Question

calcium hydroxide is not used in to remove permanent hardness of water??

Answer 1

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Removal of Hardness from Water | Water Softeners


Removal of hardness from water can be done using different methods that can be grouped into two: chemical (use of chemical softeners) and mechanical (physical) methods.

Chemical Method of Removing Water Hardness - Use of Water Softeners
Removing harness using chemical method involves the use of chemical water softeners. These are of two types: those which lead to precipitation; and those which do not precipitate.

A. Chemical Water Softeners which lead to precipitation - these are chemical substances added to the water to remove dissolved calcium, magnesium or iron(II) salt by forming precipitate or undissolved solids. The precipitate is then removed by filtration.

Examples of these chemicals are:

(i). Calcium hydroxide, Ca(OH)2 - addition of calculated amount of Ca(OH)2 will remove only temporary hardness from water. Ca(OH)2 precipitates the insoluble trioxocarbonate(IV).

Example, Ca(OH)2(s) + Ca(HCO3)2(aq) → 2CaCO3(s) + 2H2O

Note: Ca(OH)2 is slightly soluble; Ca(HCO3)2 is soluble; CaCO3 is insoluble.

Also, notice that excess of Ca(OH)2 could cause hardness, hence the amount added is measured.

(ii). Sodium hydroxide, NaOH - addition of caustic soda (NaOH) removes both temporary and permanent hardness by precipitating the metal ions which cause the hardness as insoluble hydroxides.

Example, 2NaOH(aq) + Ca(HCO3)2(aq) → 2NaHCO3(aq) + Ca(OH)2(s)

Note: Ca(HCO3)2 is soluble; Ca(OH)2 is insoluble.

2NaOH(aq) + MgSO4(aq) → Na2SO4(aq) + Mg(OH)2(s)

Note: MgSO4 is soluble; is Mg(OH)2 insoluble

(iii). Sodium trioxocarbonate(IV), Na2CO3 (in form of crystals (i.e. washing soda) and soda ash (anhydrous)) - addition of Na2CO3.10H2O (crystals) or Na2CO3 (anhydrous) will remove both temporary and permanent hardness.

Example, Na2CO3(aq) + Ca(HCO3)2(aq) → 2NaHCO3(aq) + CaCO3(s)

Na2CO3(aq) + CaSO4(aq) → Na2SO4(aq) + CaCO3(s)

(iv). Borax, Na2B4O7. 10H2O - addition of borax removes both temporary and permanent hardness.

B. Chemical water softeners which do not lead to precipitation - these are chemical softeners whose process is based on the softeners ability to sequester Ca2+, Mg2+ or Fe2+ (i.e. without precipitating them, but keep them in solution, away from precipitating soap). These substances are Mostly of the polyphosphate family, e.g., polymetaphosphate, (NaPO3)n (this is sold under the trade name Calgon); and tetrasodium diphosphate, Na4P2O7.

They remove both types of hardness. Mechanical Method Mechanical water softening include: A. Distillation method - this will remove all solid particles that had dissolved in the water, but it is a rather expensive method. Both temporary and permanent hardness are removed. B. Ion exchange method - this involves the use of porous solids ion exchangers.

The principle of this method is that, the ions, i.e. Ca2+, Mg2+ or Fe2+, which cause hardness are removed from water by exchanging them with other cations from the ion exchangers which do not cause hardness - by this, the water is free from hardness. This is a cation exchanger. Two major types of ion exchangers (also called permutit) are used in softening water.

These are: 1. The zeolites - these are naturally occurring aluminosilicate minerals, e.g., NaAl(SiO3)2 or NaAlSi2O6

2. The ion - exchange resins - these are synthetic organic polymers (e.g., polystyrene). For example, when water containing Ca2+ or Mg2+ ions is allowed to filter through thick layers of zeolite, the Na+ ions in the zeolite are replaced by Ca2+ or Mg2+ ions in the solution: 2NaAlSi2O6 + Ca2+ → Ca(AlSi2O6)2 + 2Na+ . The hardness is thus removed.

A zeolite which has been used can be regenerated by allowing it to stand in contact with conc. NaCl solution. The calcium alumino silicate is reconverted to sodium aluminosilicate: Ca(AlSi2O6)2 + 2Na+ → 2NaAlSi2O6 + Ca2+.

Hence, by this reaction, sodium zeolite is ready to serve as a water softener again - common salt (NaCl) will keep this io

Answer 2

Answer: Permanent hardness is caused by the dissolved chlorides, nitrates and sulfates of Calcium and magnesium elements.

These include $CaCl_2,Ca(NO_3)_2,CaSO_4,MgCl_2,Mg(NO_3)_2\text{ and }MgSO_4$

This hardness cannot be removed easily just by boiling. Some chemical treatment is used to remove this hardness.

For all the Magnesium salts causing permanent hardness, we require calcium hydroxide ( $Ca(OH)_2$ ) and sodium bicarbonate ( $Na_2CO_3$ ) both to form an insoluble precipitate of calcium carbonate ( $CaCO_3$ ).

$MgSO_4(aq.)+Ca(OH)_2(aq.)\rightarrow CaSO_4(aq.)+MgSO_4(aq.)$

$CaSO_4(aq.)+Na_2CO_3(aq.)\rightarrow CaCO_3(s)+Na_2SO_4(aq.)$

For all the Calcium salts causing permanent hardness, we only require sodium bicarbonate to form a precipitate of calcium carbonate ( $CaCO_3$ ).

$CaSO_4(aq.)+Na_2CO_3(aq.)\rightarrow CaCO_3(s)+Na_2SO_4(aq.)$

So, in order to remove permanent hardness from water, only calcium hydroxide cannot be used.