At the current work, the substrate phenyl hydrazine reacted with selected thiophene-2-carbaldehyde in absolute ethanol and a few drops of glacial acetic acid to give 1-phenyl-2-(thiophen-2-ylmethylene)hydrazine (PTMH) as a selective and sensitive reagent. It was synthesized and studied to determine cobalt spectroscopy in the following detail:  at a pH value of 6.0, which greatly increased the selectivity, Co(II) reacted with (PTMH) to form a 1:2 complexes, with absorption peaks at 386 nm for cobalt. Under the optimal conditions, Beers law was obeyed over the ranges of (0.14ـــ5.8) mg/L. The apparent molar absorptivity and Sandell’s  sensitivities were 7.1360×104 L.mol-1.cm-1, and 8.2585×10-4 µg.cm-2. The detection limits were found to be 0.0133 and the limit of quantification is 0.0466.

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