Hard water, soap precipitation

Hard" water contains impurities dissolved in it, such as chalk, and when ordinary soap is put into hard water, a precipitate, or scum, forms. This is not only unattractive, it is also wasteful, since it prevents the soap from doing its job until all the chemicals causing the scum have been pushed out of the water. One of the main chemical ingredients of hard water is calcium sulfate. 

Hard water is water that is rich in the salts of calcium, magnesium, and iron. The cations of hard water combine with fatty acid anions from soap to form insoluble salts known as curd or soap curd. In areas with particularly hard water, these precipitates can be seen as gray rings around bathtubs and sinks. 

Soap has been used for over two millennia, as people discovered long ago that soap could be made by heating animal fat together with wood ashes, which contain alkaline substances. Nevertheless, the usefulness of soap is diminished in the presence of water that contains high concentrations of calcium ions (Ca ) or magnesium ions (Mg ). When soap is used with such water, called hard water, a precipitate is formed as a result of the following ion exchange reaction. 

Detergents are designed to be effective in hard water meaning water containing calcium salts that form insoluble calcium carboxylates with soaps These precipitates rob the soap of Its cleansing power and form an unpleasant scum The calcium salts of synthetic deter gents such as sodium lauryl sulfate however are soluble and retain their micelle forming ability even m hard water... 

The presence of a sufficientiy strong chelating agent, ie, one where K in equation 26 is large, keeps the concentration of free metal ion suppressed so that pM is larger than the saturation pM given by the solubiUty product relation (eq. 29) and no soHd phase of MX can form even in the presence of relatively high anion concentrations. The metal is thus sequestered with respect to precipitation by the anion, such as in the prevention of the formation of insoluble soaps in hard water. 

Acylglycerols can be hydrolyzed by heating with acid or base or by treatment with lipases. Hydrolysis with alkali is called saponification and yields salts of free fatty acids and glycerol. This is how soap (a metal salt of an acid derived from fat) was made by our ancestors. One method used potassium hydroxide potash) leached from wood ashes to hydrolyze animal fat (mostly triacylglycerols). (The tendency of such soaps to be precipitated by Mg and Ca ions in hard water makes them less useful than modern detergents.) When the fatty acids esterified at the first and third carbons of glycerol are different, the sec-... 

This test measures the minimum lime soap dispersant requirement (LSDR) (in g) which keeps 100 g of sodium oleate from precipitating in hard water (333 ppm CaC03). Although this is a convenient laboratory test, it bears little relation to an real in-use situation. 

When the CMC determination is made by surface tension measurements, the resulting curve appears without minimum as a single surfactant. It is probable that an inversion takes place through the adsorption of the LSDA onto the surface of the Ca soap micelle, so that complete precipitation does not occur [23]. Zhang and Xiao [32] are of the opinion that the dispersion comes from the union of LSDA with the free ionic soap molecules. The particles from the soap-LSDA mixture are far larger than the corresponding soap molecules in soft water and therefore result in turbidity in hard water. 

Soaps are made by heating sodium hydroxide with a fat such as coconut oil, olive oil, or beef fat, which contain esters formed between glycerol and fatty acids (see Section 19.7). The sodium hydroxide attacks the esters and forms the soluble soap. In the case of beef fat, stearic acid forms the soap sodium stearate, seen in (3). Soaps, however, form a scum in hard water. The scum is an impure precipitate of calcium stearate. 

A second problem with hard water is that these metals react with soap molecules and form a scum to which bathtub rings, etc., are attributed. Hard water is therefore not the best water to use for efficient soapy water cleaning processes, since the metal-soap precipitation reaction competes with the cleaning action. Water softeners assist with solving this problem too. 

These Insoluble soaps separate as scum in water and are useless as cleansing agent. In fact these are hlnderance to good washing, because the precipitate adheres onto the fibre of the cloth as gummy mass. Hair washed with hard water looks duU because of this sticky precipitate. Dye does not absorb evenly on cloth washed with soap using hard water, because of this gummy mass. 

Detergents are like soaps, but they re derivatives of either phosphoric acid or sulfuric acid (see Figure 16-31). They ve gained popularity laecause they don t precipitate 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. 

Sometimes people refer to water as being "hard" or "soft." If someone says water is hard, he means that there are a lot of calcium (Ca2+) or magnesium (Mg2+) ions dissolved in it. Hard water causes several problems. First, it can cause scales to form on the inside of pipes, water heaters, or teakettles. These scales occur when the calcium or magnesium precipitates out of solution and sticks to the insides of pipes. The scales build up and eventually the pipes are completely clogged. Hard water also prevents soap from lathering, and it reacts with soap to leave behind a sticky film commonly called soap scum. 

In hard water areas all over the world, it is difficult to make the soap lather. Instead, the water becomes cloudy. This cloudiness is caused by the presence of a solid material (a precipitate) formed by the reaction of the dissolved substances in the water with soap (basically sodium stearate) and it is a real problem. This white precipitate is known as scum (Figure 11.35). 

Soap does not form lather in hard water. It forms a precipitate or scum. As a result, dirt sticks to clothes. 

SDS has several advantages over soap. Unlike the carboxylate anions in soap, the dodecylsulfate anion does not form insoluble precipitates with hard water ions (e.g., Ca2+), leading to the formation of soap scum. Also, the dodecylsulfate anion is the conjugate base of a very strong acid (H2S04). Therefore, SDS is a weak base, and solutions of SDS are pH neutral. 

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. ... 

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