Hydrazine Salts

Hydrazine combines with acids to form crystalline salts which in their composition show a formal similarity to the mono- and di-acid salts of a diacid base, N₂H₄.HX and N₂H₄.2HX. The evidence already mentioned, as well as that derived from the conductivity of these salts and the determinations of molecular weight in solution, renders it highly probable that the base is a monacid one, which is capable of forming acid salts (like ammonium bisulphate, NH₄HSO₄) by evaporation with excess of acid. The dihydrochloride, N₂H₄.2HCl, may be prepared by passing chlorine into an alcoholic solution of hydrazine hydrate:

2Cl₂ + 3N₂H₄.H₂O = 2(N₂H₄.2HCl) + 3H₂O + N₂.

It is also produced by precipitation of the sulphate with barium chloride. It may be crystallised from water in octahedra. The crystals melt at 180° C. with loss of HCl, and give the hydrochloride, N₂H₄.HCl. Aqueous solutions of the dihydrochloride are almost completely decomposed, giving one equivalent of free hydrochloric acid. This is proved by the velocity of inversion of cane sugar.

Also the Van't Hoff or activity coefficient "i" of a 0.5901N solution is 3.76, since nearly four ions are derived from one molecule, thus:

N₂H₄.2HCl → N₂H^•₅ + H^• + 2Cl'

represents the dissociation as determined by the lowering of the freezing-point.

The densities of the solutions have been determined, and also the refractivities.

The monohydrochloride, (N₂H₄.HCl), obtained from the di-salt by careful heating to 140° C., crystallises from water in needles which melt at 89° C., and on stronger heating decompose into NH₃, N₂, and H₂.

The molar heat of solution is -5.440 Cals., that of the acid salt, which includes the heat of dissociation, is -6.201 Cals.

The hydrobromide and hydriodide are made by similar methods. A salt, 3N₂H₄.2HI, is made by adding iodine to alcoholic hydrazine hydrate. The dihydrofiuoride is made by direct combination in aqueous or aqueous alcoholic solution. It melts at 105° C., and is more stable than the corresponding compounds with the other halogen hydracids, since it sublimes without decomposition. In dilute solution the activity coefficient is about 2; it therefore gives only two ions, no doubt on account of the polymerisation of hydrofluoric acid, i.e. N₂H^•₅ and HF'₂.

The disulphate, prepared by direct combination, crystallises in the form of tables or their prisms in the rhombic system. These melt with decomposition at 254° C. They are easily soluble in hot water, sparingly so in cold, and insoluble in alcohol. The activity coefficient in solutions containing 0.31 to 0.96 mols. to the litre is about 2. The molecule probably dissociates into N₂H^•₅ and HSO'₄. The heat of solution of 1 mol. is -8.700 Cals.

The sulphate, (N₂H₄)₂.H₂SO₄, prepared from the calculated amounts of base and acid, may be precipitated by alcohol as an oil which becomes crystalline on rubbing. The crystals melt at 85° C. The sulphates of hydrazine are decomposed by liquid ammonia, with liberation of the hydrazine. The reaction is quantitative, and may be used in preparing the anhydrous base:

N₂H₄.H₂SO₄ + 2NH₃ = (NH₄)₂SO₄ + N₂H₄.

The dinitrate, N₂H₄.2HNO₃, is prepared from the sulphate and barium nitrate, and may be crystallised in deliquescent needles which melt with decomposition. The nitrate, obtained from nitric acid and the carbonate, or by the addition of alcohol to a solution of the binitrate, may be crystallised in needles from hot alcohol. It is more stable than the binitrate, and may be heated far above its melting-point, 69° C., without decomposition (i.e. to 300° C.).

The nitrite is prepared by the general reaction

Ba(NO₂)₂ + (N₂H₄)₂H₂SO₄ = BaSO₄ + 2(N₂H₄.HNO₂).

The filtrate is concentrated in vacuo over P₂O₅, and may be crystallised from alcohol and ether. It decomposes on heating and explodes on percussion.

N₂H₄.HNO₂ = NH₃ + N₂O + H₂O.

Hydrazine forms a series of double salts with salts of metals, which resemble to some extent the corresponding ammonium double salts. Zinc hydrazine chloride, ZnCl₂.2N₂H₄.HCl, crystallises in needles; it is soluble in alcohol. Double chlorides are also formed with the chlorides of cadmium, mercury, and tin. The platinichloride, 2N₂H₅.HCl.PtCl₄, is obtained as a yellow precipitate from a concentrated aqueous alcoholic solution.

The double sulphates differ from those of ammonia by crystallising anhydrous and being less soluble in water. Those of zinc and nickel have the composition ZnSO₄.(N₂H₄)₂.H₂SO₄ and NiSO₄.(N₂H₄)₂.H₂SO₄.

By the addition of hydrazine hydrate to salt solutions, addition compounds or hydrazinates corresponding to ammines are precipitated, e.g. NiSO₄.3N₂H₄, ZnCl₂.2N₂H₄. Compounds of this type are formed also by salts of Co, Cd, Cu, and Hg, but not by those of manganese, iron, copper, and tin.