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).
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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.
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8.52 8.57 8.47 27.64 27.60 27.70 27.48 27.50 17.46
