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    Home > Chemicals Industry > Chemical Technology > Phosphorus oxo acids and their salts

    Phosphorus oxo acids and their salts

    • Last Update: 2021-06-17
    • Source: Internet
    • Author: User
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    The oxo acids of phosphorus mainly include hypophosphorous acid (H 3 PO 2 ), phosphorous acid (H 3 PO 3 ), phosphoric acid (H 3 PO 4 , orthophosphoric acid ) and their condensation products (Figure 12-12)
    .

    Figure 12-12 Structures of hypophosphorous acid, phosphorous acid and phosphoric acid

    The number of hydroxyl groups in the oxo acid of phosphorus determines how many acids it is.
    H
    4 P 2 O 7 , H 3 PO 4 , H 3 PO 3 , and H 3 PO 2 are tetrabasic acid, tribasic acid, and dibasic acid, respectively.
    , Monobasic acid

    .

    The acidic order of common phosphorus oxo acids is

    HPO 3 >H 4 P 2 O 7 >H 3 PO 3 ≈H 3 PO 2 >H 3 PO 4

    Generally speaking, the higher the oxidation number of the center, the stronger the acidity of the oxo acid
    .
    However, the acidity of phosphorous acid and hypophosphorous acid is stronger than that of orthophosphoric acid, which does not conform to the general law of acidity

    .

    1.
    Hypophosphorous acid and its salts

    Elemental phosphorus reacts with hot concentrated lye to generate hypophosphite, namely

    P 4 +3NaOH+3H 2 O=3NaH 2 PO 2 +PH 3

    Pure hypophosphorous acid is a white solid (melting point 26.
    5°C), which is easily deliquescent

    .
    Hypophosphorous acid is a strong monobasic acid (Ka
    Θ =5.
    89×10
    -2 )
    .
    Both hypophosphorous acid and its salts are strong reducing agents, especially in alkaline solutions, which have stronger reducing power

    .

    H 3 PO 2 +4Ag + +2H 2 O=H 3 PO 4 +4Ag+4H +

    Due to its strong reducing power, hypophosphite is often used in electroless plating
    .

    H 2 PO 2 - + 2Ni 2 + + PO = 6OH- .
    4
    3- + 2Ni + 4H 2 O

    2.
    Phosphorous acid and its salts

    Phosphorous acid H 3 PO 3 is a solid at room temperature (melting point 74.
    4°C)

    .
    Phosphorous acid and its salts are strong reducing agents and are easily oxidized to the highest oxidation state of phosphorus

    .

    H 3 PO 3 +2Ag + +H 3 O=H 3 PO 4 +2Ag+2H +

    Hypophosphorous acid and phosphorous acid and their salts are prone to disproportionation reactions to produce PH 3 , H 3 PO 4 or their salts
    .

    4H 3 PO 3 =3H 3 PO 4 +PH 3

    Phosphorous acid is a strong dibasic acid (Ka1 Θ =3.
    72×10-2, Ka2Θ=2.
    09×10
    -7 ), which can form two types of salts , H 2 PO 3 - and HPO 3 2-
    .

    3.
    Phosphoric acid and its salts

    Oxidizing white phosphorus with nitric acid or P 4 O 10 completely react with water to obtain phosphoric acid
    .
    Industrially produced phosphoric acid with low purity is made by the action of sulfuric acid and calcium phosphate

    .

    Ca 3 (PO 4 ) 2 +3H 2 SO 4 =3CaSO 4 +2H 3 PO 4

    The melting point of H 3 PO 4 is 42.
    4°C, which is miscible with water

    .
    Commercially available phosphoric acid is a concentrated solution containing 85% H
    3 PO 4 , and the viscosity of the phosphoric acid solution is higher due to the formation of more hydrogen bonds between molecules
    .

    H 3 PO 4 is a ternary medium strong acid, and its gradual dissociation constants are 7.
    11×10
    -3 , 6.
    34×10
    -8 , and 4.
    79×10
    -13, respectively
    .
    H
    3 PO 4 has coordination ability and generates colorless ion with Fe 3+
    .

    Fe 3+ +2PO 4 3- =[Fe(PO 4 ) 2 ] 3-

    H 3 PO 4 dehydration polymerization to obtain pyrophosphoric acid (H 4 P 2 O 7 ), chain polyphosphoric acid (H n+2 P n O3 n+1 ), cyclic polyphosphoric acid (HPO 3 ) n, etc.
    (Figure 12-13 ), cyclic polyphosphoric acid is also called metaphosphoric acid, abbreviated as HPO
    3
    .

    Figure 12-13 The structure of several phosphoric acid polymerization dehydration products

    Many types of phosphate with orthophosphates (PO .
    4
    3- ), hydrogen phosphate (HPO .
    4
    2- ), dihydrogenphosphate (H 2 PO .
    4
    - ), pyrophosphate (P 2 O .
    7
    4- ) , Chain polyphosphate, cyclic polymetaphosphate
    .

    PO .
    4
    3- hydrolysis is alkaline salt solution (the pH> 12 is), HPO .
    4
    2- hydrolyzed salt solution was slightly alkaline (pH = ~ 10.
    9), H2PO
    .
    4
    - Hydrolysis salt solution was slightly acidic (pH = 4~5)
    .
    Adding silver nitrate solution to the solution containing PO
    4 3- , HPO 4 2- , and H 2 PO 4 - all produces Ag 3 PO 4 yellow precipitate; adding silver nitrate solution to the H 3 PO 4 solution does not produce precipitation .
    Both AgPO 3 and Ag 4 P 2 O 7 are white insoluble salts .
    Ag 3 PO 4 , AgPO 3 and Ag 4 P 2 O


    7 are easily soluble in strong acid
    .

    Add a small amount of dilute HAc to the P 2 O 7 4- and PO 3 - solutions to acidify, and then drip into the dilute egg white solution respectively.
    The PO
    3 - and the solution can be condensed by the egg white solution to identify P 2 O 7 4- and PO .
    3
    -
    .

    The salts formed by PO 4 3- and HPO 4 2- with Na + , K + , Rb + , Cs + , NH 4 + are easily soluble; the salts formed by metals and H 2 PO 4 - are more soluble; PO 4 3- and Most of the salts formed by Li + and +2 valence and above metals are insoluble salts
    .

    4.
    Other oxo acids of phosphorus

    In addition to the condensation products of hypophosphorous acid, phosphorous acid, phosphoric acid and phosphoric acid, phosphorus has many other oxyacids.
    Now select a few important brief introductions.
    See Table 12-2.
    For more information on oxyacids of phosphorus, you can Read related monographs and literature

    .

    Table 12-2 Oxy acids with various oxidation numbers of phosphorus

     

     

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