Trisodium phosphate (TSP), available at most hardware stores in white powder form, is a cleaning agent, stain remover and degreaser, commonly used to prepare surfaces for painting. It can also be called trisodium orthophosphate and has the chemical formula Na3PO4; however, it is generally found in hydrated forms. It is a highly water-soluble ionic salt. Solutions of it dissolved in water have an alkaline pH.
Other sodium phosphates, sometimes know by E number E339, are food additives and used as an acidity regulator (buffering agent), emulsifier, thickening agent, nutrition enlargement agent and sequestrant (metal-chelating agent). Sodium phosphate enemas are sold over-the-counter in the United States, working as a laxative to treat constipation. However, these should not be confused with the related compounds sodium dihydrogen phosphate, also known as monosodium phosphate or MSP, and disodium phosphate.
Similar chemicals were once common in laundry and dishwashing detergents, but the phosphate, being a fertilizer, would cause algal blooms in the bodies of water that the drains led to. In the early 1970s the use of phosphate-containing products was limited. Now products sold as TSP Substitute, containing 80–90% sodium carbonate, are promoted as a direct substitute.
Cleaning products labeled as TSP may contain other ingredients as well, and may in fact be less than half TSP. So even "regular" TSP found at the hardware store may be half TSP and half "TSP substitute". Some large home improvement centers that sell paint no longer offer TSP.
Although it is the active ingredient in at least one toilet bowl cleaning tablet, TSP is generally not good for cleaning bathrooms, because it can corrode metal.
TSP is commonly used after cleaning with mineral spirits in order to then clean up all the mineral spirits. TSP may be used with household chlorine bleach in the same solution, and this is particularly good for removing mildew from wood. The TSP alone can cause dark stains on redwood, and bleach prevents it.
Also used in various forms as a boiler treatment chemical for calcium precipitation, as well as regulating the caustic effects of disodium phosphate in coordinated phosphate chemistry.
Also used to clean swimming pool polyester filter elements.