M,G, QVRYARAMAN AXD HAROLDF WALTON
7%
TABLE I METALIONCOSTENTOF RESIX, Metal ion
Equivalent f rac tion of metal in resin
Aminea
Ag Ni
etol 0.75-0.65 etol .54- .G4 en (1) .03- .24 en (2) .40- .74 cu etol .48- .53 en (1) .4G- ,SO en (2) .51- .77 Zn en .38- .69 etol = 2-aminoethanol; en = ethylenediamine. The equivalent fraction for the lowest amine concentration is placed first.
resin as its concentration is increased. The effect is shown in summary form in Table I, which also shows, for nickel, the effect of resin loading on the complex stability.
Vol. 66
Discussion The main conclusion to be drawn from these data is that the resin environment stabilizes metalethylenediamine complexes. In the light of additional evidence to be presented in a subsequent paper, it is thought probable that the diamine acts as a “cross-linking ligand,” binding metal ions together in a chain or network, and that this cross-linking occurs much more readily in a cationexchange resin, where the metal ions are close together, than it does in dilute solutions. Acknowledgments.-Most of this work was described in the Ph.D. thesis of Leone Cockerel1 (University of Colorado, 1954), and some of it was presented a t the 138th National Meeting of the American Chemical Society, New York, 1960. Part was supported by the U. S. Atomic Energy Commission, Contract AT-(11-1)-499.
METAL-AMINE COIHPLEXES IN ION EXCHANGE. 111. DIAMINE COMPLEXES OF SILVER(1) AND NICKEL(I1) BY M. G. SURYARAMAN AND HAROLD F. WALTON Department of Chemistry, University of Colorado, Boulder, Colorado Received July 6 , 1051
The stabilities of complexes of silver ions with ethylenediamine and 1,3-propanediamine have been measured in sulfonated polystyrene cation-exchange resins having different degrees of crosslinking and functional-group content. Preliminary measurements also have been made with nickel(I1) and hydrazine. In every case the complexes were more stable in the resin than in solution, the stabilization being greatest with ethylenediamine and silver ions, but the maximum degree of coordination was lese in the resin. The stability was independent of crosslinking and silver ion concentration. Internal polymerization of the metal-amine complexes is suggested as a partial explanation.
Introduction In measuring the stability of metal-amine complexes in cation-exchange resins we found2that the silver-ethylenediamine complex is much more stable within a sulfonated polystyrene resin than in aqueous solution. There were indications that a silver ion added only one molecule of ethylenediamine in the resin as compared to two molecules in aqueous solution. The tentative explanation was offered that the species in the resin was a doubly charged complex ion, +AgH21\TC2RJ&+. That such an ion exists in solution mas shown by Schwarzenbach.a He d ~ found o strong evidence for a dimer, Ag2(cn)2++(en = ethylenediamine), and suggested to us that this dimer might be formed in the resin. That the formation of Ag(enH)++ could not alone account for the complexing in the resin was easily shown by loading the resin with silver ions to more than half its exchange capacity. The Ag+: en coordination was still in the ratio 1:1. This paper describes a detailed study of the coordination of ethylenediamine with silver ions in cation-exchange resins. The variables studied included crosslinking of the resin and the degree (1) Part 11, J . Phys. Chem.. 66, 760 (1962). (2) R. H. Stokes and H. F. Walton, J . A m . Chem. Soc., 76, 3327 (1354). (3) G. Schwarzenbach, H. Sckermann, B. Ahissen and G. Bndoregg. Helu. Chzm. A d a . 86, 2237 (1352).
of sulfonation of the resin. Tests also were made with 1,3-propanediamine (pn) as the ligand. As it seemed very likely that these diamines were acting as crosslinking ligands to bind two or more silver ions together, the coordination of hydrazine also was studied in a preliminary way. Since hydrazine reduces silver ions, these tests were made with nickel ions, Experimental Materials.-The resins were all of the sulfonated polystyrene type, Dowex 5O-W, kindly supplied by the Dow Chemical Go., Midland, Michigan. They were of “white” grade, pale yellow in color, and very uniform in crosslinking and sulfonation, as shown by the flotation m e t h ~ d . ~ The particle size was 100-200 mesh. Three degrees of crosslinking were used: 2 , 4 and 8% nominal djvinylbenzene. The capacities on a dry basis were, respectively, 5.245, 5.176 and 5.153 meq./g. (H-form). In addition, two “specialcapacity” resins were used; these were made from crosslinked polystyrene by partial sulfonation and were kindly provided by Robert M. Wheaton. These had 2% crosslinking and exchange capacities of 1.629 and 2.322 meq./g. dry H-reoin; they were designated A and B, respectively. All resins were washed and air-dried before use. Ethylenediamine and 1,3propanediamine were Eastman White Label gradcs rrdistilled from barium hydroxide. The ethylenediamine boiled a t 114’ (628 mm.) (corr.), 1,3-propanediamine a t 131” (623 mm.). Hydrazine was obtained as 85% hydrazine hydrate from Arapahoe Chemicals, Inc. and diluted €or use without further purification. Nickel fluoborate, used in the experiments with hydrazine, was made from Baker and Adamson (4) hl. G. Surj;aramian and H. F. Walton, S c i e ~ c e ,131, 823 (1860).
Jan., 1962
DIAMINE COMPLEXES OF SILVER AND NICKEL
fluoboric acid (48% solution) and basic nickel carbonate; its solutions had pH 5.5. Equilibration .-Weighed portions of air-dried resin, ITform, of known moisture content were placed in flasks with measured amounts of nitric acid, silver nitrate and amine (in the hydritzine experiments, fluoboric acid, nickel fluoburate and amine) Water was added to give a convenient volume. Usually 4 meq. of resin was used with 50 ml. of solution of total normality 0.1. The flasks then were shaken or stirred a t 25 f 1’ (in one series of tests, 50 i 1’) for a t least 8 hr., preferably 24 hr. The solution then was withdrawn and its pH immediately deteirmined using a Beckman Model G pH meter and a reference electrode with saturated potassiurri nitrate solution as the salt bridge. The total base was determined by potentiometric titration to the inflection at pH 4.5-5. Silver was determined by potentiometric titration with chloride after acidification; nickel by titration with EDTA ueing murexide indicator. To interpret the data with ethylenediamine and 1,3-propanediamine one must know the distribution of singly and doubly charged cations between the solution and the resin. This was measured for each resin by shaking weighed amounts of air-dried hydrogen-form resin with nitric acid solution and enough diamine to give a mixture of the two ions, enH+ and enHn++. Four to six different proportions of amine were used in every case. After equilibration the enH+ in the solution was found by titration with standard acid, and the enHz++by difference, with checks made by titration with standard base to the poor inflection a t pH 8.5. The total amine in the resin was found by difference, and its ionic composition calculated from this value and the known ionic capacity of the resin. Water Uptake.--The amount of water taken up by the resins on swelling was determined for the different cationic forms by a modification of the “blotting” method of Bonner, et aL6 About 0.5-1 g. of moist resin was placed in a small weighing bottle and blotted dry with strips of close-textured filter paper. As each strip was withdrawn, any adhering resin was ,jcraped back into the bottle with a metal microspatula. Blotting was continued uatil a strip became only barely wet a t the edges or corners after thorough “working” in the resin. The bottle then was utoppered and weighed. Then about 0.1 ml. of water was added and the blotting and weighing repeated. The weighed, blotted resin then was analyzed for metal ion and base contents and total exchange capacity. This rather crude technique gave results reproducible within 0.01 g. HzO/meq. resin, and seems to us the best way of determining water uptake, short of elaborate isopiestic vapor pressure measurements. For tha silver-ethylenediamine and nickel-hydrazine complexes the resins were wetted with 0.1 M amine solution instead of water.
Results and Discussion The ionization constants used in the calculations all were determined by titration under as nearly as possible ithe same ionic strength as that used in the test solutions, although, as explained in the previous paper,I it is impossible to keep the ionic strength constant in all the experiments. The pK‘ values used were (see also ref. 1) enHn++, pEL’ = 7.22, pKz‘ = 10.03; pnH2++, pKl‘ = 8.88, pKz‘ = 10.64; NzH5+, K,‘ = 8.12
Ion Exchange of Diamine Cations.-Values the equilibrium quotient for the reaction enHz++ (solution)
-+ 2enHfR-
2enH+ (solution)
of K ,
+ enH2++Rl-
are given in Table I. The units are equivalents of
resin/liter of solution. The probable error is h0.05 to 0.1 in log K , which is not very good, but more than adequate for the purpose, as will be seen from Table 11. Silver and Ethylenediamine.-Table I1 gives some typical data obtained in the equilibrations as ( 5 ) 0. D. Bonner, W. J. Argersinger and A. W. Davideon, J . Am. Chem. Sac 74. 1044 (1952).
79 meq _ figf _
erors
meq ream
linkins
.
I
O
z
1
4
c
4
I 0.25 I
h e
e Q
0.28 I
o
3
L
1
4
I
\
I
h
‘\+
5
6
I
1
7
8
+‘
I 9
10
~(onl.
Fig. 1.-Silver-ethylenediamine complexes in Dowex-50 resins. The ratio Ag:resin indicated is the ratio of total silver to resin. The amount of silver in the resin varied greatly; typical data are shown in Table I for 1 meq. Ag: 1 meq. resin.
TABLE I DISTRIBUTION QUOTIINTSFOR enH +-enHz + + pnH2++EXCHANGES‘ AT 25” Amine
en
Resin crosslinking
AND
pnH +-
log K at
N = 0.7
N = 0.9
1.0 1.2 .. 1.6 2%-sp. cap. A 1.2 0.95 2%--sp. cap. B 0.9 0.8 Pn 2% .. 1.65 a en = ethylenediamine; pn = 1,3-propanediamine; AT = equivalent fraction of doubly charged ion in the resin. 2%
4% 8%
1.1 1.4
an indication of the relative magnitudes of the concentrations involved. This table also shows the very marked shift of metal ions from the solution into the resin which occurs as more amine is added, and which qualitatively is to be expected from the greater stability of the complex in the resin. Figure 1 shows the data of Table I1 along with all the other data obtained with the commercial Dowex resins. The values of “rneq. Ag+/meq. resin” are the quantities originally added; only a part of the added silver entered the resin, but this portion increased with the amount of added amine. The “solution” curve was drawn from Schwarzenbach’s valuesa for the formation constants of Ag(en)+ and Ag(en)z+. This was done for two reasons; first, the amount of bound amine in solution in our experiments usually was too low to evaluate accurately, and the concentration of dissolved silver ions often was very low too; second, silver and ethylenediamine form several different complex species in s ~ l u t i o nand , ~ the dashed curve of Fig. 1 focuses attention on only two of these. Analysis of our solutions gave values of pi, the ratio of bound amine to total silver, lying roughly along the lower part of this dashed curve. From Fig. 1 it is clear that the complex is over a thousand times as stable in the resin as it is in the solution for low values of pi, and that the association in the resin reaches a limiting value of .IZ = 1. (There is some evidence that fi in the resin rises above 1 for ethylenediamine concentrations of 0.1 M and above.) The formation curve is independent of the crosslinking and of the equivalent fraction of silver in the resin, except that with 8%
M. G.SURYARAMAN AND HAROLD F, WALTON
80
Vol. 66
TABLE I1
BINDING OF ETHYLENEDIAMINE BY SILVER IONSIN DOWEX 50W x 4" Solution eomposition titEtS1e base
PH
enHe++
enH*
Ag+
p(en)
enH+
5.09 0.092 0.007 1.001 2.931 8.77 5.63 .357 .030 1.148 2.613 7.63 5.86 .557 .054 1.246 2.392 7.13 6.06 .874 .093 1,347 2.152 6.70 6.16 1.048 ,131 1.499 1.987 6.45 ... 6.31 1.458 ,204 1.567 1.777 6.11 6.70 2.211 ,496 1.643 1.334 5.33 0.014 7.46 3.729 1.865 1.072 1.107 4.00 0.047 a All quantities in millimoles except where stated. Solution volume = 50 ml. resin were used in every case. I
.
.
... ...
I
)
.
.
...
2.0
1.5
r'
Resin Dowex SoW-XZ
o special copooity (AI
- '\,\\\
.---.
+
\,
Cowex 5OW-XI! special Lopacity
meq Ag+
I meq i meq
(81
W
05
'\\
~(enl.
Fig. 2.-Silver-ethylenediamine complexes in specialcapacity, partially sulfonated Dowex-50 resins. Resin A, capacity 1.63 meq./g. dry H-resin; resin B, capacity 2.32 meq./g.
Resin oompositionRound en Ag
enHa++
-
n +
1.710 0.194 1.093 0.18 1.412 0.751 1.411 .53 1.273 1.127 1.632 .69 1.097 1.491 1.872 .80 1.116 1.694 2.037 .83 1.006 1.995 2.247 .89 0.854 2.535 2.690 .94 0.623 2.871 2.917 .99 4.024 mmole AgNOs and close to 4 meq. of
crosslinking, high silver loading, and above 0.5, a definite drop in stability was observed. This could be due to a steric effect. As was mentioned in the Introduction, the limiting .Tz of unity must be explained by a complex (Ag en+),,. not (Ag enH)++, for the 1:l ratio of bound amine to silver is maintained even when the exchange sites of the resin are saturated with silver ions. It is possible that a dimer (Ag2enz)++ is formed in the resin as it is in solution,6 but the ring structure attributed to this dimer seems improbable in the restricted solution volume inside the resin. A more likely possibility is a linear polymer (Ag'
+HaNCHzCHiNHp + Ag+ f- HzNCHzCHzNH2)s
The stability of such a polymer would increase, the more closely the silver ions could be brought together. Increasing the crosslinking of the resin would be expected to stabilize the polymer, yet it does not. A possible explanation for this fact is that the polymer lies along the polystyrene chains of the resin, rather than bridging them. The maximum distance between the sulfonate ions of adjacent benzene rings in the polystyrene chain is about 15 A., the minimum (cis-configuration) is 2.8 A. The Ag-Ag distance in the pogtulated polymer is an intermediate value, 8.5 A. One 0.1 thus may picture the polymer entwined like a vine '3 4 5 6 7 6 9 10 around the randomly oriented polystyrene chains. pipn) Evidence that the Ag-en complex restricts the Fig. 3.-Silverl,3-propanediamine complexes in Dowex-50 swellihg of the resin in water is given in Table 111. resin, 2% crosslinking. The water uptake on swelling is distinctly less for the complex in the resin than for silver ions in the resin. Perhaps the postulated linear polymer restricts the thermal motion of the polystyrene chains and thereby restricts the volume they occupy. Even with the Ag-en complex present the 2% resin has more than twice a8 much water as the 8% resin in the swollen state, so it does not appear that the postulated polymer bridges or crosslinks the resin chains to any great extent. If a linear polymer is formed which lies along the polystyrene chains, its stability should be afI 0 I I 05 IO 13 20 25 30 33 40 45 fected by changing the spacing of the sulfonic acid P(hn1, groups along the chains. The experiments with Fig. 4.-Nickel-hydrazine complexes in Dowex-50 resin, "special capacity" resins, having less than half the 2% crosslinking. The Ni:resin ratios are ratios of total nickel to resin; some So-90% of the nickel was in the resin, normal degree of sulfonation, showed that this was the case. Figure 2 shows that the complex formed the amount increasing with decreasing p(hn).
Jan., 1962
DIAMIKE COMPLEXES OF SILVER AND NICKEL
in these resins is only one-sixth as stable as that in the fully siilfonated resins (for f i = 0.5). If a polymer of any kind is formed one would expect its stability to depend on the concentration of silver ions in the resin, yet this is not the case, except as noted with the 8% cxosslinked resin. A possible explanation is that the silver ions tend to cluster together along the resin chains rather than to be dispersed randomly; this also would help to explain the anomalous increase in affinity for silver ions found with increasing proportion of silver ions in the silver-hydrogen cation-exchange equilibrium.6 Another peculiar feature of Figs. 1 and 2 is the slope of the formation curve for a < 1. The theoretical maximum slope of iz against log (ligand concn.) for a 1 :1 complex is 2.303/4 = 0.82. The slope of these formation curves is about 0.3, considerably less than that expected for a 1:1 complex. This slope could be explained if a complex Ag(enH) + predominated in the resin in this range, and so could the lack of dependence on the silver ion concentration. One would, however, no longer have an explanation of the extraordinary stability of the complex iri the resin, unless it were the rather tenuous explanation offered in our previous paper.3 Silver and 1,3-Propanediarnine.-The formation curve for this complex in the resin is shown in Fig. 3, which also shows the formation curve for the complex in solution as determined by SchwarzenbachS6 He reported that only the 1:1 complexes, Ag(pn) + and Ag(1lpn) ++, existed (except at high silver ion concentrations), and of these the first was much the more stable. Again the Fame relationships are found, with somewhat less stabilization in the resin than with the silver-ethylenediamine complex, namely, a factor of 102J a t n = 0.5, contrasted to the factor 103.2for ethylenediamine. The slope a t fi = 0.5 seems to be greater than with ethylenediamine, which may be significant. Nickel and Hydrazine.-If the idea of a metal ion-diamine polymerization is correct, we should find that other “crosslinking ligands”’ form more stable metal complexes in an ion-exchange resin than in solution. Such a “crosslinking ligand” is hydrazine. Nickel and zinc salts are known to form insoluble compounds with hydrazine, apparently of a polymeric nature, which precipitate from aqueous solutions.* We therefore made ( 6 ) G. Schwarrenbach, B. Maissen and H. Ackcrmann, Helv. China. Acta, 36, 2333 (1952). (7) G . Schwareenbach, Angew. Chem , TO, 451 (1958).
81
prelirninarv measurements of the stabilitv of the nickel-hydrazine complex in 2% crosslinked Dowex50 resin, using solutions of nickel fluoborate, since fluoborate is one of the few anions that does not form a precipitate. The results are shown in Fig. 4. Again the dashed line represents Schwarzenbach’s formation curve for the solution.* The-association is stronger in the resin than in the solution, except for high and high loading of the resin. (With 1 meq. of Ni++ added per meq. of resin, virtually all the nickel went into the resin.) Increasing the proportion of amine displaced nickel ions into the resin from the solution. There are indications that an association of 3N2€14:1 Ni++ is preferred in the resin, by contrast with the 6 : 1 coordination in fluoborate solutions, and this is consistent with the idea of a three-dimensional network of nickel ions crosslinked with hydrazine molecules. The extremely small water uptake of the resin loaded with nickel and hydrazine (Table 111) certainly suggests a crosslinked network. TABLE I11 WATER UPTAKE BY DOWEX-50 RESINS‘ Water uptake, mg. HeO/meq. resin, for Resin crosslinking cation 2% 4% 8%
Ha0 + 1020 500 280 Af4 470 270 170 Agen + 290 160 140 enHz+ + 200 140 Ni++ 460 *. Nihn, + + 100 .. .. For the special capacity resins in HaQ+-form, the uptakes were: resin A, 680 mg./meq.; resin B, 720 mg./meq. +
..
..
Conclusions Evidence is presented that ethvlenediamine, 1,3-propanediamine and hydrazine form polymeric association complexes with metal ions in cationexchange resiins. The data for silver and ethylenediamine suggest that linear polymers are formed, but the slope of the formation curve for low amine: silver ratios is hard to explain on this view. Acknowledgment.-One of us (M. G. S.) acknowledges the tenure of a Dow Chemical Company research fellowship 1959-1 960, The work was supported by the U. S. Atomic Energy Commission, Contract AT-( 11-1)-499. Part of it was presented a t the 138th National Meeting of the American Chemical Society, New York, 1960. (8) G. Schwarzenbach and A. Zobriat, Helo. Chim. Acto, 36, 1291 (1952).
