540
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
*
Const. T =
+ RT
- 18,507 + 5.99341'
In
T + 0.002936T2
PCHd
In -,
based on the following values: ' PH, Heat of formation methane a t 18" =20,500 cal. = 6.6 0.0006T Specific heat per mole hydrogen =9.106 (20-208" C.) Specific heat per mole methqne =2.0994 4-0.0017361' Specific heat per atom carbon
+
EQUJLIBRIUM
PRESSURES
OF
METHANEIN
NITROQEN-ARQON
MIXTURES
VoI. 14,No.6
temperature is, however, still unknown and also the effect of low pressure on the specific heats. In spite of these uncertainties the error shown in the values of the constant at these two temperatures is only about double the error between the experiments a t atmospheric pressure. Mayer and Altmayer's value for the constant of 21.6 would seem therefore to be sufficiently close to 23.3, the mean found in these present experiments, to indicate that the equation as stated by them holds at least fairly closely for the reaction at the low partial pressure of 0.03 of an atmosphere. Passage of this mixture of nitrogen with 2 per cent methane through copper oxide alone having the same volume as the nickel powder yielded less than one-eighth the extent of decomposition secured by the nickel under the same conditions of temperature and flow. To demonstrate more particularly the advantage of the nickel catalyst in eliminating methane impurities from inert gases, the equation was used to calculate the decomposition for amounts less than 8 per cent methane, and at temperatures ranging up to 800" C. For the constant Mayer and Altmayer's value of 21.6 was taken. The results are given in the series of curves in the attached figure. They show,for instance, that for a gas containing as much as 1.5 per cent methane, one passage through nickel at 500" C. reduces its content to 0.24 per cent, and passage through a second similar series lowers it still further to 0.01 per cent. In each case the nickel furnace should be followed by one containing copper oxide, also at 500" C., to remove the hydrogen liberated. Instead of using sequences of such furnaces, it was found that a mixture of nickel and copper oxide in the same furnace secured equally good results. The disadvantage of such an arrangement, however, lies in the necessity of renewing the charge on exhaustion of the copper oxide. Oxidation of the exhausted mass was found to lead to oxidation of the nickel as well as of the copper, even at as low a temperature a,s 275' C. Nickel powder was found to be oxidized, though at a rateless than one-ninth that of copper powder of the same fineness. In such a mixture, nickel oxide failed to exercise any catalytic function, and the mass was found to be no more efficient than copper oxide alone in the removal of methane.
Results are given in the accompanying table. DECOMPOSITION OP METHANE (2.23 P E R CENT IN NITROGEN) BY OVER 3 Cc. OF NICKEL-ASBESTOS 0.2 N Partial Pressures Ba(0H)a at Equilibrium Gas Flow Cc. Temp. CHI H? Min. cc C. Liters 0.0075 0.0296 550 0.465 72.0 1.60 0.0071 0.0304 550 0.415 56.8 1.35 0.0087 0.0272 550 0.415 83.0 1.66 0.0066 0.0314 '550 0.440 45.5 1.35 0.0075 0.0296 550 MEAN 0.0134 0.0178 440 0.355 51.2 2.19 0.0194 440 0.380 50.3 2.20 -'0.0126 0.0121 0.0204 58.2 2.24 0.405 440 0.0102 0.0242 68.0 2.15 0.460 440 0.0127 0.0192 2.24 0.385 56.2 440 0.0210 0.0118 440 0.400 63.5 2.16 0.0125 0.0205 440 MSAN
___
..
..
..
..
,'
-
PASSAGE
CONSTANT
24.4 24.2 25.0 23.9 24.4 23.0 22.6 22.3 21.2 22.6 22.1 22.3
The values for the constant show very close agreement at any one temperature, well within the limitp of error of the previously cited experiments at atmospheric pressure. There is, however, a consistent difference between the values for 550" C. and for 440" C. As the conditions were the same in both cases, and as the difference is in the opposite direction from what would be expected if it were due to failure to reach equilibrium at the lower temperature, there would seem to be some significance to be attached t o it. In recent years the specific heat of hydrogen has been more accurately determined,' but the use of these values does not change the relationship. The change of the specific heat of methane with 4
W.Escher, Ann. Physik., 48 (1918),761.
Cleaning of Museum Exhibits Builetin 5, published by the Department of Scientific and Industrial Research, H. M. Stationery Oflice, London, is an interesting account of investigations conducted a t the British Museum under the direction of Dr. A. Scott. It deals with the cleaning and restoration of such museum exhibits as prints and enamels, silver, lead, iron, and copper articles, and prehistoric paintings on rocks. Brown spots on prints, the result of growth of mold fungi. are best removed by very dilute bleaching agents. Thymol and similar antiseptic agents have given promising results as mold preventatives, especially when aided by heat. Brown stains due t o oils or varnishes cannot be removed by bleaching agents, but yield to pyridine, which does not affect the paper. To remove the crust of copper oxides, carbonates, and oxychlorides from silver-copper alloys a warm dilute formic acid solution is a safe reagent which does not attack the silver or the alloy. Other agents for this purpose are a solution of ammonium sulfite and ammonia containing some cuprous sulfite, a solution of ammonia and ammonium formate, or zinc dust moistened with very dilute sulfuric acid. Ammonium chloride, either alone or with stannous chloride and a little hydrochloric acid, is more satisfactory than ammonia for removing superficial deposits from objects of copper, bronze, or brass. An alkaline solution of Rochelle salt will also give good results. The lichens on rock paintings from Northern Rhodesia could not be removed by mechanical means. They were softened and gelatinized by painting with dilute ammonia, and brushed off. The rock was then washed with distilled water and finally with absolute alcohol
