Pinch-cocks

Cut off the delivery tube at a distance of about 1.5 cm. from the neck and seal on a tube B of thin glass 1.3 cm. in diameter and 50 cm. to act as an air condenser, bending the tube as shown in the figure. Make a tube C shaped like a pipette the wide part of which will just slip into the neck of the flask. The lower end is of very small tubing drawn to a capillary at the tip. When the tip is almost at the bottom of the flask, the upper part of the narrow tube should be just above the outlet into the condenser tube. The wide tube should extend about 3 cm. above the end of the neck of the flask and should then contract to a short, narrow tube over which a short piece of rubber tubing, provided with a pinch cock F, is slipped. 

Procedure Set up the apparatus as shown in Fig. 10. Clamp the wash bottle in an inverted position. Attach a 2-foot piece of rubber tube to the mouth piece of the wash bottle. Slip a pinch cock over the tube, and insert a small funnel in the open end. Raise the funnel to the position shown by the dotted lines in Fig. 10. Pour water through the funnel, and fill the wash bottle nearly to the top of the vertical tube. Close the pinch cock, and replace the funnel with the capillary tip from the wash bottle. Connect the other tube of the wash bottle to the Pyrex test tube by means of an elbow tube and rubber connection. Open the pinch cock and allow water to run out of the wash bottle into the beaker. If the apparatus is tight the flow of water will stop completely after a moment. Close the pinch cock, remove the Pyrex test tube, dry it, and weigh it accurately. Place in it... 

The scale at the back is marked off in divisions of 0.1 inch. The acid is placed in the shallow dish at the left and water in the dish at the right, both being at the same temperature. The level is adjusted in both dishes so that the surface of each liquid is exactly at the zero point of the scale. Suction is applied at the top until the acid rises exactly to the 100 mark when the pinch cock is closed. The reading of the scale at the top of the water column divided by 100 gives the specific gravity of the acid. 

The number of drops formed per minute should not exceed 20 as otherwise they will not be properly formed. This rate can be controlled with the help of a pinch cock attached to a rubber tube fixed at the upper end of the stalagmometer, not shown here. 

To prevent Bumping. — In vacuum distillations a troublesome bumping (a sudden, violent ebullition) frequently occurs- To prevent this a slow, continuous current of air is drawn through the liquid, thus keeping it in constant motion. The air current, controlled by a pinch-cock, must not be allowed to enter too rapidly, otherwise. it will be difficult to maintain a high vacuum. The same effect may be obtained by placing certain substances in the liquid — splinters of wood the size of a match, capillary tubes, bits of glass, pieces of porcelain, powdered talc, scraps of platinum wire or foil. Small pieces of pumice-stone bound with platinum... 

Method of Obtaining Heat—Bottles—A Chemical Balance—Test Tube Racks—A Ring Stand—A Hydrogen Sulphide Generator — Necessary Apparatus — Test Tubes — Evaporating Dishes—Fire Clay Crucibles—Filter Paper—Graduated Cylinder—Chemical Thermometers—Pipette—Red and Blue Litmus Paper—Beakers—Pinch Cocks—Thistle Tubes—Flasks— Clamps—Chemicals for the Laboratory. Pages 40-62... 

The hydrogen sulphide is forced out of the rubber tube. When enough of the gas has been produced, the pinch-cock is closed and the pressure of the H S increases until the hydrochloric acid is forced from the chimney. 

Pinch-cocks are used in connection with rubber tubing to either cut off a supply of liquid or gas. The pinch-cock merely presses the tube together and holds it tightly so that nothing can pass. Owing to the nature of the rubber perfectly gas tight and liquid stoppage is made. 

D shows a spring clamp for holding work to be soldered or cemented. It may also be used as a pinch cock. 

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