Chemical elements
    Nitrogen Cycle
    Physical Properties
    Chemical Properties
      Hydrazine Salts
      Sulphonic Derivatives
    Nitric Acid



Organic derivatives of hydrazine have long been known and used in syntheses, as, for example, that of phenylhydrazones from hydrazine. The substituted hydrazines are obtained by the reduction of compounds containing the diazo grouping; thus, diazo-benzene chloride may be reduced to phenylhydrazine hydrochloride,

C6H5-N=N-Cl + 4HC6H5NH-NH2HCl;

azo-benzene yields diphenylhydrazine,

C6H5-N=N-C6H5 + 2HC6H5NH-NHC6H5;

nitro-urea yields semi-carbazide,

H2N-CO-NH-NO2 + 6HH2N-CO-NH-NH2 + 2H2O;

diazo-acetic ester yields hydrazo-acetic ester,

C2H5-O-CO-CH=N2 + 2HC2H5-O-CO-CH=(NH)2.

Hydrazine itself was prepared by Curtius in 1887 from di-diazo-acetic acid. Para-urazine, C2H4O2N4, derived from urea, may also be hydrolysed by dilute sulphuric acid and the sulphate of hydrazine crystallised out. When ethyl-diazo-acetate is heated with concentrated potash a coloured salt is formed. After digestion with sulphuric acid the colour disappears and hydrazine sulphate is formed.


  1. By the reduction and subsequent hydrolysis of diazo-acetic ester, or by the hydrolysis of its condensation products. Diazo-acetic ester is reduced by an alkaline ferrous solution to hydrazo-acetic ester (vide supra), which is then hydrolysed by mineral acids:

    + 2H2OC2H5OH + CHO.COOH + N2H4.

    Diazo-acetic ester and the free acid easily polymerise into bisdiazo-compounds:

    which probably undergo a tautomeric change, giving

    This compound, on boiling with mineral acids, is hydrolysed (by 4 mols. of water) to 2(COOH)2 + 2N2H4. 245 grams of the bisdiazo acid in 2 litres of water and 300 grams of concentrated sulphuric acid are heated on the water-bath until all is dissolved, then heated until effervescence (CO2) ceases. On cooling, hydrazine sulphate crystallises.
  2. By the reduction of nitro-guanidine to amino-guanidine, which is then hydrolysed. Ammonium thiocyanate is heated for some time at 170° to 180° C. The residue, guanidine-thiocyanate, NH2-C(NH)-NH2.HNCS, is treated, first with concentrated, then with fuming, sulphuric acid. This mixture, after cooling, is mixed with nitric acid of density 1.5 and poured into water. Nitro-guanidine, NH2-C(NH)-NH-NO2, separates, and is reduced with zinc dust and acetic acid. The solution of amino-guanidine, NH2-C(NH)-NH- NH2, may be hydrolysed either by acids or by alkalies. It is concentrated, mixed with sodium-hydroxide solution, and boiled for eight to ten hours. The cooled liquid, after separation from NaHCO3, is mixed with concentrated sulphuric acid, when most of the hydrazine separates as the sulphate, and may be purified by one recrystallisation:

    NH2-C(NH)-NH-NH2.HCl + 3NaOH = Na2CO3 + NaCl + 2NH3 + N2H4.
  3. By the reduction of hyponitrous acid or its sulphonic derivative. The hyponitrous acid may be reduced by zinc and acetic acid in the presence of sodium bicarbonate. Or, the potassium salt of nitroso-sulphurous acid (nitroso-hydroxylamine sulphonic acid), 2NO.K2SO3 (or KO-N=NO-SO3K), which was discovered by Davy, may be prepared by passing nitric oxide to saturation into an alkaline solution of potassium sulphite containing an excess of potassium hydroxide. The compound is deposited in a crystalline form, and may be recrystallised from hot water with some loss. A solution of this salt may be reduced with an excess of sodium amalgam:

    + 6H = + H2O + KOH,

    + KOH = N2H4 + K2SO4

    Several organic compounds containing the -N=N- group and the -SO3K group are hydrolysed in this way by alkalies.
  4. By the oxidation of ammonia with sodium hypochlorite in the presence of glue. One litre of an aqueous solution of sodium hypochlorite is mixed with 3 litres of concentrated ammonia solution containing 12 c..c. of a 5 per cent, solution of glue. The mixture is boiled in an open vessel for about half an hour, cooled in ice, and acidified with 10 per cent, sulphuric acid. The sulphate, N2H4.H2SO4, crystallises. The yield (80 to 90 grams) is about 40 per cent, of theory. The oxidation of the ammonia probably proceeds through the formation of chloramine,

    NH3 + NaOCl = NH2Cl + NaOH,
    NH2Cl + NH3 + NaOH = N2H4 + NaCl + H2O.

    The glue appears to act as a protective colloid, and also by raising the viscosity of the solution.

The Hydrate and the Free Base

Hydrazine is set free by alkalies in aqueous solution in the hydrated form. One molecule of water is firmly combined, for on distillation of the salts with solid potassium hydroxide, the hydrate. N2H4.H2O, is obtained as a fuming liquid which boils at 188° C. This is sometimes regarded as a constant boiling mixture, i.e. one of maximum boiling-point, like those of the halogen hydracids and water. It has been proved that there is dissociation in the vapour phase. The hydrate does not freeze at -40° C.; it does not exist as a definite solid compound. It has a strong smell resembling that of ammonia, and is a most penetrating reagent; it corrodes not only cork and indiarubber, but also glass.

The solutions are oxidised by atmospheric oxygen:

N2H4.H2O + O2 = N2 + 3H2O.

They also decompose spontaneously, giving ammonia, and more quickly in the presence of spongy platinum, giving nitrogen and hydrogen as well:

2N2H4 = 2NH3 + N2 + H2.

The spontaneous decomposition in the presence of a little alkali gives different proportions:

3N2H4 = 2NH3 + 2N2 + 3H2.

In connection with this instability it may be noted that the heat of formation in dilute solution (deduced from the heat of oxidation by K2Cr2O7, etc.) is –9.5 Cals.

Hydrazine itself is obtained by distillation of the hydrate with anhydrous alkalies in a reducing atmosphere and under diminished pressure. Barium oxide may be used as alkali. The partly dehydrated base may be boiled with an excess of the oxide for three hours under a reflux condenser and then distilled in hydrogen at reduced pressure.

The product may contain 99.7 per cent, of hydrazine. It may also be prepared by a similar method from the carbonate or the borate. The barium oxide may be replaced by solid sodium hydroxide. The hydrochloride may be decomposed with sodium methoxide, and the methyl alcohol removed by distillation under diminished pressure.


Hydrazine is a colourless liquid which can easily be solidified; the solid melts at +1.4° C. The liquid boils at 113.5° C. under 761.5 mm., at 56° C. under 71 mm. pressure. The density is only slightly greater than that of water. D15° is 1.0114, and is 1.0258.

The refractive index for the D line is 1.46979; for Hα it is 1.46675. The molecular refraction MD is 8.867.

Hydrazine mixes in all proportions with water, with evolution of heat, 1.919 Cals. per mol. in dilute solution.

It also mixes with some alcohols, but not with other organic solvents of the " normal " type, i.e. those which, on the evidence of many physical properties, appear to exist as simple molecules and are freely miscible with one another.

As a solvent it bears many resemblances to liquid ammonia. It dissolves sulphur and iodine, but with some chemical action. Most of the sulphur is deposited again on pouring the solution into water, but some reacts with evolution of nitrogen and formation of an unstable hydrosulphide. The alkali metals dissolve, and also react with evolution of hydrogen. Sodium gives a white explosive product, which has been described as a derivative of azoimide, or as a mono- or di-sodium hydrazate, NaN2H3 and Na2N2H2. These solutions conduct electricity; nitrogen and hydrogen are evolved at the electrodes. It also dissolves many salts, especially halides, nitrates, and ammonium salts. Typical solubilities are: NaCl, 12.2; KNO3, 21.7; KI, 135.7; Ba(NO3)2, 81.1, in 100 parts by weight of hydrazine. All these solutions conduct electricity. Many reactions take place in this solvent. Thus, hydrazine sulphide and a zinc salt give zinc sulphide.

Hydrazine as a Base

A solution of hydrazine in water reacts alkaline on account of electrolytic dissociation:

N2H4.H2OH2N.NH3 + OH'.

The constant of this dissociation is obtained from the following conductivity measurements: -

At V8163264128256

Assuming a kation mobility of 61, and that the mobility of the hydroxyl ion is 177 (both at 25° C.), the value of λ0 is 238, whence the dissociation constant is as stated in the third line. Taking into account the present accepted value of the mobility of OH', which is rather greater (ca. 190), the dissociation constant becomes 2×10-6 in round numbers; which is about 1/10 of the constant of ammonia (q.v.). The heat of neutralisation is also less than that of ammonia.

Thus the corresponding heats of formation of the salts in dilute solution are:

N2H4.HCl aq.9.600 Cals
N2H4.HNO3 aq.9.700 Cals
N2H4H2SO4 aq.11.100 Cals
NH4Cl aq.12.390 Cals
NH4NO3 aq.12.320 Cals
½(NH4)2SO4 aq.14.075 Cals

Although the salts of the diacid base are known in the solid state, they are completely dissociated in solution. Thus there is a difficulty in the addition of a hydrion to the second - NH2. This is no doubt connected with the fact that alkylation also only takes place on one -NH2. The final product is a quaternary halide of the monacid base which cannot be alkylated further:


Hydrazine as a Reducing Agent

Solutions of the free base and its salts are slowly oxidised by atmospheric oxygen. They are powerful reducing agents, and will reduce cupric and ferric salts to the cuprous and ferrous states, iodine to hydrogen iodide, and selenious acid to selenium. The noble metals are precipitated from their salts, and metallic copper is also similarly precipitated.

Derivatives of Hydrazine

In addition to the numerous organic derivatives in which the hydrogen is replaced by alkyl or aryl radicals, etc., there are a few derivatives of inorganic acids, but these also are more stable if some of the hydrogen is replaced by organic radicals.

The sulphonate, H2N-NH-SO3H, and the disulphonate are known. When air is passed through fuming sulphuric acid and then through anhydrous hydrazine, hydrazine hydrazine-sulphonate is produced:

2N2H4 + SO3 = H2N-NH-SO3H.N2H4.

Among the carboxyl and carbonyl substitution . products are hydrazine-carboxylic acid, NH2-NH-COOH, the dicarboxylic acid, HOOC-NH-NH-COOH, carbonic-acid dihydrazide, CO(NH.NH2), and the amide hydrazide or semi-carbazide, NH2-CO-NH-NH2.

Detection and Estimation

The reactions which are used in the analysis of hydrazine and its salts usually depend upon its oxidation. Thus it is determined by titration with potassium permanganate in acid solution, or with vanadic acid. Nitrogen is quantitatively evolved:

N2H4 + 2O = N2 + 2H2O.

Hydrazine is also a useful reagent in general analysis. A solution made by dissolving the hydrochloride in an excess of alkali will quantitatively precipitate copper as metal. The copper solution is added drop by drop. Copper can be estimated in this way in the presence of tin zinc.

Hydrazine alone does not reduce chlorates, bromates, or iodates, but in the presence of cupric oxide the reduction is quantitative:

2KClO3 + 3N2H4.HNO3 = 6H2O+3N2 + 3HNO3 + 2KCl.

The chloride, etc., may then be determined in the usual manner after decomposition of the excess of hydrazine with permanganate and nitric acid.

The Sulphonic and Sulphinic Substitution Products of Ammonia, Hydroxylamine, and Hydrazine.

Sulphinic Derivatives

The action of dry SO2 on dry NH3 produces a variety of coloured compounds having the empirical formulae (NH3)x,(SO2)y. It appears, however, that a little water is required in order that combination may occur freely. In ethereal dry solution a compound, (NH3)2SO2, has been isolated as a white solid, which may also be the ammonium salt of amido-sulphinic acid.

Organic substitution products of amino-sulphinic acid, R-NH-SO2H, and of thionyl imide, R-N=SO, are known.

The addition of thionyl chloride drop by drop to liquid ammonia gives a red solution, from which triammonium imido-disulphinate can be isolated. This may also be formed by the hydrolysis of imido-sulphonamide.

Oxygen and Hydroxyl Substitution Products of Hydrazine

These compounds are chiefly represented by organic derivatives, namely, (azoxy-compounds), R-N=NOH (free diazocompounds), R-NH-NO (their tautomeric forms, nitrosamines), RH=N-NO2 and R2=N-NO2 (nitramines).

Although dinitrous acid, or rather dinitronic acid, (HO-NO)2, does not exist, the intermediate oxidation products of hydrazine, namely, H2N-NO2, nitramide, HO-N-NOH, hyponitrous acid and nitrohydroxylamine, HO-NH-NO2, have been prepared.
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