1662
NOTES
n
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nitrate anions. The formation of nitrate complexes,
CKi[A]2-1so this plot extrapolates to F(A) = K1 or relatively stable ion pairs, with the foreign cation z
obviously will have an effect on the amount of CdN03+ formed from a given total amount of nitrate. If comparisons are confined to systems of the same charge types it is possible to test the reasoiiableness of this conjecture. It is easily seen that the stability constant decreases as the complexing power of the foreign cation increases for Groups Ia and IIa cations. This is also supported by the general tendency of the dissociatioll constants of the nitrate salts to fall into groups according to the valency type of the salts ~0nsidered.l~The order fouiid for Groups Ia and IIa cations is the same as that reported by Fialkov and Spivakos~ski.~ S U M M ~ OF R YRESGLTS OF POTENTIOMETRIC TITRATION OF The equilibrium constant for C d S 0 3 +was deterCERTAIN SITRATE SALTS WITH CADMIUM( 11) mined a t 25, 35 and 45’ for titrating solutioiis of Salt Temp., OC. Ki(CdN0a +) sodium nitrate and of magnesium nitrate. From 25 1.063 Sr( the slope of a plot of log K , vs. 1 / T (‘IC), AHo for Ca(h’Od4 25 0.904 the reaction v a s determined. The data of Table I WN03)2 25 .817 show that there is a very small temperature de35 .794 pendence and therefore a very lov AHo. The 45 .813 AHo values are -7.7 cal./mole and - 160 cal./mole iVaS03 25 ,747 for magnesium nitrate and sodium nitrate, respec35 .618 tively. Since K1 is very close to unity the free 45 .616 energy changes are also rather small in most of Nd(S03)S 25 .550 these cases. Thus for the sodium nitrate titration, -411XO& 25 .539 AFO = +176 cal./mole, AHo = -160 cal./mole 25 .533 La(NO8h and ASo = - 1.1 e.u. The AFO values range from LiN03 25 ,142 -36 cal./mole for strontium nitrate to $1160 The number of nitrate salts which could be used cal./mole for lithium nitrate. In coiiclusion, this study shows that the measured in the titrant solutions was restricted by several factors. Any metal ion which is able to oxidize the stability constant of the CdN03+ ion is dependent cadmium in the amalgam will lead to erratic results upon the specific nitrate used as the source of the (such were obtained with lead nitrate solutions). added nitrate ion. This shows the limitation of the Furthermore, only those nitrates were used in common assumption that, within rather broad which it is customarily assumed that the cation limits, the cation added in the titrant solution has complexes the nitrate to a “negligible” degree. no effect on the measured stability constant. E’inally, the problem of insolubility limited the It proves that even in the presence of very high conselection. Barium nitrate has a sufficiently small centrations of a swamping electrolyte, such an efsolubility in water that it is impossible to prepare a fect is present and may affect the measured stability titrating solution of anywhere near the required constant by as much as an order of magnitude. concentration. Potassium nitrate may not be used The previous report^^-^ of such phenomena seem because of the insolubility of potassium perchlorate. to be most reasonably reinterpreted in terms However, even with these limitations a reasonably of a constancy of behavior of the principal coordination center and a variation arising from changes representative series of nitrates was studied. The fact that the stability constant for CdSOB+ in the species present in the added solutions. These is affected by the presence of foreign cations is not include changes in activity coefficients due to unexpected if the charges on these are different. specific interactions. We wish to acknowledge, with thanks, the That such differences should also he fouiid with very closely related ions of identical charge is also assistance furnished in preliminary studies by the understandable and recalls Brgnsted’s concept of late hIr. J. Davis Sibley, Jr. This work 11-as supthe specific interaction.12 In the present case the ported by a grant from the U. s. Atomic Energy specific interactions of the various cations with the Commission, At-(40-1)-2676, for which we wish to nitrate ioii seem to be the most obvious cause of express our gratitude. the small, but definite and reproducible, variations (13) C. W. Davies, Endemour, 4, 114 (1945). found for the stability constant. In addition to thermodynamic factors (Le., activ- OBSERVATIONS Oh- THE DECO3IPOSITIOS ity coefficient variations), the change in the OF X-RAY IRRADIATED ,1Ptl3\IONTUM measured (apparent) stability constant of the CdPERCHLORATE K03+ ioii may be considered to arise in part from BY ELI S. FREEMAN AND DAYID -4.ANDERSOU a competition between Cd +2 and the foreign I’iii olri l i i i i c s Chcmical Rcsearch Laboratory, Picatinnu I. iseiaal, I l o b c i , cation of the titrating solution for the available S e w Jersey
(the stability constant of the first complex) when [A-] goes to zero. The stability constant for the CdKO3+ ranges from 0.142 for lithium nitrate as titrant to 1.06 for strontium nitrate as titrant. An approximate value for this constant was determined previously by Ledeng who reported a value of 1.29 a t 25’ using sodium nitrate in the titrating solution. Our values under these same conditions were 0.748 and 0.746 in duplicate determinations. A more recent polarographic study resulted in a n l u e of 0.62. l1 This was available only in abstract and the titrant was not specified. TABLE I
(11) Hung-Chi Chiang and Kuang-Hsien Hsu, K’o Hsueh Tung Pao, 397 (19591, C . A , , 66, 3241 (1961). (12) J. N. Br@nsted,J . Am. Chem. Soc., 44, 877 (1922).
Reccized A p i z l 67, 1381
Photomicrographs by Bircumshaw and Newnianl
Sept., 1961
SOTES
1663
of undecomposed and partially decomposed ammonium perchlorate crystals showed that a general clouding of the surface occurs as the crystals are 20 heated. Initially formed hemispherical opaque areas on the surface coalesce as the reaction prou gresses. These observations apparently were made under reflected light. Additional details of the surface decomposition, obscure under reflected light, may be observed readily under transmitted light. This vias found for undecomposed and partially decomposed ammonium perchlorate crystals heated a t 200°. The clouding described previously i s observed in photomicrographs taken under reflected light. Cnder transmitted light, however, additional surface structure is evident. 0 The disruption pattern on the surface of partially I 1 I I I decomposed crystals indicates preferential regions 200 260 300 350 400 of reaction along intermosaic boundaries and dislocations. In previous papers2 3 it was shown that Reference temp., "C. exposure to X-ray and y-ray radiation profoundly Fig. 1.-Differential thermal analysis at 4.4'/1nin. of 500 affects the thermal decomposition of ammonium mg. of ammonium perchlorate containing 1 mole % KCIOI. prepared by evaporating solution t o near dryness perchlorate. This effect was determined by meas- Sample and by oven drying a t 105'. The ammonium perchlorate uring loss in weight and by differential thermal mas doubly recrystallized Fisher reagent grade material. analysis. The appearance of the decomposing irradiated crystal is significantly different from the decomposing unirradiated sample. This was ob- decomposition of ammonium perchlorate containserved microscopically under reflected light. The ing C103- ion impurity occurs only following crystalcrystal was irradiated with an OEG X-ray tube line transition. Our experiments show that significontaining a molybdenum target for hr. at a cant exothermal reaction does occur prior to crystaldose rate of 1.3 X lo6roentgen per hr. On heating, line transition as is demonstrated in Fig. 1. The the striking contrast between the behavior of the apparent discrepancy may be in part due to the unirradiated and irradiated samples is that, whereas higher rate of heating used in the differential therin the former initial reaction occurs primarily a t mal analysis (DTA) experiments of the referenced the surface, the irradiated sample undergoes reac- authors.5 I n the referenced article it is also stated tion throughout the entire crystal. The surface that complete decomposition occurs during the of the irradiated crystal does not appear to be dis- exothermal reaction following crystalline transirupted or to reveal preferential sites of reaction as tion for samples containing more than 0.1 wt. yo in the case of the unirradiated .oljd. The apparent C103- ion impurity. Our DTA results shown ill homogeneous clouding of the entire crystal indicates Fig. 1 demonstrate that reaction does not go to that nuclei for reaction were produced by radiation completion during this exotherm but a t approxithroughout the entire crystal. mately 450'. IJndoubtedly the referenced obserBy differential thermal analysis it was found that vations is due to a relatively large sample size (not animoniuin perchlorate sublimate exhibits de- specified in the referenced article) and is not a true romposition characteristics similar to the irradiated chemical phenomenon. The fact that the exosample. Recently Hyde and Freeman4 have therm following transition represents a stage of identified the presence of NH,+ radical in irradiated reaction which is a function of temperature seems ammonium perchlorate by electron spin resonance. to support the idea that C103- ion provides interKeither the sublimate nor partially decomposed mediate species which may act catalytically. ammonium perchlorate exhibited resonance spec- Nevertheless the DTA decomposition pattern of tra. The sublimate did not contain NH3+radical, irradiated animonium perchlorate is strikingly although its chemical reactivity is similar to that similar in detail to that of the sublimate and of amof the irradiated crystal^.^ These results imply monium perchlorate containing C103- ion imthat radicals or positive holes may not entirely purity . account for the changes in the decomposition deInterestingly the presence of C103- ion in irraditails of irradiated ammonium perchlorate, a pos- ated ammonium perchlorate JTas confirmed by thc sibility previously suggested.3 K I test for C103- ion but it mas not found to br It was recently shon-ii that C103- ion may give present in the sublimate. This is in agreement with rise to a sharp exothermal reaction following the previously reported data6 on the decomposition crystalline transition of ammonium perchlorate.5 products formed during sublimation. ConseIt was reported in the qame paper that exothermic quently, although the decomposition characteristics (1) L. L. B i m m s l i a a and E. 11. Newman, l'roc. R o y . S o e . ( L o n d o n ) , of the irradiated salt may be partially explained 237.. 228 mm. . by the presence of the C103- ion, presumably iii ( 2 ) E. S. Freeman and D. A . Anderson, J . Pkys. Chem.. 63, 1344 solid solution, this does not account for the details (1 959). of the decomposition of ammonium perchloratc ( 3 ) E. S. Freeiiian arid D. A. Anderson. ibid., 64, 1727 (1960). ( 4 ) J. S. Hyde a n d E. S. freeman, i h d , 66, 163G (1961). sublimate. If the mechanism of the decomposition ~~
( 5 ) J. C. Petticciani, S.E. Kiber, W. H. Bauer a n d T. W. Clappcr, ibid., 64, 1309 (1960).
(6) H. M. Cassel and I. Liebmnn, J Chem. Phys., 34, 343 (1961)
1664
COMMUNICATION TO THE EDITOR
of irradiated ammonium perchlorate below 300’ involves C102 radical as an intermediate, then the role of C103- ion may principally be that of a source of radicals. The presence of C103may also, in part, account for the seemingly uniform reaction
~
~
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throughout the irradated crystal when heated a t 200’. Acknowledgment.-The authors wish to thank Dr. James Hyde of Varian Associates for conducting the electron spin resonance experiments.
~
COMMUNICATION TO THE EDITOR EVIDENCE FOR ELECTRONlC INTERACTION BETWEEN IODINE AND A SOLID SURFACE Sir:
The presence of an electric field a t the surface of a solid has been inferred from certain phenomena associated with the solid-gas interface : namely, surface potentials’ and a significant reduction in the intermolecular attraction within the adsorbed film,2J which has been traced to the effect of parallel-oriented induced dipoles. The surface fields for various solids have been estimated to lie in the range lo7 to los volts em.; a field of this order of magnitude would be expected to affect the electronic absorption spectrum of an adsorbed molecule in one or more of the following ways: shifts in the electronic states due to a highly polarized condition of the molecule; splitting of the electronic energy levels, L e . , a molecular Stark effect; ionization of the adsorbate or partial charge transfer with the conduction bands of the substrate. Since iodine is known to enter into charge-transfer complex formation readily and since its major absorption band is in the visible range, its use as an adsorbate seemed likely to offer a method whereby perturbations of its electronic spectrum by the surface could be observed easily. Preliminary experiments with adsorbed iodine have shown that the expected interaction does take place, with marked changes in the visible spectrum and in some systems with the production of strong absorption in the ultraviolet region. Spectral measurements from 300 to 700 mp were made a t room temperature by reflectance from powdered solids containing adsorbed iodine; in all cases the adsorption was reversible, the iodine being desorbed readily by evacuating the sample a t 100”. The quantity of iodine adsorbed was known only approximately, but was always less than a complete monomolecular layer; nevertheless, the colored adsorbed film was clearly visible and its departure from the violet color of iodine vapor in many cases could be detected a t once by the eye. The material that displayed the least effect among those observed is silica (when freed of adsorbed water). The principal absorption band (1) J. C. P. Mignolet, “Chemisorption.” ed. W. E. Garner, Butterworth, London, 1957, p. 118. (2) J. H. de Boer, “The Dynamioal Character of Adsorption,” Clerendon Press,Oxford, 1953,pp. 168-169. (3) E.Clark and 8.Ross,J. A n . C b m . Soc., 76,6081 (1953).
of iodine vapor, centered about, 520 mp, is shifted to about 480 mp. One specimen of silica in the form of a transparent, porous slab was suitable for transmission measurements down to 220 mp: adsorbed iodine on dry silica showed no ultraviolet absorption. Silica-alumina cracking catalysts also show an absorption maximum in the visible region a t around 450-480 mp, but have developed an increasing absorption below 350 mp. We could not investigate the absorption a t shorter wave lengths with our present apparatus. The samples mentioned above were violet to pink in color. Samples of pure alumina and boron nitride showed a gradual increase of absorption with decreasing wave length below 700 mp; they did not, however, display any absorption maximum in the visible region. The wsible color of these systems is bright yellow to brownish. A slight difference between y- and 11-alumina is apparent to the eye: the slightly redder tint obtained with y-alumina plus iodine can be traced to its greater absorptioii in the green portion of the spectrum. On a third group of solids the adsorbed iodine film is characterized by having only a slight absorption in the visible spectrum, thus appearing white or faintly yellow in color. These substances include the alkali halides, calcium fluoride, and zirconium and titanium oxides. The reflectance spectra show sharply increasing absorption below 350 rnk. The colors formed by the interaction of iodine with various solid surfaces is reminiscent of thc colors of iodine in various solvents. The greatest changes in the absorption spectra of iodine solutions are found with the more polar solvents, and presumably the same rough correlation exists with adsorbed iodine on surfaces of differing polarity. Furthermore, the charge-transfer complex mechanism postulated to account for the spectra of certain iodine solutions is also a reasonable explanation of the spectra of the adsorption complexes: if this is the case, the study of the spectra of adsorbed iodine should yield important information about the electronic nature of solid surfaces, which mould be pertinent to their action in heterogeneous catalysis. A more detailed study of such systems is now in progress. This work forms part of a program sponsored a t Rensselaer Polytechnic Institute by Esso ReResearch and Engineering Company. RENSSELAER POLYTECHNIC INSTITUTE SYDNEY Ross TROY,N. Y. JAMES P. OLIVXER JUNE 20, 1961 RECEIVED
