Spectrophotometric Determination of the Formation Constants of Cerium and Thorium Complexes of Ethylenediametetraacetic Acid

A systematic ultraviolet spectrophotometric study of the versenate complexes of cerium[III] and thorium[IV] in acidic solutions of various ionic strengths has been made. It has been found that the cerium[III]‐versenate complex in aqueous solution exhibited a characteristic absorbancy maximum at 285 mμ, and that cerium[III] and versenate ion reacted in the ratio of 1:1 to form a complex. The reaction between thorium[IV] and versenate ion has been studied by means of the cerium[III]‐versenate complex as an indicator. The result also indicated that thorium and versenate ion reacted in the 1:1 ratio. The formation constants[log K f ] obtained at ionic strength of 0.1, 0.3 and 0.5 for the complexes were extrapolated graphically to infinite dilution to give the corresponding thermodynamic constants of 16.75 and 20.13 at 25°C, respectively. For the determination of very small quantities of thorium, the spectrophotometric titration of standard cerium[III]‐versenate complex solution with thorium nitrate solution was performed by using the absorption band of cerium [III]‐versenate complex as an indicator.