Quantitative Chemical Analysis Harris | Lucy presented by Macmillan Learning Indirect EDTA determination of cesium. Cesium ion does not form a strong EDTA complex, but it can be analyzed by adding a known volume of NaBil, in cold concentrated acetic acid containing excess Nal. Solid Cs, Bi,I, is precipitated, filtered, and removed. The excess yellow Bil, is then titrated with EDTA. The end point occurs when the yellow color disappears. (Sodium thiosulfate is used in the reaction to prevent the liberated I" from being oxidized to yellow aqueous I, by 0, from the air.) The precipitation is fairly selective for Cs. The ions Lit, Na*, K*, and low concentrations of Rb* do not interfere, although TI does. Suppose that 22.00 ml of an unknown containing Cs were treated with 22.00 ml. of 0.05694 M NaBil, and the unreacted Bil, required 18.18 ml. of 0.0334 M EDTA for a complete titration. Find the concentration of Cs* in the unknown. (Cs*) = M