Muriatic acid is a corrosive acid that is manufactured by absorbing hydrogen chloride in water. Muriatic acid is often used for cleaning brick and metal and for swimming pool maintenance. The chemical is so strong it is known to etch stone and concrete, and it can remove varnish and destroy many materials including plastics and fabrics. If you cannot find muriatic acid, you can use a substitute.
Phosphoric acid is also commonly referred to as E338 and orthophosphoric acid. Phosphoric acid cleaner can be diluted with water, or it can be used full strength for heavy-duty cleaning and rust removal. Typically more mild than muriatic acid, phosphoric acid can be used both inside and outside on many different surfaces such as concrete, slate, tumbled and honed marble, granite, brick and masonry work, quarry tile and ceramic tile as well as mortar and grout surfaces. Phosphoric acid cleaner is fast acting, and it is often found as an ingredient in many types of household cleaners such as those designed to remove hard water stains, calcium buildup and rust stains.
Muriatic acid is often used in swimming pool maintenance to adjust the alkalinity of the water and prevent fluctuations in the pH balance. When the pH balance of the pool's water is disturbed, swimmers can experience an uncomfortable skin rash and pool equipment can deteriorate more quickly. The alkalinity and pH of the water can be lowered by adding muriatic acid, but sodium bisulfate is also an acceptable substitute. Sodium bisulfate is preferred for pool maintenance in many cases, because it is a dry acid that is milder than liquid muriatic acid, and it does not cause the same spikes in pool alkalinity and pH levels that muriatic acid can cause.
Trisodium phosphate comes in a granular or crystalline solid that is water soluble and makes a much safer alternative to muriatic acid. Considered an all-purpose cleaner by many painting professionals, trisodium phosphate can be used to clean many interior and exterior surfaces to remove dirt, grease and grime as well as soapscum, mold and layers of old paint and varnish. Following cleaning, the surface will be clean and etched to allow better adhesion for a new layer of paint or varnish. The chemical mixture can be diluted with water to alter the strength level, but you should avoid direct contact with your skin and eyes.