Methods of Removal of Hardness in Water

Methods of Removal of Hardness in WaterRemoval of Temporary Hardness By boiling the hard water Temporary hardness in water can be easily removed by boiling. On boiling, calcium/magnesium bicarbonate decomposes to give calcium/magnesium carbonate, which is insoluble in water. Therefore, it precipitates out. By the addition of slaked lime (Clark's process) In Clark's process, slaked lime, Ca(OH)2 is added to temporary hard water. Insoluble calcium carbonate precipitate out and no longer produce hardness. The methods used to remove permanent hardness given in the next section can also be employed to remove the temporary hardness. However, the above methods cannot be used to remove the permanent hardness. Methods to Remove Permanent HardnessBy the addition of washing soda (sodium carbonate) Calcium and magnesium ions present in hard water react with sodium carbonate to produce insoluble carbonates. The water now contains

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soluble and harmless sodium salts. Calgon process Calgon is a trade name of a complex salt, sodium hexametaphosphate (NaPO3)6. It is used for softening hard water. Calgon ionizes to give a complex anion: The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon. This results in the removal of calcium and magnesium ions from hard water in the form of a complex with Calgon. The water is softened and sodium ions are released into water. By the ion-exchange process (Permutit process) Permutit or sodium aluminium silicate is a complex chemical compound, which occurs as a natural mineral called Zeolite. Permutit or zeolites are insoluble in water and have the property of exchanging ions present in them with the ions present in the solution. Permutit or zeolites are packed in a suitable container and a slow stream of hard water is passed through this material. As a result, calcium and magnesium ions present in hard water are exchanged with sodium ions in the permutit (Na+Al-Silicate). The outgoing water contains sodium salts, which do not cause hardness. Regeneration Over a period of time permutit gets converted into a mixture of calcium and magnesium aluminium-silicates and permutit has to be regenerated for further use. This is done by packing brine - a concentrated solution of sodium chloride, through the column packed with spent permutit. The following reactions take place. The resulting calcium chloride and magnesium chloride produced, are washed out through a tap at the bottom. The regenerated permutit is reused for softening water. By the use of ion-exchange resins Giant organic molecules having acidic or basic groups are known as Ion-exchange resins. Acid resins contain the acid group (- COOH). Basic resins contain the basic group [(-NH)OH-], i.e., substituted ammonium hydroxide group. Acid and basic ion exchange resins are represented as RCOO-H+ and RNH + OH- respectively. Acid resins exchange their H+ ions with other cations such as Ca2+, Mg2+, etc., present in hard water. Acid resins are, therefore known as base-exchange resins. Basic resins exchange their OH-ions with the other anions such as HCO3-, Cl-, SO42-, present in hard water. Basic resins, therefore, are also known as acid exchange resins. Fig: 11.5 - Ion-exchange process for water softening In the ion exchange process, hard water is passed through two tanks 'A' and 'B'. Tank- A contains acid resin and tank- B is filled with basic resin. All the cations present in hard water (except H+) are removed by the acid resin present in Tank- A, and the basic resin present in Tank- B removes all the anions (except OH-) present in hard water. Water obtained after passage through both the tanks is free from all the cations and anions that make it hard. The water obtained after passing through the ion-exchangers is called deionised water or demineralised water. This is as good as distilled water. The water becomes soft after this process. Regeneration After some time, the resins require regeneration as they become ineffective. Adding a strong acid regenerates acid resin and the basic resin is regenerated by treating it with a solution of a strong base. The regenerated resins are washed to remove Ca2+, Na+, SO, Cl-etc. The resins can be used again. The water treated by ion-exchange process not only becomes soft but also free from all dissolved mineral impurities. It gets completely demineralized. Problem 4. Given that a sample of water has degree of hardness equal to 40 ppm. If the entire hardness is due to MgSO4. How much MgSO4 is present per kg of water? Solution The degree of hardness of water is defined as the number of parts by mass of calcium carbonate, equivalent to various calcium and magnesium present in one million parts of mass of water. It is expressed in ppm. Now, 106 g of water contains CaCO3 = 44 mg By definition, 1 mol of CaCO3 = 1 mol of MgSO4 Or, 100 mg of CaCO3 = 120 mg of MgSO4 Therefore 48 mg of MgSO4 is present in 1 kg of water.

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skatealexia

7th of Jul 2012

Quality of writing

Punctuation, grammar and spelling are all to a high standard and the candidate communicates in a presentation that is clear and easy to read so that someone without previous chemistry knowledge could easily understand the work.

Level of analysis

The candidate knows extensive chemical knowledge of the different methods to remove hardness in water, and shows a grasp of chemistry that I would expect from a high level chemistry A level student not a GCSE student. The chemical notation and equations are all spot on. It would be good if the candidate added an evaluation of the different techniques and which one appears best to add something to the work.

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An excellent piece. The candidate shows an excellent grasp of chemistry and the solutions that can be applied to this and shows this by the extensive detail presented in this essay.

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