The amount of good quality bleaching powder required for chlorinating a round well measuring 4 mtrs diameter and depth of water column 6 mts, to get PPM of residual chlorine when the Horrok'stest gives definite blue colour in the 3rd cup onwards, is ?

Ans. is 'a' i.e., 993.8 grams o First we will have to find the volume of water in the well. o Now you must know how to calculate the amount of bleaching powder required to disinfect the well. o In Horrock's apparatus, 6 cups are filled with water and indicator (starch iodine) is added in increasing quantity in each cup (1 unit in 1st, 2 units in 2nd, 3 units in Ihird and so on). o The first cup which shows the blue color is multiplied by 2, for example if 2nd cup shows blue colour than 4 grams (2 x 2) of bleaching powder will required to disinfect 455 litre of water. o In the question 3rd cup is showing blue color. So, 6 gms (3 x 2) of bleaching powder will be required to disinfect 455 litre of water.