3.1. Appearance of Analysis
The reference and sample were visually assessed in a horizontal direction in the light box with a white background. If the system suitability criteria are met (i.e., the solution color must be such that purified water is colorless) and the reference color solutions have detectable color and can readily be distinguished in the relative color intensities, we proceeded with the sample analysis. With the prolongation of heat treatment time, changes to the appearance were noted (Table 1).
Table 1. Analysis of sample appearance.