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Aqueous Ammonia

General Properties of Aqueous Solutions

Ammonia is easily soluble in water, with evolution of heat. The densities of the solutions are less than that of water. The solution saturated at 15° C. has a density of 0.882. Solutions saturated with the gas at 1 atmosphere contain per gram of water 0.875 grams at 0° C., 0.536 grams at 15° C., and 0.526 grams at 20° C. One volume of water dissolves at 0° C. about 1150 vols, (at N.T.P.), at 20° C. 739 vols, (at N.P.). At lower temperatures the solubilities are greatly increased. The following weights are absorbed by 1 gram of water in equilibrium with ammonia pressures of 743 to 744.5 mm.:

Temperature, ° C.-3.9-10-20-25-30–40
Grams of NH30.9471.1451.7682.5542.7812.946

Ammonia Pressures and Concentrations of Aqueous Solutions

Up to normal concentrations and the corresponding ammonia pressures, the gas dissolves nearly according to Henry's Law.

p (in mm. of mercury )5.976.719.3513.45
w (grams per litre7.7528.60211.9317.00
N (mols. per litre)0.4560.5060.7021.00

That the law does not hold for higher pressures is evident from the tables given below.

The relations between p and N have been expressed as an interpolation formula

p = 13.34N + 0.18N2.

The addition of salts to solutions of ammonia generally raises the ammonia pressure, or diminishes the solubility at a given pressure. The lithium halides are an exception to this rule and increase the solubility of ammonia. Some typical solubilities are given in the table, in which "p" denotes the ammonia pressures (mm.) in equilibrium with various concentrations of the salt in solutions which are normal with respect to total ammonia, and Δp the differences between the pressures of ammonia, i.e. 13.45 mm. in equilibrium with a normal aqueous solution, and those in equilibrium with the salt solutions normal in respect to ammonia.

Ammonia pressures of ammonical salt solutions

Salt.Concentration of Salt.
0.5 Normal1.0 Normal1.5 Normal

In the case of more concentrated solutions, the comprehensive results of Perman are available. The total pressures of ammonia and water-vapour in equilibrium with solutions of various concentrations and at various temperatures were measured by the static method, i.e. on a manometer. Table is based on these results. In another series, the partial pressures of ammonia and of water-vapour (mm.) in equilibrium with the solutions were determined by the dynamic method, i.e. by passing a known volume of air through the solutions, and then through (a) standard dilute and (b) concentrated sulphuric acid. The total increase in the weights of (a) and (b) gives the ammonia plus water, and the decrease in the titre of the sulphuric acid gives the ammonia. Table B is based on these results. By means of this series it was proved that the lowering of the vapour pressures of water in the presence of ammonia is a rectilinear function of the compositions up to 10 per cent, solutions,

p1 = P(1 - x)("p" in mm.),

in which p1 are the partial pressures of water over the solutions, p are the pressures of pure water at the same temperatures, and x is the fractional amount of ammonia in 1 gram of the solution.

Vapour pressures of aqueous ammonia (Total pressures, (H2O+NH3) in mm. of mercury.)

Temperature,°CPercentage of NH3

Vapour pressures of ammonia and of water over aqueous ammonia

Temperature, ° C.Percentage of NH3Pressure of NH3 (mm.).Pressure of H2O (mm.).

The fact that Raoult's Law is obeyed shows that the molar weight of ammonia as determined by osmotic pressure or depression of the freezing-point is normal at these concentrations. Therefore there is only 1 molecule of ammonia present in the hydrates.

The vapour pressures of more concentrated solutions are best expressed by formulae which are used for binary mixtures of liquids. Thus the pressure-composition isothermals agree with the Margules equation

in which p1 and x1 refer to the ammonia, and p2 and 1-x1 to the water. Thus for x=0.2088 (20.88 per cent. NH3 at 20° C.), p1=228.5, dp1=13, dp2=0.825:

p1(1-x)/p2x = 66.6; dp1/dp2=61.8

Table gives the total pressures of solutions containing from 0 to 30 per cent, of ammonia between 0° and 62° C. The pressures were measured by the static method, i.e. on a manometer connected with the space above the solutions, which were kept at the controlled temperatures by the vapours of various liquids boiling at atmospheric pressure.

At 100° C. the following ratios between the concentrations, C1 of ammonia in the gas phase, and those, C2, in solution, increase with increasing concentration, but the ratios K=C1/C(1-α), in which C(1-α) is the "ion residue," or total ammonia not present as ions, is nearly constant.


Table gives the experimental partial pressures of ammonia and water-vapour in equilibrium with solutions. The pressures were determined by the dynamic method already described. The total pressures found as the sums of those of the ammonia and water by the dynamic (air current) method usually agree within 1 per cent, with the corresponding pressures found by the static (manometer) method, although in a few cases there is a discrepancy of 2 to 3 per cent. These results have also been in a table which gives values at intervals of 2° C. from 0 to 60.

The partial pressures of ammonia and of water in equilibrium with the concentrated solutions have been redetermined with the aid of elaborate apparatus by Neuhausen and Patrick. The dynamic method outlined above was criticised on grounds which were indeed noted by Perman.

The static method employed gives accurately the vapour pressure of the water, which is the minor constituent in the case of these solutions. The pressure of pure ammonia gas, admitted into one tank from the reservoir of liquid, was measured against that of a similar tank containing ammonia plus water-vapour on a differential manometer with an obtuse angle of 157° (i.e. each limb 11.5° to the horizontal). Water-vapour was injected into the second tank until the pressure, which rises at first (because the volume is constant), reaches a maximum, and then falls as solution is condensed, carrying with it some of the ammonia in the dissolved state. The results are given in Table C (t= 20° and 40° C.). Those due to Berthoud are given in Table (t=0° C.).

Partial pressures of ammonia and of water, densities and contractions of aqueous ammonia (p in mm. of mercury.)

Temperature, ° C.Molecular Percentage of NH3pNH3pH2ODensity.Percentage Contraction.

The densities of aqueous ammonia solutions at 15° C. are given in the well-known tables of Lunge and Wiernik, and have been redetermined by Price and Hawkins. A selection of comparative values from both sets of observations is given in Table.

Hydrates of Ammonia

The freezing-point curve for aqueous solutions of ammonia indicates the existence of two hydrates and three eutectic mixtures. The following data are due to Rupert; those in brackets to Smits and Postma:

Molecular percentage of NH3 in the solution65.4048.59
Freezing-point, ° C-79 (-78.8)-79 (-79)

Eutectic mixtures - Solid phases NH3 and 2NH3.H2O2NH3.H2O and NH3.H2ONH3.H2O and H2O
Molecular percentage of NH3 in solution78.5 (81.4)57.5 (58.5)32 (34.7)
Freezing-point, ° C-96 (-92.5)-86.5 (-86)-115 (-100.3)

The monohydrate forms small colourless crystals and the hemihydrate larger transparent needles. Solutions containing about 33 per cent, of ammonia are very viscous at - 100° C.

Densities of aqueous ammonia

DensityPercentage of NH3 (P. and H.)Percentage of NH3. (L. and W.)DensityPercentage of NH3 (P. and H.)Percentage of NH3. (L. and W.)

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