A Greaseless High-Vacuum Valve R. H. CRIST AND F. B. BROWN, Columbia University, New York, N. Y.
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N T H E investigation of f gases, particularly in 3 the field of reaction kinetics, the impurities introduced with the stopcock lubricant have caused a great deal of difficulty. I n order to work with chlorine systems where a small amount of foreign gas may completely change the kinetics, Bodenstein (1) constructed an all-glass valve that has been used with success. Ramsperger (6)designed a valve making use of a metal diaphragm with silver chloride as the s e a t i n g surface. T h e authors have designed a more rugged and compact valve, also using silver chloride as the seating surface. The details of construction are given in Figure 1. A silver seat, B , covered with silver chloride and mounted on a metal diaphragm is pressed against the polished end of the glass valve stem at A. The glass parts of the valve are of Pyrex. The diaphragm is a platinum alloy, covered FIGURE 1 with B thin layer of silver chloride, and sealed to the outer edge of the glass by silver chloride. The glass is platinized t o give a metallic surface over which the molten chloride will flow, It was found that a properly fitted diaphragm rim accomplished the same purpose, the contraction of both metal and chloride on cooling givin a very satisfactor seal. The valve seat, B, is o f silver with silver ciloride on the inside, well compacted and shaped t o fit A . The internal diameter of A is around 3 mm., thus permitting rapid flow of gas. The entire surface of B is covered with the chloride. The silver art, C, extends through the dia hragm and is screwed into B. !‘he gas seal between B and the iiaphragm is by means of silver chloride. The brass piston, D, is screwed tight to C and is thrust downward by the spring, which has a pressure of less than 40 kg. (80 pounds) per square mm. of bearing surface on A , as it was found that the silver chloride would flow if the pressure were much greater. A lug through the outer brass case prevents twisting. The vertical motion is controlled by the thumbscrew, E,and is limited by the stop on the cap, F, to between 0.5 and 1.0 mm. When the valve is assembled the final seating is made by supporting the valve rigidly under the lower end, turning the thumbscrew to %lose” until it is free, and puttin a steel ball in the top of cap F. The ball is tapped lightly severaf times with a small hammer, 30 grams (1 ounce). The piston is raised and lowered, and the tapping is repeated. This is done several times. The extent to which the silver chloride has been compacted can be observed by the change in the position at which the thumbscrew just engages in lifting the piston. The chloride layer is about 0.5 mm. thick. If re-seating is necessary the same procedure can be used. When the valves are in the apparatus the high-tension coil cannot be used for detecting leaks because of injury to the silver chloride surface. It is best to assemble the glass parts without the valves, test as usual, and then ut in the valves. Leaks from the last operation can be detected y! some technique such as the use of the Apie~onclay-oil product. It is important to prevent solid particles, particularly lass, from being blown through the valve. If glass is to be cut, &e inside pressure should be brought t o one atmosphere.
A considerable number of these valves have been made and tested by observing the leak through the seat as measured on a McLeod gage of 300-cc. capacity. This leak was generally less than 0.0001 mm. per hour. No measurable leak that could be attributed to the outer diaphragm glass seal was observed. There was no effect of seating the valve with or without an atmosphere differential on the diaphragm. The valve pumps out quickly, there being no outgassing as is usually experienced with ordinary greased stopcocks. The layer of silver chloride on the metal parts prevents gas from coming from this source. It seems probable that the valve would be serviceable a t higher temperatures. The upper limit would be the temperature a t which silver chloride would flow under the pressure necessary for closure. Acknowledgment The authors wish to acknowledge their indebtedness to the Klett Manufacturing Company for the construction of this valve and for many useful ideas in its design.
Literature Cited (1) Bodenstein, M., Z . physik. Chem., B7, 387 (1930). (2) Ramsperger, H. C . , Rev. Sci. Instruments, 2, 738 (1931).
Improvement for a Meniscus Reader EDGAR J. BOGAN, University of Maine, Orono, Maine
READER
FISHER
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OR those doing numerous titrations and using a meniscus reader, the ease of reading the meniscus is offset by the need of removing the buret from its holder each time to slide the reader from the top to the bottom reading. This operation can be eliminated by making a slight change in the reader as shown in the diagram. The tubular rivets holding one side of the reader together are removed to free the clear celluloid from the black and white. The top rivet is replaced to hold the black and white celluloid together. One part of an ordinary snap fastener is placed on themclearcelluloid and the other part is placed on the black. This may be done by making small holes in the celluloid and tying the fastener on with a heavy thread. The fastener is placed near the edge of the reader, so that a fingernail can be inserted to pry it open. The meniscus reader can then be easily snapped on and off the buret. This improvement makes it possible for the meniscus reader to be used on gas burets as easily as on any other type. 396
