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 PH 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 CaCO  (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
                                                            =  V220  mg of CaCO3
                                                                          V1                             
Therefore, 1000ml of sample requires = V220  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
                                                            =  V320  mg of CaCO3
                                                                          V1                             
Therefore, 1000ml of boiled  sample requires = V320  X  1000 mg of CaCO3
                                                                                           V1         20

Hence, permanent hardness                         =          V3 X  1000 ppm
                                                                                    V1

Step 4 :  Temporary hardness =  Total hardness – permanent hardness

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