A New and Sharp Ternary Indicator System for Argentometric Titration of Microamounts of Iodide

 A ternary indicator system, Cadion 1B-silver ion-emulsifier octylphenyl polyglycol ether, for visual and direct titration of iodide ion with silver nitrate (as the titrant) is described. The advantages of the method are as follows: (1) when the titration reaches the equivalence point, the color of the resulting solution sharply changes from pure yellow to orange and the solution remains transparent rather than turbid; (2) the method is sensitive (the detection limit is 2 × 10⁻⁴ mol/L iodide) and accurate (the recoveries of standard addition are 99.60%–100.8%); (3) the results of titrating iodide by the method agree with those obtained by eosin B as an indicator for iodide; (4) the endpoint is much sharper than that obtained by eosin B as indicator for iodide ion; (5) the method has been successfully applied to determination of iodide in the tincture of iodine and tetrapropyl ammonium iodide samples with satisfactory results; (6) a vast amount of iodate, sulphate and fluoride ions does not interfere; (7) the method also may be automated if necessary. Received November 23, 1998. Revision November 6, 2000.