Micro scaling

The various types of Chromatographic Separation have been developed partly to avoid the above disadvantages, but (more particularly) to provide methods of separation on a micro-scale. Three methods are described below ... 

The first of the two experiments given below illustrates the separation of amino-acids, now an almost classic example of the use of paper chromatography the second illustrates the separation of anthranilic acid and iV-methylanthranilic acid. Both experiments show the micro scale of the separation, and also the fact that a mixture of compounds which are chemically closely similar can be readily separated, and also can be identified by the use of controls. 

This conversion cannot easily be carried out on a semi-micro scale by ordinary chemical means. Liberation of an acid from one of its salts by dil. H SO is feasible when the organic acid is insoluble in water (e.g. an aromatic acid) or... 

The difficulties of working with small quantities of liquids are much greater than with small quantities of solids. For example a competent worker can, and does in fact, often work with 100 mg, of solid without any special apparatus. With liquids this is often not practicable because of the much greater losses entailed, particularly when it is realised that one ordinary-sized drop weighs about 50-100 mg. The account which follows gives details of modifications of standard apparatus suitable for the semi-micro scale defined above. 

Distillation Technique. With all distillations on a semi-micro scale it is essential to heat the liquid so slowly that a cushion of its... 

Fractional Distillation. Fractional distillation on a semi-micro scale can be carried out satisfactorily with the fractionating column shown in Fig. 39. The column is 10 cm. long and is filled with pieces... 

Distillation under reduced pressure. The student should first read details of this operation on a macro-scale (p. 28). For micro-scale work the apparatus shown in Fig. 40 is very convenient. A small pear-... 

When the correct solvent for recrystallisation is not known a procedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than 10 mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(B)) and about o i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(B)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a calibrated pipette with shaking to determine whether crystallisation will now take place, indicated by a cloudiness or by the separation of solid.]... 

Molecular Weight Determinatioos. Details of the determination of molecular weights on a semi-micro scale by the Freezing-point method are given on p. 436, and by the Boiling-point method on p. 440. 

The semi-micro scale is particularly suitable for students who wish to carry out a succession of syntheses from one starting material. For this purpose the following conversions are recommended. 

Acetic Acid, CH3COOH, in Aqueous Solution, and crystalline Cupric Acetate, (CH3C00)2Cu,H20. (Semi-micro scale). 

IODOFORM FROM ACETONE. (Semi-micro Scale.) Required Acetone, 0 5 ml. 10% potassium iodide solution, 20 ml. 10% sodium hydroxide solution, 8 ml. zM sodium hypochlorite solution, 20 ml. 

Hydrolysis of Ethyl Acetate (Semi-Micro Scale)... 

Acetanilide (Semi-micro Scale) (1/40 of above scale). 

For a description of the Carius Method on a semi-micro scale, see pp. 502. et seq. 

Two simple methods for the semi-micro estimation of halogens are available, (a) the Carius method, and (b) the Parr bomb method. It should be emphasised that there are other methods available for these estimations on the true micro scale, but they do not lend themselves, by virtue of the balances, apparatus and manipulation required, to semi-micro work, or to the intermittent usage which class-work necessarily entails. 

Principle. This is essentially a small-scale modification of the macro piethod described on p. 416, the substance being completely oxidised in a sealed tube with fuming nitric acid in the presence of silver nitrate, the halogen being thus converted into silver halide. The collection and weighing of the silver halide require special techniques on the semi-micro scale. 

First, considerably greater emphasis has been placed on semimicro techniques and their application to preparations, separations, analysis and physical determinations such as those of molecular weight. We have therefore greatly expanded the section on Manipulation on a semi-micro scale which was in the Third Edition, and we have described many more preparations on this scale, some independent and others as alternatives to the larger-scale preparations which immediately precede them. Some 40 separate preparations on the semi-micro scale are described in detail, in addition to specific directions for the preparation of many classes of crystalline derivatives required for identification purposes. The equipment required for these small-scale reactions has been selected on a realistic basis, and care has been taken not to include the very curious pieces of apparatus sometimes suggested as necessary for working on the semi-micro scale. 

Quantitative estimation of cyclohexane in the presence of benzene and aUphatic hydrocarbons may be accompHshed by a nitration-dehydrogenation method described in Reference 61. The mixture is nitrated with mixed acid and under conditions that induce formation of the soluble mononitroaromatic derivative. The original mixture of hydrocarbons then is dehydrogenated over a platinum catalyst and is nitrated again. The mononitro compounds of the original benzene and the benzene formed by dehydrogenation of the cyclohexane dissolve in the mixed acid. The aUphatic compound remains unattacked and undissolved. This reaction may be carried out on a micro scale. 

There is a wide variety of impellers usiu fluidfoil principles, which are used when flow from the impeller is predominant in the process requirement aud macro- or micro-scale shear rates are a subordinate issue. 

What about the micro-scale phenomena These are dependent primarily on the energy dissipation per unit volume, although one must also be concerned about the energy spec tra. In general, the energy dissipation per unit volume around the impeller is approximately 100 times higher than in the rest of the tank. Tnis results in an rms velocity fluc tuation ratio to the average velocity on the order of 10 I between the impeller zone and the rest of the tank. 

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