Muriatic acid, also known as hydrochloric acid, is a very strong chemical that can remove things from stone, such as efflorescence (salt buildup) or iron oxide rust staining. While it works incredibly well on stains, it is very dangerous to work with, and in general, it's best to try other, safer products first, such as vinegar or phosphoric acid. When working with muriatic acid, be very careful to prevent damage to the environment or your body.
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Prepare to Handle the Product
Only work with muriatic acid outdoors. Put on proper safety gear, including long sleeves, pants, closed-toe shoes, safety goggles, gloves, and a respirator. Water down plants in the immediate area in case you spill acid on them and have a neutralizing agent on hand, such as baking soda or garden lime, so you can quickly neutralize the acid if necessary. To be ready to neutralize the acid when you are finished cleaning your stone, prepare a solution of 1 cup ammonia with 1 gallon of water and put it in a sprayer.
Dilute the Acid
Muriatic acid needs to be diluted before use. While the most common formulation consists of 31.45 percent acid and 68.55 percent inert ingredients and needs to be diluted with 10 parts water to 1 part muriatic acid, check the instructions on the product label for the proper dilution ratio.
In order to safely dilute this strong acid, pour water in a plastic or glass container and then add the acid. This will reduce the likelihood of splashing muriatic acid on your skin. Additionally, because muriatic acid heats up quickly when exposed to water, adding water to acid will result in the hot acid being trapped under cold water, which can cause the dangerous acid to explode out of the container, whereas when the acid is slowly poured into the cold water, it will cool slowly so it can be mixed safely.
In addition to never adding water to muriatic acid, never add anything else to your cleaning solution or a dangerous reaction may occur.
Apply the Muriatic Acid
You can use a sprayer or brush to apply muriatic acid depending on what will work better for you. If using a brush, use a long-handled brush to ensure the solution does not splash on you during use. Because muriatic acid damages metal, avoid using metal sprayers or brushes. Be aware that the strong acid can destroy even plastic sprayers in less than an hour, so for bigger projects, you may need to have multiple cheap plastic sprayers ready to go.
Before applying acid, wet the stone with a garden hose. Apply the acid with your sprayer or brush and then leave it on the surface between five and eight minutes before rinsing it away using a garden hose. Repeat as necessary.
When the stone is fully cleaned, spray the diluted ammonia solution over the surface to neutralize the acid.
Dispose of the Acid
Muriatic acid should never be flushed down a toilet, thrown in the trash, or poured in a storm drain. Instead, you can either dispose of it by contacting your local recycling center to see if it has a hazardous chemical drop-off site or neutralize the acid by filling a 5-gallon bucket with 4 cups of lime and a gallon of water before slowly adding the acid. Stir the solution carefully until all fizzing has stopped. Once it has been properly neutralized, the acid is no longer dangerous, and the solution can safely be poured down the sink.