ESTIMATION OF HARDNESS BY EDTA METHOD-----WATER TECHNOLOGY---CY2111 183101 –ENGINEERING CHEMISTRY –I Lecture Notes

ESTIMATION OF HARDNESS BY EDTA METHOD:

1. Aim:  To estimate the amount of hardness present in the given water sample.

2. Chemicals required:

a) Standard hard water – 1g CaCO₃ + dil. HCl – made up to 1 Lr.

b) EDTA solution – 4 gms of EDTA sodium salt / 1 Lr of water

c) EBT Indicator – 0.5g EBT / 100 ml alcohol

d) Ammonia buffer - 67.5g Ammonium chloride +570ml Ammonia-- made up to 1Lr

3. Principle:

EDTA stands for Ethylene Diamine Tetra Acetic acid. As it is insoluble in water, we use its disodium salt.

Structure of EDTA:

Na-OOC-H₂C                                                 CH₂ –COO-H                       

                             N – CH₂ – CH₂ – N

H-OOC-H₂C                                              CH₂ –COO-Na

By nature,  Eriochrome Black T indicator is blue in colour. When EBT indicator is added to water sample, it formsa wine red coloured unstable Ca-Mg-EBT complex. 

This reaction is carried out under a basic P^H of 8- 10 using ammonia buffer.

Ca²⁺ / Mg²⁺  in water  +  EBT  -à  [Ca / Mg –EBT] unstable wine red complex

When EDTA is titrated against the complex, EDTA replaces all the EBT and forms a stable Ca / Mg –EDTA complex.  The liberated EBT indicates the end point as steel blue.

[Ca / Mg –EBT]  + EDTA  à  [Ca / Mg –EDTA]    +   EBT

(Wine red/unstable)           (Stable)                         (Steel blue)

So, the end point is the colour change from wine red to steel blue.

4. Short Procedure:

S.No	Content	Titration-I Standardisation of EDTA	Titration-II Total Hardness	Titration-III permanent Hardness
1	Burette	EDTA	StandardEDTA	standardEDTA
2	Pipette(20 ml)	Std. Hardwater	Sample water	Boiled water
3	Additional solution	10ml of NH3 buffer	10ml of NH3 buffer	10ml of NH3 buffer
4	Indicator	EBT	EBT	EBT
5	Endpoint	Wine red to steel blue	Wine red to steel blue	Wine red to steel blue
6	Volume	V1	V2	V3

5. Calculation:

Step1 – Standardisation of EDTA

1 ml of Std. Hard water                    =  1 mg of CaCO₃    (Given)

So, 20 ml of Std. Hard water            =  20 mg of CaCO₃

V₁  ml of EDTA is required for        =  20 mg of CaCO₃

Therefore, 1ml of EDTA                 =  20 / V₁ mg of CaCO₃

Step 2:  Finding Total hardness:

20ml of sample water required        = V₂ ml of EDTA

                                                            =  V₂ X  20  mg of CaCO₃

                                                                          V₁                             

Therefore, 1000ml of sample requires = V₂ X  20  X  1000 mg of CaCO₃

                                                                               V₁         20

Hence, total hardness                        =          V₂  X  1000 ppm

                                                                         V₁

Step 3:  Finding Permanent hardness:

20ml of boiled water required          = V₃ ml of EDTA

                                                            =  V₃ X  20  mg of CaCO₃

                                                                          V₁                             

Therefore, 1000ml of boiled  sample requires = V₃ X  20  X  1000 mg of CaCO₃

                                                                                           V₁         20

Hence, permanent hardness                         =          V₃ X  1000 ppm

                                                                                    V₁

Step 4 :  Temporary hardness =  Total hardness – permanent hardness