Polarimetric Studies of Alkali Metal Ion Complexes of /-trans 1,2-Diaminocyclohexane=N,N,N',N'-Te traacetic Acid James D. Carr and D, G.Swartzfager Department of Chemistry, University of Nebraska, Lincoln, Neb. 68508 RECENTLY SEVERAL REPORTS have appeared on the formation of weak 1 : 1 complexes between the alkali metal ions and the aminocarboxalate multidentate ligands: ethylenediaminetetraacetic acid ( I ) , propylenediaminetetraacetic acid (2-4) and truns-l,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (5). In the present study the optical rotatory properties of t-trms-1,2-diamiriocyclohexane-N,N,N',N'-tetraacetic acid (abbreviated here as I-CyDTA or Cy) have been employed as a convenient and accurate method of monitoring the formation of these weak complexes. EXPERIMENTAL Apparatus. Polarimetric measurements were performed at 365 nm in a 10-cm cell thermostated at 25 "C in a PerkinElmer Model 141 polarimeter. All pH measurements were made with a Corning Model 12 expanded scale pH meter. Reagents. All solutions were prepared with deionized water and stored in polyethylene bottles. The I-fruns-l,2diaminocyclohexane-N,N,N',N'-tetraacetic acid was prepared by the method of Reinbold and Pearson (6). A 0.5% aqueous solution of the active acid gave a specific rotation of -53.1 at the sodium D line. Stock solutions of the cations sodium, potassium, cesium (Alfa Inorganics) and tetramethylammonium (Southwestern Analytical) were prepared from their respective hydroxides. Solutions of lithium and tetraethylammonium hydroxide were prepared via ion exchange of their chloride salts. Procedure. Working solutions were prepared volumetrically from the stock solutions. The initial concentration of I-CyDTA was about 2.0 X 10-*M in all runs. After the alkali metal ion was added (in the hydroxide form), the ionic strength was adjusted to 0.5 with tetramethylammonium hydroxide which gave an initial pH of about 13.4. The optical rotation of the solution was determined at approximately 0.2 pH unit intervals as the pH was lowered by the addition of concentrated HCI. The observed molar rotation was then calculated according to Equation 1 after the total concentration of the active species was corrected for dilution. [slobs = aoba/bC
where
(Y&
b c
(1)
the observed rotation (degrees) cell length (centimeters) = concentration (moles per liter) = =
RESULTS AND DISCUSSION The effect of a large excess of each of the cations on the observed molar rotation over the pH range from 1.5 to 13.5 is illustrated in Figure 1. Since the observed molar rotations in the presence of both tetramethylammonium and tetraethylammonium were identical within experimental error over the entire pH range, it is assumed that these cations do
" I
0
4
6
8
1 0 1 2 1 4
PH
Figure 1. Effect of a large excess (0.35M) of each of the various cations on the observed molar rotation of I-CyDTA ( A ) potassium, ( B ) sodium, ( C ) cesium, (D) tetramethylammoniumand tetraethylammonium,( E )lithium
not interact with CyDTA. The behavior in the high pH region (10 to 13.5) is generally consistent with the known complexation of lithium, sodium, and potassium. Between pH 5 and 10 there are rather dramatic differences in the molar rotations observed in the presence of sodium and lithium. This is ascribed to the formation of the protonated complexes of these metal ions. Although less obvious, it is apparent that potassium also forms a protonated complex of some stability. Only below pH 5 do the curves all become congruent indicating complete dissociation of any associated species of CyDTA and the alkali metal ions. The effect of varying the concentrations of the metal ions: lithium, sodium, and potassium are shown in Figures 2, 3, and 4, respectively. The constraints on the system are represented by Equations 2-7.
(1) G. Anderegg, Helu. Chirn. Acta, 50, 2333 (1967). (2) J. L. Sudmeier and A. J. Senzel, ANAL.CHEM., 40, 1693 (1968). (3) J. L. Sudmeier and A. J. Senzel, J. Amer. Cheni. SOC.,90,6860 (1968). (4) J. D. Carr and D. G. Swartzfager, ANAL.CHEM., 43, 583 (1971). (5) Zbid.,42, 1238 (1970). (6) P. E. Reinbold and K. H. Pearson, Ztiorg. Chern., 9, 2325 (1970). 1520
2
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
["I [CY'-1 K 4 = [HCy3-] [MCY"1 [M+l[Cy"1
Kmcy = -~
(3)
(4)
-7
I
-10 I
I
I
I
I
I
I
I
I
I
, E
-1
1
I
0
7
6
5
9
8
PH
Figure 2. Effect of varying the lithium ion concentration Concentration of lithium: ( A ) 0, ( B ) 0.048, (0 0.072, (D)0.094, ( E )0.212
PH
Figure 4. Effect of varyhg the potassium ion concentration Concentration of potassium: ( A ) 0.375, ( B ) 0.166, (C) 0.083, ( D ) 0.055, ( E )0.028, (F)0
-!
I
I
1
6
7
8
9
I
I
I
10
11
12
13
PH
Figure 3. Effect of varying the sodium ion concentration
Figure 5. Comparison between experimental values of Z and the Atted curve for sodium data
Concentration of sodium: ( A ) 0, ( B ) 0.059, ( C ) 0.099,
(D)0.148, ( E )0.237, (F)0.355 [MHCy2-l KMHCY [M+l["y -'1
[M+l
F,
+ F,
=
WIT - [CYITF~
(5)
1
which relates the observed molar rotation at a given pH to the molar rotations of the free ligand [a],, (taken as the observed molar rotation in the presence of tetramethylammonium ion) and the complexed ligand [a]
