FE > Semester 1 > Applied Chemistry 1

Marks : 03

Years : MAY 2016

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solidpolymeric or mineralic 'ion exchangers'.

Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions).

Natural water contains calcium and magnesium ions (see water analysis) which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove.

Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt.

Strongly acidic cation exchange resins (SAC, see resin types) used in the sodium form remove these hardness cations from water. Softening units, when loaded with these cations, are then regenerated with sodium chloride (NaCl, table salt).

Reactions

Here the example of calcium:

$2 R-Na + Ca^{++} \rightleftharpoons R_2-Ca + 2 Na^+$

R represents the resin, which is initially in the sodium form. The reaction for magnesium is identical.

The above reaction is an equilibrium. It can be reversed by increasing the sodium concentration on the right side. This is done with NaCl, and the regeneration reaction is:

$R_2-Ca + 2 Na^+\rightleftharpoons 2 R-Na + Ca^{++} $

The water salinity is unchanged, only the hardness has been replaced by sodium. A small residual hardness is still there, its value depending on regeneration conditions.