Phosphorous Acid (H3PO3) - Reactions, Structure, Properties, Uses, FAQs

Preparation Of Phosphorous Acid ($\mathrm{H}_3 \mathrm{PO}_3$)

Phosphorous acid or $\mathrm{H}_3 \mathrm{PO}_3$ is produced by the slow combustion reaction of phosphorus in the form of a white volatile powder. The salt of phosphorus acid is called phosphites.

• Phosphoric acid is prepared by the reaction of phosphorus trichloride that is PCl₃ with water.

By Hydrolysis of Phosphorus Trichloride $\left(\mathrm{PCl}_3\right)$

$
\mathrm{PCl}_3+3 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{PO}_3+3 \mathrm{HCl}
$

• It is also prepared by the reaction of tetra phosphorus hexoxide P₄O₆.

By Action of Steam on Phosphorus Trioxide ( $\mathrm{P}_4 \mathrm{O}_6$ )

$
\mathrm{P}_4 \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{H}_3 \mathrm{PO}_3
$

• By Oxidation of Phosphine $\left(\mathrm{PH}_3\right)$

  $
  \mathrm{PH}_3+\mathrm{O}_2 \rightarrow \mathrm{H}_3 \mathrm{PO}_3
  $
  Phosphine is oxidized with limited oxygen. This produces phosphorous acid under controlled conditions.

Toxicity Of Phosphorous Acid ($\mathrm{H}_3 \mathrm{PO}_3$)

When phosphorous acid, $\mathrm{H}_3 \mathrm{PO}_3$ contact with the eyes skin, and mucous membranes, severely causes irritation and eye damage. It is also toxic when inhaled or by absorption by the skin. When a person inhales a particular amount of phosphorus acid it will cause irritation to the nose, throat, and lungs which will then lead to coughing, wheezing, and breathing problems. It severely damages the Respiratory system.

Phosphorus acid ($\mathrm{H}_3 \mathrm{PO}_3$) is a white solid with sour order and is soluble in water. The taste of phosphorous acid eases garlic-like. From the chemical formula of phosphorus acid, we may think it is a tri-protic in nature that it ionizes two protons but actually it is diprotic. The molar mass of phosphorus acid is 81.9 g/mol with a density of 1.651g/cm³. The melting point of phosphorus acid eases 73.6 degrees Celsius and the boiling point of phosphorus acid is 200 degrees Celsius. It is soluble in water and also it is soluble in ethanol.

Structure Of Phosphorus Acid ($\mathrm{H}_3 \mathrm{PO}_3$)

The structure of $\mathrm{H}_3 \mathrm{PO}_3$ can be easily explained in this portion. Phosphorus acid ($\mathrm{H}_3 \mathrm{PO}_3$) contains one covalent bond and unity has the name trihydroxy ortho phosphorus acid suggesting that it contains three OH groups that are 3 hydrogen bond acceptors. The geometry of phosphorus acid is pseudo-tetrahedral. One oxygen forms a double bond with a Phosphorus atom, three OH groups are bonded directly with the phosphorus atom. The structure of phosphorus acid is shown below.

Phosphorous Acid

The structure of H₃PO₃ can be more accurately explained using the chemical formula HPO(OH)₂. In the solid state phosphorous acid is tetrahedrally coordinated with one P-H and PO and two longer POH bonds. While it exists in equilibrium with the tautomer form containing three POH bonds. The one containing two hydroxyl groups is phosphonic acid while the other with three hydroxyl groups is phosphorous acid. The name ending with ‘ous’ means it is in the reduced form. The following structure shows this fact.

Tautomerism in phosphorous acid

Other important oxyacids are phosphoric acid and hypophosphorous acid. These compounds also show tautomerism similar to phosphorous acid by the shift in the position of H. This is all about the structure of $\mathrm{H}_3 \mathrm{PO}_3$.

Reactions Of Phosphorus Acid ($\mathrm{H}_3 \mathrm{PO}_3$)

Acidic property of $\mathrm{H}_3 \mathrm{PO}_3$

Phosphorous acid, $\mathrm{H}_3 \mathrm{PO}_3$ has a pKa value of 1.26-1.3. And it donates two hydrogen atoms only. The H atom which is bonded directly with the phosphorus, is not readily ionized. The reaction can be represented as,

$\mathrm{HPO}(\mathrm{OH})_2 \rightarrow \mathrm{HPO}_2(\mathrm{OH})^{-}+\mathrm{H}^{+}$

Thus, one hydrogen ion is formed. The obtained product, hydrogen phosphite ion is a weak acid with a pKa value of 6.7. It will also donate one hydrogen ion to form a phosphite ion. The reaction taking place is shown below.

$\mathrm{HPO}_2(\mathrm{OH})^{-} \rightarrow \mathrm{HPO}_3^{2-}+\mathrm{H}^{+}$

HPO₃^2- is the second conjugate base of phosphorous acid with the name phosphite ion. This is the acidic property of phosphorous acid, the three hydrogens are not acidic

Redox Properties

Phosphorous acid, H₃PO₃ undergoes a disproportionation reaction at a temperature of 200 degrees Celsius to form phosphoric acid and phosphine.

$4 \mathrm{H}_3 \mathrm{PO}_3 \rightarrow 3 \mathrm{H}_3 \mathrm{PO}_4+\mathrm{PH}_3$

This reaction is used for the preparation of phosphines that is PH₃. Phosphorous acid slowly oxidizes in air to form phosphoric acid. The reaction of mercuric chloride with phosphorous acid forms mercurous chloride. While mercurous chloride reacts with phosphoric acid, $\mathrm{H}_3 \mathrm{PO}_3$ to form mercury. Thus it shows redox properties.

Basicity Of Phosphoric Acid

For phosphoric acid, H₃PO₃ three OH groups are bonded to the phosphorus atom so it is a tribasic group with a basicity of three. Basicity is the number of hydrogen atoms present in a chemical compound. Here the three hydrogen atoms are strongly bonded with an Electronegative atom of oxygen so it is acidic. While the basicity of phosphorus acid is only two. As all the hydrogen atoms are acidic it is dissociated completely in water except at low pH. When all three hydrogen ions have been removed the compound obtained is called orthophosphates. And the basicity of orthophosphoric acid is 2.

Uses Of Phosphorous Acid ($\mathrm{H}_3 \mathrm{PO}_3$).

• Phosphorous acid, $\mathrm{H}_3 \mathrm{PO}_3$ is widely used for the production of PVC stabilizers, amino methylene phosphonic acid, etc.

• Because of its strong reducing nature, it can be used as a reducing agent.

• It is used in the production of synthetic fibers and organophosphorus pesticides.

• It is used for the production of amino trimethylene phosphonic acid which is used as a water treatment agent.

• Phosphorous acid, $\mathrm{H}_3 \mathrm{PO}_3$ is used to produce the fertilizer phosphate salt.

• Phosphorous acid, $\mathrm{H}_3 \mathrm{PO}_3$ also has many applications in the medical field mostly the dental field.

• It is used for protecting ferrous materials including steel from rusting.

• Basic lead phosphite is prepared by using phosphorous acid.

Application Of Phosphorous Acid($\mathrm{H}_3 \mathrm{PO}_3$)

Phosphorous acid, also called orthophosphorous acid, is a dibasic acid with the formula $\mathrm{H}_3 \mathrm{PO}_3$. It acts as a strong reducing agent and has multiple industrial and agricultural uses.

1. Production of Phosphites

• $\mathrm{H}_3 \mathrm{PO}_3$ is used to produce phosphite salts such as sodium phosphite ($\mathrm{Na}_2 \mathrm{HPO}_3$) and potassium phosphite ($\mathrm{KH}_2 \mathrm{PO}_3$). These are used in fertilizers and as stabilizers in plastics.

2. Reducing Agent in Chemical Reactions

• Phosphorous acid is a powerful reducing agent. It can reduce gold, silver, and mercury salts to their metallic form.

3. Synthesis of Organophosphorus Compounds

• Used in the preparation of phosphonates and phosphite esters, which are important in Plastic stabilizers, Lubricant additives, Flame retardants

4. Manufacture of Plastic Stabilizers

• Acts as a stabilizer in PVC (Polyvinyl Chloride) production to prevent oxidative degradation.

5. Agricultural Use

• Phosphorous acid and its salts help control fungal and oomycete infections in crops like grapes and potatoes.

Phosphinic Acid And Phosphonic Acid

The hypophosphorous acid known as phosphinic acid is also a phosphorous acid, just like the phosphonic acid formula is $\mathrm{H}_3 \mathrm{PO}_3$. The chemical formula of phosphinic acid is H₃PO₂ and it contains one phosphorus, three hydrogens, and only two oxygen atoms. It is a monoprotic compound in which only one hydrogen is acidic while for phosphonic acid it is diprotic with two acidic hydrogen atoms. The salt of hypophosphorous acid is hypophosphite. Like phosphonic acid, it also exists in equilibrium with the tautomer, HP(OH)₂.

When phosphinic acid is heated, it undergoes a disproportionation reaction to form phosphine and phosphoric acid. So it can be used for the synthesis of phosphoric acid. It is also used for reducing nickel. Its main application is the reduction of metal salts back to the bulk metal. Organo phosphinic acids can be easily synthesized using phosphinic acids, and they have many important applications in various chemical reactions.

Some Solved Examples

Question.1 The oxidation state of phosphorus in hypophosphoric acid is +

1) 4

2) 5

3) 3

4) 1

Solution:

Phosphorus forms a number of oxoacids.

The compositions of the oxoacids are interrelated in terms of loss or gain of H₂O molecule or an O-atom. The structures of some important oxoacids are given in later pictures. In oxoacids, phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has a lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H₄P₂O₆) or P–H (e.g., in H₃PO₂) bonds but not both. These acids in the +3 oxidation state of phosphorus tend to be disproportionate to higher and lower oxidation states. For example, orthophosphorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

$4 \mathrm{H}_3 \mathrm{PO}_3 \rightarrow 3 \mathrm{H}_3 \mathrm{PO}_4+\mathrm{PH}_3$

Hypophosphoric acid is $\mathrm{H}_4 \mathrm{P}_2 \mathrm{O}_6$ oxidation state of P is +4 .
Hence, the answer is option (1).

Question. 2 Which of the Phosphorus oxoacid can create silver mirror from $\mathrm{AgNO}_3$ solution?
1)$
\left(\mathrm{HPO}_3\right)_n
$

2)$
\mathrm{H}_4 \mathrm{P}_2 \mathrm{O}_6
$

3) $
\mathrm{H}_4 \mathrm{P}_2 \mathrm{O}_5
$

4)$
\mathrm{H}_4 \mathrm{P}_2 \mathrm{O}_7
$

Solution:

Phosphorus forms several oxoacids. These are the important phosphorus oxoacids, with their formulas, methods of preparation, and the presence of some characteristic bonds in their structures.

The compositions of the oxoacids are interrelated in terms of loss or gain of H₂O molecule or O-atom. The structures of some important oxoacids are given in later pictures. In oxoacids, phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has a lower oxidation state (less than +5) include, in addition to P=O and P–OH bonds, either P–P (e.g., in H₄P₂O₆) or P–H (e.g., in H₃PO₂) bonds but not both. These acids in the +3 oxidation state of phosphorus tend to be disproportionate to higher and lower oxidation states. For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

Silver mirror test can be given by $\mathrm{P}^{+3}, \mathrm{P}^{+1}$ ox acid

$
\mathrm{H}_4 \stackrel{+3}{\mathrm{P}_2} \mathrm{O}_5+\mathrm{Ag}_2 \mathrm{O} \rightarrow \mathrm{Ag}
$
Silver mirror

Hence, the answer is the option (3).

Question.3 Phosphorous acid (H₃PO₃) is prepared by the hydrolysis of:
a) $\mathrm{PCl}_5$
b) $\mathrm{PCl}_3$
c) $\mathrm{P}_4 \mathrm{O}_{10}$
d) $\mathrm{PH}_3$

Solution:

Hydrolysis of phosphorus trichloride with water gives H₃PO₃ and HCl gas.

Hence, the correct option is (b) $\mathrm{PCl}_3$

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