Phosphoric acid, also known as orthophosphoric acid, is a common inorganic acid with a chemical formula of H3PO4. Not volatile, not easy to decompose, almost no oxidation. It has acidity and is a ternary weak acid. Its acidity is weaker than hydrochloric acid, sulfuric acid, and nitric acid, but stronger than acetic acid, boric acid, etc. Phosphoric acid is easily deliquescent in air. After heating, it will lose water to get pyrophosphoric acid, and further lose water to get metaphosphoric acid. Phosphoric acid is mainly used in pharmaceutical, food, fertilizer and other industries. Including raw materials and components as rust inhibitors, food additives, dental and orthopedic, EDIC corrosives, electrolytes, fluxes, dispersants, industrial corrosives, fertilizers, household cleaning products, and also used as chemical reagents.
Physical properties：
1. Specific conductance
At room temperature (219K), the specific conductance is the largest when the concentration of H3PO4 solution is 45%-47%.
2. Crystallization point
Phosphoric acid is a medium-strong acid, and its crystallization point is 21°C. When the temperature is lower than this temperature, hemihydrate crystals will precipitate. Of course, phosphoric acid usually does not crystallize at temperatures above 10°C or even lower. This is due to the supercooling characteristic of phosphoric acid, but as long as such a low temperature is maintained for a period of time, phosphoric acid is easy to crystallize in a static state.
Crystallization of phosphoric acid is a physical change, not a chemical one, like crystallization of other liquids. Its chemical properties are not changed by crystallization. That is to say, the properties of phosphoric acid will not be affected by crystallization, as long as it is melted at a temperature or diluted with heated water, it can still be 2used normally.
Phosphoric acid has high concentration, high purity and high crystallinity. When the temperature is around 4 degrees Celsius and the concentration is greater than 85%, its crystallinity increases. If crystalline phosphoric acid is accidentally mixed in, it will cause the phosphoric acid that was not crystallized to be immediately infected and crystallized. Moreover, the crystallization of phosphoric acid was extremely rapid, causing most of the phosphoric acid storage container to crystallize. After the crystallization of phosphoric acid, the phosphoric acid in the upper part becomes thinner, and the needle-like crystal pure phosphoric acid is deposited in the lower part. 75% phosphoric acid is also more difficult to crystallize at lower temperatures. Therefore, under lower temperature conditions, it is recommended to use 75% phosphoric acid.
Phosphoric acid crystallization is like water freezing. It is its own physical property and inherent property, which cannot be changed. Only proper preservation and treatment can prevent crystallization. Commercially available phosphoric acid is a viscous concentrated solution containing 85% H3PO4. Crystallization from concentrated solution forms hemihydrate 2H3PO4·H2O (melting point 302.3K).
Application field：
1. Agriculture
Phosphoric acid is the raw material for the production of important phosphate fertilizers (calcium superphosphate, potassium dihydrogen phosphate, etc.), and also the raw material for the production of feed nutrients (calcium dihydrogen phosphate).
2. Industry
Phosphoric acid is an important chemical raw material, the main functions are as follows:
2. 1Treat the metal surface. Forms an insoluble phosphate film on the metal surface to protect the metal from corrosion.
2.2 Mixed with nitric acid as a chemical polishing agent to improve the smoothness of the metal surface.
2.3 Phosphate esters as raw materials for the production of detergents and insecticides.
2.4 Raw materials for the production of phosphorus-containing flame retardants.
3. Food
Phosphoric acid is one of the food additives, used as sour agent and yeast nutrient in food. Coke contains phosphoric acid. Phosphate is also an important food additive and acts as a nutritional enhancer.
4. Medicine
Phosphoric acid can be used to prepare phosphorus-containing drugs, such as sodium glycerophosphate.