Abstract: UV-vis spectra of henna aqueous extract and lawsone solution of different pH mediums were analyzed to select the detection wavelength and the reference solution for determining lawsone content in henna powder by spectrophotometer.Selecting 453 nm as the detection wavelength,with henna powder extract in hydrochloric acid of pH value 2 as the reference solution,the absorbance value of the testing solution samples in NaOH solution of pH=12 as the working medium,the lawsone mass concentration in the henna powder sample was quantified based upon comparison with the standard calibration curve.The results showed that within the lawsone mass concentration range of 10~140 mg/L,the absorbance follows Lambert-Beers law;the linear correlation coefficient is r=0.999 95 and the precision RSD is less than 0.47%.The absorbancevalue of the sample solution can keep stable within 14 h,while the lawsone spiked recoveries achieves 99.0%~100.0%.The method is simple,reliable,and can be used to detect the lawsone content in henna powder.

Key words: henna powder, lawsone, UV-vis spectrum