The stability constants of Ni( II) acetylacetonates in methanol-, ethanol-, 1-propanol-, 2-propanol-, and dioxane-water and those for Co( II) acetylacetonates in methanoland 1-propanol-water have been measured a t 25' C. linear relationships were shown when log K1 or log KZ was plotted vs. 1/D. Also, the data have been discussed in terms of the effect of solvent on dissociation and chelation.DISSOCIATION constants of acetylacetone have been determined in mixed solvent systems (9). The purpose of this work is to investigate in a similar manner the solvent effect on the stepwise formation of Ni(I1) and Co(I1) acetylacetonates in mixed solvent systems a t 25" C. Acetylacetone was selected as the ligand because the resultant metal chelates are stable and readily soluble in the dielectric constant range 80 to 25. MATERIALS 1-Propanol, 2-propanol, methanol, dioxane (Fisher Certified reagents), and ethanol (Commercial Solvents absolute) were used without further purification. Refractive indices a t 25"C., n 2 , agreed closely with reported values ( I S ) , corrected (temperature corrections were based on the empirical relation dn,/dt = -0.0004 deg.) to the same temperature ( 3 ) : 1-propanol, 1.3837 (1.3834); 2-propanol, 1.3756 (1.3756); methanol, 1.3267 (1.3268); dioxane, 1.4200 (1.4197); and ethanol, 1.3592 (1.3590).Acetylacetone (2,4-pentanedione) and tetramethylammonium hydroxide (10% aqueous solution) were used without further purification. The boiling point range and the refractive index of acetylacetone were 13556.5" C. and 1.4520, respectively. Stock solutions of tetramethylammonium hydroxide were prepared from the Kodak reagent by diluting distilled water and standardized with HC1 solution. The nickel and cobalt perchlorate hexahydrates (analytical reagent grade) were obtained from the G. Frederick Smith Co. Stock solutions of these perchlorates were prepared from the above reagents as received, and then the exact concentrations of the metal ions were determined by standardizing them with EDTA solution (Fisher Certified) using murexide as an indicator ( 1 8 ) .The sodium perchlorate (G. Frederick Smith Co.) assay was accomplished by using an ion exchange technique. Sodium ion was exchanged for hydrogen ion on a purified Dowex resin column, and the eluted solution was titrated against sodium hydroxide (Fisher Certified), which had been previously standardized against potassium acid phthalate.Doubly distilled, lead-free perchloric acid (G. Frederick Smith Co.) was used without further purification. Distilled water was used throughout for preparation of aqueous solutions. Standard buffers (pH 2.0, 4.0, 7.0, and 10.0) were obtained from Beckman Scientific Co.Although the reagents used were previously shown (9-11) to be of sufficient purity for titration studies, an additional precaution was taken. The two-titration method by Calvin and Wilson (5), and modified by Calvin and Melchior (6), was used to eliminate possible errors due to contamination of the reagents used. A possible trace impurity would cause a displace...
