Water Chemistry: Hardness, Alkalinity, and Treatment
English with a size of 8.6 KB1. Sources of Water
| Source | Description | Characteristics |
|---|---|---|
| Surface Water | Rivers, lakes, ponds, reservoirs | Contains suspended, colloidal, and dissolved impurities. |
| Ground Water | Wells, springs, borewells | Contains dissolved salts (hardness) but less organic matter. |
| Rain Water | Condensed atmospheric water vapor | Purest natural form but may contain dissolved gases like CO2 and SO2. |
| Sea Water | Oceanic water | Contains ~3.5% salts; not fit for domestic use. |
2. Impurities in Water
| Type | Examples | Effects |
|---|---|---|
| Suspended Impurities | Clay, sand, silt | Causes turbidity. |
| Colloidal Impurities | Organic matter, bacteria | Makes filtration difficult. |
| Dissolved Impurities | Salts of Ca, Mg, Na, Cl-, SO42- | Causes hardness and corrosion. |
| Gaseous Impurities | CO2, O2, H2S | Causes acidity or foul odor. |
3. Hardness of Water
Definition
Hardness is the property of water that prevents lather formation with soap and forms scum instead.
Types of Hardness
| Type | Cause | Removal |
|---|---|---|
| Temporary Hardness | Due to bicarbonates of Ca and Mg: Ca(HCO3)2, Mg(HCO3)2 | Removed by boiling or lime treatment. |
| Permanent Hardness | Due to chlorides, sulfates, and nitrates of Ca and Mg: CaSO4, MgCl2 | Removed by chemical methods. |
Units of Hardness
| Unit | Equivalent of 1 Unit | Conversion |
|---|---|---|
| ppm (parts per million) | 1 mg CaCO3 per liter | 1 ppm = 1 mg/L |
| mg/L | 1 mg of CaCO3 per liter | Same as ppm |
| °Clark | 1 grain CaCO3 per gallon | 1°Clark = 14.25 mg/L |
| °French (°Fr) | 10 mg CaCO3 per liter | 1°Fr = 10 ppm |
| °German (°dH) | 10 mg CaO per liter | 1°dH = 17.86 ppm |
4. Determination of Hardness by EDTA Method
Principle
EDTA (Ethylenediaminetetraacetic acid) forms stable complexes with Ca2+ and Mg2+ ions in water.
Reaction: Ca2+ + Na2EDTA → [Ca-EDTA]2- + 2Na+
Reagents
- Buffer solution (pH = 10): Maintains alkalinity.
- Indicator: Eriochrome Black T (EBT).
- Titrant: Standard EDTA solution.
Procedure
- Take a 50 mL water sample.
- Add buffer (pH = 10) and 1–2 drops of EBT (turns wine red).
- Titrate against EDTA until the color changes from wine red to blue.
Formula
Hardness (mg/L as CaCO3) = (A × N × 50,000) / V
Where:
A = Volume of EDTA used (mL)
N = Normality of EDTA
V = Volume of sample (mL)
5. Alkalinity of Water
Definition
Alkalinity is the capacity of water to neutralize acid, due to the presence of OH-, CO32-, and HCO3- ions.
Types of Alkalinity
| Type | Cause | Indicator Used |
|---|---|---|
| Phenolphthalein Alkalinity (P) | Due to OH- and half of CO32- | Phenolphthalein |
| Total Alkalinity (T) | Due to OH-, CO32-, and HCO3- | Methyl Orange |
Formula for Alkalinity
Alkalinity (mg/L as CaCO3) = (A × N × 50,000) / V
Where:
A = mL of acid used
N = Normality of acid
V = Volume of sample (mL)
6. Boiler Problems
| Problem | Cause | Effect | Prevention |
|---|---|---|---|
| Sludge & Scale Formation | Precipitation of CaSO4, Mg(OH)2, etc. | Reduces heat transfer | Blow down, external softening |
| Priming & Foaming | High dissolved solids, rapid steam formation | Wet steam causes damage | Anti-foaming agents |
| Boiler Corrosion | Dissolved O2, CO2, acids | Weakens metal | Deaeration, use Na2SO3 or hydrazine |
| Caustic Embrittlement | High NaOH in boiler | Cracking of boiler metal | Control NaOH, add phosphate |
7. Internal Treatment of Hardness
| Method | Chemical Used | Reaction / Mechanism |
|---|---|---|
| Calgon Conditioning | Sodium hexametaphosphate (Na6P6O18) | Forms soluble complexes with Ca2+ and Mg2+ |
| Colloidal Conditioning | Organic colloids (tannins, starch) | Prevents adhesion of scale-forming salts |
| Carbonate Conditioning | Na2CO3 | Converts CaSO4 → CaCO3 (soft sludge) |
| Phosphate Conditioning | Na3PO4 | Converts Ca2+ → Ca3(PO4)2 precipitate |
8. External Treatment of Hardness
(a) Lime–Soda Method
Removes both temporary and permanent hardness by adding lime (Ca(OH)2) and soda (Na2CO3).
| Reaction | Equation |
|---|---|
| Temporary hardness removal | Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 ↓ + 2H2O |
| Permanent hardness removal | MgCl2 + Ca(OH)2 → Mg(OH)2 ↓ + CaCl2 |
| Soda for Ca-hardness | CaSO4 + Na2CO3 → CaCO3 ↓ + Na2SO4 |
Lime required (mg/L) =
Soda required (mg/L) =
(b) Zeolite Process
Uses sodium alumino-silicate (Na2Z) to exchange Na+ with Ca2+ and Mg2+.
Reaction: Ca2+ + Na2Z → CaZ + 2Na+
Regenerated by: CaZ + 2NaCl → Na2Z + CaCl2
(c) Ion Exchange Method
Uses cation and anion exchangers to produce deionized (demineralized) water.
| Step | Reaction |
|---|---|
| Cation Exchange | Ca2+ + 2H+R → CaR2 + 2H+ |
| Anion Exchange | Cl- + OH-R → RCl + OH- |
| Combined | H+ + OH- → H2O |
9. Quick Formula Summary
| Quantity | Formula / Value | Units |
|---|---|---|
| Hardness (EDTA method) | (A × N × 50,000) / V | mg/L as CaCO3 |
| Alkalinity | (A × N × 50,000) / V | mg/L as CaCO3 |
| Lime required | (Calculated based on impurities) | mg/L |
| Soda required | (Calculated based on impurities) | mg/L |
| Conversion | 1°dH = 17.86 ppm, 1°Fr = 10 ppm, 1°Cl = 14.25 ppm | ppm |