Physical Properties of Krypton

Krypton is a colourless gas. It has not been stated definitely whether it has any odour or taste, and indeed it is possible that the experiment has never been tried. There is, however, no reason to suppose that it differs in this respect from the other members of the group.

The density of krypton was originally determined by Ramsay and Travers, but as they had at their disposal only 12 c.c. of gas, weighing about 0.045 gram, their result was, naturally, only approximate. The value of this constant was redetermined with great care by Moore, using fractions 8 and 9 of his gas obtained as described above. Fraction 9 was further refractionated ten times by Ramsay and Cameron, and the density of their final product determined. The value found in the mean was 41.506 (O = 16), i.e. the weight of a normal litre of krypton is 3.708 grams.

The compressibility of krypton has been measured by Ramsay and Travers at considerable pressures (over 20 metres of mercury): their results show that the value of pv decreases markedy with increase of pressure. From a critical discussion of the available data, it seems probable that at 0° and between 0 and 1 atmospheres the compressibility coefficient of krypton is +0.00215.

Assuming the accuracy of this figure, and calculating according to the method of limiting densities, or calculating by Guye's method of critical constants for an easily liquefiable gas, we find for the molecular weight of krypton the value 82.92.

The solubility of krypton in water has been determined with two samples of the gas. The mean result obtained for the absorption coefficient was 0.1249 at 0°, 0.0788 at 20°, and 0.0823 at 50°. If the' solubility be plotted against temperature, the curve shows a distinct minimum solubility about 30°-40° C.

The viscosity of krypton has been determined by Rankine. At 10.6° C. it has a value 1.361 times that of air. Its value at 0° in absolute (C.G.S.) units is 2.334. If the increase of viscosity with temperature follows a linear law of the type

η_θ = η₀(1+βθ),

then β×10⁵ = 308.

The refractivity of krypton was first determined by the discoverers, who found it to be 1.449 times that of air. Later determinations have given the value μ - 1 = 428.74×10^-6 at N.T.P. for the green mercury line (λ = 5461); and its dispersion at N.T.P. is given by



where C = 10.6893×10²⁷

and n₀² = 12767.9×10²⁷

The passage of an electrical discharge through a vacuum tube containing krypton causes the emission of light of a pale-violet colour. It is found that the spectrum of this light is profoundly influenced by the nature of the discharge. That obtained with the direct discharge has few lines, the chief of which are in the yellow7 and blue, with a group in the green; that seen when a jar and spark-gap are used shows a large number of lines in the blue. A list of the principal lines in the visible part of these spectra is given below: -

Accurate determinations of the wave-length of certain krypton lines, by comparison with the red cadmium line by an interference method, have been made.

First Krypton spectrum (Uncondensed discharge.)

Wave-Length.	Intensity	Wave-Length.	Intensity
5871.12	10	4454.12	10
5570.50*	10	4400.11	6
5562.45	6	4376.33	10
4671.40	10	4362.83	9
4624.48	10	4319.76	10
4502.56	9	4318.74	8
4501.13	7	4274.15	10
4463.88	10

Probably the green auroral line.

Second Krypton spectrum (Condensed discharge.)

Wave-Length.	Intensity	Wave-Length.	Intensity
5633.17	6	4109.38	6
4765.90	6	4098.89	7
4762.60	5	4088.48	8
4739.16	7	4067.53	5
4659.04	5	4065.22	8
4634.05	5	4057.17	8
4619.31	6	4050.62	5
4615.46	5	4044.80	5
4577.40	6	3998.10	6
4523.32	5	3994.98	6
4475.18	7	3954.90	5
4355.67	10	3920.29	8
4317.98	5	3917.76	6
4300.67	5	3912.69	5
4293.10	6	3906.37	8
4145.28	6

In this connection it is of interest to note that the spectrum of the aurora borealis contains a number of strong lines, all of which coincide exactly with prominent lines in the spectrum of krypton. In particular this is true of the line λ = 5570, which is known to persist in the spectrum of krypton at pressures as low as 1/23×10⁶ atmosphere. The pressure of the atmosphere is of this order of magnitude at a height of 80 miles, a height within the limits (50-125 miles) at which the aurora has been observed. It is hardly possible, then, to avoid the conclusion that the northern lights are due, in part at least, to the presence of krypton in the atmosphere. The measurement of the intensity of the lines of the krypton spectrum lias been used to estimate the proportion present in various spring gases. The Zeeman effect in krypton has been investigated by Lohmann.

Krypton is easily liquefied at temperatures above that of boiling liquid air: it boils at - 151°'7 C. The vapour-pressure ratio is 0.0467. The density of the liquid at its boiling-point is 2.155 grams per c.c., whence the molecular volume = 37.84.

The critical temperature of the liquid is -62.5° C., and its critical pressure is 54.3 atmospheres.

Krypton may be easily solidified by cooling in liquid air: it melts at -169°.

The specific heat of krypton has not been determined, and formerly the only data available as to the ratio of the specific heats were obtained with mixtures of krypton with other inert gases. This ratio has now been determined with a sample of pure krypton; the method used was that described under Helium, and the following are the data obtained with air and krypton examined successively in the same tube: -

	Temperature.	Half Wave-Length.	Density
Air	19°	27.92	14.48
Krypton	19°	18.13	41.46

Hence for krypton the ratio Cp/Cv= 1.689.