Phosphoric acid is an inorganic mineral acid fundamental to countless industrial and consumer products. Unlike volatile acids such as hydrochloric or nitric acid, it is relatively non-toxic in its diluted forms. This compound is typically encountered as a colorless, odorless, and viscous liquid. Its uses range widely from metal surface treatment to serving as a common food additive.
Essential Grades and Concentrations
The suitability of phosphoric acid for a specific task depends on its grade and concentration, which determines its purity level. The two primary grades encountered are Food Grade and Technical Grade. Food Grade acid is produced through a thermal process that yields a highly pure product, with strict limits on heavy metals and other impurities, ensuring it is safe for human consumption.
Food Grade is often sold in concentrations of 75% or 85% by weight. Technical Grade is generally produced via the wet process, which is less refined and contains trace impurities. These impurities make it unsuitable for food or pharmaceutical use. It is primarily intended for industrial applications where minor contaminants do not affect performance.
The concentration of the acid is a separate but important consideration. An 85% concentration is a dense, syrupy liquid that is significantly more corrosive and hazardous to handle than a diluted solution. Although classified as a weak acid, concentrated phosphoric acid can still cause severe irritation and burns upon direct contact with skin or eyes.
Practical Retail and Commercial Sources
The source of purchase is usually dictated by the required grade and intended application. For high-purity needs, such as homebrewing, soda making, or specialized nutrient solutions, Food Grade phosphoric acid is necessary. It can be sourced from dedicated food ingredient suppliers who provide additives that meet stringent regulatory standards.
For technical applications like rust removal or metal etching, Technical Grade acid is sufficient and more readily available. This industrial product can often be found at specialized online chemical suppliers. They sell it in bulk quantities for manufacturing or commercial cleaning operations, catering to businesses and hobbyists requiring high-concentration material for non-food uses.
Consumers seeking smaller quantities of Technical Grade acid frequently find it indirectly at hardware and home improvement stores, where it is sold as a component in pre-mixed products. These products are labeled as rust removers, concrete etchers, or metal prep solutions. Agricultural supply centers and hydroponic stores also stock phosphoric acid, often in concentrated forms, specifically for pH adjustment in nutrient solutions and soil management.
Primary Household and Industrial Applications
One of the most common applications for technical grade phosphoric acid is in metal treatment, specifically for rust removal and surface passivation. The acid reacts directly with iron oxide (rust), converting it into a stable, black compound called iron phosphate. This iron phosphate layer adheres to the metal surface, creating a protective barrier that resists further corrosion and provides an excellent base for paint or other coatings.
The acid is widely used for pH control in water systems and agriculture. In hydroponics and soil-based growing, adding small amounts of phosphoric acid helps lower the water’s pH level, ensuring plant roots can absorb nutrients efficiently. As a food additive, the Food Grade acid is recognized by the code E338. It is used to provide the sharp, tangy flavor in many cola beverages and acts as an acidity regulator to inhibit microbial growth.
Due to the corrosive nature of its concentrated form, safety precautions must be followed when handling the chemical. Working in a well-ventilated area is necessary to prevent the inhalation of fumes or mists. Protective gear, including chemical-resistant gloves and safety goggles, should always be worn to prevent direct skin and eye contact with the liquid, especially when working with concentrations of 75% or higher.