Reagent A Hardness Calcium 800 comes in a 0.75 oz bottle with a drop dispenser tip designed for accurate hardness testing. It reacts with diethyl-p-phenylene diamine (DPD) in a buffered solution allowing for precise measurement using a colorimeter or photometer. For optimal performance, store reagents consistently between 36°F and 85°F (2°C and 29°C), avoid extreme temperature fluctuations, and protect them from prolonged direct sunlight.