Tetrametaphosphoric                    2. Tetrapolyphosphoric

                                                                                                     

The brown ring test is used to detect nitrate(V) ions in aqueous solutions

                                                                                                     

Procedure:

1. Mix a freshly prepared FeSO₄ solution with a solution suspected of containing nitrate(V) ions in a test tube.

2. Concentrated H₂SO₄ is added carefully along the side to the bottom of the test tube with the test tube tilted.

3. Formation of a brown ring confirms the presence of nitrate(V) ions in the solution.

   

Oxides and oxoacids of phosphorus

 

Phosphorus(III) oxide , P₄O₆ (also called tetraphosphorus hexoxide) and phosphorus(IV) oxide, P₄O₁₀ (or tetraphosphorus decoxide) are acid anhydrides of phosphorus oxyacids and hence readily react with water.

P₄O₆ + 6 HCl → 2 H₃PO₃ + 2 PCl₃

P₄O₁₀ is a particularly good dehydrating agent that can even remove water from nitric acid, HNO₃. The structure of P₄O₆ is like that of P₄ with an oxygen atom inserted between each of the P-P bonds.

The structure of P₄O₁₀ is similar to that of P₄O₆ with the addition of one oxygen bond to each phosphorus atom via a double bond and protruding away from the tetrahedral structure.

The desiccating power of P₄O₁₀ is strong enough to convert many mineral acids to their anhydrides.

Examples: HNO₃ is converted to N₂O₅; H₂SO₄ is converted to SO₃; HClO₄ is converted to Cl₂O₇.

Oxoacids of phosphorus

Oxidation State	Formula	Name	Acidic Protons	Compounds
+1	H3PO2	hypophosphorous acid	1	acid, salts
+3	H3PO3	(ortho)phosphorous acid	2	acid, salts
+5	(HPO3)n	metaphosphoric acids	N	salts (n=3,4)
+5	H5P3O10	triphosphoric acid	3	salts
+5	H4P2O7	pyrophosphoric acid	4	acid, salts
+5	H3PO4	(ortho)phosphoric acid	3	acid, salts

Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H₃PO₂. HOP(O)H₂ exists in equilibrium with the minor tautomer HP(OH)₂. Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.

8Р + 3Ва(ОН)₂ + 6Н₂О = 2РН₃­ + 3Ва(Н₂РО₂)₂

PH₃ + 2I₂ + 2H₂O → H₃PO₂ + 4I⁻ + 4H⁺

Ва(Н₂РО₂)₂ + H₂SO₄ = 2Н₃РО₂ + BaSO₄

sp³-is a hybrid state of Р due to p-bonding with О.

It is relatively strong monobasic acid :

Н₃РО₂ = Н⁺ + Н₂РО₂^-    k = 8.5^.10^-2

Anhydride of this acid is not known.

Hypophosphites are well soluble in water and have strong reducing properties:

NiCl₂ + 2  + 2H₂O = Ni¯ + 2  (phosphite) + 2H₂

­2е      ¯2е          ­1е^.2

    Н₃РО₂ can but rarely be also an oxidant:

Н₃РО₂ + 2Zn + 4HCl [H] =  + 2ZnCl₂ + 2H₂O

Hypophosphorous acid can also disproportionate when heated:

3  =  + 2

 

Phosphorous acid.

Preparation:

РX₃ + 2Н₂О = Н₃РО₃ + 3НX

Structure (see the scheme):

The bond P—H existence in the molecule signifies that phosphorous acid is a dibasic one and its neutral salts are absent.

Н₃РО₃ is an acid of intermediate strength:

Н₃РО₃ Û Н⁺ + Н₂РО₃^-             k₁ = 8^.10^-3

Н₂РО₃^- Û Н⁺ + НРО₃^2-            k₂ = 2.6^.10^-7

• It is a low active reducing agent and oxidizing properties are practically absent (E^о РО₃^2-/РО₄^3- = +0.28 V).

Hg(NO₃)₂ + H₃PO₃ + H₂O = Hg + H₃PO₄ + 2HNO₃

H₃PO₃ + I₂ + H₂O = H₃PO₄ + 2HI

• Н₃РО₃ can be an oxidising agent only with strong reducing agents:

Н₃РО₃ + 6Н (Zn+HCl) = РН₃ + 3Н₂О

• It disproportionates when heating

4Н₃РО₃ = 3Н₃РО₄ + РН₃

At heating dihydrogen phosphite salts of pyrophosphorous (diphosphorous) acid pyrophosphites can be obtained:

2NaH₂PO₃ = Na₂H₂P₂O₅ + H₂O

Pyrophosphites hydrolyse when boiling:

Na₂H₂P₂O₅ + 3H₂O = 2NaOH + 2H₃PO₃

Pyrophosphorous acid itself is of low stability. It structure is shown below:

There is also metaphosphorous acid of polymeric structure (HPO₂)_n

Phosphoric acids. Р₂О₅ interaction with water depending on the number of associated molecules of H₂O leads to the formation of a family of phosphoric acids.

 

Structure. Phosphoric acid exists in two polymeric forms:

· linear

· cyclic.

For instance, H₃PO₄ is a linear polymer, and (HPO₃)_n is a cyclical one. Let us consider some types of phosphoric acid. Some of them are formed at the beginning of a reaction (numbers indicate the order of appearance)

Tetrametaphosphoric                    2. Tetrapolyphosphoric

 

Р₄О₁₀ + 2Н₂О = (НРО₃)₄                                     (НРО₃)₄ + Н₂О = Н₆Р₄О₁₃

 

 

 

 

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