Water Hardness

WATER HARDNESS, depends on the presence of calcium and magnesium salts in it. Salts of calcium and magnesium are contained in water mainly in the form of bicarbonates and partly in the form of sulfate compounds; in significantly smaller quantities and much less frequently, chloride, nitrite, and nitrate compounds of these metals are encountered. A distinction is made between total, permanent, and removable hardness. The total hardness is the hardness of unboiled water; it depends on all the alkaline earth metal salts present in the water. Permanent hardness is the hardness of water after boiling it for an hour. When boiled, bicarbonates of alkaline earth metals decompose and turn into carbonates, which precipitate out of the water in the form of sediment. The course of the reaction is evident from the following equations: Ca(HCO₃)₂ = CaCO₃ + H₂O + CO₂; Mg(HCO₃)₂ = MgCO₃ + H₂O + CO₂. After boiling, water becomes softer, as the precipitated carbonates reduce its original (total) hardness. The arithmetic difference between total and permanent hardness is called removable hardness. Removable hardness is often also called carbonate hardness, however, it should be kept in mind that when water is boiled, the precipitation of calcium and magnesium carbonates is never complete (therefore the terms 'carbonate' and 'removable' although very close, are not entirely identical in fact). At room temperature in 1 liter of boiled water, about 16 mg of CaCO₃ can be in dissolved state, which corresponds to 0.9 German degrees of hardness, and about 1,000 mg of MgCO₃, which corresponds to 66.5 German degrees of hardness. From these data it follows that removable hardness gives a picture mainly of carbonate hardness, depending on calcium bicarbonate, but not on magnesium bicarbonate, which, when water is boiled, turns into monocarbonate and precipitates out of it only slightly. Waters taken from limestone soil layers have the greatest hardness. These include some well waters, especially artesian waters, whose hardness can reach several tens, and sometimes more than one hundred German degrees. Meteoric waters, for example rain and snow, are distinguished by negligible hardness. Waters of open bodies of water (rivers, ponds, lakes) usually have moderate hardness, as they represent a mixture of ground and meteoric waters. In spring during snowmelt, as well as in summer and autumn during rains, water in them is softest, in winter it is hardest. Well waters throughout the year give very slight fluctuations in hardness, provided the well is dug deep enough and well isolated from the inflow of surface waters. - As for the sanitary significance of hardness, waters with high hardness are undesirable both in domestic use and for industrial purposes. In hard water, food products rich in protein substances (for example meat, peas, beans) cook slowly and poorly. This is due to the fact that protein bodies form hard insoluble compounds with alkaline earth metals, which prevent water from quickly penetrating deep into the food product. According to Rubner's research, peas cooked in very hard water not only remain hard but also have an unpleasant taste and are poorly digested. Tea in hard water gives a weak infusion, becomes cloudy and loses its pleasant taste qualities. When washing the body and washing linen, hard water requires excessive use of soap due to its binding with lime and magnesia in the form of insoluble fatty acid compounds. Depending on the degree of hardness, unproductive soap consumption can reach 10-80%, causing great economic damage to the population. In samovars and steam boilers, hard water forms significant scale, which prevents rapid heating of water and thus causes excessive fuel consumption. Hard waters are considered unsuitable for many industrial purposes: in dyeing, for brewing, etc. As drinking water, hard waters usually do not cause direct harm to the body; only with significant amounts of magnesium sulfate compounds in unaccustomed people, intestinal disorders are observed. Erisman considers it acceptable in 1 liter of drinking water no more than 40-50 mg of MgO. According to hygiene requirements, water intended for supplying populated areas should not have a hardness of more than 18-20 German degrees. Due to local conditions, there are many deviations from these norms; thus, in some areas of the Caspian Sea coast, in the Transcaspian region, in the Dnepropetrovsk and Don districts, water with a hardness of 30-70 German degrees is often used. In Germany, hard water is supplied to Göttingen (45 German degrees), Würzburg (30-40 German degrees), Halle (105 German degrees), etc. However, hard waters should be tolerated only in extreme cases, and as far as possible, the above-mentioned hygienic norms should be observed. To reduce water hardness, i.e., to remove lime and magnesia salts from it, two methods are used: boiling water and treating water with chemical reagents. When water is boiled, most of the alkaline earth carbonates are removed, and through chemical substances, all alkaline earth salts without exception can be precipitated from water. As reagents for precipitation, caustic lime or a mixture of soda and caustic soda are used. Water softened by these substances usually acquires a sharp alkaline reaction and a very unpleasant taste, due to which it is suitable only for technical purposes, but not for drinking. For softening drinking water, the zeolite method can be used: filtering water through sand from natural or artificially prepared zeolite (sodium aluminosilicate Na₂Al₂Si₄O₁₂+6H₂O), 'permutite'; it is prepared by fusing 3 parts of white clay (kaolin), 3 parts of quartz sand and 12 parts water. When water is passed through zeolite or permutite, an insoluble calcium and magnesium silicate is formed, with Ca and Mg replacing Na in the zeolite, and Na forms salts with acids that were bound with lime and magnesia: 2 NaPerm. + Ca(HCO₃)₂ → Ca Perm. + 2 NaHCO₃. By this method, all calcium and magnesium salts can be removed from water and water hardness can be reduced to zero, without changing the alkalinity of the water. At the same time, iron and manganese salts are also removed from the water. The spent zeolite and permutite sand can be easily regenerated by passing a 10% solution of common salt through it: Ca Perm. + 2NaCl → CaCl₂ + 2 Na Perm. Waters with hardness above 15 German degrees require preliminary treatment with lime. This significantly reduces the practical value of the zeolite method for softening drinking water. - For determining hardness, three methods are mainly used. 1. The Clark method - titration of water with an alcoholic solution of potassium soap. The method is very simple, results are obtained of approximate accuracy. More accurate results are obtained by titrating water with an alcohol-glycerin solution of potassium palmitate (Blacher's method), however, this method is not always available, as it is not easy to obtain potassium palmitate in chemically pure form. 2. The Wart-Pfeiffer method - precipitation of calcium and magnesium salts with an alkaline mixture of equal parts of decinormal solutions of caustic and sodium carbonate, followed by titration of the unreacted mixture with hydrochloric acid; this method gives good results in waters of medium and high hardness. 3. The gravimetric method - precipitation and weighing of calcium and magnesium salts contained in the water, with calcium salts precipitated as oxalic acid compound, and magnesium salts as ammonium magnesium phosphate. With the help of this method, the most accurate results are obtained. In the practice of a sanitary physician, the Clark method and, to some extent, the Wart-Pfeiffer method (recommended by the 'standard methodology' for water research) are most widely used; in laboratory practice, the gravimetric method and the Wart-Pfeiffer method are predominantly used.