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INDUSTRIAL AND ENGINEERING CHEMISTRY
In all cases, except that of copper, the diffusion current constants in the three mineral acids and in sodium hydroxide decrease in the order hydrochloric acid >nitric acid >sulfuric acid >sodium hydroxide, which indicates that the chloro complexes in hydrochloric acid are smaller than the aquo complexes that predominate in nitric and sulfuric acids, and that these in turn are smaller than the hydroxo complex ions present in sodium hydroxide solution. The values in the tartrate media are smaller than in the other media, which is to be expected in view of the relatively large size of a tartrate complex ion. Furthermore, the diffusion current constants correspond t o practically identical values for the diffusion coefficients of the tartrate complexes of the various metals. This leveling effect is doubtless due to the fact that the coordinating tartrate ions are so large compared to the size of the central metal ion, that differences in the size of the central ion, and even in the type and orientation of the bonds, have only a slight influence on the effective sizes of the different complexes.
Vol. 15, No. 9
(3) Heyrovsw, J., and IlkoviE, D., Collection Czechoslou. Chem. Commzcn., 7, 198 (1935). (4) IlkoviE, D.,Ibid., 6,498 (1934); J . chim. phys., 35, 129 (1938). (5) Kaoirkova, K., Collection Ctechoslov. Chem. Commun., 1, 477 (1929). (6) Kolthoff, I. M., and Lingane, J. J., “Polarography”, New York, Interscience Publishers, 1941. (7) Kolthoff, I. M.,and Orlemann, E. F., J . Am. Cham. Soc., 63, 2085 (1941). (8) Latimer, W.M.,“Oxidation Potentials”, New York, PrenticeHall, 1938. (9) Lingane, J. J., Chem. Rev., 29, 1-36 (1941). (10) Lingane, J. J., IND. ENQ.CHBM.,ANAL.ED., 14,655 (1942). (11) Lingane, J. J., J . Am. Chem. Soc., 65,866 (1943). (12) Lingane, J. J., and Kolthoff, I. M., Ibcd., 61,825 (1939). (13) Lingane, J. J., and Laitinen, H. A., IND. ENQ.CREM.,ANAL. ED., 11, 504 (1939). (14) Maas, J., “De Polarografisohe Methode met de Druppelende Kwikelectrode ten Dienste van het Pharmaceutisch Onderzoek”, Amsterdam, 1937. (15) Page, J. E., and Robinson, F. A., J . Soc. Cham. Znd., 61, 93 (1942). (16) Smrz, J., Rec. trav. chim., 44,580 (1925). (17) Suohy, K.,Collection Czechoslov. Chem. Commun., 3 , 354 (1931); Chem. News, 143,213 (1929).
Literature Cited (1) Bambach, K.,IND. ENO.CHIDM., ANAL,ED., 14, 265 (1942). (2) Bayerle, V.,Rcc. tsar. chim., 44, 514 (1925).
An Apparatus for Quantitative Catalytic Hydrogenation LLOYD M. JOSHEL, Hyattsville, Md.
T
HE apparatus for catalytic hydrogenation recently described by Noller and Barusch (3) is very useful when not more than 50 ml. of hydrogen are to be absorbed; however, i t is not suitable for the many occasions when i t is necessary to meter with some degree of accuracy as much as several hundred milliliters of hydrogen. The apparatus pictured (Figure 1) fills the entire gap between a strictly micro apparatus and the standard Adams (Burgess-Parr) machine. This apparatus is a combination, with certain modifications, of features previously reported. The buret system is similar t o that described by Fieser and Hershberg (2)and the reaction flask together with the stirring device is a slight modification of the one used by Noller and Barusch (3). The 50-ml. buret is for
small-scale work and the 500-ml. buret is used for larger runs. If more than 500 ml. of hydrogen are t o be absorbed, the stirring is slackened or momentarily stopped whlle the buret is quickly refilled with hydrogen. All the stopcocks are the oblique-bore high-pressure type manufactured by Eck and Krebs, New York, N. Y., who fabricated the glass parts of this apparatus. For those who wish t o construct their own apparatus, attention is called to the pressure stopcock recently described by Connelly (I). A somewhat less elegant, although reasonably satisfactory, method is t o use rubber bands t o keep the stopcocks seated firmly. Ordinary stopcocks with none of these precautions will leak hydrogen, especially if extra internal pressure is applied during the hydrogenation as described below. Flasks of the three sizes indicated afford flexibility in the volume of liquid that can be used, and small coil springs keep the greased ground-glass joint tightly sealed. This joint is reversed to minimize the danger of contamination of the flask contents with grease. A critical dimension is the height of the apparatus, which should not be less than 1 meter, in order that the system may be subjected to either a vacuum or the pressure of an extra atmosphere without forcing mercury out of the manometer.
.
Some experimenters will prefer using water or the solvent being employed in the hydrogenation as the displacing fluid instead of mercury, especially in view of the current limited availability of mercury. Increased accuracy can probably be obtained if the amount of dead space is reduced by using capillary tubing of about 3-mm. bore in place of the IO-mm. outside diameter standard tubing indicated for the manometer and connections. Unless care is exercised, the stirring may become vigorous enough t o drive the stirrer through the wall of the flask, but encasing the iron core in a material such as Saran, instead of glass, may be at least a partial solution of this difficulty. The air can be displaced from the apparatus by opening the flask stopcock and sweeping with hydrogen; however, unless care has been taken to trap hydrogen in the buret by closing the buret stopcock a t the end of the previous hydrogenation, mere sweeping may leave a pocket of air in the buret. It is prudent, therefore, to open the buret stopcock, raise the mercury to a point near the top of the buret, close the screw clamp, and, with the flask stopcock closed, successively evacuate and flood the system with hydrogen three or four times by manipulating the three-way stopcock. Any addition of catalyst or compound after the system has been filled with hydrogen is made by washing the material through the cup into the flask with a little solvent after the internal pressure has been reduced by lowering the mercury in the buret. A small amount of solvent is allowed to remain in the cup, so that air is not admitted to the system. The sequence of addition of catalyst and reagents can be left to the discretion of the investigator; in many cases it is convenient to have the sample in the flask before attaching it t o the buret system, but where a highly volatile substance is to be hydrogenated, it is preferable to introduce it through the cup after the sweeping or evacuation procedure has been completed. The catalyst can be reduced in the presence of the sample or the catalyst can be reduced first, a reading
September 15, 1943
ANALYTICAL EDITION
taken, and then the compound added through the cup. The former procedure provides a somewhat more active catalyst, whereas the latter has the advantage of eliminating the necessity of making a blank run to determine the amount of hydrogen absorbed by the catalyst. Before the initial volumetric reading is taken, the height of the mercury in the buret is adjusted so that the pressure inside the system is equal to the atmospheric pressure, as eyidenced by the same mercury level in both arms of the
I
(ui FIGURE1
manometer. A facile way of doing this is to raise the level of the mercury in the buret until the internal pressure slightly exceeds the atmospheric pressure and then vent the excess hydrogen to the atmosphere by momentarily opening the three-way stopcock. During the reduction, up to about one atmosphere pressure can be applied by raising the mercury level in the buret and closing the screw clamp, remembering that the internal pressure must be equalized with the atmospheric pressure before reading the volume. The apparatus lends itself very easily to correction for changes in barometric pressure during the experiment, since with the aid of the manometer and leveling bulb a measured differential between the internal and atmospheric pressures can be created a t will before making the final volumetric reading. For example, if the barometric pressure increases 5 mm. during the experiment, the mercury in the buret is adjusted so that the level in the right (open) arm of the manometer is 5 mm. below the level in the left (closed) arm
591
before the volume is read. The absolute pressure in the system will then be the same as a t the beginning of the experiment, . I t is desirable to operate in a constant-temperature room but, in the absence of such a room, temperature changes during the experiment also can be compensated for by proper adjustment of the internal pressure in relation to the atmospheric pressure. If the temperature increases during the experiment, the mercury in the buret should be adjusted so that the internal pressure is greater than the atmospheric pressure before making the final volumetric reading; and, vice versa, if the temperature decreases, the internal pressure should be adjusted so that it is less than the atmospheric pressure. Calculation shows that for this purpose a pressure differential of about 2.5 mm. of mercury corresponds to a change of 1' C. If both the pressure and temperature change, the algebraic sum of both corrections is used in adjusting the manometer levels:
where AP is the increment which the internal pressure should be adjusted in excess of the atmospheric pressure a t the end of the experiment-i. e., the number of millimeters the mercury level in the open arm of the manometer must be set above the level in the closed arm. Pi is the initial barometric pressure minus the vapor pressure of the solvent a t the initial absolute temperature, T,; Pf and T, are the corresponding net pressure and temperature a t the end of the e x p e r i m e n t . S i n c e t h e s e are all known, AP is easily calculated. In this way the final volume corrected to the initial temperature and pres-. I sure is read directly. The net volume of hydrogen absorbed is then corrected to standard conditions in the usual manner. Although no rigorous comparison has been made with the rate of hydrogenation obtained when using a shaking device, it can be shown that magnetic stirring affords reasonably rapid reduction. For example, reduction of 1.5 grams of nitrobenzene in ethanol solution was complete (900 ml. of hydrogen) in 1.25 hours although the Adams catalyst used (25 mg.) was over two years old.
Acknowledgment The collaboration of W. Nudenberg, H. A. Fouche, R. E. Davis, and L. JV. But2 of the United States Department of Agriculture is gratefully acknowleged.
Literature Cited (1) Connelly, IND. ENQ.CHEM.,A N . ~ LED., . 15, 200 (1943). (2) Fieser and Hershberg, J . Am. Chem. SOC.,60, 940 (1938). (3) Noller and Barusch, IND. ENG. CHEM., ANAL. ED., 14, 907
(1942).
