High-Purity Ion Exchange Pretreatment
By David Paul
This Back to Basics article describes some of the common high-purity ion exchange configurations and pretreatment schemes. IX technology exchanges undesirable ions for desirable ions. For high-purity water applications, the desirable ions are hydrogen ions (H+) or hydronium (H3O+), and hydroxide ions (OH-). Undesirable ions in high-purity water applications are all other ions. Ions are atoms or molecules that have a net positive or negative charge. Positively charged ions are called cations because they will migrate, in water, to a negative electrode (the cathode). Negatively charged ions are called anions because they will migrate, in water, to a positive electrode (anode). For water treatment, a few of the most important positively charged atoms include sodium (Na+), calcium (Ca+2), and magnesium (Mg+2). Important positively charged molecules include the ammonium cation (NH4+), and the hydronium cation (H3O+). A few of the important negatively charged atoms include hydroxide (OH-), chloride (Cl-), and sulfide (S-). Important negatively charged molecules include bicarbonate (HCO3-), carbonate (CO3-2), sulfate (SO4-2), silicates (HSiO3-, SiO3-2, and SiO4-4), and the anions of weak organic acids, for example acetate (CH3COO-), the anion of acetic acid (CH3COOH).
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