Nuclear Magnetic Resonance Method for Determining the Moisture Holding Capacity of Cation Exchange Resins as a Function of Temperature L. S. Frankel Rohm and Haas Company, 5000 Richmond Street, Philadelphia, Pa. 19137
Nuclear magnetic resonance spectrometry has recently been utilized to study many properties of ion exchange resins ( I ) . During our work in this field, we observed some spectra which a t first sight appeared unusual. A variable temperature study gave a clear explanation and led to a very convenient, accurate, sensitive method for determining the moisture holding capacity of ion exchange resins as a function of temperature. The experimental resin of interest was made from ethyl acrylate, methacrylic acid, crosslinked with CH3CH2CR3 (R = CHZOCOCCH~CHZ) in the following ratio: 50/48/2. The Mg2+ ionic form was studied.
EXPERIMENTAL The NMR spectra were obtained on a Varian A-60 spectrometer equipped with a variable temperature probe. A portion of hydrated resin was centrifuged to remove exterior water ( 2 ) and quickly transferred into an NMR tube. An exterior solvent which does not enter the hydrated resin (1,2-dichloroethane)was subsequently added.
RESULTS AND DISCUSSION The NMR spectrum with water as the exterior solvent showed two water peaks a t ambient probe temperature (37 "C). The two water peaks correspond to water inside and outside the resin. The spectrum with 1,2-dichloroethane as the exterior solvent consisted of three peaks.
The high field peak is due to 1,2-dichloroethane outside the resin. The chemical shift between the two low field water peaks, 6 = 20.2 Hz, was almost identical to that obtained with water as the exterior solvent, 6 = 20.5 Hz. Spectra a t 21.5 "C (approximately room temperature) and 44 "C are shown in Figure 1. The spectra are completely reversible with temperature. These spectra readily explain the cause of the two water peaks. The moisture holding capacity is dependent on temperature. At higher temperature, the moisture holding capacity decreases and water leaves the resin phase and enters the exterior solvent phase. NMR can be used to determine the moisture holding capacity of cation exchange resins ( I ) . The calculation requires the integral ratio of water inside the resin to water outside the resin, the void volume of the column (usually determined by a separate NMR integral experiment), and the true hydrated density of the resin. The moisture holding capacity of the resin of interest determined via the above procedure was 63% a t 37 "C. The moisture holding capacity determined via a simple resin drying procedure is generally more accurate due to cumulative experimental errors in the above parameters. The resin moisture holding capacity as a function of temperature was calculated from the moisture holding capacity (MHC) a t 22 "C (70.4%), determined by a resin drying experiment and the NMR integral ratio of the two water peaks. The following equation is applicable: MHC ( T )= lOOW/(l +It(l - W))
10.0 Hz H
Ho
Figure 1. NMR spectra of water the acrylic resin at 21.5 and 44.0
peaks in the Mg2+ ionic form of "C
( 1 ) L. S. Frankel, Ana/. Chern., 43, 1506 (1971). (2) F. Helfferich, "Ion Exchange," McGraw-Hill N . Y . , 1962, p 230.
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ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
(1)
where It is the integral ratio of water released from the resin to water inside the resin at a particular temperature and W is the weight fraction of water in the resin a t 22 "C. The above procedure is an analytically accurate method for determining the moisture holding capacity as a function of temperature. For example, an error of *lo% in It leads to an error of approximately *0.5% in the moisture holding capacity. The void volume of the column and the true hydrated density need not be determined. In general, the experimental error will depend on the resolution between the two water peaks. For many resins, the resolution is not sufficient, over an extended temperature range, for application of this method. Values of It were determined via cutting out and weighing appropriate parts of the spectrum. The weight of both water peaks and the low field half of the interior water peak was determined and the value of It subsequently calculated. The results are summarized in Table I. The resin moisture holding capacity a t 37 "C was 65.070, in reasonable agreement with the above value (63%). The moisture holding capacity shows a large continuous variation with temperature. Creekmore and Reilley have recently reported that the moisture holding capacity of the Mg2+ ionic form of Doaex 5OW-8X also shows a significant temperature dependence (3). (3) R . W. Creekmore and C. N . Reiiley, Anal. Chem., 4 2 , 570 (1970)
The data summarized in Table I indicate that 6 and the internal molality show a significant variation with temperature. However, the molar shift is virtually independent of temperature. This implies that the water released from the resin a t higher temperature originates from the bulk water in the resin and not from the cation hydration sphere. The molar shift, 0.215 ppm, is somewhat larger than has been reported for styrene based cation resin (3, 4 ) and for that recently reported for Mgz+ ion in water, 0.153 ppm ( 5 ) . If one looks carefully a t the spectra obtained a t 21.5 “C, a small peak can be seen a t approximately the chemical shift of water outside the resin. We estimated that this small peak is approximately 1% of the major water peak. It could result from either a small temperature effect or residual water which is not removed uia the centrifuge method. No matter which explanation is indeed correct, the relative intensity implies that the centrifuge method is highly effective for isolating the hydrated resin.
Table I . NMR Results and the Moisture Holding Capacity as a Function of Temperature Temperature,
“C
It
MHCt
6
21.5 28.5 33.5 37.0 44.0 57.0
0.00 0.17 0.24 0.28 0.38 0.43
70.4 68.2 65.7 65.0 63.2 62.5
17.6 19.2 20.2 21.1 23.4
...
Internala molality
Molar shift, Hz
1.220 1.352 1.514 1.561 1.689 1.740
... 13.0 12.7 12.9 12.5 13.4
a The dry weight capacity of the resin is 5.8 mequiv/gram.
An NMR method has recently been explored for the determination of effective hydration numbers of cations in ion exchange resins ( 3 ) . Among other things, this method assumes that the molality and, therefore, the moisture holding capacity of the internal solution phase is independent of temperature. This is clearly not true for the Mg*+ form of the acrylic resin reported here and Dowex 50W8X, and may not be true for other resins.
(4) D. G . Howery and M. J. Kittay, J. Macrornoi. Sci., Part A, A ( 4 ) , 1003 (19701, (5) J. Davies, S. Ormandroyd, and M . C. R . Symans, Trans. faraday Soc., 67, 3465 (1971).
Received for review December 11, 1972. Accepted February 20,1973.
Estimation of the Chemical Shifts of Aromatic Protons Using Additive Increments Jane Beeby and Sever Sternhell D e p a r t m e n t of Organic C h e m i s t r y , U n i v e r s i t y of Sydney, Sydney, A u s t r a l i a
1.Hoffmann-Ostenhof, Ern0 Pretsch, and Wilhelm Simon L a b o r a t o r i u m fur Organische Chernie, Eidgenossische Technische Hochschule, Zurich, Schweiz
Additivity of substituent effects on the chemical shifts of aromatic protons has received a great deal of attention (1-12) and while accurate calculation of the combined effect of more than one substituent is not always possible, the principle of additivity often yields results which are precise enough for the purpose of determining the substituent patterns in polysubstitued benzenes (13, 1 4 ) . In these laboratories, we have been using the principle of “simple additivity” (see below for definition) routinely as an aid in the solution of structural problems utilizing substituent increments derived from data for monosubstituted benzenes. In this work, we have assembled in a convenient form (Table I) the literature data for monosubstituted benzenes most likely to be useful and ( 1j L. M. Jackman and S. Sternhell, “Applications of NMR Spectroscopy in Organic Chemistry,’ Pergamon Press, New York, N.Y., Chap. 3-6. ( 2 ) W . 6. Smith and J. L. Roark, d. Amer. Chem. Soc.. 89, 5018 ( 1967). (3) S. Castellano and R . Kostelnik, Tetrahedron Lett., 1967, 5211. (4) Y. Sasaki. M. Suzuki. A . Shimazu, and A. Misaki, Chem. Pharm. Bull.. 15, 1083 (1967). (5) B. Richardson and T. Schaefer, Can. J, Chem.. 46, 2195 (1968) (6) N . Van Meurs, Reci. Trav. Chim. Pays-Bas, 87, 145 (1968). (7) R . H. Cox, Spectrochirn Acta, 25A, 1189 (1969). (8) Y . Nomura and Y . Takeuchi, Org. Mag. Resonance. 1, 213 (1969). (9) W . 6. Smith, A. M . lhrig and J. L. Roark, d. Phys. Chem., 74, 812 (1970). (10) D. Aksnes and F. H. Kronhaug, Acta Chem. Scand., 25, 1871 (1971). (11) A . R . Tarpley and J. H. Goldstein, J. Phys. Chem., 75, 421 (1971). (12) K. N . Scott, J . Mag. Resonance. 6, 55 (1972). (1 3) J, J. R . Reed. Ana/. Chem.. 39,1586 (19671, (14) M. Zanger. Org. Mag. Pesonance. 4, 1 (1972).
augumented them with data obtained in these laboratories. We also report the results of a systematic statistical analysis of errors arising from the application of the simple additivity principle to the prediction of 1452 chemical shifts in polysubstituted benzenes. The data in Table I were chosen from those available in the literature (cf. column headed “Other references”) using the following criteria: (a) The solvents of choice were carbon tetrachloride, cyclohexane, and tetramethylsilane with data for deuterochloroform solutions as a second preference. (b) The data for dilute solutions, preferably data for a series of concentrations extrapolated to infinite dilutions, were chosen whenever possible. (c) Unless otherwise indicated, all data were obtained by complete analysis of the spectra or from massively deuterated samples. (d) The data are presented as differences from the chemical shifts of benzene in the same solvent and at the same concentration. Where such data for benzene were not available with sets of results which were otherwise appropriate, we have obtained the relevant chemical shifts of benzene in our laboratories using a Varian HA 100 spectrometer. All data were rounded off to the nearest 0.01 ppm because precision beyond this figure is well beyond that attainable by additivity and because differences of u p to 0.1 ppm are often observed even between sets of meticulously obtained data for dilute solutions in CC14 (15) and cyclohexane (16). (15) K. Hayamizu and 0. Yamamoto, J Mol. Spectrosc.. 28, 89 (1968). (16) Y. Yukawa, Y. Tsuno, and N. Shimizu, Bull Chem SOC.Jap.. 44, 2843 (1971).
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