Centrifugation, Craig tubes

When crystallization is complete fit the matching glass rod (a close fit is essential) into the Craig tube and secure it tightly with a rubber band (Fig. 11,2b). Place the inverted assembly in a centrifuge tube and centrifuge for a few minutes (remember to use a counter-balancing tube and solvent). 

After centrifuging, the mother liquor will have been forced into the centrifuge tube (Fig. 11.2c). Remove the Craig tube from the centrifuge tube, remove the glass rod, then tap the tube to so that the crystals fall to the bottom (Fig. 11.2d). Cover the tube with foil, and dry the crystals under vacuum. 

Now the fun part. You ve got to turn this thing upside down, put it in a centrifuge tube, and not have it come apart and spill everything. Easy to say— Solvent is now removed by inverting the Craig tube assembly into a centrifuge tube... (Mayo et al., p. 80). Easy to do. .. well ... 

Get your centrifuge tube and cut a length of copper wire as long as the tube. Make a loop in the end that s big enough to slip over the end of the Craig tube stem. Twist the wire so that the loop won t open if you pull on it a bit. 

Put the centrifuge tube upside down over the top of the Craig tube (Fig. 71b). Watch it. If the fit is snug, the copper wire could cause... 

The Three Bears of centrifuge tubes meet the Craig tube. 

After you ve centrifuged the Craig tube, and the centrifuge has stopped turning... 

Pull the centrifuge tubes out, and carefully lift out the Craig tube. The solvent will have filled the bottom of the centrifuge tube, and the crystals will be packed down near the Craig tube top (Fig. 72). 

Isolating The Crystals Craig Tube Filtration Centrifuging the Craig Tube... 

When crystallization is complete, place the Craig tube in a centrifuge tube and separate the crystals from the mother liquor by centrifugation. Follow the procedure in Technique 11, Section 11.7. 

Isolate the crystals from the Craig tube by centrifugation (see Technique 8, Section 8.7, and Figure 8.11) and allow the crystals to air-dry. Determine the weight and the melting point (164°C). 

Using the copper wire, then pull the Craig tube out of the centrifuge tube. If the crystals collected on the end of the inner plug, it is now a simple procedure to remove the plug and scrape the crystals with a spatula onto a watch glass, a day plate, or a piece of smooth paper. Otherwise, it will be necessary to scrape the crystals from the inside surface of the outer part of the Craig tube. 

When the crystals have formed and the mixture has cooled in an ice-water bath (if desired), the Craig tube is placed in a centrifuge tube and the crystals are separated from the mother liquor by centrifugation (see Technique 8, Section 8.8). The crystals are then scraped off the end of the inner plug or from inside the Craig tube onto a watch glass or piece of paper. Minimal drying will be necessary (see Section 11.9). 

Collect crystals by vacuum filtration using a Hirsch funnel or by centrifugation using a Craig tube. 

Equip the Craig tube for filtration, isolate the crystalline product by centrifugation. and transfer it to a piece of filter or weighing paper for air-drying. Recrystallize the diacid from water. 

Apparatus A dry 5-mL conical vial, gas trap, ice-water bath, 15-mL screw-cap centrifuge tube, apparatus for magnetic stirring, heating under reflux, vacuum and Craig-tube filtration, and flameless heating. 

Work-Up, Isolation, and Purification Transfer the cooled reaction mixture to a screw-cap centrifuge tube and add 1.5 mL of water. Rinse the reaction vial with 2 mL of diethyl ether, and add this rinse to the centrifuge tube. Cap the centrifuge tube, and shake the contents to mix the layers. Separate the layers and extract the aqueous layer sequentially with two additional 2-mL portions of diethyl ether. Extract the combined diethyl ether layers with two separate 2-mL portions of saturated aqueous sodium bicarbonate. Slowly add concenfrafed hydrochloric acid dropwise to the comb/ned bicarbonate washings until the solution is slightly acidic (pH = 3). Collect the precipitate by vacuum filtration, wash the filter cake with 1-2 mL of water, and air-dry the crude product. 4-Chlorobenzoic acid may be recrystallized from methanol in a Craig tube. 

Apparatus Glass apparatus from the microscale experimental procedure of Section 19.2, 3-mL conical vial, two screw-cap centrifuge tubes, 1-mL plastic syringe, Pasteur pipet with 0.5- and 1.0-mL calibration marks, ice-water bath, and apparatus for magnetic stirring, simple distillation, vacuum filtration, Craig tube filtration, and fiameless heating. 

Craig tubes are a particularly effective method for recrystallizing small quantities of reaction products. These tubes possess a nonuniform ground joint in the outer section. The substitution of Teflon for glass in the head makes these systems quite durable and much less susceptible to breakage during centrifugation (see Fig. 3.4). 

Step 7. Remove the solvent by inverting the Craig tube assembly into a centrifuge tube and spinning the mother liquors away from the crystals (Fig. 5.32). This operation should be carried out with care. First, fit the head with a thin copper wire (Fig. 5.32), held in place by a loop at the end of the wire that is placed around the narrow part of the neck. Some Teflon heads have a hole in the neck to anchor the wire. The copper wire should not be much longer than the centrifuge tube. 

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