Elmar Piel Norwich University Northfield, Vermont 05663
An Inexpensive Mercury-Free
Chemistry lahoratory procedures sometimes call for working a t specific pressures below atmospheric, hut above normal, aspirator pressures. Some monomer distillation procedures are examples. Attainment of these conditions usually requires both a manometer and a manostat. Most commonly, hoth pieces of apparatus are relatively expensive and use considerable mercury. These are big disadvantages when large classes are involved. The apparatus shown in the accompanying figure, which uses only inexpensive and common lahoratory apparatus and solvents can function hoth to hold a fixed pressure a t some chosen value and to indicate continually whatever pressure is held. I t therefore is a replacement for both manostat and manometer for this purpose. The apparatus maintains the pressure a t a fixed value by making use of t h e vapor pressure of a control liquid held a t constant temperature with the spent water from the aspirator. The vapor-flow choke limits the boiling rate of the liquid and allows appreciable fine control over the temperature of the liquid and hence the system's pressure. Appropriate control liquids may readily he chosen from a selected list on the basis of tapwater temperature and semi-log plots of vapor pressure versus temperature for the liquids (good experience for the student). Operation of the apparatus is as follows: When the aspirator is turned on, the pressure in the system soon drops to the vapor pressure of the control liquid so that it starts to boil. It should he allowed to boil at least so rapidly as to he sure that the phenomenon is that of hoiling rather than mere bubbling from the ebullition capillary. The hoiling lowers the temperature of the -control liquid, hut this temperature soon comes to equilihrium because of the aspirator water bath. The vapor flow resistance of the choke controls the rate of boiling of the control liquid, so that its temperature can he held anywhere from a fraction of a degree t o five or more degrees Centigrade below the temperature of the aspirator water. When the thermometer in the hoiling control liquid is steady, equilibrium has been reached, and the pressure of the system is equal to the vapor pressure of the control liquid at that temperature. This is found on the vapor pressure chart. Equilibrium is normally reached in 2-5 min (depending on choke). Appreciable system leaks are indicated by failure of the control liquid to boil. The choke is simply a '14-in. polyethylene tube section pulled into a capillary after Bunsen flame heating. The length of capillary suitable is selected by razor blade cut and try till the hoiling rate is satisfactory. It connects two sections of rubber hose as shown in the figure, and can conveniently be pulled out to break the vacuum. The quantity of control liquid used can be very small, especially when operating with plenty of choke. Under such conditions 20 ml of the liquid will last for several hours. It should he pointed out that the top area of the control liquid in the boiler should he as small as possible. This is
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Journal of Chemical Education
EWER AND CCNTROC LIQUID OF KNOWN V. I?
TEMPERATURE CONTROL BATH
Manostat assembly to limit evaporation from its surface so that the vaporization is mostly from hoiling. Only when the hoiling phenomenon is observed can one he sure that the vapor pressure of the liquid and the pressure in the system are the same. It is therefore usually preferable to use as a boiler a large side-arm test tube or a distilling flask filled into the neck rather than a filtering flask. An additional feature of this manostat is that the usual trap can he dispensed with when one is operating a t the higher pressures. The following solvents (and their vapor pressures a t 18°C) are suggested as covering a useful range of distillation pressures Ethyl ether carbon disulfide t-butyl chloride acetone n-hexane carbon tetrachloride trichloroethylene isopropyl acetate isooetane dioxane
Data for the above vapor pressures were taken from a semi-log chart of the vapor pressure versus temperature made up from data on the "Low Vapor Pressure of Organic Compounds" in the "Handbook of Chemistry and Physics." ..~ The author thanks senior Norwich University students Kirk Bordeaux. William Lincourt. and Howard Stranger who helped "debug" this mercury-free manostat.
