Sodium Arsenates

Sodium Orthoarsenate, Na₃AsO₄, is formed when excess of sodium hydroxide reacts with arsenic acid. It is usually manufactured by causing arsenious oxide, sodium hydroxide or carbonate and a reducible metallic oxide, for example antimony trioxide, to react in the fused condition, the sodium arsenate being extracted from the melt with water and crystallised from the resulting solution. The process may be performed in the presence of molten lead to alloy the antimony which is formed. Metallic arsenic may be used instead of arsenious oxide. Or, arsenious oxide vapour mixed with excess of air or oxygen is passed over the alkali at about 500° C.6 The mixture of alkali and arsenious oxide may be oxidised with nitric acid, but large quantities of nitrous vapours are evolved during the reaction. An improved method is to use sodium peroxide as the oxidising agent.

The arsenate may be prepared, however, without the presence of an oxidiser other than arsenious oxide if excess of the latter be used. Thus, sodium carbonate with excess of arsenious oxide heated at 500° to 550° C. yields the arsenate by the following reaction:

9Na₂CO₃ + 5As₂O₃ = 6Na₃AsO₄ + 4As + 9CO₂

Sodium orthoarsenate is also obtained electrolytically. Yields up to 100 per cent, may be obtained by employing a cell with a diaphragm between iron electrodes. The anolyte should contain sodium arsenite, or sodium hydroxide and arsenious oxide (equivalent to 150 g. As₂O₃ per litre), and the catholyte sodium hydroxide (150 g. per litre). With a current density of 3 amps, per sq. dm. the current efficiency is 100 per cent. A solid crust of sodium arsenate forms around the anode. The process may be rendered continuous by circulating the anolyte and removing the precipitated arsenate. Iron or nickel electrodes are satisfactory; the latter yield a purer product, but iron has the advantage of hindering the formation of arsine. Electrodes of Acheson graphite may also be used with advantage. The diaphragm should be of asbestos cement.

The density of anhydrous sodium orthoarsenate is 2.835. From solution it crystallises as the dodecahydrate, Na₃AsO₄.12H₂O, of density 1.76 and melting point 85.5° C. The crystals are hexagonal prisms, isomorphous with the corresponding phosphate. The following refractive indices have been determined:

Na3AsO4.12H2O	Li	Na	Tl
ω	1.4553	1.4589	1.4624
ε	1.4630	1.4669	1.4704

The molecular refraction is 61.4.

The salt dissolves in water yielding a strongly alkaline solution. Reliable determinations of the solubility have not been made; Graham stated that at 13.5° C. 100 parts of water dissolve 28 parts of the crystallised salt, but Schiff gave a much lower value.

The densities of solutions of various concentrations at 17° C. have been determined as follows:

Concentration. Parts per 100 Parts Solution.		Density
Na3AsO4.12H2O	Na3AsO4
2	0.981	1.0167
4	1.962	1.0215
6	2.944	1.0325
8	3.925	1.0435
10	4.906	1.0547
12	5.887	1.0659
14	6.868	1.0773
16	7.850	1.0887
18	8.831	1.1003
20	9.812	1.1120

The heat of formation of sodium orthoarsenate in aqueous solution from the elements is 381,500 calories, and that of the solid 360,800 calories. The heat of neutralisation (3NaOH.H₃AsO₄.aq.) is 35,920 calories, and the heat of formation from the oxides is:

3Na₂O + As₂O₅ = 2Na₃AsO₄ + 202,800 calories

Other hydrates of sodium orthoarsenate have been obtained. The decahydrate, Na₃AsO₄. IOH₂O, crystallises from a solution containing sodium monohydrogen orthoarsenate (100 g.) in 150 c.c. of 50 per cent, sodium hydroxide after keeping for some time at about 77° C.; or it may be prepared by treating a saturated solution of arsenious oxide with an excess of sodium peroxide in the cold, and concentrating on a water-bath, the crystals being deposited on cooling. The decahydrate is efflorescent in dry air. The crystals are regular, resemble those of the corresponding vanadate, and have melting point 85° C. A (2, 9)-hydrate, 2Na₃AsO₄.9H₂O, is deposited from a solution containing 100 g. of the monohydrogen arsenate in 40 c.c. of 50 per cent, sodium hydroxide when allowed to crystallise at about 86° C.

In cold aqueous solution, sodium orthoarsenate reacts with sodium hydrosulphite to form sodium arsenohydrosulphite, Na₃As(S₂O₄)₃, a creamy white granular powder. In the presence of sodium sulphite this compound decomposes, forming sodium arsenothiosulphate, an unstable intermediate product, and finally arsenious sulphide. If the reduction by sodium hydrosulphite takes place in the presence of hydrochloric acid, some arsenic subsulphide, As₃S, is also precipitated, and this product is also obtained when the orthoarsenate in aqueous solution is treated with phosphorus trichloride and the mixture saturated with sulphur dioxide.

Sodium Monohydrogen Orthoarsenate, Na₂HAsO₄, is produced when aqueous arsenic acid is mixed with a large excess of sodium carbonate, or when the normal orthoarsenate is treated in solution with a dilute acid or chlorine. It is prepared on a large scale by dissolving arsenious oxide in aqueous sodium hydroxide and adding sodium nitrate. The mixture is concentrated by boiling and finally heated in a furnace until dry and the residue calcined; this residue is extracted with dilute alkali and allowed to crystallise. The product is used in calico-printing as a substitute for cow-dung, which was formerly used for clearing the cloth after mordanting with either iron or aluminium acetate (the use of dung originated in India and its action has not been satisfactorily explained).

The anhydrous salt is obtained by heating the crystals to 120° C. If crystallisation takes place at the ordinary temperature, the dodecahydrate, Na₂HAsO₄.12H₂O, is obtained; while if the crystals are formed above 36° C. the heptahydrate, Na₂HAsO₄.7H₂O, is produced. The transition point determined from the solubility curve of sodium monohydrogen arsenate in water is at 22° C.

The dodecahydrate is efflorescent at room temperature in air. At 22° C. it becomes damp and the dampness increases as the temperature rises until finally the salt becomes completely liquid. A definite melting point is not exhibited. The liquid becomes clear at 56.2° C. and is then a true solution of the heptahydrate. When this is supercooled to about 40° C. and seeded with a crystal of the salt, crystals of the heptahydrate are deposited and the temperature rises to 56.2° C. A similar evolution of heat, but less marked, occurs at 22° C. The heptahydrate does not effloresce appreciably at room temperature in air.

The dissociation pressures of the two hydrates have been determined by a dynamical method with the following results:

For the reaction Na₂HAsO₄.12H₂O → Na₂HAsO₄.7H₂O + 5H₂O, at 14.9° C., 5.24 mm.; and at 20° C., 7.36 mm.

For the reaction Na₂HAsO₄.7H₂O → Na₂HAsO₄ + 7H₂O, at 24.92° C., 9.98 mm.; at 30° C., 14.39 mm.; and at 35° C., 20.73 mm.

The dissociation pressure-temperature curves confirm the transition point as very near 22° C.

The specific heats of the hydrates over the range +16° to -12° C. have been found to be approximately: Na₂HAsO₄.12H₂O, 0.414 calorie per gram; Na₂HAsO₄.7H₂O, 0.350 calorie per gram; the respective molecular heats being 166.3 and 109.3 calories.

The solubility expressed in grams of anhydrous salt per 100 g. of solution has been determined up to 34° C. as follows:

Temperature, ° C.	Solubility	Solid Phase.
0	5.46	Na2HAsO4.12H2O
8.6	9.66
11.8	12.53
16.0	17.30
21.0	25.75
22.5	26.75	Na2HAsO4.7H2O
28.0	30.48
30.1	32.34
34.0	35.09

Both hydrates crystallise in the monoclinic system, the crystal elements being

Na₂HAsO₄.12H₂O, a:b:c = 1.7499:1:1.4121; β = 121°49'
Na₂HAsO₄. 7H₂O, a:b:c = 1.2294:1:1.3526; β = 97°14'

and the densities respectively 1.67 and 1.88.

The refractive indices for light of various wavelengths along the three crystal axes are as follows:

		Li	Na	Tl
Na2HAsO4.12H2O	α	1.4420	1.4453	1.4482
	β	1.4462	1.4496	1.4513
	γ	1.4480	1.4513	1.4545
Na2HAsO4.7H2O	α	1.4587	1.4622	1.4655
	β	1.4623	1.4658	1.4689
	γ	1.4746	1.4782	1.4812

The equivalent conductivity is as follows (for solutions containing ½Na₂HAsO₄ in v litres):

V	32	64	128	256	512	1024	∞
Λ	79.0	84.7	88.8	92	94.4	95.9	103.8

An investigation of the temperature-concentration curves of the system Na₂HAsO₄-H₂O over the range < 0° to 120° C. reveals transition points which indicate the existence of the anhydrous salt and hydrated forms containing 0.5, 5, 7 and 12H₂O. The normal boiling point of the saturated solution is 116° C. The existence of a hemihydraie has not been confirmed, but Menzel and Hagen state that a monohydrate exists, and have determined the following transition points: anhyd. ⇔ 1H₂O, 99.5° C.; anhyd. (metastable) ⇔ 5H₂O, 68 0° C.; 1H₂O ⇔ 5H₂O, 67.4° C.; 5H₂O ⇔ 7H₂O, 56.3° C.; 7H₂O ⇔ 12H₂O, 20.5° C. The cryohydric point is -1.138° C.

Sodium Dihydrogen Orthoarsenate, NaH₂AsO₄, is formed when an aqueous solution of sodium carbonate is treated with arsenic acid until the solution gives no precipitate with barium chloride; after concentration and cooling thoroughly, crystals of the monohydrate, NaH₂AsO₄. H₂O, slowly separate. The crystals may also be obtained by fusing together equivalent quantities of arsenious oxide and sodium nitrate, dissolving the residue in water and allowing to crystallise. Rhombic crystals, isomorphous with those of the corresponding phosphate, are obtained, the axial ratios being a:b:c = 0.9177:1:1.6039. The salt is dimorphous, however, and crystallisation from warm solutions yields monoclinic crystals with a:b:c = 1.087:1:1.1588 and β = 92°22'. These crystals are unstable at the ordinary temperature, rapidly becoming turbid and passing to the rhombic form without change in weight. The indices of refraction of the latter for sodium light are α = 1.5382, β = 1.5535 and γ = 1.5607. The density is 2.67. The aqueous solution, when concentrated until the density is 1.7, deposits rhombic octahedra of the dihydrate, NaH₂AsO₄.2H₂O. These are isomorphous with those of the corresponding phosphate and have axial ratios a:b:c = 0.9177:1:1.6039 and density 2.309. The indices of refraction for sodium light are α = 1.4794, β = 1.5021 and γ = 1.5265. The dihydrate is efflorescent in air.

The monohydrate loses water of crystallisation at 130° C. and on further heating yields a product which has been described as sodium metarsenate, NaNaO₃, but which dissolves in water to form sodium dihydrogen orthoarsenate. Similarly, when sodium monohydrogen orthoarsenate is strongly heated (above 250° C.) the product appears to be sodium pyroarsenate, Na₄As₂O₇, and this re-forms the orthoarsenate on dissolution in water. The existence of the meta- and pyro-arsenates is not established. Both products when heated in hydrogen or carbon monoxide yield volatile arsenic.

Complex salts of composition Na₃H₃(AsO₄)₂.3H₂O, Na₃K₃H₆(AsO₄)₄.9H₂O and Na₃(NH₄)₃H₆(AsO₄)₄.6H₂O have been described, as also have NaKHAsO₄.nH₂O (n = 7-9), Na(NH₄)HAsO₄.4H₂O and Na(NH₄)₂AsO₄.4H₂O.

In the presence of sodium hydroxide, arsenic pentoxide reacts with the hydroxides of aluminium, chromium and ferric iron to form complex salts of composition NaH₂[Al(AsO₄)₂].0.5H₂O, NaH₂[Cr(AsO₄)₂].H₂O and NaH₂[Fe(AsO₄)₂].H₂O, respectively. Corresponding salts of bismuth, cobalt, thallium or lanthanum are not formed under the same conditions.