Craig apparatus

Many materials have such low distribution ratios that neither separatory funnels nor the older, no longer commercially available, Craig apparatus are practical to use. What is needed is a continuous extraction with an apparatus that can be left unattended for long periods of time. Several different approaches are described in this chapter. 

How does the Ito system differ from the older Craig system Do not try to answer this unless you have a diagram of a Craig apparatus. 

Table 4.2 Countercurrent Distribution of a Given Solute in a Craig Apparatus... 

These techniques were introduced to assist biotechnologists with separations on both analytical and preparative scales, but have recently been shown to apply equally well to inorganic and metal ion separations. These techniques can provide separations of thousands of stages with apparatus that does not fill a large room, as did the Craig apparatus. The Sanki instrument is quite compact, occupying a space of about a 2-ft cube, not counting the detector and other accessory equipment. 

Three steps required for multistage solvent extraction, i.e., phase mixing, phase settling, and transfer of the mobile phase, are defined clearly in the discontinuous countercurrent distribution process using the Craig apparatus. These basic requirements are essentially fulfilled by the use of a coiled tube in a continuous fashion. Solvent extraction using a coiled column is most efficiently performed with a horizontally laid coil that rotates about its own axis. In this horizontal coil orientation, the rotation induces the well known Archimedean screw force, which can be utilized for performing countercurrent solvent extraction. 

Using Craig apparatus, complex mixtures of fragile, heat labile solutes such as hormones and vitamins were routinely separated. However, because of its bulk and expense, and the time required for its separation, this technique has largely been replaced by chromatographic techniques. 

Craig apparatus filled with 100 ml upper phase and 100 ml lower phase of a mixture equilibrated at I9°C of benzyl alcohol-n-butanol-1 N hydrochloric acid-water-aq. sodium chloride (saturated at 19°C) (200 100 6 300 60). 30 shakes/ distribution at 15 min intervals. First two tubes contain solvent next three tubes contain 2 g mixture 115 transfers, 120 tubes Detection... 

While solvent extraction is often done on a small scale by synthetic lab chemists using a separatory funnel or Craig apparatus, it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, thin layer extractors, spray columns, pulsed columns, and mixer-settlers. 

Find, in a reference book, a description of the Craig countercurrent distribution apparatus and discuss its design as it relates to the description of countercurrent extraction presented in this chapter. 

Figure 4.4. Two interlocking glass units for Craig counter-current distribution, (a) Position during extraction, (b) Position during transfer. (Note by returning the apparatus from (b) to (a) the transfer is completed. The mobile phase movos on to the next unit and is replaced by a fresh portion)...

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. 

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

Apparatus for continuous addition and transfer of one phase, involving a helix of narrow-bore tubing, has been described by Ito and Bowman. With a coil of 8000 turns and continuous phase separation provided by centrifugation they obtained separations equivalent to about as many as 5000 tubes used in the conventional Craig manner this number would be expected imder some conditions if each turn in the capillary acts as one equilibrium vessel under conditions of continuous transfer. 

Craig has designed an extraction apparatus of the former type, one form of which is semiautomatic and the other form completely automatic. The semiautomatic model will be described here. The extraction tube is shown in Fig. 4-17. A series of tubes are joined together at point e and are closed off at d by means of a ground butt joint and a... 

Fig. 4-18. Countercurrent-extraction apparatus. (Courtesy of Dr. L. Craig and Interscience Publishers, Inc.)...

Craig, L. C., and O. Post Apparatus for Countercurrent Distribution. Analyt. 

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. 

Although excellent results may frequently be obtained by fractional extraction with such simple equipment as a set of separating funnels, still more efficient separations may be effected when more elaborate apparatus is used. There is commercially available a fully automatic equipment, named after Craig, which is so designed that several transfers occur simultaneously in a train of contacting tubes. There are counter-current instruments containing from a few dozen tubes up to 1000 or more. 

The development of the method of countercurrent distribution is largely due to the work of Craig and his associates (see Craig et al., 1949). The method is applicable to the separation of any substances that can be reversibly distributed between two immiscible solvents, and these, of course, include peptides. Several different types of apparatus... 

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