Tile is a popular finish material in kitchens and bathrooms. If the grout isn't completely wiped down, however, stains can remain on the tile after installation. While dried-on grout is difficult to remove, a careful application of muriatic acid will keep your tile grout looking new. Muriatic acid, also known as hydrochloric acid, is a harmful, strong chemical compound but perfect for removing stains from grout when used properly.
Fill a bucket with 1 gallon of clean water. Slowly add 1cup of muriatic acid into the water. Do not pour acid into the bucket and add water afterward, as it can create a dangerous chemical reaction. There is truth to the saying, "Do as you oughta, pour acid into water."
Apply the acid solution to the grout with the paintbrush. Allow the solution to sit for some time on the grout until the grout begins to become clean. (This time is typically measured in seconds or minutes, not hours, so don't leave the reaction unattended.)
Thoroughly rinse off the cleaning solution and apply an ammonia solution to neutralize the remaining acidity in the grout. The ammonia solution is made of 1 cup of ammonia and 1 gallon of water. Thoroughly rinse the grout with clean water after applying the ammonia solution.