REDUCTION OF DICHROMATE BY IODIDE IN PRESENCE OF CHLORIDES AND BROMIDES

The reduction of K2Cr2O7 by KI in sulphuric acid solutions in presence of varying amounts of chloride and bromide ions has been studied potentiometrically. Presence of small quantities of chloride ions (0.03 M) does not interfere with the normal course of the reaction, tha titration curve, showing only one sharp inflexion corresponding to six moles of KI per mole of K2Cr2O7. Higher concentrations of chloride ions lead to two inflexions corresponding to three and six moles, the first inflexion indicating the formation of ICl. The final inflexion is quite sharp in 4 N-H2SO4, but not so in smaller concentrations. Presence of even a small concentration of bromide (0.03 M) affects the reduction process, leading to two inflexions. The final inflexion is still at six moles KI for a rapid titration. In a slow titration there are two inflexions : the first one at three moles KI, followed by the final one, not sharp, at about six moles. The formation of IBr is thus clearly indicated