N'-(1-(Pyridin-2-yl)ethylidene)isonicotinohydrazide (2-ACPINH) is identified as a selective and sensitive analytical reagent for the determination of trace levels of me rcury (II) in environmental samples and synthetic mixtures by spectrophotometry. 2-ACPINH reacts with mercury (II) forms greenish yellow colored 2:3 (Hg-2-ACPINH) complex at pH 5.5, in aqueous DMF. The Hg-2-ACPINH complex shows maximum absorbance at λ_max 357nm. The color reaction is immediate and the absorbance remains constant for about 24h.The molar absorptivity coefficient and Sandell’s sensitivity are found to be 5.48x10⁴L.mol^-1cm^-1 and 0.0010μg cm^-2 of mercury (II) respectively. The linear regression coefficient is 0.99925 and standard deviation is 0.04175. The system obeys Beer`s law in the range of 0.2-2.0 μg mL^-1 of Hg (II). Large number of cations, anions and complexing agents do not interfere in this method. This method is successfully applied for the determination of mercury in a number of environmental water samples (potable and polluted), food samples and synthetic mixtures.