if it occurs. may be trimmed off with a razor blade. The lead ring may be reused. The potassium bromide pellet is pushed out, and the lead ring is inserted with a 180" orientation into the dic bore. RESULTS AND DISCUSSION
The necessity for employing high pressures in the satisfactory production of potassium bromide pellets n as early recognized in thi5 investigation. Kereiakes (4) showed by x-ray diffraction methods a decrease in the grain size of potassiuni chloride powder M ith increasing prebsures. Ingebrigtson and Smith concluded that both pressing pressures and prcssing times affect the quality of the pellet ( 3 ) . The higher pressures available from the briquetting p r e s produced a single-crystal, clear specimen which remained transparent for several weeks. Cloudy areas, a phenomenon occurring at low pressures with nonuniformity of powder distribution over the face of the die, were virtually nonexistent. I n the usual technique of disk fabrication, either the die required precise machining and a high surface finish, or the material was pressed into steel rings. In the latter case, tremendous tensile stresses are set up during the pressing operation n hich bring about permanent distortion of the steel ring with attendant scoring of the die bore
during ejection. After several pressings, the ring no longer fits into the die bore. Obviously, the lead not only provides a suitable support for the disk, but also affords a type of lubrication I\ hicli virtually eliminates scoring of die-bore surfaces. Of the several designs which have been used in this a ork, one die in particular has successfully produced over 200 disks without nialfunction. Rings fabricated from unalloyed lead are readily reinserted for additional use and a single ring is capable of producing 12 to 15 pressings. For quantitative applications, some degree of tolerance of ring dimension was achieved in the punch-out process by alloying with arsenic and antimony. These less pliable rings do not reinsert easily. In quantitative applications, a new ring is used for each pressing, while for general routine FT-ork, punch-out rings from ordinary 1/8-inch thick sheet lead work satisfactorily. Potassium bromide disks pressed under the conditions mentioned earlier showed maximum transmittances of 89.5%. This value compares favorably with the measured value of 90.3% a t 2 microns for single-crystal potassium bromide. Energy losses from reflection. scattcring, and absorption amount to about 10 to 11%. The disk generally exhibits 2 to 3 7 , spread betn-ecn niiniriiuni and maximum tr ansniit t a w e .
The effect of taper on absorbance measurements has been evaluated (2, I O ) . If reproducibility in transmittance becomes important, the disk is rotated to maximum transmittance prior t o the scan. ACKNOWLEDGMENT
The author desires to express his appreciation to many members of the Applied Research Laboratories, Glendale, Calif., for assisting in the developnient of this technique and for granting permission to print the line drawing of the potassium bromide die. LITERATURE CITED
(I) Anderson, D. H., Smith, R. G., A N A L . CHEM. 26. 1674 11954). (2) Browning, R.' S., hberley, S. E., Xachod, F. C., Zbzd., 27, 7 (1955). (3) Ingebrigtson, D. S . , Smith, A. L., Zt~zd.,26, IT65 (1954). (4)Kereiakes. J. G., Phiis. Rei,. 98. 553 (1955). 15) Kirkland. J. J.. ANAL.CHEX 27. 1537 ' i1955). (6) Olsen, A4.L., Ibid., 30, 155 (1958). ( 7 ) Perkin-Elmer Corp., Instrument ,+-eu,s 5 , S o . 3, 5 (1954). (8) Ryason, R., J . Opt. SOC.Am. 43, 928 (1953). (9) Scheidt, C., Reinwein, H., Z. S a h r forsch. 76, 270 (1952). (10) Spell, -4.,Rector, H. E., Pittsburgh Conference on Applied Spectroscopy and Analytical Chemistry, Pittsburgh, Pa., March 4, 1954. I
,
Plastic Diaphragm Valve for Burets Kenneth A. Allen, Oak Ridge National Laboratory, O a k Ridge, Tenn.
A
diaphragm valve for burets eliminates many of the difficulties encountercd with commercially available greaseless ~ a 1 1 . c ~Greaseless valves have considerable advantages for aqueous systems and are almost indispensable for certain nonaqueous systems -e.g., sodiuni (>thosidein benzene or perchloric acid in dioxane. Fluoroplastic plugs are difficult to fit to ordinary stopcock barrels, and tend to stick, making fine adjustments difficult. I W i glass needle valve types, liquids tend to leak past the inert' plastic around the needle and come in cont'act with the noninert plastic parts. I n addit,ion, the handles come loose. With the valve diagraninird delicate flow rate adjustments are made easily and precisely. Bubble trapping in the chamber was not a source of difficulty. The diaphragm did not set during buret storage, if the valve was left open, and the diaphragm could easily be reversed or replaced. PLASTIC
0pera.tion of the valve is apparent.
322
ANALYTICAL CHEMISTRY
The body and diaphragm are made from a resilient, inert plastic, such as Teflon
Figure 1. 1.
Diaphragm valve
Inlet and outlet tubes pressfitted into b o d y 2. Body 3. Plastic or metal retainer holds diaphragm against the b o d y with leaktight pressure 4. Diaphragm out of resilient, inert plastic 5. 'Chamber ca. 20 mils 6. Adjustor
or polyethylene. The retainer is of some stiff plastic, such as polymethacrylate or Bakelite. It could also be made of metal, because it does not come in contact nith the liquid. Turning out the body and retainer are machining jobs, but thwe are no critical tolerances. Even the chamber spacing between the diaphragm and body can be 0.020 =k 0.005 inch. ;Is long as the inner spacing between the retainer and the body is reasonably true, considerable rariation is allowable in the other dimensions. The inlet and outlet holes in the body are drilled a few mils smaller than the outside diameter of the glass tubing selected for sealing to the buret and forming the exit nozzle. The resulting press fit is satisfactory with regard to both the seal and the necessary mechanical rigidity. Suitable diaphragms can be cut from '/,,-inch sheet material with a cork borer. This automatically provides a slightly raised, sharp edge which helps make a leaktight seal against the body. Thumbscrews from standard laboratory hardware, with the ends filed smooth, make eucrllmt adjustors.
