Step 1: Prepare standard of different known lignin concentration. (For eg. 0.1mg, 0.2mg, 0.3 mg, 0.4mg, 0.5mg);
Step 2: Prepare a calibration curve with the help of above known concentration of solution using absorbance at the 280 nm of UV/Vis spectrometer.
Step 3: Plot the concentration of Unknown lignin solution (i.e. your sample) on calibration curve and estimate the lignin concentration.