Separation of Ions Using Differences in Solubility

In chemical analysis, it sometimes is necessary to remove one type of ion from solution by precipitation while leaving other ions in solution. For instance, the addition of sulfate ions to a solution containing both potassium and barium ions causes BaS04 to precipitate out, thereby removing most of the Ba ions fi-om the solution. The other product, K2SO4, is soluble and will remain in solution. The BaS04 precipitate can be separated from the solution by filtration. 

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Separation of Ions Using Differences in Solubility Fractional Precipitation Qualitative Analysis of Metal Ions in Solution... 

Change in oxidation state can be used effectively because of the large solubility changes accompanying the changes in oxidation state. Examples of this method of separation are (i) separation of cerium by oxidation to Ce4+ and (ii) reduction of Eu, Sm and Yb to divalent state. Differences in solubility products of RE(OH)3 can be profitably used in their separation by fractional precipitation. For example, Ksp values of La(OH)3 and Lu(OH)3 are 10-19 and 10 24, respectively, and these values show that it is possible to preferentially precipitate the bulk of rare earths in the presence of ammonium ion leaving La3+ in solution. 

It is useful to state that there are complete separation systems of cations devised, based entirely on the separation of metal hydroxides, omitting the use of sulphides altogether. These however will not be discussed here. As it will be seen later, the separation of ions within the third group of cations, is almost entirely based on the differences in the solubilities of their hydroxides. 

Solubility products are usually used as a criterion of reactivity and specificity of collectors. In general, the less the solubility, the more is collectivity, but if the solubility is small enough to result in slight differences in solubility for various metallic ions, the selectivity will decrease. For separation of two minerals, it has been suggested that the ratio of the solubility products of compounds of two mineral ions (pLi/pL2) or the difference (pLi -PL2) must be maintained at a suitable value [6]. 

Ion exclusion is a technique using ion exchange resins to separate ionic compounds as a cla-is from nonionic compounds as a class. It is based on the differences in solubility of ionic compounds and nonionic compounds between the resin beads and the aqueous surroundings. The only requirement is that either the cation on the resin must be the same as the major cation in the sample or the anion on the resin must be the same anion as the sample. No ion exchange takes place, so the resin never needs to be regenerated. 

PRECIPITATION AND SEPARATION OF IONS We learn how differences in solubility can be used to separate ions through selective precipitation. 

Reverse osmosis can be used for the separation of ions om an aqueous solution. Neutral membranes are mainly used for such processes and the transport of ions is determined by their solubility and diffusivity in the membrane (as expressed by the solute permeability coefficient, see eq V 162). The driving force for ion transport is the concentration difference, but if charged membranes or ion-exchange membranes are used instead of neutral membranes ion transport is also affected by the presence of the fixed charge. Teoreil [45] and Meyer and Sievers [46] have used a fixed charge theory to describe ionic transport through these type of systems. This theory is based on two principles the Nemst-Planck equation and Dorman equilibrium. 

The separation of RE metals is one of the most difficult problems within inorganic chemistry. The chemical properties of the elements are so exceptionally similar. When the separation work started with fractional crystallization and precipitation chemists used the small differences in solubility between the RE salts. To some extent the differing basicities of the oxides were also used. As soon as advanced separation techniques, ion exchange and Hquid-Hquid extraction, were developed the situation changed drastically. 

Solutes have differing solubilities in different liqnids dne to variations in the strength of the interaction of solnte molecnles with those of the solvent. Thus, in a system of two immiscible or only partially miscible solvents, different solutes become unevenly distribnted between the two solvent phases, and as noted earlier, this is the basis for the solvent extraction technique. In this context, solvent almost invariably means organic solvent. This uneven distribution is illustrated in Fig. 1.3, which shows the extractability into a kerosene solution of the different metals that appear when stainless steel is dissolved in aqueous acid chloride solution. The metals Mo, Zn, and Fe(III) are easily extracted into the organic solvent mixture at low chloride ion concentration, and Cu, Co, Fe(ll), and Mn at intermediate concentration, while even at the highest chloride concentration in the system, Ni and Cr are poorly extracted. This is used industrially for separating the metals in super-alloy scrap in order to recover the most valuable ones. 

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