Compounds in which one or two hydroxyl groups in phosphoric acid (HO) 3PO molecules are substituted with alkyl or aryl groups. According to the number of substituted alkyl groups, it can be divided into alkyl phosphonic acid RP (O) (OH) 2 and dialkyl phosphonic acid R2P (O) OH.
Pure phosphoric acid is a colorless crystal with a melting point of 42.3 degrees Celsius. It is a high boiling acid and is easily soluble in water. phosphoric acid
Commercial phosphoric acid reagents are viscous, non-volatile concentrated solutions with a phosphoric acid content of 83-98%.
Phosphoric acid is a strong ternary acid, ionized in three steps, not easily volatile, not easily decomposed, and has almost no oxidizing properties. Three step ionization constant of phosphoric acid with acidic properties
(1) Reaction with alkali NaOH+H3PO4=NaH2PO4+H2O 2NaOH+H3PO4=Na2HPO4+2H2O 3NaOH+H3PO4=Na3PO4+3H2O
(2) Reacts with certain salts NaBr+H3PO4 (concentrated)=NaH2PO4+HBr ↑ NaI+H3PO4 (concentrated)=NaH2PO4+HI ↑ Principle: Volatile acid is produced from non volatile acids
(3) Phosphate ions have strong coordination ability and can form soluble complexes with many metal ions. Fe3+and PO43- can form colorless soluble complexes [Fe (PO4) 2] 3- and [Fe (HPO4) 2] -, and using this property, PO43+is commonly used in analytical chemistry to mask Fe3+ions.
(4) Phosphoric acid dehydrates under strong heat, generating pyrophosphoric acid, triphosphate, and polyphosphoric acid in sequence. Triphosphate has a chain like structure, while polyphosphoric acid has a cyclic structure.
There are three types of phosphate salts: orthophosphates (containing PO43-), monohydrogen phosphate (containing HPO42-), and dihydrogen phosphate (containing H2PO4-). The transformation relationship between the three types of salts is as follows:
1. Solubility law: Orthosalts and monohydrogen salts: except for a few salts such as potassium, sodium, and ammonium, the rest are insoluble in water, but can dissolve in strong acids. Dihydrosalts: are easily soluble in water.
2. Mutual conversion a) Add H3PO4 dropwise to clarified lime water, oscillating while dripping.
Phenomenon: White precipitates begin to form and gradually dissolve. Reaction equation: 3Ca (OH) 2+2H3PO4=Ca3 (PO4) 2 ↓+6H2O Ca3 (PO4) 2+H3PO4=3CaHPO4 CaHPO4+H3PO4=Ca (H2PO4) 2 b) Add clear lime water dropwise to a phosphoric acid solution, and shake while adding.
Phenomenon: There is no phenomenon at the beginning, but when the clarified lime water drops to a certain amount, white precipitates are formed. phosphoric acid
Reaction equation: Ca (OH) 2+2H3PO4=Ca (H2PO4) 2+2H2O Ca (H2PO4) 2+Ca (OH) 2=2CaHPO4 2CaHPO4+Ca (OH) 2=Ca3 (PO4) 2 ↓+2H2O
3. The problem of ion coexistence
(1) H2PO4-, HPO42-, PO43- cannot coexist with H+
(2) H2PO4-, HPO42-, and OH - cannot coexist
(3) H2PO4- cannot coexist with PO43- H2PO4-+PO43-=2HPO42-
(4) H2PO4- can coexist with HPO42-, HPO42-, and PO43-compatible