Liquid Nitrogen

Cailletet first liquefied nitrogen by compressing it to 200 atmospheres and allowing it to expand rapidly. Liquid nitrogen is colourless, with a refractive index of 1.2053. The critical data of nitrogen, according to different investigators, is given in the following table:

Critical temperature: from -147.13 to -144.7
Critical pressure: from 33.49 to 35.00
Critical density: from 0.31096 to 0.32690

The following table gives the variation of the boiling-point of nitrogen with the pressure:

Variation of the boiling-point of nitrogen with the pressure

Pressure, mm. (Cath.)	Absolute Boiling- point.	Pressure, Atm. (Crommelin.)	Absolute Boiling- point.
120.9	64.51	2.507	86.21
361.6	71.54	7.370	99.51
561.3	74.83	21.82	117.62
760	77.31	33.46 crit. pt.	125.96
938.6	79.18
1591.1	84.21

The empirical formula, giving the vapour pressures of liquid nitrogen at any temperature between -208.36 to -148.08° C., maybe written as:

log p = -510.64/T + 12.381 - 0.12696T + 0.0008302T² - 0.0000019975T³, where T=t+273.09.

Densities of liquid nitrogen at various temperature

t° C.	Density of Liquid.	Density of Gas.
-148.08	0.4314	0.18620
-149.75	0.4799	0.16380
-153.65	0.5332	0.11170
-161.20	0.6071	0.06987
-173.73	0.6922	0.02962
-182.51	0.7433	0.01558
-195.09	0.8043	0.00490
-200.03	0.8265	0.00278
-205.45	0.8499	0.00136
-208.36	0.8622	0.00089

These values may be compared with those of earlier investigators and with others, from which the following densities (of liquids only) have been selected:

t

	-210.5	-205	-200	-198.3	-195.5	-195.5	-190	-185	-184
d	0.8792	0.8537	0.8309	0.8297	0.8103	0.8042	0.7874	0.7622	0.7576

The latent heats of evaporation (in calories per gram) are given in the following table:

Latent heat of evaporation of liquid nitrogen

° C	-148.08	-148.61	-149.75	-153.65	-161.20	-173.73	-182.51	-195.09	-200.03	-205.45	-208.36
T.° abs.	125.01	124.48	123.34	119.44	111.89	99.36	90.58	78.00	73.06	67.64	64.73
l cals. per gram	9.17	11.40	15.03	22.96	31.73	39.63	43.09	47.06	48.64	50.52	51.64

These latent heats may be represented by the formula

l² = 89.962(T_C - T) - 1.47242(T_C - T)² + 0.011666(T_C - T)³.

The specific heat of liquid nitrogen at -200.4° C., and also at -209° C., is 0.476.

This was first determined at -196° C. as 8.514 dynes per cm., while the variation with temperature is as follows:

t° C.	-203	-193	-183
γ dynes/cm.	10.53	8.27	6.16

The change in the molar surface tension with the temperature is given by

r(V_M)^2/3 = 2.002(124.29 - T),

in which V_M is the molar volume M/D.

The temperature coefficient 2.002 is slightly below the mean value for normal liquids. The relation is rectilinear, as usual, up to a temperature a few degrees below the critical temperature (126° to 128° abs.).