absorptivity is 1.35 X 10(5) dm(3) mol(-1) cm(-1). A highly sensitive extraction-spectropho tometric method for the determination of nickel with 5-NPA-5-NPH is pr oposed and has been successfully applied to the determination of nicke l in a tungsten steel sample.