Abstract：2-hydroxy-4-methoxybenzo phenone (UV-9 ) is one of general auxiliary reagents used in paint resin and plastics, which can delay the material degradation and ageing of high polymer. Direct determination of high polymer by ultraviolet spectrum has not been reported. We selected an efficient solvent that can dissolve the resin of high polymer, and got an even，transparent and steady mensuration system. The first derivative absorption data of 2-hydroxy-4-methoxybenzo phenone(UV-9) were treated many times with smooth handing by Origin 6.1，a famous data analysis software. At a certain wavelength, the results accord with Beer′s law from 1.000 to 20.00 μg·mL-1, thus UV-9 can be measured directly. Its absorptivity is 5.7×104 L·mol-1·cm-1. The results of synthetic samples determined by this method showed that the relative standard deviation and the relative error of UV-9 are less than 5.25% and 4.75% respectively，and the UV-9 recovery rate is 98.88%-102.04%.
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