PHOSPHOROUS ACID (H3PO3): PROPERTIES, RISKS AND USES - CHEMISTRY - 2024

The phosphorous acid, also called orthophosphorous acid, is a chemical compound of formula H ₃ PO ₃. It is one of the various oxygenated acids of phosphorus and its structure is presented in figure 1 (EMBL-EBI, 2015).

Given the compound formula, it can be rewritten as HPO (OH) _2. This species exists in equilibrium with a minor tautomer P (OH) ₃ (Figure 2).

Figure 1: Structure of phosphorous acid.

The IUPAC, 2005 recommendations are that the latter is called phosphorous acid, while the dihydroxy form is called phosphonic acid. Only reduced phosphorus compounds are spelled with a "bear" ending.

Figure 2: Tautomers of phosphorous acid.

Figure 3: H3PO3 form stabilized by resonance

Phosphorous acid is a diprotic acid, this means that it only has the ability to give up two protons. This is due to the fact that the main tautomer is H ₃ PO ₃. When this shape loses a proton, the resonance stabilizes the anions formed, as shown in Figure 3.

The P (OH) 3 tautomer (Figure 4) does not have the benefit of resonance stabilization. This makes the removal of the third proton much more difficult (Why is phosphorous acid diprotic and not triprotic ?, 2016).

Figure 4: form PO33- where it is observed that there is no stabilization by resonance.

Phosphoric acid (H ₃ PO ₃) forms salts called phosphites, which are used as reducing agents (Britannica, 1998). It is prepared by dissolving tetraphosphoric hexoxide (P ₄ O ₆) according to the equation:

P ₄ O ₆ + 6 H ₂ O → 4 HPO (OH) ₂

Pure phosphorous acid, H ₃ PO ₃, is best prepared by hydrolysis of phosphorous trichloride, PCl ₃.

PCl ₃ + 3H ₂ O → HPO (OH) ₂ + 3HCl

The resulting solution is heated to drive off the HCl, and the remaining water is evaporated until a colorless crystalline ₃ PO ₃ appears on cooling. The acid can also be obtained by the action of water on PBr ₃ or PI ₃ (Zumdahl, 2018).

Physical and chemical properties

Phosphorous acid are white or yellow hygroscopic tetrahedral crystals with a garlic-like aroma (National Center for Biotechnology Information, 2017).

Figure 5: appearance of phosphorous acid.

H ₃ PO ₃ has a molecular weight of 82.0 g / mol and a density of 1.651 g / ml. The compound has a melting point of 73 ° C and decomposes above 200 ° C. Phosphorous acid is soluble in water, being able to dissolve 310 grams per 100 ml of this solvent. It is also soluble in ethanol.

Furthermore, it is a strong acid with a pKa between 1.3 and 1.6 (Royal Society of Chemistry, 2015).

Heating phosphorous acid to about 200 ° C causes it to disproportionate into phosphoric acid and phosphine (PH ₃). Phosphine, a gas that normally ignites spontaneously in air.

4H ₃ PO ₃ + heat → PH ₃ + 3H ₃ PO ₄

Reactivity and hazards

Reactivity

• Phosphorous acid is not a stable compound.
• It absorbs oxygen from the air to form phosphoric acid.
• Forms yellow deposits in aqueous solution that are spontaneously flammable on drying.
• Reacts exothermically with chemical bases (eg, inorganic amines and hydroxides) to form salts.
• These reactions can generate dangerously large amounts of heat in small spaces.
• Dissolving in water or diluting a concentrated solution with additional water can generate significant heat.
• Reacts in the presence of moisture with active metals, including structural metals such as aluminum and iron, to release hydrogen, a flammable gas.
• It can initiate the polymerization of certain alkenes. Reacts with cyanide compounds to liberate hydrogen cyanide gas.
• May generate flammable and / or toxic gases in contact with dithiocarbamates, isocyanates, mercaptans, nitrides, nitriles, sulfides, and strong reducing agents.
• Additional gas-generating reactions occur with sulfites, nitrites, thiosulfates (to give H2S and SO3), dithionites (to give SO2) and carbonates (to give CO2) (PHOSPHOROUS ACID, 2016).

Dangers

• The compound is corrosive to the eyes and skin.
• Contact with the eyes can result in corneal damage or blindness.
• Contact with the skin can cause inflammation and blisters.
• Inhalation of dust will produce irritation of the gastrointestinal or respiratory tract, characterized by burning, sneezing and coughing.
• Severe overexposure can cause lung damage, suffocation, loss of consciousness or death (Material Safety Data Sheet Phosphorous acid, 2013).

Action in case of damage

• Make sure medical personnel are aware of the materials involved and take precautions to protect themselves.
• The victim should be moved to a cool place and the emergency medical service called.
• Artificial respiration should be given if the victim is not breathing.
• The mouth-to-mouth method should not be used if the victim has ingested or inhaled the substance.
• Artificial respiration is performed with the aid of a pocket mask fitted with a one-way valve or other suitable respiratory medical device.
• Oxygen should be given if breathing is difficult.
• Contaminated clothing and shoes should be removed and isolated.
• In case of contact with the substance, immediately rinse the skin or eyes with running water for at least 20 minutes.
• For less contact with the skin, avoid spreading the material on unaffected skin.
• Keep the victim calm and warm.
• The effects of exposure (inhalation, ingestion or skin contact) to the substance may be delayed.

Applications

The most important use of phosphorous acid is the production of phosphites that are used in water treatment. Phosphoric acid is also used to prepare phosphite salts, such as potassium phosphite.

Phosphites have been shown to be effective in controlling a variety of plant diseases.

In particular, treatment by trunk or foliar injection containing phosphorous acid salts is indicated in response to infections by plant pathogens of the phytoftera and pythium type (they cause root decomposition).

Phosphorous acid and phosphites are used as reducing agents in chemical analysis. A convenient and scalable new synthesis of phenylacetic acids, through the iodide-catalyzed reduction of mandelic acids, is based on the in situ generation of hydroiodic acid from catalytic sodium iodide. For this, phosphoric acid is used as a stoichiometric reducer (Jacqueline E. Milne, 2011).

It is used as an ingredient for the production of additives used in the poly (vinyl chloride) industry (Phosphorous acid (CAS RN 10294-56-1), 2017). Also the esters of phosphorous acid are used in various reactions of organic synthesis (Blazewska, 2009).

References

1. Blazewska, K. (2009). Science of Synthesis: Houben-Weyl Methods of Molecular Transformations Vol 42. New York: Thieme.
2. (1998, July 20). Phosphorous acid (H3PO3). Retrieved from Encyclopædia Britannica: britannica.com.
3. EMBL-EBI. (2015, July 20). phosphonic acid. Recovered from ebi.ac.uk: ebi.ac.uk.
4. Jacqueline E. Milne, TS (2011). Iodide-Catalyzed Reductions: Development of a Synthesis of Phenylacetic Acids. Org. Chem. 76, 9519-9524. organic-chemistry.org.
5. Material Safety Data Sheet Phosphorous acid. (2013, May 21). Recovered from sciencelab: sciencelab.com.
6. National Center for Biotechnology Information. (2017, March 11). PubChem Compound Database; CID = 107909. Retrieved from PubChem: ncbi.nlm.nih.gov.
7. Phosphorous acid (CAS RN 10294-56-1). (2017, March 15). Recovered from gov.uk/trade-tariff:gov.uk.
8. PHOSPHOROUS ACID. (2016). Recovered from cameochemicals: cameochemicals.noaa.gov.
9. Royal Society of Chemistry. (2015). PHOSPHOROUS ACID. Recovered from chemspider: chemspider.com.
10. Why is phosphorous acid diprotic and not triprotic? (2016, March 11). Recovered from chemistry.stackexchange.
11. Zumdahl, SS (2018, August 15). Oxyacid. Recovered from britannica.com.