EDTA (Ethylene Di amine Tetra Acetic acid) Method:-

Principle: This is a complex metric method. It is in the form of its sodium salt which yields the anion and this forms complex with Ca+2 and Mg+2 ions. (Molecular Wt- 372.24, Equivalent Wt- 186.14 i.e., M=2N)

In  order  to  determine  the equivalence point  (i.e.,  just  completion of metal-EDTA  complex formation) indicator Eriochrome Black-T or EBT (an alcoholic solution of blue dye) is employed which form unstable wine red complex with Ca+2  and Mg+2  ions. The indicator is effective at about pH 10.When EBT is added to hard water, buffered to a pH of about 10 (employing NH4OH-NH4Cl buffer), a wine red unstable complex is formed. Thus,

$\underset{complex}{M^2}+EBT \xrightarrow[]{pH=10}[M-EBT]$

(M+2 indicates Ca+2 / Mg+2 and M-EBT is un-stable wine red)

During the course of titration against EDTA solution, EDTA combines with M+2 (or Ca+2 or Mg+2) ions from stable complex M-EDTA and releasing free EBT, which instantaneously combines with M+2 ions still present in the solution, thereby wine red colour is retained. Thus, titration

$[M-EBT] complex + EDTA\rightarrow [M-EDTA] complex + EB$

nearly all M+2  (Ca+2 or Mg+2) ions have formed [M-EDTA] complex, then next drop of EDTA added drop wise displace the EBT indicator from [M-EBT] complex and wine red color changes to blue  color (due to EBT).

Thus at equivalence point,

$[M-EBT] complex + EDTA \rightarrow [M-EDTA] complex + EBT (blue)$

Thus, change of wine red to blue colour marks the end point of titration.

Steps involved:

1. Preparation of Standard Hard Water: Dissolve 1gm of pure dry CaCO3 in minimum quantity of dil. HCl and then evaporate the solution to dryness on water bath. Dissolve the residue n distilled water to make 1L solution. Each 1ml of this solution contains 1mg of CaCO3 hardness.
2. Standardization of EDTA solution: Rinse and fill the burette with EDTA solution. Pipette out 50ml of standard hard water in a conical flask. Add 10-15ml of buffer solution and 4 drops of indicator. Titrate with EDTA solution till wine red colour changes to clear blue. Let the volume used be V1ml.
3. Titration of Unknown Hard Water: Titrate 50ml of water sample just in step5. Let the volume used be V2ml.
4. Titration of Permanent Hardness: Take 250ml of water sample in a large beaker. Boil till the volume is reduced to about 50ml (all the bicarbonates are decomposed into insoluble CaCO3+Mg(OH)2) filter, wash the precipitate with distilled water collecting filtrate and washings in a 250ml measuring flask. Finally make up the volume to 250ml with distilled water. Then titrate 50ml of boiled water sample just as in step 5. Let the volume used be V3ml.

Calculations:

$50\ ml\ of\ standard\ hard\ water = V_1\ ml\ of\ EDTA.$

$∴ 50\times1\ mg \ of \ CaCO_3 = V_1\ ml \ of \ EDTA$

$ ∴ 1\ ml \ of \ EDTA = \dfrac{50}{V_1} mg \ of \ CaCO_3 \ equivalent$

$50\ ml\ of\ given\ hard\ water = V_2\ ml\ of\ EDTA=\dfrac{V_2\times20}{V_1}mg \ of \ Ca CO_3 \ eq.$

$\therefore1L\ (1000ml)\ of \ given \ hard \ water= \dfrac{V_2\times1000}{V_1} \ mg\ CaCO_3\ eq.$

$Total\ Hardness\ of\ water = 1000\times\frac{V_2}{V_1} mg/l= 1000\times\frac{V_2}{V_1}p.p.m.$

Now,

$50\ ml\ of\ boiled\ water = V_3\ ml\ of\ EDTA=\dfrac{V_3\times50}{V_1}mg \ of \ Ca CO_3$

$∴1L\ (1000\ ml)\ of\ boiled\ water = 1000\times\dfrac{V_3}{V_1}mg\ of\ CaCO3\ eq.$

$∴ Permanent\ hardness = 1000\dfrac{V_3}{V_1} p.p.m.$

$\therefore Temporary\ Hardness= Total\ hardness- Permanent\ hardness$

Advantages of EDTA method : This method is preferable because of -

1. Greater accuracy
2. Convenience
3. Rapid procedure