Preparation and Properties of Quaternary Cellulose Anion Exchangers RUTH
R. BENERITO, BETSY
B. WOODWARDtl and JOHN D. GUTHRIE
U . S. Department o f Agriculture, Southern Regional Research laboratory, New Orleans, l a .
b Quaternary cellulose anion exchangers, the existence of which has been questionable, have been prepared by reacting diethylaminoethylcellulose with either methyl iodide or ethyl bromide under completely anhydrous conditions. Experimental procedures for the preparation of, and analytical data in evidence of the existence of, the quaternary anion exchangers in the salt forms have been obtained. The difficulty with which the quaternary salts exchange the halide ions for hydroxyl ions as compared to the ease with which the weak cellulose anion exchanger can be regenerated to the base form has been demonstrated. Potentiometric and conductometric titrations of the salts and bases of both weak and strong cellulose anion exchangers have been determined under carefully controlled equilibrium conditions in inert atmospheres. Typical titration curves for the cellulose exchangers have been compared with those obtained with weak and strong anion exchange resins under similar conditions.
A
preparation of a strongly basic quaternary ammonium-type cellulose anion exchanger has been reported (4, 10, 16) and a commercially available product (9) was claimed to be a quaternary, namely triethylaminoethylcellulose (TE AE-cellulose) , various other investigators have been unable to prepare quaternary anion exchangers of cellulose. Experimental evidence confirming complete conversion of a weak base cellulosic exchanger to a quaternary salt is lacking. Jakubovic (11) reported his attempts to prepare the quaternary were unsuccessful, but that its successful preparation would find many applications in chromatography. I n a study of ion exchange properties of the product formed by reacting cotton cellulose fabric with N,N,N-tris(3chloro-2-hydroxypropyl) amine (IS,1 4 , titration curves for various anion cellulose exchangers were reinvestigated in the light of latest theories of ion exchange LTHOUGH
1 Present address, 1334 Terrace Street, Tallahassee, Fla.
resins (9, I d ) . These investigations included commercially-available diethylaminoethylcellulose (DEAE-cellulose) , TEAE-cellulose, and a product formed in the reaction between cellulose, sodium hydroxide, triethanolamine, and epichlorohydrin and referred to as ECTEOLA. I n addition, quaternary iodides and bromides of DEAE-cellulose were prepared under completely anhydrous conditions. Results of these investigations show that complete conversion of DEAE-cellulose to a quaternary salt is not realized unless certain precautions are taken. These quaternary base cellulose exchangers titrate as strong base exchangers, whereas the commercially-available TEAEcellulose, once reported to be a quaternary, is more like the weakly basic DEAE-cellulose, showing only slight conversion to a quaternary. Although commercially-available ECTEOLA-cellulose (15) is not a quaternary anion exchanger, work in this laboratory shows that a quaternary anion exchanger can be prepared under certain carefully controlled conditions by reacting cotton cellulose with triethanolamine and epichlorohydrin under basic conditions (1).
This paper reports experimental data which prove the existence of quaternary cellulose anion exchangers. Details for the preparation and complete regeneration of the quaternary to the basic form and typical titration curves of the various cellulosic anion exchangers as determined by carefully controlled potentiometric and conductometric analyses in inert atmospheres are given. The characteristics of these titration curves for quaternary cellulose exchangers are compared with those of the weak base cellulose anion exchanger (DEAE-cellulose), a strong base anion exchange resin, and a weak base anion exchange resin under similar experimental conditions. EXPERIMENTAL Materials. The following commercial grade cellulose anion exchangers were obtained : Cellex-T, Cellex-D, and Cellex-E (which were TEAE-cellulose, DEAE-cellulose, and ECTEOLA, respectively, from the Bio-Rad Co.), some TEAE-cellulose from the Gallard-Schlesinger Chemical Manu-
facturing Corp., and DEAE-cellulose (Type 20, Lot 1023) from the Brown Co. Methyl iodide and ethyl bromide were reagent grade chemicals obtained from the Tennessee Eastman Co., Division of Eastman Kodak Co. A strong anion exchange resin, Amberlite IPA-401, was a product of Rohm and Haas Co., Philadelphia, Pa. The weak anion exchange resin, Duolite A-7, was a product of the Chemical Processing Co. Conductivity water was used in the preparation of all salt solutions used in titration experiments. The pH of this water was maintained a t 7 by flushing out all titration vessels with dry nitrogen before the addition of the cellulose anion exchangers. Preparations. REGEKERATION OF DEAE-CELLULOSETO BASE FORM. The cellulose anion exchanger in the salt form was treated with excess 0.01M NaOH for several minutes; the treatment was repeated three times with fresh portions of 0.01X NaOH. The exchanger was then filtered through a sintered-glass funnel, and rinsed with COz-free water until the pH of the filtrate was 6 . When swelling occurred, the swollen material was treated with absolute ethanol to remove water. Finally, the exchanger in the base form was dried over Pz05under vacuum to constant weight. QUATERNARY CELLULOSE ANIONExCHANGERS. Quaternary salts were made from DEAE-cellulose and methyl iodide and from DEAE-cellulose and ethyl bromide. I n either instance, the commercially-available DEAE-cellulose was regenerated with 0.01Jf NaOH. (Use of stronger base was unnecessary, and with base as concentrated as 0.5M NaOH, the exchanger was too difficult to filter.) The regenerated DEAE-cellulose was washed with COz-free water until free of C1-ions and then thoroughly dried over P&&in vacuo for 2 days. WITH CHJ. Methyl iodide was dried over calcium chloride and then distilled just prior to its being used. A 10% by weight solution of CHII in fresh absolute methanol was prepared. The dried regenerated DEAE-cellulose and CHJ solution were refluxed for 2 hours. The yellow precipitate was washed in absolute ethanol and then dried over P& in vacuo. All weight gains and analyses are reported on the dry weight ’ basis. (Preliminary experiments showed that whenever one of the reactants was not completely dry, complete conversion to the quaternary iodide was not realized.) VOL. 37, NO. 13, DECEMBER 1965
1693
WITH C2H5Br. Ethyl bromide was dried over CaC12 for 2 days, distilled, and then used immediately to prepare a 20y0 by w i g h t solution in absolute ethanol. Bone-dry regenerated DEAEcellulose was refluxed with the C2H5Br solution for 4.5 hours. The white quaternary bromide was washed with absolute ethanol and dried over PZOS in vacuo before analyses were determined. SAMPLES.A11 samples were weighed by difference in closed containers to prevent moisture pickups and all analyses and capacities are reported on dry weight basis. Nitrogen analyses were determined by the micro-Kjeldahl method. Samples were oxidized in either Schoniger combustion flasks or macro-Parr bombs and then analyzed for chlorine or bromine by the method of Cheng (6) or for iodine by the method of Elek (8). Microcombustion analyses
were used to determine percentage of carbon and hydrogen of a few samples. An Titrations. POTENTIOMETRIC. accurately weighed sample of sufficient weight t o give approximately 0.8 meq. of replaceable anion was placed in a titration flask containing exactly 50.00 ml. of a salt solution prepared with conductivity water and kept in a nitrogen atmosphere. T h e sample was allowed t o equilibrate overnight in the titration flask while being stirred with nitrogen gas bubbled first into a bubbler containing the given solution before being bubbled into the titration flask to minimize concentration changes in the flask. All potentiometric titrations were carried out in specially designed cells of approximately 250-ml. capacity having four ground-glass outlets to accommodate the microburet in the center, a n inlet-outlet tube for the nitrogen,
-
Table I. Analyses of Cellulose Anion Exchangers
DEAE-celluloseb DEAE CHJ-
DEAE .C1H5Brcellulose
CaDacitv from elemental “analyses (meq./gram) N Halide 0.782 & 0.02 0.780 0.793 0.791
C 43.18 41.6
Composition, yoa H N Halogen 1.82 6.87 1.14 10.21 6.44 1.11
0.771 42.4 6.20 1.06 6.18 0.657 42.0 6.30 1.04 6.30 0.800 0.765 a Values correspond t o a degree of substitution on cellulose of 0.16 (complete substitution = 3.0). b Av. of several analyses of DEAE-cellulose samples used in preparation of quaternaries
Table II.
0.757 0.793
Summary of Initial pH Readings and Capacities of Anion Exchangers in Basic Form”
Exchanger in salt form Amberlite
Theo- NaCl reticalb mocapacity larity 3.60 1.0 1.o
1.0 Duolit e
9.65
1.0
DEAE-cellulose
0.804
0.5 2.0
TEAE-cellulose
0.839
1.0 1.o
ECTEOLAcellulose
0.1
0.5 2.0
Base groups meq./50 ml. 0.4597 0.8712 1,9324 0.5433 1.957 2.001 0.8552 1.2975 1.025 1.0625 0.6888
Initial PH 9.03 9.00 9.18 9.05 9.50 9.58 10.13 10.30 9.88 9.82 9.88
Experimental capacityC H0,ffpauirConPotentio- Guthrie ductometric method metric 2.50 9.60 0.810
0.780 0.587 0.555
...
..,
0.813
...
0.550
...
0.804 0.775 0.510
... ...
0.15 0.15 ... 0.123 9.75 ... ... 0.2613 9.86 0.767 0.724 0.730 9.32 0.8432 0.790 DEAE .CHaI... 0.745 9.10 0 6696 cellulose 0.730 0.731 0.687 9.50 0.8595 0.780 DEAE . CzHSBr0.765 ... ... 9.10 1.0235 1.o cellulose e Solutions of regenerated base in indicated NaCl concn. made in conductivity water (pH = 7) and samples allowed t o equilibrate in nitrogen atmosphere for 16 hours. b Meq./gram dry wt. basis aa detd. from nitrogen analysis on original samples. c Meq./gram dry wt. basis aa detd. by indicated method. 1694 *
0.25
0.5 0.5 0.5 1.0 0.5
ANALYTICAL CHEMISTRY
I
the glass electrode, and the saturated calomel electrode. All p H measurements were made with a Beckman Model G-S p H meter to +0.05 p H unit. Each titration required from 8 to 12 hours because of time required in some instances for establishment of equilibrium. CONDUCTOhlETRIC. accuratelyweighed sample of the exchanger containing approximately 0.8 meq. of replaceable anion was placed in 230 ml. of conductivity water containing a known amount (approximately 2 meq.) of HCl in a specially constructed conductivity cell of 300-ml. capacity, flushed with dry nitrogen previously bubbled through a solution of the same concentration as that used in the conductivity cell. The borosilicate glass conductivity cell was made with openings for the microburet, gas inlet-outlet tube, and the platinum electrodes. All titrations with standardized 1X XaOH were performed after exchangers had equilibrated for at least 6 hours. A11 conductance measurements were made a t 25’ i. 0.05’ C. with a Jones conductivity bridge in conjunction with a Hewlett - Packard (Model 2 0 1 4 ) audiofrequency oscillator and a General Radio Corp. tuned amplifier and null point detector (Type 1231 XC). Variation of pH with Per Cent Salt Form. A modification of the Hoffpauir-Guthrie (IO) method for determination of capacity and strength of the cellulose exchangers was used. The regenerated base exchangers were dried over P2OS in a desiccator and accurately-weighed samples approximating 1 gram were put in glass-stoppered Erlenmeyer flasks containing exactly 50 ml. of 1-V NaCl solutions made in conductivity water. Suitable amounts of alkali or acid in conductivity water were added to give a range of p H values and the total volumes were made up to exactly 100 ml. The flasks were flushed with nitrogen and allowed to equilibrate for 24 hours, after which the p H was determined and aliquots of the supernatant liquid were titrated to the phenolphthalein end point. RESULTS
In Table I are given a few analytical results obtained with the methyldiethylaminoethyl - cellulose iodide (DEAECHJ-cellulose) and the tribromide ethylaminoethyl - cellulose (TEAE.C2H5Br-cellulose) synthesized from DEAE-cellulose. For comparative purposes, averages of several values obtained with the original DEAE-cellulose are included. I n Table I1 and throughout the text, capacities are reported as milliequivalents on dry weight basis. The theoretical capacities mere calculated from nitrogen analyses and based on one nitrogen per one exchange group. Capacities of salts were also found by halide analyses. Experimental capacities were also determined with either potentiometric or conductometric methods. Those values listed are values obtained on samples of ex-
H
7t
K L
f
4
2o
0.1
ae
0.3
0.4
as
0.6
a7
a0
as
I
MEQ. HCL/g. EXCHANGER Figure 1 . Potentiometric titration curve of a partially regenerated quaternary cellulose anion exchanger (0.37 meq./gram based on iodide, and 0.43 meq./gram based on hydroxide) vs. standard 0.1M HCI in the presence of various concentrations of NaCl X , 0.5M NaCl 0 , 1M NaCl 0 , 2 M NaCl A, COz-free water
changers which were completely regenerated into the basic form. I n those instances, to be explained later, where the quaternary salts were not completely regenerated to the base form, only a capacity equivalent to the basic fraction was obtained on titration. Regeneration of Base from Salt Forms of Exchangers. The cellulose exchangers were regenerated with aqueous N a O H varying in concentration from 0.005 t o 0.1M h'aOH. Attempts to regenerate the exchangers in more concentrated base, as is usually done with resin-type exchangers, presented too difficult a task because of swelling of the cellulosic exchangers. After regeneration in base and washing and drying of the base form, the exchangers were again analyzed for nitrogen and halide ions. In general, the DEAE-cellulose, commercial samples of TEAE-cellulose, and ECTEOLA-cellulose were easily regenerated in very dilute NaOH. Water of almost neutral p H could be used to regenerate the weak base exchangers. Even after only one treatment with 0,005M base, the weak base exchangers showed no trace of halide ions on analysis. I n contrast, both the quaternary iodide and bromide were very difficult to regenerate. Even after two treatments with 0.01;M NaOH, and one treatment with 0.05JI S a O H , the yellow color was still visible in the quaternary iodide. Both the quater-
nary bromide and iodide required three treatments with fresh portions of 0.05-1.1 NaOH to change them completely into the basic form. The commercially available TEhE-cellulose, which contained 0.499 meq. C1- gave no trace of C1- ions after regeneration
Table 111.
Analytical Data Illustrating Differences in Ease of Regeneration of Base Forms"
Molarity of
Cellulose exchanger Quaternary iodide
Quaternary bromide
Exchange exsolution changer N&OH NaOH NaOH NaOH NaOH NaOH NaOH NBOH NaOH
NaOH Quaternary iodide DEAEcellulose
with only one treatment of 0.05 or 0.5M KaOH. Analytical data in Table I11 are given to illustrate the difficulty with which the I- ions of the quaternary exchanger are exchanged for OH- ions. The Br- ions of the quaternary, while difficult to exchange, are more easily replaced than the Iions. In contrast, the OH- ions of the base form are readily exchanged by halide ions and the halide ions of the weak base exchangers are most easily exchanged by OH- ions as evidenced by the fact that the weak cellulose salt exchangers can be titrated conductometrically under conditions where the OH- ion is present in very small concentrations. I n another series of experiments, accurately weighed samples approximating 1 gram of the quaternary iodides and bromides were equilibrated with 50.00 ml. of standardized KaOH of approximately 0.1JIin an inert atmosphere for 24 hours. Aliquots of the supernatant liquids were analyzed for OH- ions and the meq. of OH-/gram of sample exchanged were calculated to be 0.651 for the iodide and 0.753 for the bromide, values which represented a n 85y0 exchange of the iodide and 100% exchange of the bromide. A quaternary cellulose anion exchanger, analyzed as 0.800 meq./gram based on nitrogen analyses and 0.780 meq./gram based on iodide and total iodine analyses, was partially regenerated by one treatment with 0.Ol.W XaOH. Samples of the partially regenerated base, which on analysis was shown to contain 0.790 meq./gram based on nitrogen and 0.370 meq./gram
NaiS04 Na2S04
NBOH NaOH
NO. of exchanges
0 005
0.010 0.010 0.010 0.05
0.05 0.10
0:0&
0.010 ... 0.10
...
0.1
0.1 o:O05 0.01
0 1 1 0 1 0 1
2
0 1 1 0 1
Meq. of element/gram residue N Halide SO1-* 0.721 0.745 ... 0.725 0.374 ... 0.793 0.380 ... 0.689 0.099 ... 0.696 0,040 ... 0.720 0.062 ... 0.725 0.003 ... 0.104 0.720 ... 0.793 0.790 .,. 0.793 0.423 ... 0.800 0.184 ... 0.680 0.670 , , . 0.680 0.0 ... 0.721 0.767 0 0.635 0.121 0.721 0.018 0.749 0.798 ... ... 0.798 0.0 ... 0.796 0.0 ... 0.836 0.499 ...
Halide replaced, % 0.0
50.0 50.0 87.0 94.7 92.0 99.6 86.0 0.0
46.9 77.0 0
100.0 ... 84.2 85.1 0.0
100.0 100.0
TEAE ... 0.0 (chloride) NBOH 0.01 0.802 0.0 ... 100.0 5 Approximately 1 gram of cellulose exchanger shaken with 100 ml. of aqueous solution indicated in NZ atmosphere for 5 hours.
VOL. 37,
NO. 13,
DECEMBER 1965
1695
0
I
0.1
I
0.2
I
I
0.3
0.4
I 0.5
I 0.6
I
I 0.7
I 0.9
0.8
1.0
MEII. HCL/g. EXCHANGER Figure 2. Potentiometric titration curve of quaternary base completely regen erated from quaternary iodide vs. standardized 0.1 M HCI in presence of 0.1M NaCl Capacity based on N analysis 0 . 7 2 meq./gram; 0.66 meq./gram based on iodine
based on iodide, were equilibrated with aqueous solutions ranging from zero to 2M with respect to NaCl and then titrated potentiometrically against standardized 0.1M HC1. Typical results illustrated in Figure 1 show that almost identical curves were obtained with C02-free water as with 2M NaC1. The identical pH values in the presence of 2-11 YaC1 as well as C02-free water obtained with the quaternary base are in contrast to results obtained with the basic form of the weak cellulose anion exchanger. With the latter, higher pH values were obtained in the presence of salt than in C02-free water. Figure 2 shows the potentiometric titration curve of a sample of DEAE. CHJ-cellulose which had been completely converted to the quaternary base by treatment with 0.05M YaOH and then equilibrated with 0.1J.1 NaCl before being titrated with HCl. The theoretical capacity of this quaternary base exchanger was 0.720 meq. N/gram and there was no trace of iodide found in the basic form. The four potentiometric titration curves in Figure 3 were obtained with samples of a quaternary iodide regenerated to the base form to the extents of 29,4T, 57, and 99%, respectively, and then equilibrated with 2 . 0 X NaCl and titrated us. 0.1-11 HCl. These curves show that the OH- ions of the quaternary base are easily exchanged by C1ions. The capacity determined potentiometrically agreed with the amount of regenerated base in every case and the difference between total capacity as determined by nitrogen analyses and 1696
ANALYTICAL CHEMISTRY
titrations was equal to meq./gram based on iodine found in the exchanger before titration. A potentiometric curve for a completely regenerated basic form of a quaternary bromide (capacity of 0.702 meq./gram) equilibrated in 0.1M NaCl and then titrated against 0.1M HC1
,I0
is shown in Figure 4 as Curve A . The titration of the chloride form in excess HCl against 0.1M XaOH is shown in Curve B. These curves demonstrate that the OH- ion of the base is easily exchanged by the C1- ions as the theoretical capacity is the same as the midpoint of the sharp break in Curve A . The basic strength of the groups on the exchanger is revealed by the absence of a plateau a t pH 9 in Curve B. Anion Exchange Resins. For comparative purposes, a strong base anion exchange resin, Smberlite, and a weak base anion exchange resin, Duolite, were potentiometrically titrated under conditions similar t o those used with the cellulose anion exchangers. I n each case, an amount of sample equivalent to 0.8 meq. of exchangeable ion was used. The initial p H of basic form of the Amberlite resin and shapes of its titration curves were essentially the same as those obtained with the quaternary cellulose anion exchangers. I n contrast, titration curves for the regenerated bases of the Duolite started a t a higher pH (9.7.9, and the pH changed gradually until the end point was reached. Shapes of the titration curves for the Duolite resin in both the base and salt forms were similar to those obtained with DEAE-cellulose and not like those obtained with the quaternary cellulose bases or salts. Weak Base Cellulose Exchangers.
Potentiometric curves for the basic forms of DEAE-cellulose, commercially available TEAE-cellulose, and
I I I I I I I I 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 I ! MEII. HCLjg. EXCHANGER Figure 3. Potentiometric titration curves of quaternary iodides which were regenerated to the basic form to various extents I 0.1
1 0.2
A, 29.8%; X , 47.6%; NaCl and then titrated
VI.
1
0.3
0 , 57.0y0; and 0 , 100%. Titrations of exchangers equilibrated with 2.OM standardized 0.1 M HCI. Theoretical capacity 0.745 meq. N/gram
a product obtained as a result of a n unsuccessful laboratory preparation of a quaternary cellulose bromide (all reagents were not completely anhydrous) were determined in COz-free water and in the presence of salts for comparative purposes. Variation of pH with Percentage Exchanger in Salt Form. T h e variation of p H with percentage of cellulose exchanger in the salt form was found experimentally by use of the modified Hoffpauir-Guthrie method. These experimentally determined values were compared with values determined from the potentiometric titration curves for each type of exchanger. For each exchanger, the values from the two methods were in agreement as illustrated in Figure 5 . The lines represent variation of pH with per cent of exchanger in salt form as determined from potentiometric titration curves. The points represent experimental values determined by the Hoffpauir-Guthrie method. Open circles are typical values for quaternary cellulose base exchangers regenerated from either the quaternary iodide or quaternary bromide. Shaded circles show variations observed for DEACcellulose. Similar data obtained on commercially available TEAEcellulose and ECTEOLA-cellulose are not given. However, it! should be noted that their curves are almost superimposable on the line for DEAE-cellulose of Figure 5 and indicate them to be weak cellulose anion exchangers. Variation of p H with per cent salt form found in this investigation differs from that previously reported (IO). How-
Figure 4.
0
0
I 25
I 50
I 75
I
100
J
’/.SALT FORM OF EXCHANGER Figure 5.
Variation of pH with percentage of exchanger in salt form
Open circles and closed circles are respective experimental values for quaternary base and weak base cellulose exchangers obtained by Hoffpauir-Guthrie method. Line through open circles colculated from titration curve for quaternary cellulose base completely regenerated from quaternary iodide, and other line is calculated from titration curve of weak base completely regenerated from DEAE-cellulose, both in 0.5M NaCl VI. standardized 0.1 M HCI
ever, in the present study, regenerated base samples were not allowed to airdry, conductivity water was used in all experiments, and nitrogen gas was used during all equilibration periods. Conductometric Titrations. D a t a in Figure 6 illustrate t h a t a weaklybasic cellulose anion exchanger can be titrated conductometrically. A typical graph, 0-A-B-C, for a regenerated basic form of DEAEcellulose in excess HCl us. 1 M NaOH is given. Point A , the intersection of the first two lines a t 0.72 meq. of NaOH, corresponds to the neutraliza-
MEQ. TITRANT/g. EXCHANGER Potentiometric titration curve for a quaternary bromide
Curve A i s titration o f the regenerated base equilibrated with 0.5M NaCl VI. standardized 0.1 M HCl. Curve B i s the titration curve o f the excess HCI and salt form with excess standardized 0.1M NaOH. Capacity was 0 . 7 4 3 meq. N/gram and 0.702 meq. Br/gram exchanger
tion of the excess HC1 not used in exchanging OH- ions. The difference between the total meq. of HCl added and the excess, 1.36 meq., gives the meq. of OH- ions exchanged by the 1.6947-gram sample, in this instance 0.81 meq./gram of exchanger. The second intersection point, B , a t 1.98 meq. of NaOH, represents the amount of base used for resin and excess HCl. The difference, 1.26 meq., or 0.74 meq. per gram of cellulose exchanger, also gives the capacity of the DEAE-cellulose. With the weak exchanger, both points A and B can be used to find the capacity of the exchanger. The other graph in Figure 6, X-Y-2, is that typical of the quaternary cellulose anion exchanger, It has only one point of intersection a t 1’. The capacity of the exchanger can be found only by a titration of the excess HC1 not exchanged by the strong base. I n this instance, an experimental capacity of 0.687 meq./gram compared to a theoretical capacity of 0.700 meq. per gram was obtained. There was no second break for the quaternary salt, as the C1- ions of the strong exchanger were not readily exchanged by the very dilute base used in these conductometric experiments. Therefore, the second segment of this graph is parallel to the third segment of the graph for the weak exchanger and denotes simply addition of OH- ions to the solution. I n addition, use of a quaternary iodide and a bromide in a conductometric titration us. 1 X S a O H gave, in each case, two intersecting lines corresponding to only 45% replacement of Br- ions and 5070 replacement of I- ions. These values are in agreement with amounts of halide ions displaced from quaternary salts by dilute base as shown by analytical data given in Table 11. VOL. 37, NO. 13, DECEMBER 1965
1697
DISCUSSION
For the quaternary cellulose anion exchanger, as with strong base anion exchange resins (Q), the titration curve is similar to that of a dissolved strong base except that the initial pH of the aqueous phase is less with the exchanger than with a soluble base, since some OH- ions of the basic groups are in the resin rather than in the aqueous phase. Nevertheless, the milliequivalents of original OH- ion per gram of resin are equivalent to the milliequivalents of titrant per gram of resin and there is a sharp change a t the end point in the titration curve only; before the end point, the pH is relatively constant. For the weak base cellulose anion exchanger, the pH of the aqueous phase changes noticeably even in the early stages of titration as H + ions are added and the equilibrium is shifted more toward completion (9). Nevertheless, there is a sharper change in pH at the end point where the resin is converted entirely to the salt form, and this end point gives the milliequivalents per gram of resin. The pK value of the exchanger cannot be obtained directly from the titration curve. First, the relationship between the pH of the aqueous phase and the pK of the ionogenic group must be established. It is only for weak base groups (9) that the following equation can be used :
The S d e n o t e s the pH within the resin and is not necessarily that in the external aqueous phase. For strong base exchangers, Equation 1 cannot be used; a t best, it gives only a lower limit to the pK value. Our findings with quaternary cellulose anion exchangers are analogous to the findings recently reported by Boyd (3). I n his work with weakly crosslinked strong base anion exchangers of the polystyrene quaternary ammonium type, he found large selectivity coefficients and little dependence of the coefficient on the equivalent fraction of the preferred ion in the exchanger. I n contrast with analogous work with cation exchangers, he found that the ion binding was quite strong between the anion and the structurally-bound tetraalkylammonium ion. Diamond (7) has explained Boyd’s observations in terms of a new type of ion pairing between large unhydrated univalent ions. This explanation is also applicable to selective bindings found with our cellulose quaternary anion exchangers. The tetrasubstituted ammonium ions on the cellulosic backbone do not attract water molecules as strongly, as the latter molecules are coordinated into the bulk of the water structure. Thus, the water 1698
a
ANALYTICAL CHEMISTRY
NaDH(meq.1 ADDED Figure 6. Conductometric titration graph of regenerated basic forms of cellulose exchangers in presence of excess standardized 0.1 M HCI vs. standardized 1 .OM N a O H line 0 - A - 6 - C for weak base DEAE-cellulose (1.6947-gram sample of 0.790 meq. N/gram exchanger in presence of 2.082 meq. HCI.); line X-Y-2 for quaternary base cellulose (0.8595-&ram sample of 0.72 meq. N/gram exchanger in presence of 1.04 meq. HCl)
structure around the hydrophobic ion is tightened as the large hydrophobic cation creates its cavity in water. Similarly, an unsolvated anion, such as a large iodide ion, also causes considerable loss in water-water interaction on entering solution. Therefore, there would be a tendency for the water structure to force a large anion, such as the iodide, and a large cation, such as the tetrasubstituted ammonium ion, into a single large cavity in order to decrease the disturbance to itself. This type of ion pairing, called waterstructure-enforced ion pairing by Diamond, is not primarily because of an electrostatic ion-ion interaction but is an association forced by the water structure. Such a theory explains the ease with which OH- ions in the regenerated quaternary cellulosic bases are replaced by halide ions, the difficulty experienced in this investigation in regenerating the basic form of the quaternary cellulose exchangers, and the ease with which the weak or hydrogen bonded cellulose anion exchangers are regenerated into the basic form. I n general, with quaternary cellulose
anion exchangers, the larger and more hydrophobic the anion, the greater the disturbances in the water structure, and the more readily the larger ion leaves the external solution and passes into the less structured resin phase solution within the quaternary cellulosic exchanger (6). I n addition, the highly hydrated hydroxyl ions prefer the external aqueous phase as their transfer to the internal quaternary ammonium resin phase necessitates a loss in hydration energy. Within the quaternary exchanger, the water structure is so badly disrupted that the internal water phase offers less opposition to the entrance of a large hydrophobic ion such as the iodide than does the external water solution. CONCLUSIONS
Quaternary cellulose anion exchangers, while not thermally stable a t very high temperatures, are stable chemically and show no hysteresis effects during several reversible cycles of exchange. Therefore they should be of value in various chromatographic applications, particularly in the field of
protein chemistry. The difference in ion exchange behavior of the weak and strong base cellulose anion exchangers centers predominantly about the ease with which the hydroxyl ion can be exchanged. The attainment of equilibria a t times is long and tedious. It is difficult to regenerate the basic forms of the quaternary cellulose exchanger from its halide, very easy to regenerate the basic form of the weak cellulose exchanger, and relatively simple to replace the hydroxyl ions from both the strong and weak cellulosic anion exchangers with less hydrated anions. ACKNOWLEDGMENT
Grateful acknowledgment is given to Lawrence E. Brown and Alva F. Cucullu for microanalyses.
LITERATURE CITED
(1) Berni, R. J., Benerito, R. R., McKelvey, J. B., Calamari, L. D., Am. Dyestuf Reptr. 54, 426 (1965). (2) Bio - Rad Laboratories, Richmond, Calif., ‘‘Ion Exchange Materials,” p. 20, 1961. (3) Boyd, G. E., Lindenbaum, S., Myers, G. E., J . Phys. Chem. 65, 577 (1961). (4) Champetier, G., Kelecssenji-Dumesnil, E., Montegudet, G., Petit, J., Compt. Rend. 242, 269 (1956). (5) Cheng, F. W., Microchem. J . 3, 537 (1959). (6) Chu, B., Whitney, D. C., Diamond, R. M., J . Inorg. 1vucl. Chem. 24, 1405 (1962). (7) Diamond, R. M., J . Phys. Chem. 67, 2513 (1963). (8) Elek, A., Harte, R. A., ANAL.CHEM. 9, 502 (1937). (9) Helfferich, F., “Ion Exchange,” McGraw Hill, New York (1962).
(10) Hoffpauir, C. L., Guthrie, J. D., Teztile Res. J . 20, 617 (1950). (11) Jakubovic, A. 0.. Brook. B. N.. Polymer 2, 18 (1961).‘ (12) Kunin, R., McGarvey, F. X., ANAL. CHEM.36(5), Pt. 1, 142R (1964). (13) McKelvey, J. B.,. Benerito, R. R., Berni, R. J., Burgis, B. G., Teztile Res. J . 33, 273 (1963). (14) McKelvey, J. B., Webre, B. G., Benerito, R. R., J . Org. Chem. 25, 1424 (1960). (15) Pet‘erson, E. A., Sober, H., J . Am. Chem. SOC.78, 751 (1956). (16) Porath, J., Arkiv Kemi 11,97 (1957). RECEIVEDfor review May 21, 1965. Accepted September 14, 1965. Presented in part before the Division of Cellulose, Wood, and Fiber Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965. Mention of a company and/or product by the Department does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.
Nuclear Magnetic Resonance Analysis of Alpha-Olefins P. W. FLANAGAN and H. F. SMITH Research and Development Department, Continental Oil Co., Ponca City, Okla. A method for the determination of olefinic impurities in a-olefin samples has been developed. The method is based on quantitative nuclear magnetic resonance spectrometry; and i s particularly applicable to the analysis of a-olefin samples containing 1,2-disubstituted ethylenes and 1 ,I disubstituted ethylenes. Samples containing a wide range of molecular weight species are examined with no prior separation and, indeed, no preparation at all. The method is relatively rapid; a complete determination requires about 30 minutes.
The NMR method has the advantage that signal positions and intensities are essentially independent of molecular weight; thus it is not necessary to separate a sample into homogeneous molecular weight fractions, nor to have
any knowledge of the molecular weights present. Additional advantages are lack of interferences, rapidity, the very small number of standard samples required, and the fact that the mole ratios of the olefin types are obtained directly.
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A
in interest in a-olefins as articles of commerce has led to several papers on the analysis of a-olefin systems (9, 6, 8). Saier, Cousins, and Basila (6) have developed an infrared method that uses gas chromatographic data to correct for changes in molar absorptivities with molecular weight. Suatoni (8) has described a lengthy hydrobromination procedure for the determination of normal aolefins. The analysis scheme used in a pilot plant operation has been described in a news release (2). This work was directed toward development of a simple, rapid method for determining the relative amounts of the olefin types present in a mixture that is predominantly a-olefin. The method utilizes the quantitative aspect of high-resolution nuclear magnetic resonance (KMR) spectrometry (3, 4). RECENT IKCREASE
Figure 1 .
Vinyl region of the NMR spectrum of pure 1 -hexene VOL. 37, NO. 13, DECEMBER 1965
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