“Analysis of Water Hardness in Lapindo Mud Sources: Identification of Ca²⁺ and Mg²⁺ using the EDTA Method as a Science Learning Medium”, JPSE, vol. 1, no. 1, pp. 52–59, Nov. 2025, Accessed: Jul. 28, 2026. [Online]. Available: https://journal.kopialinea.com/index.php/JPSE/article/view/12