Craig tube

A one-piece filtration apparatus, as shown in Fig. 4.12a is very useful for small scale filtration of solutions prior to recrystallization. It is often used in conjunction with a Craig tube (Fig. 4.12b, see Chapter 11 for more detail). A sintered funnel (Fig. 4.13) for filtration under inert atmosphere is also useful (see Chapter 6 for more details). 

Remove the apparatus from the bath, wipe the bulb to remove oil or water, and quickly filter the hot solution into a clean receiver by pressurizing the vessel using hand bellows or an inert gas line. Filtering under pressure in this way avoids the problem of unwanted crystallization, and reduces transfer losses. The hot solution can be filtered into a small conical flask, but on a scale of lOOmg or less, a Craig tube (Fig. 11.2) gives better recovery because it allows the crystals to be recovered without another filtration. 

Filter the hot solution into a suitably sized Craig tube and cover the tube with aluminium foil while crystalhzation occurs (Fig 11.2a). 

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. 

If necessary the product can be recrystallized again in the same tube by repeating the procedure above. The crystals can be dissolved in the Craig tube, in the minimum volume of hot solvent as usual, and the neck of the tube will act as a condenser. Care is required though, and the tube should be no more than one third full of solvent in order to avoid losses. This process can be carried out several times with minimal losses. 

Crystallizations on a small scale are most conveniently carried out using a Craig tube apparatus as described in Chapter 11. 

I ve put drawings of microscale equipment I ve had occasion to use in this section, along with some discussion ofthe 0-ring seals, conical vials, drying tubes, and so on. I ve put full descriptions of certain microscale apparatus with the operations they re used in. So Craig tubes show up with recrystallization the Hickman still is with distillation. 

Finally, although I experimented a bit with this gas collection setup, mostly I read up on the technique in Mayo et al. By now they should have another edition out, but check the photo on the cover of the 1985 work There it is—the entire gas collection setup in full-color glory—with the end of a Craig tube sitting on the capillary gas collection tube. I guess the batteries are extra even if you re the group that designed the batteries. 

After a recrystallization, you usually collect the new crystals by suction on a Buchner funnel (see Chapter 13, Recrystallization ). For microscale quantities you may have to use a Hirsch funnel—a tiny Buchner funnel with sloping sides and a flat porous plate (see Chapter 5, Other Interesting Equipment ). Or you might need the high-tech power of Craig tubes (see Chapter 14, RecrystalUzation Microscale ). Or you might be able to get away with a Pasteur pipet (Fig. 42). 

Usually this is called Craig tube crystallization, because you ve pipet filtered your hot solution into the bottom of a Craig tube. So if you ve recrystalhzed in something else, re-dissolve the crystals and get this solution into the bottom of a Craig tube (Fig. 68). 

Carefully put the upper section of the Craig tube into the bottom section. Let the solution slowly cool to room temperature and then put the tube in an ice-water bath. 

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. 

Hold the bottom of the Craig tube between your thumb and forefinger. Put the loop over the end of the tube. 

Once this is adjusted, tightly press the wire to the glass, so that the wire will keep the Craig tube closed when you turn the tube upside down. 

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... 

Craig tube. You can t push it up to the bottom with your fingers. 

You ll have to lower the craig tube to the bottom once this is upside down... 

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). 

I d get a glass plate or watch glass, open the Craig tube, and put both sections on the glass to dry. (CAUTION—rolling Craig tubes will spread your product on the bench.)... 

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

V is the ratio of the upper phase volume to the lower phase volume and is usually equal to 1 because both volumes in the Craig tubes are usually equal. D is the distribution ratio and equals K, the molecular partition coefficient in the absence of secondary equilibria, as discussed in Chapter 3. If we introduce a mixture of solutes to stage 0, each solute will have its own value for D and hence a unique value for p and q. For example, for a mixture containing four solutes, we would realize a fraction p for the first solute, a fractionp for the second solute, and so forth. In a similar manner, we would also realize a fraction q for the first solute, a fraction q for the second solute, and so forth. 

We seek now to show how the ps and s of Table 4.2 were obtained. We also wish to show how to apply the information contained in Table 4.2. We then extrapolate from the limited number of Craig tubes in Table 4.2 to... 

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