Muriatic acid is a liquefied form of hydrogen chloride gas. This highly corrosive liquid is frequently used to balance the pH of swimming pools as well as to remove excess mortar from bricks and etch concrete. Muriatic acid is destructive to cellulose and will break up the cellulose chain into smaller units, leading to hydrolysis. Because of this, muriatic acid may be used as a weed killer. Because muriatic acid is highly corrosive, you should read all warnings included with the chemical and take the advised precautions when using the acid as an herbicide.
Mix a weak solution of muriatic acid that contains 1 part acid to 20 parts water in a bucket.
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Dip a sponge in the solution. Squeeze the sponge so that it is saturated, but not dripping.
Swipe weeds with the sponge to saturate the weed with the acid solution. This method of weed treatment will prevent the acid from dripping into the soil and potentially contaminating the soil or harming nearby plants.
Pour the remaining solution into a spray bottle.
Spray weeds that appear in the cracks in a sidewalk until they are coated with acid. The solution will kill the weeds through the cracks and prevent them from returning. Surround the weed with a drop cloth to prevent overspray onto the sidewalk. Muriatic acid can etch a sidewalk if it is oversprayed.
Observe the weed. It should turn brown and brittle. If the weed does not die, mix a slightly stronger solution of weed killer and re-treat the weeds.