ADSORPTION OF THORIUM X BY FERRIC HYDROXIDE A T DIFFERENT pH BY I-’.
KURBATOW
The Properties and the Methods of obtaining highly Emanating Preparations Hydroxides of different elements containing radium or thorium X uniformly distributed throughout their entire mass are able to give off quantitatively emanation which is formed in the process of radioactive disintegration. Such compounds of air-dried hydroxides with radium received the name of highly emanating preparations in the science of radioactivity. The methods of their preparation and 1 heir properties have been systematically studied by 0. Hahn and his co-workers in recent years. Their research resulted in important changes and in the simplification of the technical methods of obtaining emanation. By the use of these preparations it is now possible to obtain emanation, as for instance for medicalpurposes, in exactly measured quantities. At the same time a nearly quantitative utilization of the produced emanation is realized. The whole technical equipment has been very much simplified and to obtain emanation it is now necessary only to attach the gold emanation needle to the apparatus. In spite of the practical importance of these compounds the method of their preparation remained purely empirical. The only theory which received general attention was proposed by 0. Hahn’ some five years ago. The procedure followed in preparing highly emanating compounds consisted according to Hahn in pouring a barium-radium chloride solution containing ferric ions into a large excess of ammonia and ammonium carbonate or sulfate. The purpose of ammonium carbonate or sulfate was to obtain insoluble radium salts which are adsorbed on ferric hydroxide. The investigation of Hahn and Heidenheim led the authors to the following conclusion: “Insoluble salts of radium are quantitatively precipitated from the solution, even if their solubility product has not been exceeded, if the precipitation is carried out in presence of a large excess of ferric hydroxide.”? Erbacher and Kadig have given another method of separating the highly emanating substances. They first precipitate ferric hydroxide and add then radium salt solution. The whole is poured into an excess of ammonium carbonate. The authors noticed that in alkaline solutions radium was quanti0. Hahn and Heidenheim: Ber., 59, 284 (1926). According to this point of view radium is precipitated as carbonate if ammonium carbonate is present, etc. Ferric hydroxide is considered to be only a carrier of the radium containing substance. See: Hahn and Muller: Z. Elektrochemie, 27, 189 (192). Hahn: Ann., 440, I Z I (1924);Hahn: Saturwissens-haften, 12, 1140 (1924);Hahn, gfbacher, Feichtinger: Ber., 59, 2014(1926);HahnandZ. Biltz: Z.physik. Chem., 126, 323; Blltz: 356 (1927);Hahn: Ann., 462, 174 (1928);Hahn: Naturwissenscheften, 15, 295 (1930). 2
IW. KURBATOW
I242
tatively removed from the solution even before addition of ammonium carbonate. Nevertheless, they think that it is necessary to add some anions forming insoluble radium salts to prevent a subsequent desorption of radium from the ferric hydroxide. Another theory can be proposed which is consistent with the preparation method, and is based on the colloidal properties of the precipitates. The hydroxides contain, as it is well known, varying quantities of anions (SOa--) in bound condition, depending on the pH of the solution. We may assume then that the adsorption of radium depends on the presence of these anions, The hydroxide resembles then a labile compound of changing composition corresponding to the pH of the solution in which it is present. According to this view it is natural to expect that radium would be uniformly distributed throughout the precipitate an! would not be present in the form of submicroscopic crystals adhering to the hydroxide. This theory has received now a satisfactory experimental confirmation as will be seen from the following:
The Experimental Part For the following experiments not radium but its isotope thorium X was chosen because a rapid determination of the radio-active content of the solution and of the precipitate by the emanation method is then possible. It seemed to be desirable to establish first the fact whether the ions SO4-and HC03- have any influence on the amount of radium precipitated on ferric hydroxide. A series of experiments with this purpose in view has been made and the results will be clear from the following examples. Two solutions were prepared containing each: Fe+++: 0,0797 gr., T h X : 2.84 X IO-^ gr.$ and C1- and NOS- ions in 500 cc. The second of these solutions contained some SO,-- ions in addition. Ferric hydroxide was precipitated from both solutions by adding ammonia. After a period of four hours samples of solution were withdrawn. No T h X could be detected showing that the whole of it was adsorbed on the precipitate. To the remainder of the solutions hydrochloric acid was added until ferric hydroxide became flocky while remaining still in the precipitate. After another four hours samples of the clear solution were again withdrawn and T h X determined. The following Table I shows the results:
TABLE I Effect of SO4-- on the Adsorption of T h X Contents before the experiment in one cc. of solution I. solution 5.68 X IO-^ T h X
T h X (Th-units) after the experiment in one cc. of solution 2.92
X
IO+
gr.
solution (containing Sod--) 5.68 X IO-^ T h X 4.80 X IO-^ gr.
x
T h in % of tota1 quantity (2.84 X IO-3 gr.) in solution in precipitate
51.4
48.6
2.
84.5 15.5 T h X is given here and in the following in T h units which give grams of thorium in equilibrium with the present quantity of Th X. All experiments in the following were performed using portions of the same standard solution containing Th, MsthI, Rdth and T h X in radioactive equilibrium.
ADSORPTION OF THORIUM X
1243
It will be seen that the presence of S04--does not increase the amount of T h X precipitated with ferric hydroxide. The following experiment shows whether the presence of S04--has any influence on the free exchange of T h X bet,ween the precipitate and the solution or not. A solution containing Fe+++ : 0.0793 gr.; T h X : 2.84 X IO-* gr. C1-, NO3and 0.1normal SOa-- ions in 500 cc. was made. Strong ammonia was added and 50 cc. of the solution analyzed for T h X. Found 1.2 X IO& gr. T h X per cc. or 21y0of the total T h X in solution. The remaining solution was then neutralized with HCl and T h X again determined. Found: 5.15 X IO-^ gr. T h X per cc. or 9 2 . 7 % of it in solution. Strong ammonia was added to the rest again and a determination of T h X made. No T h X could be found in solution. It is seen thus that the ion SO,-- does not prevent free exchange of T h X between the precipitate and the solution. In the same manner it has been established that the HCO3- ion does not prevent this exchange either. A solution containing Fe+++ 0.0793 gr. T h X 2.84 x 10-3gr., C1-, NO3- in 500 cc. was made. I t was boiled and the precipitation carried out with a large excess of ammonia. All reagents were carefully purified from traces of CO? and all work was carried out in a closed space free of Cot. No T h X could be detected in 50 cc. of solution, showing that it was quantitatively precipitated with ferric hydroxide. A solution identical with the preceding one was now made and precipitation made without adding an excess of ammonia. Found: 4.9 X IO+ gr. T h X per cc. or 86.3yo of it in solution. Another such solution was precipitated with a small amount of dilute ammonia to which 10% ammonium carbonate was added. Found 4.7 X IO--^ gr. per cc. or 82.8% T h X in solution. A large excess of ammonia was later added to this solution. N o T h X could be detected in solution. The preceding experiments show clearly that adsorption of T h X-and therefore of Ra-is determined by the alkalinit,y of the solution above. In the following experiments the dependence of T h X adsorption upon the pH of solutions was quantitatively studied. The experiments were carried out in the following manner. To a solution containing known quantities of ferric ion and Th X some ammonia was added. A sample of the solution was withdrawn and T h X and pH determined. More ammonia was added to the remainder and a sample again withdrawn. In such a manner from 3 to 6 determinations of T h X at varying pH could be made with one solution. The determination of T h X in sample solutions was made as follows. The sample was poured into a special glass vessel, acidulated with HC1 and made up to a constant volume. Air was blown through the solution and then through the ionization chamber a t constant rate. The ionization was measured with a unifilar electrometer. The apparatus was calibrated before and after each determination by determining the ionization produced by a standard solution of thorium in equilibrium with its disintegration products. A sample determination: I. Natural ionization before the experiment: 2.36 scale divisions per minute or 0.036 volts per minute (Electrometer sensitivity 65.0 divisions per volt).
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IW. KURBATOW
2. Natural ionization after the experiment 2.19 scale divisions per minute or 0.034 volts per minute (Electrometer sensitivity 64.5 scale divisions per volt). 3. Ionization produced by the standard T h solution containing 1 . 5 7 X 10-3 gr. T h X (in T h units of course) 0.36 volts per minute (Mean of 0.372 and 0.348). 4. The unknown solution produced ionization equivalent to 0.042 volts per minute. It contains therefore: 1.57 X IO-^ X 0.042 = 1.84 X 1o-O gr. T h X
0.36
The pH of solutions was determined with the aid of quinhydrone calomel electrodes, A potentiometer and a Hartmann and Braun mirror galvanometer were used. I n the following Table I1 are given the results of these experiments.
TABLE I1 The Dependence of T h X Adsorption on the pH of Solutions t = 17OC. T h X used for the experiment T h X found in the solution % T h Total
PH
in
I
cc.
Solution No. 6 contains: 5.09 6.48 7.6
7 . 8 X IO-^ 5 . 3 4 X IO-^ 4.195 X IO-^
in
X 1.70 X
cc.
IO-$
IO-'
x
%of total in prequantity cipitation
a) T h X-7.80 X b) Fe-0.0793 gr.
3,12-~ 2.72
I
2.97-5 2 . 4 0 X IO-^ 0 . 7 4 X IO-^
gr. 100.
88.2 33.17
a) T h X-3.90 X IO-^ gr. b) Fe-0.0793 gr. 1 . 5 6 X IO-^ 1 . 5 2 X IO-^ 100. 1 . 5 6 X IO-^ 1 . 7 3 X IO-: 100. 1 . 3 9 X IO-^ 1 . 4 3 X 10-j 100.
11.8 66.83
Solution No. 7 contains: 4.24 4.37 5.00
7.20
3 . 9 X IO-^ 3 . 1 2 X 10-3 1 . 4 6 X IO-^ 1 . 9 5 X IO-^
1.39 X
10-j
0 . 7 0 X IO-'
50.25
__ --
__ 49.75
a) T h X-7.8 X IO-^ gr. Solution No. 8 contains: b) Fe-o.I j86 gr, 5,36 7.68 7.53 8.25
7.8
X IO-^
4 . 5 1 X IO-^ 2 . 4 3 X IO-^ 1 . 9 8 X IO-^
3.12 2.61 1.64 1.46
X 10-j X IO-^ X IO-' X 10-j
2.88 X
IO+
0.60 X
IO-6
0.22
X
a) T h X-19. Solution No. 9 contains: b) Fe-o.1586 4.7 5.99 5.99 6.23 6.90 7.54
19.5 15.6 12.55 9.60 6.70 4.68
x
IO-^
X X X X X
IO-^ IO-^ IO-^ IO-^ IO-^
3.9 3.81 3.69 3.62 3.40 2.92
X IO-' X IO-^ X IO-^ X IO-' X IO-$ X 10-j
IO-j
-
j
92.5 23.1 13.15
-
7.5 76.9 86.85 IO0 . o
X IO-^ gr. gr.
4.010-~ 3 . 8 0 X IO-' 3 . 7 0 X 10-j 3 . 4 0 X IO-^ 2 . 0 4 X IO+ 0 . 4 2 X IO-$
100.
-
100.
__
100. 94. 60. 14.4
6. 40. 85.6
ADSORPTION O F THORIUM X
1245
The last two columns of the table give the percentage of T h X in solution and in the precipitate. Since, however, the experiments were performed with different amounts of T h X and ferric hydroxide present, the following Table I11 gives the results recalculated for the case of a solution containing the same quantity of ferric hydroxide, 0.I 586 gr.
TABLE I11 ‘rh X adsorbed at Different pH PH 5.99 6.23 6.48 6.90
T h X in I cc. of solution
3.80 X IO-^ 3.4
x
2.4
X IO-^
2.04
x
7.20
0.70
7.54 7.60
0.42
7.68 7.53 8.25
X X
0.74 x 0.60 X 0.22
x
10-6
10-5
IO+ IO-’
IO-’ IO-’ 10-5
__
T h X in 0.1586gr. of of ferric hydroxide
-
0.58 X I . 26 X 2.62 X 1.94X 3.49 X 5.60 X 3.47 X 2.11 X 3.80 X
IO-^ IO-^ IO-^ IO-^ IO-^ IO&
IO-^ IO-^ IO-^
The precipitated ferric hydroxide contains Rdth from the standard T h X solution. The question arises whether T h X produced by Rdth is freely exchanged with the solution or whether it is retained in the precipitate. I n the first case one should expect that, when a solution is maintained a t constant pH, the amount of T h X in solution will not change with time, in the second case it should decrease due t o its radioactive decay. A solution identical with solution No. I described on the previous pages and containing ferric hydroxide on the bottom was kept at pH 6-7 for a longer period of time. It contained a t the beginning of the experiment 2.55 X 10-3 gr. T h X. The following results were obtained: TABLE IV The Change of T h X in Solution with Time Time from the beginning of the experiment
A.
45 hours 167 hours 307 hours
B. 307 hours
Volume of the Solution
900 cc.
800cc. 700 cc. 600 cc.
T h X in I cc. of solution
Th X in the whole solution (@.)
(u.1
2.88 X
z.j9 X IO-^ 2 . 7 1 X IO-^ 1.29X 1 0 - ~ 2 . 2 8 x IO-^
2.64 X IO-^ 2.6j X IO-’ 2.67 X 10-8 2.49 x IO-^
(u.1
IO-^ 3.39 X IO-^ 1.83X IO-^ 3.81X IO-^
Calculated uan tity of TB X -
Before taking the first and second samples of this experiment the solution was shaken. For the third sample it was not shaken while for the fourth it was warmed up and shaken. The calculation of T h X in solution (last column)
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IW. KURBATOW
was made assuming a free exchange between precipitate and solution. typical calculation is given below for the 45 hour sample: I.
The quantity of T h X remaining in the solution considering the decay: 1.79 X ~ o - ~ g r .
2.
T h X produced by 2.84 X
3.
The total T h X in solution: 2.64 X IO-^ gr.
IO+
A
gr. Rdth (45 hrs.) 0.85 X IO-^ gr.
The agreement of calculated and determined quantities shows that the concentration of T h X in solution is maintained by T h X rising from Rdth in the precipitate. Four solutions were now made containing equal quantities of ferric ion and T h X. They were precipitated with different amounts of ammonia. After thirteen and fifteen days two samples were taken from each solution (after shaking) and T h X and pH determined. The following Table V shows the results:
TABLE V The Amount of T h X in Solution as a Function of Time and pH No. of the solution
T h X (Th-units) used for the experiment
X
gr. IO-^ gr.
pH of the solution
7.6 7.62
I
I .92
2
I
3 4
1.92 X ~ o - ~ g r . 5 . 4 2 1.92 X I ' ~ - ~ g r . 4.6
.92 X
Quantity of Th X (Th-units) found in the solution
Remarks
not found not found
pH and T h X measured after 13 days
1.71 X Io-*gr. 1.82 X ~ o - ~ g r .
p H and T h X measured after 15 days
It will be seen that T h X is freely exchanged between the precipitate and the solution when the solution has a pH between 5 and 6 but that the exchange is absent when pH exceeds 7. These experiments were performed in absence of S04--and HC03- ions. Discussion of the Results The experiments described on the previous pages show quite conclusively that the opinion, that ferric hydroxide (and similarly other hyrdoxides) is simply a carrier of a distinct radioactive compound, is untenable. Instead it must be assumed that the precipitation of radium together with ferric hydroxide in alkaline media depends on the formation of a salt-like compound in which radium acts as a cation and ferric hydroxide as anion. This is estab' lished by the observations that the presence of neither S04--nor HCO - ions is necessary to produce highly emanating preparates. Thus the procedure of obtaining these is simplified still further. A more detailed study shows, that a t pH higher than 7,'Th X is firmly adsorbed by the hydroxide. At pH lower than 7 T h X originally adsorbed or produced by Rdth is freely exchanged with the solution. The exact point on the pH scale a t which T h X (or radium) is adsorbed on ferric hydroxide or is desorbed from it can not be given, of course.
ADSORPTION OF THORIUM X
1247
The experiments here described show also that in order to obtain the highly emanating preparates the solubility product of pure radium salt (or of the isomorphous Ba-Ra salts) should not be exceeded in the solution. This is contrary to the earlier views on the subject but is in accord with the observations of Erbacher and KAdig (loc. cit.) who found that by precipitating R a and Ba salts with large amounts of ammonium chromate preparates of diminished emanating activity were obtained. By exceeding the solubility product of Ra salts they obtained these in the form of microscopic crystals instead of having Ra uniformly distributed throughout the mass of the hydroxide, a state which obtains when Ra is fixed on the ferric hydroxide in presence of ammonium chromate. This work has been carried out in the Chemical Laboratory of the University of Moskau. summary
The properties and the methods of preparation of the highly emanating Ra preparates are described. 2. It is shown that the fixation of T h X (and Ra) on ferric hydroxide does not depend on the presence of some anions forming insoluble Ra salts but is entirely determined by the pH of the solution. 3 . The structure of the highly emanating preparates is described. I.
