Separation of cations into groups

8 SEPARATION OF CATIONS INTO GROUPS Once the sample is dissolved, the separation of cations into groups can be attempted. This can be done according to the scheme outlined in Table V.12. 

Before describing the general scheme for the separation of the metal ions into groups, the following facts are brought to the notice of the student as it is believed that by their proper understanding and appreciation, many of the usual pitfalls will be avoided. 

The analysis should not be conducted with large quantities of the substance because much time will be spent in filtering the precipitates and difficulty will be experienced in washing and dissolving them. It is therefore recommended that 0 5—1 g should be employed for the analysis. After a little experience the student will be able to judge from the relative sizes of the precipitates, the relative quantities of the various components present in the mixture. 

The conditions for precipitation and for solution must be rigidly followed. 

All precipitates must be washed to remove adhering solution in order to avoid contamination by the metals remaining in the filtrate. The first washings should be added to the solution from which the precipitate has been filtered later washings may be discarded. 

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Table V.12 Separation of cations into groups (Anions of organic acids, borates, fluorides, silicates, and phosphates being present). Add a few drops of dilute HC1 to the cold solution. If a ppt. forms, continue adding dilute HC1 until no further precipitation takes place. Filter. (1)...

Table V.12 Separation of cations into groups (contd.)...

When a suitable solvent has been found, prepare the solution for analysis using about 50 mg of the solid the volume of the final solution should be 1-1-5 ml. Use this solution for the separation of cations into groups according to Section VI.9. 

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. 

Separation of cations into groups can be carried out according to the instructions of Table VI. 11. Note, that this method is suitable only if anions of organic acids, borate, fluoride, silicate, and phosphate are absent. Modifications of the separation scheme in the presence of these anions are described in Section VI. 17. 

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.

For the separation of cations into Groups IIA and IIB the prescriptions of Table VII.2 should be followed. 

Table VII13 Separation of cations into groups (anions of organic acids, fluoride, and phosphate being absent) (contd.)...

TABLE 16.5 Separation of Cations into Groups According to Their Precipitation Reactions with Various Reagents... 

FIGURE 1 Simplified strategy for separation of cations into groups for qualitative analysis. 

These operations are all incorporated into Table V.12, describing the general separation scheme of cations into groups. 

V.10 SEPARATION OF GROUP II CATIONS INTO GROUPS IIA AND IIIB Having precipitated the sulphides of the Group II cations (cf. Section V.8, Table V. 12) the next task is to separate these into Groups IIA (Hg2+, Bi3+, Pb2+, Cu2+ and Cd2+) and IIB (As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+). There are two methods recommended for the purpose, one making use of (yellow) ammonium polysulphide, the other making use of potassium hydroxide. 

For the separation of Group II cations into Groups IIA and IIB two methods were suggested (cf. Section V.10, Tables V.14 and V.15). To separate and identify the Group IIB cations, one of the two recommended procedures must be followed, according to the method chosen in the previous step. If the ammonium polysulphide method (method a) has been followed in Section V.10, then method (a) has to be applied here. Method (b), on the other hand, has to be used if the potassium hydroxide method was followed to separate Groups IIA and IIB. 

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. 

Table VII.2 Separation of Group II cations into Groups HA and IIB in the presence of...

The classic scheme for qualitative analysis of a mixture containing all the common cations (listed in Fig. 8.13) involves first separating the cations into five major groups based on solubilities. (These groups are not directly related to the groups of the periodic table.) Each group is then treated further to separate and identify the individual ions. We will be concerned here only with separation of the major groups. 

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. 

Poly(trifluorochloroethylene), known as Daiflon , can be treated with tributyl phosphate, trioctylamine, or acetylacetone for example, to separate many cations into several groups (327). This material has been used in electrolytic cells and appears to have interesting environmental applications because of its inert nature. Similarly, a porous tetrafluoroethylene membrane infiltrated with 8-hydroxyquinoline can extract 95% of copper(II) in 2 h (328). A commercial... 

Two points regarding qualitative analysis must be mentioned. First, the separation of the cations into groups is made as selective as possible, that is, the anions that are added as reagents must be such that they will precipitate the fewest types of cations. For example, all the cations in group 1 also form insoluble sulfides. Thus, if HjS were... 

Similar reactions form the basis of the separation of cations. An example of the separation of inorganic anions at the ppm level is shown in Figure 3(b). Size exclusion chromatography (SEC). This is suitable for mixtures of solutes with relative molecular masses (J(MM) in the range lOMO Da. Stationary phases are either microparticulate cross-linked co-polymers of styrene and divinyl benzene with a narrow distribution of pore sizes, or controlled-porosity silica gels, usually end-capped with a short alkyl chain reagent to prevent adsorptive interactions with solutes. Exclusion is not a true sorption mechanism because solutes do not interact with the stationary phase (Topic D2). They can be divided into three groups ... 

The strategy for the separation of cations involves then-division into a set of groups by treatment with a particular selective reagent. After solution of the unknown in water, a reagent is employed to cause all cations of the group to precipitate, with all other ions remaining in solution. A typical overall scheme (simplified) is depicted in Fig. 1. After separation, the precipitate is reexamined for specific... 

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