Microscale Work

For the distillation of small amounts of material the apparatus of Pozzi and E. cot [11] is very convenient, a.s the steam-boiler simultaneously serves as heater for the distillation vessel (Fig. 222). Parnass and Wagner [121 offer an arrangement for microscale work. 

As an example of the benefits of microscale work, consider the t)q)ical Kjeldahl reaction, which uses mercury as a catalyst. The mercury waste produced by this procedure creates a difficult disposal problem. Converting to micro-Kjeldahl equipment and quantities reduces the waste by 90%, which could result in a reduction of several liters of waste per day in laboratories that routinely run Kjeldahl reactions. 

In microscale work, it may sometimes be more convenient to use a column prepared with a Pasteur pipette to separate fine particles from a solution. The Pasteur pipette is packed with alumina or silica gel, as shown in Figure 8.6. 

In microscale work, the apparatus with the round-bottom flask can be modified by replacing the round-bottom flask with a conical vial. The glass tubing is connected by vacuum tubing to either an aspirator or a vacuum pump. A convenient alternative, using a side arm test tube, is also shown in Figure 11.8B. With either apparatus, install a water trap when an aspirator is used. 

Gas chromatography is one of the most useful instrumental tools for separating and analyzing organic compounds that can be vaporized without decomposition. Common uses include testing the purity of a substance and separating the components of a mixture. The relative amounts of the components in a mixture may also be determined. In some cases, gas chromatography can be used to identify a compound. In microscale work, it can also be used as a preparative method to isolate pure compounds from a small amount of a mixture. 

Another method for collecting samples is to connect a cooled trap to the exit port of the column. A simple trap, suitable for microscale work, is illustrated in Figure 22.12. Suitable coolants include ice water, liquid nitrogen, or... 

Commercial sublimation adapters such as those shown in Figure 2.59 are available for microscale work. A Winston adapter is normally used with a sublimation chamber such as a 3- or 5-mL conical vial (Fig. 2.59a), whereas adjustable adapters may be used with 5- or 10-mL round-bottom flasks (Fig. 2.59b). In both cases a cold-finger that may be connected to a vacuum source is fitted to the chamber containing the substance to be sublimed, and the connection is secured with a standard-taper joint that is often accompanied by an O-ring and threaded collar (Fig. 2.59). The cold-finger can be filled with ice or dry ice. The advantage of using... 

Solvent extraction is frequently used in the organic laboratory to separate or isolate a desired compound from a mixture or from impurities. Solvent extraction methods are readily adapted to microscale work because small quantities are easily manipulated in solution. Solvent extraction methods are based on the solubOity characteristics of organic substances in the solvents used in a particular separation procedure. Liquid-liquid and solid-liquid extractions are the two major types of extractions used in the organic laboratory. 

In microscale work, never pour a recovered liquid product from the rotary flask. Always use a Pasteur pipet. 

The choice of the method suitable for the solution of a certain problem is significantly influenced by the scale on which the analyst can or must work in a given case. A limited amount of sample, a very low concentration of the substance to be detected, or the presence of impurities in the sample exclude the application of common macro-methods, but compels the analyst to work on a semimicro, micro, or ultramicro scale. The limits of these individual scales cannot be determined unambiguously and they vary from method to method therefore, we shall indicate them in each particular method. We consider it necessary to point out that throughout the world the development of analytical chemistry is characterized by a trend toward microscale work and even lower, because this usually leads to an increase in the speed of analytical work, even though training in work on such a scale seems rather tiresome. Nowadays this transition is facilitated by the per-... 

If the 3,5-dinitrobenzoates have melting points which are too high, benzoic acid esters are used for the identification of alcohols, namely glycols. On the other hand, in cases where the 3,5-dinitrobenzoates are low-melting substances, the alcohols are identified in the form of urethanes, preferably naphthylurethanes. Identification procedmes are also elaborated for microscale work a partial disadvantage consists in the fact that the samples for identification must be anhydrous. 

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