How to Use Sulfuric Acid for Drain Cleaning

When you have a drain clog, you can bust it by using a snake or taking the pipes apart, but most people would rather just pour a chemical down the drain. Three broad classes of chemicals are available for this purpose. The most common are caustic ones, particularly sodium hydroxide, which is the main ingredient in Drano and other widely used drain cleaners, but enzymatic chemicals that digest the material that blocks pipes are also easy to find and are effective, although they tend to work slowly.

Acids are the third class of clog-eating chemicals, and the best acids for cleaning plumbing pipes include hydrochloric acid and sulfuric acid. Acidic drain cleaners should be a last resort and only used when other drain cleaning chemicals don't work fast enough or can't do the job. They're hazardous and not particularly good for the pipes or the environment, and some countries, such as the UK, prohibit their use by anyone other than licensed professionals.

In the United States, you can buy sulfuric acid drain cleaner in big box stores under brand names such as Kleen-Out, Clean Shot and Liquid Lightning. These are 93 to 95 percent sulfuric acid solutions, which means they're highly concentrated, so you must treat them with respect. Follow the directions on the product label carefully and observe the standard safety precautions for handling an acid.

The first thing you learn in any chemistry class is to never pour water into an acid. If you have to dilute the acid, pour it into the water — never the other way around. The reason is that the hydration reaction of sulfuric acid is highly exothermic, which means it releases heat, and the mixture can boil spontaneously. Acid is denser than water, so it sinks, and the explosive bubbles spray the acid solution in all directions.

There's a potential for spitting and spraying even when you pour acid into water, so when you use a sulfuric acid drain cleaner, you need to wear protective eyewear and cover your hands and other parts of your body. Clear the area around the sink and cover the faucet before using the drain cleaner because the corrosive mixture can damage any metal with which it comes in contact. The Clean Shot Instant Drain Opener directions recommend coating the drain assembly and all other metal parts of the sink with petroleum jelly.

Because it corrodes metal, sulfuric acid can damage chrome, stainless steel and galvanized steel plumbing pipes. It's recommended for use only if you have plastic plumbing pipes, and even these can sustain heat damage if you introduce the drain cleaner too quickly or use too much of it. Pour the product slowly down the drain, using only the amount recommended on the label.

Sulfuric acid will eat through a number of drain-clogging materials that other drain cleaners, including those that contain sodium hydroxide, won't. These materials include paper, grease and hair, which are the main ingredients in many drain clogs. If your pipes are draining slowly, it's safer to use an enzymatic cleaner and let the situation slowly improve, but if the pipes are completely blocked and you can't use the sink, sulfuric acid can provide instant relief.

If you have a plunger, you may be able to clear the clog without using chemicals at all, which is the safest of all options. However, if you plan to plunge, do it before you use sulfuric acid and never after. Remember that the drain cleaner produces heat, and sloshing the hot mixture around in the pipes is a recipe for disaster as far as the pipes are concerned.

Sulfuric acid, like caustic cleaning agents such as sodium hydroxide, isn't safe for septic systems. It kills beneficial bacteria in the tank and reduces the system's ability to digest waste. Use sulfuric acid only if your house is connected to a municipal sewer system.

The time to use sulfuric acid is when all efforts to mechanically clear the drain, including plunging and snaking, have failed. Don't use sulfuric acid if you've recently introduced another drain-cleaning chemical because the chemicals could combine to create toxic fumes. After putting on protective clothing, covering vulnerable parts of the sink and drain and reading the application instructions on the product label, use the following procedure:

1. Open windows to provide ventilation.
2. Pour the recommended amount of drain cleaner (about 200 ml or about 7 ounces) slowly into the drain.
3. Wait for about 10 seconds, then turn on the faucet and allow water to flow slowly into the drain. If the drain backs up, turn off the water and wait a bit longer.
4. Pour more acid into the drain — up to 500 ml or about 16 ounces — if the clog doesn't clear. Wait even longer – several minutes to an hour — to give the acid time to work.
5. Flush the drain with plenty of water when the clog clears and the drain begins to flow.
6. Neutralize the acid by mixing a solution of 1/4 cup baking soda per quart of water and pouring it down the drain. You can use this solution to neutralize any spills that occur.

The safest, easiest and most effective way to clear a toilet clog is to use a plunger. When that doesn't work, you can be sure the clog is massive. Sulfuric acid is strong enough to eat through massive clogs, but it's generally not recommended for toilets, especially if the house is on a septic system. However, there's at least one sulfuric acid drain cleaner that's safe for septic systems.

Liquid Lightning drain opener is a buffered sulfuric acid solution that cleans scale and dissolves pipe clogs. It's widely available and costs about $10 for a 32-ounce container. It isn't safe for cast iron or galvanized steel pipes, though, so it's best not to use it if you have older plumbing.

To use it, pour between 12 and 16 ounces, but no more, slowly into the toilet water, being careful not to splash it on the floor or anywhere else. Then, wait 15 minutes. When the waiting period has passed, close the toilet lid and flush the toilet three times. Alternatively, pour 3 to 4 gallons of cold water directly into the bowl using a bucket.