INDUSTRIAL A N D ENGINEERING CHEMISTRY
84
Vol. 17. No. 1
Vapor Pressures of Some Liquid and Solid Metals’*g By Russell W. MilIar PACIFIC EXPERIMENT STATION, BURLAUOF MINES,BERKELEY, CALIF.
UMEROUS determinations of the vapor pressures of liquid metals have been made, usually of the boiling point at one atmosphere, but a knowledge of the boiling points at other pressures-that is, of the temperatures at which the vapor pressures are other than one atmosphere-is equally important. The difficulty of measuring the vapor pressures of metals has limited the number of metals for which complete vapor pressure curves are known to a few of those that are relatively volatile at low temperatures. I n 1917Johnston3reviewed in detail the data existing at that time, and by a method of extrapolation obtained vapor pressures at various temperatures which are sufficiently accurate for many purposes. Eldebrand,*using a method based upon his modification of Trouton’s rule,S has calculated another set of values, using mercury as a reference substance. Both writers were able to plot the boiling points of a given liquid metal at various pressures from a single experimental point, but neither was able to calculate the sublimation pressures of the solid except a t the triple point, where it is equal to the vapor pressure of the liquid. I n the author’s calculations the triple point and the melting point are taken as equal, which is true well within the limits of experimental error. Recently, the author6 described a method by means of which it is possible to calculate the vapor pressures of a liquid metal and the sublimation pressures of the solid metal at any temperature from a single determination of the vapor pressure of the liquid or of the solid at ‘one temperature, the specific heats of the solid and liquid, the heat of fusion, and the melting point. He showed that the values calculated for liquid mercury, sodium, and zinc, and for solid zinc agreed with the experimentally determined values within less than the probable experimental error. The equation has the form Log p = A log T B (log T)2 C
N
+
+$+
where A , B, and C are simple functions of the specific heat and entropy of the solid or liquid metal, and of the molecular weight of the gaseous metal. The agreement of the calculated and measured vapor pressures of sodium, mercury, zinc, and cadmium justifies the assumption that in all cases the metallic vapors are monomolecular. 2 is a constant determined by one value of the pressure at one temperature, p is the pressure, and T the temperature. The vapor pressure a t the melting point may be obtained by calculation from the equation for the liquid, and the values of A , B, and C for the solid from the necessary thermal data. 2 for the solid can then be calculated. The sublimation pressures of the solid can be calculated from the equation so obtained. For a full discussion of the derivation of the equation and the method of evaluating the constants, the reader is referred to the article mentioned above.6 Table I gives, in the third and fourth columns, the temperature and pressure by means of which 2 was evaluated for the Received July 25, 1924. Published by permission of the Director of the U. S. Bureau of Mines. The calculations presented here comprise a part of a review of the vapor pressures of metals made for the Pacific Experiment Station of the Bureau of Mines while the author was acting as consulting chemist for this bureau during the summer of 1923. 8 THIS JOURNAL, 9, 873 (1917). J . A m . Chem. SOC.,40,45 (1918). 8 Zbid., 37, 970 (1915). @ Zbid., 48, 2323 (1923). 1
liquid metal. The following columns give the values of A , B, and C, and the values of 2 when p is expressed in millimeters of mercury. The calorimetric data used in the calculations were obtained from Lewis, Gibson, and Latimer;’ Wust, Meuthen, and Durrer;8 8chUbel;Q Williams;lo and Roos.“ I n case the heat capacity of the condensed phase is constant, B becomes zero, and the (log T ) 2term does not occur in the equation. TABLEI-CONSTANTSOR METAWUSEDIN CALCULATING VAPOR PRESSURES CONDENSED METAL PHASE T(OAbs.). 9ntm A -E c -z 1039 760 3.150 0.748 5.761 5790 Cadmium Liquid Solid ... 1 472 0.362 7.701 5950 1393 i 0 6 -1:5 13.081 7640 Magnesium gA@d 4.4 0:838 2.909 7690 2Of3 i s 0 4.640 0.817 1.648 10680 Aluminium . . . 3.387 0.667 4.467 11000 Manganese Liquid 18.621 12700 2173 760 2.963 Gold 2470 760 18.41 21687 -23.44 12900 Liquid 2583 760 -1.625 ... 14.241 15010 ; & i d Copper ... -0.81 11.811 15210. Liquid Nickel 2610 30 -1.448 13.451 18340 2648 30 -1.865 Cobalt 14.881 1S61Q 1,iquid 2720 36 -1.715 14.221 18440 Iron Liquid a The boiling points in Table I were obtained from the following sources: Cadmium Heycock and Lamplough Proc. Chem. SOC.(London) 28 4 (1912); magnesiu& aluminium manganese ’and copper Greenwood P>or.’Roy. SOC. (London), 83, 483A (19’10); nickel,’ cobalt, and’iron, Ruff akd Borman, .Z. anorg. allgem Chem 88, 397 (1914): gold, Tiede and Birnbrauer, Ibzd., 87, 129 (1914j give h O O a C. as the boiling point of gold, at one atmosphere but a considerhtion of the melting points of cogper, silver, and gold, and of the boiling points of the first two points to 2200 C. as a more probable value.
1
...
{
...
... .... .. ...
A series of vapor pressures of each metal was calculated by means of the constants of Table I and plotted on a large scale as the logarithms against the reciprocals of the corresponding absolute temperatures. From the curves, which were nearly straight lines, the boiling points at a series of pressures were read off. Though the curves obtained by plotting log p against 1/T could in many cases be described by a somewhat less complicated equation than that used in these calculations, the constants of such an equation could not be calculated from calorimetric data, and consequently more than one vapor pressure datum would be necessary to establish the complete vapor pressure curve. Table I1 contains the boiling points in degrees Centigrade a t a series of pressures. Table I11 contains the sublimation pressures at various temperatures of four of the metals. TABLE11-BOILINQ POINTS 760 500 mm. mm.
METAL
o c .
QC.
Cadmium Magnesium Aluminium Manganese Gold Copper Nickel Cobalt Iron
766 1120 1800 1900 2200 2310 3075 3185 3235
729 1075 1715 1820 2100 2220 2950 3050 3097
LIQUIDMETALS AT VARIOUS PRESSURES 300 100 50 10 1 0.1 mm. mm. mm. mm. mm. mm. QC. o c . OC. OC. OC. OC. 686 609 565 481 386 321 1015 910 855 735 615 515 885 1635 1475 1390 1220 1030 1720 1555 1465 1285 1080 925 2005 1835 1745 1560 1350 1175 2120 1930 1825 1625 1400 1220 2805 2560 2415 2135 1840 1605 1885 1645 2890 2635 2485 2195 1900 1655 2930 2670 2520 2225
OF
The calculated vapor pressures of liquid cadmium agree well with the experimentally determined values of Folger and Rodebush,12 while those of solid cadmium agree with the measurements of Egertonls and of Pilling.l* For each of
2
J . A m . Chem. Soc., 44, 1008 (1922). “V. D. I. Forschungsarbeiten,” 1918, p. 204. 0 2. anorg. allgem. Chem., 87, 81 (1914). 10 Thesis for the doctorate, University of California, 1932. 11 2.anorg. allgem. Chem., 94, 329 (1916). 12 J . A m . Chem. SOC., 48,2080 (1923). 18 Phil. Mag., 33,33 (1917). 14 Phus. Rev., 18, 362 (1921). 7
January, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the other metals but one measurement has been made, which, with the exception of iron, cobalt, and nickel, has been the boiling point at one atmosphere. It is difficult to estimate the accuracy of these boiling points, but it is probable that all are somewhat too high. The boiling point a t one atmosphere of iron, calculated from its boiling point at 36 mm., seems too far above its melting point, but this boiling point is perhaps not unreasonable when we consider that, according to Langmuir and MacKay,lSthe boiling point of platinum, 3707" C., is about 2100" C. above its melting point, and that in general the difference between melting points and boiling points increases with the former. The only previous measurement for iron was 2450' C. at one atmosphere, by Greenwood,lg which is undoubtedly too low. It should be pointed out that large percentage errors in the heat capacity data used cause but small percentage errors in the calculated vapor pressures. The long extrapolation of the specific heat of the liquid metal, made necessary several times by insufficient data, is therefore justified, and unless the data used are very seriously in error, the con16 16
P h y s . Rev., 4, 377 (1914). Proc. Roy. Sac. (London), 85,4838 (1910).
35
stants A , B, and C are sufEciently accurate. However, a n y more accurate measurement of the one boiling point from which Z was calculated would necessitate its recalculation and a resulting change in the calculated vapor pressures. TABLE III-vAPOR
PRESSURES O F SOLID METALS Aluminium r ( o C.) Pmm l(0 C.) Omm 25 1.45 X 10-11 25 7.0 X lo-** 169 0.000040 300 3.4 x 10-11 3.0 X 10-8 210 0.00054 500 272 0.0126 659 m. p. 0.00062 302 0.0457 321 m. p. 0.098 Magnesium Copper 25 1.7 x 10-17 300 1.1 x 10-17 300 0.000025 500 6.2 X 10-11 380 0.0011 727 1.4 X 10-1 651 m. p. 2.28 1083 m. p. 0.012 Cadmium
Since the specific heats of the solid metals, even a t temperatures as high as the melting points, are quite well known, the vapor pressure equations for the solid metals are more accurate than those for the liquid metals except for the constant 2, which depends upon the calculated or measured vapor pressure at the melting point.
Use of Iron or Nickel Crucibles for Alkali Determinations'Dz By Alice W. Epperson and R. B. Rudy BUREAUOF STANDARDS, WASHINGTON, D. C.
HE necessity of using platinum crucibles for fusions in the determination of alkali by the J. Lawrence Smith method has made the method an expensive one for routine work and somewhat impractical for plant laboratories. A recent articlea showing satisfactory results obtained by the use of nickel instead of platinum dishes for the ashing of saccharine products, led t o an investigation of the possibility of using nickel or iron crucibles for J. Lawrence Smith fusions. Nickel and iron crucibles in the regulation form-a long tapering cylinder-were not obtainable. New iron crucibles of about 25 cc. capacity, 3.8 em. top diameter, and 3.2 cm. high, and new nickel crucibles of about 30 cc. capacity, 3.6 cm. top diameter, and 4.9 em. high were used for the tests. All crucibles had close-fitting covers. The usual procedure was f o l l ~ w e d except ,~ that in three single fusions in iron the crucibles were placed on triangles and heated over a low free flame, the top part of the crucible not being allowed to become red. No determination was made of loss in weight of the crucibles, though after having been used for eight fusions the crucibles could be cleaned with sand and were apparently in as good condition for use as at the beginning. The nickel crucibles were more satisfactory because they suffered less attack and were more easily cleaned. The iron crucibles had scaled off and rusted, but on cleaning there was little difference in appearance after the first and eighth fusion. I n both cases the nickel or iron which was removed by the fusion was eliminated by the first filtration, and gave no troub7e in subsequent operations. Because of the low, shallow type of crucible used, some difficulty was experienced in obtaining complete disintegration of the silicate without some volatilization of the alkali. When only one fusion was used, results were low for a sample of glass containing about 15 per cent of alkali (NazO and KzO).
T
Received August 8, 1924. a Published by permission of the Director, U. S. Bureau of Standards. 8 Whaley, J . Assoc. Oflcial Agr. Chem., 6 , 870 (1923). 4 U.S.Ged.Survey, Bull. 700, p. 207. 1
This difficulty was overcome by keeping the temperature a t the bottom of the crucible at low red heat during fusion and after extraction with .water igniting the residue gently and again fusing with calcium carbonate and ammonium chloride. It is believed, however, that less care would be necessary in the fusion and a single fusion might be sufficient, even for materials containing high percentages of alkali, if crucibles of the regulation form or a form approaching it were used. While a few specially formed crucibles of this type mightibe expensive, their use in quantity for routine work would eliminate this objection. Samples of argillaceous limestone and of two types of glam, chosen to represent varying percentages of alkali in silicates, were used for the determinations. Results obtained by two operators are given in the accompanying table. The results show that either nickel or iron crucibles can be used with entire satisfaction for fusions in determinations of the alkalies by the J. Lawrence Smith method, and that the accuracy of the results is comparable to that obtained by the use of platinum. The initial cost, danger of loss by theft, care required, etc., are, of course, far less. RESULTSOB ALKALIDETERMINATIONS MADE IN PLATINUM,IRON, AND NICKEL CRUCIBLES PER CENTCOXBINED NACLAND KCL Platinum Crucible -Iron Crucible-Nickel Crucible1 fusion 1 fusion 2 fusions 1 fusion 2 fusions Argillaceous Limesfone 2.33 2.47 2.40 2.48 2.28 2.42 2.47 2.47 2.35 2.40 2.42 2.40 2 27"
2.36" 2,23a 8.52 8.53 8.43 27.64 27.70 27.76
Pyrex Glass 8.49 8.53
7.70 7.75 7.76
Light Crown Glass 27.64 27.23 27.70 27.33 27.08 Crucibles heated over free dame.
.
8.52 8.57 8.47 27.64 27.60 27.70 27.48 27.50 17.46
