X1 was the absorbanceof all DCPI, and X2 was the absorbance value of the remaining DCPI,
after its reaction with AA. X2 values were similarly recorded for the
other standard AA solutions (20, 30, 40, and 50 ppm). X1 X2 values
were the absorbance of each working standard. The calibration
graph was constructed by plotting the absorbance values versus
the concentration (ppm) of standard AA solutions (r2 = 0.9951)