What is Hypophosphorous Acid?
Hypophosphorous acid (HPA), which is also called phosphinic acid, is defined as a phosphorus oxyacid and a powerful reducing agent having the molecular formula H3PO2. It is a low-melting and colourless compound, soluble in dioxane, water, and alcohols. Generally, the formula for this acid is written as H3PO2, but a more descriptive presentation can be HOP(O)H2, which highlights its monoprotic character. Salts that are derived from this acid are known as hypophosphites.
Structure of Hypophosphorous Acid
The structure of Hypophosphorous acid is pseudo-tetrahedral, which is a molecular shape:
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Preparation and Availability
First, the hypophosphorous acid was prepared by the French chemist named Pierre Louis Dulong (1785–1838) in 1816.
Industrially, the acid is prepared via a two-step process: Firstly, the alkali and alkaline earth metals' hypophosphite salts result from the white phosphorus reaction with a hot aqueous solution of the appropriate hydroxide, for example, Ca(OH)2.
P4 + 4 OH− + 4 H2O → 4 H2PO-2 + 2 H2
Then, the salt is treated with a non-oxidizing and strong acid to give the free hypophosphorous acid as:
H2PO−2 + H+ → H3PO2
Usually, HPA is supplied as a 50 percent aqueous solution. Also, anhydrous acid can't be obtained by simple evaporation of the water because the acid ready oxidizes to phosphoric acid, phosphorous acid, and also disproportionates to phosphine and phosphorous acid. Pure anhydrous hypophosphorous acid may be formed by the continuous extraction of the aqueous solutions with diethyl ether.
Organic Chemistry
In organic chemistry, H3PO2 may be used for the reduction of arene diazonium salts by converting ArN+2 to Ar–H. When diazotized in the hypophosphorous acid's concentrated solution, an amine substituent can be removed from the arenas.
Owing to its ability to function as an oxygen scavenger and mild reducing agent, sometimes, it is used as an additive in the Fischer esterification reactions, where it prevents the coloured impurities formation.
It can be used to prepare the derivatives of phosphinic acid.
What are Hypophosphite Salts?
Sodium hypophosphite (NaPO2H2, which is also called sodium phosphinate) is defined as the sodium salt of hypophosphorous acid, and often, it is encountered as the monohydrate, NaPO2H2·H2O. At room temperature, it is solid, appearing as odourless white crystals. And, it is soluble in water and can absorb moisture from the air easily.
Eventually, the name has changed from sodium hypophosphite to hypophosphorous acid.
Sodium hypophosphite or the hydrated sodium hypophosphite must be kept in a cool and dry place, isolated from the oxidizing materials. It decomposes into a phosphine that is irritating to disodium phosphate and the respiratory tract.
2 NaH2PO2 → PH3 + Na2HPO4
Uses
Mainly, the sodium hypophosphite may be used for electroless nickel plating (Ni-P). With this particular method, a durable nickel-phosphorus film may coat objects with irregular surfaces, such as in aviation, petroleum fields, and avionics.
Sodium hypophosphite is also capable of reducing the nickel ions in solution to the metallic nickel on metal substrates and plastic substrates as well. The latter needs that the substrate can be activated with fine particles of palladium. The resulting nickel deposit contains around 15 percent of phosphorus.
It may be used as a food additive.
Industrial Uses
Sodium sulfate is essential in textile manufacturing, especially in Japan, where it is such a largest application. Sodium sulfate also helps in "levelling," reducing the negative charges present on fibres so that the dyes may penetrate evenly. It does not corrode the stainless steel vessels used in dyeing, unlike the alternative sodium chloride. This application consumed approximately 100,000 tonnes in Japan and the US in 2006.
Small-scale Applications
Anhydrous sodium sulfate is widely used as an inert drying agent in the laboratory for removing traces of water from the organic solutions.
It is more slower-acting but efficient compared to the similar agent magnesium sulfate. Only it is effective below up to a temperature of 30 °C, but it may be used with various materials since it is chemically fairly inert. Sodium sulfate can be added to the solution until the crystals, together, no longer clump.
Phosphorous vs. Phosphoric acid
Phosphoric acid (H3PO4) should not get confused with phosphorous acid (H3PO3). The first is a completely hydrated and oxidised form of P, while the second is a partially hydrated and oxidised form. Thus, phosphorous acid can be described as a powerful reducing agent, whereas phosphoric acid is not. Polyphosphoric acid is an odourless liquid with high viscosity and hygroscopic properties.
Phosphoric acids having less than 95 percent of H3PO4 (68 percent of P2O5) contain simple orthophosphoric acid. In higher concentrations, the acid holds a combination of pyro, ortho, tetra tri, and phosphoric acid that is highly concentrated. For this specific reason, acids with a concentration of more than 68%, and P2O5 is better referred to as polyphosphoric acid. Polyphosphoric acid can be insoluble in water, hydrolyzing into orthophosphoric acid by producing heat. They are insoluble in both hydrocarbons and halogenated hydrocarbons.
FAQs on Hypophosphorous Acid and Hypophosphite Salts
1. What is hypophosphorous acid?
Hypophosphorous acid, with the formula H3PO2, is a phosphorus oxyacid, also known by its IUPAC name, phosphinic acid. It is a powerful reducing agent and is unique among phosphorus acids because it is monobasic, meaning it only donates one proton in reactions despite having three hydrogen atoms.
2. What are hypophosphite salts?
Hypophosphite salts are chemical compounds that contain the hypophosphite anion (H2PO2⁻). These salts are formed when hypophosphorous acid reacts with a base. A common and commercially important example is sodium hypophosphite (NaH2PO2).
3. How does the structure of hypophosphorous acid explain why it's monobasic?
The structure of hypophosphorous acid is key to its properties. The central phosphorus atom is directly bonded to two hydrogen atoms (P-H bonds) and one hydroxyl group (P-OH group). Only the hydrogen atom in the hydroxyl (P-OH) group is acidic and can be released as a proton. The two hydrogens attached directly to phosphorus are not acidic, which is why H3PO2 is considered monobasic.
4. Why is hypophosphorous acid a good reducing agent?
Hypophosphorous acid acts as a strong reducing agent because of the two P-H bonds present in its molecule. These bonds can be readily oxidised, which allows the acid to donate electrons and reduce other substances. During these reactions, hypophosphorous acid itself is typically oxidised to phosphorous acid or phosphoric acid.
5. What is the difference between hypophosphorous acid and phosphoric acid?
The main differences are in their structure, basicity, and chemical properties:
- Basicity: Hypophosphorous acid (H3PO2) is monobasic (one acidic hydrogen), while phosphoric acid (H3PO4) is tribasic (three acidic hydrogens).
- Structure: H3PO2 has two P-H bonds and one P-OH bond. In contrast, H3PO4 has three P-OH bonds and no P-H bonds.
- Chemical Property: The presence of P-H bonds makes H3PO2 a strong reducing agent. Phosphoric acid, lacking these bonds, does not act as a reducing agent.
- Oxidation State: The oxidation state of phosphorus is +1 in H3PO2 and +5 in H3PO4.
6. What are the main applications of sodium hypophosphite?
Sodium hypophosphite is primarily used as an industrial reducing agent. Its most significant application is in electroless nickel plating (ENP), a process that deposits a uniform layer of a nickel-phosphorus alloy onto metal or plastic surfaces without using electricity. It is also used as a stabiliser in some polymers and as a catalyst in certain chemical reactions.
7. What happens when hypophosphorous acid is heated?
When heated, hypophosphorous acid undergoes a disproportionation reaction, where the same element (phosphorus) is both oxidised and reduced. It decomposes to form two products: phosphoric acid (H3PO4), where phosphorus has a higher oxidation state (+5), and toxic phosphine gas (PH3), where phosphorus has a lower oxidation state (-3).
8. What is the oxidation state of phosphorus in H3PO2?
To find the oxidation state of phosphorus in hypophosphorous acid (H3PO2), we assign +1 to hydrogen and -2 to oxygen. If 'x' is the oxidation state of phosphorus, the equation is: 3(+1) + x + 2(-2) = 0. This simplifies to 3 + x - 4 = 0, which means x = +1. Therefore, the oxidation state of phosphorus is +1.
