Effect of Anions. T h e effect of various anions on t h e slope of t h e depolarization curves is shown i n Table 11. For constant, reproducible results, some iodide ion must be present in solution. If t h e original sample is present as the chloride or bromide, 1 ml. of a 10-351 solution of potassium iodide should be added. Larger amounts of chloride or bromide do not affect the results. Cyanide and very large amounts of iodide do affect the accuracy of the results and should be absent.
V. Determination of Substrate [ E ]= 0.02 mg. [ E ]material per ml. of solution; pH = 7.40; T Acetylthiocholine Error, Butyrylthiocholine Present Found % Present Found Table
1.oo 2.00 5.00 8.00 10.0
1.01 2.00 5.05 8.00 10.0 15.2 30.0 50.0 66.3
+1 .o 0.0 +l.O 0.0 0.0 + I .4
2.00 4.00 6.00 8.00 10.0 22
n
C. Error,
= 27"
70
2.00 4.04 5.97 8.00
io
0.0 $1.0 -0.5 0.0
o
0.0 0.0 +2.0 u = 1.0
0
= 0.9
RESULTS
The results of the determination of samples of enzyme and substrate are indicated in Tables 111, IV, and V. I n general, samples of cholinesterase of specificity activity 0.27 to 14.0 pmoles per minute were analyzed, with a deviation of approximately 0.8y0(Table 111). Acetylcholinesterase, specific activity 0.29 to 29.3 pmoles per minute, was determined Kith a deviation of 0.8% (Table IV). Samples of acetylthiocholine of 1 to 65 mg., and butyrylthiocholine from 2 t o 85 mg. were analyzed with a standard deviation of about 1.0% (Table V). Application of this system to other substrates, as (D-thioethy1)diethyl methyl ammonium acetate, propionate, butyrate, and benzoate esters, and (&
thioethy1)ethyl dimethyl ammonium iodide esters, has been shown. Likewise, application to other enzymes (propionyl- and butyrylcholinesterase), other thioesters, and even other enzymesubstrate systems should be possible. ACKNOWLEDGMENT
The authors thank James D. Mokren, who assisted in some phases of the work. LITERATURE CITED
(1) Blaedel, W. J., Hicks, G. P., ANAL. CHEM.34, 388 (1962). (2) Ellman, G. L., Biochem. Pharmacal. 7, 88 (1961). 13) Gal. E.. Roth, E.. Clin. Chim. Acta 2 , 316 (1957). I
,
(4) Heilbronn, E., Acta Chem. Scand. 10, 337 (1956). (5) Ibid., 12, 1481, 1492 (1958). ( 6 ) Heilbronn, E., Scand. J . Clin. & Lab. Invest. 5,308 (1953). (7) Mendel, R., Rudney, H., Biochem. J. 37, 59 (1943). (8) Michaelis, L., Menten, T., Biochem. 2.49. 333 (1913). (9) Purhy, W. C.,'U. S. Army Contract DA49-193-MD-2052,Quarterly Rept., December 1961. (10) Stedman, E., Stedman, E., Easson, L. H.. Biochem. J . 26. 2056 (1932). (11) Willstaetter, R., 'Kuhn,' R., 'Lind, O., Memmen, F., 2. physiol. Chem. 167, 303 (1927).
RECEIVEDfor reviex January 9, 1962. Accepted April 12, 1962. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962.
Chelation Ion Exchange Properties of a Salicylic Acid-Formaldehyde Polymer R. C. DeGEISO, L. G. DONARUMA, and E. A. TOMIC
E . 1. du Pont de Nemours & Co., Inc., Wilmington, Del.
b The purpose of this study was to develop an evaluation procedure for determining the selectivity of chelating polymers prepared in this laboratory. This paper discusses this procedure as applied to a salicylic acid-formaldehyde polymer (SFP). The procedure involved contact of a sample of the polymer with various metal ions in several steps over a wide p H range. Each equilibration at a given p H is followed by determination of unchelated metal in solution. The results, expressed in a plot of distribution ratios of metal ions between the solution and the polymer, show a high selectivity of SFP for UOz+2and FefS and also suggest optimum p H values for sorption and elution of these metals for separations from each other and other metal ions.
I
LAST TWO DECADES interest in the chemistry of coordination compounds has increased considerably. Most of the work has been done on monomeric chelates, but papers have also been published concerning polymeric ligands (1, 14, 16). Many of the workers in the field of polymeric chelating compounds were primarily interested in the synthesis of chelating resins. A relatively small number have studied selectivity extensively. A variety of methods was used t o determine the selectivity of polymers. Gregor, Luttinger, and Loebl (11) used a modification of the Bjerrum titration which allowed the calculation of apparent chelate formation constants. However, different values were found b y the same author for a solution of the polymer and the solid (1.2). Ultraviolet N THE
spectroscopy was utilized for the same purpose (10, 16),and solutions of chelating polymers were employed. This method cannot be used with solids. Gregor, Taifer, Citarel, and Becker (IS) and Pennington and Williams (17') used batch equilibration of chelating polymers with solutions of metal ions containing buffers of high capacity to determine selectivity and capacity of chelating polymers. Buffer systems offer the convenience of holding the equilibration slurry at constant p H but suffer from the fact that buffer-metal complexes form which influence the position of the metal-polymer equilibrium. Column experiments also have been used to estimate selectivity (18). We decided t o use batch equilibration of polymer samples with metal VOL. 34,
NO. 7, JUNE 1962
0
845
ions over as wide a p H range as the hydiolytic stability of the metal ion and the solubility of the polymer in the We aqueous solution permitted. avoided, however, the use of buffers in our systems. The experimental results obtained by this method do not permit the calculation of stability constants unless assumptions about the nature of the formed chelates and the order of magnitude of the activity coefficients are made and the equilibration is carried t o completion of metal uptake. Batch equilibration, however, yields a very accurate measure of the selectivity of the polymer for various metal ions. EXPERIMENTAL
Polymer. All experiments were carried o u t with highly purified S F P (8). It is of extreme importance t h a t only highly purified polymer be used (molecular weight distribution 6000 t o 7000). Polymer samples containing fractions of lower molecular weight gave erratic results. Apparatus. All equilibrations were carried out in 400-ml. beakers covered with tight fitting Lucite disks through which four holes were drilled. T h e tip of a microburet nas inserted through one of t h e holes for the addition of base or acid during equilibration. Through another hole, samples were withdrawn by means of a n Emich filter stick fitted to a 1-ml. pipet. T h e other two holes accommodated a glass and calomel electrode couple which were connected through a Beckman multielectrode switch to a Beckman Model H 2 p H meter. Six equilibrations were carried out a t one time. The slurries in the beakers were stirred continuously b y magnetic stirrers. Most analyses were carried out by means of gamma-scintillation counting in a Harsham m-ell-type TlCl activated N a I crystal connected to a Radiation Counter Laboratories. Inc.. single channel differential analyzer consisting of probe, power supply, preamplifier, amplifier, analyzer, and scaler. The other determinations ti-ere carried out photometrically on a Beckman Model B spectrophotometer. Cells, 1 cm., were used. Influence of Electrolyte. Samples of S F P , 1 gram each, were equilibrated with solutions containing 3 meq. of Fe+3 (marked with Fe-59) in 200 ml. of 0.01, 0.05) 0.1, 0 . 5 , 1.0,V NaC104, KaN03, XaC1, CH,COOXa, and Na2S04, respectively, a t p H = 2.5 for 24 hours. From the Fe-59 content of the solution, the amount of Fe+3sorbed on the polymer was calculated. R a t e s of Metal Uptake. One gram of SFP ground to pass a 100-mesh screen was slurried overnight in 10 ml. of 1M ?r’aN03, p H = 3. T h e slurry was centrifuged, and t h e clear solution was discarded. Ten milliliters of a 0.05M solution of U02+2, Fe+3, Cu+z, and Mn+2 ions, respectively, in the same electrolyte solution were added to the polymer in centrifuge tubes. The tubes were stoppered and shaken. A similar 846
ANALYTICAL CHEMISTRY
experiment was carried out with a 1ik’ U02+z solution also. With each cation, seven samples were prepared and shaken for I/Z, 1, 2, 4, 7 , 24, and 48 hours, respectively, at 25” C. The p H was checked periodically and adjusted to 3 when necessary with 0.1,V NaOH. After equilibration, the slurry mas centrifuged, and the clear solution was analyzed. The amount of metal ion adsorbed by the polymer was calculated. Screening Procedure. T h e polymer was finely ground t o pass a 100mesh screen a n d dried in vacuo a t 50” C. for 24 hours. Samples, 2 grams each, were weighed into a 50-ml. glass stoppered Erlenmeyer flask. Fifty milliliters of a 1M NaNO3 in HzO of p H = 2.5 ( p H = 3 in earlier runs) adjusted b y addition of I N HNO, or 1N NaOH, was added to the polymers. The polymer m-as left in contact with this solution for 24 hours. This treatment “swelled” the polymer. Two solutions were prepared of each metal ion. For this purpose a stock solution of known metal concentration containing, wherever available, a radioactive isotope of the metal was diluted with 1N K a N 0 3 to contain 5 meq. of metal in 200 ml. The p H of these metal solutions n-as 1 with the exception of tungstate and molybdate solutions where equilibration was started a t p H = 7 and 5 , respectively, and p H values of 6, 5 , 4, etc., were adjusted in the course of equilibration. One of the two metal solutions was kept as the reference solution. The other solution was added to the presoaked polymer which had been freed of the N a X 0 3 solution as completely as possible, and the slurry was transferred quantitatively into a 400-ml. beaker for the subsequent equilibration. The slurries were adjusted, initially, to p H = 1.0 and 7 and 5 , respectively, b y addition of liV “ 0 3 or 1N NaOH where necessary. The p H value, once adjusted, x i s maintained at this value by addition of 1N acid or base during 1 hour of equilibration. After one hour, a sample of the solution was taken. Gamma scintillation counting or photometric techniques for metal for which no suitable tracer is available (9, 19, 60) were used to
I
determine the metal content of the samples. From these values, the distribution ratio was calculated. Then the p H was adjusted to the next higher or lower value b y addition of IN acid or base and again kept there for exactly 1 hour. The procedure of sampling, p H adjustment, and equilibration for 1 hour was repeated as often as the stability of the metal ion in aqueous solution and the stability of the polymer permitted. RESULTS AND DISCUSSION
Chelation reactions can be carried out most efficiently in the presence of a buffer system of high capacity t o maintain a certain p H value. Buffers, however, form complexes-e.g., acetateor even chelates-e.g., phthalate. Metal-buffer reactions can shift the position of the equilibrium of metalpolymer interaction especially where chelates of low stability are formed. We established, therefore, the influence of the acetate, nitrate, chloride, sulfate, and perchlorate ions at various concentrations on the position of the equilibrium. We restricted this investigation t o the interaction of Fe+3 with SFP. Iron(II1) is one of the few ions whose complex formation constants with various anions as well as the stabilitj- constants with the resin monomer, salicylic acid, are well investigated. Iron also offers the further advantage of havislg a convenient isotope, Fe59, for use as a tracer. The influence of the nature and concentration of the various anions on the position of the equilibrium between Fe+3 and the polymer is given in Figure 1. At basic electrolyte concentrations below O.O5N, not enough anions are present to complex Fe + 3 completely, and hydrated Fe + 3 exists besides the Fe+3 anion complexes. I n this concentration range, experimental results were not reproducible and were therefore omitted in Figure 1. A t an electrolyte concentration of O.O5N, an excess of anions is present. From here on, the F e f 3 uptake by the polymer increases with increasing concentration of C1-, ?SO3-, and Clod-, but decreases with increasing concentration of CH3COO- and This can be explained in terms of stability constants of the complexes which iron(II1) forms with these ligands: 2 to sulfate and acetate K 4 at p = 1 to p = 0 (6); CH3COO-: high but uncertain (9)1 form rather strong complexes with Fe +3, while chloride, nitrate, and perchlorate [for (FeC1)+2,K1 = 0.1 to 1.5 a t p = 0.6 to p = 0 (6); for (FeN03)+2, K1 = -0.5 to 1.0 at p = 1 to p = 0 (4), and for (FeC104)+2, K1 = -0.32 t o 1.28 a t p = 0 ( 7 ) ] only give weak complexes and therefore cannot be expected t o influence the position of the iron(II1)-
-
Figure 1 . Influence of basic electrolyte on sorption of Fe+3 by SFP 3-meq. of Fefa were used at p H 2.5 in various electrolyte solutions in contact with 1 gram SFP
4 Figure 2. Rates of metal uptake o 0.05M solutions f SFP a tofp HU02+2, 3 for Fef3, Cuf2, and 1 .OM U 0 2 + 2 nitrates
‘
o
‘
o
a
o
~
-
and 1000
t
D
Fe
b Figure 3. Distribution ratios of metal ions between an aqueous solution and SFP at various p H values Values were determined for 200 ml. of solution, 1M in NaNOs and containing 5 meq. of metal HOURS
01
salicylate equilibrium [K, = 15.8 (S)] as much as sulfate and acetate. Since C1- complexes with many metal ions other than Fe+3 are much stronger than either NO3- and C104complexes and since determination of metal ions will occasionally require evaporation t o dryness, x03- as 1M N a N 0 3 or KNO, was used for all further studies in perference to Clod- and C1-. The rates of metal adsorption of S F P , were determined for Fe+3, C U + ~U02+2, and h I n f 2 t o establish the shortest period of time for which equilibration could be carried out while operating as close to equilibrium conditions as possible. The term “rate” refers merely t o the speed of*changes in the concentration of the metal ions in the aqueous solution which is in contact with S F P . Figure 2 shows the results obtained for the first 7 of 48 hours of equilibration only, since after 7 hours no changes in the concentration of metal in the solution could be observed. Curves for the 0.05M and 1.OM UOzf2 coincide which shows independence of rates from concentration. The equilibrium between polymer and metal reached approximately 90% in the first hour of equilibration. For practical screening purposes, a 90% attainment was considered satisfactory. Changes of the p H of the polymer slurry in the metal solution during equilibration were compensated b y addition of acid or base, respectively. If the p H did not change b y more than +0.2 p H at any particular value for 1 hour after the final adjustment, a sample of the solution wa.s withdrawn
and analyzed. The obtained value was considered the equilibration concentration a t the defined pH. The slurry was then adjusted to the p H value desired next, and the procedure for p H adjustment and sampling was repeated. The equilibrations were carried out over as wide a p H range as the hydrolytic stability of the metal ion permitted. From the known starting concentration and the amount of metal in solution after each equilibration, the distribution ratio of the metal between the solution and the polymer was calculated according t o
D = amount of metal on the polymer
I
1
I
J
,
2
,
3
,
v ,
4
r 5
, 6
, 7
8
P H --c
coordination requirements which, as a model of the polymer shows, is easily accomplished. LITERATURE CITED
(l)ZBerlin, A. ii.,Prog. Chem. (U.S.S.R.) 29, 277 (1960). (2) Bjerrum, J., Schwarzenbach, G., Sillen, L. G., “Stabi1i;y Constants of Metal-Ion Complexes, Vol. I, p. 3, The Chemical Society, London, 1958. (3) Ibid., p. 53. (4)Ibid., Vol. 11, p. 5 5 . (5) Ibid., pp. 82-83.
amount of metal in the solution X volume of solution weight of polymer
(6) Ibid., p. 9 i . ( 7 ) Ibid., p. 111. ( 8 ) DeGeiso, R. C., Donaruma, L. G., Tomic, E. A., J . Org. Chem., in press. (9) DeSesa, M.il., Neitzel, 0. A.? ASAL.
The units of D are: ml. of solution per gram of dry polymer. The distribution ratios for various metal ions are plotted on a logarithmic scale us. p H in Figure 3. This plot reveals the selectivity of the highly purified polymer for uranyl ion over all other metal ions investigated. The plot also makes it possible to select the optimum p H range for sorption and elution of mixtures of metal ions. The slope for the Fe+3, lower than the expected value of 3, could be attributed to steric hindrance imposed b y the polymer matrix which inhibits the completion of the Fe(II1) d2sp3hybrid with 3 mers of the salicylic acid as opposed t o the formation of the chelate of Fe(II1) with monomeric salicylic acid. U02+2only needs 2 mers of salicylic acid for completion of its
110) Gill, S. J., Kall, F. T., J . Phys. Chem. ‘ 58, 1128 (1956). (11) Gregor, H. P., Luttinger, L. B., Loebl, E. &I., Ibtd., 5 9 , 34 (1955). (12) Ibzd., p. 336.
CHEM.29, 756 (1957).
(13) Gregor, H. P., Taifer, Mark, Citarel, Louis, Becker, Ernest, Ind. Eng. Chem. 44, 28.14 _ _ - (1952) (14)xIenney, C. S., Chem. & Ind. 1960, \ - - - - I .
2x0
(15)killar, J. R., Ibid., 1957, 606. (16) Morawetz, J . Polymer Sci. 17, 442 (1955). (17) Pennington, L. D.. Williams, M. B., Ind. Eng Chem. 51, 759 (1959). (18) Petit, J., Lumbroso, R., Con@ rend. 250, 2568 (1960). (19) Snell, C. T., Snell, F. D;,, “Colorimetric Methods of Analysis, 3rd ed., Vol. 11,p. 248, Van Nostrand, Princeton, N. J., 1948. (20) Ibid., p. 453. RECEJVED for review November 8, 1961. Accepted March 22, 1962. VOL. 34, NO. 7, JUNE 1962
a47
