Solved: Indirect EDTA determination of cesium. Cesium ion does not form a strong EDTA

Chapter 11, Problem 11-38

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Indirect EDTA determination of cesium. Cesium ion does not form a strong EDTA complex, but it can be analyzed by adding a known excess of NaBiI4 in cold concentrated acetic acid containing excess NaI. Solid Cs3Bi2I9 is precipitated, filtered, and removed. The excess yellow BiI4 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 I2 by O2 from the air.) The precipitation is fairly selective for Cs. The ions Li, Na, K, and low concentrations of Rb do not interfere, although Tl does. Suppose that 25.00 mL of unknown containing Cs were treated with 25.00 mL of 0.086 40 M NaBiI4 and the unreacted BiI 4 required 14.24 mL of 0.043 7 M EDTA for complete titration. Find the concentration of Cs in the unknown. 11-39. The