As always when working with chemicals, wear a ventilation mask, protective goggles, long-sleeved clothing, and rubber gloves. Since the toilet bowl already has water in it, it doesn’t need to be diluted. Add a few tablespoons to a cup of muriatic acid directly into the toilet bowl. Let it sit for 10-20 minutes. Flush the toilet three times to ensure the diluted acid gets into the pipes, so you can clean them.
It has been rumored that muriatic acid is strong enough to clear up a clog in a pipe or toilet. This is a myth. The problem isn’t the toilet or the pipes, it what is clogged in the pipe to begin with. If a homeowner’s water supply has high levels of lime built up in drain lines, any contact with muriatic acid could cause an explosive reaction.
If you decide to clean your toilet with muriatic acid, make sure to choose a brush that won’t get eaten away. Be sure you are away of the pipes you’re dealing with before you go this route—you don’t want to cause further problems down the road.
So, mystery solved: muriatic acid is okay (if used correctly) for cleaning a toilet, but much better or cleaning concrete and masonry.