The phosphoric acid changes the reddish-brown iron oxide.
Multiple applications of phosphoric acid may be required to remove all rust.
Among many applications, phosphoric acid is used:
As a solution for anodizing
As an external standard for phosphorus-31 Nuclear magnetic resonance (NMR).
As a buffer agent in biology and chemistry; For example, a buffer for high-performance liquid chromatography.
As a chemical oxidizing agent for activated carbon production, as used in the Wentworth Process.
As the electrolyte in phosphoric acid fuel cells.
With distilled water (2–3 drops per gallon) as an electrolyte in oxyhydrogen generators.
As a catalyst in the hydration of alkenes to produce alcohols, predominantly ethanol.
As an electrolyte in copper electropolishing for burr removal and circuit board planarization.
As a flux by hobbyists (such as model railroaders) as an aid to soldering.
In compound semiconductor processing, phosphoric acid is a common wet etching agent: for example, in combination with hydrogen peroxide and water it is used to etch InGaAs selective to InP.
Heated in microfabrication to etch silicon nitride (Si3N4). It is highly selective in etching Si3N4 instead of SiO2, silicon dioxide.
As a cleaner by construction trades to remove mineral deposits, cementitious smears, and hard water stains.
As a chelant in some household cleaners aimed at similar cleaning tasks.
In hydroponics pH solutions to lower the pH of nutrient solutions. While other types of acids can be used, phosphorus is a nutrient used by plants, especially during flowering, making phosphoric acid particularly desirable.
As a pH adjuster in cosmetics and skin-care products.
As a dispersing agent in detergents and leather treatment.
As an additive to stabilize acidic aqueous solutions within a wanted and specified pH range.
As a sanitizing agent in the dairy, food, and brewing industries