Magnesium in hard water

The use of sulphonation chemistry together with the availability of cost effective hydrocarbon feedstock led to the production and wide-scale use of synthetic surfactants in detergents during the 1950s. In particular, tetrapropylene benzene sulphonate rapidly replaced soap-based products in many applications because it does not react with calcium and magnesium in hard water to form scum . The benefits of this new surfactant together with an increasing consumer demand resulted in a substantial diffuse release of this substance into the environment. 

Still, the most important use of sodium chloride by far is as a raw material in the production of other compounds. In 2004, 65 percent of all the sodium chloride consumed in the United States was used in the production of sodium hydroxide, sodium carbonate, hydrogen chloride, sodium metal, chlorine gas, and other chemical products. The next most important use of sodium chloride is in water conditioners. The compound is used in such devices because the sodium in sodium chloride will replace the calcium and magnesium in hard water (water in which it is hard to make suds). By softening water with sodium chloride, clothing and other materials can be cleaned more efficiently at lower cost. The Salt Institute claims that sodium chloride has more than 14,000 distinct uses. Some of the most important of those uses include ... 

The difference in solubility between carbonates and hydrogen carbonates is responsible for the behavior of hard water, which is water that contains dissolved calcium and magnesium salts. Hard water originates as rainwater which dissolves carbon dioxide from the air and forms a very dilute solution of carbonic acid. 

A very important ligand (or chelating agent) for titrimetric analysis is the ethylenediaminetetraacetate (EDTA) ligand. It is especially useful in reacting with calcium and magnesium ions in hard water such that water hardness can be determined. The next section is devoted to this subject. 

A problem exists with this procedure, however, in that at basic pH values, many metal ions precipitate as the hydroxide, e.g., Mg(OH)2, and thus would be lost to the analysis. This occurs with the magnesium in the water hardness procedure alluded to earlier. Luckily, a happy medium exists. At pH =10, the reaction of the metal ion with the predominant HY - and Y4- species (Figure 5.21) is shifted sufficiently to the right for the quantitative requirement to be fulfilled, while at the same time the solution is not basic enough for the magnesium ions to precipitate appreciably. Thus, all solutions in the reaction flask in the water hardness determination are buffered at pH = 10, meaning that a conjugate acid-base pair... 

The contents of tap water vary among communities. In some areas, the water is hard. Hard water is water that contains large amounts of calcium or magnesium ions. Hardness can be measured in milligrams per liter (mg/L) of calcium or magnesium ions. Hard water makes it difficult to get hair, clothes, and dishes clean. In this lab, you will learn how hard water is softened and how softening water affects its ability to clean. You will also collect, test, and classify local sources of water. 

Hard water contains calcium and magnesium Ions. These Ions form Insoluble calcium and magnesium soaps respectively when sodium or potassium soaps are dissolved in hard water. 

Soaps react with the calcium and magnesium ions in hard water to produce soap curd that greatly reduces its effectiveness. The curds are actually insoluble calcium and magnesium salts. Synthetic laundry detergents have replaced soap for cleaning clothes in the last half century. Synthetic detergents are made from petroleum. They work like soap except they do not react with magnesium and calcium ions to form insoluble precipitates and salts. 

The impact of water hardness due to calcium or magnesium ions on detergents was explained in Section 7.3.1 The source of most Ca2+ and Mg2+ in hard water is the dissolution of limestone (CaCOs) or dolomite [CaMg(COs)2]. Magnesium carbonate is fairly soluble (1.26 mmol L 1 at ambient temperature), but CaCOs is much less so (0.153 mmol L 1). However, if the water contains dissolved CO2 (as indeed it will if it is exposed to the air see Exercise 14.9), the relatively freely soluble Ca(HCOs)2 forms, and the limestone slowly dissolves away ... 

If used in beverages, tartaric acid must be perfectly pure and guaranteed for food use. One problem that may need to be addressed is that tartaric acid salts, particularly the calcium and magnesium tartrates, have lower solubility than citric acid. Consequently, there is a tendency for unsightly precipitates of insoluble tartrates to form in hard water, and in such conditions it is preferable to use citric acid. 

Interest in detergent products derived from renewable resources and with better biodegradability has driven evaluation of oxidized sugars and starches as builders or co-builders in detergents.113 Builders and co-builders complex calcium and magnesium ions in hard water to prevent sealing or deposits due to precipitation of insoluble carbonate salts. In current powder detergents, the builders are usually zeolites used in combination with polycarboxylate polymers derived from synthetic acrylic-maleic acid copolymers.114... 

A soap is the sodium salt of carboxylic acid attached to a long, nonpolar hydrocarbon chain. When a soap is placed in hard water, the sodium cations exchange with cations such as Ca2+ and Mg2+. The resulting calcium and magnesium salts are insoluble in water and precipitate to form soap scum. ... 

The usefulness of soaps is limited by their tendency to precipitate out of solution in hard water. Hard water is water that is acidic or that contains ions of calcium, magnesium, or iron. In acidic water (such as the acid rain of environmental concern), soap molecules are protonated to the free fatty acids. Without the ionized carboxylate group, the fatty acid floats to the top as a greasy acid scum precipitate. 

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