Also known as hydrochloric acid, muriatic acid is a chemical frequently used to etch concrete, clean brick or metal and adjust the pH of swimming pools. Most toilet bowl cleaners contain muriatic acid as the active cleaning ingredient to eat away at stains and scum that, over time, build up within your toilet bowl. Muriatic acid isn't appropriate for every toilet as toilets are made of porcelain and the chemical is particularly harsh on porcelain surfaces.
Purchase a brand of toilet bowl cleaner, such as Lysol, that contains muriatic acid as the active ingredient. It will be listed as hydrochloric, rather than muriatic, on the bottle.
Open a window. Muriatic acid gives off dangerous fumes that you should avoid inhaling, if possible.
Put on protective eyewear and gloves. Muriatic acid is corrosive and will burn you if it makes contact with your skin.
Lower the toilet bowl's water level by turning off the water to the toilet and flushing it. Your cleaner is already diluted to the proper level. This prevents water within the toilet from diluting the muriatic acid even further.
Apply an ample amount of toilet bowl cleaner to the toilet brush. Scrub around the rim of the toilet with the brush, working your way down into the bowl.
Apply the muriatic acid-based cleaner directly to stubborn stains and allow it to sit for a minute or two before scrubbing again with the toilet brush.
Turn the water to the toilet back on and flush the toilet several times to clear away any residual acid-based cleanser.
A pumice stone works well for scrubbing away stubborn toilet bowl stains and won't scratch the toilet bowl's porcelain interior.
Do not use undiluted muriatic acid to clean your toilets. Not only is this dangerous for your health, it can damage your toilet bowl and pipes. Muriatic acid will damage nylon carpet. If you have nylon carpet in your bathroom, cover the surfaces around the toilet with a rubber mat before you begin the cleaning process.