Hardness and its units
Definition
Hardness of water is the property of water that prevents it from producing lather readily with soap and causes the formation of insoluble scum due to the presence of dissolved multivalent metal ions, mainly calcium (Ca²⁺) and magnesium (Mg²⁺).
Hardness is commonly expressed as equivalent concentration of calcium carbonate (CaCO₃), even when other salts are actually present.
Main Content
1. Types of hardness
Temporary hardness
Temporary hardness is caused by the presence of bicarbonates of calcium and magnesium, such as calcium bicarbonate and magnesium bicarbonate. It is called temporary because it can be removed by boiling. When water is boiled, bicarbonates decompose and form insoluble carbonates or hydroxides that can be separated.
Example:
- Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ + H₂O
- Mg(HCO₃)₂ → Mg(OH)₂ ↓ + CO₂
Permanent hardness
Permanent hardness is caused by chlorides, sulfates, and nitrates of calcium and magnesium, such as CaSO₄, MgSO₄, CaCl₂, and MgCl₂. It cannot be removed by boiling because these salts do not decompose easily on heating. It requires chemical treatment methods such as lime-soda process, ion exchange, or zeolite process.
Example:
- CaSO₄, MgCl₂, Ca(NO₃)₂
2. Causes and sources of hardness
Natural sources
Hardness mainly arises when rainwater or surface water passes through soil and rocks containing minerals like limestone, dolomite, gypsum, and magnesite. These minerals dissolve slowly and release calcium and magnesium ions into the water.
Industrial and environmental sources
Effluents from industrial activities, mining, fertilizer use, and sewage discharge may increase the hardness of water. Groundwater is usually harder than surface water because it remains in contact with mineral-bearing rocks for a longer period.
3. Units and expression of hardness
Hardness is expressed as CaCO₃ equivalents
Since water may contain different hardness-producing salts, their concentrations are converted into an equivalent amount of calcium carbonate. This makes comparison and treatment calculations easier because CaCO₃ has a standard equivalent weight basis.
Common units of hardness
Hardness is reported in mg/L as CaCO₃, which is numerically equal to ppm as CaCO₃ for dilute water. Other units may also be used in some countries or applications, such as:
- Degrees of hardness
- French degrees (°fH)
- German degrees (°dH)
- Clarke degrees (°Cl)
However, the most widely used and accepted unit in water chemistry is mg/L as CaCO₃.
Conversion idea
The hardness contributed by a salt is found by converting the mass of that salt into an equivalent mass of CaCO₃ using the formula:
Hardness as CaCO₃ (mg/L) =
Here, 50 is the equivalent weight of CaCO₃.
Example:
If water contains 100 mg/L of CaSO₄, the hardness as CaCO₃ can be calculated by converting based on its equivalent weight. This allows uniform reporting of all hardness-causing ions.
4. Classification based on measurement
Total hardness
Total hardness is the sum of temporary hardness and permanent hardness. It represents the overall concentration of calcium and magnesium salts in water.
Calcium hardness and magnesium hardness
Sometimes hardness is separately reported as calcium hardness and magnesium hardness because these ions contribute differently in certain treatment and industrial processes. Calcium hardness is often determined by titration methods, and magnesium hardness is obtained by subtracting calcium hardness from total hardness.
Carbonate hardness and non-carbonate hardness
Carbonate hardness is associated with bicarbonates and carbonates of calcium and magnesium, while non-carbonate hardness is due to chlorides, sulfates, and nitrates. Carbonate hardness is usually the same as temporary hardness, and non-carbonate hardness corresponds to permanent hardness.
Working / Process
1. Collection of water sample and selection of method
A representative water sample is collected in a clean container. The analyst then decides whether hardness will be estimated by laboratory titration, instrumental analysis, or field kit methods. For accurate results, the sample should be well mixed and free from contamination.
2. Determination of calcium and magnesium ions
In the common EDTA titration method, the sample is treated with a buffer to maintain the required pH and an indicator such as Eriochrome Black T. EDTA forms stable complexes with calcium and magnesium ions. The amount of EDTA used is directly related to the hardness in the water. If calcium hardness is required separately, a different indicator and pH condition may be used.
3. Calculation and expression as CaCO₃
The volume of EDTA consumed is used to calculate hardness in mg/L as CaCO₃. The result is then interpreted as soft, moderately hard, hard, or very hard water depending on standard classification ranges. This converted value is the standard reporting unit used in water analysis and treatment design.
Advantages / Applications
Helps in water quality assessment
Hardness measurement is essential in determining whether water is suitable for drinking, washing, laboratory use, or industrial processing. It gives a quick indication of the amount of calcium and magnesium salts present.
Useful in designing water treatment systems
Knowledge of hardness and its units is necessary for designing softening plants, boiler feed water treatment, ion exchange systems, and lime-soda treatment units. Proper measurement ensures efficient treatment and cost control.
Prevents operational problems in industry
Hard water causes scale formation, boiler troubles, reduced heat transfer, pipe blockage, soap wastage, and damage to industrial equipment. Measuring hardness helps industries select the correct treatment method and maintain smooth operation.
Summary
- Hardness is the property of water that prevents soap from lathering easily due to calcium and magnesium ions.
- It is mainly caused by bicarbonates, chlorides, sulfates, and nitrates of calcium and magnesium.
- Hardness is commonly expressed as mg/L as CaCO₃, which is the standard unit in water analysis.
- Accurate hardness measurement is important for domestic use, boiler operation, and industrial water treatment.
Important terms to remember
- hardness, temporary hardness, permanent hardness, total hardness, CaCO₃ equivalent, mg/L, ppm, calcium hardness, magnesium hardness