Capillary Seals for Dropping Mercury Electrode Assembly R. G . Barradas and J. L. A. French Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, Toronto 5, Ontario, Canada
A LARGE VARIETY of polarographic dropping mercury electrodes (DME) have been described and reviewed in the literature (1-3). The task of constructing an all-glass DME system presents a problem in sealing the capillary to the rest of the assembly when the composition of the capillary differs from the other glass components. Capillaries of borosilicate glass as recommended by Milner (3) are not always easily prepared with uniform internal diameters, which are very convenient for adjustment of drop rates by cutting the capillary to an estimated length. Commercially available capillaries (E. H. Sargent and Co. Nos. S 29417 and S 29419) as recommended by Meites (2) are very satisfactory for the above mentioned purpose. Unfortunately these capillaries are made from a high lead content type of glass [60 silica, 10 soda, 4 z alumina, 2 z potash, and 23 lead oxide (4)] which is difficult to seal on to borosilicate glass or soda glass tubing that contains the mercury head. In conventional analytical polarography slight variations in drop times ( t ) are not of serious consequences, but in electrocapillary work where drop times should be reproducible the presence of epoxy cement, rubber, to =tO.OOl second (3, Tygon, Teflon, or other comparable union between the capillary and the rest of the tubing containing the mercury column may lead to trace contamination which significantly affects the precision and accuracy o f t determinations. The problem is easily overcome by the following simple and novel technique that we have found to be satisfactory.
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PROCEDURE
Union of Sargent Lead Glass Capillary to Soda Glass Tubing. Sargent capillaries are 21 cm in length with an internal diameter of about 20 to 30 microns. The first step is to widen this narrow capillary orifice to a workable size of about 2 mm in diameter. The procedure is to support the capillary in a vertical position with its lower end immersed to about 1 mm of conductivity water in a beaker. The water usually rises to about two thirds of the length of the upright capillary. The open end is sealed with the use of a microhand torch in the following manner. A small hot flame applied directly to the top of the open end is preferred to heating the side walls of the capillary. When sealed and (1) J. Heyrovsky and J. Kuta, “Principles of Polarography,”
Academic Press, New York, 1966, pp. 41-2 (and the references contained therein). (2) L. Meites, “Polarographic Techniques,” 2nd Ed., Interscience, New York, 1965, pp. 73-83. (3) G. W. C. Milner, “Principles and Applications of Polarography and Other Electroanalytical Processes,” Longmans, Green and Co., London, 1957, pp. 19-20. (4) E. H. Sargent and Co. of Canada Ltd., private communication, 9 Milvan Drive, Weston, Ontario, March 6, 1967. ( 5 ) R. G. Barradas and F. M. Kimmerle, Can. J. Chem., 45, 109 (1967).
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ANALYTICAL CHEMISTRY
cooled, the capillary is removed from the beaker, inverted and held vertically with an asbestos covered clamp. The open end is sealed as before with water trapped within the capillary. With both ends sealed the entire length of the capillary is warmed gently with a wide but soft yellow flame for about 1 to 2 minutes. While the tube is hot (ca. 200” C), cautiously reheat the upper end with the micro-hand torch allowing the small hot flame to play on shoulders as well as on the top of the closed capillary. Expansion of the entrapped water vapor will create a glass bubble in the region of the softened glass. When the bubble is approximately 2 mm wide, rapidly heat the center of the thin glass bubble for a brief moment. Remove flame before perforation of the bubble, and the expanding water vapor pressure will explode an approximately 2-mm diameter hole in the entire bubble area. The edges of this hole are carefully fire-polished using the oxidizing region of the flame in order to avoid blackening of the glass due to reduction of the lead oxide present. A piece of soda glass tubing of desired length is then sealed to a short length of Corning lead-containing glass (Code OOlO), which is then easily sealed to the prepared capillary as described above. The closed end of the capillary is then opened and shortened by cutting with a sharp knife to an appropriate length corresponding to the approximately desired drop time. Union of Sargent Lead Glass Capillary to Borosilicate Glass Tubing. This can be done by directly sealing the borosilicate glass tube to the 0010 end of a standard grade Corning multiseal tubing 6460. For economical reasons (representing about ‘/E, to 1 / 2 ~ of the cost of the 6460 seal), it is recommended that the Corning Tubing Graded Seal Bundle No. 6462 be used instead. Each bundle contains six distinct glasses labelled Nos. 1 to 6 ranging from borosilicate to a lead-containing glass. The individual glasses are sealed in sequential numerical order and each sectional seal should ideally be approximately the diameter of the tubing itself. After construction of the six-graded seal, unite a proper length of borosilicate glass tubing to end No. 1 and add an inch of 0010 lead glass to end No. 6. The latter procedure helps to ensure thermal stability, thereby reducing a possibility of shattering the entire multisealed glass. The final step is to proceed as described for the soda glass case by attaching the 2-mm wide end of the Sargent capillary to the 0100 end of the prepared tubing. Regulation of Mercury Head. For an all glass DME assembly, mercury is forced through the capillary by purified nitrogen pressure control in a manner analogous to that described for a Lippmann capillary electrometer electrode (6, 7). RECEIVED for review March 29, 1967. Accepted April 19, 1967. Work supported in part by the National Research Council of Canada and the Ontario Government Department of University Affairs. (6) R. G. Barradas and P. G. Hamilton, Can. J . Chem., 43, 2468 (1965). (7) K. M. Joshi and R. Parsons, Electrochim. Acta, 4, 189 (1961).
