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.
47	22.1
51	24.5
60	26.2
90	31.7
110	35.6
125	38.5
203	49.0
250	53.5
290	57.1
346	62.1
360	63.4
425	68.5
500	72.7
580	77.2
675	82.5

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.
86	9.53
100	11.77
130	18.79
160	28.96
190	49.34
220	72.07
250	93.03
256	100.00

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

Densities and percentages of nitric acids

Density	Percentage HNO3
1.000	0.10
1.005	1.00
1.010	1.90
1.020	3.70
1.030	5.50
1.040	7.26
1.060	10.68
1.080	13.95
1.100	17.11
1.120	20.23
1.140	23.31
1.160	26.36
1.180	29.38
1.200	32.36
1.250	39.82
1.300	47.49
1.350	55.79
1.400	65.30
1.450	77.28
1.475	84.45
1.500	94.09
1.520	99.67

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, D_4°^15° = 1.4960.

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

Percentage N2O4.	HNO3, Increase in Density.
0.5	0.00075
1.0	0.00300
2.0	0.01050
3.0	0.01800
4.0	0.02525
5.0	0.03225
6.0	0.03950
7.0	0.04650
8.0	0.05325
10.0	0.06600
12.0	0.07850

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 D_4°^18° = 1.5126 and D_4°^15° = 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 HNO3	Contraction.
19.37	0.03685
25.50	0.04650
30.17	0.05171
33.87	0.05571
42.40	0.06690
52.80	0.07040
60.60	0.06592
65.80	0.06104
78.22	0.04528
87.90	0.02500
92.34	0.01626
95.62	0.00672
99.97	0.00008

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 H2O	18° C.	10° C
0.5	2.00	2.03
1	3.29	3.34
1.5	4.16	4.16
2	. . .	4.86
2.5	5.27	. . .
3	5.71	5.76
4	. . .	6.39
5	6.66	6.76
6	. . .	6.98
8	. . .	7.22
10	7.32	7.27
20	7.46	7.36
40	7.44	7.27
80	7.42	. . .
100	7.44	7.21
160	7.45	. . .
200	. . .	7.18
320	7.49	. . .

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
2.19	1.336208
6.76	1.342298
18.27	1.358541
32.60	1.377798
41.45	1.388788
50.00	1.398435
70.04	1.406094
78.07	1.405091
87.20	1.402412
95.60	1.398148
98.67	1.396980
99.87	1.397167

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 HNO3	98.5	82.0	70.0	65.0	50.0	30.0	10.0
η at +15° C.	0.548	1.036	1.277	1.300	1.144	0.822	0.655
η at -15° C.	0.833	2.240	3.268	3.304	2.369	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 Composition	Percentage HNO3	Specific Rotation at 15° C.	Molecular Rotation.	Molecular Rotation less that due to H2O.
HNO3 + 0.019H2O	99.45	0.5292	1.226	1.207
HNO3 + 2.701H2O	56.44	0.8042	3.678	0.977
HNO3 + 7.311H2O	32.36	0.9066	8.163	0.852
HNO3 + 9.555H2O	26.81	0.9238	10.360	0.805
HNO3 + 12.030H2O	22.54	0.9350	12.783	0.753

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

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

	0.0157	0.0154	0.0152	0.0147	0.0143
At V =	∞	1000	100	10	2

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.
18	λ=346.4	353.7	365.0	371.2	377
25	λ=385.0	393.3	406.0	413.7	421
100	λ=728.0	760.0	786.0	806.0	826

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. HNO3	Temp., °C.
0.0	0.0
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 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.).

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 litre	0.001054	0.003158	0.007378	0.01153	0.05103	0.1059
Depression of F.P. (Δt)	0.0040	0.0119	0.0276	0.0430	0.1890	0.3735
van't Hoff factor (i)	2.008	1.994	1.979	1.973	1.958	1.866
Degree of dissociation (α)	100.8	99.4	97.9	97.3	95.8	86.6

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

HNO3 in mols. per litre	0.25	0.5	1.0	1.5	2.0	2.5	3.0
Depression of F.P. (Δt)	0.75	1.810	3.796	5.938	8.347	11.046	13.909

The specific heats of nitric acids of various concentrations

Percentage HNO3.	Mean Specific Heat.	Temperature of Determination.
1.72 (200 mols. H2O)	0.982	18° c
3.40 (100 mols. H2O)	0.963	18° c
15.00 (20 mols. H2O)	0.849	18° c
26.00 (10 mols. H2O)	0.768	18° c
10.00	0.900	20° C
25.57	0.787	20° C
40.00	0.669	20° C
60.52	0.637	20° C
81.80	0.575	20° C
92.15	0.500	20° C
98.15	0.475	20° C