Clark’s Method of Treating Hardness of Water

Clark’s method is one of the solutions for treating hard water by adding slaked lime into the water. Hardness of water can be defined as the dissolved content of divalent metal cations in water. In Clark’s method, hard water is treated with Ca(OH)2. This method removes hardness by conversion of bicarbonates into carbonates.  

Water hardness 

Calcium and magnesium are primarily interconnected to bicarbonate, sulphate or chloride, which is found in natural water. Bicarbonate turns to carbonate and precipitates out with Ca++ to produce calcium carbonate (CaCO3) scales when hard water evaporates or is heated over 61°C/141°F. 

Water hardness is commonly measured in mg/l as CaCO3 equivalent. But, CaCO3 is not present in water. There are other classification techniques for indicating the degree of hardness. But mostly soft water has less than 60 mg/l CaCO3  compared to hard water, which has more than 120 mg/l CaCO3. Local sources of water may contain hard water. In certain places, water drawn from various sources is hard water. Hardness or softness of water depends on the source of the water. Water drawn from the ground is normally hard water.

Hard-water minerals, such as calcium carbonate, also operate as pH buffers. But soft water is minimally buffered. It is more susceptible to acidification by acid-forming chemicals. Due to the presence of pH in soft water, it tastes better. The pH of most water sources is between 6.5 and 8.0. 

Water hardness depends on the amount of calcium and magnesium dissolved in the water. Hard water contains high amounts of mineral ions. The longer the plumbing system has been installed, and exposure to heat can have a major impact on hard water. Acidic water speeds up corrosion, shortening the life of plumbing materials and causing fixture stains and discolouration. 

Also, the efficiency of most chemicals, such as reagents, disinfectants, and pharmaceuticals, depends on the pH level of the water. Hard water with less pH (+0.6) is favourable for use in most water facilities. If a water supply contains hard water, plumbing should be done very carefully.  

Hardness is present in practically every water source due to the presence of calcium and magnesium bicarbonates. It is often known as temporary hardness or carbonate hardness. Acidic rainwater, which is carbon dioxide-laden, reacts with naturally occurring minerals in the ground, like limestone, to produce chemicals that harden the water. 

For example:  

CO2 + H2O → H2CO3

H2CO3 + CaCO3 → Ca(HCO3)2

How can water hardness be measured? 

Hardness of water can be determined by the amount of calcium and magnesium ions present in the water. It can also be ascertained by the concentration of calcium carbonate in it. The potentiometric method can be used to calculate the hardness of water by adding a hardness buffer to the water sample till the pH level of the water sample reaches 10. This method uses ethylene diamine tetra acetic acid titration to measure the hardness. A liquid ion-exchange electrode is used as an ion-selective electrode, which responds to the divalent magnesium and calcium ions. 

How can you remove hardness? 

You can remove temporary hardness by two methods: 

• Boiling

• Clarks method

Boiling: Temporary hardness of water can be removed by boiling water. During boiling, bicarbonates are processed to form hydroxides and calcium bicarbonates and then they are converted to form carbonates. Temporary hardness can be eliminated by filtering these precipitates.

Clark’s method: Thomas Clark is known in chemistry for introducing a method for softening hard water known as Clark’s method. His method involves softening calcareous water by removing carbonic acid from them. His method consists of adding milk of lime to hard water until the formation of precipitation of calcium and magnesium carbonates.   

Removing water hardness by Clark’s method

Minerals, salts, metals, cations, anions and other dissolved solids in water can be removed to reduce water hardness. One of the common methods of water softening is Clark’s method by which the temporary hardness of water can be removed by using cold lime. 

Clark’s method helps in the preparation of water in a  direct cooling tower makeup. This method can be used as a first-stage treatment before ion exchange for boiler makeup or recycling of RO reject water. Lime or a combination of lime and soda ash is added to treat the water (carbonate ions). They generate insoluble compounds when they come in contact with the hardness and natural alkalinity of the water. Sedimentation or calcification can be further removed from the water by precipitating them. Many companies use this method to treat water with moderate to high levels of hardness and alkalinity of about  150-500 ppm. 

What is cold lime softening ?

Cold lime softening refers to the process of softening lime at room temperature. The reaction that occurs when Ca(OH)2 (slaked lime) is added to water can be expressed by the following equation: 

CO2 + Ca(OH)2 → CaCO3  + H2O

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3¯ + 2H2O

Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2 + 2CaCO3 + 2H2O

Conclusion

Hard water is softened with the help of Ca(OH)2 (slaked lime) in Clark’s water softening process. Clark’s reagent is calcium hydroxide. It converts bicarbonates to carbonates to eliminate hardness from water. Clarke’s technique uses calcium hydroxide (lime) to soften water.