<prompt> Horroks apparatus determines Chloride which has to have a holding level of0.1 mgL1 mgL0.01mg L0.5 mgL  <response>: Horrocks 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 mglitre at the end of one hour contact. 