Phosphoric Acid: Food, Fertiliser and Industrial Grades Explained

Phosphoric acid (H3PO4, CAS 7664-38-2) is a mineral acid that is far milder than sulfuric or hydrochloric acid, yet still corrosive at strength. It is usually sold as a syrupy, colourless solution at 75 to 85 percent, though it also comes at other strengths and as a crystalline solid. Its mildness is why it can appear in foods, and also why it is easy to underestimate.

Applications

• Fertilisers, the biggest market, for making ammonium phosphates (DAP and MAP) and triple superphosphate.
• Food and drinks: an acidulant in cola drinks, jams and processed cheese, using food-grade acid (E338).
• Rust removal and metal treatment: converting iron oxide to iron phosphate and as a phosphating step before painting.
• Detergents and water treatment, producing phosphates and as a descaling agent.
• Dental and medical products, such as etching gels and cements.
• Industrial: flame retardants, catalysts, and electronics (etching in semiconductor and display manufacture).
• Animal feed, as a phosphorus supplement and in silage.

Technical, food and electronic grades

The main thing to understand is that grade drives the specification. Technical or fertiliser grade (often made by the wet process from phosphate rock) contains impurities such as fluoride, sulphate, iron and heavy metals, and is not suitable for food. Thermal-process or purified acid is cleaner, and food grade must meet limits for arsenic, lead and fluoride set in food additive specifications. Electronic grade is very high purity for semiconductor use. Your certificate of analysis should state the grade and the relevant standard so buyers can place it correctly. Our guide to purity grades explains the categories.

Hazard classification

Phosphoric acid is classified as corrosive to skin and eyes (category 1B, H314) at higher concentrations, usually above about 25 percent, and as an irritant at lower concentrations (H315, H319 between roughly 10 and 25 percent), and below around 10 percent it may fall out of classification. It is also commonly labelled corrosive to metals (H290). The signal word is Danger and the pictogram is corrosion (GHS05). These cut-offs mean that 85 percent technical acid and 10 percent diluted acid receive different labels. Check the current harmonised limits and classify each strength you sell.

Handling

The viscous solution burns skin and eyes, so use goggles or a face shield, acid-resistant gloves and an apron. Heating produces irritating fumes, and hot concentrated acid is very aggressive to skin. Dilute by adding acid to water slowly. Section 8 should recommend ventilation and, for hot processes, local exhaust. Because phosphoric acid is a weak oxidiser compared with nitric, it is less reactive with organic material, but it still reacts with metals to release hydrogen.

Storage

Use containers made of polyethylene, polypropylene, fibreglass, stainless steel (suitable grades) or rubber-lined steel, and avoid ordinary carbon steel and aluminium for prolonged storage. Concentrated acid can crystallise or thicken in cold weather, so tanks may need gentle heating. Keep it away from bases, cyanides, sulphides, chlorates and strongly reactive metals. See the storage compatibility guide.

Transport and paperwork

In most transport codes, phosphoric acid solution is a Class 8 corrosive, though some low-concentration solutions are exempt, and solid phosphoric acid has a separate entry. Check the current UN numbers. For documents, supply an SDS for each strength and a specification or certificate of analysis for each batch. For food applications, add a food-grade statement and the compliance standard. The data we hold is on the phosphoric acid page.