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ANALYTICAL. CHEMISTRY, VOC. 51, NO. 13, NOVEMBER 1979
determines the length of the mercury plug. The valves are then switched to the gas flushing sequence whereby the inert gas finishes the coating and dries the column.
RESIJLTS AND DISCUSSION The system designed has been tested repeatedly and offers several advantages over other reported apparatus. It depends on valves rather than on using a moving capillary to introduce the mercury plug. The coating phase is physically separated from the mercury and, thus, the mercury supply remains essentially noncontaminated. The mercury is isolated from the liquid phase container. Only the delivery Teflon pathway toward the column presents a contact of liquid and mercury. The apparatus handles mercury safely making it difficult to spill mercury during such operations as loading, cleaning, and pressurizing the assembly. The same apparatus can be used to rinse the column or for the passage of deactivating solutions. It is possible to expand the unit by attaching an additional glass cylinder tu the unused ports of one valve. Thus the second glass container can be
used exclusively for washing or deactivation treatments. This would reduce further the possibilities of contamination. The additional cylinder can be made of plastic and used for etching the columns with liquid solutions ( 5 ) or by attaching a gas generator. Finally, the assembled unit as shown in Figure 1 has components which are found in most separation laboratories and has a cost of less than $150. We have repeatedly used this system in coating columns obtaining theoretical plates number of 2000-3000 for k = 14-20 (C-14) and internal column diameters of 0.25 mm.
LITERATURE CITED (1) Jennings, W. "Gas Chromatography with Glass Capillary Columns"; Academic Press: New York, 1978; p 31 (2) Sandra, P.; Verzele, M. Chromatographia 1977, 10, 419-425. (3) Onuska, F. I.; Comba, M. E.; Bistriki, T.; Wilkinson. R. J. J . Chromatcgr. 1977, 142, 117-125. (4) Schomburg, G.; Husmann, H. Chromatographia 1975, 8,517-519. (5) Heckman, R . A . ;G-een, R.; Best, F Anal. Chem. 1978, 50, 2157-2158.
RECEIVED for review April 23,1979. Accepted August 6,1979.
Surge Control Unit for Mercury Manometers Jesse S. Ard Eastern Regional Research Center, Agricultural Research, Science and Education Administration, 600 E. Mermaid Lane, Philadelphia, Pennsylvania 19 1 !8
The reading of a mercury meiiiscus can be critical to obtaining correct pressure and volume measurements. Sometimes correction9 have to be applied for local gravity, temperature, capillary depression, the height of a cylinder equivalent to the volume of a curved section, glass refraction, static electricity, and dryness of the gas exposed ( I , 2). These factors can produce errors, but these can be reduced or eliminated by the use of wide-diameter glass tubing where the meniscus is read. Such tubing is also desirable for obtaining large displncernent volumes for McCleod gauges, hydrogenation apparatus, Toepler pumps, and other gas handling systems. However, glass wnlls of large chambers cannot well survive the liquid hammering action of large masses of surging mercury, whicli often occurs through careless handling of valves or other accidents. The usual provision for surge control has been a constriction in the lower connections, but this, if small enough, niay rrtard normal flow and may result in long delays in reading the menisccls. A simple glass unit (Figure 1)that provides division and reunion of a liquid stream has been designed tu overcome these difficulties. Diameters of the channels can be chosen to fit particular circumstances. Normally, the unit would be placed in the lower part of' a glass structure. Connecting units in tandem may be used t o enhance the dampening effect. Glass tubing may he bent to form a unit. A somewhat less efficient unit may be made more easily by blowing a bulb, flattening it, and pushing in each side with carbon to form an island-like structure encircled by channels The glass units should be annealed well to overcome local stresses that are likely to occur if one branch contracts more than the other. The unit does not trap bubbles, avoids the need for narrow constrictions, and offers resistance to flow that is an increasing function of the mercury velocity. Thus, surging iu prevented without appreciable effect on normal flow rates. Resistance to mercury motion approaches zero as the velocity approaches zero, and the meniscus settles to the correct level with negligible hysteresis. Surges upward and downward are dampened alike. For example, on upward flow, collision of the mercury streams produces a resistance to flow a t the top
Figure 1. Glass unit for mercury surge control by divergent-convergent actions
juncture. A t the same time, the velocity is attenuated a t the bottom juncture because of the division of the mercury into two streams. This attenuation not only minimizes the forces from the moving mercury but also nearly balances the lateral forces through opposing symmetry. In actual operation, no tendency for the unit to jump could be observed, and the small glass structure withstood the internal forces well. These units have served for manometers, hydrogenation apparatus, and large fixed McCleod gauges. For example, a surge control unit measuring 40 mm across the squared part and containing channels 2.7 mm in diameter for the divided stream proved satisfactory in controlling mercury hammer in a 200-mL volume bulb and overshoot in the comparison column of a McCleod gauge.
LITERATURE CITED (1) "International Critical Tables," H. H. Kimball, Ed., No. 1, Maple Press CO.. York, Pa., 1926, p 68. (2) W. Cawood and H. S. Patterson, Trans. Faraday Soc.,29, 514 (1933).
RECEIVED for review July 19, 1979. Accepted August 23, 1979.
This article riot subject to U S . Copyright. Published 1979 by the American Chemical Society
