I. The Adsorption of Thorium B and Thorium C from,Solution. - The

I. THE ADSORPTION OF THORIUM B AND THORIUM C FROM SOLUTION J. F. KING

AND

ALFRED ROMER

Thompson Chemical Laboratory, Williams College, Williamstown, Mass. Received January 20, 19% INTRODUCTION

Paneth (1) in his Baker Lectures has shown how, with the development of the knowledge of the chemistry of the socalled radioelements, there has come about an increasing use of these elements in studying some of the well-known chemical problems. Fajans (2), Paneth (3), and Hahn (4)l have made use of these radioelements in studies of the causes and mechanism of adsorption from solution. The phenomenon of adsorption is one of the most studied of chemical problems and the chemical literature is full of observations of the adsorption of many adsorbed substances by numerous adsorbents. A great deal of this work on adsorption has been carried out with difficultly defined adsorbents and with more or less complex adsorbed substances. Although these studies give valuable data because they record observations of common chemical systems as they actually exist, yet the number of variables involved makes the interpretation of the data in simple terms very difficult, The work of Fajans and others has reduced the study of adsorption to a simple system, that of a salt-like crystal as adsorbent and a single simple ion as the adsorbed substance. Because methods of analysis limit the accuracy of a quantitative study of the amount of ions removed by an adsorbent, these workers have made use of the electroscope for measuring quantitatively the number of radioactive ions which have been removed from solution. By use of this method it has been possible to measure on a rather small surface of a crystal adsorbent not only the amount of radioactive ions which have been removed from solution but also indirectly (using the radioactive ions as indicators) the amount of the ions of some of the common elements and of radicals which could not be detected in micro quantities by the ordinary methods of analysis. From the results of the experiments just referred to, Fajans, Paneth, and Hahn have formulated rules for adsorption from solution. (For a state1 In references 1to 4 are t o be found most of the references t o work which has been done in the field of the articles to be presented.

663 THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXVII, NO.

6

664

J. F. KING AND ALFRED ROMER

ment and discussion of these rules see reference 2.) In these rules there is shown a relation between adsorption and mixed crystal formation, adsorption and solubility (see also Taylor and Beekley ( 5 ) ) and more recently (by Fajans in his expanded rule) adsorption and weakness of dissociation of the compound formed by the adsorbed ion and the oppositely charged ion of the adsorbent. It is the purpose of this series of three papers to report work which was started under the direction of Professor Fajans2 in his laboratory in Munich and continued here in this laboratory, giving results which add additional information to the subject of the cause and mechanism of the adsorption of ions from solution. This first paper is to deal in general with the special technique used in all the work and to take up some of the sources of error in measurements in which the thorium B and thorium C equilibrium is concerned. The second paper will report results of the adsorption of thorium B ions on thallium bromide and iodide and the influence of different ions in the solution on this adsorption. The third paper will be concerned with the adsorption of thorium B ions on silver bromide from solutions of varying concentrations of different bromides. Thorium B, an isotope of lead, is a desirable ion for use in a study of the adsorption of ions on insoluble halide crystals because:1. Its half-life of 10.6 hours permits it to be used in an adsorption experiment and then easily determined by the electroscopic method. 2. It can be very easily obtained from the emanation from a sample of radiothorium. 3. Its halides are insoluble. For this work a 8-ray electroscope was used.3 Since the beta particles emitted from thorium B are so slow that 99 per cent of them can be absorbed by 0.4 mm. of aluminum ( 6 ) , they cannot conveniently be used for determining the amount of thorium B present. From the disintegration table (7)it can be seen that from the successive disintegration products of thorium B, there are two, thorium C and thorium C1,which emit alpha particles and two, thorium C and thorium Cz, which emit beta particles. When radioactive equilibrium has been established, the activity due to thorium Czis about 5 per cent of the total activity and its half-life is short enough to bring it to equilibrium quickly. The thorium C1comes into equilib2 The senior author of this article (and of two t o follow) takes this opportunity to express his gratitude to Professor Dr. K. Fajans, Director of the Institute for Physical Chemistry of the University of Munich, for his suggestions and his kind help and consideration in preparing the material for these papers, not only in his own private laboratory in Munich in 1928, but also while he was Baker Professor of Chemistry in Cornell University in 1930 and twice visited this laboratory. 3 This electroscope was of design similar to that described in Ostwald-Luther’e Physiko-Chemische Messungen, 4th ed., chap. 21. 1

I. ADSORPTION OF THORIUM B AND C FROM SOLUTION

665

rium instantaneously with thorium C and need not be considered as separate from it. By either alpha or beta particle measurements, then, the whole activity may be considered to be that of thorium C. From the relations given by Rutherford can be constructed (1) the activity curve for thorium B alone; (2) the activity curve for thorium B if thorium B and thorium C were in equilibrium with a constant source of thorium B and that source were removed; (3) the activity curve of thorium B if thorium B were pure a t zero time and the equilibrium between thorium B and thorium C were then reached. I t can be calculated, or roughly seen from the curves, that after eight hours the course of the three curves is the same t o better than 1per cent and that after ten hours to better than 0.2 per cent. Any measurement, whether of alpha or beta particle activity after these periods of time, will indicate the amount of thorium B present to the accuracy mentioned. The measurements in the present work were made on an equilibrium mixture of thorium B and thorium C. Since with thorium B we are dealing with a lead isotope and with thorium C with a bismuth isotope, consideration will have to be given to the chemistry of these common elements. The adsorption experiments were conducted on thorium B ions, but the disintegration product, thorium c, was used to determine the amount of thorium B present. In handling the radioactive solutions, therefore, attention had to be given to all conditions which disturbed an equilibrium mixture of thorium B and thorium C ions. This first paper is to consider factors involved in the preparation and handling of the radioactive solutions for the subsequent adsorption measurements. I. THE COLLECTION OF THORIUM B

The source of active material for the measurements, which extended over a period of three years, was a sample of radiothorium of about 3 milligram equivalence, kindly furnished us by the Welsbach Company; later to this was added another sample of 1.5 milligram equivalence procured from the University of Missouri. The emanation from this radiothorium was collected on platinum loops in the collector shown in figure 1. The scale gives the sizes of the different parts. This collector was connected through a water resistance to a 125- or a 300-volt battery. Assuming the radiothorium is in equilibrium with thorium X and assuming that all the thorium A from the decomposition of the thorium emanation is deposited on the loops, the time for thorium B on the loops to reach equilibrium can be calculated from the decay constants. Sixty-four hours is the time required t o deposit 98.4 per cent of the equilibrium amount.

666

J. F. KING AND ALFRED ROMER 11. THE PREPARATION O F RADIOACTIVE SOLUTIONS

The active deposit was dissolved off the platinum loops with nitric acid. A study was made of the rate of solution of this active deposit in 1N , 0.2 N and 0.1 N nitric acid. From this study it was found that standing in 0.2 N nitric acid at room temperature for from five to ten minutes was sufficient t o dissolve off most of the active deposit. Since the acidity of the radioactive solution had to be controlled, the amount of 0.2 N nitric acid used depended on the subsequent use which was made of the solution; this

P

B

II

&9 F

I

FIG.1. EMANATION COLLECTOR A, platinum loops; B, heavy brass cylinder; C, lead dish containing radiothorium preparation (the oxide or hydroxide); D, removable wood top; E, aluminum foil cut away around the binding post; F, brass base.

will be described later. This nitric acid solution was then transferred to a 100-cc. volumetric flask and made up to volume with distilled water. In order to be sure that the solution as prepared had the desired amount of activity, a 1-cc. sample was pipetted (after rinsing the pipette several times with the solution) on a watch glass and evaporated to dryness. The activity of this sample was measured by use of the beta ray electroscope and the remaining solution was then diluted with nitric acid of the same concentration t o give the desired activity.

667

I. ADSORPTION OF THORIUM B AND C FROM SOLUTION

The electroscope The beta ray electroscope used was purchased through Bender and Hobein of M ~ n i c h . ~ An important aid in the measurements was an eyepiece scale consisting of vertical lines bisected by a horizontal line. The gold leaf could then be timed as it passed an intersection. All samples measured were in the form of thin salt layers on 6-cm. watch glasses of slight concavity and of uniform thickness. A special watch holder and guide held the sample in the same position under the aluminum window so that reproducible results were possible. It was found that the aluminum window (which was 0.07 mm. in thickness) needed to be larger than the watch glasses used because of the effect of the scattering of the beta particles. The scale of the electroscope was calibrated against the constant activity of a radioactive preparation. The natural fall of the electroscope was measured each day and this correction was made on the observed fall. To eliminate the “soaking in effect” of the insulation of the gold leaf system (an amber bead), which gives the gold leaf an abnormal natural fall after it has stood in a discharged condition for some time, the electroscope was charged up about an hour before using. The possible error due t o this effect is illustrated by the following measurements: Time elapsed since the electroscope was charged

,

Natural fall of the gold leaf i n sea2e divisions per minute

10 minutes.. .................................................... 34 minutes.. .................................................... 1 hour, 3 minutes.. ............................................. 1 hour, 43 minutes.. ............................................

0.71 0.63 0.51 0.50

The activities of all the samples of a set of measurements were calculated according to the disintegration formula back to the time of measurement of the first sample measured, to.

The acidity of the solutions

It has been observed that the acidity of the solutions plays an important part in the reproducibility of results in adsorbing radioactive ions on various materials. By controlling the acidity, the thorium B solutions could be diluted to any dcsired activity. For instance, two solutions of different activity were prepared and then. the solution of higher activity was diluted with the calculated amount of acid to give the activity of the solution of lower activity. The measurements on both solutions are as follows:-

668

J. F. KING AXD ALFRED ROMER Scale divisions per minute

A. Samples of solution of lower activity

B. Samples from the solution of higher activity after it had been diluted with the calculated amount of acid

............ ............ 3.. . . . . . . . . . .

106 106 107

...........

111 108

In another experiment 10 cc. of a solution which had an activity of 68.6 scale divisions per minute (when evaporated on a watch glass) was divided into two parts, and then 5 cc. of the solution was diluted with 5 cc. of acid. This 10 cc. when evaporated t o dryness had an activity of 34.5 scale divisions per minute (calculated activity would be 34.3). It was noted a t the beginning of the measurements that there was an apparent adsorption of the radioactive material on the walls of the flasks and pipettes used in the measurements. It is known that glass is negatively charged against a neutral solution and that this negative charge decreases with increasing acidity of the solution. If the adsorption of the ions is due to the charge on the glass, this adsorption should decrease with jncreasing acidity of the solution. A series of glass bottles containing an equal number of glass beads were prepared. Into each was pipetted 10 cc. of a radioactive solution and varying amounts of nitric acid and water to bring the total volume up to 25 cc. These bottles were then shaken and 10 cc. of the equilibrium solution was then withdrawn and evaporated to dryness on a watch glass; the activities of the deposits on the watch glasses were immediately measured. These activities were compared with the activity of the original solution and the per cent adsorption was computed. The data follows. Normality of acid

Per cent adsorption

1... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.004 0.004

66.4 64.7

3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.044 0.044

61.8 61.6

5... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.084 0.084

60.6 61.2

7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.125 8.. . . . . . . . . . . . . . . . . . . . . . . . . . . . ,. . . . . . . . . . . . . . . . . . . .0.125

58.6 59.2

The same series of experiments was carried out except that the activity measurements were made after the watch glasses had stood overnight (Le., more than eight hours).

669

I. ADSORPTION OF THORIUM B AND C FROM SOLUTION

Aclizitv to to in scale divisions per minute

Original solution.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Original solution.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33.1 32.6

Normality oj acid

0.0016 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31.4

0.0097... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 0 0 9 7 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33.3 32.4

0.0179... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 0 1 7 9 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33.4 32.7

0 . 0 4 2 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 0 4 2 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32.4 33.4

0 . 0 8 2 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.082. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34.3 33.2

0.205... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.205. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34.4 34.5

0.408... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 4 0 8 . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34.8 34.7

0.61 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 6 1 . . ..............................................................

32.4 32.8

These two different series of experiments indicate that there is a selective adsorption of thorium C on the walls of the glass vessels and that the increase of acidity decreases this adsorption. In the case of any adsorption of thorium B, the increase of acidity in this experiment did not change the adsorption. The results of a third series of experiments are given in figure 2. In this series of measurements the activity of the watch glasses was followed over a period of tjme. Curve A represents the activity of a solution pipetted out 30 minutes after to and curve B represents the activity of the same solution from a sample pipetted out 33 hours after to. Curve C represents the activity of the same solution after it had been shaken for 70 minutes in a soft glass bottle with glass beads and pipetted out 3 t hours after to. The final observations are given in table 1. From this data it is- obvious that thorium B is not adsorbed by the glass. The normality of the acid used in this series of measurements was 0.0016. Comparing these curves with the theoretical ones (see page 665), it can be seen that there must have been a partial adsorption of thorium C on the walls of the pipettes and on the walls of the bottles. I

670

J. F. KING AND ALFRED ROMER

/o

9

N

I

I2

FIG.2. ADSORPTIONOF THORIUM C ON GLASS TABLE 1 BOLUTION A

I

I

1

TIME

ACTIVITY

hours

scale divisions per minute

3.02 3.37 3.58 4.33 4.63 4.88 5.17 5.38 5.75 5.98 6.28 8.17 8.47 9.55 9.95 10.32 10.65 11.10

306 306 303 292 288 283 278 276 273 273 263 231 231 214 210 207 202 196

*

BOLUTION B

'

SOLUTION C

TIME

ACTIVITY

TIME

ACTIVITY

hows

scale divisions per minute

hours

scale divisions

288 288 286 278 278 270 273 268 268 234 232 218 212 210 204 197

3.65 4.38 4.68 4.95 5.22 5,43 5.78 6,:03 6.35 8.20 8.52 9.62 10.02 10.35 10.72 11I 1 8

236 257 259 260 260 260 259 260 257 236 230 2 18 212 208 204 198

3.53 4.37 4.67 4.92 5.18 5.42 5.77 6.00 6.30 8.18 8.48 9.58 9.98 10.35 10.68 11.15

'

per minute

,

I. ADSORPTION OF THORIUM B AND C FROM SOLUTION

671

Another experiment was carried out to determine the selective adsorption of glass for thorium C. A sample of 10 cc. of an active solution was allowed to stand on a clean watch glass for 20 minutes. The glass was then rinsed with distilled water, drained, a.nd dried on a steam bath. The decay curve for the residue on the watch glass was then determined as accurately as the weakness of the activity of the residue would permit. The times and the log, of the activities are given in table 2. These data are plotted in figure 3. The solid line is the theoretical decay curve for thorium C. Thus it has been shown that in the handling of the radioactive solutions there is a selective adsorption of thorium C by the glass walls of the vessels used and that this adsorption is affected by the acidity of the solution. Herta Leng (8) has made a study of the adsorption of thorium B and thorium C on filter paper, dialyzing tubes, different kinds of glass, and paraffin. We plotted a number of her results showing the relation between TABLE 2 TIME

LOCe OF ACTIVITY

hours

.50 .70 .93 1.10 1.53 1.78 2.17 2.43

1.55 1.43 1.35 1.24 1.01 .85 .73 .53

hydrogen-ion concentration and the amounts of thorium B and thorium C adsorbed on different kinds of glass. The results from these curves show:I. For thorium C (isotope of bismuth): (1) there is a maximum adsorption a t a pH of from 5 to 7; (2) the adsorption drops off rapidly around a pH of from 2 to 3. 11. For thorium B (isotope of lead): (1) there is a maximum adsorption at a pH of about 9 to 10; (2) there is a sharp dropping-off of adsorption a t a pH of about 7 to 8. It has been suggested by a number of workers that thorium B, an isotope of lead, and thorium C, an isotope of bismuth, exist in solution both in the ionic and in the colloidal condition and that the equilibrium is determined by the hydrogen-ion concentratipn. It is easy to understand that the colloidal form of thorium C, bismuth oxysalt, could occur in a more acid solution than the colloidal form of thorium B, some basic salt of lead. Thus the effect of the acidity on the amount of adsorption would be caused

672

J. F. KING AND ALFRED ROMER

by a change in (1) the charge on the glass and its attraction for a trivalent and a bivalent ion, and (2) the nature of the radioactive material in solution. Hahn (9) calls attention t o the colloidal nature of lead, thorium B, and bismuth, thorium C, in slightly acid solutions. According t o the adsorption rule thallium (thorium C,) would be more strongly adsorbed on silver bromide than lead and lead more than bismuth. In slightly acid solutions

Erne in hours FIQ.3. DECAY CURVE

FOR

THORIUM c

this adsorption is just the reverse, Bi > Pb > T1. In neutral solution this difference is more pronounced. He explains this by the supposition that in slightly acid solution bismuth and lead do not exist as ions but as so-called “radiocolloids.” These radiocolloids may consist of particles of dust, decomposition particles of glass, etc., which have adsorbed the products of hydrolysis of bismuth and lead salts and behave as true colloidal particles of bismuth (thorium C) and lead (thorium B). Hahn suggests that thorium

I. ADSORPTION OF THORIUM B AND C FROM SOLUTION

673

B can be obtained in colloidal form or ionic form depending on the concentration of the acid used to dissolve it from the wire of the collector and also depending on the way that the solution is handled or diluted before adsorption is allowed to take place. He gives a series of experiments to show the possibility of the handling effect and suggests that this phenomenon warrants detailed study. The adsorption experiments take place in concentrations less than are usually considered necessary to exceed the solubility product of the salts concerned. The amount of adsorption would, in the case of these radiocolloids or pseudocolloids, depend on the number of ions in one of the particles of hydrolysis and the charge on the colloidal dust on glass decomposition particles. All of this would be a function of the hydrogen-ion concentration. The pseudocolloidal form of these two radioactive elements best explains the selective adsorption of thorjum C, for it is not easy to explain this selective adsorption as due to the attraction of the negatively charged glass walls for the trivalent thorium C ion in preference to the divalent thorium B ion. Rahn was able to remove on a glass container the greater portion of the activity due to thorium B and thorium C by evaporating an active solution to dryness and then treating the residue with water. In the following work on the adsorption of thorium B on the halides of silver and thallium, this effect was controlled by using sufficiently acid solutions to reduce this anomalous adsorption to a minimum. SUMMARY

1. The technique used in the preparation and handling of the radioactive solutions used in the measurements in the two papers that are to follow has been described. 2. A study has been made of the selective adsorption of thorium C from a solution containing thorium B and thorium C on the walls of the glass vessels used in the work and the effect of the acidity of the solutions on this adsorption. REFERENCES (1) PANETH,FRITZ:Radioelements as Indicators. McGraw-Hill Book Co., New York (1928). (2) FAJANS,K., AND E R D E Y - G R ~T.: z , Z. physik. Chem. 168,97 (1931). FRITZ:Z. physik. Chem. 89,513 (1915). (3) HOROWITZ, KARL,AND PANETH, (4) HAHNAND IMRE,L. : Z. physik. Chem. 144, 161 (1929). J. s.,AND TAYLOR, H. s.:J. Phys. Chem. 29,942 (1925). (5) BEEKLEY, (6) RUTHERFORD, E . : Radioactive Substances and their Radiations. Cambridge University Press, Cambridge (1913). (7) RUTHERFORD, CHADWICK, AND ELLIS : Radiations from Radioactive Substances. MacMillan, New York (1930). (8) LENG,HERTA:Sitzber. Akad. Wiss. Wien l l a , 136, 19 (1927). (9) HAHN,OTTO,AND IMRE,LUDWIG: Z. physik. Chem. A144, 161 (1929). Seep. 172.