Measurements of Emanating Power.
149. Emanating power. The compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions. It is convenient to use the term emanating power to express the amount of emanation given off per second by one gram of the compound. Since, however, we have no means of determining absolutely the amount of emanation present, all measurements of emanating power are of necessity comparative. In most cases, it is convenient to take a given weight of a thorium compound, kept under conditions as nearly as possible constant, and to compare the amount of emanation of the compound to be examined with this standard.
In this way comparisons of the emanating power of thorium compounds have been made by Rutherford and Soddy[[244]], using an apparatus similar to that shown in [Fig. 51] on page [240].
A known weight of the substance to be tested was spread on a shallow dish, placed in the glass tube C. A stream of dry dust-free air, kept constant during all the experiments, was passed over the compound and carried the emanation into the testing vessel. After ten minutes interval, the current due to the emanation in the testing vessel reached a constant value. The compound was then removed, and the standard comparison sample of equal weight substituted; the saturation current was observed when a steady state was again reached. The ratio of these two currents gives the ratio of the emanating power of the two samples.
It was found experimentally that, for the velocities of air current employed, the saturation current in the testing vessel was directly proportional to the weight of thorium, for weights up to 20 grams. This is explained by the supposition that the emanation is removed by the current of air from the mass of the compound, as fast as it is formed.
Let i1 = saturation current due to a weight ω1 of the standard,
i2 = „ „ „ „ ω2 of the sample to be tested.
(emanating power of specimen) i2 ω1
Then ------------------------------- = --- ---
(emanating power of standard) i1 ω2
By means of this relation the emanating power of compounds which are not of equal weight can be compared.
It was found that thorium compounds varied enormously in emanating power, although the percentage proportion of thorium present in the compound was not very different. For example, the emanating power of thorium hydroxide was generally 3 to 4 times greater than that of ordinary thoria, obtained from the manufacturer. Thorium nitrate, in the solid state, had only ¹⁄₂₀₀ of the emanating power of ordinary thoria, while preparations of the carbonate were found to vary widely among themselves in emanating power, which depended upon slight variations in the method of preparation.
150. Effect of conditions on emanating power. The emanating power of different compounds of thorium and radium is much affected by the alteration of chemical and physical conditions. In this respect the emanating power, which is a measure of the rate of escape of the emanation into the surrounding gas, must not be confused with the rate of decay of the activity of the emanations themselves, which has already been shown to be unaffected by external conditions.
Dorn (loc. cit.) first observed that the emanating power of thorium and radium compounds was much affected by moisture. In a fuller investigation of this point by Rutherford and Soddy, it was found that the emanating power of thoria is from two to three times greater in a moist than in a dry gas. Continued desiccation of the thoria in a glass tube, containing phosphorus pentoxide, did not reduce the emanating power much below that observed in ordinary dry air. In the same way radium chloride in the solid state gives off very little emanation when in a dry gas, but the amount is much increased in a moist gas.
The rate of escape of emanation is much increased by solution of the compound. For example, thorium nitrate, which has an emanating power of only ¹⁄₂₀₀ that of thoria in the solid state, has in solution an emanating power of 3 to 4 times that of thoria. P. Curie and Debierne observed that the emanating power of radium was also much increased by solution.
Temperature has a very marked effect on the emanating power. The writer[[245]] showed that the emanating power of ordinary thoria was increased three to four times by heating the substance to a dull red heat in a platinum tube. If the temperature was kept constant the emanation continued to escape at the increased rate, but returned to its original value on cooling. If, however, the compound was heated to a white heat, the emanating power was greatly reduced, and it returned on cooling to about 10% of the original value. Such a compound is said to be de-emanated. The emanating power of radium compounds varies in a still more striking manner with rise of temperature. The rate of escape of the emanation is momentarily increased even 10,000 times by heating to a dull red heat. This effect does not continue, for the large escape of the emanation by heating is in reality due to the release of the emanation stored up in the radium compound. Like thoria, when the compound has once been heated to a very high temperature, it loses its emanating power and does not regain it. It regains its power of emanating, however, after solution and re-separation.
A further examination of the effect of temperature was made by Rutherford and Soddy[[246]]. The emanating power of thoria decreases very rapidly with lowering of temperature, and at the temperature of solid carbonic acid it is only about 10% of its ordinary value. It rapidly returns to its original value when the cooling agent is removed.
Increase of temperature from 80° C. to a dull red heat of platinum thus increases the emanating power about 40 times, and the effects can be repeated again and again, with the same compound, provided the temperature is not raised to the temperature at which de-emanation begins. De-emanation sets in above a red heat, and the emanating power is then permanently diminished, but even long-continued heating at a white heat never entirely destroys the emanating power.
151. Regeneration of emanating power. An interesting question arises whether the de-emanation of thorium and radium is due to a removal or alteration of the substance which produces the emanation, or whether intense ignition merely changes the rate of escape of the emanation from the solid into the surrounding atmosphere.
It is evident that the physical properties of the thoria are much altered by intense ignition. The compound changes in colour from white to pink; it becomes denser and also far less readily soluble in acids. In order to test if the emanating power could be regenerated by a cyclic chemical process, the de-emanated thoria was dissolved, precipitated as hydroxide and again converted into oxide. At the same time a specimen of the ordinary oxide was subjected to an exactly parallel process. The emanating power of both these compounds was the same, and was from two to three times greater than that of ordinary thoria.
Thus de-emanation does not permanently destroy the power of thorium of giving out an emanation, but merely produces an alteration of the amount of the emanation which escapes from the compound.
152. Rate of production of the emanation. The emanating power of thorium compounds, then, is a very variable quantity, much affected by moisture, heat, and solution. Speaking generally, increased temperatures and solution greatly increase the emanating power of both thorium and radium.
The wide differences between the emanating powers of these substances in the solid state and in solution pointed to the conclusion that the differences were probably due to the rate of escape of the emanation into the surrounding gas, and not to a variation of the rate of reaction which gave rise to the emanation. It is obvious that a very slight retardation in the rate of escape of the thorium emanation from the compound into the gas, will, on account of the rapid decay of activity of the emanation, produce great changes in emanating power. The regeneration of the emanating power of de-emanated thoria and radium by solution and chemical treatment made it evident that the original power of thorium and radium of producing the emanation still persisted in an unaltered degree.
The question whether the emanation was produced at the same rate in emanating as in non-emanating compounds can be put to a sharp quantitative test. If the rate of production of emanation goes on at the same rate in the solid compound where very little escapes, as in the solution where probably all escapes, the emanation must be occluded in the compound, and consequently there must be a sudden release of this emanation on solution of the compound. On account of the very slow decay of the activity of the emanation of radium, the effects should be far more marked in that compound than in thorium.
From the point of view developed in [section 133], the exponential law of decay of the emanation expresses the result that Nt the number of particles remaining unchanged at the time t is given by
where N₀ is the initial number of particles present. When a steady state is reached, the rate of production q₀ of fresh emanation particles is exactly balanced by the rate of change of the particles N₀ already present, i.e.
q₀ = λN₀,
N₀ in this case represents the amount of emanation “occluded” in the compound. Substituting the value of λ found for the radium emanation in [section 145],
N₀
---- = 1/λ = 463,000.
q₀
The amount of emanation stored in a non-emanating radium compound should therefore be nearly 500,000 times the amount produced per second by the compound. This result was tested in the following way[[247]].
A weight of ·03 gr. of radium chloride of activity 1000 times that of uranium was placed in a Drechsel bottle and a sufficient amount of water drawn in to dissolve it. The released emanation was swept out by a current of air into a small gas holder and then into a testing cylinder. The initial saturation current was proportional to N₀. A rapid current of air was then passed through the radium solution for some time in order to remove any slight amount of emanation which had not been removed initially. The Drechsel bottle was closed air-tight, and allowed to stand undisturbed for a definite time t. The accumulated emanation was then swept out as before into the testing vessel. The new ionization current represents the value of Nt the amount of emanation formed in the compound during the interval t.
In the experiment t = 105 minutes,
and the observed value
Nt
---- = ·0131.
N₀
Assuming that there is no decay during the interval,
Nt = 105 × 60 × q₀.
N₀
Thus -------- = 480,000.
q₀
Making the small correction for the decay of activity during the interval,
N₀
---- = 477,000.
q₀
We have previously shown that from the theory
N₀ 1
----- = --- = 463,000.
q₀ λ
The agreement between theory and experiment is thus as close as could be expected from the nature of the experiments. This experiment proves conclusively that the rate of production of emanation in the solid compound is the same as in the solution. In the former case it is occluded, in the latter it escapes as fast as it is produced.
It is remarkable how little emanation, compared with the amount stored up in the compound, escapes from solid radium chloride in a dry atmosphere. One experiment showed that the emanating power in the dry solid state was less than ½% of the emanating power of the solution. Since nearly 500,000 times as much emanation is stored up in the solid compound as is produced per second, this result showed that the amount of emanation which escaped per second was less than 10-8 of that occluded in the compound.
If a solid radium chloride compound is kept in a moist atmosphere, the emanating power becomes comparable with the amount produced per second in the solution. In such a case, since the rate of escape is continuous, the amount occluded will be much less than the amount for the non-emanating material.
The phenomenon of occlusion of the radium emanation is probably not connected in any way with its radio-activity, although this property has here served to measure it. The occlusion of helium by minerals presents almost a complete analogy to the occlusion of the radium emanation. Part of the helium is given off by fergusonite, for example, when it is heated and all of it when the mineral is dissolved.
153. Similar results hold for thorium, but, on account of the rapid loss of activity of the emanation, the amount of emanation occluded in a non-emanating compound is very small compared with that observed for radium. If the production of the thorium emanation proceeds at the same rate under all conditions, the solution of a solid non-emanating compound should be accompanied by a rush of emanation greater than that subsequently produced. With the same notation as before we have for the thorium emanation,
N₀ 1
----- = --- = 87.
q₀ λ
This result was tested as follows: a quantity of finely powdered thorium nitrate, of emanating power ¹⁄₂₀₀ of ordinary thoria, was dropped into a Drechsel bottle containing hot water and the emanation rapidly swept out into the testing vessel by a current of air. The ionization current rose quickly to a maximum, but soon fell again to a steady value; showing that the amount of emanation released when the nitrate dissolves, is greater than the subsequent amount produced from the solution.
The rapid loss of the activity of the thorium emanation makes a quantitative comparison like that for radium very difficult. By slightly altering the conditions of the experiment, however, a definite proof was obtained that the rate of production of emanation is the same in the solid compound as in the solution. After dropping in the nitrate, a rapid air stream was blown through the solution for 25 seconds into the testing vessel. The air stream was stopped and the ionization current immediately measured. The solution was then allowed to stand undisturbed for 10 minutes. In that time the accumulation of the emanation again attained a practical maximum and again represented a steady state. The stream of air was blown through, as before, for 25 seconds, stopped and the current again measured. In both cases, the electrometer recorded a movement of 14·6 divisions per second. By blowing the same stream of air continuously through the solution the final current corresponded to 7·9 divisions per second or about one-half of that observed after the first rush.
Thus the rate of production of emanation is the same in the solid nitrate as in the solution, although the emanating power, i.e. the rate of escape of the emanation, is over 600 times greater in the solution than in the solid.
It seems probable that the rate of production of emanation by thorium, like the rate of production of Ur X and Th X, is independent of conditions. The changes of emanating power of the various compounds by moisture, heat, and solution must therefore be ascribed solely to an alteration in the rate of escape of the emanation into the surrounding gas and not to an alteration in the rate of its production in the compound.
On this view, it is easy to see that slight changes in the mode of preparation of a thorium compound may produce large changes in emanating power. Such effects have been often observed, and must be ascribed to slight physical changes in the precipitate. The fact that the rate of production of the emanation is independent of the physical or chemical conditions of the thorium, in which it is produced, is thus in harmony with what had previously been observed for the radio-active products Ur X and Th X.