Phosphorous Acid, H₃PO₃

The production of an acid liquid by the slow combustion of phosphorus in the presence of water was observed by le Sage in 1777. The acid was prepared by Pelletierby drawing a slow current of air over phosphorus enclosed in fine tubes which dipped into water. The distinction between this acid and phosphoric acid was recognised in the early nineteenth century, as also was the fact that the slow combustion of phosphorus could yield a mixture of the two acids.

The method of preparation just mentioned is not a good one. Phosphorous oxide dissolves only slowly in cold water and phosphoric acid is continuously produced from the first, while hot water gives a variety of products. A better method is that recommended by Davy, namely, by the hydrolysis of phosphorus trichloride. Some white phosphorus is placed in a deep cylinder and covered with water. The element is melted and chlorine is passed in through a tube which dips well into the phosphorus. Large quantities may be quickly obtained by this method, but the product contains phosphoric acid. Regulated action of PCl₃ on water may be effected by passing a current of dry air through the trichloride kept at 60° C. and then through two wash- bottles containing water. The action is also much less violent if an acid such as concentrated aqueous hydrochloric acid is used in place of the water. Oxalic acid is most suitable, since it is dehydrated with decomposition when heated in a flask with phosphorus trichloride. The flask is furnished with a reflux condenser. Much of the HCl is evolved and a concentrated solution of phosphorous acid remains:—

PCl₃ + 3H₂C₂O₄ = H₃PO₃ + 3CO + 3CO₂ + 3HCl

In these preparations it is usually necessary to remove the hydrochloric acid which is produced; this may be done by distilling the solution up to a temperature not exceeding 180° C. A syrupy liquid is then obtained which, after cooling, crystallises quickly, or at any rate within a few hours. The process may be hastened as usual by seeding.

The crystalline acid was found to melt at 70.1° C., 74° C. The density of the liquid supercooled at 21° C. was 1.651. The latent heat of fusion of the acid was found to be 7.07 Cals. The heat of solution of the acid per mol dissolved in 400 mols or more of water was +233 Cals. The heat of formation of the crystallised acid has been given as +227.7 Cals.

The conductivities of phosphorous acid are lower than those of hypophosphorous acid at corresponding dilutions, showing that the former acid is less dissociated, as appears from the following values:—

Molar conductivities of phosphorous acid at 25° C

V	2	4	8	16	32	64	128	256	512	1024
μ	129	156	187	222	252	292	318	337	351	358

Since phosphorous acid is dibasic, the dissociation constant was not calculated from the conductivities, but from the hydrogen-ion concentrations set up during neutralisation, which may be expressed by a neutralisation curve. This gave K₁ = 0.05 and K₂ = 2.4×10^-5 or K₁ = 0.016 at c = 0.001 to K₁ = 0.062 at c=0.l and = 0.7×10^-6.

As in the case of hypophosphorous acid, the molar conductivity, while increasing at first with temperature, reached a maximum at about 70° C. and then diminished.

The lowering of the freezing-point, -Δt, and the elevation of the boiling-point, +Δt, of water, which is made c-normal with respect to phosphorous acid, are taken as proving that polymerisation had taken place as well as electrolytic dissociation.

Lowering of freezing-point of water by H₃PO₃

c	1.186	0.533	0.296	0.148
-Δt, °C	2.941	1.538	0.835	0.455
i	1.128	1.143	1.515	1.650

Raising of boiling-point of water by H₃PO₃

c	0.976	0.488	0.244	0.122
+Δt, °C	0.51	0.28	0.16	0.13
i	0.969	1.063	1.215	1.972

The factors or activity coefficients i are calculated on the assumption that the original molecules dissociating are the simplest, i.e. H₃PO₃.

In the neutralisation of the acid by NaOH in dilute solution it was found that

H₃PO₃ + NaOH = NaH₂PO₃ + H₂O +148 Cals.
H₃PO₃ + 2NaOH = Na₂HPO₃ + 2H₂O + 2×142 Cals.
H₃PO₃ + 3NaOH = Na₂HPO₃ + 2H₂O + NaOH + 3×96 Cals.

whence only two of the hydrogens are ionisable. So far as the formulae of phosphites are known, they contain a maximum of two equivalents of a base combined with 1 mol of the acid. Attempts to prepare Na₃PO₃ have not been successful. Dibasicity is also confirmed by a study of the neutralisation curves.

Phosphorous acid and the phosphites are not quite such strong reducing agents as hypophosphorous acid and the hypophosphites. The free acid undergoes self-oxidation and -reduction at a higher temperature than hypophosphorous acid.

Atmospheric oxygen does not oxidise the acid at ordinary temperatures, nor is such oxidation catalysed by iodine in the dark. In the light, however, hydrogen iodide is formed according to the equation

H₃PO₃ + I₂ + H₂O = H₃PO₄ + 2HI

being reoxidised rapidly by oxygen, thus

2HI + ½O₂ = I₂ + H₂O

The oxidation of phosphites by iodine was found to proceed to completion in neutral solution. Over a narrow range of concentrations the reaction was found to be uni-molecular with respect to iodine and phosphorous acid. It is said to be catalysed by hydrogen ions which are formed as the reaction proceeds. A further study of the velocity constants showed that the mechanism was more complicated than had previously been supposed, and that the two tautomeric forms participated in different ways. A solution of iodine in potassium iodide contains the ions I^- and I₃^- and also molecular iodine, I₂. The latter reacts directly with the normal form of phosphorous acid and this reaction is repressed by hydrogen ions. Simultaneously, the phosphorous acid changes into another form with which the I₃^- ion reacts. This second reaction is accelerated by hydrogen ions either directly or, more probably, because they accelerate the tautomeric change.

Various products were obtained when phosphorous acid was heated with halogens in a sealed tube. Iodine gave phosphoric and hydriodic acids, phosphonium iodide and an iodide of phosphorus, whilst bromine gave phosphoric acid, phosphorus tribromide and hydrobromic acid. A dry ether solution of the acid was not oxidised by bromine or dry palladium black, but oxidation proceeded readily in the presence of moisture.

Phosphorous acid also reduced sulphurous acid, the end products being sulphur and phosphoric acid, thus:—

2H₃PO₃ + H₂SO₃ = 2H₃PO₄ + S + H₂O

Sulphuric acid, which dissolved phosphorous acid in the cold, was reduced to sulphur dioxide on heating.

Salts of the noble metals, including copper and mercury, were found to oxidise phosphorous to phosphoric acid. The metal was precipitated from silver salts and also from gold salts. It is generally agreed that copper is precipitated from copper sulphate, while cuprous oxide or hydrogen may also be liberated, according to the conditions:—

3H₃PO₃ + CuSO₄ + 3H₂O = Cu + 2H₂ + 3H₃PO₄ + H₂SO₄
H₃PO₃ + CuSO₄ + H₂O = Cu + H₃PO₄ + H₂SO₄

The reduction of mercuric chloride gave mercurous chloride when the mercuric chloride was in excess, and mercury when the phosphorous acid was in excess, while the total reaction can be represented by the equations

2HgCl₂ + H₃PO₃ + H₂O = H₃PO₄ + 2HgCl + 2HCl (1)
HgCl₂ + H₃PO₃ + H₂O = H₃PO₄ + Hg + 2HCl (2)

The mechanism has not been completely elucidated and may be very complex. The reaction has been classified as of the third order in dilute solution, and as of the first order with respect to HgCl₂. But it has also been stated that the reaction which is chiefly responsible for the observed velocity is the conversion of a first or normal form of H₃PO₃ into a second or active form, which then reacts according to equation (2) above. In the absence of extraneous chloride ions these are produced by another reaction, thus

HgCl⁺ + H₃PO₃ (normal) + H₂O = H₃PO₄ + Hg + 2H⁺ + Cl^-

Many other oxidising agents are capable of oxidising phosphorous acid. The reaction with potassium persulphate is very slow, but in the presence of hydriodic acid it is much accelerated. This is a good example of a coupled reaction.

Phosphorous acid forms esters by direct union with several alcohols. With ethyl alcohol it gave diethyl phosphite. Ethyl derivatives of phosphorous and phosphoric acids have been made by the action of bromine on sodium diethyl phosphite in ligroin. They are separated by fractional distillation.

Two series of phosphites are known, the primary phosphites, MH₂PO₃, and the secondary, M₂HPO₃, M being a univalent metal. Crystalline salts have also been prepared containing an excess of phosphorous acid. The phosphites of the alkali metals and ammonia are soluble, those of the alkaline earths sparingly soluble, while those of other metals are only very slightly soluble. They may be prepared by the usual methods:—

1. By neutralising phosphorous acid to the appropriate end point with alkali hydroxides and evaporating to crystallisation.
2. By neutralising a solution of phosphorous acid, or one made from PCl₃, with ammonia and adding a salt of the required metal.
3. By dissolving the hydroxide of the base in phosphorous acid.

The phosphites are fairly stable in the absence of oxidising agents and dilute solutions may even be boiled without decomposition. More concentrated solutions may decompose, giving hydrogen; thus

Na₂HPO₃ + NaOH = Na₃PO₄ + H₂

The solid salts decompose when heated, giving phosphine or hydrogen or both, and leaving the ortho- or pyrophosphate of the metal; thus

2BaHPO₃.H₂O = Ba₂P₂O₇ +2H₂
5PbHPO₃ = Pb₂P₂O₇ + Pb₃(PO₄)₂ +PH₃ + H₂

The formulae of typical phosphites are as follows:— LiH₂PO₃; Li₂HPO₃.H₂O, four-sided plates; NaH₂PO₃.2½H₂O, monoclinic prisms; Na₂HPO₃.5.5H₂O, rhombic bipyramidal needles; KH₂PO₃, monoclinic prisms; K₂HPO₃; NH₄H₂PO₃, monoclinic prisms; (NH₄)₂HPO₃.H₂O, four-sided prisms; Ag₂HPO₃, white crystalline precipitate; 2CaHPO₃.2H₂O, white precipitate; Ba(H₂PO₃)₂; BaHPO₃.½H₂O, white crystals; Ba₂(H₂PO₃)₃.5H₂O; Mg(H₂PO₃)₂; MgHPO₃.6H₂O; CuHPO₃.2H₂O, blue crystals; ZnHPO₃.2½H₂O; MnHPO₃.H₂O, reddish-white precipitate; Pb(H₂PO₃)₂; PbHPO₃; nFeO.P₂O₃, crystalline mineral; CoHPO₃.2H₂O, red crystals, blue when dried; (NH₄)₂[Co₃(HPO₃)₄].18H₂O, and a corresponding nickel compound.

Ammonium phosphite in its hydrated form, (NH₄)₂HPO₃.H₂O, or as the anhydrous salt, has been made by saturating phosphorous acid with ammonia, or by passing ammonia over the hydrogen phosphite at 100° C. It easily loses both water and ammonia when heated or kept in a vacuum.

The hydrogen phosphite NH₄H₂PO₃ is rather more stable, and has been prepared by the neutralisation of phosphorous acid with ammonia to methyl orange, followed by careful evaporation. It melts at about 120° C. and decomposes at about 140° C., giving ammonia, phosphine and phosphorous acid. The form and constants of the crystals have been described.

Hydroxylamine phosphite, (NH₂OH)₂.H₃PO₃, was prepared by double decomposition of one mol of Na₂HPO₃ with two mols of NH₂OH.HCl. The sodium chloride was crystallised out by evaporation in vacuo, and the very soluble hydroxylamine salt crystallised from alcohol. It melts easily, is inflammable and is a strong reducing agent. Hydrazine phosphite, N₂H₄.H₃PO₃, has also been prepared, from barium phosphite and hydrazine sulphate.

Structure of the Hypophosphites and Phosphites.—The monobasicity of hypophosphorous acid points to the unsymmetrical formula. The probable existence also of a proportion in the symmetrical form may be indicated by the ease with which the acid undergoes self-oxidation and -reduction on heating, thus



Hypophosphorous acid easily adds on benzaldehyde, and the product, di[oxybenzene] phosphorous acid, must have the unsymmetrical formula

HO-PO=[CH(OH)C₆H₅]₂

Alkylphosphinic acids should also be considered as compounds in which the alkyl is directly attached to phosphorus, since they have been obtained by the oxidation of primary alkylphosphines with fuming nitric acid. (CH₃)₂=PO-OH is hardly an acid, but rather resembles a higher aliphatic alcohol in its waxy appearance and melting-point (76° C.). This compound sublimes without decomposition.

Ester acids of phosphorous acid are known, such as CH₃.PO(OH)₂ (m.pt. 105° C.) and C₂H₅.PO(OH)₂ (m.pt. 44° C.), which are capable of giving mono- and dithyl esters.

The tautomerism of phosphites has been proved by the preparation of two triethyl phosphites. The one, prepared by the action of phosphorus trichloride on sodium ethoxide, was probably the symmetrical ester, being formed according to the equation

PCl₃ + 3NaOC₂H₅ = P(OC₂H₅)₃ + 3NaCl

Its density, D_0°^17°, was 0.9605, it boiled at 156° C., was insoluble in water, soluble in many organic solvents, and had a molecular weight of 154 in benzene. It reduced mercuric chloride. The other, prepared by the action of lead phosphite on ethyl iodide, or phosphorous oxide on ethyl alcohol, had a density, D_0°^21°, of 1.028, boiled at 198° C. and did not reduce mercuric chloride. This was probably diethylethyl phosphite, having the formula C₂H₅-PO=(OC₂H₅)₂. A diethyl ester has also been prepared which boils at 187°-188° C. and may have either formula

HO-P(OC₂H₅)₂ or H-PO(OC₂H₅)₂

The unsaturated character of these trialkyl esters was shown by the ease with which they were attacked by nitric acid, but still more clearly by the formation, with evolution of heat, of stable crystalline addition compounds when they were mixed with cuprous halides. Thus CuCl.P(OC₂H₅)₃ was described as consisting of colourless crystals melting at 190° to 192° C. and soluble in organic solvents. This property they share with phosphine, alkylphosphines and phosphorus trihalides. The phosphoric esters were quite indifferent to cuprous halides. Nor were such addition compounds formed either by phosphorous acid itself or by the dialkyl esters, which may show that the latter compounds have the unsymmetrical formula. Phosphorous acid probably exists in both forms, but first as P(OH)₃, i.e. when produced from PCl₃ and H₂O. This may be transformed into the unsymmetrical form through an addition compound HCl.P(OH)₃, and probably also exists in the form of complex molecules, such as H₄(H₂P₂O₆), the existence of which was demonstrated by the freezing-points of concentrated solutions.

The X-ray K absorption spectra of phosphorous acid and the phosphites of Na, Al, Mn, Fe^••, Fe^•••, Ca, Ni and Cd were nearly the same, the head of the absorption band lying at λ =5754.1 X-ray units. The band of silver diethyl phosphite was at 5760.4. The values for phosphorus in the elementary state and in different forms of combination were as follows:—

Hypophosphites	Phosphites	Phosphates	Violet Phosphorus	White Phosphorus
5757.5	5754.1	5750.7	5771.5	5776.9

It was stated that the wavelengths of the absorption bands of elements are higher than those of their compounds, and that the bands pass to shorter wavelengths as the valency rises, provided that the successively attached atoms or radicals are the same. The general results showed that the structural formulae of the phosphorous diesters, triphenyl-methylphosphorous acid and ferric monopropyl phosphite were (RO)₂=PO-(H), sodium diethyl phosphite (EtO)₂PO(Na), silver diethyl phosphite (EtO)₂P(OAg), monoacetylphosphorous acid (HO)₂PO(Ac). In solution, the diesters and metal esters contained a mixture of tautomeric forms.