Ammonium Arsenates

Ammonium Orthoarsenate, (NH₄)₃AsO₄.3H₂O, is precipitated on saturating an aqueous solution of arsenic acid, or of either ammonium mono- or di-hydrogen orthoarsenate, with ammonia; or it may be obtained by cooling a warm concentrated solution of arsenic acid in aqueous ammonia. The deposit consists of rhombic crystals and may be recrystallised from hot aqueous ammonia. The salt rapidly loses ammonia both in air and in solution, the dihydrogen arsenate ultimately being formed. The anhydrous salt, (NH₄)₃AsO₄, is formed when the dry mono- or di-hydrogen salt absorbs ammonia under a pressure of 8 atmospheres; the absorption should be allowed to continue for 12 hours at about 50° C. This product also is unstable and rapidly loses ammonia to form the monohydrogen arsenate.

Ammonium Monohydrogen Orthoarsenate, (NH₄)₂HAsO₄, may be prepared by the addition of ammonia to a concentrated aqueous solution of arsenic acid, the precipitate first formed being dissolved by warming and the salt then obtained either by crystallisation or by precipitation with ethyl alcohol. The dry salt, when gently heated, loses ammonia to form the dihydrogen arsenate, as also does the aqueous solution; in the cold, however, the dry salt is stable in dry air. The crystals are monoclinic prisms, with a:b:c = 0.918:1:1.1715 and β = 91°13'; density 1.99.

Ammonium Dihydrogen Orthoarsenate, NH₄H₂AsO₄, may be prepared by mixing aqueous ammonia and arsenic pentoxide in theoretical proportions. It has also been obtained by heating arsenious oxide with ammonium nitrate and by gently heating ammonium mono- hydrogen orthoarsenate. It crystallises in long non-deliquescent prisms belonging to the tetragonal system, the axial ratio a:c being 1:1.0035. The salt is isomorphous with the corresponding potassium salt and with the corresponding ammonium and potassium phosphates. The density at 20° C. is 2.340. When heated above 300° C. it loses ammonia and an almost insoluble acid metarsenate, NH₄H(NaO₃)₂, is obtained -

2NH₄H₂AsO₄ = NH₃ + 2H₂O + NH₄H(NaO₃)₂

which is stable up to 425° C., no trace of arsenious oxide or arsenic being formed at this temperature. When boiled with water the metarsenate reverts to the orthoarsenate.

The solubility in water of ammonium dihydrogen orthoarsenate and the densities of the saturated solutions have been determined with the results shown in the following table:

Solubility of NH₄H₂AsO₄ in water

Temp., ° C.	g. per 100 g. H2O	Density
0	33.74	1.1814
20	48.67	1.2280
40	63.83	1.2821
60	83.05	1.3464
80	107.25	1.4200
90	122.40	1.4623

The following values for the heats of neutralisation at 15° C. of arsenic acid by aqueous ammonia, and for the heats of dissolution of the ammonium arsenates, have been obtained. The solutions of arsenic acid for the determination of the former contained 1/6 mole of H₃AsO₄ per litre, and those of ammonia were of such concentrations that 1 volume of the acid was neutralised by an equal volume of the base.

Heats of Neutralisation

H₃AsO₄.330H₂O + NH₃.330H₂O = NH₄H₂AsO₄.660H₂O + 13,750 calories
H₃AsO₄.330H₂O + 2NH₃.330H₂O = (NH₄)₂HAsO₄.660H₂O + 24,300 calories
H₃AsO₄.330H₂O + 3NH₃.330H₂O = (NH₄)₃AsO₄.660H₂O + 25,100 calories

Heats of Dissolution

NH₄H₂AsO₄ solid + 660H₂O = NH₄H₂AsO₄ dissolved - 4250 calories
(NH₄)₂HAsO₄solid + 660H₂O = (NH₄)₂HAsO₄ dissolved - 3150 calories
(NH₄)₃AsO₄solid + 1500H₂O = (NH₄)₃AsO₄dissolved - 8400 calories
(NH₄)₃AsO₄.3H₂Osolid + 1200H₂O = (NH₄)₃AsO₄.3H₂Odissolved -13,700 calories

From the above data, and assuming the heat of dissolution of arsenic pentoxide to be +6000 calories, the following values for the heats of formation have been calculated:

Heats of Formation

½(As₂O₅solid + 3H₂Oliquid) + NH₃gas = NH₄H₂AsO₄solid + 29,480 calories
½(As₂O₅solid + 3H₂Oliquid) + 2NH₃gas = (NH₄)₂HAsO₄solid + 47,410 calories
½(As₂O₅solid + 3H₂Oliquid) + 3NH₃gas = (NH₄)₃AsO₄solid + 61,940 calories
½(As₂O₅solid + 3H₂Oliquid) + 3NH₃gas + 3H₂Oliquid = (NH₄)₃AsO₄.3H₂Osolid + 67,240 calories

The values of Q₀ from Nernst's formula



for the di- and tri-ammonium salts are respectively 15,500 and 13,070 calories; these values represent the heats liberated in fixing the second and third molecules of ammonia. A study of the dissociation of the triammonium arsenate suggests that the successive withdrawal of ammonia gives rise to equilibria which indicate the existence of the following compounds; Q₀ represents the heat changes of the equilibria.

(NH₄)₆As₂O₈ ⇔ (NH₄)₅HAs₂O₈ + NH₃, Q₀ = 13,000 cal.
(NH₄)₅HAs₂O₈ ⇔ (NH₄)₄H₂As₂O₈ + NH₃, Q₀ = 14,000 cal
(NH₄)₄H₂As₂O₈ ⇔ (NH₄)₃H₃As₂O₈ + NH₃, Q₀ = 15,600 cal
(NH₄)₃H₃As₂O₈ ⇔ (NH₄)₂H₄As₂O₈ + NH₃, Q₀ = 16,200 cal
(NH₄)₂H₄As₂O₈ ⇔ (NH₄)H₅As₂O₈ + NH₃, Q₀ = 16,700 cal

Mixed orthoarsenates of ammonium and sodium have been obtained, namely, diammonium sodium orthoarsenate, (NH₄)₂NaAsO₄.4H₂O, and ammonium sodium hydrogen orthoarsenate, (NH₄)NaHAsO₄.4H₂O. The latter forms monoclinic crystals when a solution containing equivalent quantities of ammonium and sodium monohydrogen arsenates is allowed to crystallise, or when an ammoniacal solution containing ammonium chloride and sodium dihydrogen arsenate (1:6) is concentrated. When ignited, water and ammonia are expelled and sodium metarsenate remains. The diammonium sodium salt may be obtained by allowing to crystallise a solution of the ammonium sodium hydrogen salt in concentrated aqueous ammonia. The tabular crystals, when confined over dilute sulphuric acid, lose ammonia and revert to ammonium sodium hydrogen orthoarsenate.