What Is Phosphoric Acid (H₃PO₄)?

Phosphoric acid — chemical formula H₃PO₄ — is a colorless, odorless, non-volatile mineral acid. In its pure anhydrous form, it exists as a white crystalline solid at room temperature; however, it is almost universally handled, transported, and sold as a concentrated aqueous solution, most commonly at 85% w/w.

It belongs to a category of chemicals known as mineral acids, alongside sulfuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl). Unlike these stronger acids, phosphoric acid is classified as a medium-strength acid — it is triprotic, meaning it can donate three protons (H⁺) in solution, giving it a distinctive buffering capacity that makes it uniquely valuable across a diverse range of industries.

Globally, over 85% of all phosphoric acid produced is consumed in the manufacture of phosphate fertilizers — primarily DAP (diammonium phosphate), MAP (monoammonium phosphate), and TSP (triple superphosphate). The remainder finds application in food and beverage processing, metal surface treatment, water treatment, cleaning product formulation, and as a precursor for industrial phosphate salts.

Chemical and Physical Properties

Understanding the fundamental properties of phosphoric acid is essential for proper handling, storage, and application selection.

Phosphoric Acid Grades and Specifications

Phosphoric acid is sold in multiple grades, each defined by impurity limits, intended application, and regulatory certifications. Selecting the wrong grade can lead to product failure, regulatory non-compliance, or unnecessary cost.

Food Grade (E338 / FCC / USP)

Food-grade phosphoric acid is governed by standards including FCC (Food Chemicals Codex), USP (United States Pharmacopeia) — note: this refers to purity specifications for food additive use, not for pharmaceutical manufacturing — and EU Regulation 231/2012 (additive E338). Key specifications:

Applications: Carbonated beverages (cola), jams and jellies, cheese processing, vegetable oils refining, sugar refining, brewing pH adjustment.

Technical / Industrial Grade

Technical-grade phosphoric acid is designed for non-food industrial applications. It is typically produced from purified wet-process acid and has wider impurity tolerances than food grade.

Applications: Metal phosphating, rust removal and conversion coatings, industrial cleaning, water treatment, refractory materials, activated carbon production.

Fertilizer Grade (Merchant Grade Acid / MGA)

This is the most voluminous grade by far. It is sold on a P₂O₅ content basis (typically 52–54% P₂O₅, which corresponds to ~72–75% H₃PO₄).

Applications: DAP/MAP/TSP production, NPK compound fertilizers, liquid fertilizers.

Industrial Applications of Phosphoric Acid

1. Fertilizer Production — The Dominant Market

Fertilizer manufacturing consumes over 85% of all phosphoric acid produced globally. Phosphorus is one of the three macro-nutrients (N-P-K) essential to plant growth, and phosphoric acid is the gateway to virtually all commercial phosphate fertilizers.

DAP and MAP together represent approximately 70% of global phosphate fertilizer consumption. They are produced by reacting phosphoric acid with ammonia under controlled conditions.

2. Food and Beverage Industry

Phosphoric acid is the defining acidulant in cola-type carbonated beverages. It provides a sharper, more persistent "bite" than citric acid and does not introduce fruity flavor notes. Typical usage levels in finished beverages range from 0.05% to 0.08%.

Beyond soft drinks, food-grade phosphoric acid is used for:

Cheese manufacturing— pH adjustment in processed cheese and emulsifying salts production

Edible oil refining— degumming crude vegetable oils (removing phospholipids)

Sugar refining— pH control during clarification

Brewing— mash acidification and yeast nutrient phosphate source

Jams and preserves— gelling agent activation (pectin requires acidic pH)

Bakery leavening agents— precursor for sodium aluminum phosphate (SALP) and monocalcium phosphate (MCP)

3. Metal Treatment and Rust Removal

Phosphoric acid is one of the most widely used chemicals in industrial metal finishing. It serves two distinct functions:

Rust Removal (Chemical Derusting)

Phosphoric acid reacts with iron oxide (rust) to form water-soluble iron phosphates:

Fe₂O₃ + 2H₃PO₄ → 2FePO₄ + 3H₂O

This converts loose, flaky red rust into a dark, tightly adherent iron phosphate layer that can be rinsed away, leaving clean metal underneath. Commercial rust removers typically use phosphoric acid at concentrations of 10–30%, often blended with surfactants and inhibitors.

Phosphating (Conversion Coating)

Phosphoric acid-based zinc, manganese, or iron phosphate coatings are applied to steel surfaces before painting to:

Improve paint adhesion

● by creating a microcrystalline surface with 10–100× the surface area of bare metal

Provide under-paint corrosion resistance

● by forming a chemically inert barrier

Reduce friction

● during cold forming operations (wire drawing, tube bending)

This process — known historically as Parkerizing or Bonderizing — is standard across the automotive, appliance, and construction steel industries.

Typical phosphating bath composition:

● Phosphoric acid: 5–15 g/L

● Zinc oxide or manganese carbonate: 2–8 g/L

● Accelerators (nitrates, nitrites, chlorates): 0.5–3 g/L

● Temperature: 45–70°C

● Immersion time: 3–15 minutes

4. Water Treatment

Phosphoric acid and its salts are essential chemicals in industrial and municipal water treatment:

Corrosion inhibitor for drinking water distribution systems

● — orthophosphate (from phosphoric acid neutralization) forms a protective film inside lead and copper pipes, dramatically reducing metal leaching. This is the primary lead corrosion control strategy used by water utilities across North America.

Boiler water treatment

● — trisodium phosphate and disodium phosphate (produced from phosphoric acid) precipitate calcium and magnesium hardness as soft sludge rather than hard scale.

Cooling tower water conditioning

● — phosphate-based programs prevent calcium carbonate and calcium sulfate scale formation.

Wastewater nutrient addition

● — biological treatment systems often require supplemental phosphorus to maintain adequate microbial activity, particularly in food processing and pulp & paper wastewater.

5. Cleaning Products and Detergents

Phosphoric acid is a key ingredient in numerous industrial and household cleaning formulations:

Toilet bowl cleaners

● — 10–25% phosphoric acid removes hard water scale, rust stains, and mineral deposits

Bathroom tile and grout cleaners

● — dissolves calcium and iron stains without attacking ceramic surfaces

Concrete and masonry cleaners

● — etches and cleans concrete surfaces before coating or sealing; removes efflorescence

Dairy and food plant cleaners

● — acid-based CIP (Clean-in-Place) cycles for removing milkstone (calcium phosphate/protein deposits)

Brewery and winery sanitation

● — acid rinses to remove beerstone and tartrate deposits