92
C. R. MAXWELL, D. C. PETERSON AND P. M. WATLINGTON
Vol. 62
THE FORMATION OF A FERRIC-GLYCINE COMPLEX BY CHARLES R. MAXWELL, DOROTHY C. PETERSON AKD PERCY M. WATLINGTON Department of Health, Education and Welfare, Public Health Service, National Institutes of Health, National Cancer Institute, Radiation Branch, Bethesda, Maryland Received August 14,1967
The ultraviolet absorption spectra of solutions of glycine and Fe(C104)rwere found to exhibit a maximum at 285 mp which could be explained only b the formation of a ferric-glycine complex. Measurements of the absorption a t this wave length as a function of glycine, %e+++and H + concentrations, showed that the complex was formed by the reaction F e + + + -1NHZCH~COO- (FeNH&HzCOO)++. The reaction was found to have an equilibrium constant of 20 5 2 I./mole and the complex a molecular extinction coefficient of 5750 cm. -1.
*
Introduction In order to explain certain aspects of the kinetics of the reaction of Fe++, Fe+++, 0 2 , HzOt and NH2CHzCOOH in aqueous solution, we recently postulated' the formation of a complex between the ferric ion and glycine according to eq. 1. The equilibrium constant for the reaction was evaluated as 10 l./mole
trode and a calomel reference cell. To prevent the clodion from interfering with the proper functioning of the calomel cell, this cell was immersed in a KC1 solution which was connected to the test solution with a KC1-agar salt bridge. The system was calibrated with a standard buffer solution at p H 4.0. All solutions were within 0.02 pH unit of the reported value. All measurements were made at room temperature, 22 f2'.
Reagents. HCIO(.-Fisher reagent grade. Glycine.-Nutritional Biochemical Corp.; recrystallized from water and CHsOH and again from water. Fe( C104)3.-Fe( OH)* was preci itated from reagent grade FeNH4(S04)2.12Hz0with " 4 0 3 filtered, washed and dissolved in 2 M HC104. A portion oothis solution was diluted and storedwasapprox. 0.1 M Fe(C104)*, 0.5 M HC104. A second portion was partially neutralized with NaOH and stored as approx. 0.1 M,Fe(ClO&, 0.1 M HC101, 0.4 M NaC104. P4 hProcedure.-The"stockTsolutions of Fe(C104)cwere assayed spectrophotometrically at 304 mp in 0.8 N HzS04. Aliquots were diluted with water to intermediate concentrations of 1.35, 2.7 and 3.5 X 10-8 M Fe(C104)8. The final solutions were prepared by the addition of 25 ml. of glycine solution or water (each containing HClOl to 2 ml. of one of the intermediate concentration of Fe(Cld)3. The optical densities were measured immediately in a 1 cm. cell with a Beckman DU spectrophotometer. All optical densities reported, with the except,ion of those in the absence of glycine a t pH 2.95, were constant for at least 30 minutes following the final dilution, thus indicating the rapid formation of a stable equilibrium. In the absence of glycine at pH's greater than 3.0 the absorption spectra of the Fe( C104)ssolutions changed with time after dilution, indicating the onset of ferric hydrolysis reactions other than reaction 2 below. The value reported at pH 2.95 was measured less than 2 minutes after the final dilution and any change is believed to be negligibly small. The pH of each solution was measured with a Beckman pH meter using a glass elec-
FUNCTION O F THE GLYCINE CONCENTRATION AND O F THE pH
__
Results and Discussion The ultraviolet absorption spectra of mixtures of F e + + + NHzCHZCOOH Complex (1) Fe(C104)s,HC104and NH:CH2COOH are shown in Since a separate proof of the existence of such a Fig. 1. The absorption in the Fe(CIOd)asolutions complex would support the validity of the reaction near 240 mp is due to the Fe+++ ion and the peak mechanism proposed for the above system the in- near 295 mp is due to the FeOH++ ion which is vestigation reported here was undertaken. Be- formed according to equation 2.2 The increased cause most of the common anions form complexes absorption on the addition of glycine to the ferric with Fe+++ which absorb in the near ultraviolet, F e + + + + HzO FeOH++ + H+ (2) the ultraviolet absorption spectra of mixtures of solutions is considered positive evidence for the Fe+++ and NH&H&OOH were examined for eviformation of a ferric-glycine complex of some nadence of a ferric-glycine complex. The perchlorate salt was used because it does not form a complex ture. The characteristics of the complex have been with the ferric ion. An absorption peak attributable only to a ferric-glycine - complex was ob- investigated by measuring the absorption a t 285 served a t -285 mp. The properties of this com- mp of various combinations of Fe(C104)sand NHY plex were then investigated by measuring the CH2COOH concentrations a t four pH levels. The absorption of solutions as functions of the Fe+++, data are shown in Table I. NH2CHzCOOHand H+ concentrations. TABLE I Experimental THEOPTICALDENSITYOF A Fe( C104)3SOLUTION AS A
+
(1) C. R. Maxwcll and D. C. Peterson, J . Am. Cham. 1067).
800..
79, 5110
M 1.00 0.60 .35 .20
.IO .05 .02
,OO a
-
Optical density at 285 mp-I cm. cell Fet = 1 X 10-4 M Fet = 2 X 10-4 M PH 2.55 2.9z 2.00 2 . 0 0 2.55 2.954 1.51 1.51 0.860 0.960 1 . 0 0 1.09 0.435 0.477 0.600 0.516 .475 .430 .467 ,375 .868 0.923 0.955 .740 .620 .753 ,820 .850 .310 .375 .410 .430 .480 .630 .700 .726 .235 .316 .355 .371 .325 .475 .559 .682 .160 .235 .281 .299 ,488 ,I1 1 .169 ,225 .253 .225 .342 .453 .361 * 080 .I78 .I53 .126 .206 .293 .380 ,062 .lo4 .143 .187
..
Stock solution 0.1 M Fe+++, 0.1 M HClO4,0.4 NaC104.
These data have been analyzed in terms of the principle of additive absorbancies by assuming reaction 1 and the applicability of Beer's law. AIthough the absorption at 285 mp is due to three species-Fe+++, FeOH ++ and the ferric-glycine complex-the data for any given pH can be treated in terms of only two variables since the ratio of Fe+++ to FeOH++ is k e d by the ferric hydrolysis reaction 2. With the symbols defined below, eq. I follows from the principle of additive absorbancies and eq. I1 is the expression for the equilibrium constant for 1. Equation I11 followsfrom eq. I and 11. (2) E. Rabinowitoh and W. H. Stockmayer, $%id., 64, 335 (1942).
,
FORMATION OF A FERRIC-GLYCINE COMPLEX
Jan., 19.58 Fet C
X G
ODb OD, Eb
Eo
Eo
= total ferric concn. = concn. of ferric-glycine complex = C/Fet = fraction of Fet complexed by glycine = concn. of glycine > >C = optical density with glycine absent = optical density with glycine present = ODb/Fet = apparent molecular extinction coefficient for ferric iron = OD,/'Fet = apparent molecular extinction coefficient for soliis. containing glycine = molecular extinction coefficient for the ferricglycine complex
Eo = XEo
(1
- x)Eb
(1)
93
0
0 w J
" + %
F
/.OM
2.5X/O-'
10M
LS
0 0
Q
06
0 4
Eo
- Eb
= K I E ~ K1Eo
(111)
Thus, if equation 1 describes the formation of the ferric-glycine complex, a plot of (E. - &)/G against E, will be a straight line with a slope equal to -K1 and an intercept on the EOaxis equal to E,. Figure 2 shows the data for Fet = 1 X M in Table I plotted in this manner. Similar plots for M are almost identical. Fet = 2 X The straight lines in Fig. 2 support the thesis that the ferric-glycine complex is formed by equation l and give a value of 5750 for the molecular extinction coefficient of the complex. However, the fact that K1 varies with the pH shows that equation 1 is not a complete description of the formation of the complex. The correct equation can be shown to be 4, with an equilibrium constant K4, where the reacting species are the ferric cation and the glycine anion. The concentration of these two ions are controlled in this pH region by equations 2 and 3 and the associated equilibrium constants Kz and K3. Equation I V follows and K4 can be evaluated from the measured values of K1in Fig. 2, if values of K Zaiid K3 are known. The dissociation constant NHzCHzCOOH C-,NHzCHzCOOFe+++
+ NH2CH2COO-
+ H+
(3) Fe(NH2CH2COO)++ (4)
for glycine, K3, is known quite accurately and is given by Edsall and Blanchard3as 4.9 X mole/ 1. Only the approximate value of the hydrolysis constant for the ferric ion, Kz, is known. Lamb and Jacques4 and Bray and Hershey6 give values of 3.5 X and 6 X 10-3, respectively, for this constant. Therefore, the thesis upon which eq. I V is based has been tested by seeing what, if any, value for KP will give a value for K4 which is independent of pH. The results of calculations with several assumed values of Kz are shown in Table 11. Since the first four values of Kd in column 6 based on the data in this paper are constant within the limits of error in evaluating K1 and since the value of 4 X for Ka used there is comparable with the reported values of Ka we have concluded that equation 4 describes the formation of the ferric-glycine complex and that the equilibrium constant for the reaction is 20 2 l./mole a t 25'. A value of K:! =
*
(3) J. T. Edsall and M. H. Blanchard, THISJOURNAL, 66,2337(1933). (4) A. B. Lamb and A. G.Jacques, ibid., 60, 967, 1215 (1938). (5) W. C. Bray and A. V. Hershey, ibid., 66, 1889 (1934).
02
0 240
280
260
300
340
320
W A V E LENGTH
360
300
mp.
Fig. 1.
E,
x
10-3.
Fig. 2.
0.3 X which brings the value of K4 for pH 3.5 in closer agreement with the other values is incompatible with the reported values for K2 and with the degree of hydrolysis indicated by the absorption spectra of Fe(C104)3shown in Fig. 1. Furthermore the spread in the values calculated for K4 at the lower pH levels is much greater than the experimental error in the values of K I reported here. TABLE I1 VALUESOF K4 CALCULATED FROM THE OBSERVED VALUES OF K1BY EQUATION IV USING K s = 4.9 X MOLE/L. AND THE INDICATED VALUESOF K2 KI
l./rnble
Kn X 100 (mole/l.)
pH
0.3
1.0
2.61 1.51 19.7 20.1 3.95 2.00 12.4 13.2 4 . 7 0 2.55 8 . 2 10.0 3.87 2.95 6 . 0 9 . 0 10.2" 3 . 5 21.0 4 4 . 0 a Data from reference 1.
2.0
4.0
6.0
8.0
20.7 22.0 23.2 2 4 . 5 14.3 16.8 1 9 . 2 2 1 . 6 12.7 18.0 23.3 2 8 . 0 13.3 22.0 3 0 . 7 39.2 79.0 145.0 220.0 280.0
The difference in column G between the average value of 20 I./mole for K4 over the pH range 1.5 to 3.0 and the value of 145 a t pH 3.5 is believed to be due to an error in the value reported for K1 (based on a much more devious method) and not to a fail-
94
ALoUIN F.GREMILLION AND ERNABELLE BOULET
ure of the mechanism represented by equations 2, 3 and 4. Unfortunately a value of K I could not be obtained at a p H of 3.5 by the method used here because of the onset of additional ferric hydrolysis reactions a t these ferric concentrations in the absence of glycine.
Conclusion The existence of the ferric-glycine complex postulated as a part of a reaction mechanism proposed
Vol. 62
earlier' for the Fe+++, Fe++, Hz02, 0 2 , NHzCHZCOOH system has been confirmed and the equilibrium constant for the reaction describing its formation evaluated. Although this is considered supporting evidence for the general validity of the proposed mechanism, the difference between the values for the equilibrium constant determined here and the one reported earlier suggests that the original mechanism may be incomplete in some detail.
AN X-RAY STUDY OF SOME VINYLIDENE CHLORIDE-ACRYLONITRILE COPOLYMERS BY ALCUINF. GREMILLION AND ERNABELLE BOULET Bipphysics Laboratory, Tulane University, N e w Orleans 18, La. Received August $6, 1967
An X-ray investigation of some vinylidene chloride-acrylonitrile copolymers containing from zero to 40% acrylonitrile has been conducted. Membranes cast from solutions of the resins have also been studied and evidence is presented to show that these polymers are ordered to a small extent. Certain halos in the patterns are associated with interchain distances.
Alfrey, Bohrer and Mark1 have discussed the arrangements and sequences of comonomer units in copolymer chains and have pointed out some of the properties attendant with each of several arrangements. However, without detailed knowledge of the polymer chains formed and of the reactions giving rise t o a copolymer, the possibilities of sequences of monomer units of various lengths and various distributions of the sequences are recognized. In addition one recognizes for each of the head-to-head and head-to-tail configurations three possible kinds in a binary copolymer. Thus, although certain modes of combination of comonomer units are favored, within a given copolymer it might be found that various repeat periods exist. I n so far as halos in the X-ray diffraction pattern of an unoriented copolymer are due to repeat distances, a multi-halo pattern may arise. Krimm and Tobolsky2have suggested that some outer halos in polystyrene X-ray patterns arise from distances along chains. Furthermore, Simard and Warren3 have stated that interchain distances are responsible for halos in some cases. Thus it is not unlikely that X-ray diffraction patterns of some unoriented, non-crystalline copolymers should show a number of facets. This is the case with the copolymers at hand. Experimental A number of vinylidene chloride-acrylonitrile copolymers have been studied using both high-angle flat-casette and small-angle X-ray scattering techniques. Experiments have been performed on several resins in both powder form as available from the Dow Chemical Company, Midland, Michigan, and in membrane form as cast from various solutions. Table I lists the mean compositions of these resins and the number by which each is hereafter designated. The distributions of compositions about the mean compositions (1) T. Alfrey, Jr., J. J. Bohrer and H. Mark, "Copolymerieation,'I Interscience Publishers, Ino., New York, N . Y.,1952, Chapter VII. (2) S. Krimm and A. V. Tobolsky, Tezlile Research J . , 21, 805 (1951). (3) G. L. Sirnaid and B. E. Wawen, d . A m . Chem. Soc., 88, 507 (1936).
are unknown, except that the distribution for resin R6 is sharper than that of resin R5. In addition Table I lists the designation of each membrane cast from solution as well as solvents used and bake-out treatment for each mem-
TABLE I COMPOSITION-DESIGNATION CHARTFOR RESINS AND MEMBRANES Designation of Mean powdered oomCompn. merpial as % ' resins VCN
Designation of membranes
Bake-out treatment
Solvent"
3 min. a t 195'F. a 3 min. at 220'F. b 3 min. a t 195'F. 5 R2 M2D 3 min. a t 220°F. a 3 min. at 195°F. 10 R3 M3T 3 min. a t 220°F. 3 min. at 195°F. b M3D 3 min. a t 220'F. 4 min. at 195°F. C 15 R4 b44K 2 min. a t 220'F. a R14T 3 min. a t 195'F. 3 min. a t 220'F. C 4 min. at 195°F. M5K-1 5 min. at 220'F. 4 min. a t 195'F. C 20 R5 M5K-2 5 min. at 220°F. 4 min. a t 250'F. c 4 min. a t 195°F. M5K-3 3 min. a t 220'F. 10 min. a t 250°F. b 3 min. at 195'F. 20 R6 M5D 3 min. at 120°F. C 3 min. a t 195°F. 40 R7 M7IZ 3 min. a t 220°F. 3 min. a t 195'F. a M7T 3 min. a t 220°F. a Solvent a = tetrahydrofuran; b = dioxane; c = methyl ethyl ketone. 0
R1
M2T
