TOSHIRO NAKATA AND SANJURO MATSUSHITA
458
Infrared Studies of Intermediates of the Ammonia Synthesis on Iron by Toshiko Nakata and Sanjuro Matsushita FacultP of Science, Hokkaido University, Sapporo, Japan
(Received March $1, 1967)
Infrared spectra were measured in the region 4000-1300 cm-' of the surface species on iron which were formed when ammonia or a mixture of hydrogen and nitrogen came in contact with the metal a t 20-500". The iron was dispersed in silica and reduced a t 550". NHa always gave absorption bands a t 3380, 3290, and 1610 cm-l. By making use of variously deuterated ammonia, the species which gave these bands was identified as the NH2 radical on the surface. The mixture of hydrogen and nitrogen, after heating at 500" for 30 min, gave the same three bands and one new band a t 3200 cm-l, the latter being attributed to the N H radical. Temperature dependence of the rate of surface amino formation from this mixture corresponded to an activation energy of about 28 kcal. The spectrum of the iron sample itself was found to shift reversibly when the sample was heated with this mixture. The probability of the NH2 radical being an intermediate of the synthesis reaction is discussed.
Infrared spectroscopic observations were made on chemical species existing on an iron surface that had been exposed to ammonia gas or heated with a mixture of hydrogen and nitrogen. The results of such investigations will provide a useful basis in discussing the mechanism of the ammonia-synthesis reaction by this catalyst.
Experimental Section The methods of the experiment and the derivation of the final results are, in the respects not described in this report, generally the same as used in our previous work.' Preparation of Sample. Iron, in the nitrate, was dispersed in Aerosil silica. This sample was, after being calcined, pressed into a disk2 and reduced in the observation cell set in the path of the spectrometer. The concentration of iron in the sample was just 10% in weight after the reduction. The thickness of disks adopted was 20 or 25 mg/cm2, but! thinner samples were also employed complementarily in certain cases where we can make more accurate determinations with thinner ones. The structure and arrangement of the cell were, generally, the same as used previously (the nichrome ribbon for heating the sample is wound outside the cell so that gas in the cell never comes into contact with the ribbon). The quartz cell and fluorite sample table permitted the sample to be maintained a t 600" for a long time. The reduction was carried out a t 550" with 99.99% pure hydrogen by the static method; this was continued for about 60 hr with hydrogen renewed a t varying appropriate intervals. The final evacuation of hydrogen to make the sample ready for the adsorption experiment was made a t 570" for 1 hr, when a The Journal of Physical Chemistry
vacuum of mm was attained. The complete reduction and evacuation of the sample was essential in the present experiment to prepare a sample that gave a stable spectrum and distinct adsorption effects. One sample thus prepared could be used, with reduction interposed, three or four times for the adsorption experiment without marked lessening in the effects. The possibility that iron silicate was produced seems very small. Before the experiment reported below, we carried out the same one using the sample reduced a t 350", the temperature at which formation of iron silicate is most improbable; the results of ammonia adsorption then obtained were qualitatively identical with the present ones. AnX-ray diffraction study of the present sample showed no evidence for the existence of iron silicate. Measurement of Spectra. A double-beam instrument was used, but no adsorbent was put into the reference cell, The measurement was made in many cases with the whole frequency range divided into two or three regions for each of which the transmission scale was magnified from 1.5 to 6 times. A fluorite prism was used. In this work, spectra were often measured after or between various heat treatments of the sample, but the measurement itself was always made at 20". Among the spectral changes caused by the contact of sample and gas, those arising from the presence of silica were examined by blank tests using the pure silica sample. After various observations were made on the spectral changes due to adsorption, a check was (1) S. Matsushita and T. Nakata, J . Chem. Phys., 3 2 , 982 (1960); 36, 666 (1962). (2) In the present work about 4 tons/oma pressure was used.
IRSTUDIESOF INTERMEDIATES OF THE AMMONIA SYNTHESIS ON IRON invariably made to ascertain that by complete evacuation the spectrum was restored to that produced before gas admission.
459 FREQUENCY
(cm-')
Results and Discussion
I . Experiments with Ammonia. The spectrum of a sample which had been completely reduced and evacuated (20 mg/cm2 thick) is shown in the lower curve in Figure 1. The limit of transmission due to the silica at 1300 cm-I was also the frequency limit of this work. The band at 3750 cm-' is attributed to OH radicals on the silica surface. When this sample was fed with ammonia, it gave the spectrum shown in the upper curve. This was measured after admitting ammonia of 7 cm pressure into the cell, leaving it for 2 hr, and then evacuating the cell, all at 20". The reason for making this evacuation, despite the use of reference cell,3 is that, with the gas present in the gas phase, ammonia is adsorbed physically on silica surfaces and interacts with OH radicals, giving broad and strong absorptions which mask the effects of the adsorption on the metal; fortunately, these absorptions due to physically adsorbed ammonia were removed by evacuation at 20" ;4 nonetheless, effects of the adsorption on iron remained stable after that. The transmission of a bare sample and that after its contact with the gas are expressed by TO and T , respectively; t,he quantity (To - T)/TO shall be denoted by A. The A curve determined from Figure 1 is shown in Figure 2. This is regarded to be the spectrum of the species produced on the sample surfaces by its contact with ammonia. The absorptions in the figure indicated by the arrow and s appeared also in the blank test and are to be attributed to a species, involving hydrogen, formed on the silica surface; such absorptions due to the silica, although appearing at different frequencies when deuterated ammonia was used, will hereafter only be indicated in the figures by the above-mentioned symbols. Except these, the bands at 3380, 3290, and 1610 cm-l are absorptions of chemical species adsorbed on iron surfaces. Also in the test in which the sample in contact with the gas was heated at 250" for 1 hr before the observation at 20", the number and the frequencies of the bands observed were the same as in the adsorption at 20". Another experiment, in which heating was made at 500" for 30 min, again gave the result of absorptions at the same frequencies, although in this case hydrogen and nitrogen were found in large quantities in the gas pumped out from the cell after the heating. In the case of 20", such decomposition products were not detected even by a McLeod gauge (put after a liquid-nitrogen trap) in the gas pumped out just before the spectrum measurement. Under the conditions of this experiment, the equilibrium pressure of hydrogen anld nitrogen was about 1.2 cm.
I
Figure 1. Spectra of the sample itself (20 mg/cm2 thick, containing 10% reduced iron) measured in the well-evacuated state (lower curve) and after NHs being adsorbed (upper curve).
Figure 2. Spectrum of the surface species produced on iron by its contact with "8. (The absorptions indicated by s, which are due to the presence of silica in the sample, should be ignored.) The species is concluded to be "2.
After the measurement of the adsorption effects, the sample was evacuated at 550" for 1 hr, and then the spectral changes were checked; it was found that not only had the three bands in question disappeared, but over the whole frequency range, the transmission of the sample was completely restored to that seen before the gas admission! In Figure 3 is shown the result of a similar experiment using deuterioammonia ("3). Since the OH radicals on the silica surface easily exchange hydrogen atoms with ammonia, in carrying out this experiment they had beforehand been replaced with OD's; this was effected by heating a sample, already well reduced, in NDa gas at 250" for 1 hr, then reducing it again with D2. The sample thus prepared had no band at 3750 cm-l, but did have one at 2760 cm-l. (3) The reference cell was vacuous in all main observations reported in this paper, although frequent subsidiary tests were made with both cells filled with the same sample gas. (4) N. W. Cant and L. H. Little, Can. J. Chem., 43, 1252 (1966). (5) The two absorptions due to silica, indicated by 8 in Figure 2, are more difficult t o remove than the adsorption bands due to the metal.
Volume 78, Number 2 February 1968
TOSHIKO NAKATA AND SANJURO MATSUSHITA
460
I
1
Figure 3. Spectrum similar to Figure 2 but with NDa. The bending bands fall outside the region observed.
This hydrogen exchange with ammonia is very rapid even at room temperature, although that with hydrogen gas is far more difficult. The distinct band at 2515 cm-l in Figure 3 is reasonably considered to be due to the 3380-cm-' band in Figure 2 having been shifted to this position by deuteration. The one corresponding to the 3290-cm-1 band in Figure 2 also seems to have appeared near 2400 cm-', but the complete form of the curve in this region could not be determined reliably because the spectrometer was much less sensitive in that region, due to the presence of atmospheric carbon dioxide. The counterpart of the 1610-cm-' band in Figure 2 is not found in Figure 3, but if this absorpt,ion is also concerned with hydrogen atoms in adsorbed species, it is natural that the band should shift to outside the observation range used in this work. I I . Discussion of the Above Results. In view of the fact that by deuteration the bands shifted by reasonable wavenumbers, the three bands at 3380, 3290, and 1610 cm-l in Figure 2 are to be attributed to vibrations concerned mainly with hydrogen atoms; and almost certainly they are the stretching and bending vibrations of the N-H bond. It is unlikely that these bands are due to the absorption of hydrogen dissociated from ammonia and directly adsorbed onto the surface metal atoms. When hydrogen gas alone was allowed to come into contact with this sample, and many pieces of evidence showed that chemisorption had actually occurred, no specific band in the infrared was observed with that sample. Thus we must now consider what the surface species is that involves the N-H bond and gives these absorptions. First, it might be asked if the species is not ammonia adsorbed physically. However it is an experimental fact that when a sample containing adsorbed ammonia was evacuated at) 200" for 1 hr, the adsorption bands diminished, but still remained. This indicates that the species giving rise to these bands is a chemically adsorbed one. Now it becomes a question of composition: was the chemisorbed species in the form of "8, or was it the NH2 radical, or the NH radical? In view of the frequency regions in which the bands appeared, especially the region of the stretching vibrations, the present absorptions are different from those of NH3+ as in amino acids, or of the ammonium ion. The Journal of Physical Chemistry
The possibility of the observed bands resulting from the coexistence of two or all of the above three species must also be considered. An experiment was carried out such that, with a sample giving the three bands, an evacuation a t 200" for a short time and a spectrum observation at 20" were repeated alternately; it was found that the ratios of the intensities of the bands remained nearly the same during their shrinkage owing to the successive evacuations. Also in the other cases using different samples or different sample treatments in the course of this work, these three bands had always behaved as a unit. We regard, therefore, that these bands arise from a single species on the surface. We may then eliminate the NH radical from immediate consideration because there are two bands of stretching vibration and the band of bending is very strong. III. Further Experiments with Ammonia. Whether the adsorbed species is in the form of NH3 or NHz is now considered. If it be NH3, assuming it has the same degeneracy as the free ammonia molecule, the bands which should appear in the region now studied agree in number with those that are observed, Le., two stretching bands and one bending band. The numbers of bands the amino radical gives are also the same, however. Also, it appears difficult t o distinguish clearly NH8 from NH2 merely on the basis of locations of the bands. Therefore, in order to determine which species was involved, we carried out an experiment using partially deuterated ammonia as the sample gas. It was impractical to use any one of the pure deuterated species alone, e.g., NH2D alone containing no NHDz or ND3, because ammonia molecules easily exchange their hydrogen atoms with each other on glass or silica surfaces at room temperature. Accordingly, a mixture of four kinds of ammonia, "3, NHzD, IYHDz, and NDa, equilibrated in the isotope exchange, was used. In this case, if the molecules are adsorbed without decomposition, the surface species produced must be of four kinds, whereas, if they are adsorbed as amino radicals, only the three kinds of "2, NHD, and ND2 are to be formed. It is expected
3000
s"
NHZD
"4
'II ' 11
I
ND3 "e
NHD N D,
II I
FREQUENCY (cm-I) 2000 I
I II II
I II
Figure 4. Locations of the absorption bands of variously deuterated ammonia molecules and amino radicals, shown to be compared with Figure 5.
IRSTUDIES OF INTERMEDIATES OF THE AMMONIA SYNTHESIS ON IRON FR E O U E N C Y
46 1
(ern-')
Figure 5 . Spectrum taken, for the purpose of identifying the surface species given by ammonia, using a mixture of “3, NHzD, NHD2, and NDa.
therefore, that the number of bands that will appear is far greater in the former case and that we can distinguish the surface species by the number of observed bands. For convenience, in Figure 4 are shown the locations of the absorption bands of the four kinds of ammonia and the three kinds of amino, cited from the literature (the data for ammonia are of solidified ammonia6 and for amino those of aniline are used’). Each of the four kinds of ammonia has its symmetrical deformation band below 1000 cm-l, but that region is omitted in the figure. I t cannot be expected, of course, that ammonia or amino as adsorbed species give bands exactly coincident in frequency with those in Figure 4, but the figure will be useful in viewing the numbers and the approximate locations of the bands that are to arise from similar modes of vibrations of these adsorbed species. I n the adsorption spectrum , there should also appear absorptions due to the modes of motion of the NH3 or NH2 group moving as a single rigid body, but it is almost certain that they do not occur a t as high frequencies as appear in Figure 4. In relation to this inference, we may refer to data of analogous vibrations in various amines and ammine Incidentally, as for ammine in the complexes, the main features of its spectrum in the regions shown in the figure are generally the same as those of ammonia, except that its symmetrical deformation band is shifted up to near 1300 C ~ - ’ . ~ ~ ~ Therefore, O if we take it for the basis of comparison, we must only consider the possibility that such NH3 might give still more bands in the bending region now observable. I n Figure 5 is shown the result of the experiment using the mixture (total pressure 14 cm). In the N-H stretching region, absorptions are perceived a t 3380, 3330, and 3290 cm-l, apart from the one at 3450 cm-1 arising from the adsorption to silica, two of the initial three being already noted in the case of NHs alone. In the N-D stretching region, there are at least two bands attributable to species on iron, a t 2515 and 2430 cm-l, the former again being already noted in a preceding experiment. I n comparison with Figure 4, these results support amino for the surface species. However, these data might appear unsatisfactory, owing to
bands overlapping too much or lowered spectrometer sensitivity in a part of the region. We shall examine the bending region below 1700 cm-l. The four kinds of ammonia have six bands in all, due to deformation vibrations degenerate or split in the region 1700-1100 cm-l, as seen in Figure 4. Even if the adsorbed species be ammonia, however, all these six bands corresponding would not be observed in the present experiment, since also in that case the lower band of NHDz and the band of ND3 probably occur outside the observable range. Thus the number of bands to be observed when the species is ammonia is most probably four. If it should happen that the lower band of NHzD is shifted downward as much as 100 cm-’ by adsorption, it also goes out of the observable range; even in that case, three bands to be attributed to ammonia on iron must be observed. I n Figure 5, absorptions in this region are found a t 1610,1550, and 1400 cm-l, among which the 1550-cm-l band is already proved to be due to the adsorption to silica. This 1 5 5 0 - ~ m -band ~ is far weaker than the other two and never large compared with that observed when NH3 alone was used. In the region 17001400 cm-’, the sample had high transparency and the instrument high resolution, by virtue of which the measurement in this region was quite precise, with no probability of mistaking overlapped bands for a single band. Thus the number of the bands due to species on iron in this region is certainly two, indicating that the surface species in question is not ammonia but the amino radical. The frequencies of the observed bands also are fit for the assignment to amino.12 The gas used in this experiment was prepared by mixing an equal amount of NH3 and NDs and bringing (6) F. P. Reding and D. F. Hornig, J. Chem. Phys., 23, 1053 (1955). (7) J. C. Evans, Spectrochim. Acta, 16, 428 (1960). (8) J. E.Stewart, J . Chem. Phys., 30, 1259 (1959). (9) M. Tsuboi, Spectrochim. Acta, 16, 505 (1960). (10) K.Nakamoto, “Infrared Spectra of Inorganic and Coordination Compounds,” John Wiley and Sons, Inc., New York, N. Y.,1963. (11) R. J. H. Clark and C. S. Williams, J. Chem. SOC.,Sect. A , 1425 (1966). (12) L. J. Bellamy, “The Infra-Red Spectra of Complex Molecules,” 2nd ed, Methuen Co. Ltd., London, 1964.
Volume 78, Number d
February 1988
TOSHIKO NAKATA AND SANJURO MATSUSHITA
462
-0,l
'
1
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0
m
1
1
'
"
I
I
"
"
I
-
c
"
"
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1
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I
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: ,
\
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Figure 6 . Spectrum of surface species on iron, obtained after heating the iron with hydrogen and nitrogen, The four bands are attributed to the NH2 and NH radicals. For the curve lying in the negative region of A , see the text.
it into equilibrium in hydrogen exchange,13so that the ratios of NH3, NHzD, NHDz, and NDI in it must be approximately 1:3 :3 : l.14 From this, it follows that NHD produced on the surface was about twice as much as NHZ (ignoring the isotopic reaction-rate difference), but it must be remembered that the specific absorption coefficient is smaller in NHD than in NHZ. I V . Experiments with .Mixtures of Hydrogen and Nitrogen. The gas of a 3: 1 mixture of hydrogen and nitrogen at a total pressure of 30 cm was admitted to the cell containing the prepared sample (25 mg/cmz thick). The sample was heated at 300" for 3 hr and then a t 400" for the same period; immediately after each heating, the spectrum was checked, with no particular change observed. In the spectrum xeasured after heating at 500" for 30 min, however, absorptions attributed to adsorbed species appeared. The A curve obtained from this last observation is shown in Figure 6. In the present experiments with hydrogen and nitrogen, unlike those with ammonia, measurements of the spectra after admitting the gas was usually made with all the gas left in the cell. I n the above experiment, formation of ammonia in the cell was not detected. In the spectra measured after the heatings, there were observed no absorptions of gaseous ammonia itself, nor the absorptions due to ammonia physically adsorbed to silica which should appear strongly when ammonia gas is present, in the cell, nor those hitherto indicated by s.lS All these facts can be seen also in Figure 6. The bands appearing in Figure 6, except the one at 3200 cm-l, are all in agreement with those appearing in Figure 2. (The swelling around 3600 cm-I is effected by silica in the sample. When the sample of silica alone, which has been well evacuated at 570", is heated with hydrogen, spectral changes occur at the 3750-cm-l OH band and its base. The same changes gave this swelling.) Many attempts were made to discover the set of experimental conditions under which the 3200cm-l band alone arises. These were not successful, but it was found that the intensity of this band relative to the others varied considerably under varied conditions. The Journal of Physical Chemistry
As for the origin of this fourth band, in view of the fact that it appears independently of the other three (that are due to the NH2 radical), and singly near the low-frequency edge of the N-H stretching region, it is reasonable to attribute it to the i\" radica1,ll which is one of the most probable species to be formed on the surface. It is also known that the bending band of the NH radical is so weak that in many cases it cannot be detected. A test was made in which the sample was left at 300" for 100 hr with the same sample gas of the same pressure as was used in the short-term experiment. In the spectrum measured after the heating, unquestionable increases in absorption, smaller than those in Figure 6, were detected at the frequencies where the NHz bands occur. In contrast, the same amount of increase as these was found after 10 min of heating a t 500". If, based on these facts, "the activation energy of formation of surface amino" is computed by simply taking the logarithm of the ratio of 10 min to 100 hr, it comes to about 28 kcal. The significance of this figure, however, should be reexamined after making kinetic studies that permit further analysis. Another fact to be noted in Figure 6 is that, in the region higher than 2000 cm-l, A is negative; its absolute value is greater with higher frequency, except for in the region of the three peaks. This is an inevitable effect produced after heating the sample with hydrogen and nitrogen and is not a contingent experimental error. Also in this experiment, the final evacuation at 550" for 1 hr restored the spectrum of the sample completely to that measured before gas admission; this description means that, if said on the A curve, not only did the four peaks vanish but over the whole frequency range A returned to zero by that evacuation. A conceivable reason for the negative A observed (13) The pretreatment of adsorbent for the silica hydroxyl was also made with this mixture. (14) More exact values of these ratios determined by statistical mechanics are 1: 2.51 : 2.73 : 0.92, to which figures the vibrational partition functions contribute negligibly. (15) Under the conditions of this experiment a t 500°,the equilibrium pressure of NHs is on the order of 0.2 mm.
IRSTUDIESOF INTERMEDIATES OF THE AMMONIA SYNTHESISON IRON was the temperature difference that might exist between the bare and the gas-adsorbed samples, since when their spectra were measured, the former was under vacuum and the latter in the gas while receiving the incident beam. This difference in temperature must cause a change in the conduction-electron absorption in the iron. Theoretically,le it can be shown that, if A is determined solely by that change, A is negative at about 3000 cm-l and its frequency derivative is also negative. Experimentally also, it was confirmed that the "transmission" of a bare sample decreased there with elevating the temperature, at least up to 150",despite increasing radiation from the sample itself. Of course, negative A results from the sample transmission increasing by contact with the gas. However, if the temperature difference is the sole cause for the observed transmission increase, no such increase is to be observed if the spectrum of the gas-adsorbed sample is measured without the gas-phase gas. In fact, however, if the spectrum is measured again, this time with the cell reevacuated, it is found that the increase over this range remains, together with the four bands, and is diminished only to a small extent (1/10-3/~0 of the initial increase). The major reason for this transmission increase cannot definitely be explained at present. It may be a prerequisite to analyze quantitatively the mechanism of an infrared beam being transmitted through a sample that involves dispersed fine particles of a metal.
Discussion Regarding Intermediates For the reaction producing gaseous ammonia from gaseous hydrogen and nitrogen on the catalyst, there are a good many conceivable reaction routes. It was found in this work that NH2 and NH can be formed on the iron surface. The surface species NH2 is denoted by NH2(a) hereafter. These are species that occur on some of the above conceivable routes. (Really, they have frequently been assumed in kinetic studies of this reaction.) Since, however, the existence of a speciesmay not necessarily mean existence of a route involving the species, we shall consider the possibility that the reaction occurs via the observed species. At this time discussion will be confined to only NH2(a) that was observed in every experiment in this paper. First, we shall summarize the experimental facts relating to the growth or shrinkage of the NH2(a) bands, mainly those not yet mentioned. I n the experiment with ammonia, under the conditions where generation of hydrogen and nitrogen was not detected, distinct absorptions of NH2(a) were observed in the spectrum measured 1 or 2 hr after the gas admission; a measurement made after leaving the sample for 1 day with readmitted gas of the same pressure showed that these absorptions had not become larger than observed a t first. Although they were not removed by the 20" evacuation, they shrank a t an appreciable
463
rate by evacuation at temperatures as low as loo", while a t 300" an evacuation for 1 hr removed them completely. When the mixture of hydrogen and nitrogen was used, the bands observed after heating a t 500" for 30 min were larger than those after the initial 15 min, but after 1 hr no growth of the bands was perceptible within the accuracy of the measurement. When these observations were made, ammonia had not been formed. I n this experiment, it was noted that the bands disappeared by evacuation at 300" for 30 min. Even if "%(a) is easily formed, if it is so stable as not to change further, it is not reasonable t o assume it an intermediate. However, NH2(a) was removed by the evacuation with ease as mentioned above. This indicates that individual NH2(a) molecules are always speedily changing to others at such temperatures as those used in the evacuation. The fact that the growth of NH2(a) bands almost ceased in a certain time probably should be accounted for, not by the species covering the whole surface, but by the balance of formation and consumption. Though variations in concentration of the surface species with temperature also comes into question, NH2(a) must exist on the surface, even if in small quantities, at higher temperatures." Now, since it is found that h"z(a) can be formed both from ammonia and from hydrogen and nitrogen and besides that NH2(a) so formed changes to evaporable species again, we may recognize the existence of a reaction route connecting ammonia with hydrogen and nitrogen which passes through NH2(a). Indeed, it is also possible that NHZ(a) is not an intermediate but a blind-alley product which is formed from a (set of) species on the route and is in equilibrium with it. However, since hydrogen and nitrogen gave NH2(a) without producing ammonia, this supposition must involve the assumption that there does not exist the elementary reaction that directly connects NH2(a) and ammonia (adsorbed or gaseous). This assumption seems very unnatural. Of course, there has never been observed the least sign that would suggest that when NH2(a) is produced from ammonia any other species exists as an intermediate between them. It is true that, even if the existence of a route is recognized, whether it is the main route of the reaction is another question. However, which is the main route depends, in principle, on the conditions under which the reaction proceeds. For these questions to be answered exactly, a large number of infrared measurements made in the method of reaction kinetics are required. However, since in the course of this work no (16) E.Q., F. Seitz, "The Modern Theory of Solids," McGraw-Hill Book Co., Inc., New York, N. Y., 1940. (17) Observations of the "$(a) bands made a t temperatures where
the reaction proceeds appreciably will be reported in a separate paper. Volume 72, Number d
February 1.968
JAMESN. SPENCER AND ADOLFF. VOIGT
464 evidence has been found to suggest that the decomposition or synthesis of ammonia occurs faster than the formation of NHz(a), there is a strong probability that NHZ(a) is an intermediate in the main route under con-
ditions not widely different from those in this work. Anyway, to ascertain as many intermediates and routes as possible would be an approach to the systematic analysis of the mechanism of reaction.
Thermodynamics of the Solution of Mercury Metal. I. Tracer Determination of the Solubility in Various Liquids’ by James N. Spencer and Adolf F. Voigt Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 60010 (Received April 8,1967)
The temperature dependence of the solubility of mercury in 17 solvents has been measured using a radiotracer technique. The solubilities at 25” were found to vary from about mole/l. for aromatic and aliphatic hydrocarbons and ethers to about 10-7 mole/l. for water and perfluorodimethylcyclobutane. The solubility parameter equations developed by Hildebrand and Scatchard give calculated values of the mercury solubilities at 25” which agree well with the experimental in many systems. That this is so appears to result from a cancellation of several factors, since these equations do not take the metallic nature of mercury into account. The partial entropies and heats of solution of mercury were determined from the temperature dependence of the mercury solubilities. The experimental entropies were found to differ from ideal.
Introduction The limited solubility of mercury metal offers an excellent system for an appraisal of solute-solvent interactions. The solutions formed are sufficiently dilute so that solute-solute interactions are negligible. As it is a monatomic solute, mercury offers no rotational or vibrational complications and it also does not react chemically with the solvent. A determinfition of the solubility of mercury as a function of temperature permits the evaluation of most of the thermodynamic properties of mixing. From these properties, a comparison with theory may be made, and an elucidation of the interactions between solute and solvent is possible. The theory of regular solutions as developed by Hildebrand is perhaps the most widely used for predictions of the properties of nonelectrolyte solutions. The experimental data obtained are compared to the predictions of solubility parameter theory and regular solution theory. The data are also used in the following paper to obtain heats and entropies of vaporization of the solute.2 of the solubility of mercury Early were made by amalgamation or electrodeposition of the The Journal of Physicat C h m k t r y
dissolved mercury or by calorimetric methods. Moser and Voigte and then Klehr and Voigt7 measured mercury solubilities by determining directly the specific activity of the solution using a radiotracer technique. Kuntz and Mains8 more recently determined the solubility of mercury by combining optical data with a known mercury solubility. They report that saturation was obtained in 20 min, while the solutions of this work were found to require about 24 hr of shaking for equilibration.
(1) Work performed at Ames Laboratory of the U. S. Atomic Energy Commission, Contribution No. 2072. (2) J. N. Spencer and A. F. Voigt, J. Phys. Chem., 7 2 , 471 (1968). (3) H. Reichardt and K. F. Bonhoeffer, 2. Physik, 67, 780 (1931). (4) A. Stock, F. Cucuel, F. Gerstner, H . Kohle, and H. Lux, Z. Anorg. Allgem. Chem., 217, 241 (1934). (5) J. C. Pariaud and P. Archinard, Bull. SOC.Chim. France, 454 (1952). (6) H. Moser and A. F. Voigt, United States Atomic Energy Commission Report ISC-892,Iowa State College, Ames, Iowa, 1967. (7) E. Klehr and A. F. Voigt, “Radioisotopes in the Physical Sciences and Industry,” Vol. 3, IAEA, Vienna, 1962, p 617. (8) R. R. Kuntz and G. J. Mains, J . Phys. Chem., 68,408 (1964).
