Congo red as adsorption indicator

Abstract

1. A new adsorption indicator 'Congo red' has been described for argentometric titraitions. 2. The new indicator differs from all those described earlier in having both acidic as well as basic character, on account of which it is adsorbed with different colours, by both the positively and negatively charged halide bodies. 3. The adsorbability of the indicator by both the positively as well as the negatively charged halide bodies, endows it with a sharp colour change at the end point, a ready reversibility and a large range of applicability in the argentometric titrations. 4. The great usefulness of the indicator lies in its being applicable both in the titrations of halide ions by silver ions and also in the reverse estimations of silver ions by halide solutions. 5. The end point is particularly sharp as the colour change occurs from one end of the spectrum to the other: blue to red and vice versa. 6. The effect of the addition of a protective colloid 'dextrin' has been found to make the end points even sharper.