Manostat Lewis

The Lewis manostat operates on the same principle, except that a sintered glass disk is substituted for the capillary, and a column of mercury takes the place of the diver. The bulbs A and B are nearly half-filled with mercury, which should be boiled tmder reduced pressure to remove trapped gases. In operation, both stopcocks are left open until the pressure in the distillation system is slightly above the desired value, and then they are closed. The reference pressure above the mercury in A will cause it to flow into B as the pressure decreases, until the sintered glass plate is covered by the mercury. At this point, the system is isolated from the pump. As the pressure increases, the mercury flows back into A and pumping resumes. 

In addition to these considerations, it is necessary to make provisions for controlling and measuring the vacuum produced by the pump this requires that a manostat and a manometer be incorporated into the system. Excellent Cartesian diver manostats are available commercially at reasonable prices ca. 50), and these and the Lewis manostat (Fig. 2-20) are very satisfactory for most organic distillations since pressure variations within the system will be very small during their operation. It is desirable to protea the manostat from the system with a Dry Ice trap to avoid fouling of the mercury by contact with organic vapors. 

In using a Lewis manostat, the large tubes are half-filled with mercury, and both stopcocks are opened. When the pressure is slightly above that desired, they are both closed, and the manostat controls the pressure in the system by opening and closing it to the pump as required. 

A manostat (one pressure) is a device for maintaining any pressure desired from 760 to 2 torr with an accuracy of 1 to 2 torr. Suppose you had prepared /w-nitrotoluene and wanted to purify it. At 20 torr, the compound boils at 112.8 C. How can you maintain a pressure of 20 torr in your system during the entire distillation so that you can be sure that you collect only the pure material A manostat is a device that allows you to do this. The two most common types are the Cartesian diver and the Lewis-Nester. 

Goggles, safety Hammer Holders, clamp Manometer, U-tube, 1 m Manostat, Lewis-Nester Mantle, heating, 250 mL Motor, magnetic stirring... 

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