Ventilate the room. When working indoors, open all windows and use a fan to help circulate the air and blow fumes away from your work area.
Fill a 5-gallon bucket with 1 gallon of water. Put the water into the bucket before the acid to avoid a dangerous chemical reaction.
Slowly pour 1 cup of muriatic acid into the bucket of water. Use a plastic or glass container to measure the acid; never use a metal container. Using a metal container can cause a chemical reaction that results in the acid fizzing and giving off dangerous fumes.
Brush the acid mixture onto the stained tiles using a large paintbrush. If any of the acid mixture spills onto another surface, immediately pour baking soda onto the spill to neutralize the acid.
Allow the muriatic acid and water mixture to sit on the tiles for two to three minutes.
Rinse the acid off the tiles with plenty of water. Use a stiff brush to scrub the tiles and ensure that all of the muriatic acid residue has been removed.