Chemical Properties of Helium

Chemists generally accept the view that helium and the mother members of the group are incapable of entering into chemical combination, but it may serve a purpose to collect in this place the evidence for and against this belief.

Firstly, it is of interest to know in what state helium exists in the minerals which contain it, and several investigators have, at different times, directed their efforts to the solution of this problem. At an early date it was shown that when fergusonite is heated to 500°-600° C. it suddenly becomes incandescent and evolves a considerable amount of gas, which consists, as the following analysis shows, chiefly of helium: -

99.99%

	Volume of Gas. c.c. per Grata.	Percentage of Total Gas.
Helium	1.080	75.5%
Hydrogen	0.078	5.47%
Carbon dioxide	0.245	17.14%
Nitrogen	0.027	1.88%
Total	1.430

The weight of helium evolved is 0.0326 per cent, of the weight of the mineral.

Fergusonite is a niobate of yttrium containing some uranium; it occurs in felspar and mica deposits, but it is doubtful whether it is of igneous origin or is deposited by water. It is macro-crystalline, but under the microscope shows no trace of crystalline structure, and appears to be homogeneous and free from cavities. By heating the mineral with a hydrogen flame burning in oxygen in a specially constructed calorimeter, it has been shown that the heat evolved during the change is 809 cal. per gram; moreover, the evolution of helium is accompanied by an increase of volume (a decrease might, conceivably, account for part of the heating). A similar liberation of helium with evolution of light and heat has been noticed in the case of a sample of fluor-spar containing fluorides of cerium and yttrium.

Travers has pointed out, however, that many minerals which contain no helium exhibit a similar incandescence on heating, and this fact rather discounts the significance of the foregoing observations.

Another fact which might be considered to point to the existence of compounds of helium in minerals is that the gas is completely eliminated from samarskite by heating in carbon dioxide, but only partially by heating in hydrogen. A similar phenomenon has been observed in the liberation of nitrogen from certain nitrides, and Kohlschutter suggests that this may be due to the reduction by the hydrogen of the higher oxides which otherwise provide the oxygen required to expel the helium or nitrogen from its compounds.

More definite conclusions may be deduced from experiments in which pitch blende was ground in vacuo Helium was thifs liberated in amounts which were approximately a constant proportion (1.1 to 1.2 per cent.) of the total amounts that could be obtained by heating the mineral with potassium hydrogen sulphate.

A slightly different method of attacking this problem was followed by Gray, who powdered thorianite in an agate mortar, sorted the resulting powder into various grades of fineness by elutriation with water, dried the resulting fractions, and determined the amount of helium which was liberated from each on heating with nitric acid in an exhausted tube. By comparing these amounts with the total amount of helium evolved when the unground material was subjected to similar treatment with acid, it was possible to ascertain the proportion of the total helium which had been liberated by grinding to a degree of fineness which was determined in each case by microscopic examination of the powder. It appears that very little gas is liberated until the particles have a diameter as small as 10 μ; from this point the proportion of helium liberated increases with the fineness of the powder until a temporary limit is reached with particles of an average diameter of 3 μ, from which about 28 per cent, of the total helium has been given off.

Both these sets of experiments lead to the conclusion that the helium is contained in a structure (e.g. of cavities) which, though invisible under the microscope, is large compared with the molecular structure.

A similar conclusion is reached as the result of experiments upon the liberation of helium from monazite and thorianite by prolonged heating at various temperatures from 500°-1200°. With the latter mineral there was a practical limit to the evolution of helium at any given temperature. The following are typical results: -

0.7 per cent, of helium content liberated in 5 hours at 300°
8.5 per cent, of helium content liberated in 80 hours at 500°
62.3 per cent, of helium content liberated in 320 hours at 750°
100.0 per cent, of helium content liberated in 30 hours at 1000°

It seems probable, therefore, that in thorianite, while a small proportion of the helium may be diffused throughout the mineral - possibly in solid solution - the greater part of it is concentrated in minute cavities. If this is the case, the pressure of the gas in the cavities is certainly very great: something of the order of 200 atmospheres at 0° C.

It is stated that considerable amounts of helium are absorbed by the finely divided platinum produced by the so-called "electrical vaporisation" of the platinum electrodes of a vacuum tube, and also by magnesium electrodes in vacuum tubes, but it seems probable that this absorption, in so far as it actually exists, is a physical phenomenon, and, as the extent of the absorption was measured chiefly by the changes in the spectrum - a method which has been shown to be unreliable in the case of the inert gases - too much importance must not be attached to these results.

The most exhaustive series of attempts to bring about the combination of helium with various elements and compounds was made by Ramsay and Collie. A measured amount of helium was circulated over the substance at a bright-red heat; then the whole apparatus was allowed to cool, the residual helium pumped off and measured, and finally the reagent was reheated, and any gas evolved pumped off and examined.

No change in the volume of the helium was observed in any of the experiments, and the reagent in each case appeared to be unattacked.

From experiments that have been made during the isolation of helium it is certain that it does not combine with oxygen, nitrogen, hydrogen, etc., and it has been proved that even at high temperatures it does not pass through iron or platinum and cannot, therefore, form any compound or solid solution with these elements. Further, helium has no measurable solubility in solid or liquid copper, silver, gold, nickel, iron, palladium, aluminium, magnesium, uranium, or tantalum.

One possibility that remains is that an endothermic compound of helium might be obtained by the action of the silent electric discharge. Berthelot stated that when helium was thus treated in presence of benzene and mercury, a green glow was seen, which gave the spectrum of mercury, and a large proportion of the gas was absorbed (68 per cent, in 210 hours) with production of a resinous solid. This solid, when heated, gave off a gas which, when purified from carbon monoxide, etc., showed exactly the same behaviour as the original gas when submitted to the discharge with benzene and mercury. Moreover, the residual gas from the first experiment could be made to undergo further contraction by the action of the discharge in presence of fresh benzene and mercury.

These results are so startling that one is naturally disinclined to accept them without confirmation, and this, hitherto, has not been forthcoming. On the other hand, Ramsay and Collie1 submitted mixtures of helium with benzene vapour and with chlorine to the action of the silent electrical discharge for many hours, but could in no case detect any alteration in the volume of the helium or any other sign of chemical combination. Strutt also has found that helium is not absorbed when subjected to the silent discharge with benzene or carbon bisulphide.

There appears, therefore, to be good ground for the statement that no exothermic compound of helium exists, and that neither high temperatures nor the silent discharge are capable of causing the formation of endothermic compounds of the element.