I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
418
shows the end of t h a t oxidation with standard potassium dichromate solution. The volume of solution used between the two color changes is equivalent to the iron being determined.
The authors are indebted to H. H. Willard of the University
of Michigan for the electrolytic iron.
Literature Cited (1) Syrokomsky and Stiepin, J. Am. Chem. Soc., 58, 928 (1936). (2) Wu, J. Bid. Chcm., 43, 189 (1920).
Acknowledgment J*
Vol. 13, No. 6
aided the senior author during the
preliminary work with the method.
C ~ N T R I ~ ~6.3 T Ifrom ~ N the Chemioal Laboratories of the University of Utah.
Pressure-RegulatingApparatus for Vacuum Systems FREDERICK M. LEWIS United States Rubber Company, Passaic, N. J.
F
REQUENTLY in practical laboratory work i t is advantageous to employ a regulating device for vacuum systems in cases which do not warrant the expense or complication of a precision apparatus. Several vacuum regulators are described in the literature, representing a considerable range of complexity and precision. So far as is known none of these operate on a principle similar to that indicated below. The instrument described here is intended to combine simplicity with a moderate degree of precision. It is not difficult to construct or operate, is self-contained and positive in action, and will maintain a desired pressure in a reasonably tight system within k0.2 mm. in the range 5 to 100 mm. Higher operating limits require modifications. Two types are illustrated; these differ only in the manner of controlling the operation point. In operation, a high vacuum, chamber A , is allowed to act periodically through a sintered-glass disk, on a system, B, in which it is desired to control the pressure. The operation is controlled by means of the mercury column in the open arm of a manostat, chambers A and B being alternately connected and sealed aa the manostat fluctuates in response to pressure differences in B. The level difference, h (Figure I), or the pressure, p (Figure 2), is then the operating pressure of the system.
n SIU TERED-
GLASS DISK
FIGURE 1
Figure 1 illustrates a unit in which the desired pressure is maintained by rotatin the entire apparatus on a pivot until the porous disk is just sealecfby one level of the mercury, as the difference in the levels between the two mercury columns equals the operating pressure. In practice this is not measured on the apparatus but is read on a manometer connected to B. An increase in pressure in B will result in a movement of mercury in the manostat tube, so as to open the disk between A and B. The stopcock, D,is not essential, but facilitates rapid evacuation of the system at the start. It is closed during operation.
h
ONE /NcH
FIGURE 2 Figure 2 indicates a device employing a residual pressure-in chamber C, rather than a balancin mercury column, to set the oint at which the porous disk wilf be sealed by the manostat %he pro er pressure is obtained in C by leaving open sto cocks D and Zuntil the auxiliary manometer connected to B inxicates the desired operation point, and then closingthem. It isadvisable to insert a stopcock (not shown) between the apparatus and pump ,to aid in this last adjustment; it should be closed as the pressure approaches the desired point to allow the pressure to become equalized. Further drop in pressure in A cannot affect the pressure in B unless a leak or gas evolution in B raises the pressure so as to open the sealed porous disk. Stopcock F is provided only to prevent rapid surges of mercury when air is allowed to enter the a paratus at the completion of an operation. Fine adjustments o f the operating pressure are best made by slightly tilting the unit. A 0.75-inch disk constructed of 60-mesh glass or a commercial G-3 disk appears satisfactory for operation in the range 5 to 100 mm. Hi her working pressures may be attained by using a h e r sintered-gyms partition than specified. Lower maximum operation ressure allows coarser disks, because of lower pressure differentiaracross the disk. A satisfactory artition may be constructed of other porous materials cementexin position. Smooth operation is facilitated by mounting the disk in a slanting position. The mercury placed in the manostat should be boiled out under vacuum, so that bubbles of gas will not rise in the closed arm of the manostat tube and cause a drift of operation pressure. The unit should be protected by a dry ice trap to avoid clogging of the disk. If uncondensed vapors pass through the trap an absorption tube may be added to the assembly.
Acknowledgment The author wishes to thank W. A. Gibbons and Clyde Coleman of the U. Rubber Company for their interest and permission to publish t,his work.
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