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    Nitric Acid
      Manufacture of Nitric Acid
      Physical Properties
      Action on Metals

Physical Properties of Nitric Acid






Pure nitric acid only exists at -41° C. in the form of snow-white crystals, which decompose slightly on liquefying, into nitrogen pentoxide, N2O5, and water. Passing a current of dry air through this liquid removes the N2O5, and leaves a colourless liquid with a 98.67 per cent, content of nitric acid.

Distillation of concentrated nitric acid with concentrated sulphuric acid in an atmosphere of carbon dioxide gives a product containing 98 to 99 per cent. HNO3. Nitric acid containing 99.8 per cent. HNO3 is slightly yellow, with a pungent smell, and fumes strongly in air.

A study of the freezing-point curves shows two maxima, and Pickering isolated two crystalline hydrates corresponding to HNO3.3H2O at -18° C., and HNO3.H2O at -36.8° C. Kuster and Kremann give these two maxima as -18.5° C. and -38° C.

Roscoe gave the boiling-point of the purest liquid nitric acid under atmospheric pressure as 86° C., but slight decomposition begins below this temperature.

Creighton and Githens have determined the boiling-points of nitric acid of 99.79 per cent, purity under different pressures, which are given in the following table:

Pressure, mm. Hg.Boiling-point, ° C.
4722.1
5124.5
6026.2
9031.7
11035.6
12538.5
20349.0
25053.5
29057.1
34662.1
36063.4
42568.5
50072.7
58077.2
67582.5


An aqueous solution of constant boiling-point (120.5° C.) is obtained when any solution of nitric acid is distilled. A concentrated solution will give a distillate of decreasing nitric acid content until, under atmospheric pressure, the residue has a concentration of 68 per cent. Similarly, a dilute solution will eventually reach the same concentration, when it will boil unchanged. The density of this constant boiling solution is 1.414 at 15.5° C., and Roscoe found that the composition varied with pressure, which indicates that this constant boiling solution is not a definite hydrate of nitric acid.

The vapour pressures of solutions of nitric acid of different concentrations and at different temperatures have been determined by Creighton and Githens and Saposchnikoff. Reference has already been made to the necessity of keeping the temperature as low as possible during the preparation of nitric acid from sodium nitrate and sulphuric acid. The thermal decomposition of nitric acid is shown in the following table:

Temperature, ° C.Percentage Decomposition.
869.53
10011.77
13018.79
16028.96
19049.34
22072.07
25093.03
256100.00


The following table shows the variation of density with concentration of aqueous solutions of nitric acid:

Densities and percentages of nitric acids

DensityPercentage HNO3
1.0000.10
1.0051.00
1.0101.90
1.0203.70
1.0305.50
1.0407.26
1.06010.68
1.08013.95
1.10017.11
1.12020.23
1.14023.31
1.16026.36
1.18029.38
1.20032.36
1.25039.82
1.30047.49
1.35055.79
1.40065.30
1.45077.28
1.47584.45
1.50094.09
1.52099.67


The effect of dissolving nitrogen peroxide in nitric acid is to raise the density, and the table below gives the increase in density of nitric acid, D15° = 1.4960.

Influence of nitrogen peroxide on nitric acid (D=1.4960)

Percentage N2O4.HNO3, Increase in Density.
0.50.00075
1.00.00300
2.00.01050
3.00.01800
4.00.02525
5.00.03225
6.00.03950
7.00.04650
8.00.05325
10.00.06600
12.00.07850


The average increase in density per cent, from 1 to 5 per cent, of nitrogen peroxide is 0.00585, and from 5 to 12 per cent, is 0.00660.

The average increase in density from 1 to 5 per cent, of nitrogen peroxide on nitric acid D18° = 1.5126 and D15° = 1.5180 is 0.0035 per 1 per cent, nitrogen peroxide, and from 5 to 20 per cent, of nitrogen peroxide is 0.0040 per 1 per cent, nitrogen peroxide.

Pascal and Gamier found that the maximum weight of nitrogen peroxide dissolved was 42.5 per cent., which corresponds to a hydrate of the formula N2O5.N2O4.H2O, which was stable below -48.5° C.

Contraction in volume occurs when nitric acid and water are mixed, and the maximum contraction occurs corresponding to the ratio 2HNO3:3H2O.

The following table gives the contractions for various concentrations of nitric acid:

Percentage HNO3Contraction.
19.370.03685
25.500.04650
30.170.05171
33.870.05571
42.400.06690
52.800.07040
60.600.06592
65.800.06104
78.220.04528
87.900.02500
92.340.01626
95.620.00672
99.970.00008


The heats of formation have been determined by Berthelot:
H + N + O3 = HNO3 (gas) +34,000 calories.
H + N + O3 = HNO3 (liquid) +41,600 calories
H + N + O3 = HNO3 (solid) +42,200 calories
H + N + O3 = HNO3 (solution) +48,800 calories

The values obtained by Thomsen are:
H + N + O3 = HNO3 (liquid) +41,610 calories.
H + N + O3 = HNO3 (solution) +49,090 calories
NO + O2 + H = HNO3 +63,185 calories
NO2 + O + H = HNO3 +49,735 calories
½[N2O4 + O2 + H2] = HNO3 +42,935 calories

The molecular heat of fusion =601 calories.
The molecular heat of evaporation =7250 calories

The following table gives the values of the heat of solution in Cals. as determined by Thomsen and Berthelot, when 1 gram molecule of nitric acid is dissolved in n gram molecules of water:

No. of molecules of H2O18° C.10° C
0.52.002.03
13.293.34
1.54.164.16
2. . .4.86
2.55.27. . .
35.715.76
4. . .6.39
56.666.76
6. . .6.98
8. . .7.22
107.327.27
207.467.36
407.447.27
807.42. . .
1007.447.21
1607.45. . .
200. . .7.18
3207.49. . .


It will be seen that the maximum heat of dilution occurs when the ratio of nitric acid to water is 1:20, so that further dilution will not cause temperature change.

The heat of neutralisation of nitric acid with potassium hydroxide is 13.770 Cals., and with sodium hydroxide 13.680 Cals.

The refractive indices of nitric acid of different concentrations have been studied by Veley and Manley, and the following table shows the values of n for sodium light at 14.2° C.:

Percentage HNO3.nD
2.191.336208
6.761.342298
18.271.358541
32.601.377798
41.451.388788
50.001.398435
70.041.406094
78.071.405091
87.201.402412
95.601.398148
98.671.396980
99.871.397167


The values for the refractive index increase up to about 70 per cent, concentration, and then decrease gradually to 98.67 per cent., and then again increase slightly. The value of n for the anhydrous acid is practically the same as that for the 50 per cent. acid.

The viscosities, η, of nitric-acid solutions as determined by Kuster and Kremann at 15° C. and -15° C. show that the values reach a maximum in each case where the concentration is 65 per cent. The viscosity of water at 0° C. is taken as unity.

Percentage HNO398.582.070.065.050.030.010.0
η at +15° C.0.5481.0361.2771.3001.1440.8220.655
η at -15° C.0.8332.2403.2683.3042.369 1.635. . .


The magnetic rotatory power of nitric acid is given in the following table, and a gradual decrease in the value of the molecular rotation occurs, corresponding with increase in dilution:

HNO3 + 2.701H2O
Molecular CompositionPercentage HNO3Specific Rotation at 15° C.Molecular Rotation.Molecular Rotation less that due to H2O.
HNO3 + 0.019H2O99.450.52921.2261.207
HNO3 + 2.701H2O56.440.80423.6780.977
HNO3 + 7.311H2O32.360.90668.1630.852
HNO3 + 9.555H2O26.810.923810.3600.805
HNO3 + 12.030H2O22.540.935012.7830.753


The temperature coefficient of conductivity, = 0.0162, varies with the dilution.

The mean coefficient per 1° between 18° and 52° C. is:

0.01570.01540.01520.01470.0143
At V = 1000100102


The values of the molar conductivities A have been obtained up to 100° C.

t° C.C=0.1 mols./litre.0.05 mols./litre.0.01 mols./litre.0.002. mols./litre.0.000 mols./litre.
18λ=346.4353.7365.0371.2377
25λ=385.0393.3406.0413.7421
100λ=728.0760.0786.0806.0826


The conductivities of acids from 20 to 89.4 per cent, have also been determined at -16° C.

Freezing-points and Compositions of the System HNO3-H2O:

Mols. per cent. HNO3Temp., °C.
0.00.0
4.5-10.3
9.5-27.1
12.2-43.0*
15.8-30.6
19.8-22.9
25.0-18.5*
33.3-26.6
37.7-35.4
40.6-42.0*
44.4-39.4
48.9-37.9*
50.3-38.2*
61.3-44.4
67.1-55.3
73.2-64.5
80.0-55.3
100.0-41.2*


Freezing-point of nitric acid
Freezing-point curve of aqueous nitric acid.
The values marked * are fixed temperatures or halt-points of crystallisation, i.e. either eutectics or melting-points of compounds. As in the case of other strong mineral acids, the freezing-point composition curve (fig) shows the existence of a series of hydrates and eutectics. The hydrates are HNO3.3H2O (melting-point -18.5° C.) and HNO3.H2O (melting-point -38° C.). The freezing-point curve also shows three eutectics as minima between the four separate components, these eutectics corresponding to the cryohydrates ice + HNO3.3H2O (melting-point -43° C.), HNO3.3H2O+HNO3.H2O (melting-point -42° C.), and HNO3.H2O+HNO3 (melting-point -66° C.).


Freezing-points and Dissociation in Dilute Solutions

Nitric acid ranks with hydrochloric acid as being one of the strongest acids. This is due to the high concentration of hydrogen ions in dilute solution resulting from the far-reaching dissociation of nitric acid in water. The degree of dissociation calculated from the depression of the freezing- point of water is shown in the following table:

HNO3 in mols. per litre0.0010540.0031580.0073780.011530.051030.1059
Depression of F.P. (Δt)0.00400.01190.02760.04300.18900.3735
van't Hoff factor (i)2.0081.9941.9791.9731.9581.866
Degree of dissociation (α)100.899.497.997.395.886.6


The depression of the freezing-points of more concentrated solutions of nitric acid are given below:

HNO3 in mols. per litre0.250.51.01.52.02.53.0
Depression of F.P. (Δt)0.751.8103.7965.9388.34711.04613.909


The specific heats of nitric acids of various concentrations

Percentage HNO3.Mean Specific Heat.Temperature of Determination.
1.72 (200 mols. H2O)0.98218° c
3.40 (100 mols. H2O)0.96318° c
15.00 (20 mols. H2O)0.84918° c
26.00 (10 mols. H2O)0.76818° c
10.000.90020° C
25.570.78720° C
40.000.66920° C
60.520.63720° C
81.800.57520° C
92.150.50020° C
98.150.47520° C
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