The salts (hypophosphites, formates, a phosphite, a phosphate, and an oxalate) were X-irradiated, whereby hydrogen formed chemically by a radiolytic process becomes trapped in the solid. By room temperature vacuum extraction, the kinetics for the evolution of this trapped hydrogen was studied mass spectrometrically. All salts except two exhibited second-order kinetics. The two exceptions (NaH2PO2(H2O) and K2HPO4) showed first-order kinetics. Based on experimental results, the escape of hydrogen involves three steps: the diffusion of hydrogen atoms from the bulk to the surface, association of these atoms on the surface (rate controlling step for second-order hydrogen evolution), and the desorption of molecular hydrogen from the surface. The hydrogen does not escape if the irradiated salt is stored in air, apparently because adsorbed air molecules occupy surface sites required in the escape mechanism

Marsik, S. J.

May, C. E.

Philipp, W. H.

English

NASA Technical Reports Server

AND
NASA TECHNICAL NOTE NASA TN D-7413
(N'fq'ASA-TN-D--7$13) TRAPPING OF HYDROGEN N74-13 825
ATOMS IN X-IRRADIATED SALTS AT ROOM
TEMPERATURE AND THE DECAY KINETICS (NASA)
--' p HC $2 75 CSCL C7E Unclas
1__ I/06 25272
TRAPPING OF HYDROGEN ATOMS IN
X-IRRADIATED SALTS AT ROOM TEMPERATURE
AND THE DECAY KINETICS
by Charles E. May, Warren H. Philipp,
and Stanley J. Marsik
Lewis Research Center
Cleveland, Ohio 44135
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION * WASHINGTON, D. C. * JANUARY 1974
https://ntrs.nasa.gov/search.jsp?R=19740005712 2020-03-23T12:04:26+00:00Z
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TN D-7413
4. Title and Subtitle 5. Report Date
TRAPPING OF HYDROGEN ATOMS IN X-IRRADIATED SALTS AT January 1974
ROOM TEMPERATURE AND THE DECAY KINETICS 
6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.
Charles E. May, Warren H. Philipp, and Stanley J. Marsik E-7540
10. Work Unit No.
9. Performing Organization Name and Address 501-21
Lewis Research Center 11. Contract or Grant No.
National Aeronautics and Space Administiation
Cleveland, Ohio 44135 13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address Technical Note
National Aeronautics and Space Administration 14. Sponsoring Agency Code
Washington, D. C. 20546
15. Supplementary Notes
16. Abstract
Ten salts (hypophosphites, formates, a phosphite, a phosphate, and an oxalate) were X-
irradiated, whereby hydrogen formed chemically by a radiolytic process becomes trapped
in the solid. By room-temperature vacuum extraction, the kinetics for the evolution of this
trapped hydrogen was studied mass spectrometrically. All salts except two exhibited
second-order kinetics. The two exceptions (NaH2 PO2 *H2 0 and K2HPO4) showed first-order
kinetics. Based on experimental results, we postulated that the escape of hydrogen involves
three steps: the diffusion of hydrogen atoms from the bulk to the surface, association of
these atoms on the surface (rate controlling step for second-order hydrogen evolution), and
the desorption of molecular hydrogen from the surface. The hydrogen does not escape if the
irradiated salt is stored in air, apparently because adsorbed air molecules occupy surface
sites required in the escape mechanism.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Trapped hydrogen atoms; Radiation; Decay Unclassified - unlimited
kinetics; Mass spectroscopy
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price*Domestic, $2.75
Unclassified Unclassified --it / Foreign, $5.25
* For sale by the National Technical Information Service, Springfield, Virginia 22151
TRAPPING OF HYDROGEN ATOMS IN X-IRRADIATED SALTS AT
ROOM TEMPERATURE AND THE DECAY KI NETI CS
by Charles E. May, Warren H. Philipp, and Stanley J. Marsik
Lewis Research Center
SUMMARY
Ten salts (hypophosphites, formates, a phosphite, a phosphate, and an oxalate)
were X-irradiated, whereby hydrogen formed chemically by a radiolytic process be-
comes trapped in the solid. By room-temperature vacuum extraction, the kinetics for
the evolution of this trapped hydrogen was studied mass spectrometrically. All salts
except two exhibited second-order kinetics. The two exceptions (NaH2PO2 H20 and
K2HPO 4 ) showed first-order kinetics. Based on experimental results, we postulated
that the escape of hydrogen involves three steps: the diffusion of hydrogen atoms from
the bulk to the surface, association of these atoms on the surface (rate controlling step
for second-order hydrogen evolution), and the desorption of molecular hydrogen from the
surface. The hydrogen does not escape if the irradiated salt is stored in air, apparently
because adsorbed air molecules occupy surface sites required in the escape mechanism.
The effect of two parameters (sample size and presence of unirradiated salt) on the
decay kinetics of irradiated nickel hypophosphite was also studied.
INTRODUCTION
During the y-irradiation of hypophosphites (refs. 1 and 2), phosphites (ref. 3), and
formates (refs. 4 and 5) atomic hydrogen (H') appears to be produced:
hv
H 2PO - HPO+ H (1)
hv
HP03 - PO3 + H (2)
I
hv
HCO2 - C02 + H- (3)
These studies identified the ion radical produced, by means of electron paramagnetic
resonance (EPR). It was assumed (ref. 4) that the hydrogen formed is lost rapidly from
the irradiated salts as molecular hydrogen. Recent studies at Lewis Research Center
(ref. 6) have shown that such reactions (eqs. (1) to (3)) probably take place during X-
irradiation of the appropriate salts and that the hydrogen which is formed is trapped in
the solid for considerable time periods (over 15 days).
The purpose of this present investigation was to study the nature of the trapping of
the hydrogen in these salts. The study involved the measurement of the kinetics of hy-
drogen evolution from various X-irradiated salts. This measurement was accomplished
by room-temperature vacuum extraction, with the rate of hydrogen evolution being de-
termined mass spectrometrically. This investigation also included a study of the rate
constant of hydrogen evolution for nickel hypophosphite as a function of sample weight.
APPARATUS AND PROCEDURE
Ten X-irradiated salts were studied: nickel hypophosphite (Ni(H2PO2 )2 *6H2 0),
sodium hypophosphite (NaH2PO2'H 2 0), ammonium hypophosphite (NH 4H2PO2 ), cobalt
hypophosphite (Co(H 2PO2 )2), calcium hypophosphite (Ca(H2 P0 2)2), anhydrous disodium
hydrogen phosphite (Na 2HPO 3), nickel formate (Ni(CHO2 )2 " 2H20), sodium formate
(NaCHO2), nickel oxalate (NiC204 2H2 0), and dipotassium hydrogen phosphate (K2HPO4 ).The salts had in general about a 40-micrometer particle size.
The general format of the experimental investigation was as follows: A known
quantity of the compound (less than a gram) was placed in an aluminum boat (inside di-
mensions, 5 cm by 0.6 cm by 0. 6 cm; wall thickness, 0. 1 cm). The use of a platinum
or gold boat in place of an aluminum boat did not alter the results. The sample was ex-
posed for about 1 hour to 250-kilovolt, 10-milliampere X-rays from a tungsten target
tube with the sample 10 centimeters from the target. This dose rate was found to be
660 000 roentgens per hour. The actual exposure time (which determined the dose) and
the actual weight of the sample were selected to produce easily measurable rates for the
hydrogen gas evolution.
Within 5 minutes after irradiation the measurement of the gas evolved at room tem-
perature (about 230 10 C) by vacuum extraction was begun. For this purpose a modifica-
tion of the method used in an earlier work (ref. 6) proved to be satisfactory. The
aluminum boat containing the irradiated sample was placed in a 10-centimeter-long glass
tube with a 19/38 standard taper joint at the top. This tube was connected to the mani-
2
fold of the apparatus as depicted in figure 1 and evacuated by the auxiliary pump for
15 seconds. Immediately after this, the evolved gas was pumped into the expansion
volume of the mass spectrometer by means of a three-stage mercury diffusion pump. A
liquid nitrogen trap between the pump and the sample prevented condensible vapors such
as water from entering the expansion volume. After the gas had been collected for a
known period of time (length of first collection, several minutes), the amount of hydro-
gen present in the collection volume was determined quantitatively with the mass spec-
trometer. The collection sample was then discarded (pumped out). The second collec-
-tion of hydrogen was begun immediately, and after another known (longer) period of
time the amount of hydrogen was determined. This procedure was repeated until the
rate of hydrogen collection approached the low value for the collection system alone
(blank value). Generally, the procedure involved about 20 collections during the day
followed by a final overnight collection. The collected gas was composed of essentially
pure hydrogen.
A Consolidated Electrodynamics Corporation Model 21-614 mass spectrometer was
used. 'The calibrated system, discussed previously (ref. 7), was sensitive to nanogram
quantities of hydrogen. The amount of hydrogen collected during each time period was
corrected for both the mass spectrometer background (about 4 ng of hydrogen per de-
termination) and the collection system blank (about 24 ng/hr).
The effect of two parameters on the kinetics of hydrogen evolution was studied using
X-irradiated Ni(H2 PO2) 2* 6H2 0. These parameters were sample size (0.0039 to 0.67 g)
and presence of some unirradiated Ni(H2 PO2) 2 *6H 20 (0. 014 g irradiated mixed with
0.2 g unirradiated).
RESULTS
Figure 2 shows typical hydrogen evolution data for small samples of X-irradiated
Ni(H2 PO2) 2 . 6H 2 0. These data indicate second-order kinetics:
or (4)
dC -_ kC2
dt
3
where, k is the rate constant. The square root of the hydrogen evolution rate -dC/dt
is plotted against CO - C and C, where C is the hydrogen concentration remaining in
the sample and CO is its initial concentration. Initially the curve deviates from linear-
ity. This initial deviation is more pronounced for larger samples and for the mixture of
irradiated and unirradiated material. Other than this, data for all the samples of ir-
radiated Ni(H 2PO2 )2 6H 20 tested had the same general appearance shown in figure 2,
that is, second-order kinetics for the evolution of hydrogen.
From a graph of - dC/dt against Co - C, such as figure 2, the initial concentra-
tion CO is readily obtained from the X-intercept. The rate constant k is likewise
readily obtained from such a plot (fig. 2) by squaring the slope. See equation (4). In
figure 3 the values of k for Ni(H2 PO2) 2 6H 2 0 are plotted as a function of the sample
weight. Except at low sample weights, k is essentially constant (3.7x10 4
gsamp(gH2 )-(hr)-1) with good reproducibility. However, for low sample weights, k
appears to be larger and less reproducible.
The kinetics found for the evolution of hydrogen from each salt investigated are
given in table I. With two exceptions (i.e., NaH 2PO2*H20 and K2HPO 4), the kinetic
behavior was second order. Irradiated NaH2PO2 *H20 and K2HPO4 showed first-order
behavior for hydrogen evolution
dC = kC (5)
dt
This behavior is depicted graphically in figure 4 for NaH2 PO2 H20. The rate constants
for both the first- and second-order kinetic processes are listed in table I. The
second-order constants ranged from 104 to 107 grams of sample per gram of hydrogen
per hour.
DISCUSSION
The fact that the hydrogen evolution kinetics for most of the irradiated salts is
second order led us to postulate the following escape mechanism:
H (trapped in bulk) - H- (surface) (6)
2H" (surface) -- H2 (surface) (7)
H2 (surface)-- H2 (gas) (8)
4
with equation (7) generally being the rate controlling step. But irrespective of the exact
mechanism, second-order kinetics strongly suggests that atomic hydrogen is involved in
the escape mechanism.
Because the amount of hydrogen liberated from a sample is too large to be account-
able to purely surface adsorption, we are led to postulate that most of the hydrogen is
held within the bulk of the crystal. By using a value of 54x10 - 1 2 gram of H- per gram
of Ni(H2PO2) 2 . 6H 2 0 (ref. 6), a typical dose of 106 roentgens, and an actual crystal size
of 40 micrometers, it can be shown that 150 atoms of hydrogen would have to be adsorbed
on each surface molecule. This ratio of adsorbed H, per molecule of substrate is far
too high for a solely surface adsorption phenomena.
Because the association of hydrogen atoms on the surface (eq. (7)) is rate con-
trolling, the mobility of hydrogen atoms from inside the crystal to the surface must be
high enough to prevent this process (eq. (6)) from significantly affecting the overall
kinetics. Likewise the removal of hydrogen from the surface must also be relatively fast.
For the two salts, which exhibit first-order kinetics for hydrogen evolution,
NaH2 PO2*H 20 and K2HPO 4, the mechanism proposed in reactions (6) to (8) can also be
applied. However, for these salts, reaction (8) (a first-order reaction) is most likely
the rate controlling step.
This trapping of hydrogen atoms in irradiated nonmetallic solids is not unique.
Other investigators have identified, by EPR, trapped hydrogen in y-irradiated frozen
solutions (about 70K) of sulfuric acid (ref. 8), perchloric acid (ref. 9), and phosphoric
acid. Trapped hydrogen atoms have also ban detected by EPR at room temperature in
y-irradiated phosphates (ref. 10). Our present study, however, is the most definative
one on the decay kinetics of the trapped hydrogen atoms.
We have reported previously (ref. 6) that hydrogen formed during the irradiation of
salts (e.g., Ni(H2 PO2) 2 * 6H 20) remains trapped in the solid for a considerable time.
Practically no trapped hydrogen is lost when X-irradiated Ni(H2PO2) 2 6H 2 0 stands for
15 days in air. Nevertheless, trapped hydrogen is rapidly evolved when irradiated
salts are subjected to vacuum conditions. Apparently the adsorption of air molecules
(e.g., N2 , 02, CO2, and/or H20) on the solid inhibits hydrogen atoms present in the
interior of a crystal from occupying the surface sites necessary for hydrogen escape.
As mentioned previously, the hydrogen atoms are quite mobile within the bulk.
Their mobility on the surface should be even greater because of shallower potential
wells, assuming of course vacuum conditions so that surface sites are not blocked by air
molecules. Under such conditions, migration of hydrogen atoms from one crystal to
another is possible if the crystals are in contact. By such a process, a sample ori-
ginally nonuniform in hydrogen concentration (e.g., large samples of ref. 6 or in-
tentionally prepared nonuniform mixtures) could become more uniform. When near
uniformity is achieved, second-order kinetics for hydrogen evolution should be observed.
5
However, prior to this condition, diffusion along the surface may be so rapid that the
hydrogen activity on the surface is not in equilibrium with the hydrogen concentration
in the bulk. Under such circumstances, deviation from second-order kinetics should
be expected. Our results show that indeed large initial deviations from second-order
kinetics are found for large (nonuniform) samples and for mixtures purposely made non-
uniform in hydrogen atom concentration.
The rate constant k for second-order hydrogen evolution from Ni(H2PO2)2 " 6H 20(fig. 3) appears to increase abruptly from a constant value (k = 3.7x10 4
gsamp(gH2)- 1 (hr)-1 as the sample weight decreases below about 0.05 gram. We
attribute the high values of k to experimental circumstances and not to an actual in-
crease in the rate constant. We believe that some of the hydrogen may be desorbed via
aluminum boat and glass surfaces. For large samples the percent contribution would
likely be small. However, for small samples, such an effect may contribute noticeably
to the overall evolution of hydrogen. The net effect would be a high apparent value of k
for small sample weights.
RESUME OF PROCESS INVOLVED
When certain salts (e.g., hypophosphites) are X-irradiated, hydrogen atoms are
produced. At atmospheric pressure, adsorbed air molecules prevent their association
and so atoms of hydrogen are trapped within the crystalline lattice. Under vacuum con-
ditions, the hydrogen atoms diffuse to the surface (being very mobile), and recombine at
surface sites; the resultant hydrogen molecules escape as a gas. For most salts in-
vestigated the recombination is the slowest step; this results in second-order kinetics.
Lewis Research Center,
National Aeronautics and Space Administration,
Cleveland, Ohio, October 4, 1973,
501-21.
REFERENCES
1. Morton, J. R.: The E.S.R. Spectrum of Irradiated Ammonium Hypophosphite.
Molecular Phys., vol. 5, no. 3, May 1962, pp. 217-223.
2. Atkins, P. W.; Keen, N.; and Symons, M. C. R.: Unstable Intermediates.
Part XVI. Hyperfine Coupling from a-Protons in Non-Planar Free Radicals: The
HPO2 Radical. J. Chem. Soc., pt. I, 1963, pp. 250-254.
6
3. Horsfield, A.; Morton, J. R.; and Whiffen, D. H.: Electron Spin Resonance and
Structure of the Ionic Radical, PO3 . Molecular Phys., vol. 4, no. 6, Nov. 1961,
pp. 475-480.
4. Ovenall, D. W.; and Whiffen, D. H.: Electron Spin Resonance and Structure of the
CO 2 Radical Ion. Molecular Phys., vol. 4, no. 2, Mar. 1961, pp. 135-144.
5. Brivati, J. A.; Keen, N.; Symons, M. C. R.; and Trevalian, P. A.: Radicals in
Irradiated Formates and Oxalates. Chem. Soc. Proceedings, Feb. 1961, pp. 66-67.
6. May, Charles E.; Philipp, Warren H.; and Marsik, Stanley J.: Developable Images
Produced by X-Rays Using the Nickel-Hypophosphite System. III - The Latent
Image And Trapped Hydrogen. NASA TN D-7412, 1973.
7. Otterson, Dumas A.; and Smith, Robert J.: Determination of Hydrogen in Milligram
Quantities of Titanium and Its Alloys. NASA TN D-7326, 1973.
8. Sprague, E. D.; and Schulte-Frohlinde, D.: Electron Spin Resonance Investigation
of the Disappearance of Trapped Hydrogen Atoms in y-Irradiated Sulfuric Acid
Glasses. J. Phys. Chem., vol. 77, no. 10, May 10, 1973, pp. 1222-1225.
9. Ultee, C. J.; and Kepford, C. R.: ESR Study of the Behavior of H Atoms Trapped in
Frozen Perchloric Acid Solutions. J. Chem. Phys., vol. 52, no. 7, April 1, 1970,
pp. 3462-3467.
10. Virmani. Y. P.; Zimbrack, John D.: and Zeller, E. J.: An Electron Paramagnetic
Resonance Study of Hydrogen Atoms Trapped in y-Irradiated Lithium Phosphates.
Jour. Phys. Chem., vol. 77, no. 22, Oct. 25. 1973, pp. 2622-2628.
7
TABLE I. - KINETIC DATA FOR HYDROGEN EVOLUTION
FROM X-IRRADIATED SALTS
Salt Reaction Rate constant, k
order
order sampH2 -1 ( h r ) -  hr-1
Ni(H2 PO 2 ) 2 '6H 2 0 Second 3.7x10 4
NaH 2 PO 2 H 2 0 First ------- 0.5
NH 4 H 2 PO2  Second 10 ---
Co(H 2 PO 2 ) 2  6x10 4  ---
Ca(H2 PO 2 ) 2  6x10 4
Anhydrous Na 2 HPO3  106 -
Ni(CHO2 )2 2H 2 0 106
NaCHO2  106
NiC2 04* 2H 2 0 107--
K2 HPO4 First ------- 3
Standard
taper,12/3taper / Connection to
12130 auxiliary pump-
ing system 1.
\-Vacuum stopcock l
S (4-mm bore) /
- 24140 Desorption chamber
Aluminum boat /
. ............ a nd sam ple -
Cold trap (cooled
by liquid nitrogen)
Three-stage Water
mercury jacket
diffusion
pump
Strips of
copper
Inlet to mass
spectrometer
Figure L - Schematic drawing of desorption chamber and pumping system.
-L
24x104
2.5x10 - 2  22
20 -
c. 2.0-9
O 18
1. O _14O
S. ~ 12 -
a' I I l l c
o 
-. 12
0C
0 2 4 6 8 10 12x10-5  8
Initial hydrogen concentration minus
hydrogen concentration, Co - C, g/g
10x10 5  8 6 4 2 0 6
Hydrogen concentration, C, g/g
Figure 2. - Rate of hydrogen evolution from X-irradiated 4 _
Ni(H P02)2 .6H20. Sample size, 0. 0105 gram; voltage
250 kilovolts; current, 10 milliamperes; dose, 1. 3x10
roentgens.
0 .1 .2 .3 .4 .5 .6 .7
Sample weight, g
Figure 3. - Effect of weight of X-irradiated Ni(H 2 PO2 )2 .6H20
on rate constant for hydrogen evolution.
9x10- 6
0 O
8-
7
6
5 -
4- O
33-
2
1
0 4 8 12 16 20 24 28x10 - 6
Initial hydrogen concentration minus hydrogen concentration,
I I I COl-C, g/g I I
24x10 -6  20 16 12 8 4 0
Hydrogen concentration, C, g/g
Figure 4. - Rate of hydrogen evolution from X-irradiated
NaH2PO2 H20. Sample size, 0.0195 gram; voltage, 250
kilovolts; current, 10 milliamperes; dose, 6.6x105 roentgens.
NASA-Langley, 1974 - 6 E-7540


Trapping of hydrogen atoms in X-irradiated salts at room temperature and the decay kinetics

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