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Water : Waters, Hard And Soft |
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The presence in water of bicarbonates of calcium and magnesium produces what is called temporary hardness; such water destroys soap without producing a lather, but is "softened" by boiling. The different varieties of hard soap consist of the sodium salts of organic acids derived from fats. Soft soap consists of the potassium salts of these acids. These salts are soluble in water, and are slightly decomposed by the latter, giving caustic soda and an acid soap. E.g., with sodium palmitate: 2C15H31CO2Na + H2O <=> (C15H31CO2)2HNa + NaOH. This decomposition of a salt by water, with production of free acid and base, is called hydrolysis. The soap in solution considerably lowers the surface tension of water, so that soapy water readily froths, and particles of dirt and grease tend to accumulate in the liquid. The detergent action of soap is thus an example of separation by surface tension effects. Expt 2. - Wash lampblack (fine soot) with ether to remove grease, and dry in a steam oven. If the fine powder is shaken with water, the suspension settles on standing. But in a 1 per cent, soap solution an inky suspension is formed which does not settle. The action of soap in removing dirt depends on this action. The calcium and magnesium salts in hard water cause a larger waste of soap than corresponds with the production of the insoluble calcium and magnesium salts of the fatty acids:CaCO3 (dissd.) + 2NaCO2.C15H31 = Ca(CO2.C15H31)2 (ppd.) + Na2CO3. About 0.17 lb. of soap is required for 100 gallons of water containing 1 grain of CaCO3 per gallon, instead of 0.075 lb. (theoretical). The slimy precipitate of calcium salts carries down with it some of the soap and renders it useless. It also adheres tenaciously to the skin or fabric, and interferes with washing. The hard water does not acquire the smooth feeling characteristic of a soft water (free from dissolved calcium and magnesium salts), which is intensified by traces of alkali from the excess of soap, but retains its harsh feeling until excess of the soap has been added. Ferrous carbonate dissolves in water containing carbonic acid, forming ferrous bicarbonate, Fe(HCO3)2. On boiling, a_ reddish-brown precipitate of hydrated ferric oxide is thrown down, since the ferrous carbonate is readily oxidised by the dissolved oxygen: 4FeCO3 + 6H2O + O2 = 4Fe(OH)3 + 4CO2. A similar ochre-like deposit is formed by oxidation of ferruginous water in streams. If such water is used for washing, the_ slimy salts formed with soap carry down brown ferric hydroxide, which adheres to the fabric in spots, forming "iron-mould." This may be removed by oxalic acid solution. Temporarily hard waters deposit a crust or scale of calcium carbonate when boiled in kettles or boilers, which dissolves in hydrochloric acid with effervescence. Waters containing magnesium and calcium carbonates held in solution by carbonic acid, when they fall in drops from the roofs of caves, lose the carbonic acid by evaporation and deposit the insoluble salts in the form of pendants, made up of several concentric layers, and known as stalactites.
The drops falling on the floor of the cave also deposit salts, and another concretion called a stalagmite, growing upwards to meet the stalactite, is formed. Small stalactites formed under brickwork arches, even in localities where the water is soft, are derived from the calcium carbonate in the mortar, which is dissolved by the carbon dioxide in rain. Temporarily hard water may be softened by the addition of exactly the right amount of lime in the form of lime-water, or milk of lime (Clark's process, 1841). Calcium bicarbonate is precipitated as carbonate by adding an equivalent amount of lime: Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O. When magnesium bicarbonate is present, double the amount of lime must be added, when the sparingly soluble magnesium hydroxide is formed: Mg(HCO3)2 + 2Ca(OH)2 = Mg(OH)2 + 2CaCO3 + 2H2O. Magnesium carbonate is appreciably soluble in water. One gm. dissolves per litre, as compared with 0-013 gm. per litre in the case of calcium carbonate; the bicarbonates are about thirty times as soluble. The normal carbonate would not be precipitated, but magnesium hydroxide is much less soluble (0.01 gm. per litre). The precipitates are allowed to settle, and the softened water run off for use. A different kind of hardness is due to sulphates or chlorides of calcium and magnesium, derived from the soil. These are not precipitated on mere boiling, and cause permanent hardness. The water may at the same time possess temporary hardness. If such waters are evaporated in boilers, calcium sulphate, CaSO4, is deposited as a very hard crystalline scale, which may give rise to overheating of the metal. This scale does not effervesce with hydrochloric acid unless carbonates are also present. Such waters cause waste of soap in laundry work for the same reason as temporarily hard water. Permanently hard waters may be softened by adding a mixture of caustic soda and sodium carbonate (soda-ash, or else washing-soda, Na2CO3,10H2O), when both temporary and permanent hardness are removed, but sodium carbonate and sulphate remain in solution, so that this process is not used with drinking water: CaSO4 + Na2CO3 = CaCO3 + Na2SO4; Ca(HCO3)2 + 2NaOH = CaCO3 + Na2CO3 + 2H2O. In softening water for use in boilers, both lime and sodium carbonate, in special apparatus, are applied. The lime is first treated with part of the water to produce a clear lime water, which then mixes with the raw water, and a solution of sodium carbonate is at the same time added. The water, after some solid has deposited, then passes through a filter. The whole operation proceeds continuously. A method applied in domestic water softeners and also on the larger scale is the permutit process. Permutit is the trade name for a sodium aluminium silicate allied to the natural mineral zeolites, e.g. natrolith, Na2Al2H4(SiO4)3 = Na2P. On percolating hard water through granules of permutit, the calcium and magnesium ions in the water art exchanged with an equivalent of sodium ion from the permutit, and hence the water is softened: Na2P + Ca++ = 2Na+ + CaP. The change is reversible, and when the permutit ceases _to function it is percolated with a solution of common salt, which displaces the calcium and magnesium from the permutit and replaces them by an equivalent of sodium: 2Na+ + CaP = Na2P + Ca++ A permutit containing manganese dioxide oxidises dissolved iron and manganous salts, which are injurious in laundering as they form spots, and precipitates them as oxides. This permutit may be regenerated by percolating with permanganate solution. Other materials used on the small scale in laundering are: ammonia, which acts similarly to caustic soda, and borax, Na2B4O7,10H2O, which precipitates calcium borate, Ca(BO2)2, and also forms a little caustic soda by hydrolysis: Na2B4O7 + 3H2O <=> 2H3BO3 + 2NaBO2; NaBO2 + 2H2O <=> NaOH + H3BO3. Hardness is not known to be injurious to water for drinking purposes (potable water) - in fact the presence of bicarbonates gives the water a refreshing taste, and prevents its corrosive action on lead pipes. The hardness of a water is usually expressed in parts of calcium carbonate, CaCO3, equivalent to the calcium and magnesium salts, per 100,000 parts of water, or else in grains per gallon (parts per 70,000). |
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