The municipal water processing plants of communities with hard water often treat

Question

The municipal water processing plants of communities with hard water often treat the water supply with slaked lime, Ca(OH)2 , in order to remove Ca2+ from the water. The slaked lime reacts with bicarbonate from the dissolved metal bicarbonates in the water according to the following reaction:

      (Ca(OH)2  + 2 (HCO3)^-   --> CaCO3  (s) + (CO3)^2-  + H2O)     

The carbonate produced then reacts with Ca^2+originally in the water to precipitate as CaCO3 (s).

Thus calcium ions from slaked lime are added to the hard water in order to remove calcium ions from

the hard water!

If hard water has a calcium ion concentration of 1.8 x 10^-3 M and you want to reduce that concentration to 0.6 x 10^-3 M, what must be the concentration of the slaked lime? If you didn%u2019t do this calculation and you simply used a saturated solution of slaked lime, what do you think the consequences might be? Support or refute your prediction by quantitatively assessing the likelihood

of your predicted consequences.

Explanation / Answer

Calcium ion conc in hard water = 1.8 x 10-3 M

required calcium ion conc = 0.6 x 10-3 M

So conc of calcium to be removed = (1.8- 0.6) x 10-3 M

COnc of calcium ion to be removed = 1.2 x 10-3 M

The reaction between Calcium ion and carbonate is

Ca+2 + C03 2- = CaC03

So 1 M of calcium ion reacts with 1 M of carbonate ion

1.2 x 1-3 M of calcium ion reacts with 1.2 x 10-3 M of carbonate ion

So reuired carbonate ion conc = 1.2 x 10-3 M

These carbonate ions comes from the reaction between slaked lime and metal bicarbonates .

Ca(0H)2 + 2HC03 - --> CaC03 + C03 2- + H20

from the above reaction

1 M of carbonate ion is obtained from 1 M of slaked lime

So 1.2 x 10-3 M of carbonate ion is obtained from 1.2 x 10-3 M of Slaked lime .

SO Conc of Slaked lime should be 1.2 x 10-3 M

The over softening of water by slaked lime can have an adverse effect on the corrosion rate of cast iron and galvanised steel pipes. Very soft water can also hasten the decay of concrete tanks .Normally, the inside of ferrous pipes are protected by a layer which builds up as water flows, this layer contains CaCO3 and Fe. If very soft water is passed down a galvanised pipe the zinc layer is slowly removed exposing the steel. By adding more slaked lime more Ca+2 ions are removed . if there is not enough Ca2+ ions in the water ,calcium compounds can leach out of concrete causing a deterioration in it's structural integrity.Therefore care should be taken .

The concentration of calcium ions (Ca2+) in freshwater is found in a range of 0 to 100 mg/L, and

usually has the highest concentration of any freshwater cation. A level of 50 mg/L is

recommended as the upper limit for drinking water. High levels are not considered a health

concern; however, levels above 50 mg/L can be problematic due to formation of excess calcium

carbonate deposits in plumbing . If the calcium-ion concentration in freshwater drops below 5 mg/L, it can support only sparse plant and animal life.

Tha maximum conc of Ca+2 ions in drinking water = 1.25 x 10-3 M

The minimum conc of Ca+2 ions in drinking water to support human life = 0.125 x 10-3 M

So minimum amount of slaked lime to be added to make it possible for drinking = (1.8 -1.25 ) mM

So minimum amount of slaked lime to be added to make it possible for drinking = 0.55 x 10-3 M

Maximum amount of slaked lime that can be added = (1.8- 0.125) mM

Maximum amount of slaked lime that can be added = 1.675 x 10-3 M

So the amount of slaked lime to be added should be in the range of 0.55 mM to 1.675 mM

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