Cations exchange

Ion-exchange processes on clay minerals moving from riverwater to seawater (Section 6.2.3) remove about 26% of the river flux of Na+ to the oceans and are significant removal processes for K+ and Mg2+ (Table 6.3). To balance this removal, clay minerals exchange a significant amount of Ca2+ to the oceans, adding an extra 8% to the river flux (Table 6.3). These modern values are, 

Minus sign indicates removal from seawater. 

It was pointed out earlier that the balancing cations associated with the negatively-charged particles consist of 

Reactions of this type are known as cation exchange, sometimes incorrectly referred to as base exchange . Note that the reaction is reversible. 

For most types of cation, at least, the amount of cation exchange is a definite and constant quantity, called the cation exchange capacity, and is measured in milli-equivalents (m.e.) of exchangeable 

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Pure water for use in the laboratory can be obtained from tap water (hard or soft) by distillation if water of great purity is required, distillation must be carried out in special apparatus, usually made of quartz, not glass or metal precautions must be taken to avoid any spray getting into the distillate. Water which is sufficiently pure for most laboratory purposes can, however, be obtained by passing tap water through cation-exchangers and anion-exchangers as described above, when the water is deionised . 

Alternatively an ice-cold dilute solution of sodium peroxide is passed through a column containing a cation-exchanger of the synthetic type (p. 274) where the cation is hydrogen (i.e. H3O then exchange occurs ... 

Conversion of a sodium salt of a carboxylic acid into the free acid e.g., if R-SO H represents the cation exchange resin ... 

When either of the reactants is sensitive to mineral acids, the esterification can often be successfully accomplished with the aid of a cation exchange resin (hydrogen form) in the presence of benzene. Zeo-Karb 225/H, a unifunctional sulphonated polystyrene resin in the hydrogen form, may be used. Thus good yields of isopropyl lactate may be obtained ... 

The exchange resins 6nd application in (i) the purification of water (cation-exchange resin to remove salts, followed by anion-exchange resin to remove free mineral acids and carbonic acid), (ii) removal of inorganic impurities from organic substances, (iii) in the partial separation of amino acids, and (iv) as catalysts in organic reactions (e.g., esterification. Section 111,102, and cyanoethylation. Section VI,22). 

Sulfonate exchangers contain the group —SO3, which is strongly acidic and completely dissociated whether in the H form or the cation form. These exchangers are used for cation exchange. 

Cation exchange resins — gel type—strongly acidic—sulfonic acid functionality ... 

Dowex 50W-X2 0.6 0.70 Strongly acidic cation exchanger with S-DVB matrix for separation of peptides, nucleotides, and cations. Molecular weight exclusion <2700. 

Bio-Rex 70 2.4 0.70 Weakly acidic cation exchanger with car-boxylate groups on a macroreticular acrylic matrix for separation and fractionation of proteins, peptides, enzymes, and amines, particularly high molecular weight solutes. Does not denature proteins as do styrene-based resins. 

Cation exchange resin— -macroreticular type- — sulfonic acid functionality... 

Weak cation exchanger—macroreticular type—carboxylic acid or phenolic functionality... 

AMP-1 4.0 Microcrystalline ammonium molybdo-phosphate with cation exchange capacity of 1.2 mequiv/g. Selectively adsorbs larger alkali metal ions from smaller alkali metal ions, particularly cesium. 

Carboxylate exchangers contain —COOH groups which have weak acidic properties and will only function as cation exchangers when the pH is sufficiently high (pH > 6) to permit complete dissociation of the —COOH site. Outside this range the ion exchanger can be used only at the cost of reduced capacity. 

Ion-Exchange Equilibrium. Retention differences among cations with an anion exchanger, or among anions with a cation exchanger, are governed by the physical properties of the solvated ions. The stationary phase will show these preferences ... 

Example Shaking 50 mL of 0.001 Af cesium salt solution with 1.0 g of a strong cation exchanger in the H-form (with a capacity of 3.0 mequiv g ) removes the following amount of cesium. The... 

Accurately weigh about 6 g NaCl and dissolve in distilled water. Pass the solution through a well-rinsed cation exchange column (Dowex 50W) in the hydrogen form. The equivalent amount of HCl is washed from the column (in 10 column volumes) into a volumetric flask and made up to volume. Equivalent weight is the formula weight. 

Water. Distilled water must be (a) redistilled in an all-Pyrex glass apparatus or (b) purified by passage through a column of cation exchange resin in the sodium form. For storage, polyethylene bottles are most satisfactory, particularly for very dilute (0.00 lAf) EDTA solutions. 

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