Q. Discuss the EDTA method of determining hardness of water.

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What is hardness of Water? How it is determined by EDTA method? Enumerate units of hardness.

                                             OR

Give the chemical reactions involved in determination of hardness of water by EDTA titration.

Hardness of water can be determined by EDTA (complex metric) method:

Ans. EDTA (ethylene diamine tetra acetic acid) is an extraordinary strong complexion agent. As it is not very soluble in water and hence EDTA in the form of its disodium salt is normally used in all comlexometric titrations. This acts as a tetra/hexes dentate legend and attaches itself with metal ions (through O or N atoms) to give very stable, soluble and colourless complexes. Since hardness of water is due to Ca⁺² and Mg⁺² mainly, their presence can be conventionally determined by titrating a water sample against standard solution of Na₂H₂Y using a suitable ion indicator to note the point. When hard water is titrated against EDTA then EDTA or its sodium salts form stable complex ions with calcium (Ca⁺²) magnesium (Mg⁺²) ions in water.

                            Ca⁺² + EDTA = [Ca⁺²                     EDTA] complex

                            Mg⁺² + EDTA = Mg⁺²                    EDTA] complex

The titration is carried out in presence of indicator, Erichrome black-T. The indicator when added in small amount of hard water, buffered to a pH value of about 10, combines with a few of Ca⁺², Mg⁺² of ions to form a weak complex of wine-red colour which changes to blue when an excess drop of EDTA is added.

Ca⁺² + Erichrome black-T = [Ca⁺² Erichrome black-T]

Mg⁺² + Erichrome black-T = [Mg⁺² Erichrome black-T]

                                                              (Unstable complex) (Wine red)

= [Ca⁺² + EDTA] complex + Erichrome black-T

   [Mg⁺² + EDTA] complex + Erichrome black-T

    (Stable)    (Blue)       (Free indicator)

Thus, change of wine red colour to distinct blue, marks the end point of titrations. Various steps of involved in this method are:

1.      Standardization of EDTA solution.

2.      Determination of total hardness of water.

3.      Determination of permanent hardness of water.

Calculation:

50 ml of standard hard water = V₁ ml of EDTA

50* 1 mg of CaCO₃ = V₁ ml of EDTA

1 ml of EDTA = 50/V₁ mg of CaCO₃ eq.

Now 50 ml of given hard water = V₂ mg of EDTA

V₂*50/V₁ mg of CaCO₃ Eq.

1 L of given hard water = 1000V₂ mg of CaCO₃ eq. /V₁

Total hardness = 1000V₂ mg/l or ppm. /V₁

Now 50 ml of boiled water = V₃ ml of EDTA = V₃*50 mg of CaCO₃ eq. /V₁

1000 ml of boiled water = 1000V₃ mg of CaCO₃ eq. /V₁

Permanent hardness = 1000V₃ppm/V₁

Temporary hardness = (Total-Permanent) Hardness

Temporary hardness = 1000(V₂-V₃) ppm/V₁

Units of hardness and their inter-relations

(1)         Parts per million (ppm):

                                              It is defined as the number of parts by weight of CaCO₃ equivalent present in per million (10⁶) parts by weight of water.

1 ppm = 1 part of CaCO₃ equivalent hardness in (10⁶) parts of water.

2. Milligrams per litre (mg/L):

It is defined as the number of milligrams of CaCO₃ present in one equivalent hardness litre of water.

1mg/L = 1 mg of CaCO₃ equivalent hardness present per little of water.

It can be easily proved that 1 mg/L = 1 ppm, for water

Weight of 1 litre of water = 1 Kg = 10³ g = 10⁶ mg =1000 * 1000g = 10⁶ mg

1 mg/L = 1 mg of CaCO₃ eq. Hardness per 10⁶ mg

1 mg/L = 1 part of CaCO₃ eq. Hardness per 10⁶ parts of water

1 mg/L = 1 ppm

Degree Clarke (⁰Cl):

It is defined as the parts of CaCO₃ equivalent hardness pert 70,000 part of water or it is number of grains (1/7000lb) of CaCO₃ equivalent hardness per gallon (10 lb or 70,000 grains) of water.

1⁰Cl = 1 part of CaCO₃equivalent Hardness per 70000 parts of water.

Degree of French (⁰Fr):

It is defined as the parts of CaCO₃ equivalent hardness per lace (10⁵) parts of water.

1 ⁰Fr = 1 part of CaCO₃ eq. Hardness per 10⁵ parts of water.

Mille equivalent per Litre (meq/L):

It is defined as the number of milli equivalents of hardness present per litre.

 1meq. /L = 1meq. Of CaCO₃ per L of water

                    = 10^-3* 50 mg of CaCO₃ eq. per litre of water

                     = 10^-3*1000* 50 mg of CaCO₃ eq. per litre of water

                    = 50 mg of CaCO₃ eq. per litre of water

                    = 50 mg/L of CaCO₃ eq. = 50 ppm

                      = 50 mg of CaCO₃ eq. Hardness per 10⁶ parts of water

                      = 1 mg of CaCO₃ eq. Hardness per 10⁶/50 mg of water

                      = 1 part of CaCO₃ eq. Hardness per 20,000 parts of water