Non-toxic to the environment and to humans, citric acid effectively removes hard water stains and long-lasting residues. Size: 700 g. Uses: For descaling, dilute 30 ml (2 tbsp) of citric acid in 1 L of water and let stand 15 to 30 minutes. Rinse well. For toilet bowl stains, add 3 tbsp of citric acid in toilet bowl and let sit overnight. Scrub the following day.