CHEMICAL ACTION OF ALPHA PARTICLES FROM RADON ON AQUEOUS SOLUTIONS’
F. C LAKKISG
AKD
c.
LIND
School of Chemzstry, Cntuerszty of Mznnesota, Mznnenyolc F, Iftnnesota
Receued Aprzl $2, 1958 INTRODUCTIOK
The action of radon on aqueous solutioiis ha3 lxen relatively neglected Theoretically, since alpha particles have no selective effect on the separate components of a mixture, one mould expect that in dilute or moderately concentrated solutions thcl principal chemical effect would be exerted upon the solvent water It is well known that alpha particles decompose n ater into hydrogen and oxygen and form a little hydrogen peroxide If a reducing agent such as hydrogen iodide be present in solution, the evolulioii of oxygen is suppressed and the equivalent aniouiit of hydrogen iodide is oxidized to free iodine, hydrogen being liberated as gas The converse case of the acceptance of hydrogen by a n oxidizing agent present in solution 11ith liberation of free oxygen has not been previously investigated, but is now demonstrated and quantitatively studied for aqueous solutioiis of iodine and of potassium permanganate Besides the theoretical interest pertaiiiing to such reactions as just described, there is the practical problem of finding a convenient method of calibrating alpha-ray bulbs containing radon. By calibration is meant ascertaining how much ionization will bP produced outqide such a thin bulb per millicurie of radon contained Various means of calculation or measurement have been employed The calculation must assume the bulb to be a sphere of uniform all thickness Mund recently devised a very elegant method of making this calculation, but the present experiments show that the assumption of uniform wall thickness is sometimes so far from the truth as to render the application uncertain Evidently if one has a solution readily acted on by alpha particles to give a reaction of known yield per ion pair (M,”), it nould o n l y b e necessary to dip the bulb containing radon into the solution for a suitable interval, titrate the amount of change, aiid calculate N I t may be said a t the outset that no 5olUtioll n a s found capable of Submitted by F C L a m i n g to the Giaduate School of the University of Minnesota in partial fulfillment of the iequirements for the degree of Doctoi of Philosophy 1223
1230
F. C. LANNINQ AND 8. C. LIND
yielding a result so simply. Complications, such as reverse reaction, made it necessary to measure the evolved gases also. This complicated the manipulation so that it was found simpler to calibrate the bulb in a separate experiment. A new method2 was employed in decomposing gaseous ammonia and measuring the hydrogen and nitrogen left on freezing out the ammonia, which was used in such large excess that the back reaction mas negligible. Besides the employment of alpha-ray bulbs, the radon was also sometimes introduced directly into the water or the solution. As is well known, this method involves a knowledge of the distribution of radon between the liquid and gas phases and also the calculation of recombination of hydrogen and oxygen in the gas phase due to the radon then present. In this connection, it was found that when the neck separating the liquid and gas is narrow (so as to give a sharp demarcation of volumes), the distribution of radon according to the static distribution coefficient cannot be assumed. Apparently the evolved hydrogen and oxygen are carrying radon out through the neck faster than it can redissolve at the small surface, so that the equilibrium is dieplaced toward excess of radon in the gas phase, calling for a greater correction for back reaction, hence a high yield of the decomposition. QUALITATIVE RESULTS
Aside from the decomposition of water brought about by radioactive salts dissolved in water, little has been done in the field of radiation of liquids. It is, of course, well known that the radiations produced by a radioactive salt dissolved in water produce decomposition of the water, yielding oxygen, an excess of hydrogcn, and a small amount of hydrogen peroxide. In 1907 Cameron and Ramsay (2) studied the decomposition of mater brought about by the radiation from radon dissolved in water. Their results indicate that the primary products of decomposition are hydrogen and oxygen in equal quantities and that a secondary reaction ensues with the formation of a small amount of hydrogen peroxide, so that the gas evolved contains a slight excess of hydrogen. They showed that the volume of gas produced was proportional to the amount of radon present in the water, also that the reaction was half completed in 3.8 days, which is approximately the half-life of radon. Duane and Schcuer (3) showed that the hydrogen excess, amounting to as high as 36 per cent in the initial stage of the radiation, diminished steadily as the experiment proceeded. At very low temperature (183OC.) no excess hydrogen was formed. They concluded that the formation of hydrogen peroxide, and consequent hydrogen excess, was the result of the 2
Kindly suggested by Dr. J. C. dungere.
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIONS
1231
reaction of nascent oxygen on water. Their calculated values of - M / N , the number of molecules of water decomposed per ion pair formed, were between 0.86 and 1.05. Iiernbaum (6) measured the amount of hydrogen peroxide formed in radon water solution and showed it to be equivalent bo the hydrogen excess. He further showed that the diminishing hydrogen excess was due to establishment of equilibrium in the formation and decomposition of hydrogen peroxide. Lind (7) found that the rate of decomposition of potassium iodide in water, decomposed by means of radon dissolved in the solution, increased with the concentration. Potassium iodide in 2 normal sulfuric acid was decomposed still more rapidly. Nurnberger (9), by a direct method, found - M / N for water to be between 0.7 and 0.9. - M / N for the oxidation of ferrous sulfate, determined by bot>h direct and indirect methods, increased with the concentration, but at higher concentration the - M / N values (approximately 8) are too great to be accounted for by the action of liberated oxygen on ferrous sulfate to form ferric sulfate. Qualitative experiments were conducted on aqueous solutions of hydrogen iodide, hydrogen bromide, iodine, and iodine in potassium iodide to determine their suitability for quantitative study, the first two as oxygen acceptors, the last two as hydrogen acceptors. The iodine solutions were studied to ascertain if hydrogen is accepted (resulting in synthesis of hydrogen iodide), since this would have an important bearing 011 the hydrogen iodide decomposition reaction. A 0.5 normal solution of hydrogen iodide in water was exposed to the radiations from radon confined in an alpha-ray Eulb mounted on the ground-glass stopper so that it could bt! introduced into the liquid. Air above the solution was displaced by means of nitrogen. During the course of the reaction the solution mas shaken greatly to dislodge any gas bubbles from the alpha ray and also to prevent the accumulation of decomposition products immediately next to the ionization source. After the lapse of a suitable interval of time the alpharay bulb was removed, and the solution was titrated with sodium thiosulfate. By an arrangement similar to that given above for the decomposition of hydrogen iodide, it was shown that iodine, in aqueous solution, accepts hydrogen and is converted into hydrogen iodide. The following data ’irere obtained : .................... . . . . . . 140 Time of radiation, in hours.. . . . . . . . . . . . . . . . . . . . . . . . . 22 Ml. of 0.0241 N sodium thiosulfate.. . . . . . . . . . . . . . . . . . . 1 . 9 M1. of 0.0335 N barium hydroxide. . . . . . . . . . . . . . . . . . . . 0.75 125 ml. of 0.00108 N iodine = 5.6 ml. of 0.0241 N sodium thiosulfate
1232
F. C. LANNING AND S. C. LIND
Calculations : (1) From the sodium thiosulfate titration:
5.6 - 2(1.9)0.024 X 127.94 0 . 0 0 3 g. of hydrogen iodide 1000 (2) From the barium hydroxide titration: 2 X 0.75 X 0.0335 X 127 94 = 0.0064 g. of hydrogen iodide 1000 These results (although they do not agree exactly) indicate that there is a t least some reaction betiveen hydrogen from the primary decomposition of water and iodine in neutral solution with the formation of a n equivalent amount of hydrogen iodide. This experiment was next repeated with iodine in potassium iodide, to ascertain if iodine will combine to form hydrogen iodide. The results from four experiments shon-ed no loss of iodine and no formation of hydrogen iodide. I t mas therefore concluded that there should bc no back reaction in the decomposition of hydrogen iodide, since the iodine n.ould, in all probability, exist in combination with hydrogen iodide as €113, as it does with potassium iodide as KIB. Hydrogen iodide n-ould, therefore, appear suitable for more extended study as oxygen acceptor. It should be noted that, in the results to be reported later in this reaction, the hydrogen iodide is 0.5 N (more than twice as concentrated as the potassium iodide in these preliminary experiments), and, further, that the iodine formed is always much less concentrated than the iodine used in the potassium iodide solutions. QUANTITATIVE RESULTS
1. Decomposition of water and aqueous hydrobromic acid solutions by the
direct method The reactions were carried out in the apparatus shown in figure 1 B is the reaction vessel containing the solution It is connected to the vessel a by means of a capillary tube of 3-mni. diameter. The volume of B = 56.0 ~ m The . ~ vessel a during the experiment will contain evolved gases and radon in equilibrium with the radon in the solution The vessel carries sealed-in tungsten electrodes for sparking evolved gases The volume of the vessel, together with connecting capillary tubes to the manometer, is 4.29 ~ r n .The ~ liquid in B can be agitated by means of a glass-covered bead of soft iron actuated by a n electromagnet. The procedure followed was to introduce the radon into the evacuated vessels, A and B, by means of the special device, D, which consists of a metal rod with a n eye at the top through which the radon capsule wao inserted By turning the metal cup to which the rod is attached the capsule may be
ACTIOE; OF ALPHA PARTICLES
os
AQUEOUS SOLUTIONS
1233
broken, allowing the radon to diffuse up into the reaction vessel, from G, through the connecting tubes and E. The coursc of the reaction was followed manometrically, by taking pressure readings a t suitable inter\-als, both before and after sparking the evolved gases in the vessel A. At the conclusion of the hydrogen bromide experiments, the liquid was withdrawn and analyzed for liberated bromine. Potassium iodide mas added to the hydrogen bromide solution, and the liberated iodine was
FIG.1. Apparatus for the direct radiation of water and hydrobromic acid
titrated with standard sodium thiosulfate solution. Calculations for both water and hydrogen bromide were made on the manometric data obtained. Before calculations could be made it was necessary to determine the distribution of radon between the liquid and gas phases. The conditions under which these experiments were performed made it impossible to use the distribution coefficient of St. Meyer and Schweidler. In order to define the gas volume with sufficient accuracy to make possible suitable
1234
F. C. LANNING AND S. C. LIND
manometric measurements on the hydrogen and oxygen evolved, a smallbore tube, connecting vessels A and B, had to be used. The evolution of the hydrogen and oxygen in the liquid, and diffusion into the vessel above, TABLE 1 Dtrect method for decomposition of water by radon Experiment I: initial quantity of radon = 86 7 mc.; T = 13°C. _ I _ _ _
TIME
____
em
9 5 18 5 25 35 45.5 59 70 82.5 90.5 105 117 130.5 139.5 161.5 185.5
352 8 97 12 89 17 35 22 34 27 79 32 53 36 46 39 07 43 02 47 14 50 45 51 80 57 60 62 32
€1 5 106 0
408 386
* PI =
Pa V 1 I T I I
1 -___ Pi'
hour6
'
'
0 71 1 39 2 22 3 56 5 2 7 25 8 88 10 35 11 34 12 85 14 58 16 07 16 50 18 86 20 97
592 1 6 1 4
0 249 0 635 0.912 1.23 1.58 1.97 2.30 2.58 2.805 3.085 3,388 3.62 3.725 4.13 4.47
I
'
111
1
Mz
N
I
0 95 1 95 2 84 3 60 4 20 4 97 5 79 6 40 6 69 7 80 8 70
0 0 0 0 0
I
1
0 0 0 0 0 0
093 212 299 389 442 534 610 ti87 737 853 956
1.69 3.78 5.23 6.17 7.03 8.08 9.10 10.06 10.68 12.38 13.48
I 1
total pressure of hydrogen produced, in centimeters.
P2 = total pressure of oxygen produced, in centimeters. V = volume a t standard conditions of hydrogen produced. 1019 = molecules of hydrogen in V . ,Ifz X 1019 = molecules of mater resynthesized. Jf x 1019 = molecules of water decomposed = M I MP. N X 1019 = number of ion pairs produced in water. T = average temperature. T'olume of liquid in each case = 56.0 cc. Rn = per cent of radon in the liquid phase. Rg = per cent of radon in t h e gas phase.
,TIlX
+
not only prevented the prompt establishment of equilibrium of the radon between the gas and liquid phases, but also displaced the equilibrium toward the gas phase. This necessitated making actual measurements of radon distribution throughout the course of the experiment. This was
1235
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIONS
done by x-ray comparison, using a Lind electroscope with heavy lead shielding first between A, then B, and the electroscope. The data on the water and hydrobromic acid experiments are given in tables 1 and 2. TABLE 2 Direct method for decomposition of hydrobromic acid by radon Exaeriment 111: radon in 0.397 N hydrobromic acid
-_-I_____ TIME
hours
15 29 40 51 63 75 89
5 16 25 35 44 54 63
1 '
1 I
_ _ _____ _______
53 45
WOWS
16
j
[
N
MI
58
15 85 41 04
RW
41.2
0.221 3.185 1.048 0.388 0.121 6.885 1.152 3.12 0.120 2.86 1.792 1.73 5.56 2.46 0.247 3.53 8.51 3.14 0.419 5.37 7.20 11.04 3.415 0.587 4.42 13.47 8.85 0.783 ___ I $1 = 8.85 X 1019 0.783 X lolo M = 9.633 X l O I 9 Rs M H ? - 9.633 X 10" = 0.716 55.8 I "'0 1.347 X
1.367 1.473 1.867 2.147 2.367 2.63 2.95 -__
DIBTRIBUTION Or RADON
__
MI
cm.
I
1
v
PZ
PI'
_- 1 -
'
1
+
A. From titration of bromine liberated. . , , , , . . , . , . , . , , , , . . . . . , . B. Prom hydrogen liberated.. . . . . . . . . . . . , . , . . . . . . . , , . . . . . . . . . . . Per cent oxygen utilized =
* Se,e footnote t o
1.155 1.187
1.155 X 100 = 80.5. 2 X 0.716
-___-
table 1.
M I is the number of molecules of hydrogen produced as determined by the gas measurements. M2 is the number of molecules of hydrogen resynthesized in the gas phase. M i = V X Avogadro's number 22,400
V is the volume (under standard conditions) of hydrogen produced.
P X Y X 3.85 M z = 5.34 X loi3 X 2.4 X lo4 X mc. X P X i X 760
1236
F. C. LANNING 9 N D S. C. LIpiD
In thib expression mc. is the number of millicuries of radon decayed in the gas phase, p is the average path, in centimeters, that an alpha particle would travel, P i3 the average pressure, in millimeters, over the period calculated, z is the specific molecular ionization of the gas mixture referred to air as unity, Y is a factor to take into account the ionization produced by recoil atoms, and 3.85 represents the number of molecules of water produced per ion pair p is equal to 0.61 times the radius of the sphere The term z is calculated using the average pressures of hydrogen and oxygen during the period.
Y is given by the expression R --,+ a .in which R reprea
sents the ionization produced by recoil atoms and a the ionization produced by alpha particles The ionization NHIOi s given by the following expression, in which a is the number of alpha particles per millicurie disintegrated, mc. is the riumber of millicuries of radon destroyed in the liquid, P1is the number of ion pairs produced per alpha particle, and KS is the molecular ionization of water (0.82). = a
“$0
X mc. X 1’1 X K S
Calculation of - MHB~/”~o: (1) from gas measurements: To obtain -MHBr from the gas measurements, it is necessary to correct only Ml(H2) for the unused oxygen and multiply by 2. 2 X total cm. pressure of oxygen produced Total em. pressure of hydrogen produced (2) from titration: The titration was 23.1 ~ 1 1 1of. ~ 0.01483 N sodium thiosulfate.
S H 2 = ~
-A
-
18.01 X 10’’
~ H B ~20.8
x lo”
-
N H ~ O 18.01 X 1019 Owing t o the difficulties encountered in the measurements of distributioil of radon between the liquid and the gas and in the calculation of thc amount of recombination of the gases under the influence of the radon mixed with them, this direct method proved to be unsatisfactory. Since the use of an alpha-ray bulb eliminates both of these problems, the remainder of the work here reported was done by this indirect method. As will be pointed out later, it does require a careful calibration of the alpharay bulb.
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLVTIONS
1237
3. Standardization of alpha-ray bulbs and determination of
-M / N f o r water
The use of alpha-ray bulbs for the quantitative investigation of the effect of alpha particles on chemical reactions, particularly in aqueous solutions, possesses certain advantages as well as certain distinct disadvantages. In the first place, there is obviously no problem of the distribution of the radon. It remains always in the alpha-ray bulb and one needs only to
T O VACUUM PUMPS
FIG. 2. Apparatus for the indirect radiation of ammonia, water, and potassium permanganate solutions
measure the quantity of radon initially and, by means of the Kolowrat table, to calculate the quantity of radon which decays during the interval of the experiment. The alpha particles are completely absorbed by the solution before reaching the walls of the vessel, so that the average path travelled by the alpha particles from radon, radium A, and radium C in equilibrium with i t need not be considered. Mund (8) developed a method for the calculation of the efficiency of an alpha-ray bulb as an ionization source, from measurements of the diame-
1238
F. C . LANNING AND S. C . LIND
ter, wall thickness, and tip and steni corrections. Our invcstigatioii of the thickness of the walls of many alpha-ray bulbs, both by microscope and the interferometer, conviriced us that the assumption of uniformity of wall thickness is not justified, and that the bulb must be calibrated by using it as an ionization source in a reaction whose ion yield is accurately known. The ammonia decomposition, suggested by Jungers, lends itself readily for this standardization. The apparatus used, together with that used for the potassium permanganate reduction, is shown in figure 2. Thc
I
No
TIME ELAPBED FROM START
DURATION OF EACH
hours
hours
% ; E
2
1 1 h
MANOYETER READINQ
'
mm.
10
646 0
15
659.5
'
1' PRESSURE OF GASE8 PRODQCED
662 0
17.5
47.5
___ me
I
-
RADON
ID
~
I'~
~
'Ir ~
c
~
-!!IN D
~
EXPEBIMEVT
'
mm
me.
45 0
9 09
49.5
0.868*
5q 5
12 25
i 97
0.880
I
i
I
62 0
'
12 42
'
mm
0.862
4 98
Av.
I
=
0.870
_ .
~
Radiation of water
_
11
130'
-1911
II
I
61
_ _ _ _ I _
_ _After _ _ sparkrng ___
649
49
17 3
i I8
,
630 30 _ 1_ 1 __ __ (1) (4 18 X 1023)/4 95 = 0 868; ( 2 ) (4 IS X I 023)/4.87 = 0 R80; (3) (4.18 X 1023)/4 98 = 0 862.
* Calculations:
reaction vessel, A, in the center of which the alpha-ray bulb is suspended, is of sufficiently large diameter to insure complete absorption of all aparticles by the ammonia a t the pressures used. At intervals the ammonia is frozen down in C and the gases, nitrogen and hydrogen, are pumped out and measured. The vessel, H, is a Dewar flask to which the sinal1 bulb, I, is attached. A side tube on I makes it possible to seal in the glass rod carrying the alpha-ray bulb which has been calibrated by the method just described. The whole is then sealed on to the Toepler pump and measuring apparatus at X.
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIOXS
1239
In table 3 are given the data on three experiments on ammonia decomposition and one on water, all using the alpha-ray bulb containing, initially, 125.9 mc. of radon. The calculation of - M / N for water, using a bulb calibrated in this manner, is as follows: Inspection of the decomposition equations 2NHa -+ Nz
+ 3H2
2Hz0 -+ 2H2
+
0 2
(1)
(2)
shows that the total volume of gas evolved in reaction 1 is twice that of the hydrogen only, in reaction 2, per molecule decomposed. Coniparison is made on this basis because of the fact that hydrogen is always in slight excess and rnorf’ accurately represents the total reaction than the volume of oxygen or the mixture of hydrogen and oxygen. I t happens that the molecular ionization of ammonia and that of water are the Sam?, so that this factor (Bragg’s K S ) need not be taken
Calculation
- 19.63 (V.P.) = 10.37 mm. of excess hydrogen 49 - 10 37 = pressure of the electrolytic mixturc 2/3 (49 - 10.37) = 25.8 mm., pressure of hydrogen
30
i n the electrolytic mixture 25.8 + 10.37 = 36.17 mm., total hydrogen 36.17 X 2 = 72.34 mm., total pressure of hydrogen and nitrogen which would havc hcen formed 17 3 me. of radon decayed during the interval
7234 = 4.18 mm. per millicurie 17.3 This value from the ammonia decomposition using thc ~ ~ 1 1 1bulb ~’ is 45.0/9.09 = 4.95. Thus 4.95:4.18 = 1023:X, in which 1.023 is the value adopted, from the work of Jungers ( 5 ) , for - M / N for ammonia and X is the value of - M / N for water Thus - M / N for water = 0.868. This value agrees well with those between 0.86 and 1.05, which were obtained by Duane and Scheuer (3) Again, it should be pointed out that some resynthesis of water occurs within the bubbles clinging to the alpha-ray bulb, and, while it is impossible to estimate with any degree of accuracy the magnitude of the error which this introduces, it is safe to conclude that the true value of - M / N for the decomposition of liquid water is very close to unity and that at least the net reaction is the decomposition of one inolecule of water into hydrogen and nascent oxygen per ion pair. In the two reactions now to be described, the alpha-ray bulbs used were calibrated by the water decomposition, measuring the volumc of hydrogen evolved, assuming the value of -M/hr for 1%ater to hc 0 8’7
1240
F. C. LANKING AND S. C . LIKD
3. Deconapositior: of hydrogen iodide
In the preliminary studies it was shown that hydrogen iodide solutions, under the influence of radiations from radon, are decomposed with the liberation of iodine and hydrogen. The iodine may be conveniently titrated, and the hydrogen is readily measured manometrically. It was further shown that there is very probably no back reaction under the conditions of the experiments here reported. All the iodine formed has a high probability of forming I;, which was proved to be stable under alpha radiation. The apparatus and method are essentially the same as that TABLE 4 Determiniition of - J t H J N H 2 0 with a bulb standardized against water Experiment I: initial quantity of radon = 88 mc.; T = 32°C. Pi
I
Plmc
d”,::E:‘,“,
EXPERIMEKT I
Standardization against water hours
hours
14 24.5 38 50
____
1 ~
1
14 24 5 38 50
mm
mm
1
1
17 35 42 56
7 85 5 7
1
15 26 31 40
1
35 6 45
I
mc
mm
8
mm
1
I
1
69 04
I
1
27 3
2 53
_____
Radiation of 0 5 N hydrogen iodide solution
____________ 62 96 5
_________
I
1 _______________ I
1
0 34.5
1
38 05
I
______________-___ From gas measurements From titration (5.0
I
i --_-
38 05 41.09 14 97 _ _ _ ~ - - - _ _ _ _ _
2 X 2.74 X 0.87
(--___ 2.53
- 0 . 2 = 4.8 ml. of 0.01538 Nsodium thiosulfate)
2 74
-~‘HI/NH~o I
I
1.88 1.87
described for the hydrogen bromide decomposition, except that the radon is confined in an alpha-ray bulb located in the center of the reaction vessel, and thus no measurements on the distribution of radon are necessary. The alpha-ray bulb is calibrated against water, using the - M / N value (for water) previously found by standardization against ammonia. The results are given in table 4. As will be seen in table 4, there was no loss of pressure on sparking the gases. This indicates that all of the oxygen evolved for the primary decomposition of water is utilized in decomposing hydrogen iodide. The table also shows that the calculated - MHI,”H~Ovalues from the hydrogen
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIONS
1241
pressure measurements and for the titration of liberated iodine, respectively, agree closely. Note that this agreement is quite independent of the calibration against water also reported in table 4. The absolute value of -MRI,”H1o (1.88) does, of course, depend on this calibration and should, as indicated in the calculations given herewith, be twice that for - M / N (HzO). (This value for the ammonia decomposition comparison is 0.87.) That is, the value should be 1.74 if the theory of secondary utilization of oxygen is correct as indicated in the equations:
+0 HzO + 12
HzO -4 Hz 2HI
+ 0-4
The fact that it is somewhat higher is probably brought about because there is less recombination of hydrogen and oxygen in the gas bubbles formed on the alpha-ray bulb, since the oxygen is so promptly absorbed by hydrogen iodide, although there is undoubtedly some recombination in the bubbles even here. (Any oxygen actually in a bubble as it breaks away from the surface of the alpha-ray bulb is apparently completely absorbed before it escapes from the liquid.) It seems altogether probable, therefore, that the true values are:
- (forHzO) = 1 2 N
and
- JlHI
-= 2 ”20
4. Determination of - M B M ~ O , / N H ~ ~ The apparatus is the same as that used for ammonia and pure water, and is shown in figure 2, H and I being sealed on a t X, as described previously. The potassium permanganate solution to be radiated is made by pipetting the desired quantity of a stock solution of aqueous potassium permanganate into water and adding the desired amount of aqueous C.P. sulfuric acid. The water used is conductivity water. A sample identical with that radiated is stored under vacuum until the end of the experiment and is then titrated in the same way as the radiated sample. The procedure is to radiate a solution of potassium permanganate by means of an alpha-ray bulb (calibrated against water in the same apparatus). The evolved gases are measured before and after sparking. The solution is withdrawn and potassium iodide added, after which it is titrated with sodium thiosulfate. Results are shown in tables 5, 6, and 7. The calculation of - M K M , , O , / N H ~from O the data given in table 5, having to do with the radiation of potassium permanganate, will now be given. The number of molecules of potassium permanganate which have been reduced is given by the following equation
- NKMnO,
5.85 X 0.01426 X 6.06 X lozaX 0.9377 = o,945
= -
5
x 1000
1242
'F. C. LANNING AND 8. C. LIND
TABLE 5 Determination of - M K M , , O , / N ~ ~ ~ with a bulb standardized against water Experiment I: initial quantity of radon = 89.3 me.; 2' = 26°C. -__ I I Po,/mc. P2
DEBTROYED
Standardization against water
1 !r I
?lOUTS
30
mm.
__
1
mm.
1
38
25.81
mm.
1
12.81
'
mc.
I
18.3
mm.
0.712
Radiation of 0.3 N potassium permanganate solution, normal in sulfuric acid
1
48 149.5
-__
0 101 5
71
~
-
__
-
1
I
1
68
1
62
I
33.1
i
1875
TABLE 6 with a bulb standardized against wafer Experiments I1 and 111: initial quantity of radon = 134.3 me.; T = 23°C.
Determination of
TIXE ELAPSED
1 1
-dlK,t,O,lN,,,
l;",","T E:gy
1
SPARKING liE6'ORE SPARKING AFTER
hours
hours
1
22
1
I
'
RADON
' Po2/mc.
STROYED DE-
OXYGEN
DIFFERENCE IN PERMANGANATE TITRATION8
1 ~
______ Standardization against water
~_~___-.__
22
ipRODUCLd 1 i
ID'JRATIONI R p2 OF E A C H P R 6 6 1 U R E ! PRESSURE' PRESSrRE' OF
mm.
1 56.8
1
mm.
31.9
1
mm
19.12
mc
I
,
mm.
1
20 45
~~
1
0.931
1 1
Radiation of 0.0848 N Dotassium Dermannanate, 1.25 N in sulfuric acid 2 6 1 43 ,
0
17
1
i
49.0
1 40 I 20 3 0
1
1
14.08
1.44
1
5.85 ml. of 0.01426 N sodium thiosulfate
Radiation of 0.371 N potassium permanganate, 1.25 N in sulfuric acid ____ 72 0 9 ml. of 0.01426 N 98 , 26 j 43.0 41.33 3 1 . 7 13.9 2.49 sodilm thiosulfate
'
1
KMnOd -.__
N
__
1
IrtsoI
I
1.25 1.o 1.27 ____
j
0.0818 0.3 0.37 .
1
j
~
-M/N
I
FROM TITRATION
.?I
j i
1
- ~ / ~ ~ n GASo r
~
0.516 0.864 .
__
0'.536 0,907
0.922
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIONS
1243
in which 5.85 is the difference in volume of 0.01426 N sodium thiosulfate solution required by the unradiated and radiated solutions. 0.9377 is a correction term to take account of the time elapsing while the apparatus is being assembled, evacuated, etc. During this time the potassium permanganate is being acted upon, but the collection of gases cannot be started. The number of molecules of water decomposed in the water standardization is given by the following equation: 17 X 2 X 6.06 X 10" - 2.31 - M H z O = 19.12 X 760 X 22,400 __c_______
10~9
Using the value - f i l ~ ~ ~ / N = ~0.87, , o found above by ammonia standardization,
The number of millicuries of radon decayed for the production of this number of ion pairs was 20.45. For the 17-hr. interval during which potassium permanganate was being radiated, 14.08 mc. of radon decayed. Consequently, the corresponding number of ion pairs produced was 2.65
x
io19
14 08 x= 1.83 x 20.45
1019.
Theref ore,
-M / N values for the other concentrations of potassium permanganate calculated in the same way are shown in table 7. Discussion of potassium permanganate reaction
As will be seen, the value of - M / N increases with the concentration. For the more concentrated solutions the value is approximately that found for water. If our previous conclusion that the true value for water is unity is correct, that should also be the value for - M K M ~ O J N Hin~ the O more concentrated solutions. This is 2.5 times as large as can be accounted for on the assumption that the entire primary reaction is HzO -+Hz
+0
and that potassium permanganate is reduced solely by hydrogen MnOd-
+ 2.5 H2 + 3H+ -+
Mn++ + 4H20
It is obvious that some other reaction occurs which results in the reduction of potassium permanganate. The following series of equations accounts for the results in a fairly satisfactory manner:
1244
E. C. LANNIKG AND 6. C . L I S D
1. Primary reaction:
5(Hz0)2
+ 5(E)
--j
5H202
+ 10H
2. Secondary reactions:
2Mn04-
+ 1OH + 6H+ -+
+ 8Hz0 + 5 H z 0 ~+ 6H+ -+2Mn-+ + 8HzO + 502
2Mn04-
2Mnff
Thus there should be 0.8 as many potassium permanganate molecules decomposed as water molecules, by the primary reaction. In other words, - M / N for potassium permanganate should be 0.8 of the value of - M / N for water. Our experiments are not as close as desirable for upholding the theory, but are, nevertheless, within reason. The low results for the inore dilute solutions can be explained by less utilization of the hydrogen. The more dilute the solution, the less t h e * probability of the hydrogen being used before it reaches the gas phase. This conclusion is justified by the results, as the excess hydrogen was larger in the case of the dilute solutions. The assumption that thp primary decomposition of water into hydrogen peroxide is made here, since it seems unlikely that if the decomposition were into hydrogen and oxygen, the oxygen would react quantitatively with water to form hydrogen peroxide, as would be necessary to explain the results Further rvidence for this conclusion is given in the section on mechanisms. 6. Mechanisms Until the permanganate solutions were studied, all the effects produced in solution could be explained by the classical theory that water is decomposed directly into hydrogen and oxygen on being radiated by alpha particles. This theory is based on some results of Duane and Scheuer (3). They concluded that hydrogen peroxide was formed by the secondary reaction of nascent oxygen on water, and they based their conclusion on the experimental fact observed by them that the gases liberated a t - 183°C. under alpha radiation consisted wholly of electrolytic gas. More recent work b y Bonhoeffer and Reichardt (1) adds new light to this question and undermines the classical theory. They radiated water vapor at a temperature of 1000° to 1600°C. with ultraviolet light and obtained spectra for free hydroxyl, With this evidence, along with further considerations, they concluded that the mechanism for the decomposition of water under these conditions is given by the following equation: 2H20 -+ €12
+ 20H
Two free hydroxyl groups obviously correspond to a dissociated hydrogen peroxide molecule This leads to the conclusion that the decomposi-
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLUTIONS
1245
tion of water in the liquid state produced hydrogen and hydrogen peroxide. Water in the liquid state is largely associated into (H20)2molecules. It is rather difficult to understand how one of the water molecules in the associated molecule could be influenced by the ionization without the other one being influenced, but it is easy to see how the two could be decomposed into hydrogen and hydrogen peroxide. E , in the equations, represents the energy necessary to produce one ion pair. (H20)2
+E
-+ H2
+ H2Oz
Considering the evidence of Bonhoeffer aIld Reichasdt (l),the necessity of using this mechanism for explaining the permanganate reaction and the fact that it is a logical one leads one to accept the above equation as being the true mechanism. The adoption of this new mechanism in no way affects the discussions and conclusions made from the classical theory as the net result is the same, since hydrogen peroxide is an unstable intermediate product. Probably some of that formed decomposes almost instantly. H2Oz + H2O + 0 The equation for the net result of water decomposition is then as f0llOM~S: HzO E + H 2 0
+
+
This is the same equation as is obtained by the classical theory. Before considering the mechanisms for the hydrogen bromide and hydrogen iodide decompositions, the work of Harteck and Kopsch (4)on reactions with atomic oxygen should be mentioned. With electrically produced atomic oxygen they obtained spectra showing reactions between the oxygen and halogens. HX+O-tOH+X Thus we can conclude that the decomposition of the halogen acids can be due to both hydrogen peroxide and nascent oxygen liberated from the peroxide. The primary reaction, in the case of the hydrobromic acid, must be between it and hydrogen peroxide, and there must also be some reaction between it and nascent oxygen liberated from hydrogen peroxide. HzOz
+ 2HBr -+2H90 + Br2
0 + 2HBr -+ HzO
+ Br2
(1)
(2) Bromine reacts with water to form hydrobromic acid and hypobl;pmous acid, which is unstable and decomposes to hydrobroniic acid and oxygen. Br2
+ H2O -+ HOBr + HBr HOBr
---f
HBR
+0
I246
k’. C. LANXING AND S. C . LIED
Then a n equilibrium is set up, as shown in the following equation: 2HBr
+ 0 a HzO + Brz
The per cent of oxygen utilized evidentIy depends on the concentrations and on the equilibrium constant, which depends on the temperature. In the casc of the hydriodic acid decomposition, the reaction goes to completion and the reactions taking place are as follows:
+ HzOa --+ 2HzO + 2HI + O+HzO + Hz
2HI
Ip
(1 1
(2) The first reaction is the chief one. The permanganate decomposition requires two reactions to explain the production of 2.5 atoms of oxygen per permanganate molecule decomposed.
+ 5HP02+ 6H+ 2Mn04- + 10H + 6H+
2Mn04-
--+
2Mn++ + SHzO + 5 0 2
4
2Mn++ + SH20
These reactions must also be equally probable to explain the results in the less dilute solutions. Furthermore, the reaction between the hydrogen peroxide and permanganate is assumed to take place before the hydrogen peroxide has a chance to decompose otherwise. The fact that the value drops for more dilute solutions is explained by the of MXJfnO,/NHIO increased probability of hydrogen escaping from the solution before it takes part in the second reaction. SCMMARY
h study litis been made of alpha radiation of water and of certain aqueous solutions, with the following results: 1. Hydrogen iodide is decomposed into hydrogen and iodine; hydrogen bromide into hydrogen and bromine. 2. Iodine in pure water is shown to accept hydrogen and form hydrogen iodide. 3. lodinc in potassium iodide solutions is shown not to be a hydrogen acceptor. 4. Potassium permanganate solutions containing sulfuric acid are decomposed. Alanganous sulfate is formed. 5. By the direct method, with the radon dissolved in water, the value of -,If,” for vater was found to be 0.739;for hydrogen bromide, 1.187. 6. B y the indirect method, with the radon in a n alpha-ray bulb stnndardized by ammonia decomposition, the value of - i I f / N for water was found to be 0 87. 7 . I t is concluded that the true value of - M / N for water = 1. By the indirect method, - M / N for hydrogen iodide = 1.86. If the true value of -.W,/N for water is 1, the true value of - M / N for hydrogen iodide is 2.
ACTION OF ALPHA PARTICLES ON AQUEOUS SOLU’TIONS
1247
8. By the indirect method - M / N for potassium permanganate is 0.516 in 0.0848 N potassium permanganate and 0.893 in 0.37 N potassium permanganate, each potassium permanganate solution being approximately normal in sulfuric acid. 9. The primary effect is the decomposition of water, and the effects on solutes are produced purely by secondary reactions instigated by the decomposition products of water. 10. The evidence seems to indicate that the primary effect is the decomposition of water into hydrogen peroxide and hydrogen:
(HzO)2
+E
4
Hz
+ HzOz
REFERENCES (1) BONHOEFPER AND REICHARDT: Z. physik. Chem. A139, 75 (1928). A. L., AND RAMSAY, Wx.:J. Chem. SOC.91,193 (1907); 91,1266 (1907); (2) CAMERON, 92, 966 (1908). (3) DUAXE,W.,AXD SCHEUER, 0.: Le radium 10, 33 (1913). (4) HARTECK AND KOPSCH:Z. physik. Chem. B12, 327 (1931). (5) JUKGERS, J. C.: Bull. soc. chim. Belg. 41, 389 (1932). (6) KERNBAUU, M.: Compt. rend. 148, 705; Le radium 8, 225 (1909). (7) LIND,S. C.: Le radium 8, 289 (1911). (8) MUXD,W.:Bull. SOC. chim. Belg. 39, 518 (1930). (9) XURSBERGER, C. E.: J. Phys. Chem. 38, 47 (1934).
