Nitrate is a crucial parameter for assessing water quality, owing to its dual function in ecological systems. These functions can be beneficial or detrimental depending on whether nitrate concentrations are low or high, respectively. The ultraviolet spectrophotometric method (standard as 4500-NO[subscript 3]--B) is a classic method for determining nitrate concentration in natural water. It employs the formula A[subscript correction] = A[subscript 220 nm] -- 2A[subscript 275 nm], where the absorption of nitrate at 220 nm is corrected by the removal of the non-negligible interference emanating from dissolved organic matter. The simple operation and high accuracy of this standard method on uncontaminated natural water samples render it suitable for routine experiments in undergraduate Environmental Science education. Although undergraduate teaching effectively conveys the methodology, the rationale behind the inclusion of the constant "2" in the formula is rarely discussed and remains unclear. Herein, an experiment was designed for sophomore students to enhance their practical skills and to cultivate their analytical thinking. Throughout the experiment, students were guided to explore the origin and reliability of the constant "2". Additionally, by linking the reliability of the method to the absorbance ratio at 275 nm to that at 220 nm (A[subscript 275 nm]/A[subscript 220 nm]), a "rough estimate" strategy was uncovered. Students utilized this strategy to determine the potential contamination of water samples and then assess the necessity for preliminary treatment of the samples. The experiment yielded profound engagement from students, fostering a deep understanding of the underlying principles of the routine experiment. Additionally, their capacity to derive accurate conclusions and tackle real-world problems demonstrated remarkable progress. Encouragingly, the constructive feedback received from these students confirms the successful achievement of the intended learning objectives.