Crystallization Craig tube

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

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. 

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. 

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). 

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). 

Two procedures are given here for crystallizing sulfanilamide a semimicroscale procedure using an Erlenmeyer flask and a Hirsch funnel (Experiment 3A) and a microscale procedure with a Craig tube (Experiment 3B). Your instructor may assign both or just one of these procedures. 

Technique 11, Section 11.7, Part B). When it appears that no further crystallization is occurring at room temperature, place the Craig tube in an ice-water bath using a beaker (Technique 6, Section 6.5). Be sure that both water and ice are present and that the beaker is small enough to prevent the Craig tube from tipping over. 

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. 

The decolorized acetaminophen is collected on a Hirsch funnel. It is further purified by crystallization from a methanol/water mixture. There are two procedures given in this experiment. Experiment llA involves crystallization using a Craig tube, whereas Experiment IIB is a larger-scale reaction involving an Erlenmeyer flask and Hirsch funnel for crystallization. 

Purify all of the crude chalcone from hot 95% ethanol or hot methanol using the semimicroscale crystallization procedure (Technique 11, Section 11.3). Alternatively, purify part of the chalcone using a Craig tube (Technique 8, Section 8.7, and Technique 11, Section 11.4), as follows ... 

If you started with aniline, transfer the solid to a 10-mL Erlenmeyer flask and recrystallize the product from 95% ethanol (see Technique 11, Section 11.3, and Figure 11.4). Filter the crystals on a Hirsch funnel and dry them for several minutes with suction. The brominated products from either acetanilide or anisole should be crystallized using a Craig tube (Technique 11, Section 11.4 and Figure 11.6). Use 95% ethanol to crystallize the acetanilide product and hexane to crystallize the brominated anisole compound. Allow the crystals to air-dry and determine the weight and melting point. 

Weigh the crude product and crystallize it from hot water (use 1.0 to 1.2 mL water/O.l g) using a Craig tube (Technique 11, Section 11.4, and Figure 11.6). Step 2 in Figure 11.6 (removal of insoluble impurities) should not be required in this crystallization. Let the purified product dry until the next laboratory period. 

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). 

Esterification Crystallization Use of a Craig tube Nuclear magnetic resonance Critical thinking application... 

Using a Craig tube, recrystallize the crude product from hot 95% ethanol. Allow the crystals to dry. Weigh the dried crystals, calculate the percentage yield, and determine the melting point (literature value is 90-91 °C). Determine the infrared spectrum of the product using the dry film method. Determine the proton NMR spectrum of the product in CDCI3 solution. 

During a crystallization procedure, you often must remove excess solvent from the solution. If a Craig tube is being used for the crystallization, the excess solvent can be removed directly from the Craig tube (see Technique 11, Section 11.4). The Craig tube is placed in a warm water bath or warm sand bath. Alternatively,... 

Filtration is a technique used for two main purposes. The first is to remove solid impurities from a liquid. The second is to collect a desired solid from the solution from which it was precipitated or crystallized. Several different kinds of filtration are commonly used two general methods include gravity filtration and vacuum (or suction) filtration. Two techniques specific to the microscale laboratory are filtration with a filter-tip pipette and filtration with a Craig tube. The various filtration techniques and their applications are summarized in Table 8.1. These techniques are discussed in more detail in the following sections. 

Craig tubes Used to collect a small amount of crystals resulting from crystallizations in which the volume of the solution is less than 2 mL. 8.7... 

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. 

These steps are illustrated in Figure 11.4. It should be pointed out that a microscale crystallization with a Craig tube involves the same four steps, although the apparatus and procedures are somewhat different (see Section 11.4). 

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