DISCUSSION
Table 111. Spectrophotometric Data for the Determination of log KZuHY at 25 "C and Ionic Strength 0.1 A
Absorbance at 1 cy x 103 255 nm [H+] x 10(ec"Y2-.Cc" - A ) 0.228 0.339 8.64 0.259 0.708 10.3 0.276 1.oo 11.4 0.298 1.48 13.4 0.326 2.63 17.2 0.350 4.17 22.7 ccu= 1.38 x 1 0 - 4 ~ ; cy = 1.70 x 1 0 - 3 ~ .
[CUHY-] = [B+] - Cy - Ccu + [H+] -
The first dissociation constant of the tetraprotic acid (EDPA), CH3C(OH)(POaH&, is too large to be measured potentiometrically; the three successive dissociation constants that have been measured are well separated (Table I). This simplifies the problems that are encountered in complex formation studies with copper(I1) since only two, or at the most, three anionic species of the ligand will have to be taken into consideration over a reasonably wide pH range. It has been shown experimentally that in the pH range 6.9 to 8.8, the predominant metal complex species that is present in solution is CuYz-, which has a formation constant of 1011.8*. This formation constant is much greater than the constant that would be expected if a four-membered chelate ring was formed. The metal chelate, CuY2-, probably has a more stable six-membered ring configuration. 0
HO
P-0
\
C '/
E
0-
\ /
/ \ / H3C P-0 / \
KCuHzY =
0
The equilibrium constants in Equations 22 and 29, obtained from measurements with a glass and saturated calomel electrode, are related to those in Equations 8 and 9, obtained from measurements with the solid state cupric ion-selective electrode. The relationships are:
H
K C ~ H= Y
HY KcuHy/K&y.k4
(32)
The constants obtained are summarized in Table I. An examination of the absorption spectra of solutions containing Cu2+ and EDPA in a 1 :10 ratio showed that a marked change in the absorbance at 255 nm occurred in the pH region 7.5 to 6.0. The potentiometric data obtained in this pH region indicate that the species CuY+ and CuHYare the important copper(I1) complex species present in the solution. Consequently, it should be possible to measure the equilibrium constant KguHY by a spectrophotometric method. Table I11 shows the variation of absorbance with pH in a solution containing Cu2+and EDPA in a ratio 1 :10. The value of log KgUHYwas calculated from a plot of 1 CY us. where A is the measured ab(CCu.ECuY2- - A ) W+I' sorbance and e C u Y 2 - is the molar absorptivity of CuYz-. This value of log KEuHyis included in Table I for purposes of comparison with the rest of the tabulated equilibrium constants. ~
cu
0-
This chelate can add two protons successively to give the complex species, CuHY- and CuH2Y. The addition of a proton to the dianion, CuY 2-, should take place more readily than the addition of a proton to the monoanion, especially if there is some degree of delocalization of the negative charges in the metal chelate. This is reflected in the formation constants of CuHY- and CuHzY as well as in the protonation constants of these two complexes (Table I). The maximum meta1:ligand ratio employed in this work was 1 :lo. Under these conditions, no insoluble complexes of copper(1I) were formed. It was observed, however, that if the metal: ligand ratio was decreased to about 2 : 1, precipitation occurred, probably due to the formation of a binuclear complex. It would be of interest to determine the structure of the insoluble copper(I1) complex, especially since it has been proposed that the insoluble thorium(1V) complex forms four-membered chelate rings with this ligand (I).
RECEIVED for review December 2, 1970. Accepted February 11, 1971. This work was supported by U. S. Atomic Energy Commission Grant No. AT(11-1)-1654.
Correct ion High Sensitivity Internal Reflection Spect roelect rochemist ry for Direct Monitoring of Diffusing Species Using Signal Averaging In this article by Nicholas Winograd and Theodore Kuwana [ANAL.CHEM. 43, 252 (1971)] the following credit should be added on page 259, column 2: The authors acknowledge the financial support of the National Institutes of Health (GM14036) and the National Science Foundation (GP9306).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
755
