Engineering Chemistry S Datta

366 ENGINEERING CHEMISTRY
(b) Reverse osmosis. Osmosis describes the flow of solvent from dilute to concentrated
solution through a semipermeable membrane. Whereas
Pressure
reverse osmosis describes the flow of solvent in opposite
direction i.e., from concentrated solution to dilute solution
across a semipermeable membrane by applying hydrostatic
pressure in excess of osmotic pressure.
Piston
The cellulose acetate and the more recently used
polymethacrylate and polyamide membranes do not allow
the solute pass, while the solvent is forced through and
Sea water
Stout semipermeable
collected as a pure solvent in a direction as shown in
Fig. 17.8. There are many advantages of using reverse
membrane
osmosis as the purification process for water which
include (i) removal of ionic, non-ionic, colloidal and high
Pure water
molecular weight solutes from water, (ii) regeneration of
Pure
water out
the process involves the easy replacement of the
semipermeable membrane, (iii) easy maintenance and Fig. 17.8 Reverse osmosis cell.
economical, as the membrane lifespan is high, (iv) uninterrupted supply of large volume of
water for industrial or domestic purpose can be obtained. Reverse osmosis is largely used
for purification of sea water for domestic use.
Chemical Analysis of Water
1. Estimation of free chlorine. The residual free chlorine remaining after the municipal
processing of domestic water is injurious to health and hence estimation of this free chlorine is
essential. The principle is the treatment of water containing free chlorine with KI solution.
The chlorine present liberates an equivalent amount of iodine which can be estimated with
standard thiosulfate solution using starch as an indicator.
Cl 2
+ 2KI ⎯⎯→ 2KCl + I 2
I 2
+ 2Na 2
S 2
O 3
⎯⎯→ 2NaI + Na 2
S 4
O 6
I 2
+ Starch ⎯⎯→ Deep-blue complex
Method: To 50 ml of water sample in a iodine flask, 10 ml of 10% KI solution added,
shaken and waited for some time in the dark and finally titrated with N/50 sodium thiosulfate
solution using starch solution as indicator towards the end. The end point indicates a change
from deep-blue to just colorless solution.
Calculation:
Volume of N/50 Na 2
S 2
O 3
required = V ml
V × 35.5 × 10
∴ Strength of free chlorine = ppm.
2500 × 1000
2. Alkalinity. The alkalinity of water is estimated by estimating (a) OH – and CO
2–
3
ions
and (b) HCO
–
3
ions.
(a) OH – + H + ⎯⎯→ H 2
O
CO 2– 3
+ H + ⎯⎯→ HCO
–
3
(b) HCO– 3
+ H + ⎯⎯→ H 2
O + CO 2
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