Semimicro scale

XII,2. SOME TYPICAL OPERATIONS ON THE SEMIMICRO SCALE 1. Distillation at atmospheric pressure.—A simple apparatus is shown in Fig. XII, 2, 1 the pear-shaped distilling flask with long side... 

Chapter XII is concerned with Semimicro Technique. There can be little doubt that preparations on a smaller scale than has hitherto been customary have many advantages particular reference may be made to cost, time and bench space, all of which are important factors in teaching laboratories and also in training for research. Once the student has mastered the special technique, no difficulty should be experienced in adapting most of the preparations described in the book to the semimicro scale. A few examples of small-scale preparations are included together with a suggested list of experiments for an elementary course. 

When necessary very small amounts of titrants may be generated this dispenses with the difficulties involved in the standardisation and storage of dilute solutions and the procedure is ideally adapted for use on a micro or semimicro scale. 

The oxidation of N-acylglycopyranosylamines on a semimicro scale is straightforward and overoxidation does not occur this was evident in the oxidation of an ZV-benzoyl-L-rhamnopyranosylamine10 and of the anomeric pair of IV-acetyl-D-galactopyranosylamines.66 These anomers were obtained by deacetylation of the pentaacetates of a- and /3-D-galactopyranosylamine. On periodate oxidation, the dialdehydes obtained (73 and 74) showed specific rotations of [a]D +60° and [a]D —96°, respectively. 

Vacuum distillation on the semimicro scale (1-8 ml) is conveniently carried out using the apparatus illustrated in Fig. 2.111. Although this design is not now available commercially, many research laboratories still have this useful piece of... 

For these, and also other, reasons many laboratories now employ semimicro analysis, particularly for the elementary courses. Both macro and semimicro procedures will be given separately in this book in order that the requirements of all types of students may be met. Nevertheless, when the semimicro technique is adopted, students are recommended to read the sections dealing with macro technique. It may be said that when the general technique of semimicro analysis has been mastered and appreciated, no serious difficulty should be encountered in adapting a macro procedure to the semimicro scale. Apart from drop reactions, few applications of the micro technique will be described in the text. 

The test may be performed on the semimicro scale with the aid of the apparatus shown in Fig. III.3. Place 10 drops of the test solution in the semimicro test-... 

The apparatus of Fig. III.3 may be used on the semimicro scale. Place 1 ml test solution in the test-tube, add some pure aluminium turnings, and 1 ml 2m potassium hydroxide solution. Gentle warming is usually necessary. A yellow or grey stain is produced after several minutes. 

VI.2 THE STUDY OF REACTIONS OF CATIONS AND ANIONS ON THE SEMIMICRO SCALE Before attempting to analyse unknown samples with semimicro techniques, it is worth while to study, in semimicro scale, some of the reactions of cations and anions described in Chapters III and IV. To illustrate the manner in which the simple reactions should be carried out, a few selected examples will be given. 

VI.3 SYSTEMATIC ANALYSIS ON THE SEMIMICRO SCALE. GENERAL CONSIDERATIONS The object of systematic qualitative analysis on the semimicro scale is not only to detect the constituents of a given mixture, an equally important aim is to ascertain the approximate relative amounts of each component. For this purpose about 0-2 g of material is usually employed for the analysis the relative magnitudes of the various precipitates will provide a rough guide as to the proportions of the constituents present. Systematic semimicro analysis is carried out in the following steps ... 

VI.4 PRELIMINARY TESTS ON THE SEMIMICRO SCALE In most cases samples submitted for analysis are solid. The preliminary tests 1-8 described in this section are to be carried out on solid samples. If the sample is liquid (e.g. a solution), these tests can be omitted, though tests 3-8, with little modifications, might be useful even for such samples. Preliminary tests to be carried out with liquid samples are described under test 9. 

VI.5 TESTING FOR ANIONS IN SOLUTION ON THE SEMIMICRO SCALE The preliminary tests 7 and 8 with dilute sulphuric acid and with concentrated sulphuric acid will have provided useful information as to many anions present. For more detailed information, it is necessary to have a solution containing all (or most) anions free from heavy metal ions. This is best prepared by boiling the substance with concentrated sodium carbonate solution double decomposition occurs (either partially or completely) with the production of the insoluble carbonates of the metals (other than alkali metals) and the soluble sodium salts of the anions, which pass into solution. Thus, if the unknown substance is the salt of a bivalent metal M and an acid HA, the following reaction will occur ... 

VI.6 CONFIRMATORY TESTS FOR ANIONS ON THE SEMIMICRO SCALE The tests in the preceding section will indicate the anions present. In general, these should be confirmed by at least one distinctive confirmatory test. The following are recommended. Full experimental details will be found in Chapter IV under the reactions of the anions, the reference to these will be abbreviated as follows thus (IV.2, 7) is to be interpreted as Section IV.2, reaction 7. It will of course be realized that the tests in Chapter IV refer to reactions on the macro scale the student should have no difficulty in reducing these to the semimicro scale once the technique described in Section II.4 has... 

VI.7 SPECIAL TESTS FOR MIXTURES OF ANIONS ON THE SEMIMICRO SCALE The subject is treated fully in Section IV.45, but on a macro scale. The student should be able to adapt these to semimicro work. Some typical semimicro separations of mixtures of anions are given below. The quantities are for guidance only and can be reduced, if desired. If the mixture is insoluble in water it is often convenient to employ the soda extract (Section VI.5) or the neutralized soda extract (Section VI.5, test 5). The special gas testing apparatus of Fig. 11.30 will, of necessity, find application here. 

VI.8 PREPARATION OF SOLUTION FOR CATION TESTING ON THE SEMIMICRO SCALE... 

VI.9 SEPARATION OF CATIONS INTO GROUPS ON THE SEMIMICRO SCALE Once a solution is produced, the systematic search for cations can be started. As the first step, cations should be separated into groups later separations within the individual groups must be carried out. When making these separations, the results of preliminary tests must always be kept in mind. If we know for sure that certain cations are present or absent, we can make appropriate simplifications in our separation procedures, which will result in considerable gain of time. 

Table VI.ll Separation of cations into Groups on the semimicro scale (anions of organic adds, borate, fluoride, silicate, and phosphate being absent) Add 2 drops (1) of dilute HC1 to 1 ml of the clear solution in a 3 ml centrifuge tube (or a 4 ml test-tube). If a ppt. forms, stir and add a further 1-2 drops to ensure complete precipitation. Centrifuge (2) wash the ppt. with a few drops of cold water (3) and add washings to centrifugate.

Table VI.ll Separation of cations into Groups on the semimicro scale (anions of organic acids, borate, fluoride, silicate, and phosphate being absent) (contd.)...

VI.10 SEPARATION AND IDENTIFICATION OF GROUP I CATIONS ON THE SEMIMICRO SCALE The separation of Group I cations can be carried out according to the scheme outlined in Table VI. 12. 

VL11 SEPARATION OF GROUPS IIA AND IIB AND SEPARATION AND IDENTIFICATION OF GROUP IIA CATIONS When describing these separations in macro scale, two alternative methods have been suggested (see Section V.10), one being based on the use of ammonium polysulphide (see Table V.14) and the other on potassium hydroxide (Table V.15). In semimicro scale the potassium hydroxide method is more suitable and this will be presented here. This does not mean however that the ammonium polysulphide... 

Table VI.12 Separation of Group I cations on the semimicro scale The residue may contain PbCl2, AgCl, and Hg2Cl2. Add 1 ml hot water to the ppt., place the tube in a boiling water bath for 1-2 minutes, and stir continuously. Centrifuge rapidly separate the solution from the residue with a capillary pipette and transfer the clear solution to a centrifuge tube.

Table VI.13 Separation of Groups IIA and IIB as well as separation of Group IIA cations on the semimicro scale The ppt. obtained with H2S in the presence of dilute HC1 ( 0-3m) may contain the sulphides HgS, PbS, Bi2S3, CuS, and CdS, and also As2S3, Sb2S3, and SnS2(l). Treat the ppt. with 1-5 ml of 2m KOH solution, and heat in a boiling water bath for 3 minutes with occasional stirring (CAUTION see Note 2). Add 4 drops freshly prepared saturated H2S water stir and centrifuge.

VI.12 SEPARATION AND IDENTIFICATION OF GROUP IIB CATIONS ON THE SEMIMICRO SCALE For this separation the centrifugate from Group IIA is used. The separation scheme outlined in Table VI. 14 is therefore linked directly to Table VI in Section VI.11. If the ammonium polysulphide method has been adopted for the separation of Groups IIA and IIB, the student should scale down the scheme given in Table V.18 (Section V.12) to semimicro scale. 

Table VI.15 Separation of Group IIIA cations on the semimicro scale...

VI.14 SEPARATION AND IDENTIFICATION OF GROUP IIIB CATIONS ON THE SEMIMICRO SCALE The separation scheme outlined in Table VI. 16 commences with the sulphide precipitates obtained according to the prescriptions of the general separation table (Table VI. 11 in Section VI.9). It is a semimicro adaptation of the hydrochloric acid-hydrogen peroxide method, described in Table V.24 (Section V.15). 

Table VI.16 Separation of Group IIIB cations on the semimicro scale The ppt. may contain CoS, NiS, MnS, and ZnS. If it is not black, CoS and NiS are absent. Stir the ppt. in the cold with 1 ml of M HQ (1 volume of concentrated acid 10-12 parts of water) for 1-2 minutes. Centrifuge.

As in the macro scale, two methods are recommended for the separation of Group IV cations the sulphate method is the generally accepted procedure even in semimicro scale, but the nitrate method is equally suitable. The two methods are outlined in Tables VI. 17 and VI. 18 respectively. 

Table VI.17 Separation of Group IV cations with the sulphate method, on the semimicro scale The precipitate may contain BaC03, SrC03, and CaC03. Treat the ppt. with 0 5 ml dilute acetic acid and stir. Place in a hot water bath until the ppt. has dissolved. Dilute with 0-5 ml water. Test 3-4 drops of the hot solution for barium by adding a drop or two of K2Cr04 solution. A yellow ppt. (BaCr04) indicates Ba present.

VI.16 IDENTIFICATION OF GROUP V CATIONS ON THE SEMIMICRO SCALE Apart from a partial separation of magnesium, it is not possible to separate the Group V cations from one another it is however simple to carry out individual tests for each of them. As we have used ammonia and ammonium salts in the previous separations, we cannot test for ammonium ions in the... 

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