Hypophosphoric Acid: Formula, Structure, Preparation & Uses

Hypophosphoric acid is a mineral acid containing phosphorus in the +4 oxidation number. Furthermore, this acid can be found in solid form as the dihydrate and has the molecular formula H₄P₂O₆.2H₂O. It is evident from the formula that the phosphorus atoms are similar and the P-P bond connects them strongly. Hypophosphoric acid can be produced by combining red phosphorus and sodium chlorite at room temperature.

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Key Terms: Hypophosphoric Acid, Sodium Chlorite, Red Phosphorus, Diphosphoric Acid, Ethane

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Structure of Hypophosphoric Acid

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As shown in the diagram below, this is the chemical orientation of hypophosphoric acid. It is made up of oxonium ions and is usually illustrated as [H3O⁺]₂ [H₂P₂O₆]^2-. This acid is isostructural with diammonium salt which is composed of the anion [HOPO₂PO₂OH]^2-. This anion has a P-P bond length of 219 pm.

Structure of Hypophosphoric Acid

The HOPO₂PO₂OH^2- anion in Na₂H₂P₂O₆.6H₂O possesses an unsymmetrical, clumsy structure resembling that of ethane. It also has a P-P bond with a length of 219 pm. Every phosphorus atom in this acid has two P-O bonds having a length of 151 pm and a P-OH bond with 159 pm in length.

We must also note that its isomeric state called Isohypophosphoric acid has a non-identical chemical orientation with phosphorus atoms that do not resemble one another. One of these atoms already has a bond with an H-atom and has an oxidation state of +3. Further, it is linked by a P-O-P bridge to the next phosphorus atom, which has an oxidation state of +5.

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Preparation of Hypophosphoric Acid

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In the laboratory preparation of hypophosphoric acid, it can be manufactured by mixing red phosphorus with sodium chlorite at room temperature. The equation representing this chemical reaction is:

2P + 2NaClO₂ + 2H₂O \(\to\) Na₂H₂P₂O₆ + 2HCl

When white phosphorus is partly immersed in water, it oxidizes in the air, producing a mixture of hypophosphoric acid, phosphorous acid and phosphoric acid. In the case of hot hydrochloric acid, hypophosphoric acid is unbalanced. In 4M HCl, it hydrolyzes to produce H₃PO₃ + H₃PO₄.

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Physical & Chemical Properties

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Hypophosphoric acid is sometimes also known as Diphosphoric acid. The physical and chemical properties of hypophosphoric acid are given below:

1. It has a solid appearance and appears to be white in color. 
2. The chemical formula for this acid is H₄O₂P₆. 
3. Apart from this, when talking of the melting point, it can be observed that this acid bears a melting point at 54°C. 
4. Hypophosphoric acid has a molar mass of 161.98 g/mol. Furthermore, this acid is water-soluble.

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Uses of Hypophosphoric Acid

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Following are the uses of hypophosphoric acid:

• Hypophosphoric acid serves as a bleaching or decolorizing agent required for plastics, synthetic fibers, polyester fiber, polyacrylonitrile, epoxy resins, fatty alcohols, and glycerols.
• It serves as a wetting agent and this property is used in the process of electroplating.
• Hypophosphoric acid also has the potential to serve as a stimulant in pharmaceutical agents. 
• It also serves as a reducing agent. 
• It can be used as a tetrabasic acid.

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Health Effects

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The fluid hypophosphoric acid is corrosive in nature. This makes it dangerous and can possibly cause chemical burns to the eyes and skin if it comes in contact. Furthermore, if inhaled, this acid could lead to irritation in the nose and throat. Apart from this, it can also irritate the eyes and eyelids. Upon any accident with this acid, It is always recommended to use water for removing it. If not taken care of on time, eye contact with this acid can even lead to long-term or irreversible vision impairment. Further, avoid ingestion because it will harm the mucous lining of the mouth as well as the throat and stomach.

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Things to Remember

• Hypophosphoric acid is also referred to as Diphosphoric Acid.
• Hypophosphoric acid is represented with the formula H₄P₂O₆. 
• It has four acidic hydrogens that can be made at room temperature by mixing red phosphorus and sodium chlorite together.
• The chemical equation for laboratory preparation of hypophosphoric acid is 2 P + 2 NaClO₂ + 2 H₂O\(\to\) Na₂H₂P₂O6 + 2 HCl.
• Hypophosphoric acid shows a basicity of 4 and has a corrosive nature.
• Hypophosphoric acid is a white solid in appearance. 

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