Spectrophotometric reaction of 9-(2-pyridyl-azo)-10-phenanthrol with copper, nickel and zinc
1973 Volume 22 Issue 7 Pages 860-866
Details
1973 Volume 22 Issue 7 Pages 860-866
9-(2-Pyridylazo)-10-phenanthrol, PAP, has been proposed as a sensitive reagent for metal ions in the previous paper. It has been recommended as an extractive spectrophotometric reagent since it has greater formation constants of metal chelates and sensitivities than PAN has.
More detailed stady is needed concerning the reagent and its metal chelates, because there has been no ditailed study of its behaviour at various pH, equilibrium forms, dissociation constants, metal ligand ratios, formation constants of the chelates in various dioxane concentrations, which are important in analytical chemistry.
PAP can be regarded as a bibasic acid. The acid dissociation constants of the reagent are determined spectrophotometrically at different dioxane concentrations. The value of pKa1 decreases and that of pKa2 increases with increasing mole fraction of dioxane. The plots of pKa1 and pKa2 against mole fraction of dioxane, n2, are linear. The equations for the straight lines are pKa1=-5.875 n2+2.87
pKa2=6.497 n2+12.80
PAP reacts with copper, zinc, and nickel to form red or violet, water-insoluble chelates. The results of the continuous variation method indicate that 1 to 1 chelates of copper and zinc and a 1 to 2 chelate of nickel are formed in dioxane-water mixtures. However, in solution containing more than 100-fold amounts of nickel, PAP forms a 1 to 1 chelate. The formation constants at different dioxane concentrations for the reactions between PAP and metal ions are determined spectrophotometrically. The formation constants increase with increasing mole fraction of dioxane, and the plots of log KRMR for 1 to 1 chelates against mole fraction of dioxane are linear. The equation of the straight lines are given by the equations :
log KRCuR =7.208n2+ 16.48
log KRNiR =10.56 n2+12.90
log KRZnR =8.657 n2+ 10.35
The distribution coefficient of the reagent between carbon tetrachloride phase and aqueous phase is determined spectrophotometrically. The logarithmic value, log KDR, is found to be 5.06+0.02, which is about 1.1 units greater than that of PAN. This fact suggests that the extension of benzene ring affects the aqueous solubility of the reagent. A study is made of the extraction of the chelate of copper and PAP from water into carbon tetrachloride. Since the slopes of the extraction curves, log [HR]0 vs. log D[H+]2 and pH vs. log D[H+]2-, are both 2. The results indicate that copper react with two molecules of PAP and two protons are released in forming the extractable species. The values of logKex and log KDCβ2 are found to be -1.65±0.02 and 36.13±0.02, respectively.
