The decreasing trend is similar to that of the absorbance change depicted in Fig. 2(b). In Fig. 4(c), we plotted the color intensity as a function of the absorbance for each of the standard solutions tested; the linear relationship (except for the blank) shown here illustrates the direct correlation between color intensity and absorbance data. The akin color intensity and absorbance results not only confirm the accuracy of the colorimetric protocol but also constitute a validated calibration curve (Fig. 4b) for the analysis of unknown samples.