Phosphorus oxo acids and their salts

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.

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
.

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
.

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)
.

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
.

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
.

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}
.

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