Concept of Chlorine demand and Horrocks Apparatus

1. Concept of Chlorine demand
and Horrocks Apparatus
Dr. Anuj Singh
Asst. Professor
Community Medicine Dept
UIMS , Prayagraj

10. “If the value is zero, the water is already free
of pathogenic microorganisms”.

12. Breakpoint Chlorination

13. Breakpoint chlorination is defined as the point where
enough chlorine has been added to a quantity of water to
satisfy its disinfecting demand. In other words, it is the
point where all undesirable contaminants have been
removed from the water. At breakpoint chlorination, all
chlorine added to the solution is consumed by chemical
reactions with the contaminants, resulting in no free
available chlorine (FAC) in the water

14. Breakpoint Chlorination:
Combined chlorine (C.C.) is free available chlorine that has
combined with wastes containing nitrogen or ammonia
(also known as chloramines).
Combined chlorine will cause a noticeable "chlorine
odour" and can cause skin, eye, and mucous membrane
irritation. It also has little disinfecting power. Breakpoint
chlorination is used to get rid of combined chlorine.

15. To determine the amount of combined chlorine use the
following equation:
Combined Chlorine = Total Chlorine - Free Available Chlorine
Breakpoint chlorination is a technique used to remove
combined chlorine by adding free available chlorine. To
reach breakpoint, sufficient chlorine must be added to the
pool to raise the free available chlorine level to 10 times
the amount of combined chlorine.

17. Horrocks Apparatus

18. Horrock’s apparatus:-
used for estimating the dose of bleaching
powder needed to disinfect 455 Liters of
water.
It is used mostly while disinfecting the
water in a well.

19. Contents of ‘Horrocks Apparatus’
• 6 white cups, each of 200 ml capacity
• One black cup with a circular mark on the inside
• 2 metal spoons (each holds 2g of bleaching powder
when filled up to the brim)
• 7 glass stirring rods
• One special pipette
• Two droppers
• Starch - iodide indicator solution
• Instruction folder

20. Steps in Estimating the Required Dosage of
Bleaching Powder for disinfecting water:-
1. One level spoonful (2g) of bleaching powder is placed in
the black cup and made into a thin paste with a little
water. More water is added to the paste to bring up the
volume up to the circular mark. The solution thus
prepared is the ‘Chlorine Stock Solution’
2. The six white cups are filled with the water to be tested
3. Using the pipette provided with the apparatus, add one
drop of the stock solution to the first cup, two drops to
the second cup, three drops to the third cup, and so on
till adding six drops to the sixth cup.

21. 4. Stir the water in the cups; use a separate rod for each
cup
5. Wait for half an hour
6. Add three drops of starch-iodide indicator to each of
the white cups
7. Development of blue color indicates the presence of
free residual chlorine.
8. The first cup which develops this color gives an estimate
of the amount of bleaching powder needed to disinfect
455 L of the sample water.

22. If first cup develops blue colour: 1 spoonful (i.e. 2gm) of
bleaching powder would suffice for 455 L of water
If the second cup develops blue colour: 2 spoonfuls (4 gm)
of bleaching powder for 455 L of water
Third cup develops blue colour: 3 spoons (6 gm), so on till
the sixth cup when it means 6 spoons (12 gm) of bleaching
powder is needed for 455 liter of water
If none of the cups develop a blue colour, this means that
the chlorine demand of the water is very high and it is
better to use some method of clearing the water prior to
chlorinating it