Fig. 78.
The results are shown graphically in [Fig. 78]. The maximum value of the activity is, for each time of exposure, taken as 100. The theoretical and observed values are shown in the Figure.
208. Analysis of the decay and recovery curves of Th X. The peculiarities of the initial portions of the decay and recovery curves of Th X and thorium respectively (Curves A and B, [Fig. 47], p. 221), will now be considered. It was shown that when the Th X was removed from the thorium by precipitation with ammonia, the radiation increased about 15 per cent. during the first day, passed through a maximum, and then fell off according to an exponential law, decreasing to half value in four days. At the same time the activity of the separated hydroxide decreased for the first day, passed through a minimum, and then slowly increased again, rising to its original value after the lapse of about one month.
When a thorium compound is in a state of radio-active equilibrium, the series of changes in which Th X, the emanation, and thorium A and B are produced, go on simultaneously. Since a state of equilibrium has been reached for each of these products, the amount of each product changing in unit time is equal to the amount of that product supplied from the preceding change in unit time. Now the matter Th X is soluble in ammonia, while thorium A and B are not. The Th X is thus removed from the thorium by precipitation with ammonia, but A and B are left behind with the thorium. Since the active deposit is produced from the emanation, which in turn arises from Th X, on the removal of the parent matter Th X, the radiation due to this active deposit will decay, since the rate of production of fresh matter no longer balances its own rate of change. Disregarding the initial irregularity in the decay curve of the active deposit, its activity will have decayed to half value in about 11 hours, and to one quarter value at the end of 22 hours. As soon, however, as the Th X has been separated, new Th X is produced in the thorium compound. The activity of this new Th X is not, however, sufficient to compensate at first for the loss of activity due to the change in the active deposit, so that, as a whole, the activity will at first decrease, then pass through a minimum, then increase again.
The correctness of this point of view has been tested by Rutherford and Soddy[[307]] as follows: If the precipitated thorium hydroxide after the removal of Th X is put through a series of precipitations with ammonia at short intervals, the Th X is removed almost as fast as it is formed, and, at the same time, the activity of thorium B in the thorium decays.
The following table indicates the results obtained. A portion of the precipitated hydroxide was removed after each series of precipitations and its activity tested in the usual way.
| Activity of hydroxide per cent. | |
|---|---|
| After 1 precipitation | 46 |
| After 3 precipitations at intervals of 24 hours | 39 |
| After 3 more precipitations at intervals of 24 hours and 3 at intervals of 8 hours | 22 |
| After 3 more each of 8 hours | 24 |
| After 6 more each of 4 hours | 25 |
Fig. 79.
The differences in the last three numbers are not significant, for it is difficult to make accurate comparisons of the activity of thorium compounds which have been precipitated under slightly different conditions. It is thus seen that as a result of successive precipitations, the activity is reduced to a minimum of about 25 per cent. The recovery curve of the activity of this 23 times precipitated hydroxide is shown in [Fig. 79]. The initial drop in the curve is quite absent, and the curve, starting from the minimum, is practically identical with the curve shown in [Fig. 48], which gives the recovery curve of thorium hydroxide after the first two days. This residual activity—about 25 per cent. of the maximum—is non-separable from the thorium by any chemical process that has been tried.