PHOSPHOROUS ACID
PHOSPHOROUS ACID
Phosphorous acid is an oxoacid of phosphorus with an oxidation number of +3, whose chemical formula is H3PO3.
Phosphorous acid’s molecular weight is 82.00 g/mol and its density is 1.65 g/cm3.
Phosphorous acid is obtained by hydrolysis of phosphorus trichloride.
CAS Number: 13598-36-2
EC Number: 237-066-7
Molecular Formula: H3O3P
Molar Mass: 81.99 g/mol
Synonyms: Phosphonic acid, Phosphorous acid, Dihydroxyphosphine oxide, Dihydroxy(oxo)-λ5-phosphane, Dihydroxy-λ5-phosphanone, Orthophosphorous acid, Oxo-λ5-phosphanediol, Oxo-λ5-phosphonous acid, Metaphosphoroic acid, Phosphonic acid, Phosphorous acid, Dihydroxyphosphine oxide, Dihydroxy(oxo)-λ5-phosphane, Dihydroxy-λ5-phosphanone, Orthophosphorous acid, Oxo-λ5-phosphanediol, Oxo-λ5-phosphonous acid, Metaphosphoroic acid, Phosphonsaeure, phosphorige Saeure, trihydroxidophosphorus, Phosphorous acid, ortho, O-PHOSPHOROUS ACID, trioxophosphoric(3-) acid, H2PHO3, (HO)2HPO, HPO(OH)2, hydridodihydroxidooxidophosphorus, (PHO(OH)2), P(OH)3, trihydrogen trioxophosphate(3-), DTXCID5015511, DTXCID6029674, hydridotrioxophosphoric(2-) acid, (P(OH)3), dihydrogen hydridotrioxophosphate(2-), PHOSPHONIC ACID, PHOSPHORUS ACID, Phosphorous acid 99%, Phosphonsure, Phospohorous acid, Phosphorous Acid Crystal, AURORA KA-1076, orthophosphorus, ORTHOPHOSPHOROUS, Phosphorous acid,
Phosphorous acid (chemical formula H₃PO₃ or more precisely HPO(OH)₂) is a diprotic oxyacid of phosphorus, known for its strong reducing properties and versatility in both industrial and agricultural applications.
Phosphorous acid is a colorless, crystalline solid at room temperature and is highly soluble in water, forming an acidic solution.
Structurally, phosphorous acid consists of one P=O double bond, one P–H bond, and two hydroxyl groups, making it chemically distinct from phosphoric acid (H₃PO₄), despite their similar names.
This unique structure allows phosphorous acid to act as a reducing agent, capable of converting metal ions to their lower oxidation states and being oxidized itself to phosphoric acid.
Industrially, Phosphorous acid is widely used in the synthesis of phosphonates, which serve as effective scale inhibitors, chelating agents, and water treatment chemicals.
In agriculture, phosphorous acid and its salts (phosphites) are used as systemic fungicides, particularly effective against oomycete pathogens like Phytophthora and Pythium, enhancing plant defense responses while also supplying phosphorus in a bioavailable form.
Additionally, phosphorous acid finds utility in the production of plastic stabilizers, flame retardants, and as an intermediate in chemical manufacturing.
Though not commonly found in nature, Phosphorous acid is typically produced industrially by the controlled hydrolysis of phosphorus trichloride (PCl₃).
Phosphorous acid’s handling requires caution, as it is corrosive and can cause severe burns upon contact.
The dual role of phosphorous acid as both a nutrient source and a protective agent makes it an important compound across various sectors, from agriculture and water treatment to chemical synthesis and materials science.
Phosphorous acid is the compound described by the formula H3PO3.
Phosphorous acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula.
Phosphorous acid is an intermediate in the preparation of other phosphorus compounds.
Organic derivatives of Phosphorous acid, compounds with the formula RPO3H2, are called Phosphorous acids.
Phosphorous acid which is also called Phosphorous acid is a colourless oxyacids of phosphorus.
Phosphorous acid is produced in the form of a white volatile powder by the slow combustion of phosphorus.
Phosphorous acid’s salts are called phosphates.
Phosphorous acid is conveniently prepared by allowing phosphorous trichloride to react with water.
In inorganic chemistry, Phosphorous acid is a phosphorus oxoacid with a formula of H3PO3.
Phosphorous acid exists in solution as two tautomers, the major one being HP(O)(OH)2 and the minor one P(OH)3.
In organic chemistry, a Phosphorous acid is a compound with the general formula RP(O)(OH)2.
An example of an organic Phosphorous acid is Foscarnet.
An oligoPhosphorous acid refers to a few molecules of Phosphorous acid condensed into a molecule with the loss of water.
A general formula for such oligoPhosphorous acids is (HPO)nOn-1(OH)2, where n = 2, 3, 4, etc., oligo-.
A polyPhosphorous acid can have dozens of such Phosphorous acid units condensed in a row with the loss of H2O for each unit added on.
An example that incorporates triPhosphorous acid: ethane-1,1,2-triPhosphorous acid.
In some phosphonic anhydrides (RPO2)3, R can be tBu, 2-methylphenyl, 2,4,6-trimethylphenyl
Phosphorous acid is a common inorganic acid with the chemical formula H3PO4.
The appearance of Phosphorous acid is colorless, transparent and syrupy liquid.
Phosphorous acid is odorless, sour, easily soluble in water and ethanol, etc.
Phosphorous acid appears as a white or yellow crystalline solid (melting point 70.1 deg C) or a solution of the solid.
Density of Phosphorous acid is 1.651 g /cm3.
Phosphorous acid is a diprotic phosphorus oxoacid that exists as two tautomers while in solution.
Phosphorous acid is the compound described by the formula H3PO3.
Phosphorous acid is one of the oxoacids of phosphorus, other important members being phosphoric acid (H3PO4) and hypophosphorous acid (H3PO2).
Note that only the reduced phosphorus compounds are spelled with an “ous” ending.
Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide.
HP(O)(OH)2 is Phosphorous acid of the hydrolysis of its acid anhydride, P4O6: P4O6 + 6 H2O → 4 HP(O)(OH)2
An analogous relationship connects H3PO4 and P4O10.
Phosphorous acid is an oxoacid of phosphorus with an oxidation number of +3, whose chemical formula is H3PO3.
Phosphorous acid’s molecular weight is 82.00 g/mol and Phosphorous acid density is 1.65 g/cm3.
Phosphorous acid is obtained by hydrolysis of phosphorus trichloride.
In solution, it shows tautomerism with Phosphorous acid.
In organophosphorus chemistry, Phosphorous acid is the generic name for a series of organophosphorus compounds with the general formula R-P(=O)(OH )2, where R is an organic group.
Phosphorous acid, H3PO3, is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula.
Phosphorous acid is as an intermediate in the preparation of other phosphorous compounds.
Because preparation and uses of “Phosphorous acid” actually pertain more to the major tautomer, Phosphorous acid.
Phosphorous acid has the chemical formula H3PO3, which is best expressed as HPO(OH)2 to show Phosphorous acid diprotic character.
Phosphorous acid is a phosphorus oxoacid.
Phosphorous acid is a conjugate acid of a dihydrogenphosphite.
Phosphorous acid is a tautomer of a Phosphorous acid.
Phosphorous acid is deliquescent.
Phosphorous acid absorbs oxygen from the air very readily to form phosphoric acid.
Phosphorous acid is soluble in water.
Phosphorous acid is a non flammable.
Phosphorous acid is a common inorganic acid with the chemical formula H3PO4.
Phosphorous acid is acids containing phosphorous.
Phosphorous acid is able to release protons when in aqueous solutions.
Phosphorous acid is phosphorous containing acids that have many industrial applications such as in the production of fertilizers.
Phosphoric acid is a phosphorous containing acid having the chemical formula H3PO4.
The IUPAC name of Phosphorous acid is trihydroxidooxidophosphorus.
Phosphorous acid is a triprotic acid because Phosphorous acid can release three protons (hydrogen ions) in an aqueous medium.
The molar mass of phosphoric acid is 97.99 g/mol.
Phosphoric acid is available as a white solid that is deliquescent or as a syrupy liquid that has a high viscosity.
However, Phosphorous acid odorless.
The melting point of Phosphorous acid is 42.35◦C and the boiling point is 213◦C, but at high temperatures, Phosphorous acid decomposes.
Phosphorous acid is an acid containing phosphorous and the chemical formula is H3PO3.
Although this chemical structure contains three hydrogen atoms, Phosphorous acid is a diprotic acid.
A diprotic acid is an acid that is capable of releasing two hydrogen ions (protons) to an aqueous medium.
Phosphorous acid is also called orthophosphorous acid.
The molar mass of Phosphorous acid is 81.99 g/mol.
At room temperature, Phosphorous acid is a white solid that is deliquescent (absorb water from the air when exposed and dissolve).
The melting point of Phosphorous acid is 73.6◦C and the boiling point is 200◦C.
At temperatures above the boiling point, the compounds tend to decompose.
When considering the chemical structure of the Phosphorous acid, it has a phosphorous atom as the central atom bonded with two –OH groups and one oxygen atom bonded via a double bond and a hydrogen atom bonded via a single bond.
This structure is known as a Pseudo-tetrahedral structure.
The Phosphorous acid is made via hydrolysis of the anhydride of the acid:
P4O6 + 6 H2O → 4 H3PO3
But in industrial scale productions,phosphorous chloride (PCl3) is hydrolyzed by steam:
PCl3 + 3 H2O → H3PO3 + 3 HCl
Phosphorous acid is used as a reducing agent in chemical analysis.
This acid readily converts into phosphoric acid when heated to about 180◦C.
The salts formed by Phosphorous acid are known as phosphates.
The most common application of phosphorus acid is that; Phosphorous acid is used in the production of basic lead phosphite (a stabilizer in PVC).
Phosphorous acid, with the molecular formula H₃PO₃, is a dibasic oxyacid of phosphorus and a key chemical intermediate with widespread industrial, agricultural, and chemical significance.
More accurately represented by the structure HPO(OH)₂, Phosphorous acid consists of one hydrogen atom directly bonded to the phosphorus atom (a P–H bond), along with two hydroxyl groups and a doubly bonded oxygen atom, giving it a pyramidal geometry around the phosphorus center.
This structure distinguishes it sharply from phosphoric acid (H₃PO₄), as only two of its hydrogen atoms are ionizable (acidic), making phosphorous acid diprotic, not triprotic.
As a strong reducing agent, phosphorous acid readily oxidizes to phosphoric acid and is used to reduce metal salts, such as silver nitrate or mercury(II) chloride, to their elemental forms or lower oxidation states.
Phosphorous acid is typically prepared by the hydrolysis of phosphorus trichloride (PCl₃) with water, a reaction that also produces hydrochloric acid as a byproduct.
In agriculture, phosphorous acid is highly valued for its ability to act as both a fungicide and a plant nutrient.
Phosphorous acid’s salts—known as phosphites or phosphonates—are used to control soil-borne and systemic plant pathogens, especially oomycetes like Phytophthora infestans (which causes late blight in potatoes and tomatoes) and Pythium species.
Unlike phosphate fertilizers, phosphite ions are readily absorbed and translocated in plants, offering both protective and growth-promoting benefits.
In industrial applications, phosphorous acid is a precursor to numerous organophosphorus compounds, including plastic additives, flame retardants, and chelating agents for water treatment.
Phosphorous acid plays a significant role in the synthesis of aminomethylene phosphonates, which are widely used in detergents and industrial cleaners for their excellent scale inhibition and metal ion complexation properties.
Phosphorous acid is a colorless, hygroscopic crystalline solid, highly soluble in water and polar solvents.
Phosphorous acid decomposes upon heating, releasing phosphine (PH₃) gas under certain conditions, which is highly toxic and flammable.
Phosphorous acid is corrosive to skin and eyes and should be handled with proper safety precautions.
From a regulatory perspective, Phosphorous acid’s use in food production is limited, but its roles in chemical manufacturing, pesticide formulations, and metal finishing processes are indispensable.
In academic research, Phosphorous acid is also studied as a ligand precursor and a reagent in coordination chemistry due to its ability to bind metal ions through oxygen and phosphorus atoms.
1. Synthesis of Organic Phosphorous acids:
Organic Phosphorous acids are derivatives in which a hydrogen atom on a phosphorus atom is replaced by an alkyl group.
The general formula is expressed as R-P(=O)(OH) 2.
Foscarnet, an antiviral drug, is an example of an organic Phosphorous acid.
Examples of organic Phosphorous acids are CH3P (O)(OH )2 (methylPhosphorous acid) and C6H5P (O)(OH )2 (phenylPhosphorous acid).
The trialkyl phosphite esters spontaneously transfer the alkyl group from the oxygen atom to the phosphorus atom in an isomerization reaction, yielding the alkylPhosphorous acid dialkyl ester.
P-alkylPhosphorous acid diesters can be synthesized from phosphite tri-esters and alkyl halides.
This reaction is called the Michaelis-Arbuzov Reaction.
2. Reaction of Phosphorous acid:
Phosphorous acid is used as a raw material by taking advantage of the reactivity of the P-H bond.
They are alkylated by the Kabachnik-Fields reaction or the Pudovik reaction to yield aminophosphonates useful as chelating agents.
For example, nitrilotris(methylene Phosphorous acid) can be synthesized industrially.
Alkylation of Phosphorous acid by Michael addition of an acrylic acid derivative yields a Phosphorous acid with a carboxy group.
Uses of Phosphorous acid:
The most important use of Phosphorous acid is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers.
Phosphorous acid is also used in preparing PMIDA which is a very important intermediate of herbicide glyphosate.
Phosphorous acid is used in the production of basic lead phosphonate PVC stabilizer, aminomethylene Phosphorous acid and hydroxyethane diPhosphorous acid.
Phosphorous acid is also used as a strong reducing agent and in the production of Phosphorous acid, synthetic fibres, organophosphorus pesticides, and the highly efficient water treatment agent ATMP.
Ferrous materials, including steel, may be somewhat protected by promoting oxidation (“rust”) and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating.
Industry: Phosphorous acid is used in removing dust from the metal surfaces.
Phosphorous acid is uiron,or steel tools and other surfaces that are rusted.
Phosphorous acid is helpful in cleaning the mineral deposits, cement nous smears and hard water stains.
Food: Phosphorous acid is used to acidify the foods and beverages such as colas.
In industrial synthesis PCl3 is sprayed into steam at 190oC the heat of reaction is used to distill off the hydrogen chloride and excess water vapour.
Medicine: Phosphorous acid is an important ingredient in over the counter medications to combat nausea.
Phosphorous acid is used in the production of basic lead phosphonate PVC stabilizer, aminomethylene Phosphorous acid and hydroxyethane diPhosphorous acid.
Phosphorous acid is used as a strong reducing agent.
Phosphorous acid is used in the production of high efficient water treatment agent amino trimethylene Phosphorous acid.
Sometimes confusingly, both these names are also used to refer to H3PO3 in general, i.e. both tautomers.
Phosphorous acid’s industrial applications include use in the production of basic lead phosphite and controlling plant diseases.
What’s more, Phosphorous acid has many important applications in food, medical, fertilizer and other industries.
So Phosphorous acid can be used as food additives, dental and orthopedic surgery, rust inhibitors, electrolytes, fluxes, dispersants, industrial corrosives, fertilizer raw materials and household cleaning products.
Phosphorous acid is mainly used in the manufacture of various phosphate, such as ammonium phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, etc and condensation phosphate class.
As a chemical reagent uses of Phosphorous acid: Phosphorous acid is used in chemical reactions as a reducing agent that is somewhat less vigorous than the related hypophosphorous acid.
Phosphorous acid is a diprotic phosphorus oxoacid that exists as two tautomers in solution, with the major one being HP(O)(OH)2 and the minor one being P(OH)3.
Phosphorous acid applications include use in basic lead phosphite production and controlling plant diseases.
Phosphorous acid is an intermediate in the preparation of other phosphorous compounds.
Phosphorous acid is a raw material to prepare phosphonates for water treatment such as iron and manganese control, scale inhibition and removal, corrosion control and chlorine stabilization.
The alkali metal salts (phosphites) of Phosphorous acid are being widely marketed either as an agricultural fungicide (e.g. Downy Mildew) or as a superior source of plant phosphorous nutrition.
Phosphorous acid is used in stabilizing mixtures for plastic materials.
Phosphorous acid is used for inhibiting high-temperature of corrosion-prone metal surfaces and to produce lubricants and lubricant additives.
Phosphorous acid is used to produce phosphonates like ATMP, HEDP, PBTC which are used as scale inhibitor or corrosion inhibitors in water treatment.
Phosphorous acid is used to prepare phosphite salts which are used in controlling microbial plant diseases.
Phosphorous acid has strong reducing properties and can be used as a reducing agent in electroless plating.
Silver and copper can be deposited from aqueous solutions of silver nitrate andcopper sulfate, respectively, for plating.
Phosphorous acid is used to produce the fertilizer phosphate salt like potassium phosphite, ammonium phosphite and calcium phosphite.
Phosphorous acid is actively involved in the preparation of phosphites like aminotris(methylenePhosphorous acid) (ATMP), 1-hydroxyethane 1,1-diPhosphorous acid (HEDP) and 2-phosphonobutane-1,2,4-tricarboxylic Acid (PBTC), which find application in water treatment as a scale or corrosive inhibitor.
Phosphorous acid is also used in chemical reactions as a reducing agent.
Phosphorous acid’s salt, lead phosphite is used as PVC stabilizer.
Phosphorous acid is also used as a precursor in the preparation of phosphine and as an intermediate in the preparation of other phosphorus compounds.
Phosphorous acid (additive E338) is used to acidify foods and beverages such as various colas.
Phosphorous acid provides a tangy or sour taste.
Phosphorous acid may be used to remove rust by direct application to rusted iron, steel tools, or other surfaces.
Phosphorous acid is used in dentistry and orthodontics as an etching solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed.
Phosphorous acid is sued as an additive to stabilize acidic aqueous solutions within a wanted and specified pH range
Phosphorous acid is used as a dispersing agent in detergents and leather treatment
Phosphorous acid is used as a pH adjuster in cosmetics and skin-care products
Dentistry uses:
Phosphorous acid is mixed with zinc powder and forms zinc phosphate, and Phosphorous acid is useful in temporary dental cement.
In orthodontics, zinc is used as an etching solution to help clean and roughen the surface of teeth.
Fertilizer uses:
Phosphorous acid is used as reaction fertilizer in the soil around a granule acidification is generated that improves the utilization of phosphorus applied and available in the rhizosphere.
Due to Phosphorous acid nitrogen content (present as ammonia), Phosphorous acid is good for crops that require these nutrients in Phosphorous acid initial phase
Conversion to phosphine uses:
Phosphine, being a flammable and toxic gas, is inconvenient to store.
Fortunately this useful species is readily prepared by thermal decomposition of Phosphorous acid, which degrades at about 180°C:
4 HP(O)(OH)2 → PH3 + 3 H3PO4
Since Phosphorous acid is a syrupy non-volatile liquid, the gaseous PH3 is readily separated.
In agriculture uses:
A large quantity of Phosphorous acid is used as phosphatic fertilizer.
Pure Phosphorous acid is also used for preparing phosphite salts, such as monopotassium phosphite or aluminum phosphonate.
These salts, as well as aqueous solutions of pure Phosphorous acid, have shown effectiveness in controlling a variety of microbial plant diseases—in particular, treatment using either trunk injection or foliar sprays containing Phosphorous acid salts is indicated in response to infections by phytophthora and pythium-type plant pathogens (both within class oomycetes, known as water molds), such as dieback/root rot and downy mildew.
Phosphorous acid may be used as one of the reaction components for the synthesis of the following:
α-aminomethylPhosphorous acids via Mannich-Type Multicomponent Reaction
1-aminoalkanePhosphorous acids via amidoalkylation followed by hydrolysis
N-protected α-aminoPhosphorous acids (phospho-isosteres of natural amino acids) via amidoalkylation reaction
Industrial uses:
This collector was developed recently and was used primarily as specific collector for cassiterite from ores with complex gangue composition.
On the basis of the Phosphorous acid, Albright and Wilson had developed a range of collectors mainly for flotation of oxidic minerals (i.e. cassiterite, ilmenite and pyrochlore).
Very little is known about the performance of these collectors.
Limited studies conducted with cassiterite and rutile ores showed that some of these collectors produce voluminous froth but were very selective.
Properties of Phosphorous acid:
Phosphorous acid has a melting point of 70.1°C and is a colorless, tideless crystal; when heated to 200°C, Phosphorous acid decomposes to form phosphine andphosphoric acid.
Phosphorous acid is insoluble in water except for its alkali and calcium salts.
The acid dissociation constants of Phosphorous acid are pKa = 1.5 and 6.79.
Chemical Properties:
Phosphorous acid is a white crystalline deliquescent solid that can be prepared by the action of water on phosphorus( III) oxide or phosphorus(III) chloride.
Phosphorous acid is a dibasic acid producing the anions H2PO3- and HPO3 2- in water.
Phosphorous acid and its salts are slow reducing agents.
On warming, Phosphorous acid decomposes to phosphine and phosphoric(V) acid.
Phosphorous acid is used to prepare phosphite salts.
Phosphorous acid is usually sold as a 20% aqueous solution.
Phosphorous acid has strong reducing properties it tends to be converted to phosphoric acid.
On being heated dry Phosphorous acid disproportionates to give phosphine and phosphoric acid.
H3PO3 + 3H3PO3 → PH3 + 3H3PO4
Phosphorous acid reacts with a base like sodium hydroxide forms sodium phosphate and water.
H3PO3 + 3NaOH → Na3PO3 + 3H2O
Physical Properties:
Phosphorous acid is a white crystalline mass.
Phosphorous acid is deliquescent.
Phosphorous acid is garlic-like odor.
Phosphorous acid is density 1.651 g/cm3 at 21°C.
Phosphorous acid is melts at 73.6°C.
Phosphorous acid is decomposes at 200°C to phosphine and phosphoric acid.
Phosphorous acid is soluble in water, about 310 g/100mL.
Phosphorous acid is K1 5.1×10-2 and K2 1.8×10-7.
Phosphorous acid is soluble in alcohol.
Redox Properties:
On heating at 200 °C, Phosphorous acid disproportionates to phosphoric acid and phosphine:
4 H3PO3 → 3 H3PO4 + PH3
This reaction is used for laboratory-scale preparations of PH3.
Phosphorous acid slowly oxidizes in air to phosphoric acid.
Both Phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react.
They are oxidized to phosphoric acid or its salts.
Phosphorous acid reduces solutions of noble metal cations to the metals.
When Phosphorous acid is treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms:
H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2 HCl
Mercurous chloride is reduced further by Phosphorous acid to mercury on heating or on standing:
H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2 HCl
Acid-base Properties:
Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable.
Chemistry examinations often test students’ appreciation of the fact that all three hydrogen atoms are not acidic under aqueous conditions, in contrast with phosphoric acid.
HP(O)2(OH)− is a moderately strong acid.
HP(O)(OH)2 → HP(O)2(OH)− + H+ pKa = 1.3
HP(O)2(OH)− → HPO32− + H+ pKa = 6.7
The monodeprotonated species, HP(O)2(OH)− is called the phosphite ion.
The IUPAC (mostly organic) name is Phosphorous acid.
This nomenclature of Phosphorous acid is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester.
For example, (CH3)PO(OH)2 is “methylPhosphorous acid”, which may of course form “methylphosphonate” esters.
Both Phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react.
They are oxidized to phosphoric acid or its salts.
Phosphorous acid reduces solutions of noble metal cations to the metals.
Preparation of Phosphorous acid:
Phosphorous acid can be prepared by the reaction of phosphorus trichloride with water:
PCl3 + 3H2O → H3PO4 + 3HCl
The reaction is violent.
Addition of PCl3 should be extremely cautious and slow.
The addition can be carried out safely in the presence of concentrated HCl.
Alternatively, a stream of air containing PCl3 vapor is passed into icecold water and solid crystals of H3PO4 form.
Alternatively, Phosphorous acid can be prepared by adding phosphorus trichloride to anhydrous oxalic acid:
PCl3 + 3(COOH）2 → H3PO3 + 3CO + 3CO2 + 3HCl
In this reaction, all products except Phosphorous acid escape as gases leaving the liquid acid.
Dissolution of phosphorus sesquioxide in water also forms phosphorus acid.
When shaken with ice water, Phosphorous acid is the only product.
P4O6 + 6H2O → 4H3PO3
However, in hot water part of the Phosphorous acid disproportionates to phosphoric acid and phosphorus or phosphine.
Although commercially available, the acid is most commonly prepared by hydrolysis of phosphorus trichloride with water or steam:
PCl3 + 3 H2O → HP(O)(OH)2 + 3 HCl
Potassium phosphite is a convenient precursor to Phosphorous acid:
K2HPO3 + 2 HCl → 2 KCl + H3PO3
In practice aqueous potassium phosphite is treated with excess hydrochloric acid.
By concentrating the solution and precipitations with alcohols, the pure acid can be separated from the salt.
On an industrial scale, Phosphorous acid is prepared by hydrolysis of phosphorus trichloride with water or steam:
PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
HPO(OH)2 could be produced by the hydrolysis of phosphorus trioxide:
P4O6 + 6 H2O → 4 HPO(OH)2
Production of Phosphorous acid:
The production of phosphoric acid is done via two main ways:
Wet process and thermal process.
The wet process involves the production of phosphoric acid from fluorapatite.
Phosphorous acid is known as phosphate rock and the chemical composition is 3Ca3(PO4)2.CaF2.
This phosphate rock is finely ground to increase the surface area and is reacted with concentrated sulfuric acid that gives phosphoric acid and gypsum (CaSO4.2H2O) as products.
Ca5(PO4)3F + 5H2SO4 + 10H2O → 3H3PO4+ 5CaSO4·2H2O + HF
The thermal process of phosphoric acid production includes the burning elemental phosphorous to obtain very pure phosphoric acid.
The burning of elemental phosphorous gives phosphorous pentoxide (P2O5).
Phosphorous acid is then hydrated to produce phosphoric acid.
P4 + 5O2→ 2P2O5
P2O5 + 3H2O → 2H3PO4
Major applications of phosphoric acid are in fertilizer production.
Phosphoric acid is used to produce three types of phosphorus fertilizers; triple superphosphate, diammonium hydrogen phosphate, and monoammonium dihydrogen phosphate.
Structure of Phosphorous acid:
The differential formula of Phosphorous acid is HP(=O)(OH )2, and the presence of the P-H bond is evident from physical measurements and the fact that only mono- and di-substituted salts are formed, and no trisubstituted salts are obtained.
The shape of the molecule is tetrahedral.
Phosphorous acid is in tautomeric equilibrium with Phosphorous acid.
The chemical formula for phosphite is P(OH) 3, and Phosphorous acid is predominant in equilibrium.
In organophosphorus chemistry, Phosphorous acid is a general term for organophosphorus compounds that have a phosphorus-hydrogen bond and a phosphoryl group.
Organic derivatives of Phosphorous acid include alkyl Phosphorous acid, in which the hydrogen atom on the phosphorus atom is replaced by an alkyl group, and alkyl Phosphorous acid, in which the hydrogen atom on the hydroxy group is replaced by an alkyl group.
Alkyl Phosphorous acid includes monoesters, in which only one alkyl group is substituted, and diesters, in which both alkyl groups are substituted.
Nomenclature And Tautomerism of Phosphorous acid:
Solid HP(O)(OH)2 has tetrahedral geometry about the central phosphorus atom, with a P−H bond of 132 pm, one P=O double bond of 148 pm and two longer P−OH single bonds of 154 pm.
In common with other phosphorus oxides with P−H bonds (e.g.hypophosphorous acid and dialkyl phosphites), Phosphorous acid exists in equilibrium with an extremely minor tautomer P(OH)3.
(In contrast, arsenous acid’s major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form P(OH)3 be called Phosphorous acid, and the dihydroxy form HP(O)(OH)2 Phosphorous acid.
Only the reduced phosphorus compounds are spelled with an “-ous” ending.
PIII(OH)3 ⇌ HPV(O)(OH)2
K = 1010.3 (25°C, aqueous)
Tautomerization:
Phosphorous acid is better described with the structural formula HP(O)(OH)2.
This species exists in equilibrium with a minor tautomer P(OH)3.
The latter is called Phosphorous acid.
Phosphorous acid is sometimes called Phosphorous acid or orthophosphorous acid.
Phosphorous acid has been shown to be a stable tautomer.
The dihydroxy form, HP(O)(OH)2, is called Phosphorous acid.
Many of the reduced phosphorus acids are subject to similarly complicated equilibria involving shifts of H between O and P.
In the solid state, HP(O)(OH)2 is tetrahedral with one shorter P=O bond of 148 pm and two longer P-O(H) bonds of 154 pm.
Polymerization of Phosphorous acid:
An oligophosphorous acid of the Phosphorous acid tautomer refers to a few molecules of Phosphorous acid condensed into a molecule with the loss of water.
A general formula for such oligophosphorous acids is (HPO)nOn-1(OH)2, where n = 2, 3, 4, etc., oligo-.
A polyphosphorous acid can have dozens of such Phosphorous acid units condensed in a row with the loss of H2O for each unit added on.
For the Phosphorous acid tautomer, an oligophosphorous acid also refers to a few molecules condensed into a molecule with the loss of H2O as each unit of P(OH)3 is added on, but the general formula differs:
(HO)2PO[P(OH)O]n-2P(OH)2,
where n = 2, 3, 4, etc., oligo-.
Here for both tautomers the repeat unit is (HPO2)n-2.
Again, a polyphosphorous acid can have dozens of units condensed in a row.
Regardless of the value of n, both polyPhosphorous acid and polyphosphorous acid have the same chemical formula for any specific n, e.g., triphosPhosphorous acid is H5P3O7 and triphosphorous acid is H5P3O7 for n=3.
In oligophosphorous acids of sufficient size, there are multiple OH that can result in the condensation of a cyclophosphorous acid that does not have multiple (HPO3) metaphosphoric acid units.
However, the usual referral to a cyclophosphorous acid (cyclophosphonates or cyclophosphites) may be misnomers wherein the cyclic portion is carbon-based with a Phosphorous acid side chain of one or more molecules, or one or a limited number of either of the two tautomers included in the ring but as a minority contributor.
For example the effect of varying ring size on the phosphonate-phosphite tautomerism of cyclophosphorous acids has been shown.
But the cyclophosphorous acids are biheteroorganic.
Branching can also occur in either oligophosphorous or polyphosphorous tautomer.
These are ultraoligophosphorous or ultrapolyphosphorous acids, ultraoligophosphonates and ultrapolyphosphonates, or ultraoligophosphites and ultrapolyphosphites, respectively for the phosphonic and phosphorous tautomers.
Reactions of Phosphorous acid:
Acid–base properties:
Phosphorous acid has a pKa in the range 1.26–1.3.
HP(O)(OH)2 → HP(O)2(OH)− + H+ pKa = 1.3
Phosphorous acid is a diprotic acid, the hydrogenphosphite ion, HP(O)2(OH)− is a weak acid:
HP(O)2(OH)− → HPO2−3 + H+ pKa = 6.7
The conjugate base HP(O)2(OH)− is called hydrogen phosphite, and the second conjugate base, HPO2−3, is the phosphite ion.
(Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively).
The hydrogen atom bonded directly to the phosphorus atom is not readily ionizable.
Chemistry examinations often test students’ appreciation of the fact that not all three hydrogen atoms are acidic under aqueous conditions, in contrast with H3PO4.
As A Ligand, Phosphorous acid:
Upon treatment with metals of d6 configuration, Phosphorous acid is known to coordinate as the otherwise rare P(OH)3 tautomer.
Examples include Mo(CO)5(P(OH)3) and [Ru(NH3)4(H2O)(P(OH)3)]2+.
Heating a mixture of potassium tetrachloroplatinate and Phosphorous acid gives the luminescent salt potassium diplatinum(II) tetrakispyrophosphite:
2 K2PtCl4 + 8 H3PO3 → K4[Pt2(HO2POPO2H)4] + 8 HCl + 4 H2O
Organic Derivatives of Phosphorous acid:
The IUPAC (mostly organic) name is Phosphorous acid.
This nomenclature of Phosphorous acid is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester.
For example, (CH3)PO(OH)2 is “methylPhosphorous acid”, which may of course form “methylphosphonate” esters.
Handling And Storage of Phosphorous acid:
Conditions for safe storage, including any incompatibilities:
Storage conditions:
No metal containers.
Tightly closed.
Dry.
Store under inert gas.
Air sensitive.
Stability And Reactivity of Phosphorous acid:
Reactivity:
Phosphorous acid decomposes when heated to form phosphine, a gas that usually ignites spontaneously in air.
Absorbs oxygen from the air to form Phosphorous acid.
Forms yellow deposits in aqueous solution that are spontaneously flammable upon drying.
Phosphorous acid reacts exothermically with chemical bases (for example: amines and inorganic hydroxides) to form salts.
These reactions can generate dangerously large amounts of heat in small spaces.
Dissolution in water or dilution of a concentrated solution with additional water may generate significant heat.
Phosphorous acid reacts in the presence of moisture with active metals, including such structural metals as aluminum and iron, to release hydrogen, a flammable gas.
Phosphorous acid can initiate the polymerization of certain alkenes.
Phosphorous acid reacts with cyanide compounds to release gaseous hydrogen cyanide.
Phosphorous acid 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).
Chemical stability:
Phosphorous acid is chemically stable under standard ambient conditions (room temperature).
Possibility of hazardous reactions:
No data available
Conditions to avoid:
No information available
First Aid Measures of Phosphorous acid:
General advice:
First aiders need to protect themselves.
If inhaled:
After inhalation:
Fresh air.
Call in physician.
In case of skin contact:
Take off immediately all contaminated clothing.
Rinse skin with water/ shower.
Call a physician immediately.
In case of eye contact:
After eye contact:
Rinse out with plenty of water.
Immediately call in ophthalmologist.
Remove contact lenses.
If swallowed:
After swallowing:
Make victim drink water (two glasses at most).
Call a physician immediately.
Do not attempt to neutralise.
Indication of any immediate medical attention and special treatment needed:
No data available
Fire Fighting Measures of Phosphorous acid:
Unsuitable extinguishing media:
For this substance/mixture no limitations of extinguishing agents are given.
Further information:
Prevent fire extinguishing water from contaminating surface water or the ground water system.
Accidental Release Measures of Phosphorous acid:
Environmental precautions:
Do not let product enter drains.
Methods and materials for containment and cleaning up:
Cover drains.
Collect, bind, and pump off spills.
Observe possible material restrictions.
Take up dry.
Dispose of properly.
Clean up affected area.
Exposure Controls/Personal Protection of Phosphorous acid:
Personal protective equipment:
Eye/face protection:
Use equipment for eye protection.
Tightly fitting safety goggles
Skin protection:
Handle with gloves.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0,11 mm
Break through time: 480 min
Splash contact:
Material: Nitrile rubber
Minimum layer thickness: 0,11 mm
Break through time: 480 min
Body Protection:
protective clothing
Control of environmental exposure:
Do not let product enter drains.
Identifiers of Phosphorous acid:
Appearance: white/light yellow liquid
Grade: Food Grade
Type: food additive
Molecular formula: H3PO4
HS code: 2809201900
CAS code: 7664-38-2
EINECS: 231-633-2 UN: 1805
Certificate: SGS/ISO certificate
Other names: orthophosphoric acid phosphoric(V) Acid
Molecular Weight: 81.99580
Exact Mass: 82.00
EC Number: 233-663-1
UNII: 95E079716M
UN Number: 2834
DSSTox ID: DTXSID7035511
HScode: 2811199090
PSA: 81.00000
XLogP3: -0.63930
Density: 1.651
Melting Point: 73 °C (approx)
Boiling Point: 200ºC
Flash Point: 200ºC
Water Solubility:
H2O: soluble
Storage Conditions: 0-6ºC
Air and Water Reactions: Deliquescent.
Properties of Phosphorous acid:
Chemical formula: H3PO3
Molar mass: 81.99 g/mol
Appearance: white solid deliquescent
Density: 1.651 g/cm3 (21 °C)
Melting point: 73.6 °C (164.5 °F; 346.8 K)
Boiling point: 200 °C (392 °F; 473 K) (decomposes)
Solubility in water: 310 g/100 mL
Solubility: soluble in ethanol
Acidity (pKa): 1.1, 6.7
Magnetic susceptibility (χ): −42.5·10−6 cm3/mol
Structure:
Molecular shape: pseudo-tetrahedral
Chemical formula: H3PO3
Molar mass: 81.99 g/mol
Appearance: white solid deliquescent
Density: 1.651 g/cm3 (21 °C)
Melting point: 73.6 °C (164.5 °F; 346.8 K)
Boiling point: 200 °C (392 °F; 473 K) (decomposes)
Solubility in water: 310 g/100 mL
Solubility: soluble in ethanol
Acidity (pKa): 1.1, 6.7
Magnetic susceptibility (χ): −42.5·10−6 cm3/mol
Odour: Sour odour
Appearance: White solid, deliquescent
Covalently-Bonded Unit: 1
Hydrogen Bond Acceptor: 3
Complexity: 8
Solubility: Soluble in water
Physical state flakes
Color: white
Odor: odorless
Melting point/freezing point:
Melting point/range: 63 – 74 °C at 1.013 hPa
Initial boiling point and boiling range: 259 °C at 1.013 hPa
Flammability (solid, gas): No data available
Upper/lower flammability or explosive limits: No data available
Flash point: Not applicable
Autoignition temperature: not auto-flammable
Decomposition temperature: No data available
pH: at 20 °C acidic
Viscosity
Viscosity, kinematic: No data available
Viscosity, dynamic: No data available
Water solubility: soluble
Partition coefficient: n-octanol/water: Not applicable for inorganic substances
Vapor pressure < 0,1 hPa at 20 °C Density: 1,651 g/cm3 at 25 °C - lit. Relative density No data available Relative vapor density: No data available Particle characteristics: No data available Explosive properties: No data available Oxidizing properties: Phosphorous acid has been shown not to be oxidizing Other safety information: No data available Molecular Weight: 81.996 g/mol XLogP3-AA: -1.6 Hydrogen Bond Donor Count: 2 Hydrogen Bond Acceptor Count: 3 Rotatable Bond Count: 0 Exact Mass: 81.98198095 g/mol Monoisotopic Mass: 81.98198095 g/mol Topological Polar Surface Area: 57.5Ų Heavy Atom Count: 4 Complexity: 26.3 Isotope Atom Count: 0 Defined Atom Stereocenter Count: 0 Undefined Atom Stereocenter Count: 0 Defined Bond Stereocenter Count: 0 Undefined Bond Stereocenter Count: 0 Covalently-Bonded Unit Count: 1 Compound Is Canonicalized: Yes Absorbs oxygen from the air very readily to form phosphoric acid. Soluble in water. Reactive Group: Acids, Weak Other Names: orthophosphoric acid; Anhydrous phosphoric acid CAS No.: 7664-38-2 Molecular Formula/MF: H3PO4 EINECS No.: 231-633-2 Classification: Biochemical & chemical Grade Standard: Food grade Purity: 85% min Odor: Odorless Appearance: Colorless, transparent and syrupy liquid Density: 1.65 g/cm³ Molecular Weight/ Molar Mass: 82 g/mol Boiling Point: 200 °C Melting Point: 73.6 °C Chemical Formula: H3PO3 Odour: Sour odour Appearance: White solid, deliquescent Covalently-Bonded Unit: 1 Hydrogen Bond Acceptor: 3 Complexity: 8 Solubility: Soluble in water Melting point: 73 °C Boiling point: 200 °C Density: 1.651 g/mL at 25 °C(lit.) vapor pressure: 0.001Pa at 20℃ Flash point: 200°C storage temp.: 0-6°C solubility: DMSO (Slightly), Methanol (Slightly), Water (Sparingly) form: Crystals pka: pK1 1.29; pK2 6.74(at 25℃) Specific Gravity: 1.651 color: White Water Solubility: SOLUBLE Sensitive: Air Sensitive & Hygroscopic Merck: 14,7346 Stability: Stable. Incompatible with strong bases. Hygroscopic. CAS DataBase Reference: 13598-36-2 EWG's Food Scores: 1 FDA UNII: 35V6A8JW8E NIST Chemistry Reference: (HO)2HPO(13598-36-2) EPA Substance Registry System: Phosphorous acid (13598-36-2)