Separation and Identification of Cations

PURPOSE OP EXPERIMENT Separate and identify aqueous cations from group 1, group 2, or from groups 1 and 2 combined of one conventional qualitative analysis scheme. 

Cations often interfere with each other in the final tests designed to detect the presence of specific cations. Therefore, cations must first be separated before identification can be accomplished. In fact, as with many chemical mixtures, separation of cations may be considerably more difficult than identification. Careful work is again very important if the separations are not clean, results in identification tests may be masked by interfering cations. Separation of a complex mixture of cations is by no means simple and is generally broken down into several parts. Each part involves a fairly small group of cations which can be isolated from the mixture on the basis of some property which is common to the ions in the group and then studied as a separate set. After isolation, the cations within a group are further resolved by means of a series of chemical reactions into soluble and insoluble fractions which are sufficient to allow identification of each cation by one or more tests specific to that ion once interferences have been removed. Various types of chemical reactions will be used for separations and identifications in this experiment precipitation reactions, acid-base reactions, complex ion formations, and oxidation-reduction reactions. 

You will study two groups of cations in this experiment. Group 1 cations form chloride salts that are insoluble in water. Be careful not to confuse these cations with the alkali metal cations from Group I of the periodic table which all form soluble salts. Group 1 includes Ag , Hg2 or Hg(I), and Pb2+. They are separated from a mixture of cations because they precipitate white insoluble chlorides, for example, silver chloride, AgCl, when a small excess of Cl is added. 

Care must be taken to ensure that only a small excess of Cl is added because a large excess of Cl causes the formation of soluble complexes, for example, [AgCl2] , 

Once isolated as a group, Ag , Hg2, and Pb2+ are separated from each other and identified as follows. Lead(II) chloride, PbCl2/ dissolves in hot water while AgCl and mercury(I) chloride, Hg2Cl2/ remain insoluble. The hot solution of Pb2+ is separated from the solids, and Pb t is identified by the formation of yellow, insoluble lead(II) chromate, PbCr04. 

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In the laboratory you will most likely carry out one or more experiments involving the separation and identification of cations present in an "unknown" solution. A scheme of analysis for 21 different cations is shown in Table A. As you can see, the general approach is to take out each group (I, II, III, IV) in succession, using selective precipitation. 

VIII.8 SEPARATION AND IDENTIFICATION OF CATIONS IN SOLUTION The preliminary tests and tests for anions may have indicated the presence of certain elements (e.g. As, Cr, Mn) which are normally identified as cations. This information should always be kept in mind when the separation and identification of cations is to be attempted. 

V.9 SEPARATION AND IDENTIFICATION OF THE GROUP I CATIONS (SILVER GROUP) The residue after filtering off the precipitate obtained with dilute hydrochloric acid (cf. Table V.12 in Section V.8) may contain lead, silver, and mercury ions. Their separation and identification can be carried out according to the scheme shown in Table V. 13. 

Table V.13 Separation and identification of Group I cations (silver group) The ppt. may contain PbCl2, AgCl, and Hg2Cl3. Wash the ppt. on the filter first with 2 ml of 2m HC1, then 2-3 times with 1 ml portions of cold water and reject the washings with water. Transfer the ppt. to a small beaker or to a boiling tube, and boil with 5-10 ml water. Filter hot. (1)...

For the separation and identification of Group IIA cations (Hg2+, Pb2+, Bi3+, Cu2+, and Cd2+) two, somewhat different methods are recommended. From the reagents used, these are called (a) sulphuric acid method and (b) sodium hydroxide method. 

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... 

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. 

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). 

VII.l INTRODUCTION In the previous chapters the discussions were restricted to those cations and anions which occur most often in ordinary samples. Having studied the reactions, separation, and identification of those ions, the student should now concentrate on the so-called rarer elements. Many of these, like tungsten, molybdenum, titanium, vanadium, and beryllium, have important industrial applications. 

Table VII.l Separation and identification of Group I cations in the presence of Tl and W...

VII.14 SEPARATION AND IDENTIFICATION OF GROUP II CATIONS IN THE PRESENCE OF MOLYBDENUM, GOLD, PLATINUM, PALLADIUM, SELENIUM, AND TELLURIUM The first step in this separation process is to separate cations into Groups IIA and IIB. 

The separation and identification of Group IIB cations can be carried out by following the prescriptions given in Table VII.4. 

Table VII.4 Separation and Identification of Group IIB cations in the presence of Pt, Au,...

If Pt, Au, Se, and Te are known to be absent, the procedure can be simplified. This simplified procedure is given in Table VII.5. Table VII.5 Separation and identification of Group IIB cations in the presence of Mo Boil the Group ppt. with 5 ml concentrated HC1 for 5 minutes, dilute with 2-3 ml water, pass H2S for 1 minute (to reprecipitate small amounts of As that may have dissolved) and filter. ... 

The precipitation of Group IIIA cations with ammonia solution is carried out according to the procedure given in Table V12. The separation and identification of Group IIIA cations in the presence of some less common ions, as described in Table VII.6 commences with the precipitate obtained with ammonia. 

Section VIII.4), followed by testing for anions (Sections VIII.5-VIII.7), and by separation and identification of the cations present (Section VIIL8 and VIII.9). Some teachers of qualitative inorganic analysis may prefer to start with the separation of cations - in this case a test for phosphate and fluoride has to be carried out before attempting the separation of Group III cations. 

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