Phosphoric acid, also known as orthophosphoric acid, is a common inorganic acid, medium strong acid, chemical formula H3PO4, molecular weight 97.994. It is not easy to volatilize, decompose, and has almost no oxidation. It has the common property of acid. It is a ternary weak acid, which is weaker than hydrochloric acid, sulfuric acid, nitric acid, but stronger than acetic acid, boric acid, etc. Phosphoric acid is easily deliquescent in air. Pyrophosphoric acid is obtained by heating, and metaphosphoric acid is obtained by further dehydration. Phosphoric acid is mainly used in pharmaceutical, food, fertilizer and other industries, including as rust inhibitor, food additive, dental and orthopedic surgery, EDIC corrosion agent, electrolyte, flux, dispersant, industrial corrosion agent, fertilizer raw materials and components, household cleaning products, as well as chemical reagents.
Orthophosphate is a phosphoric acid composed of a single phosphorus oxygen tetrahedron. In the phosphoric acid molecule, the P atom is sp3 hybrid, and three hybrid orbitals form three with the oxygen atom σ The other P-O bond is composed of σ And two d-p bonds from oxygen to phosphorus. σ The coordination bond is formed by the coordination of a pair of lone electrons on the phosphorus atom to the empty orbital of the oxygen atom. D ← p bond is formed by overlapping two pairs of lone electrons on the py and pz orbitals of oxygen atom and dxz and dyz empty orbitals of phosphorus atom. Since the 3d energy level of phosphorus atom is much higher than the 2p energy level of oxygen atom, the molecular orbital formed is not very effective, so the P-O bond is triple bond in number, but between single bond and double bond in terms of bond energy and bond length. Both pure H3PO4 and its crystal hydrate have hydrogen bonds, which may be the reason why the concentrated phosphoric acid solution is viscous.
Specific conductance
At room temperature (219K), when the concentration of H3PO4 solution is 45% - 47%, the specific conductance is the maximum.
Crystallization point:
Phosphoric acid belongs to medium strong acid, and its crystallization point (freezing point) is 21 ℃. When it is lower than this temperature, it will precipitate hemihydrate (ice) crystals. Of course, phosphoric acid usually does not form (ice) crystals at temperatures above 10 ℃ or even below. This is because phosphoric acid has the characteristics of supercooling, that is, when the actual phosphoric acid on the market is below 21 ℃, it will deviate from its formation (ice) crystal point and will not form (ice) crystals immediately. However, as long as such low temperature is maintained for a period of time, phosphoric acid is easy to produce ice crystals under static state.
Phosphoric acid crystal, like other liquid crystals, belongs to physical change rather than chemical change. Its chemical properties will not change due to crystallization, that is, the properties of phosphoric acid will not be affected due to crystallization. It can still be used normally as long as temperature is given to melt or heated water is used to dilute and dissolve.
Crystallization characteristics: high phosphoric acid concentration, high purity and high crystallinity. According to experience, when the temperature is above or below 4 ℃ and the concentration is greater than 85%, its crystallinity increases. If it is accidentally mixed with crystalline phosphoric acid, it will cause the phosphoric acid without crystal (ice) to immediately infect and form crystal (ice). Moreover, the phosphate crystal (ice) is unusually fast, which will cause most of the phosphoric acid storage containers to form crystal (ice). After the phosphoric acid crystals (ice) are formed, the phosphoric acid in the upper part becomes dilute, and the acicular crystal pure phosphoric acid is deposited in the lower part. According to experience, 75% phosphoric acid is also difficult to form (ice) crystals at low temperature (around 4 ℃), so it is recommended to use 75% phosphoric acid at low temperature.
Like water freezing, phosphoric acid (ice) crystal is its own physical property and inherent attribute, which cannot be changed. Only proper preservation and treatment can prevent (ice) crystal formation. Commercial phosphoric acid is a viscous concentrated solution containing 85% H3PO4. Crystallization from concentrated solution will form the semi hydrate 2H3PO4 · H2O (melting point 302.3K).
Agriculture: phosphoric acid is the raw material for the production of important phosphate fertilizer (calcium superphosphate, potassium dihydrogen phosphate, etc.), and also the raw material for the production of feed nutrient (calcium dihydrogen phosphate).
Industry: Phosphoric acid is an important chemical raw material, and its main functions are as follows:
1. The metal surface is treated to generate insoluble phosphate film on the metal surface to protect the metal from corrosion.
2. It is mixed with nitric acid as a chemical polishing agent to improve the smoothness of metal surface.
3. Phosphate ester, the raw material for the production of washing products and pesticides.
4. Raw materials for production of phosphorus containing flame retardants.
Food: Phosphoric acid is one of the food additives. As a sour agent and yeast nutrient in food, cola contains phosphoric acid. Phosphate is also an important food additive, which can be used as a nutrient enhancer.
Medicine: Phosphoric acid can be used to produce phosphorus containing drugs, such as sodium glycerophosphate.