Spectropolarimetric Determinations of the 3d Transition Metals Utilizing a Stereospecific Ligand R. J. Palma, Sr., P. E. Reinbold, and K. H. Pearson Department of Chemistry, Texas A 6 M University, College Station, Texas 77843
A spectropolarimetric method of analysis was developed utilizing the stereospecific ligand, D-(-)-l,2propylenediaminetetraacetic acid, D(-)PDTA. The optical rotation of the solution was monitored with a photoelectric polarimeter to determine the end point of the titration. The metal complexes and the titrant are optically active and self-indicators; therefore, it is possible to utilize the maximum quantitative pH range for the determination of each metal. All the metals were analyzed using three buffers. Because D(-)PDTA is a stereospecific ligand, it ensures maximum utilization of the rotations of the metal complexes. The observed optical rotation is linear with respect to concentration of the metal complexes; consequently, it was possible to perform straight line extrapolations to the end point. The effects of pH, wavelength, dilution, and diverse ions were investigated and the optimum conditions for the determination of each metal were established. The average deviation for all the metal ion titrations was 0.19%.
RECENTDEVELOPMENTS in the construction of photoelectric spectropolarimeters ( I ) and in the availability of commercial spectropolarimeters ( 2 , 3 )have resulted in a renewal of interest in extending the range of spectropolarimetric titrimetry in analysis. These instrumental developments make use of electronic polarimeters and spectropolarimeters as practical detectors in titrimetry with dissymmetric substances. Spectropolarimetric titrimetry is a new analytical technique, first described by Kirschner and Bhatnagar ( 4 ) . Basic to the technique is the use of photoelectric polarimeter to monitor the change in optical rotation of a solution upon the addition of titrant. The system must be chosen such that a recognizable change in optical rotation occurs at the titration end point. Applications to both acid-base titrimetry and the determination of metal ions have been described (5). Only three metal ion ligand titrations have been described to date: the titration of L-(+)-histidine monohydrochloride with copper(I1) chloride ( 4 ) , the titration of disodium EDTA with zinc(I1) nitrate using L-(+)-histidine as an asymmetric indicator (3, and the titration of D-( -)-1,2-propylenediamine with nickel(I1) perchlorate (5). A simple spectropolarimetric method for the determination of the 3d transition metals was developed. This method is uncomplicated, sensitive, rapid, and versatile. In order to obtain this simple spectropolarimetric method, a strong stereospecific chelating agent, D-( -)-1,2-propylenediaminetetraacetic acid, D(-)PDTA, was selected as the titrant. In this paper a spec-
(1) M. J. Albinak, D. C. Bhatnagar, S.Kirschner, and A. Sonnessa in “Advances in the Chemistry of the Coordination Compounds,” S. Kirschner, Ed., Macmillan, New York, N. Y., 1961. (2) P. Crabbe, “Optical Rotatory Dispersion and Circular Di-
chroism in Organic Chemistry.” Holden-Day. San Francisco. Calif., 1965. (3) C. Djerassi, “Optical Rotatory Dispersion,” McGraw-Hill, New York, N. Y., 1960. (4) S. Kirschner and D. C. Bhatnagar, ANAL. CHEM.,35, 1069 (1963). ( 5 ) K. H. Pearson and S. Kirschner, And. Chim. Acta, in press. _ ,
_
tropolarimetric method is described for the determination of the 3d transition metals in the range of 10-l to lO+M. No systematic study had been made to determine the potentialities and limitations of this technique. It appears to offer many advantages over present techniques. D( -)PDTA was prepared from D-(-)1,2-propylenediamine by a modified method of Dwyer and Garvan (6). This compound is a powerful chelating agent, similar in structure to ethylenediaminetetraacetic acid; however, it is optically active and stereospecific in its reaction with metal ions (7). It has been estimated that the free energy difference due to a methyl-acetate interaction in the D*-isomer, (structure 11), compared to the L*-isomer, (structure I), approaches 3 kcal/
I
II
mole (7), giving a minimum of 99.9% stereospecific reactions. The greater stability of the L*-isomer assures complete stereospecificity and therefore maximum rotations are obtained for the m e t a b - ( -)PDTA complexes. Comparisons of the available stability constants indicate larger stability constants for racemic PDTA than EDTA for many metals in solution. The observed metal-chelate and ligand rotations are simple linear functions of their concentrations. Preliminary calculations have shown that this linearity extends over many orders of magnitude of concentration. EXPERIMENTAL
Apparatus. The spectropolarimetric titrations were carried out at room temperature using a Perkin-Elmer Model 141 photoelectric polarimeter. This polarimeter has a digital readout of the observed optical rotation with a sensitivity of O.O0lo. The radiation sources are a sodium vapor lamp and a mercury vapor lamp. These sources, with appropriate filters in a filter wheel, permit the selection of the following wavelengths: 589, 578, 546, 436, and 365 nm. The polarimeter cell used in this study was a 5-ml flow-through cell with optically inactive glass endplates. The polarimeter cell had a path length of 1 decimeter. The flow-through cell was connected to a stirbar pump, the titration vessel, with two 20-cm portions of Tygon tubing. Mallinckrodt Q D connectors were placed in the Tygon tubing to facilitate handling and cleaning. The stirbar pumps were fabricated from 125ml Erlenmeyer flasks, with two attached glass tubes, one at the bottom center, and the other ‘ 1 5 the distance up the
I
(6) F. P. Dwyer and F. L. Garvan, J . Amer. Chem. SOC.,81, 2955,
(1959).
( 7 ) F. P. Dwyer and D. P. Mellor, “Chelating Agents and Metal Chelates,” Academic Press, New York, N. Y., 1964, p 208.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
47
Table I. Conditions for Spectropolarimetric Titrations Wavelength, nm Comments Metal PH Sea+ 5.0 365 365 Ti 4+ 1.4 546 5.0 VOt+ 365 Cr a+ 5.0 back titration with zinc 5.0 365 Mnt+ 1.3 546 Fe a+ 5.0 365 Cot+ 10.0 Ni I+ 365 pH = 5.0 buffer may be used cua+ 10.0 436 may also be titrated at 365 nm with pH = 5.0 buffer ZnZ+ 5.0 365 pH = 10.0 buffer may also be used
side. The titration vessel was positioned over a magnetic stirrer and a magnetic stirbar, placed in the vessel, pumped the solution through the polarimeter cell. With this apparatus, the response time was less than 5 sec. All pH measurements were made with an Orion Model 801 digital pH meter. Reagents. All solutions were prepared with demineralized water and stored in polyethylene bottles. Chemicals were reagent grade. D-(-)-1,2-propylenediaminetetraacetic acid monohydrate was prepared by a modification of the method of Dwyer and Garvan (6). A 0.5% aqueous solution of D-( - )-1,2-propylenediaminetetraacetic acid monohydrate gave [a]22689= -46.9’. Approximately 0.25 mole of the acid monohydrate and 0.50 mole of NaOH pellets were dissolved in 500 ml of water. The resultant straw colored solution was filtered through a fine sintered glass filter. The NazD( -)PDTA solution was then standardized by titrating it against a standard zinc solution using EBT as the indicator (8), and standard lead solutions using xylenol orange as the indicator (8). The titer and optical rotation of the standard N a 2 ~-)PDTA ( solutions remained constant during a fourmonth period. The pH = 10 buffer was prepared by dissolving 280 grams of ammonium chloride in water, adding 568 ml of concentrated ammonium hydroxide, and diluting the resultant solution to a final volume of 1 liter. The pH = 5 buffer was prepared by dissolving 102 grams of sodium acetate in 500 ml of a solution containing 28 ml of glacial acetic acid. The resultant solution was diluted to a final volume of 1 liter. Solutions titrated below pH = 2 were buffered with concentrated nitric acid. Standard solutions of EDTA were prepared from G. F. Smith dried primary standard NazHzEDTA 2H20. The standard cobalt(II), nickel(II), copper(II), and zinc(I1) solutions were prepared from perchlorate salts and analyzed by EDTA titrations (9, IO). The standard titanium(II1) solution was prepared from TiC14 and standardized by determining the Ti02 content of aliquots gravimetrically. The standard vanadium(1V) solution was prepared from VOS04 and standardized by EDTA titrimetry (21). The standard chromium(II1) solution was prepared by reduction of primary standard potassium dichromate with sodium sulfite in dilute sulfuric acid solution. Standard iron(II1) solution was prepared by dissolving ACS (A.R.) grade iron wire (99.9z)in dilute HC1 containing a few drops of bromine water. The solution was then heated to remove the excess bromine. The standard
-
__
~~
(8) G. Schwarzenbach, “Die Komplexometrische Titration,”
Ferdinand Enke Verlag, Stuttgart, 1955. (9) W.Biedermann and G. Schwarzenbach, Chimiu, 2, 56 (1948); Chem. Abstr., 42,3694 (1948). (10)H.Flaschka and H. Abdine, Chemist-Analyst, 45,2 (1956). (11) Kuang Lu Chen and R.H. Bray, ANAL. CHEM., 27,782(1955). 48
scandium(II1) solution was prepared from Sc203and dilute nitric acid and the standard manganese(I1) solution was Mn(NO& solution. Both solutions prepared from 52 were standardized by EDTA titrations (9). Procedure. The polarimeter was allowed to warm up for a period of about 30 minutes and the digital readout was set to zero against an air blank. The flow-through cell was attached to the stirbar pump and measured aliquots of the standard metal ion solutions were pipeted into the titration vessel. The appropriate volume of water and buffer were then added to the titration vessel. This was done to simplify dilution corrections. With the aid of a magnetic stirrer, the solution was pumped through the flow-through cell positioned in the polarimeter. The digital readout of the polarimeter was then rezeroed with the metal ion-buffer solution serving as a blank. The solution was titrated with a relatively concentrated solution of NanD(-)PDTA, using a microburet readable to 0.001 ml. After each addition of titrant, the polarimeter digital readout was allowed to stabilize, usually requiring 15 seconds. Readings near the end point are not desirable because equilibrium may be obtained slowly. Usually, it was only necessary to obtain five points before the five points after the end point for each determination. The end point was determined graphically on paper readable to 0.001 unit. It is highly desirable that the titration points graphed be far removed from the equivalence point, points should not be graphed within the first 20% of the excess titrant. This is necessary because the curves tend to curve downward toward the volume axis in the vicinity of the equivalence point. Table I lists the optimum conditions for the spectropolarimetric titrations of the transition metals with D(-)PDTA. Procedure for Chromium(II1). A solution containing Cr3+ and excess D(-)PDTA was buffered with the acetic acidsodium acetate buffer and boiled. This solution was cooled, and diluted to volume in a 250-ml volumetric flask. Fiftyml aliquots of this solution were titrated in the same manner as the other metal ions except that a standard zinc solution was used as the titrant.
RESULTS AND DISCUSSION Effect of Wavelength. In order to obtain results of analytical significance, there should be significantly different optical rotations of D( -)PDTA and the metal complex formed at the wavelength chosen for the spectropolarimetric titration. If the optical rotations are not significantly different, the end point becomes indistinct and there is a large uncertainty in the value obtained for the extrapolated end point. Another criterion in the selection of the wavelength used in a spectropolarimetric titration is that the absorbance of the solution should be low. Highly absorbing solutions decrease the sensitivity of the polarimeter, leading to errors in the measured rotation. The optical rotatory dispersion and absorption spectra for the metal complexes and D( -)PDTA were examined to determine the wavelength giving the maximum optical rotational difference with suitable transmittance. The absorption of D(-)PDTA was never the limiting factor in the selection of a suitable wavelength. The molecular rotations for the metal complexes and D( -)PDTA were obtained by preparing solutions either 0.01 or 0.001M. These were adjusted to the desired pH value with acetic acidsodium acetate buffer, ammonia-ammonium chloride buffer or concentrated nitric acid. Measurements were made at ambient temperatures in a one-decimeter polarimeter cell (Table II). Most of the metal complexes exhibited their maximum rotations at 365 nm while D(-)PDTA had a relatively small rotation at 365 nm. Consequently, 365 nm was chosen as the analytical wavelength for most of these titrations.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
z
- 200
i
+
Z
0
0 -
+ Q
400
k
0
2
-300
t-
-400
-400 CT
6
[r
Q
1 3
3
-500
0
0
5
6
0 2
w
-600
2 I
I
3.0
5.0
I
I
7.0
9.0
I
11.0
1
PH
Mna+ Fe 3+ co2+
Ni2+ cu2+ cu2+
I
4.0
1
I
I
I
6.0
8.0
10.0
12.'
PH
Figure 2. Effect of pH on the molecular rotations of the nickel(II), copper(III), and zinc(I1) complexes O f D( -)PDTA
Effect of pH. pH was found to affect the optical rotations of D(-)PDTA and its metal complexes. The optical rotation of D(-)PDTA and its complexes are sensitive to small changes in structure. The protonation or deprotonation of the acetato groups changes the charge on D( -)PDTA and results in a change in the optical rotation. A spectropolarimetric titration of D( -)PDTA with standard NaOH solution was performed to determine the effect of pH on the molecular rotations of D( -)PDTA (Figure 1). The abrupt changes in molecular rotations occur at the pK values for racemic PDTA. The rotation of D( -)PDTA in the range of 8-11 is very sensitive to small changes in pH. Because of this sharp slope in the molecular rotation pH curve near pH = 10, solutions to be titrated with D( -)PDTA at this pH must be heavily buffered and 25 ml of the ammonia-ammonium chloride buffer are recommended. The use of acetic acidsodium acetate buffer is highly recommended for spectropolarimetric titrations with D( -)PDTA, because the rotational change of D(-)PDTA is negligible over the pH range of 3-6. The maximum buffering capacity of this buffer is within this range and the acetate ion is a poor complexing agent. It was found that 20-40 ml of the acetic acid-sodium
vo2+
1
2.0
Figure 1. Effect of pH on the molecular rotation of D( -)PDTA
Metal complexes sc3+ Ti 4+
-1200
-1 600
I
Cr a+
-800
_J
acetate buffer was satisfactory for titrations performed at pH = 5, and 1-2 ml of concentrated nitric acid was required for titrations below pH = 2. The effect of pH on the molecular rotations of the nickel(II), copper(II), and zinc(I1) complexes of D(-)PDTA is less pronounced than the effect of pH on D(-)PDTA itself. Figure 2 shows the effect of pH on the molecular rotations of these complexes. There is an initial change in rotation due to the dissociation of the metal complexes below the minimum pH value for complete formation. After this minimum quantitative pH value, the curves are nearly linear over the entire quantitative pH range for each of these metal complexes of D(-)PDTA. This indicates that these molecular rotations are not very dependent on pH in the range of 4-10. This is advantageous because the other metals were titrated in this range or at a pH where the molecular rotations were linear with concentration. Dilution Effects. The observed optical rotation readings were corrected for dilution by multiplying by the factor (Vi u)/Vi,where Viis the initial volume of the solution and u is the volume of the titrant added. In cases where the observed rotations were very small, the volume corrections were
+
Table 11. Molecular Rotations of D( -)PDTA and Transition Metal Complexes of D( -)PDTA Molecular rotation (deg ml dm-I mole-') 589 Nm 578 Nm 546 Nm 436 Nm 365 Nm PH D -465 - 803 - 1284 5.0 -408 +1565 1.40 - 269 -114 -253 -259 +500 -60 4.88 +lo50 $1060 +970 +lolo - 1900 4.90 -720 -5170 +IO0 +238 +430 +730 4.80 +I99 +207 b 1.32 - 127 -130 - 130 -40 -798 -822 -950 -542 - 1618 4.37 b - 372 - 396 -773 -1371 4.30 - 1273 -1213 - 1013 - 766 -772 9.67 b $629 +413 +153 +283 4.23 - 148 - 154 - 174 -291 -449 9.60
Zn2+ Ligand D( -)PDTA -159 - 165 - 186 D( -)PDTA -117 - 122 - 138 - 191 -215 D( -)PDTA - 183 a Not recorded. * Solution absorbs too strongly at this wavelength for accurate readings.
- 305 - 230 - 355
-455
- 354 - 534
2.01 5.07 10.04
Molarity 0.01 0.01 0.01 0.001 0.01 0.01 0.01 0.01 0.01
0.01 0.01 0.01
0.01 0.01
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
49
\
Z
0 -
5
-0.500
E
-1.000
k-
> LT
w
m
m
0 -1. 500 1.000 2.000 3.000 V O L U M E OF D ( - ) P D T A ml
1 .OOO VOLUME
2.000 3.000 OF D ( - ) P D T A ml
Figure 3. Spectropolarimetric titration of 0.1042M manganese(I1) at pH = 5 and 365 nm
Figure 4. Spectropolarimetric titration of 0.1097M scandium(II1) at pH = 5 and 365 nrn
negligible, therefore it was possible to ignore the dilution effect without loss of accuracy. In this study a relatively concentrated solution of the titrant was selected in order to minimize the dilution effect. The molar concentration ratio of NaD(-)PDTA titrant to metal ion solution was approximately 50:l. Effect of Diverse Ions. The relationship between the observed optical rotation of an optically active complex in solution and the concentration and nature of optically inactive ions present is not well known (12). It was found in this study that ammonium, perchlorate, sulfate, chloride, and acetate ion had no effect on the observed end point. The presence of ammonia does affect the observed optical rotation of some of the metal complexes. The molecular rotations of zinc(II), copper(II), and nickel(I1) complexes of D(-)PDTA are altered in the presence of ammonia. Ammonia, a fairly strong complexing ligand for certain metal ions, replaces one or more of the bonded water or acetato groups in the complex. Therefore, the resultant complex, because of the difference in dissymmetry, will have a different optical rotation. This does not preclude the use of ammonia as a buffer in these titrations however. The presence of ammonia was found to have a marked effect on the molecular rotation of the copper(I1) complex of D(-)PDTA. The
molecular rotation of the complex at pH = 10 and 365 nm changes from $296" to -772" upon addition of excess ammonia. It is surmised that the complex changes from an octahedral structure with quinquedentate D( -)PDTA and one molecule of water to a structure in which D(-)PDTA is quinquedentate or quadridentate with one or two ammonia groups replacing the water, one of the acetato groups, or both. Conclusions. Table I11 gives the results of the metal ion titrations. Each reported value is the average of at least three individual titrations. The results obtained for the analyses of all the metals titrated gave an average deviation of 0.19%. The maximum time required for the titration and graphical evaluation of the data was in no case greater than 15 minutes. Typical titration curves showing the three general types of end points-positive, negative, and back titration-are presented in Figures 3, 4, and 5 , respectively. In Figures 3, 4, and 5 the initial branch of the curve corresponds to the formation of the metal complex of D(-)PDTA. In Figures 3 and 4, the second branch of the curve corresponds to the excess NazD( -)PDTA titrant. In Figure 5 , the second branch of the curve corresponds to the excess zinc ion titrant. In both cases, the equivalence point is taken
-0.380
1
- 0,400
1
(12) H. L. Smith and B. E. Douglas, J. Amer. Chem. SOC.,86, 3885 (1964).
Table 111. Results of Spectropolarimetric Titrations No. of mg Deviation Metal Taken Found mg 7z -0.11 -0.22 49.20 f 0.06 49.31 sc a+ -0.19 -0.12 62.29 f 0.02 62.41 Ti4+ V02+
Cr 3+
Mn2+ Fe 3+ COS+
Ni 2+ cuz+ Zn2+
50
39.89 8.371 57.23 51.33 64.14 6.514 56.88 56.88 5.688 45.71 45.71 4.571 63.29
39.92 f 0.06 8.404 f 0.025 57.10 f 0.11 51.30 f 0.06 65.05 f 0.08 6.510 f.0.006 56.92 f 0.03 56.90 f 0.03 5.713 f 0.006 45.61 =k 0.06 45.58 f 0.03 4.552 f 0.006 63.34 f 0.09
$0.03 +0.033 -0.13 -0.03 -0.09 -0.004
+O. 39 -0.23 -0.06 -0.14 -0.06
+O.M
+O .07
+0.02 +0.025 -0.10 -0.13 -0.019 +O. 05
$0.03 +O. 44 -0.22 -0.28 -0.41 +0.08
+0.07
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
0
-0.440
w
>
-0.480
t 1 0.500 VOLUME
1.000 1.500 Z I N C ml
OF
Figure 5. Back spectropolarhetric titration of 0.1610M chrornium(II1) at pH = 5 and 365 nm
as the intersection of the two branches. Nickel(II), copper(II), and zinc(I1) can be titrated at either pH = 5 or pH = 10. A direct titration of chromium(II1) with D(-)PDTA was attempted using a thermostated flow-through cell. The titration was performed at 60 "C, but the results were unsatisfactory due to the very slow kinetics of the reactions. Advantages. The main advantages of this technique are its versatility, simplicity, and the elimination of the subjectivity associated with visual end points. Only three buffers are required for spectropolarimetric titrimetry with D(-)PDTA. Because a stereospecific titrant, D(-)PDTA, is used, the maximum rotations of the metal D(-)PDTA complexes are obtained, allowing high sensitivity. Thus the titrant and the complexes formed serve as selfindicators permitting the maximum quantitative pH range of the metal complexes to be utilized. Because the determination of the end point is obtained through the relative measurements of optical activity, the spectropolarimetric titrations
are usually capable of achieving greater precision than visual methods. I n the construction of the titration plots, the best straight line is drawn through a number of experimental points in order to minimize the spectropolarimetric error associated with each point. The extrapolated end point is not adversely influenced by high electrolyte concentrations; therefore, this technique is suitable for use with many separation schemes. Present investigations in this laboratory are being carried out to determine the feasibility of using a polarimeter with a wider wavelength range to determine metals in the microgram range. RECEIVED for review July 30, 1969. Accepted November 3, 1969. Presented in part at the 157th National Meeting of the American Chemical Society, Division of Analytical Chemistry, Minneapolis, Minnesota, April 1969. This work was supported by The Robert A. Welch Foundation Fellowship Grant A-262.
Kinetics of Ligand Exchange Reaction of Ethylenediaminetetraacetate Ion with Ethylenediaminetetraacetatonickel(11) James D. Carr Department of Chemistry, Unioersity of Nebraska, Lincoln, Neb. 68508
C . N. Reilley Department of Chemistry, University of North Carolina, Chapel Hill, N . C . 27514
The rate for the ligand exchange reaction of ethylenediaminetetraacetate ion (EDTA) with the nickel(l1) complex of EDTA is measured by NMR techniques with deuterated EDTA. The pH dependency of the reaction rate is analyzed and the rate constant for the EDTA tetraanion attack on nickel-EDTA determined to be 1.95 X 10-3 M-lsec-l at 33 OC. The activation energy of this reaction is 14.0 kcal/mole and the log of the frequency factor is 7.1 (M-lsec-1). A rate determining step is assigned which is also consistent with the analogous EDTA ligand exchange reactions with Ca2+, Cd2+,and SrZf and with the metal exchange reaction *Ni2+ Ni-EDTA + *Ni-EDTA Nie.
+
+
SEVERAL PAPERS have appeared recently discussing the symmetric ligand exchange reactions of EDTA (ethylenediaminetetraacetate ion or Y 4-) with a metal complex of EDTA (Equation l with charges omitted for clarity) where M has been calcium ( I ) , cadmium (2),lead (3),and strontium (4), all labile, diamagnetic ions. Each of these reactions was studied by nuclear magnetic resonance (NMR) line-broadening techniques. ky""
Y*
+ MY 1_ MY* + Y
(1)
(1) R. J. Kula and G. H. Reed, ANAL.CHEM.,38, 697 (1966). (2) J. L. Sudmeier and C. N. Reilley, Znorg. Chem., 5 , 1047 (1966). (3) J. D. Carr, Kenneth Torrance, C. J. Kruz, and C. N. Reilley, ANAL.CHEM.,39, 1358 (1967). (4) R. J. Kula and D. L. Rabenstein, J . Amer. Chem. Soc., 89, 552 (1967).
In order to measure the rate of Equation 1 for nickel(II), a paramagnetic, sluggish, transition metal ion, it became necessary to devise a different measurement technique. Cook and Long have previously measured the rate of the symmetric electrophylic substitution, or metal exchange of nickel-EDTA (Equation 2) by use of radioactive 63Ni(5). They found several reaction pathways depending on hydrogen ion concentration. Ni*
+ NiY
__
kNih'iY
Ni*Y
+ Ni
(2)
The rate constant for exchange without hydrogen ion involvement was later shown to fit into a series of rate constants of various metal ions substituting nickel in nickel-EDTA (6). This latter study showed that the rate determining step in the reaction is the breaking of the nickel to nitrogen bond when the leaving nickel is bonded to EDTA through a glycine segment and the entering nickel bonded through a iminodiacetate segment.
EXPERIMENTAL Method of Following Kinetics. EDTA was prepared 98% deuterated in the acetate positions as described by Terrell and Reilley (7). They showed that the exchange of acetate methylene hydrogens with solvent water is quite slow even at near boiling temperatures both for uncomplexed ligand ( 5 ) C. M. Cook and F. A. Long, J . Amer. Chem. SOC.,80,33 (1958). (6) D. W. Margerum, Rec. Chem. Progr., 24, 237 (1963).
(7) J. B. Terrell and C. N. Reilley, ANAL.CHEM., 38,1876 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
51
