The color intensity of each solution is represented by its absorbance, which is plotted on the vertical axis of the calibration curve. Note that the absorbance increases from 0 to about 0.72 as the concentration arsenic increases from 0 to 25 ppm. The concentration of arsenic in each standard solution corresponds to the verti grid lines of the calibration curve. This curve is then used to determine the concentration of the two unknown solutions shown on the right. We first find the absor bances of the unknowns on the absorbance axis of the plot and then read the corresponding concentrations on the concentration axis. The lines leading from the cuvettes to the calibration curve show that the concentrations of arsenic in the two deer were 16 ppm and 22 ppm respectively. Arsenic in the kidney tissue of an animal is toxic at lev els above about 10 ppm, so it was probable that the deel were killed by ingesting an arsenic compound.