Water Treatment Chemistry: Hardness Testing and Boiler Management

Posted on Jan 23, 2026 in Chemistry

Estimation of Water Hardness by EDTA Method

Principle of EDTA Titration

• Hardness of water is due to the presence of Ca²+ and Mg²+ ions.
• In the EDTA method, a solution of Ethylenediamine Tetraacetic Acid (EDTA) is used as a titrant.
• EDTA forms stable, soluble complexes with Ca²+ and Mg²+ ions.
• At the endpoint, all Ca²+ and Mg²+ ions are complexed, and the indicator shows a color change.

Chemical Reactions:

Ca²+ + EDTA&sup4;− → [Ca−EDTA]²−

Mg²+ + EDTA&sup4;− → [Mg−EDTA]²−

Reagents Required

1. EDTA solution – Standard 0.01 M solution.
2. Buffer solution – Maintains pH around 10 (usually ammonium chloride + ammonium hydroxide).
3. Indicator (Eriochrome Black-T, EBT) – Forms a wine-red complex with Ca²+/Mg²+ ions. At the endpoint, when all Ca²+/Mg²+ are complexed by EDTA, the solution turns blue.

Procedure Steps

1. Take a measured volume (usually 50 mL or 100 mL) of the water sample.
2. Add a few mL of buffer solution to maintain pH = 10.
3. Add a pinch (or 2–3 drops) of EBT indicator.
4. The solution turns wine-red due to the Ca²+/Mg²+–EBT complex.
5. Titrate with standard EDTA solution from the burette.
6. At the endpoint, the color changes from wine-red → pure blue, indicating all hardness-causing ions are complexed.

Calculations for Hardness

• If V mL of EDTA of molarity M is required for V&sub1; mL of water sample, then:

Hardness (mg/L as CaCO&sub3;) = (1000 × M × V × 50) / V&sub1;

(Here, 50 is the molar mass equivalent of CaCO&sub3; per mole of EDTA reaction.)

Advantages of the EDTA Method

• Very accurate and reliable.
• Can determine total, temporary, and permanent hardness (by pretreating the sample – e.g., boiling removes temporary hardness).
• Quick and simple.

Water Hardness: Causes and Distinction

How Water Gets Hardness

• Water becomes hard when it dissolves salts of calcium (Ca²+), magnesium (Mg²+), and sometimes iron (Fe²+/Fe³+) while passing through soil and rock strata containing minerals.
• The major hardness-causing salts are:
  • Calcium bicarbonate [Ca(HCO&sub3;)&sub2;]
  • Magnesium bicarbonate [Mg(HCO&sub3;)&sub2;]
  • Calcium sulfate (CaSO&sub4;)
  • Magnesium chloride (MgCl&sub2;)
• Hardness is of two types:
  1. Temporary hardness → due to bicarbonates of Ca and Mg, removable by boiling.
  2. Permanent hardness → due to chlorides, sulfates, and nitrates of Ca and Mg, not removed by boiling.

Difference between Hard Water and Soft Water

Boiler Corrosion: Causes and Prevention

Definition of Boiler Corrosion

Boiler corrosion is the gradual destruction of boiler material (mainly iron and steel) due to chemical or electrochemical reactions with dissolved gases, salts, or acidic substances present in boiler water. It reduces boiler efficiency, causes leakage, and may even lead to boiler failure.

Causes of Boiler Corrosion

1. Dissolved Oxygen (O&sub2;)

   • Oxygen reacts with iron to form hydrated ferric oxide (rust).
   • Leads to pitting and rusting of boiler tubes.

2. Dissolved Carbon Dioxide (CO&sub2;)

   • CO&sub2; dissolves in water forming carbonic acid, which attacks the boiler metal.
   • (Reactions were implied here in the source.)

3. Dissolved Salts (e.g., MgCl&sub2;)

   • Magnesium chloride hydrolyzes to produce hydrochloric acid, which is highly corrosive.
   • (Reactions were implied here in the source.)

4. Acidic Water (Low pH)

   • Low pH of water accelerates corrosion.
   • Acids react directly with boiler metal to form soluble salts of iron.

Consequences of Boiler Corrosion

• Formation of holes and cracks in boiler plates.
• Reduction in boiler life and efficiency.
• Leakage and safety hazards.
• Frequent breakdowns and costly repairs.

Prevention of Boiler Corrosion

1. Chemical Methods

   • Oxygen removal: Add sodium sulphite (Na&sub2;SO&sub3;) or hydrazine (N&sub2;H&sub4;) to remove dissolved O&sub2;.
   • Carbon dioxide removal: Add ammonia or sodium carbonate to neutralize carbonic acid.
   • Maintain alkaline pH (8–9) using sodium hydroxide (NaOH) or phosphates.

2. Mechanical Methods

   • Deaeration: Removing dissolved gases (O&sub2;, CO&sub2;) by preheating feed water.
   • Use of water softening processes (lime-soda, zeolite, ion-exchange) before feeding into boiler.

3. Protective Methods

   • Applying protective coatings on boiler surfaces.
   • Regular monitoring and maintenance.

Boiler Operational Issues: Priming and Foaming

Priming

• Definition

  Priming is the carryover of small droplets of water along with steam during boiler operation.

• Causes

  • High water level in boiler.
  • Sudden boiling or rapid steam production.
  • Presence of oil, dissolved salts, or suspended solids.

• Effects

  • Leads to wet steam, reducing efficiency.
  • Causes deposits in turbine blades.
  • Increases chances of boiler corrosion.

• Prevention

  • Proper boiler design and maintenance.
  • Maintaining correct water level.
  • Using anti-foaming agents.

Foaming

• Definition

  Foaming is the formation of stable foam or bubbles on the water surface inside the boiler, due to high concentration of dissolved solids, oils, or alkalis.

• Causes

  • Presence of soap, oil, or alkali in boiler water.
  • High concentration of dissolved salts.

• Effects

  • Foam particles are carried with steam, worsening priming.
  • Leads to wet steam and lower efficiency.
  • Damages turbines and reduces boiler life.

• Prevention

  • Removal of oil and grease from feed water.
  • Chemical treatment to control alkalinity.
  • Blowing down boiler water at intervals.

Importance of Controlling Priming & Foaming

1. Both reduce steam quality → lowering efficiency of turbines and engines.
2. Cause mechanical damage (turbine blades erosion, scaling).
3. Promote corrosion inside boiler.
4. Increase operational costs and maintenance.

Sludge and Scale Formation in Boilers

Introduction to Boiler Deposits

When hard water is used in boilers, dissolved salts of calcium, magnesium, and other impurities precipitate upon heating. Depending on their solubility and nature, they form either sludge (soft, loose deposits) or scale (hard, adherent deposits) inside the boiler. Both are undesirable as they reduce efficiency and may damage the boiler.

Sludge Formation

1. Definition: Sludge is a soft, loose, slimy deposit that forms inside the boiler due to the precipitation of substances that are more soluble in hot water than in cold water.
2. Examples of sludge-forming substances:
   • Magnesium chloride (MgCl&sub2;)
   • Magnesium sulfate (MgSO&sub4;)
   • Calcium chloride (CaCl&sub2;)
3. Problems caused by sludge:
   1. Chokes the pipes and tubes.
   2. Reduces heat transfer efficiency.
   3. Causes wastage of fuel.

Scale Formation

• Definition: Scale is a hard, adherent, crystalline deposit on the inner walls of boilers formed due to precipitation of salts that are less soluble in hot water than in cold water.
• Examples of scale-forming salts:
  • Calcium sulfate (CaSO&sub4;)
  • Calcium carbonate (CaCO&sub3;)
  • Silica (SiO&sub2;)
• Problems caused by scales:
  1. Wastage of fuel: Scales are bad conductors of heat, so more fuel is needed.
  2. Overheating & boiler damage: Metal may crack due to uneven heating.
  3. Reduced efficiency: Narrowing of boiler tubes decreases efficiency.
  4. Danger of explosion: Excessive overheating can cause boiler bursts.

Comparison of Sludge and Scale

Prevention and Removal

1. External treatment of water – Lime-soda process, Zeolite process, Ion exchange.
2. Internal treatment – Adding chemicals like phosphates, carbonates, tannins, lignins.
3. Blow-down operation – Removing concentrated water with sludge.
4. Mechanical removal – Scraping and wire brushing for scales.

Boiler Troubles and Their Treatment

Boiler troubles are the problems caused in boilers due to the use of untreated or improperly treated water. They reduce the efficiency, safety, and life of boilers.

1. Scale and Sludge Formation

• Scale: Hard, adherent deposit formed on boiler walls due to precipitation of salts like CaSO&sub4;, CaCO&sub3;, Mg(OH)&sub2;.
• Sludge: Soft, loose, and slimy deposit that settles at the bottom of the boiler.

Disadvantages

• Wastage of fuel (reduces heat transfer).
• Overheating and damage to boiler material.
• Choking of pipes → boiler explosion risk.

Treatment

• External treatment: Lime-soda, Zeolite, Ion-exchange processes to soften water.
• Internal treatment: Addition of phosphate (Na&sub3;PO&sub4;), carbonate, or calgon (sodium hexametaphosphate) inside boiler to convert scale-forming salts into non-adherent sludge.
• Blow-down operation to remove sludge.

2. Priming and Foaming

• Priming: Carryover of water droplets with steam.
• Foaming: Formation of stable foam over water surface due to oil/grease or dissolved solids.

Disadvantages

• Wet steam → lowers efficiency.
• Damage to turbine blades.

Treatment

• Proper boiler design, maintaining correct water level.
• Adding anti-foaming agents like castor oil.
• Mechanical steam separators.
• Removal of oil and grease from boiler feed water.

3. Boiler Corrosion

• Destruction of boiler metal due to dissolved oxygen, carbon dioxide, or acidic salts (like MgCl&sub2;).
• Key Reactions:
  • 4Fe + 2H&sub2;O + O&sub2; → 4Fe(OH)&sub2; (rust)
  • MgCl&sub2; + 2H&sub2;O → Mg(OH)&sub2; + 2HCl

Treatment

• Mechanical deaeration (removal of O&sub2;/CO&sub2;).
• Adding chemical oxygen scavengers (Na&sub2;SO&sub3;, hydrazine).
• Maintaining pH of boiler water (8–9).

4. Caustic Embrittlement

• Localized corrosion of boiler due to high concentration of NaOH in stressed parts of boiler.
• Reaction:
  Fe + 2NaOH → Na&sub2;FeO&sub2; + H&sub2;

Treatment

• Use of sodium phosphate instead of sodium carbonate.
• Adding tannins, lignin, or sodium sulfate to block cracks.
• Controlling NaOH concentration.

Summary Table of Boiler Troubles

The Zeolite or Permutit Process for Water Softening

The Zeolite or Permutit process is a method of water softening that uses a hydrated sodium alumino silicate, known as zeolite (Na&sub2;Ze), to exchange its sodium ions with the hardness-producing ions present in the water.

Description of the Process

When hard water is passed through a bed of zeolite, the calcium (Ca²+) and magnesium (Mg²+) ions in the water are retained by the zeolite, and the sodium ions from the zeolite are released into the water. The hardness-causing ions are retained as calcium zeolite (CaZe) and magnesium zeolite (MgZe).

Chemical Reactions (Softening)

Na&sub2;Ze + Ca(HCO&sub3;)&sub2; → CaZe + 2NaHCO&sub3;

Na&sub2;Ze + MgCl&sub2; → MgZe + 2NaCl

The exhausted zeolite can be easily regenerated by treating the bed with a concentrated sodium chloride (NaCl) solution, also known as brine.

Chemical Reactions (Regeneration)

CaZe or MgZe + 2NaCl → Na&sub2;Ze + CaCl&sub2;

Advantages of the Zeolite Process

The zeolite process offers several benefits for water softening:

• The equipment is compact and easy to handle.
• It requires a short time for the softening process.
• There is no sludge formation, making the process clean.
• The zeolite can be easily regenerated using a brine solution.
• It can remove any type of hardness without modifications.
• It produces water with a very low residual hardness of about 10 ppm.

Disadvantages of the Zeolite Process

Despite its benefits, the zeolite process has certain limitations:

• The softened water contains more sodium salts compared to the lime-soda process.
• The process replaces only calcium and magnesium ions but leaves other ions like bicarbonate (HCO&sub3;−) and carbonate (CO&sub3;²−) in the water. When this water is boiled, it can release carbon dioxide (CO&sub2;), which causes corrosion, and can also lead to caustic embrittlement.
• Turbidity in the water can clog the pores of the zeolite, making it inactive.
• Ions like manganese (Mn²+) and iron (Fe²+) form stable complexes with zeolite that are difficult to regenerate.

Ion Exchange Process for Water Demineralization

The ion-exchange process is a method of water demineralization that uses special resins to remove dissolved minerals. The process utilizes two main types of resins: cation-exchange and anion-exchange resins.

The Demineralization Process

The demineralization process involves two steps:

1. Cation Exchange

Hard water is passed through a bed of cation-exchange resin (RH+), which is a long-chain organic polymer with acidic functional groups (like -SO&sub3;H or -COOH). This resin exchanges its H+ ions with the cations (e.g., Ca²+, Mg²+) present in the hard water, forming an insoluble complex.

2RH+ + Ca²+ → R&sub2;Ca²+ + 2H+

2RH+ + Mg²+ → R&sub2;Mg²+ + 2H+

2. Anion Exchange

The water is then passed through a bed of anion-exchange resin (ROH−), which has basic functional groups (like -NH&sub2;). This resin exchanges its OH− ions with the anions (e.g., SO&sub4;²−, Cl−) in the water.

2ROH− + SO&sub4;²− → R&sub2;SO&sub4;²− + 2OH−

ROH− + Cl− → RCl− + OH−

The H+ ions released from the cation exchanger combine with the OH− ions from the anion exchanger to form water, thus producing demineralized water:

H+ + OH− → H&sub2;O

When the resins are exhausted, they can be regenerated by treating the cation-exchange resin with a dilute acid (HCl or H&sub2;SO&sub4;) and the anion-exchange resin with a dilute base (NaOH).

Advantages of Ion Exchange Process

• Produces very pure water (conductivity < 1 µS/cm, comparable to distilled water).
• Can remove both temporary and permanent hardness, as well as acidic and basic impurities.
• Works even for highly saline or acidic water, where lime-soda or zeolite methods fail.
• Equipment is compact and easy to operate.

Disadvantages of Ion Exchange Process

• Expensive compared to lime-soda or zeolite processes.
• Requires skilled supervision for regeneration and operation.
• Resins may get damaged by turbidity, organic matter, or bacterial contamination.
• Needs regular regeneration using acids (for cation resin) and alkalis (for anion resin).

Demineralization of Brackish Water by Reverse Osmosis (RO)

1. Introduction to Brackish Water

• Brackish water: Water that contains more dissolved salts than fresh water but less than seawater (salinity between 1000–10,000 ppm).
• Such water is not suitable for drinking or industrial use.
• Reverse Osmosis (RO) is a modern and effective method for desalination and demineralization of brackish water.

2. Principle of Reverse Osmosis

• Osmosis: When two solutions of different concentrations are separated by a semi-permeable membrane, water naturally flows from the dilute solution to the concentrated one until equilibrium is reached.
• Osmotic pressure: The pressure required to stop this natural flow.
• Reverse Osmosis: If a pressure greater than the osmotic pressure is applied on the concentrated side, the water is forced to move in the reverse direction—from the concentrated solution to the dilute side—leaving salts and impurities behind.

3. Process of RO for Brackish Water

• Feed water (brackish water) is passed through a semi-permeable membrane made of cellulose acetate or polyamide.
• High pressure (15–60 atm) is applied using pumps, greater than the osmotic pressure of brackish water.
• Pure water molecules pass through the membrane, while dissolved salts, ions, and impurities are retained and discharged as brine (reject water).
• The collected water is demineralized and potable.

4. Advantages of RO

• Removes 90–99% of dissolved salts.
• Produces high-quality potable water.
• Requires less chemical treatment compared to ion exchange.
• Simple and continuous operation.

5. Disadvantages of RO

• Expensive setup due to high-pressure pumps and membranes.
• Membranes require frequent cleaning/replacement.
• Reject water disposal may cause environmental concerns.