Horrok's apparatus determines Chloride which has to have a holding level of –: (A) 0.1 mg/L, (B) 1 mg/L, (C) 0.01mg /L, (D) 0.5 mg/L
Answer is D. Horrock's apparatus is used to find out the dose of bleaching powder required for disinfection of water by chlorination.
The principle in chlorination is to ensure a free residual chlorine of 0.5 mg/litre at the end of one hour contact.