1484
D. M. HARRIS,M. L.NIELSEN AND GORDON B. SKINNER
Implications concerning the mechanism of corrosion inhibition of F e can, of course, only be made here on the assumption that a similarity exists between the mechanism of adsorption of the aromatic amines on Hg and on Fe, supported by the parallelisms shown in Fig. 11. On this basis, it appears that the most necessary structural property of an inhibitor of the aromatic amine type in acid eolution is simply that of a high degree of adsorbability and that specific attachments to "active centers" on the electrode surface are not of primary importance. The degree of adsorbability on H g appears to depend mainly upon the degree of conjugation of the molecules. These considerations of the molecu-
Vol. 63
lar structure required by an inhibitor must clearly be balanced against other secondary features, c.g., solubility. Acknowledgments.-The authors' thanks are due to E. I. du Pont de Nemours and Company, Inc., and to the Esso Research .and Engineering Company, who individually gave financial support for different portions of this work. The authors wish to express their appreciation to Professor A. R. Day and Dr. F. Brutcher for discussion of electronic aspects of organic structures, Their thanks are also due to Mrs. Maire Blomgren and to Mrs. Catherine Jesch for assistance in the experimental work.
THERMAL DISSOCIATION OF TiI? BY D. M. HARRIS,M. L.NIELSEN AND GORDON B. SKINNER Research and Engineering Division, Monsanto Chemical Company, Dayton, Ohio Received March 2 , 1960
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The principal reaction of the Ti-I system a t 1723 "K. and ressures of 3 to 15 mm. is Ti12(g) = .Ti(!) 21(g). Under these conditions K , = 8.2 x 10-2atm.; A.F" = 8.6 f 0.6 kcaf/mole. The calculated heat of reaction is 49 f 4 kcal./mole based on a calculated molar entropy of 101 f 2 e.u. for titanium diiodjde gas a t 1723 OK.
Introduction Thermodynamic data have been published for the titanium iodides only in the range 750 to 902 The present work was undertaken to obtain thermodynamic data at a higher (1723 OK.) temperature, a t which titanium metal can be deposited. Other investigators have done qualitative work in this temperature range. 2-4 Experimental Materials.-Titanium tubing was obtained from the Superior Tube Company. Titanium rod, obtained from the same source, was turned on a lathe to prepare 0.005 inch thick metal turnings. Titanium diiodide was made by heating Ti11 or Izwith a 10-fold excess of titanium metal at 550O.6 Apparatus and Procedure.-Equilibrium mixtures of TiI4, lower iodides and titanium metal for 1723°K. and two different pressures were obtained by vaporizing the lower iodides in the presence of titanium turnings. The resulting mixtures were swept into a collection zone and analyzed to obtain the titanium-to-iodine rat>ios. The apparatus consisted of a 0.75 inch by 8.5 inch long titanium tube closed a t one end, surrounded by a quartz tube (37 mm. 0.d.). The quartz tube was connected to a vacuum source and auxiliary equipment for maintaining a controlled pressure over the range 1 to 25 mm. (i0.1 mm.). For a run, about 0.8 g. of Ti12 was charged into a small titanium holder and placed in the bottom of the larger titanium tube. The larger tube was filled with titanium turnings and the tuhe covered by a cap having an '/e inch hole in the center. The cap prevented heat loss bv radiation from that end of the tube- The capped end of the tube was heated first by an induction coil 8 inches in length to brine the metal to the desired temperature. The coilhas then gradually lowered to Gaporize the iodide. There was, of course, a severe temperature gradient in the bottom half of the tube, so that the bottom end was hundreds of degrees cooler than the top part. The ( I ) A. Herceog and L. Pidgean, Can. J. Chem., 84, 1687 (1956). (2) 0. Runnalls and L. Pidgeon, J . Metals, 4, 843 (1952). (3) A. Loonam ( t o Chilean Nitrate Sales Corporation), U. S. 2,694,652 (Nov. 16, 1954) (4) A. Loonam ( t o Chilean Nitrate Sales Corporation), U, S, 2,694,653 (Nov. 18,1954). (5) J. Fast, Rec. trau. chim.,58, 174 (1939).
rate of evaporation of iodide could be controlIed by adjusting the position of the induction coil relative to the tube, by use of a screw mechanism to give smooth, accurate positioning. During the operation, a stream (about 36 ml./min. at S.T. P.) of argon purified by gettering with titanium at 9001000°~was swept past the titanium tube to transport the mixture of iodine and iodides issuing from the tube into a Dry Ice-cooled collection trap. The progress of the reaction could be followed visually by watching the build-up of deposit in the Dry Ice trap, and the flow of reaction products being swept along in the tube leading to the trap. The time a t which reaction started was noted for each run, and t h e position of the induction coil was adjusted from time to time to give a roughly constant rate of product evolution throughout each experiment. Finally, the products in the trap were dissolved by treatment with 0.5 M H 2 S 0 4 to dissolve the iodides, followed by ethanol to dissolve free iodine. The combined acid-ethanol solution was analyzed for total iodine and titanium. To determine whether the iodine-to-titanium ratios were equilibrium values, the rate of vaporization of the iodide was varied. Table I lists the average rate of flow of vapors over the hot titanium turnings, in cc./sec. It is important to note that the total volume of the titanium tube was about 40 cc., while the total volume of vapors liberated (calculated as Ti12 I as discussed below) was of the order of thousands of cc. It is clear then that the argon originally filling the tube was swept out by the first few per cent. of vapors evolved. Moreover, all of the vapors emerged through a hole of about 0.08 sq. em. area with a flow velocity varying between 14 and 300 cm./sec., which seems high enough to prevent diffusion of argon back into the tube during the experiment, While the total volume of the titanium tube was about 40 cc., on1 the top thed was at the uniform 1723°K. temperature, so tce residence time of vapors in the hot zone varied between 0.5 and 12 seconds. Since direct measurement of the reaction temperature during the dissociation was not convenient, the oscillator settings were calibrated in terms of the tube temperature. This was done by heating the tube containing titanium turnings in the absence of iodides and measuring the temperature by means of a calibrated optical pyrometer sighted on the turnings through the hole in the cap. During this operation the pressure and rates of flow of purified argon were the same as for actual runs. The settings were reproducible to within
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(6) M. Mallett. Znd. Eng. Chem., 49, 2095 (1950).
k
1485
THERMAL DISSOCIATION OF TITANIUM DIIODIDE
Sept., 1959
TABLE I SUMMARY O F EXPERIMENTAL DATAANTI CALCULATIONS, TEMPERATURE = 1723 OK. Experiment no.
1
2
3
4
5
Pressure, mm. Total Ti12 charged, g. Total wt. evaporated, g. Total reaction time, sec. Total vol. of gas, cc. Rate of flow of gas, cc./sec. I/Ti atomic ratio K , X 102 for reaction TiI?(g) = Ti(s) 2I(g) K , X 104 for reaction TiI3(g) = 3%) K , X 109 for reaction TiId(g) = Ti(,) 4 W
15.1 1.9 0.0358 1,500 1,640 1.1 6.96 8.2
16.2 0.8 0.313 2,300 14,200 6.2 6.60 7.5
15.1 0.7 0.667 1,200 29,300 24.4 5.53
3.4 0.9 0.223 5,000 51,800 10.4 20.4 7.8
3.4 0.8 0.161 4,500 37,800 8.4 23.4 9.1
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10.0
8.7
3.1
3.7
9.7
7.7
1.2
1.5
=t5 “K. For purposes of calculation, the error was assumed to be f 10”.
In order to establish that black body temperatures were being read, a portion of the titanium turnings was replaced by a titanium block with a hole l / g inch by 8/4 inch to sight on with the optical pyrometer. The temperature reading was unaffected by this substitution. Measurements were made at 1723and 1420 OK.; only the data for the higher temperature are reported since data obtained a t 1420 “K. were not reproducible, presumably owing to inability t80obtain equilibrium mixtures.
Results and Discussion The experimental results are summarized in Table I. Since the lowest I/Ti ratio was 5.53, it is clear that some titanium deposited on the titanium turnings in all experiments. In run 3, with the fastest flow rate, equilibrium does not seem to have been reached, but in runs 1 and 2 the substantial difference in flow rate caused no significant change in composition. A flow rate of about 10 cc./sec., corresponding to a residence time of just over a second, seems slow enough to give equilibrium data within the accuracy of our analyses a t 1723°K. At lower temperatures, of course, a longer time would be required. T o determine which titanium iodide is the major species under these conditions, the equilibrium constants may be calculated. If TiIz is the major species, then equilibrium (a) will govern the comTiIa(g) = Ti@) + 2I(g) (4 position, (since there is no appreciable amount of undissociated 1 2 under these conditions), with corresponding equations for TiI, and TIL. Since the calculated equilibrium constants (see Table I) are nearly constant for equation (a) and not for the others, Ti12 is indicated as the major titanium species in the vapor phase. The free energy of reaction (a), as calculated from the average value of the equilibrium constant, 8.2 X 10+ atm. a t 1723°K. is 8.6 f 0.6 kcal./ mole. The estimate of error is based on a f 10” uncertainty in temperature, and f 10% equilibrium composition. This free energy value is in rough agreement with the value of 21 f about 15 kcal./mole calculated from Brewer’s tables.’ As stated in the Experimental Section, attempts to obtain satisfactory data a t temperatures other than 1723°K. failed. Therefore, the molar en(7) L. Brewer, “The Cheniistry and Metallurgy of MisceUaneous Materials: Thermodynamios,” McGraw-Hill Book Co., Ino., New York, N. Y., 1950, pp. 60-275.
tropy of TiI2(g), 101 f 2 e.u. at 1723 OK., was cdculated using estimated molecular dimensions and vibra.tional frequencies of TnbleII. TABLE I1 MOLECULAR DIMENSIONSAND FREQUENCIES FOR Ti12 Iodine-titanium-iodine an le, degree Iodine-titanium distance, Vibrational frequencicsJncm.-1
I!kTIMATED
1.
VIBRATIONAL
110 2.50
1 320 2 130 3 340 a An error of 30q‘ in vibrational frequencies will give rise to an error of f 2 . 3 entropy units in the molar entropy of TiI?.
Because of its unused pair of “valence” electrons, the TiI, inolecule is probably bent, as are HsO, SO2 and ClzO, which have somewhat similar electronic configurations.8 The average bond angle of these bent molecules is about 110”. Pilcher and H. A. Skinner have also assumed t1i:tt TiCl, is a bent molecule with the slightly 1:trgcr ClLTi-C1 angle of 120°.9 For the Ti-I distance we have assumed a covalent bond. Since the Ti-Cl distance in Tic14 is 2.18 A.l0and the larger covalent radius of iodine is about 0.34 than that of chlorine,” the distance of 2.50 A. seems reasonable. If the Ti12 molecule were ionic, tJhe Ti-I distance would be 2.84 .\., according to I’auling’s tables, but this seems rather unlikely. Even as “ionic” a molecule as Na,Clhas a bond distance of only 2.51 8.in the vapor state,12whereas the sum of Pauling’s ionic radii is 2.76 8., and Heraberg lists many other halides and oxides where the sami effect is observed. In estimating trhe vibmtional frequencies, the known frequencies of TiCl4,I3 TiBr4I3and TiC112 were used, along with informa-
4..
(8) G . Herzborg, “Infrared and Rarnan Spectra of Polyatomic h4olecules,” D. Van Nostrand Co., New York, N. Y., 1945. (9) G. Pilcher and H. A. Skinner, J. Inow. Nuclear Chem., 7 , 8 (1968). (IO! If. W.Lister and L. E. Sutton, Trans. Faradau 9 o e . , 37, 393 (1941). (11) L. Pauling, “The Nature of the Chemical Bond,” Cornell University Press, Ithaca, N. Y., 1940. (12) G. Hereberg, “Molecular Spectra and Molecular Structure. I. Diatomic hlolecules.” Prentice-Hall, Inc., New York, N. Y., 1039. (13) M. L. Delwaulle end F. Franoois, Compt. Tend., 220, 173 (1945); J . p h y s . radium, 7 , 15,53 (1946).
1486
D. H. BRADHURST AND A. S. BUCHANAN
Vol. 63
tion on bent triatomic molecules given by HerzThe AS of reaction (a), 23.4 e.u., was then calberg? The estimates of the vibrational fre- culated using the published entropies of titanium quencies introduces the largest uncertainty into and iodine.14 The heat of reaction, then, is 8.6 the entropy calculation. An error of 30% in the (1723 X 23.4)/1000 or 49 f 4 kcal./mole. frequencies give rise to an error Of *2*3 (14) National Bureau of Standards, Selected Values of Chemical entropy units at 1723 OK. Thermodynamic Properties. Series I11 (1952-54).
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THE SURFACE PROPERTIES OF LIQUID LEAD IN CONTACT WITH URANIUM DIOXIDE BY D. H. BRADHURST AND A. S. BUCHANAN Chemistry Department, University of Melbourne, Australia Received March IO, 1060
The sessile drop technique has been used to measure surface tensions and contact angles of liquid lead on uranium dioxide surfaces. The influence on these properties of the metallic solute bismuth and the non-metallic solutes oxygen, sulfur, selenium and tellurium has been investigated. Oxygen is clearly the most effective in reducing the surface tension and contact angle of the molten lead.
Introduction
ments (magnification 30 times) of these plates, and using Dorsey'sd method of calculation, surface tensions could be reproducibly d:termined to within i2%, and contact angles Figure 1 shows that the measurements to within f 2 taken for calculation of surface tension are independent of the over-all height of the drop and its angle of contact with the plaque surface, and may be preciseIy determined. The equation used was T = gdP(0.05200/f 0.1227 0.0481j) where T = surface tension, dyne. cm.-1 f y/r 0.4142 g = acceleration due to gravity, cm. sec.-2 d = density of the lead, g. crn.-a r = radius of drop, cm. y = diatance from drop apex to the point of intersection of the two 45' tangents.
The present study forms part of an investigation on the stability of suspensions of uranium dioxide in liquid metals. As a preliminary approach to this problem it was decided to investigate the wetting of uranium dioxide by lead, and for this purpose the sessile drop technique of Humenik and Kingery1+2appeared admirably suited, since both contact angle of the liquid with the solid, and the surface tension of the former could be measured in the one experiment. Abrahams, Carlson and Flotow3have used this method for an investigation of the sodium-potassium alloy-uranium dioxide system. The sessile drop method has been adapted to the requirements of the lead-uranium dioxide system, and the influence of variables such as the Lead samples (99.999%, supplied by the Metallurgy Deconstitution of the liquid, the stoichiometry of the partment, University of Melbourne) were prepared in cyhnuranium dioxide, and its density; nature and drical pellet form (3.5 by 6.5 mm.) using a stainless steel . The lower surface of the pellet so obtained was pressure of the surrounding atmosphere, and the emispherical, ensuring a uniform advancing angle of contemperature, have been investigated. tact on melting. The lead surface was scraped clean with a
.
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3
gunoh
Experimental The apparatus consisted of a 6" by 1" dia. molybdenum tube furnace, heated by induction and enclosed within a water-cooled Pyrex glass or silica tube. This tube was equipped with optical flats at each end, enabling the sessile drop and uranium dioxide laque to be photographed under any desired conditions. h e induction heater was of the radiofrequency type and enabled temperatures of greater than 1500" to be maintained indefinitely. The temperature was measured using a Cambridge optical pyrometer, and a Cu-advance alloy thermocouple, the latter checked in each run against the melting point of lead. A vacuum line was connected to the furnace tube, and evacuated by a mercury diffusion pump and backing pump, mm. Purified argon, carbon giving a pressure of about monoxide or hydrogen atmospheres could be introduced through a purification line consisting of a li uid oxygen trap, sodium-potassium alloy trap and two PzOs%rying columns. A magnified image of the sessile drop and uranium dioxide plaque was focussed on to a photographic plate by an f4.5, 8"focal length lens and microscopic eyepiece, mounted on an optical bench. Using an exposure of 60 seconds at f23, very sharp images were obtained. Measurements of drop dimensions and contact angle were made on traced enlarge( 1 ) M. Humenik and W. D . Kingery, THISJOURNAL,67, 359 (1953). (2) W. D. Kingery, USAEC report, NYO-3144. (3) B. M. Abrahams, R. D. Carlson and H. E. Flotow, USAEC report, NESC-104. 1957.
stainless steel blade before each pellet was made, after which ivory tipped force s were used for handling. The extent of oxidation during Randling was negligible. The pellet was weighed, then reweighed in the cases where surface active agents had been added. The additives were confined in a small hole drilled in the surface of the pellet and concen- . trations in the range 0.003 to 0.05 molal with respect to PbX (X = 0, S, Se, Te) were used. Uranium dioxide plaques were prepared by igniting "Baker's Analyzed" uranyl acetate at 1 1 0 0 O in air, then completely reducing the UsOs formed to brown lJ0z.00 in purified hydrogen a t 900". A weighed sample of this product was heated in air for 10 minutes at 200" during which time the composition was altered to U02.1~. This nonst,oichiometric oxide sinters more readily than UO:.oo,S and plaques of density 8.5 to 9.3 were obtained by cold presslng at 100,000 p.s.i. and sintering at 1400-1500" in dynamic vacuum of approximately 10-8 mm. For the study of the effect of plaque density on surface tension and contact angle, specially prepared samples of uranium dioxide of density 10.8 were obtained from the Industrial Group, U. K. Atomic Energy Authority. The laque surfaces were ground flat and lightly polished on a sificon carbide stone, then reduced to stoichiometric UOz prior to each run as describe! above. The plaque and preweighed pellet were inserted into the molybdenum SUSceptor of the induction furnace and levelled horizontally and (4) N. E. Dorsey, J . W m h . Acad. Sci.. 18, 505 (1928). ( 5 ) P. Murray, E. P. Rodgers and A. E. Williams, AERE report, M/R-893, 1952.
