Selectivity And General Considerations

As noted, ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase reactions. For example:

In this reaction, the nickel ions of the nickel sulfate (NiS04) are exchanged for the calcium ions of the calcium hydroxide [Ca(OH)2 ] molecule. Similarly, a resin with hydrogen ions available for exchange will exchange those ions for nickel ions from solution. The reaction can be written as follows:

R indicates the organic portion of the resin and S03 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni+2). Trivalent ferric ions would require three resin sites. As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference, or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction

is expressed as: K = (concentration of B+ in resin/concentration of A+ in resin) x (concentration of A+ in solution/concentration of B+ in solution).

The selectivity coefficient expresses the relative distribution of the ions when a resin in the A+ form is placed in a solution containing B+ ions. Table 2 gives the selectivity's of strong acid and strong base ion exchange resins for various ionic compounds. It should be pointed out that the selectivity coefficient is not constant but varies with changes in solution conditions. It does provide a means of determining what to expect when various ions are involved. As indicated in Table 2, strong acid resins have a preference for nickel over hydrogen. Despite this preference, the resin can be converted back to the hydrogen form by contact with a concentrated solution of sulfuric acid (H2S04):

As we noted above, but a little differently, this step is known as regeneration. In general terms, the higher the preference a resin exhibits for a particular ion, the greater the exchange efficiency in terms of resin capacity for removal of that ion from solution. Greater preference for a particular ion, however, will result in increased consumption of chemicals for regeneration.

Resins currently available exhibit a range of selectivity's and thus have broad application. As an example, for a strong acid resin, the relative preference for divalent calcium ions (Ca+2) over divalent copper ions (Cu+2) is approximately 1.5

to 1. For a heavy-metal-selective resin, the preference is reversed and favors copper by a ratio of 2.300 to 1.

Table 2. Selectivity of ion Exchange Resins in Order of Decreasing Preference.

Strong acid cation exchanger

Strong base anion exchanger

Barium

Iodide

Lead

Nitrate

Calcium

Bisulfate

Nickel

Chloride

Cadmium

Cyanide

Copper

Bicarbonate

Zinc

Hydroxide

Magnesium

Fluoride

Potassium

Sulfate

Ammonia Sodium

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