It was crucial to maintain the sample and smartphone in the same positions during all image acquisitions (Figure ). Each group used one smartphone and maintained consistent camera settings throughout the entire process. The average R-values for each image were converted to absorbance using Equation . Subsequently, the average R-value of each standard solution was plotted against the 160 concentration of copper(II) ions (in ppm). A linear trendline was generated to produce the calibration curve, as shown in Figure . Additionally, a graph of absorbance versus copper(II) concentration (in ppm) was produced.