I'm with a project using arduino, and I need to measure the hardness of water. I've been online and I can´t find anything. Besides the unique processes for measuring hardnessi s getting a little water and do it manually, but I need something that arduino can do alone.
I think you need a sensor called a calcium-ion selective electrode. They are not cheap. You may also need to measure magnesium concentration, but googling suggests the hardware to do this is more research than product....