Ion Exchange Basics

 

Ions in Water: How They Work and Why They Matter

Ions are electrically charged atoms or molecules that are dissolved in water, making them soluble, ionised substances. These ions play a crucial role in water chemistry and treatment processes.

There are two primary types of ions:

• Cations — positively charged ions (e.g., Na⁺, Ca²⁺)
• Anions — negatively charged ions (e.g., Cl⁻, SO₄²⁻)

Water is electrically neutral overall, meaning that the total positive charge equals the total negative charge. Without this balance, water would conduct electricity dangerously — imagine getting a shock just by touching it!

Types of Ions

Ions can carry varying amounts of charge, typically between +1 to +3 or -1 to -3. Structurally, ions fall into two categories:

• Monoatomic ions — made up of a single atom.
• Polyatomic ions — composed of multiple atoms bonded together, behaving as a single charged particle.

Common Examples of Ions in Water

Ion Type	Example & Formula
Monovalent monoatomic cation	Sodium ion (Na⁺)
Divalent monoatomic cation	Calcium ion (Ca²⁺)
Monovalent polyatomic cation	Ammonium ion (NH₄⁺)
Monovalent monoatomic anion	Chloride ion (Cl⁻)
Monovalent polyatomic anion	Nitrate ion (NO₃⁻)
Divalent polyatomic anion	Carbonate ion (CO₃²⁻)
Divalent polyatomic anion (metallic)	Chromate ion (CrO₄²⁻)
Trivalent monoatomic cation (in acid)	Aluminium ion (Al³⁺)

Note: There are no monoatomic divalent or trivalent anions present in normal water.

Behavior of Ions in Water

Ions in water are free to move — they do not form permanent pairs. The water remains balanced because the total sum of positive and negative charges stays constant (see Figure 1).

Dissolution of Salts

Salts are crystalline compounds consisting of a fixed ratio of cations and anions. For example:

• Table salt (NaCl): Dissolves into Na⁺ and Cl⁻ ions, which disperse freely in the water.

When dissolved, ions become hydrated:

• Cations are attracted to the oxygen atoms of water molecules.
• Anions are attracted to the hydrogen atoms.

(See Figure 2 for hydrated ions in solution.)

Examples of Dissolved Salts in Water

• Magnesium sulphate (MgSO₄): Contains equal numbers of Mg²⁺ and SO₄²⁻ ions.
• Calcium chloride (CaCl₂): Two Cl⁻ ions balance each Ca²⁺ ion.
• Sodium carbonate (Na₂CO₃): Two Na⁺ ions balance each CO₃²⁻ ion.

Total Dissolved Solids (TDS)

As water evaporates, dry residue remains, primarily salts, silica, and organic compounds:

• Seawater: High TDS (35–40 g/L).
• River/tap water: Lower TDS (50–500 mg/L).

This is measured as Total Dissolved Solids (TDS) — a key indicator of water quality.

Ion Exchange and Equivalent Mass

To remove dissolved ions, water treatment often employs ion exchange. In this process, the critical measurement is equivalent mass, not just molarity or total mass.

Formula:
1 equivalent (eq) = 1 mole ÷ valence

Examples:

• Sodium (Na⁺): 1 mole = 23 g → 1 eq = 23 g.
• Nickel (Ni²⁺): 1 mole = 58.7 g → 1 eq = 29.35 g.

Understanding equivalent mass is essential for precise ion exchange system design and operation.

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Ion Exchange: How It Removes Impurities from Water

What Are Impurities in Water?

Water naturally contains trace amounts of foreign substances, many of which are harmless — and in some cases even beneficial. For example, moderate salinity in drinking water can be healthier than ultra-pure water, which lacks essential minerals.

However, for specialized applications (such as industrial processes, pharmaceutical production, or semiconductor manufacturing), even small amounts of dissolved substances are considered impurities and must be removed.

Types of Impurities in Water

• Insoluble particles (sand, sediment, organic debris):
  These can be removed by various filtration technologies, ranging from simple sand filters to advanced ultrafiltration systems that can trap particles smaller than one micron.

• Soluble substances:

  • Non-ionised substances require techniques like activated carbon filtration or reverse osmosis.

  • Ionised substances (ions) are best removed through ion exchange — a highly effective water treatment method.

What Is Ion Exchange?

Ion exchange is a process where undesirable ions dissolved in water are replaced with more desirable ions using a special material known as ion exchange resin.

This technology is commonly used for:

• Water softening

• Demineralisation

• Selective ion removal

• Industrial water treatment

Ion Exchange Resins: How They Work

Ion exchange resins are tiny porous plastic beads (approximately 0.6 mm in diameter). Each bead is:

• Made from a durable polymer matrix (similar to other plastics).

• Contains fixed charged groups permanently attached to the resin structure.

• Holds mobile counterions that can move freely in and out of the resin bead.

The resin beads also retain water within their porous structure, referred to as moisture content.

Maintaining Electrical Balance

Each fixed ion in the resin is balanced by a corresponding counterion to maintain electrical neutrality:

• When an ion from water enters the resin bead, an ion of the same charge leaves the bead.

• Only ions of the same electrical charge can be exchanged (cation for cation, anion for anion).

Types of Ion Exchange Resins

Cation Exchange Resin

• Example functional group: Sulphonate group (SO₃⁻)

• Common mobile ion: Sodium (Na⁺)

• Example resin: Amberjet™ 1000 (typically supplied in sodium form)

Anion Exchange Resin

• Example functional group: Quaternary ammonium cation

  • Chemical structure: CH₂—N⁺(—CH₃)₃

• Mobile ions exchanged: typically Cl⁻, SO₄²⁻, NO₃⁻, etc.

Important:
You cannot use a single resin to exchange both cations and anions simultaneously — otherwise, the fixed positive and negative charges would neutralize each other and no exchange would occur. This is why separate cation exchange resins and anion exchange resins are used in ion exchange systems.

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Summary of Ion Exchange Principles