Abstract: The reversible  detection for hydrazine hydrate (N₂H₄·H₂O) and hydrogen peroxide (H₂O₂) in P₂Mo₁₈ solution could be realized by ultraviolet-visible (UV-Vis) spectrum based on the reversible redox and allochroic properties of Dawson type phosphomolybdate (NH₄)₆P₂Mo₁₈O₆₂（P₂Mo₁₈）. After adding the same  amount of N₂H₄·H₂O  to P₂Mo₁₈ solution, the P₂Mo₁₈ was reduced,  and the solution changed from light yellow to blue-green. The absorption peak in the visible region of 400—800 nm gradually increaseed with the increase of N₂H₄·H₂O concentration. After adding the same amount of H₂O₂  to the reduced P₂Mo₁₈ solution, the reduced P₂Mo₁₈ was oxidized again and the solution changed from blue-green to light yellow. The absorption peak in the visible region gradually decreased with the increase of H₂O₂ concentration. The linear equation, linear detection range and detection limit of N₂H₄·H₂O and H₂O₂ were obtained by drawing concentrations of N₂H₄·H₂O and H₂O₂ with the absorbance at 750 nm. The response time, reversibility and anti-interference of the probe were studied by UV-Vis kinetic method  and spectral method. The results show that the detection limits for N₂H₄·H₂O and H₂O₂ are 7.24×10^-5， 2.15×10^-4 mmol/L,  the response time for N₂H₄·H₂O  and H₂O₂ detection are 44.8,23.9 min, respectively.

Key words:  , polyoxometalate； hydrazine hydrate； stimuli-response； reversible allochroic property； spectral detection