Hardness and pH of Water
Purity Criteria of Surface Water
Before using surface water for industrial and agricultural purposes, it is essential to determine its purity criteria. The primary parameters used to evaluate water quality are pH, hardness, DO (Dissolved Oxygen), BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), and TDS (Total Dissolved Solids). Let us discuss these parameters in detail.
Hard Water
If fresh water contains a significant amount of dissolved divalent cations, such as calcium (Ca2+), magnesium (Mg2+), and iron (Fe2+) ions, it is called hard water. These cations react with the organic anions of soap to form an insoluble, sticky, floating precipitate known as soap scum, which results in a significant wastage of soap.
Hardness of Water
The characteristic property of water arising from the presence of excess divalent cations (primarily Ca2+, Mg2+, and Fe2+ ions) that prevents the ready formation of lather with soap is defined as the hardness of water. Water hardness is broadly classified into two types:
- a) Permanent Hardness
- b) Temporary Hardness
a) Permanent Hardness
Permanent hardness occurs when the chlorides and sulfates of Ca2+, Mg2+, and Fe2+ are dissolved in water. This type of hardness cannot be removed simply by boiling the water. It requires chemical treatment or ion-exchange methods.
Removal of Permanent Hardness:
(i) Washing Soda Method: In this chemical method, washing soda or sodium carbonate (Na2CO3 · 10H2O) is added to hard water. This converts the soluble calcium and magnesium salts into insoluble carbonate precipitates, which can then be filtered out.
b) Temporary Hardness
Temporary hardness is caused by the presence of dissolved bicarbonate salts of Ca2+, Mg2+, and Fe2+ ions. When this water is heated to high temperatures, the soluble bicarbonates thermally decompose into insoluble carbonates, which precipitate out. This readily transforms hard water into soft water.
Removal of Temporary Hardness:
(i) Boiling Method: Heating the temporarily hard water decomposes the soluble bicarbonates of calcium and magnesium into their respective insoluble carbonate salts:
*Note: Because Magnesium Carbonate (MgCO3) remains slightly soluble in water, temporary hardness due to magnesium bicarbonates cannot be completely eliminated by boiling alone.
(ii) Clark’s Method: In this method, a precisely calculated commercial amount of slaked lime or calcium hydroxide [Ca(OH)2] is mixed into the water and stirred thoroughly. This converts the soluble bicarbonates into insoluble precipitates:
Since MgCO3 retains partial solubility, it further reacts with the added Ca(OH)2 to form highly insoluble magnesium hydroxide [Mg(OH)2], which settles down completely:
If the amount of slaked lime used is insufficient, magnesium hardness will not be entirely removed because 1 mole of Mg(HCO3)2 requires 2 moles of Ca(OH)2 for complete precipitation. Conversely, an excess of slaked lime introduces unwanted Ca2+ ions, inducing artificial hardness. Thus, adding a stoichiometric, measured quantity is essential in Clark’s method.
Simultaneous Removal of Temporary and Permanent Hardness
In industrial plants, both temporary and permanent hardness of surface water are simultaneously removed using the Permutit or Cation Exchange Resin Method. Hard water is passed through a synthetic cation exchange resin embedded with sodium sulfonate groups (R-SO3–Na+), exchanging divalent heavy ions with harmless sodium ions:
Once exhausted, the inactive ion-exchange resin can be readily regenerated and reactivated by flushing it with a concentrated brine solution (aqueous NaCl).
Advantages of Hard Water
- Bone & Teeth Development: Essential calcium salts in hard water actively aid the physical growth and skeletal health of children.
- Safety from Lead Poisoning: Lead dissolves significantly less in hard water compared to soft water. Hence, passing hard water through lead pipes prevents toxicity.
- Medicinal Benefits: Rich mineral profiles can act as natural remedies for certain skin ailments, rheumatism, and indigestion.
Disadvantages of Hard Water
- Wastage of Soap: Reacts immediately to form scum, destroying the cleansing action of soap in laundry.
- Boiler Scale Formation: Deposition of insoluble metallic scale acts as a thermal insulator, multiplying fuel costs and risks of explosions.
- Industrial Damage: Dissolved ferrous (Fe2+) ions leave persistent brown stains on paper and rayon, while ruining dyes.
- Culinary Issues: Rice and pulses do not boil easily, altering the color, texture, and natural flavor of vegetables.
Water pH
The pH of water is mathematically defined as the negative base-10 logarithm of the molar concentration of hydrogen ions (H+) present in the solution.
While pure water maintains a neutral pH of 7.0, natural surface water dissolves atmospheric carbon dioxide to form weak carbonic acid (H2CO3). As a result, typical surface water has a slightly acidic pH ranging from 6.0 to 6.5.
Standard Guidelines: According to the World Health Organization (WHO), the standard permissible limit for drinking water pH at 25°C is 6.5 – 8.5 (at which water remains completely colorless and odorless). For sustaining aquatic biodiversity and fish populations, the optimum water pH is strictly 7.0 – 7.5.
