This study aims to develop a low-cost, eco-friendly, and sensitive method for detecting trace phosphate in water by improving the conventional phosphomolybdenum blue (PMB) method. Water quality in Hsinchu, Taiwan, is often affected by upstream pollution and eutrophication, yet conventional phosphate detection methods remain costly and environmentally harmful due to high reagent consumption and the use of toxic antimony ions. To address these limitations, the research systematically optimized reagent ratios through a nine-grid experimental design, varying ammonium molybdate and vitamin C concentrations while controlling acidity with sulfanilic acid instead of strong mineral acids. The optical absorbance between 600 and 1000 nm was analyzed using a spectrophotometer to distinguish the characteristic peaks of molybdenum blue (MB) and phosphomolybdenum blue (PMB). The study successfully established reaction conditions that suppressed MB interference and shortened reaction time, resulting in a reliable calibration curve at 810 nm for phosphate concentrations between 0.045 and 0.101 ppm. Compared with the traditional PMB method, the improved process reduced chemical pollution by approximately 45 times, eliminated antimony use, and retained strong colorimetric sensitivity suitable for semi-quantitative field detection. When applied to municipal tap water across several Hsinchu districts, the method revealed consistent phosphate levels with clear visual color discrimination, confirming its applicability for rapid community water monitoring. The approach was also tested on household shampoos and adapted into alginate-based “phosphate detection beads” capable of visually indicating phosphate presence in river samples. Spectral analysis verified that moderate acidity and optimal reagent ratios (10⁻² M molybdate, 5×10⁻⁴ M ascorbic acid) enhanced PMB selectivity and minimized MB interference. The study concludes that this simplified, antimony-free protocol achieves comparable accuracy to standard spectrophotometric methods while substantially lowering environmental and economic costs. Its portability, visual readability, and educational potential make it a promising tool for citizen science, environmental education, and sustainable water-quality management.