Unit1 - Subjective Questions

Difference between Temporary and Permanent Hardness:

Principle of EDTA Method:

1. EDTA (Ethylene Diamine Tetra Acetic acid) forms stable complexes with metal ions (, ) responsible for hardness.
2. Indicator: Eriochrome Black-T (EBT) is used as an indicator. At pH 10, EBT forms an unstable wine-red complex with metal ions.
3. Buffer: A buffer solution of is added to maintain the pH at roughly 10, as the complexation is pH-dependent.

Reactions:

2. During titration, EDTA extracts metal ions from the weak M-EBT complex.
   

End Point: The color changes from Wine Red to Steel Blue.

Alkalinity is the measure of the water's ability to neutralize acids. It is primarily due to hydroxide (), carbonate (), and bicarbonate () ions.

Determination:

1. Phenolphthalein End Point (P): Indicates the neutralization of all and half of (conversion to ). Color change: Pink to Colorless.
2. Methyl Orange End Point (M): Indicates the neutralization of the remaining (now ) and original present. Color change: Yellow to Red.

Calculation Table:

Step 1: Convert impurities into equivalents.
Multiplication Factor =

• (MW 146): ppm
• (MW 162): ppm
• (MW 120): ppm
• (MW 111): ppm

Step 2: Calculate Types of Hardness

• Temporary Hardness: Due to bicarbonates.
  

• Permanent Hardness: Due to sulfates and chlorides.
  

• Total Hardness: Temporary + Permanent
  

Distinction:

Disadvantages of Scale:

1. Wastage of Fuel: Scales are poor conductors of heat, requiring more fuel to heat water.
2. Overheating of Boiler Material: The metal overheats and becomes soft/weak, leading to distortion.
3. Explosion Danger: Cracks in scale can allow water to hit overheated metal, causing sudden high pressure.
4. Decrease in Efficiency: Reduces the flow area and heat transfer rate.

Definition: Caustic embrittlement is a type of boiler corrosion caused by the use of highly alkaline water in high-pressure boilers, leading to the brittleness of boiler metal.

Mechanism:

• In high-pressure boilers, hydrolyzes to form .
  
• Water flows into minute hair-cracks or crevices by capillary action. The water evaporates, increasing the concentration of significantly inside the crack.
• Concentrated attacks the iron of the boiler material (dissolving it as sodium ferroate), causing inter-crystalline cracking.

Prevention:

1. Using Sodium Phosphate: Use phosphate instead of carbonate for softening.
2. Adding Tannin or Lignin: Blocks the hair cracks.
3. Addition of : Maintains a specific ratio of Sodium Sulfate to NaOH to prevent cracks.

Principle: Zeolites (hydrated sodium aluminosilicate, ) exchange their ions with hardness-producing ions () present in water.

Process:

1. Hard water is passed through a bed of Zeolite.
2. Softening Reactions:
3. The outlet water contains sodium salts (soft water).

Regeneration: When Zeolite is exhausted, it is treated with 10% Brine () solution.

Limitations:

1. Water must be turbidity-free (pores get clogged).
2. Water should not be acidic (Zeolite decomposes).
3. Cannot be used for treating brackish water (contains Na salts).

Principle: Removes all inorganic salts (both cations and anions) producing deionized water.

Process:

1. Cation Exchange Column: Contains resin containing ions (). Exchanges cations () with ions.

   • Water becomes acidic.

2. Anion Exchange Column: Contains resin containing ions (). Exchanges anions () with ions.

3. Combination: from step 1 and from step 2 combine to form water.

Regeneration:

• Cation Exchanger: Regenerated by passing Dilute or .
• Anion Exchanger: Regenerated by passing Dilute .

1. Convert to equivalents:

• :
• :
• :
• :
• :
• : Ignore (does not cause hardness).

2. Lime Requirement Formula:

3. Soda Requirement Formula:

Definition: Reverse Osmosis is a membrane filtration process used to remove ions, molecules, and larger particles from drinking water by applying pressure.

Principle:

• Osmosis: The natural flow of solvent from low concentration to high concentration across a semi-permeable membrane.
• Reverse Osmosis: If a hydrostatic pressure greater than the osmotic pressure is applied on the high-concentration side (brackish water), the solvent (pure water) flows against the concentration gradient towards the low-concentration side.

Process:

1. Brackish water is pressurized against a semi-permeable membrane (e.g., Cellulose Acetate, Polyamide).
2. The membrane allows water molecules to pass but blocks dissolved salts and impurities.
3. Pure water collects on one side, and concentrated brine is rejected.

Advantages: Removes ionic, non-ionic, colloidal, and organic matter. Simple operation. Low energy cost compared to distillation.

Priming:

• Definition: The production of wet steam (steam carrying liquid water droplets) during rapid boiling.
• Causes: High water level, rapid boiling, presence of salts.
• Remedies: maintain low water level, use mechanical steam purifiers, control dissolved salts.

Foaming:

• Definition: The formation of persistent foam or bubbles on the water surface which do not break easily.
• Causes: Presence of oils, greases, or alkalis which reduce surface tension.
• Remedies: Adding anti-foaming agents like Castor oil or Polyamides. Removing oil from feed water using coagulants (Sodium aluminate).

Concept:
When chlorine is added to water, it reacts with ammonia and organic impurities first. The point where the demand for chlorine is fully met, all combined chlorine (chloramines) is oxidized, and free residual chlorine begins to appear is called the Break Point.

Graph Stages:

1. Stage 1: Chlorine destroys bacteria and oxidizes reducing agents (Fe, Mn). Residual Cl is zero.
2. Stage 2: Chlorine forms Chloramines (Combined residual chlorine). Curve rises.
3. Stage 3: Chloramines are oxidized/destroyed. Residual Cl decreases (Curve drops).
4. Break Point: Minimum point on the curve. All impurities destroyed.
5. Stage 4: Addition of Cl beyond this point increases Free Residual Chlorine linearly.

Significance: Ensures complete disinfection, removes bad taste/odor due to organic matter, and leaves residual chlorine to prevent re-contamination.

Units of Hardness:

1. Parts per million (ppm): Parts of equivalent hardness per parts of water.
2. Milligrams per litre (mg/L): Mass of equivalent in mg per litre of water.
3. Degree Clarke (): Parts of equivalent per 70,000 parts of water.
4. Degree French (): Parts of equivalent per parts of water.

Relationship:

Combined Relation:

Principle: Electrodialysis involves the separation of ions from water by pulling them through ion-selective membranes under the influence of a direct electric field.

Construction:

• A cell consists of an anode and a cathode.
• Between them, multiple Cation Selective Membranes (CSM) and Anion Selective Membranes (ASM) are placed alternately.
• CSM allows only cations () to pass; ASM allows only anions () to pass.

Working:

1. Saline water enters the compartments.
2. When DC current is applied, cations move toward the cathode and anions toward the anode.
3. Due to the arrangement of membranes, ions are trapped in alternate compartments (Concentrate stream), while the remaining compartments become depleted of ions (Dilute stream/Pure water).
4. The pure water is collected.

Application: Used for desalination of brackish water (up to 5000 ppm).

According to IS 10500:2012, the desirable limits for drinking water are:

1. pH: 6.5 to 8.5
2. Total Hardness (as ): Max 200 mg/L (Permissible up to 600 in absence of alternate source).
3. Total Dissolved Solids (TDS): Max 500 mg/L.
4. Chlorides (): Max 250 mg/L.
5. Fluoride (): Max 1.0 mg/L (Excess causes Fluorosis).
6. Nitrate (): Max 45 mg/L (Excess causes Blue Baby Syndrome).
7. Turbidity: Max 1 NTU.
8. Pathogens (E. coli): Should be absent in 100 ml sample.

Given:

Analysis:

• Total volume for Methyl Orange () = .
• Here, and .
• Relationship: (Since ).

Conclusion based on Alkalinity Table:

• When , only Carbonate () alkalinity is present.
• , .

Calculation:

• Carbonate Alkalinity =

Internal Treatment: Adding chemicals directly into the boiler to prevent scale formation by converting scale-forming salts into sludge or soluble complexes.

1. Calgon Conditioning:

• Chemical: Sodium Hexametaphosphate (), trade name 'Calgon'.
• Mechanism: Forms a soluble complex with ions, preventing precipitation.
• Used for high-pressure boilers.

2. Phosphate Conditioning:

• Chemicals: Sodium Phosphate ().
• Mechanism: Reacts with and salts to form soft, non-adherent sludge of Calcium/Magnesium Phosphate.
• Choice of phosphate depends on pH: Trisodium phosphate (for acidic water), Disodium hydrogen phosphate (for weak alkaline), Sodium dihydrogen phosphate (for alkaline).

Boiler Corrosion: Decay of boiler material due to chemical or electrochemical attack by the environment.

1. Dissolved Oxygen:

• reacts with iron at high temps to form rust.
• Removal:
  • Adding Hydrazine: (Best for high pressure, no salts).
  • Adding Sodium Sulphite: .

2. Dissolved Carbon Dioxide:

• forms Carbonic acid (), causing slow acidic corrosion.
• Source: Decomposition of bicarbonates.
• Removal: Adding Ammonia () to neutralize acidity.

Step 1: Calculate total NaCl used for regeneration.
Total NaCl = Volume Strength

Step 2: Convert NaCl to equivalents.
Reaction:
Basic Equivalency: g g
g g
g

Step 3: Calculate Hardness.
This amount of equivalent was removed from 10,000 Litres of water.
Hardness

Hardness