INDUSTRIAL AND ENGINEERING CHEMISTRY
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material (limited only by the weighing accuracy of the available microchemical balance), and the comparative sturdiness of the apparatus. The less commonly used principle of percolation gives better service with respect to general applicability than siphoning (Soxhlet). The apparatus developed by Titus and Meloche is easier to manipulate than the one introduced by Gorbach. One feature, however, which is seldom applied in any of the known macro- or microextractors is incorporated in the Gorbach apparatus-namely, the use of a platinum filter dish as a container for the material to be extracted. Filter devices of metal permit the highest precision in weighing and maximum percentage of recovery of the residual matter without a n y contamination from the filtering material. The weighing of milligram amounts of residue on comparatively large filter paper surfaces (extraction thimbles) will not give reliable results, and the microanalyst has to face the fact that all extractors of the siphoning type using paper thimbles are limited t o the determination of the extractable matter. The difficulties in weighing the residual material in the apparatus examined prevented the development of a general formula for the efficiency of an extraction process and of numerical terms for the evaluation of the various factors influencing the extraction. To accomplish this, a satisfactory microextractor should use the principle of percolation of the solvent through the sample which is held in a platinum container provided with a filter layer of platinum-iridium sponge. The sample should be agitated by means of hot solvent vapors in order to increase the temperature, and condensation of the solvent effected by means of a short-stem water condenser having a device for distributing the refluxed solvent. For convenience the sample container should be attached to the condenser, with the latter connected to the outer shell by means of an outside ground joint. The receiver for the extract should be of low weight, preferably of platinum, and of larger top surface to allow fast evaporation from the top without loss due to creeping. Protection from the influence of atmospheric moisture must also be considered. The ratio of solvent volume to solvent in the thimble should be about 50 to 1, with the ratio of
Voi. 13, No. 2
chamber volume to initial volume of solvent as small as practical. 9 circular hinged heater supplying uniform heat to all sides is recommended for the proposed extractor.
Acknowledgment The authors express their thanks to Arthur Os01 of the Philadelphia College of Pharmacy and Science for his constructive criticism and to Ellice RicDonald, director of the Biochemical Research Foundation, for his interest and support throughout the work.
Literature Cited (1) Alber, H. K., IND.ENQ.CHEM.,Anal. Ed., 12, 764 (1940). (2) Alber, H. K., J . FranklinZnst., 226,813 (1938). (3) Alber, H. K., Mikrochemie, 25, 47 (1938). (4) Alber, H. K., and Harand, -J., J . FranklinZnst., 224, 729 (1937). (5) Blount, B. K., Mikrochemie, 19, 162 (1936). (6) Browning, B. L., Paper Trade J . , 105, No. 10, 38 (1937); Mikrochemie, 26, 55 (1939). (7) Colegrave, E. B., Analyst, 60, 90 (1935). (8) Donau, J.,Monatsh., 60, 129 (1932). (9) Erdos, J., and Pollak, L., Mikrochemie, 19, 245 (1936). Anal. Ed., 9, 437 (1937). (10) Fulton, R. A., ISD.ESQ. CHEM., (11) Garner, W., Znd. Chemist, 3, 289 (1928). (12) Gettens, R. J., Tech. Studies F i e l d o f F i n e Arts, 2, 107 (1933). (13) Gorbach, G., iMikrochemie, 12, 161 (1933). (14) Hetterich, H., Zbid., 10, 379 (1932). (15) Jendrassik, L., and PetrAs, P., Biochem. Z., 226, 381 (1930). (16) Lehrecke, H., Angew. Chem., 49, 620 (1936). (17) Prausnita, P. H., “Glas- und keramische Filter im Laboratorium”, Leiprig, Akademische Verlagsgesellschaft,1933. (18) Slotta, K. H., private communication. (19) Sperry, W. M., Mikrochemie, 12, 151 (1933). (20) Titus,L., and hleloche, V., IND. ENG.CHEM., Anal. Ed., 5, 286 (1933). (21) Wasitzky, A,, Mikrochemie, 11, l ( 1 9 3 2 ) . (22) Weissberger, A., and Proskauer, E., “Organic Solvents”, Oxford, Clarendon Press, 1935. PRESENTED before the Division of Microchemistry a t t h e 96th Meeting of the American Chemical Society, Milwaukee, Wis. Abstract of a portion of a thesis submitted to the faculty of the Philadelphia College of Pharmacy and Science b y William G. B a t t in partial fulfillment of the requirements for the D.Sc. degree.
Pressure Regulator for Microdetermination of Carbon
and Hydrogen
T
HE modified flowmeter
JOHN E. VANCE Yale University, New Haven, Conn.
described here h a s proved satisfactory as a pressure regulator for microcombustions. The chief advantages are that it avoids the use of a hydrostatic pressure head and permits a permanent adjustment of the pressure to be made, if desired. It is particularly suitable for undergraduate students taking their first course in analytical chemistry, and it has been used in this laboratory with success over a period of years. The regulator is shown in the accompanying figure. The oxygen gas enters a t A and, depending upon the height of mercury in the overflow tube, B, will exert a certain pressure on the capillary, D. This pressure is registered on the mercury manometer, C, which
f
)
I
W
0 1 1
has a paper millimeter scale behind it. The rate of flow through F to the furnace depends upon the size of the capillary as well as upon the head of mercury. A convenient size for the capillary is 0.2 to 0.3 mm.; this will give a flow of 4 to 6 cc. per minute with an appropriate pressure. The capillary may be drawn out, and the piece of glass tubing into which it is sealed held in the manometer by means of rubber tubing. The rate of flow is then measured; the capillary is removed for alteration and the flow remeasured until the proper size has been found, after which the glass is sealed in place. Once fixed, there should be no need for change, and since the gas flows upward through the capillary, it cannot become clogged by foreign matter. If desired, the device may be calibrated by measuring the flow under a few different pressure heads. The pressure may be varied by use of the bulb, E. The results of such a calibration are reproducible and, when plotted, may be interpolated to give an exact gage of the flow for any pressure. This is a convenience if the sweeping out process a t the end of a combustion is to be hastened. If a single flow is sufficient, there is no need for E, and the overflow tube may be sealed off a t the bottom and filled with the proper amount of mercury. In operation, the oxygen is permitted to overflow in B at a rate of about one bubble a second. At this rate of overflow there is no appreciable variation in the pressure.
