1852
G. R. BELTON A N D R. L. R~ICCARRON
A comparison of the last two columns in Table I V shows that the reaction between glycylglycine and FDXB occurs principally, if not entirely, in the aqueous phase when the detergent is SDS. The concentration of anions, even at an ionic strength of 0.1, must be markedly reduced near the micelle surface for a radial distance of at least the length of the glycylglycine niolecule. The reverse effect is seen in the presence of the cationic detergent cetyltrimethylammonium bromide. The preliminary experiments indicate that glycylglycine reacts with FDNB coiisiderabIy inore rapidly than does glycineamide as a result of the reverse distribution in the double layer. The binding of the glycineamide cation to a micelle of SDS does not alter the kinetics of the FDNB reaction since it is only the free base forni of the amine which enters into the reaction. In the case of the cationic detergent and the glycylglycine anion, the charge interaction is with the carboxyl group and the effective c( iicentration of the amine will be increased at the micelle surface.
It is unfortunate that the low partition coefficieiits cannot be measured more accurately. The conclusions about the surface nature of the reaction depend very largely on the assumption of complete exclusion of one of the reactants. The thioether portion of anisylthioethane resembles the side chain of methionine while the benzene ring provides a chromophore whose absorbance is sensitive to its environment. The measured partition coefficient indicates that such a conipound, were it part of a polymer chain, would contribute significantly to the stabilization energy of a folded structure which would resemble a micelle. In so doing, its spectral characteristics would be changed but its cheinical reactivity would depend very much on the exact nature of the reagent used in studying it. In this sense the detergent systems have reproduced many of the characteristics frequently found in proteins. However, the micelles appear just as complicated as the proteins for which we had hoped to use them as models.
The Volatilization of Tungsten in the Presence of Water Vapor
by G. R. Belton and R. L. McCarron Department of .lfetalZurg~, University of Pennsylvania, Philadelphia, Pennsylvania (Received Februarv 6 , 1964)
h study has been made, using the transpiration technique, of the enhanced volatility of tungsten in mixtures of hydrogen and water vapor. The volatilization is shown to occur 3Hz(g). The standard free energy 4H*O(g) = WO,H,O(g) by the reaction W(s) change for the reaction between 1200 and 1500° is given by: A F o = 26,700 - 5.56T cal.
+
Introduction The enhanced volatility of tungsten oxides in the presence of mater a t 1000" arid 1 atm. total pressure was first reported by AZillner and Neugebauer.l Froin their data Grossweiner aiid Seifert2 later showed that the gaseous species W03H20 could be responsible for the enhancrd volatility. Gleinser and his co-n orkers have used a transpiration technique to study in more detail the volatility of solid W 0 3 in nitrogen-water The Journal of Physical Chemistry
+
vapor mixtures at 1100-1200 "3 and in oxygen-water vapor mixtures at 900-1100".4 AIeyer, et al.,5 have (1) T. hlillner and J. Neugebauer, N a t u r e , 163, 601 (1949). (2) L. Grossweiner and R. L. Seifert, U. S. Atomic Energy Commission Unclassified Paper AECU-1573. (3) 0. Glemser and H. G. Voltz, Third International Congress on t h e Reactivity of Solids, Madrid, April, 1956: also Araturwissenschaften, 43, 33 (1956). (4) 0. Glemser and R. Haeseler, 2. anorg. allgem. Chem., 316, 163 (1962).
1853
VOLATILIZATIOX OF TTJNGSTEN KN THE PRESENCE OF WATERVAPOR
also made a similar study using both gas mixtures over the temperature range 1120-1250". These a ~ t h o r s s -found ~ a linear dependence of volatility on the pressure of water vapor alone and concluded, in accord with Grossweiner and Seifert, that a volatile hydrated oxide of tungsten was formed by the following over-all reaction
However, although the variation in oxygen pressure achieved by these workers is sufficient to show that the species must stoichiometrically contain WOS and HzO, the possibility of more than one WOi unit in the species cannot be ruled out. The derived values for the standard enthalpy change of the above reaction are not in agreement, the value of Glemser and Haeseler4 being 39.9 kcal. and that of Jleyer, et u L . , ~ being 25.8 kcal. The purpose of the present investigation has been to prove conclusively the formula of the volatile species and to obtain thermodynamic data for the forniation of the species directly from the metal over a substantial temperature range, This has been achieved by using the transpiration technique to measure the apparent vapor pressure of tungsten in the presence of hydrogenwater vapor mixtures insufficiently oxidizing to permit the formation of condensed oxide.
Experimental Materials. High purity tungsten wire approximately 0.38 nim. in diameter and with a minimum tungsten content of 99,95y0 was used in the experiments. The tungstien specimens which were constructed of two concentric coils of this wire were approximately 30 mm. long by 5 nim. in diameter and weighed about 1.3 g. Prior to beginning a series of experiments, the specimens were heated for several hours in the equilibrium gas mixture a t the teniperature of the experiments. The maximum impurity contents of the hydrogen used in the preparation of the equilibrium gas and of the argon-10% hydrogen used in the heating and cooling cycles are listed in Table I. The argon-10% hydrogen mixture was subjected to further purification by passing through titanium sponge held a t approxiniately 850". KO further purification of the hydrogen was carried out. Apparatus and Procedure. The apparatus was substantially the same as that described in detail by Belton and Richardson6 with a few minor niodifications Briefly, hydrogen was led through a capillary flow meter and a commercial integrating gas meter to a water presaturator maintained a t a few degrees below the boiling
Table I
0 2
N2 Hydrocarbons HzO
Hydrogen
Argon10% hydrogen
10 p.p.m. 600 p.p.m. 60 p.p.m. 17 p.p.m.
6 p.p.m. 65 p.p.m. 9 p.p.m. 16 p.p.m.
point of water. The gas was then passed through a condenser system submerged in a water bath niaintained to within =kO.O5" of the desired saturation temperature and from there to the reaction tube situated in the furnace tube of the equilibriuiii furnace. The alumina furnace tube had an inside diameter of 26 mm. and a length of 60 em. and the dense, high purity alumina reaction tube had an inside diameter of 5 111111. and a length of 33 cm. The length of the even temperature zone of the furnace was approximately 3 . 5 em. and within this zone the temperature fluctuations did not exceed *2O over periods of several hours. In order Bo prevent condensation. heated glass tubing n as used to conduct the water vapor-hydrogen mixture and the furnace was made niovable to make possible the withdrawal of the reaction tube. By a suitable arrangement of stopcocks, it was possible to pass either the equilibrium gas or the argon-hydrogen mixture through the reaction tube and the argon-hydrogen mixture through the space between the reaction tube and the furnace tube. A weighed sample was placed in the end of the reaction tube, and, with the argon-hydrogen gas mixture passing over the sample, the furnace was nioved forward until the joint between the furnace tube and the gas train was made. The argon-hydrogen mixture was then diverted to pass around the reaction tube and the water vapor-hydrogen mixture passed over the sample for between 30 and 200 niin. The argonhydrogen was again passed over the saimple and the furnace was moved back to allow the sample to cool. The sample was withdrawn and reweighed. The handling errors of the tech,nique u.ere established by carrying out several zero-time experimcnts in which the sample was immediately quenched without passing the water vapor-hydrogen mixture. These experiments showed that there were no measurable handling errors. ( 5 ) G. Meyer, J. G . Oosterom, and W.J. Oeveren, Rec. traa. chim., 78, 117 (1959). (6) G. R. Belton and F. D. Richardson, Trans. Faraday Soe., 5 8 , 1562 (1962).
Volume 68, A'urnber 7
J u l y , i964
Results The results obtained in the experiments are shown in Fig. 1, where the weight of tungsten lost per liter (STP) of the ingoing gas mixture is plotted against the flow rate. I n order to illustrate the experimental scatter by means of an expand-& scale, the values obtained with very low flow rates ai iiot shown. For example, in the experiments at 1499O, a flow rate of 60 ml./min. gave a weight loss of 102 X 10-5 g./l. The curves are similar in form to those obtained and discussed earlier,6 equilibrium between the gas and the solid being attained in the region where the weight loss curves are horizontal. 30
."
i
I
I V'.
I
I
I
Table 11: The Volatility of Tungsten in the Presence of Water Vapor Mean weight of tungsten per S T P liter of
Temp., =C.
1206 1209 1297 1300 140% 1499
gas,
PHZO/PHZ g. x
0,412 0,228 0,238 '0.310 0.173 0.247
105
29.30 3.82 5.76 17.70 3.72 20.40
Total apparent vapor pressure, atm. X 106
35.70 4.65 7.02 21.57 4.53 24.85
Apparent vapor pressure of hydrated species, atm. X 108
35 4 6 21 4 24
67 6% 98 50 46 10
pressures thus calculated were deducted from the total apparent pressures to give the apparent pressures of the hydrated species shown in column 5 of Table 11.
Discussion 0 1
g35 I
I
I
I
I
1
I
I
I
I
I
1
1
I
0
100
I
I
200
300
I
I
400
600
600
Flow rate, ml./min.
Figure 1. Results obtained from transpiration experiments, gas volumes all a t STP. Values of gas composition (pHgo/ pH9) and temperature respectively, are: 0 , 0.288, 1209"; 0,0.238, 1297"; A, 0.310, 1300"; X, 0.412, 1206"; .I,0.173, 1405"; 0, 0.247, 1500".
From the equilibrium results, the total pressures of the volatile species containing tungsten were calculated on the assumption that one atom of tungsten was present in each volatile molecule. The pressures thus calculated are shown in column 4 of Table 11. The average value of the mean deviations from the equilibrium results is 5.8%. The pressure of WOajg), the only oxide of significant pressure under the conditions of the experiments, must be subtracted from the total apparent pressure to give the apparent pressures of the hydrated species. Considering the reaction
W
+ 3HzO(g)
=
WOs(g)
+ 3Hz(g)
(2)
and utilizing the standard free energy of formation data from the literature,' the pressure of WOS was calculated for each experimental condition The sinal1 The Journal of Physical Chemistry
Errors. The maximum uncertainties in the nieasurements are considered to be as follows: temperature, k 3 " ; H20/H2 ratio, =kO.5%; weight losses, + l % ; gas volumes, f1%. The uncertainty of f3 O in the temperature results from the measured fluctuations of +2' in the even temperature zone of the furnace and an uncertainty of & l o in the Pt/Pt-lOyc Rh thermocouples used. The uncertainty in the HzO/Hz ratio was checked by experiments on the saturator system. I n the opinion of the authors, the effect of these uncertainties is to cause an uncertainty in the derived vapor pressures of about =k6.5y0which is consistent with the measured standard deviation of =k 5.87,. Five specimens with a combined weight of approxjmately 7 g. were used during the experiments. The combined weight loss in 70 experiments excluding the weight losses occurring during cleaning runs and several exploratory experiments totaled approximately 0.28 g., or about 4%. The specified maximum impurity content of the tungsten was 0.05%. One of the used specimens a d a piece of unused tungsten wire were subjected to spectrographic analysis and, within the limits of analytical error, the amounts of the major impurities (Fe, Si, and Xi) in each sample were found to be the same. It is clear from the foregoing that the impurities present in the tungsten could not significantly affect the results. Con$rmation and Thermodynamic Properties of ths Gaseous Species WOa.HzO. If only the present study is considered, the possible compounds may be determined ( 7 ) "Joint Army, N a v y , and Air Force Thermochemical Tables," Dow Chemical Co., 1962.
VOLATILIZATION OF TIJXGSTEX IN THE PRESENCE OF WATERVAPOR
from the manner in which the apparent pressures of the volatilizing species (apart from the gaseous oxide) vary with the pressures of water vapor and hydrogen. Table 111 summarizes the calculations on three species which most nearly satisfy the measurements, on the as~~
~~~~~~
K
log K
Table 111: Results of Calculations on Choice of Volatile Species from W 4-Hz HzO Experiments
+
Gas
mixture dependence
( p W 0 3 H ~ ~ ( p H z ) ~ / ( p H z o ) ~(4)
=
-5840 (*370) T
Ca1cd.b
AF'
sumption that the species indicated are solely responsible for vaporization. The small temperature corrections necessary were determined froiii corresponding log K us. 1 / T plots. Calculations are also summarized for the species (W03)2+H20 which would have satisfied the measurements of the previously cited workersa-j on the apparent pressure of WOa in the presence of steam. Column 3 shows the losses in weight calculated for the condition when lHzOI/ [Hz] is equal Lo 0.412 a t 1206" from the results obtained at 1209" when [H20]/ [Hz]was equal to 0.228. The observed value is shown a t the bottom of column 3. Column 4 shows the calculated and observed values for a ratio of 0.310 at 1300", the calculated values being derived from the results obtained a t a ratio of 0.238 a t 1297 '. The uiicertainties listed for the calculated weight losses were derived by combining the uncertainty in the measured weight losses ( f5.8yo)with the uncertainty in the water vapor-hydrogen ratio. The species W0,H20 fully satisfie,s the calculations and the species ( W 0 3 ) 2 . H z 0is clearly ruled out. The two remaining species are in reasonable agreement with the experimental data. If the species WOJ3ZO is considered, the reaction in the presence of Hz and HZO would be
+ 4HdXg)
=
(5)
W03HzO(g) f 3Hz(g) (3)
If K is the equilibrium constant and the gases are assumed ideal
=
26,700 - 5.56T
(6)
From this, the standard free energy change for reaction 1 can be estimated by addition of standard free energy equations as
AF' = 26,700 - 5.56T AF' = -179,720 40.33T AF' = 195,280 - 55.95T
5From results a t 1209" and [H%O]/[Hz] = 0.228. *From results a t 1297" and [HzO]/[Hz] = 0.238.
W
+ 1.216 (f0.23)
The standard free energy change for reaction 3 would then be
Calculated weight loss (E. X 10') per STP liter of gas attributable t o the volatile species 1206O, 1300°, [HZOI/[HII [HzOI/[HzI = 0.412 = 0.310
Calcd.=
=
Values of log K are shown plotted against 1/T( OK.) in Fig. 2. The best straight line, calculated by the method of least squares, is
~
Supposed species
1855
+
AF'
=
42,260 - 21.18T
(7)
From this equation one can readily calculate that at 1200°,unit activity of W 0 3 ,and 150 min. of HzO pW03H20= 3.25 mm. The measured value of Meyer, et aZ.,j under these conditions is 1.62 mm. and that of Glemser and Voltz3 is 1.,5 m i x (interpolated), The pressure calculated from the equation of Gleiiiser and Haeseler is 2.07 mm. Hence, the predicted value and the measured values are in adequate agreement, since a 2 kcal. error in eq. 7 would account for the difference.
-2.8
-2.6 bi
$
CI
-2.4
-2.2
- 2.0 6.8
6.6
6.4 6.2 ( I / T O K . ) x 104.
6.0
5.8
5.6
Figure 2. Temperature dependence of equilibrium conetant for the reaction W ~ H z O= WOaHzO 3Hz.
+
+
Volume 68,Number 7
J u l y , 1.9154
1856
G. R. BELTOX AND R. L. ~ICCARRON
-
The linear dependence of the volatility of W03 on HSO found by the above ~ o r k e r s ~rules - ~ out the species VCr,(OH), and W z 0 3 '2Hz0and the impossibility of these species is further demonstrated if calculations of the type shown for WO3&0 are carried out. The species IV(ON), and W2(OH)4would yield 1.58 X and mm., respectively, and the species W03. 2Hz0and WS03Hz0 mould yield 269 nim. and 3 X mm. for the volatility of WOS a t 1200" in the presence of 150 nim. of H,O. From these considerations, the second and third supposed species of Table III are effectively ruled out. Thus, there is coiiclusive evidence that the major gaseous species is stoichiometrically W03Hz0and that the vaporization reaction in the presence of water vapor and hydrogen is given by eq. 3. The most probable values for the standard free energy change of reaction 3 are given by the equation AFO = 26,700
value of Glemser and Haeseler must be at least *1 kcal. in view of the restricted temperature range used (200"). Hence, the two heats are in experimental agreement. The value of' Meyer, et al., definitely lies outside the experimental error. Table IV : Available Standard Free Energy Equations for the Reaction WOa(s) HnO(g) = WOaHzO(g)
+
Temp. range,
AFO,
Investigators
"C.
eel.
Glemser and Toltz Meyer, Oosterom, and Oeveren Glemser and Haeseler Calculated from this investigation
1100-1200 1120-1250
38,100 - 16 7T 25,800 - 8 4T
900-1100 1200-1500"
39,900 - 18 8 T 42,260 - 21 18T
a
Melting point of WOSis 1472".
- 5.56T (.t500 cal.)
over the temperature range 1200- 1500O. Combination of the well-known standard entropies of W, H,O(g), and Hf(g) from the literatures with eq. 6 yields the standard entropy of the species WO3HzO. The most probable value at 1600O K . (approximately the mean temperature of the calculations) is
So = 136.7 i 1.05 e.u. The Vaporization o j WOa in thP Presence of Water Vapor, The straight line equations for the standard free energy change of reaction 1 from the direct determinations and calculated from the present work are shown in Table IV. If no account is taken of the errors in the heats of formation of HzO(g) and JTOs(g), the probable error in the value of AH," derived from the present work is 1.1.7 kcal. The uncertainty in the
The Joz~rnaloj" Physical Chemisfral
The small difference in standard free energy change for reaction 1 between the present work and that of Glemser and Haeseler (about 1 kcal. a t 1l5Oo) may be completely accounted for by the experimental error and the uncertainty in the data for WOa(g).7 Hence, in view of the direct nature of their determinations, the equation of Glemser and Haeseler must be preferred for the vaporization of solid tungsten oxide in the presence of water vapor.
Acknowledgment. This study, a contribution from the Laboratory for Research on the Structure of Malter, University of Pennsylvania, was supported by the Advanced Research Projects Agency. Office of the Secretary of Defense. (8) "Selected Chemical Thermodynamic Properties," Series 111, Vol. 1, National Bureau of Standards, Washington 25, D. C.
