Engineering Chemistry S Datta

368 ENGINEERING CHEMISTRY
Erichrome black T is used as an indicator and pH is maintained at pH 10 using NH 4
OH/
NH 4
Cl buffer. Erichrome black T forms unstable complex with Ca 2+ , Mg 2+ giving wine red
colour. When EDTA added and the total Ca 2+ , Mg 2+ forms complex with it and the indicator
becomes free, the colour of the solution changes from wine red to blue at the end point.
Method: Indicator solution 0.5 gm. Erichrome black T dissolved in 100 ml alcohol.
Standard EDTA solution (N/100): 4 gm of EDTA dissolved with 0.1 gm of MgCl 2
in
1000 ml water.
An aliquot ≈ M
100 Zinc acetate solution (primary standard) is taken, to it 5 ml of NH 4 Cl–
NH 4
OH buffer and a few drops of EBT indicator are added and titrated to blue end point with
EDTA and the strength of EDTA is calculated.
This same procedure is repeated with 50 ml water sample. Let the volume of titrant be
V ≈ (f) M/100 EDTA solution.
∴ Permanent hardness = 0.001 × V × f × 10 6
ppm.
50
The EDTA method for the determination is preferred to other methods since the method
has greater accuracy, simplicity and rapidness.
Buffer solution: A mixture of 142 ml of conc. NH 4
OH and 17.5 gm NH 4
Cl is diluted to
250 ml distilled water.
Highlights:
• Softening methods include:
(i) Hot and cold lime soda process.
(ii) Zeolite or permutit process.
(iii) Ion exchange or demineralization.
• Micro-organisms from potable water can be removed by:
(i) Boiling for 10 to 15 minutes
(ii) Bleaching powder treatment
(iii) Chlorination with Cl 2
(iv) Chloramine treatment
(v) Ozonization.
• Desalination of brackish water done by:
(i) Electrodialysis
(ii) Reverse osmosis.
• Exhausted ion-exchange resins are regenerated by:
(i) Cation exchangers regenerated by passing strong acid solution through the bed
(ii) Anion exchangers regenerated by passing strong base solution through the bed.
• Hardness of water can be estimated by:
(i) Permanent hardness determined by titrating with standard Na 2
CO 3
solution.
(ii) Complexometric titration by EDTA
• Units of hardness are:
(i) ppm
(ii) mg/litre (1 ppm = 1 mg/litre)
(iii) Clark’s degree (1 ppm = 0.07° Cl)
(iv) degree French (1 ppm = 0.1° Fr).
(Contd.)