Healthy Living

Phosphoric Acid

Phosphoric Acid

Key Takeaways

  • Phosphoric acid is a mineral and inorganic acid. 
  • The chemical formula for phosphoric acid is H3PO4.
  • Phosphoric acid can be used for rust removal.
  • Phosphoric acid forms three classes of salts.
  • It can be harmful, so it should be handled with care.

What is phosphoric acid?

Phosphoric acid is a mineral acid. This means it is an inorganic type of acid. These are derived from one or more inorganic compounds. In the case of phosphoric acid, they are derived from inorganic phosphate rocks. Acid of higher purity can also be made, by using white phosphorus. Other examples of mineral acids are sulfuric acid, hydrochloric acid, and nitric acid. When dissolved, mineral acids form hydrogen ions and conjugate base ions.

Its IUPAC name is orthophosphoric acid. IUPAC stands for International Union of Pure and Applied Chemistry. It is the universally-recognized authority on chemical nomenclature and terminology. This system allows them to establish unambiguous, consistent, and uniform nomenclature and terminology for specific scientific fields. Through this, confusion is avoided and identification of substances are simplified.

Phosphoric acid, in its pure form, is a crystalline solid, but it is easily dissolved into a viscous liquid. In this latter form, it is a colorless and odorless solution. When sufficiently diluted in water, it has a sour taste.

There are two methods used in the production of phosphoric acid. The difference between the two is the purity of the product produced.

  • The first method is the WET METHOD. It is the less expensive process of the two. Concentrated sulfuric acid (93%) is added to tricalcium phosphate rock. This is typically found in nature as the substance fluorapatite. The products of this reaction are phosphoric acid and calcium sulfate or gypsum, along with other impurities. At this stage, water is added so that gypsum and other insoluble materials are removed through filtration.
  • This is then purified through the evaporation of water. This removes fluoride to produce an animal-grade phosphoric acid, or further purified through solvent extraction and arsenic removal to produce food-grade phosphoric acid.
  • Generally, due to its economical nature, wet-process phosphoric acid is widely used in the commercial sector. It is typically used for the manufacture of fertilizers.
  • In other instances, nitric acid is used instead of sulfuric acid. This method co-produces calcium nitrate, which is another plant fertilizer. However, this method is very rarely used.
  • The second is the THERMAL METHOD. This process uses two key reactants: phosphorus and air. Phosphorus is sprayed into a furnace, with the air temperature at 1800 to 3000 K. The process usually employs moist air and involves the addition of steam. This allows a film of condensed polyphosphoric acids to be produced and maintained. This helps protect the stainless steam burner tower which is also externally water-cooled. The products from the burner tower head directly into a hydration tower. In here, the gaseous phosphorus oxide is absorbed in recycled phosphoric acid. The product is a purer food-grade acid with a concentration of 85%. This method is typically used in the food industry.

Phosphorous also creates other oxyacids. These include phosphorous acid (H3PO3) and hypophosphorous acid (H3PO2).

Phosphoric acid uses

Phosphoric acid forms three classes of salts. This corresponds to the number of hydrogen atoms replaced. Some phosphate salts are listed below.

  • Sodium dihydrogen phosphate (NaH2PO4). It is used for the control of the acidity of solutions through their hydrogen ion concentration.
  • Disodium hydrogen phosphate (Na2HPO4). It is used as a precipitant for highly charged metal cations in the process of water treatment.
  • Trisodium phosphate (Na3PO4). This is used in soaps and detergents.
  • Calcium dihydrogen phosphate Ca[H2PO4]2. It is also called calcium superphosphate. It is used as a primary ingredient for fertilizers. Phosphorus is an essential nutrient that plants take up through its roots. Phosphoric acid is used in the manufacture of fertilizers to address phosphorus deficiency in soil.
  • Calcium monohydrogen phosphate (CaHPO4). It is used as a conditioning agent for salts and sugars.

It is also used in rust-proofing and in dental cement manufacturing. A very small amount is also added in food products, particularly soft drinks, as tart or acidic fruit-like flavor.

Phosphoric acid MSDS

The Material Safety Data Sheet (MSDS) is a document that provides safety and health information about certain products, substances or chemicals that are classified to be hazardous or dangerous goods.

Here are the details for phosphoric acid.

Product Information.

Trade Name: Phosphoric Acid

Chemical Name: Phosphoric Acid

Synonyms: Monophosphoric acid, orthophosphoric acid

Physical Data.

Physical State: At 20°C, liquid (viscous)

Color: Colorless

Odor: None

Specific Gravity: Liquid (25°C); 1.5-1.7, variable to phosphoric acid strength

Solubility in Water: Completely soluble at 20°C

Solvents: Soluble in alcohols

Preparation Information.

CAS No.: 7664-38-2

UN Code: UN 1805

H S Code: 2809 20 01

Hazards Information.

Appearance and Color: Clear, colorless, odorless, syrupy liquid

Health Effects: Corrosive liquid may cause skin and eye burns.

Physical & Chemical: May form explosive and flammable hydrogen via corrosion of metals.

Hazards at High Temperature: Thermal decomposition may occur and produce corrosive products.

Fire and Explosion Hazard.

Suitable Extinguishing Media: In case of fire nearby, use foam, dry powder, carbon dioxide (CO2) media.

Special Hazards: Non-flammable product. Through corrosion of metals, it may form flammable and explosive hydrogen. Temperature above 200°C. Formation of polyphosphoric acid at high temperature (dehydration). Phosphorus oxides, corrosive products due to thermal decomposition.

Specific Methods: In case of fire, remove exposed containers. Cool with water spray.

Special Protective Equipment for Firefighters: Self-contained breathing apparatus and acid-resistant clothing.

Reactivity Data.

Conditions to Avoid: Store in an area that is protected from heat and moisture.

Materials to Avoid: Bases, Quicklime — exothermic reaction, violent; Alcohols, amines ketones — exothermic reaction; Water — very exothermic reaction and possibility of splitting; Chlorates, nitrates, calcium carbide — explosive reaction.

Flammability: Combustible materials — overheating and ignition; Metals, finely divided metals.

Hazardous Decomposition Products: Unknown

Toxicology Properties.

Acute Toxicity: May be harmful by inhalation, ingestion or skin absorption. Material is destructive to eyes, upper respiratory tract, skin, and tissue of the mucous membranes.

Inhalation: May result to spasming, inflammation and edema of the bronchi and larynx, chemical pneumonitis, and pulmonary edema. Symptoms of exposure include a burning sensation, wheezing, coughing, shortness of breath, laryngitis, headache, vomiting, and nausea.

Chronic Effects: Target Organ/s — liver, bone marrow, blood

Additional Toxicological Information: Chronic and acute toxicity of this substance is not fully known. No classification data on this material’s carcinogenic properties is available from EPA, IARC, NTP, OSHA or ACGIH.

Preventive Measures.

Technical Measures/Precautions: Corrosive, appropriate exhaust and ventilation at machinery must be ensured. Provide eye baths and showers.

Safe Handling Advice: Do not pour water. Avoid splashing when handling.

Storage Information: Keep containers closed tightly in a cool and well-ventilated area. Store in a well-insulated area. Protect from heat and moisture. Preserve at temperature above 16°C. Provide an impermeable corrosion-resistant flooring with drainage to a neutralization tank within a bunked area. Provide a catch tank. Provide anti-corrosion electrical equipment.

First Aid Measures.

General Advice: Take off all contaminated clothing immediately. Including shoes.

Inhalation: Get fresh air. If necessary, provide oxygen or artificial respiration. Hospitalize.

Skin Contact: Wash immediately, abundantly and thoroughly with water. If possible, rinse with a bicarbonate solution.

Eye Contact: Wash with water for at least 15 minutes, immediately and abundantly. Consult an ophthalmologist at once.

After Swallowing: Do not induce vomiting. Instead, rinse mouth and lips with plenty of water. If the subject is conscious, hospitalize immediately. If subject is unconscious, do not give anything through the mouth.

Phosphoric acid hazards

Due to its acidic nature, phosphoric acid should be handled with extreme care. It is a corrosive liquid that can cause eye and skin burns. Through the corrosion of metals, it may also form flammable and explosive hydrogen. At high temperatures such as 200°C, thermal decomposition can occur. This may result in corrosive products.

Is phosphoric acid strong or weak?

Phosphoric acid commonly comes in an 85% aqueous solution. This is a viscous solution that is colorless, odorless, and non-volatile. Its viscosity is similar to that of a syrup, but is still pourable. It is a weak acid.

The strength of an acid is determined by several factors, namely:

  • Electronegativity. Strong acids have higher electronegativity in their conjugate base in the same period. The more electronegative A- an acid has, the more acidic it is.
  • Atomic Radius. The higher the atomic radius of an acid, the greater its acidity is.
  • Charge. A strong acid is positively charged. This means that the more positively charged a species is, the stronger it is as an acid.
  • Equilibrium. In strong acids, equilibrium lies far to the right in its dissociation reaction.

Compared to other acids, phosphoric is classified as a weak acid. To provide perspective, common strong acids are hydrochloric acid and sulfuric acid. However, phosphoric acid is still an acid and can be corrosive.

Phosphoric acid formula

The chemical formula for phosphoric acid is H3PO4.

This is used to denote the composition of phosphoric acid. This means that each phosphoric acid molecule is composed of 3 hydrogen atoms, one phosphorus atom, and four oxygen atoms.

Molar mass of phosphoric acid

The molar mass of phosphoric acid is 98 g/mol.

This is calculated using the atomic masses of the molecules that comprise it. The atomic mass of hydrogen is 1.00794, oxygen’s is 15.9994, and that of phosphorus is 30.973761. The atomic mass is multiplied by the number of atoms present in phosphoric acid. There are three hydrogen atoms, one phosphorus atom, and four oxygen atoms. Hence, the computation follows: 1.00794*3 + 30.973761 + 15.9994*4. This yields our molar mass of 97.995181 or 98 g/mol.

Phosphoric acid rust removal

Phosphoric acid is an effective rust remover. It acts on the rust by converting rust (Fe2O3) to a form that can be dissolved in water — ferric phosphate (FePO4). It may be applied directly to rusted iron or steel and other surfaces. It is typically commercially available in the form of a gel.

However, one must exercise caution when using rust removers. Some formulations contain hydrofluoric acid which is extremely dangerous. It can penetrate the skin, destroy bone and cause a heart attack without your knowledge. Be sure to read the label on the product you intend to buy and steer away from products that contain hydrofluoric acid.