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 BiI4 required 14.24 mL of 0.0437 M EDTA for complete titration. Find the concentration of Cs+ in the unknown.