Physical Properties of Argon

Argon, like the other members of the group, is a colourless, odourless, tasteless gas.

Many determinations of its density have been made; the more important results are tabulated below (referred to O₂ = 32): -

(a) 19.940, (b) 19.957, (c) 19.945, (d) 19.95.

(a) This result is the mean of a number of density determinations made during the earlier experiments on argon, and refers to "atmospheric argon" containing all the helium, neon, etc., present in the original air, and is probably accurate for that mixture. It is noticeable that it differs little from the accepted figure for pure argon. The second figure (b) is for the middle fractions obtained in the fractional evaporation of liquefied atmospheric argon, and the third (c) is a more recent determination made on material similarly prepared. The last number (d) is of special interest in that it was obtained with argon purified by fractional crystallisation. Identical results were obtained with the samples of gas obtained from the solid and liquid fractions.

The accepted figure for the density of argon is 19.95, i.e. the weight of a normal litre of argon is 1.782 gm. Calculating according to Guye's method of critical constants, we find the molecular weight = 39.90.

Argon, like helium, expands normally with increase of temperature from 0° to 280°: its coefficient of expansion is 0.003668.

The pv isothermals for the gas between -150° and + 20° and the reduced equation of state have been studied by Onnes and Crommelin. The compressibility coefficient of the gas at 0° C. and between 0 and 1 atmosphere has been calculated by Watson to be +0.00093.

The solubility of argon in water is about 2½ times that of nitrogen - roughly 4 vols, per 100 vols, of water at ordinary temperatures.

The absorption coefficient, measured according to the method of Estreicher, is 0.0561 at 0° and 0.02567 at 50°, and appears to show a steady fall with rise of temperature. Fox has shown, however, that these results may be in error by as much as 5 per cent., and greater weight must therefore be attached to the determinations of Antropoff, which give for the solubility coefficient the values 0.0561 at 0°, 0.0379 at 20°, and 0.0343 at 50°: the results show a distinct minimum at 40°.

Argon does not conform strictly to Graham's Law, but diffuses through a minute hole in a platinum plate 3½ per cent, faster than would be anticipated when compared with oxygen. This behaviour may be explained by taking into account the high ratio of the specific heats. Argon diffuses through a caoutchouc membrane 100 times as fast as carbon dioxide.

The viscosity of argon is high - about 1.21 times that of air - and in respect of this property it heads the list of the principal gases. The coefficient of viscosity η in absolute (C.G.S.) units has been determined by different investigators, with the results given below: -

η×107	Temperature	Observer
2203	14.7°	Schultze
2200	12.7°	Tanzler
2746	99.6°
3231	183.0°
2201	15.5°	Rankine
2102	0.0°	Rankine

Rayleigh observed that the viscosity of argon increased with rise of temperature more rapidly than did that of the common diatomic gases, and Rankine showed that if this increase followed a linear law of the type

η_θ = η₀(1+βθ)

then β, the temperature coefficient of increase of viscosity of the gas, has the value 283×10^-5.

The refractivity of argon has been determined by methods essentially similar to those described under Helium: the principal results are tabulated below: -

Wave-Length (λ)	(μ-1)×107
White light	2808
6563	2829
5896	2837
6439	2796
5790-70	2803
5461	2816
4359	2851
5461	2823

Burton expressed his results by the formula

μ = 1.0002792+1.6×10^-14/λ²

but C. and M. Cuthbertson give preference to a formula of the Sellmeier type,

μ – 1 = C/(n₀² - n²)

At N.T.P. their values for the constants in this equation are given in the following table, together with those calculated from other results, for the sake of comparison: -

Author	C×10-27	n02×10-27
Burton	9.124	16335
Ahrberg	7.437	13516
C. and M. Cuthbertson	9.43264	17009

Argon has a very low dielectric cohesion (38) - one-fifth that of air (205), and one-eleventh that of hydrogen (419). In this respect it shows a resemblance to the monatomic vapour of mercury, the dielectric cohesion of which is 0.85 that of air - a very low figure considering the density of the gas. The dielectric cohesion of argon is unaffected by change of pressure, but is markedly increased by admixture with "other gases, whether diatomic gases like oxygen and nitrogen or the vapour of mercury.

The spark-gap (sparking distance) is about 40 per cent, greater in argon than in air, hydrogen, etc., under comparable conditions. The lines seen in the spectrum of argon vary with the conditions. If a discharge at 2000 volts be passed from a storage, battery (or other similar source of continuous current) through the gas under reduced pressure, it emits a red glow; but when the discharge is oscillatory (e.g. with a spark- gap and condenser interposed) the colour of the light at once changes to blue. Proximity to a Herz oscillator will bring about a similar change in the character of the light, and a Geissler tube containing rarefied argon may, consequently, be used to detect electrical waves. Stead has stated, as the result of investigations with a lime cathode, that the red spectrum is the spectrum of the positive column - even when produced under very low pressures - while the blue spectrum is that of the cathode beam.

A very large number of lines in the spectrum have been measured, but only the more important of these are given in the following list. The reader desirous of further detail is referred to the original memoirs.

"Red" spectrum of Argon (Uncondensed discharge)

Wave-Length	Intensity	Wave-Length	Intensity	Wave-Length	Intensity
7066.6	7	4272.30	8	4182.00	7
6964.8	8	4266.43	8	4164.30	7
5607.44	8	4259.49	9	4158.72	10
4510.85	7	4200.80	10	4044.56	8
4348.11	8	4198.16	10	3949.11	8
4345.32	7	4191.84	10	3834.77	8
4333.71	8	4190.84	7	3567.79	7
4300.25	8

"Blue" spectrum of Argon (Condensed discharge)

Wave-Length	Intensity	Wave-Length	Intensity	Wave-Length	Intensity
5559.02	8	3781.02	7	3285.91	7
4880.00	8	3729.45	9	2942.94	7
4806.17	8	3638.02	7	2806.3	8
4609.74	7	3588.63	9	2769.7	8
4426.17	8	3582.55	7	2753.9	8
4348.22	9	3576.81	8	2744.9	8
4104.11	7	3561.21	7	2708.4	8
4014.00	7	3559.70	8	2647.6	8
3928.75	7	3546.01	7	2516.8	8
3868.72	7	3545.79	7	2515.6	8
3850.72	8	3491.72	9

The Doppler effect has been observed for certain lines in the spectrum of argon.

Argon is diamagnetic.

The thermal conductivity of argon, K = 0.00003894 at 0° C. When this value is substituted in the equation

K = f.η.c_v,

where η - the viscosity and c_v the specific heat of the gas at constant volume, we find (as in the case of helium, q.v.) that

f = 2.501.

This value approximates closely to that obtained theoretically by the development of Maxwell's theory, and therefore affords evidence for the simple nature of the argon molecule.

Determinations of the thermal conductivity of argon at very low pressures gave peculiar results which are at present unexplained.

Direct determination of the specific heat of argon at very high temperatures (1300°-2500° C.) have been made by exploding the gas with a known amount of electrolytic gas in a large spherical bomb provided with a special device for detecting and recording pressure variations. It was thus found that at constant volume the molecular specific heat is 2.977 cals.

The specific heats at constant pressure and constant volume at ordinary temperatures have not been determined, but the ratio between these quantities has been found by the method of Kundt's tube (vide Helium, p. 316). A particular tube, which gave in air the value λ/2 = 34.67 mm., gave in argon λ/2 = 31.68 mm.; whence γ = 1.65.