Cation Exchange Elution of Magnesium Ion by Hydrochloric and Perchloric Acids CHARLES
K. MANN
Department o f Chemistry, Florida State University, Tallahassee, Fla.
b A detailed investigation of the elution of magnesium(l1) by perchloric and hydrochloric acids from sulfonic acid cation exchange resin i s reported. Quantitative data are presented to describe the resin-solution distribution of eluate, the effective height of a theoretical plate, and the eluate band width as a function of eluent concentration for hydrochloric acid and for perchloric acid elution. Data concerning variation of distribution coefficient with eluate concentration and with flow rate are given. The effect of variation in resin cross linkage is described in terms of distribution coefficient and theoretical plate height for elution of magnesium(l1) by hydrochloric and perchloric acids, ammonium chloride, and ammonium perchlorate.
W
chemical methods for the determination of magnesium, whether gravimetric. titrimetric, or colorimetric, are subject to numerous interferences n hich often necessitate prior separations. Separation techniques frequently used-e.g., precipitation, extraction, and electro1)-sis-generally involve removal of specific interferences from the sample, possibly leaving magnesium in a solution of high ionic strength and uncertain composition. Use of a cation exchanger involves a reaction of magnesium, permitting the removal of that ion from the bulk of the sample. Subsequent elution of the metal from the resin yields a solution of known composition. Hydrochloric acid is of interest as a n eluent because of the slight tendency of magnesium to form chloride complexes as compared with many other cations. Perchloric acid is of interest because of its slight tendency to form complexes uith any of the cations. Thus with hydrochloric acid, magnesium may be expected to show a stronger prcference for the resin than do many other cation;. The reverse would be wpected to be true with perchloric acid. Ohtaki and Yamasaki (7) have studied the static equilibration of macro amounts of magnesium between hydrochloric acid solutions and a n 8% divinylbenzene IDS'B) cation exchange resin. ET
Diamond (1) has studied the hydrochloric acid elution characteristics of tracer concentrations of beryllium and the alkaline earths from resins of varying cross linkage. This report describes a detailed investigation of the elution of magnesium ion by hydrochloric and perchloric acids. I n doing this, an attempt has been made to study effects due to equilibrium changes and those attributable to variation in the rate of reaction. For the former, one must consider eluent concentration, magnesium ion concentration. and resin cross linkage. These parameters are important because of mass action effects and because of their influence on the activities of the reactants in both phases. Concerning reaction rate, the nature and concentration of the eluent will affect the diffusivity of the exchanging ion per se and \Till alter the resin matrix through n hich it must move. A similar effect is obtained by varying the resin cross linkage. EXPERIMENTAL
Resin. Dowex 5OW sulfonic acid cation exchange resin of analytical grade was obtained from Bio Rad Laboratories, Richmond, Calif. Samples of 2, 4, 8, 12, and 16% DVB, 100- to 200-mesh dry, were used as received. Reagents. All reagents were of analytical reagent grade and were used without further treatment except magnesium perchlorate, which was recrystallized once. Magnesium solutions were standardized by titration (ethylenedinitri1o)tetraacetic with acid and acid solutions were standardized by titration with a base. Potassium chloride solutions \vere prepared directly from the dried reagent grade salt. Ammonium chloride and perchlorate solutions were prepared and standardized by boiling with an excess of standardized base and back-titrating the excess base. Columns. The columns used had 1.0-sq. cm. cross-sectional areas with sintered-glass disks sealed a t the bottom. For elution with 0.5M hydrochloric acid, a similarly constructed column of 0.30-sq. cm. area was used. Resin bed depths in this work ranged from 5 to 14 cm. Columns were packed by adding resin in a water slurry t o the desired depth. The bed was equilibrated with approximately 20 column volumes of the
Table I. Effect of Flow Rate Variations (Elution of 0.100.1f MgCl? from 12% DVB resin) DistrihuFlow tion Rate, Coeffi- EHTP, Ml./Sq. Eluent cient hIm. Cm./hlin. 1AIHCl 9.21 0.91 1.27
103f HC1
9 16 9.lG 9.06 0 21 0 27
0.28
0 GO
0.6i 0.48
G 1 4.6
5.1
0.70 0.36 0.18
0.74 0.28 0.18
eluent in question and then backwashed with the same solution. As the resin settled after backnashing, the column was tapped with a mechanical vibrator t o facilitate formation of a n evenly packed bed. Resin in the potassium or ammonium form 11as prepared by displacement of hydrogen mith a solution of the appropriate chloride salt until the effluent p H equaled that of the influent. Excess salt was aashed out and colunins n-ere prepared as describecl above. Elution Technique. Prepared columns rvere loaded with 1.00 ml. of solution (0.30 ml. for 0.5Jf hydrochloric acid) containing the eluate and the same concentration of eluent t o be used. After allowing the initial volume t o drain through, the column above the bed was rinsed and the washings were discarded. Elution was carried out in a room t h a t showed maximum temperature variations from 24" to 28" C., using a drop-counting fraction collector. A Mariotte-type reservoir mas used to maintain constant head and the f l o ~rate was controlled with a stopcock a t the bottom of the column. After the system reached equilibrium, the drop time n ould usually vary no more than 0.5 second. The column tips used gave drop times of 3 to 15 seconds for the flow rates used (Table I). Fraction volumes were determined by TT eighing, usuallS7 every fifth or tenth fraction. Effluents were analyzed for magnesium colorimetrically using a Bausch and Lomb Spectronic 20 spectrophotometer (6), and for hydrogen using a Beckman Model G p H meter. Free column volumes were determined by passing a band of sulfuric acid, potassium sulfate, or ammonium sulfate of the same normality as the solution used to equilibrate the bed. The VOL. 32,
NO. 1 ,
JANUARY 1960
67
breakthrough of sulfate was detected by allowing the efRuent to drain into a previously weighed barium chloride solution. A second weighing permitted the calculation of free column volume. Calculations. Distribution of t h e sorbate between resin and solution phase is described in terms of the distribution coefficient, a dimensionless number defined a s : moles per milliliter of resin D = moles per milliliter of solution’ and meaiured
-
(effluent volume to band peak) (free column volume) bed volume
Eluate band widths are described in terms of the effective height of a theoretical plate (EHTP), calculated according to the method of Glueckauf ( d ) . The number of theoretical plates in a bed may be expressed by the following relationship: S = S(V/p)*, where S is the number of plates, V is the peak volume, and p is the band width in units of volume measured a t l / e of the maximum eluate concentration. To emphasize effects due t o the overall rate of the exchange reaction as distinguished from effects due to equilibrium changes, normalized band n idths, B(d)-1’2 where d is the resin bed depth, are used. This can be done because all esperiments, with the exception noted, involved columns of the same crosssectional area. Band shapes are described for some representative runs by giving the elution constants pertaining to band head and tail, Ehand E,. E is defined as the ratio of bed volume to effluent volume (4). DISCUSSION
Comparison of Hydrochloric and Perchloric Acid Elution. The variation of distribution coefficient with hydrochloric acid concentration for elution of 0.10M magnesium chloride from 2 and 127, DVB resin is shown in Figure 1. The results are qualitatively similar to t h e behavior of beryllium in hydrochloric acid reported by Diamond (1). The distribution coefficient for the 27, DVB resin is less in dilute acidic solutions than for the 12% DVB resin because of the smaller concentration of exchange sites in the resin of lower cross linkage. The decrease in distribution coefficient with increase in hydrochloric acid concentration may be due to a combination of the mass action effect and the formation of a chloride complex of magnesium Ohtaki and Yamasaki ( 7 ) postulated a chloride complex to explain their results. The difference in slopes of these curves and the crossover a t high hydrochloric acid concentrations reflects the greater shrinkage of 2Y0 DVB resin as the ionic strength increases. 68
4
ANALYTICAL CHEMISTRY
OS
10
20 MOLARITY
10
40 6 0 HCI
Figure 1 . Elution of 0.1 OOM magnesium chloride by hydrochloric acid
08 I
I
!
2 MOLARITY
I
I
4 6 HCI O 4
8
Figure 2. Elution of 0.100M magnesium perchlorate by perchloric acid The relationship between distribution coefficient and perchloric acid concentration for elution of 0.10M magnesiuni perchlorate from 2 and 12% DVB resin is shown in Figure 2. I n this case the curve for 2% DVB resin reaches a minimum a t 6M and is slightly higher a t 8M perchloric acid. This, together with the fact that the distribution coefficient is significantly higher a t all concentrations for perchloric than for hydrochloric acid, suggests that complex formation is not a n important factor in perchlorate solutions. However, under identical conditions except for small differences in flow rates, the normalized band widths (Table I) for the elution of magnesium from 127, DVB resin are about the same for dilute hydrochloric as for dilute perchloric. By contrast, the values for 4, 6, and SAMperchloric are increasingly greater than those for the corresponding concentrations of hydrochloric. The same is true for elution from 27,
DVB resin. The increasing band width a t higher acid concentrations is caused by a decrease in the over-all rate of exchange coupled with a constant flow rate of solution through the bed resulting in a spreading of the sorbed band. Assuming the process is particle diffusion controlled a t these concentrations i8), then the rate-determining step is diffusion of the magnesiuni ion through the resin particle, as it has been established that divalent ions diffuse more sloivly than monovalent ions (9). The rate of diffusion of the magnesium ion exchanging for hydrogen in strong hydrochloric acid solutions is substantially greater than the rate of diffusion of magnesium exchanging for hydrogen in perchloric acid solutions of the same concentration. Polarographic measurements have shown that the rate of diffusion of chloro complexes is higher than that of corresponding aquo- ions ( 5 ) . It seems reasonable, therefore, to suggest the esistence of a chloro complex in concentrated hydrochloric and of aquo- ions in perchloric and dilute hydrochloric acid. The curve for perchloric acid elution from 12% DVB resin does not show a n upturn a t high perchloric concentrations. Probably, it would have done so if the experiments, Tyhich n ere discontinued because of limited solubility of magnesium perchlorate in strong perchloric acid, had been continued a t higher acid concentrations. Elution constants for band head and tail as a function of acid concentration, shown in Figures 3 and 4, allorr an approximation of breakthrough and band tail volumes for these two eluents. I n addition, they emphasize the effect on EHTP of variation in acid strength. If the distortion inherent in logarithmic plotting is taken into considerationLe., the separation betneen Eh and E , for 2% DS‘B a t low acid concentration is magnified-the separation of the Eh from the E t curve is a n approsiniation of the EHTP. Thus, 15 ith 1 . O M hydrochloric acid a t approsimately similar flow rates, E H T P of about 0.7 mm. for 12% DS’B and 1.7 mm. for 2% DVB nere obtained. With 1 2 X hydrochloric, the values are 21 mm. for 12% DYB and 5 min. for 27, DVB. Large values of E H T P are associated with very pronounced band tailing resulting in a n increased rolunie of solution containing the hand. Small values of EHTP obtained with dilute hydrochloric result from slon- movement of the band through the resin bed and correspond to large elution volumes and considerable dilution of the sorbed ion. -1s a measure of the extent of dilution in the eluate, the normalized band width is more meaningful than the E H T P because the latter is affected both by the band width and by the peak elution volume. Xornialized band nidth as a
Table II. Normalized Elution Band Widths [Elution of 0.100M MgClz or Mg(ClO&]
Resin
Eluent HC1 HCl HCIOd HClOi
x12 x2 x12 92
0
5
1
2
MOLlRlTY
4 H CI
2
1 9.4 4.0 9.8 3.9
2.7 2.7 3.2 2.3
Acid Concentration, 4 6 1.1 1.0 1.1 0.9 2.0 ‘2.7 1.6 1.7
10
Figure 3. Elution constants for band head and tail 0.1 OOM magnesium chloride eluted b y hydrochloric acid
I
“ I
’
15IO-
8 y HCI
-*
08-
01
06 -
02
03
04
05
INITIAL M g MOLARITY
z c
2
Figure 5. Change in magnesium concentration
04-
z 0
Elution of magnesium chloride from 12% DVB resin by hydrochloric acid
z 023 J
Table 111. 0061 /’
. I
I
2 MOLARITY
4
I
I
6
8
HC104
Figure 4. Elution constants for band head and tail 0.1 OOM magnesium perchlorate eluted b y perchloric acid
function of acid concentration is s h o m in Table 11. Variation in Amount of Exchanged Ion. Dependence of elution characteristics on t h e amount of sorbate is illustrated in Figure 5 . These variations in elution volume occur when a n appreciable fraction of t h e resin bed is occupied by t h e sorbate. Conditions leading t o low EHTPe.g., low sorbate concentration and high distribution coefficient-which are most satisfactory for difficult separations, cause greatest variation in elution volumes with sample size. This variation in distribution coefficient is caused by a change in breakthrough volume of the band; the elution volume of the band tail remains essentially unchanged. This is illustrated in Table 111,in IThich the breakthrough elution constant varies by about a factor of two IThile E t changes little. The data of Table I11 pertain to the experiments described for curve 1of Figure 5. This same behavior obtains when the sample volume is
Change in Eluate Concentration
Initial M g
Molarity 0.0174 O.OX70 . 0.100 0.348 0.522
Eh 0.11 0.14 0.14 0.18 0.20
Table IV.
16 12 --
Et 0.072 0.068 0.065 0.074 0.073
12 -~ 2
16
12 16 12 16 12
8 1.7 0.9 4.3 3.2
10 1.8 1.3
12 2.8 1.7
conditions of this n-ork is s h o n n in Table I. Resin Cross Linkage. A comparison of elution characteristics of resins of different cross linkage is shown in Table IV. For elution of niagnesium chloride by hydrochloric acid, t h e normal increase in selectivity is observed for 2 , 4,8, and 12% DVB resins. The increase in D is accompanied by successive decrease in EHTP, reflecting decreased band speed and consequent increased elution volumes. Going from 12 to 1670 DVB, the distribution coefficient shows only a slight increase while the EHTP shows a definite increase-that is, a reversal of the trend. Elution of magnesium ion by perchloric acid from 12 and 16% D’I’B resin in the hydrogen form shows a definite reversal in both D and EHTP. The same is true for elution of magnesium ion by ammonium chloride and by ammonium perchlorate from the ammonium form of the resin. Reversal of normal elution order for hydrogen and the alkalies on very highly cross-linked resins 11as observed by Gregor and Bregman (3). This reversal T\ as attributed to removal of part of the ionic hydration shell. Because hydrogen is normally more highly hydrated
Effect of Resin Cross Linking on Elution
D
Sorbate
Eluent 0.99X HC1
0 . l O O M lIgCl2
2 . O O M HClO4
0 , 100AVM g ( ClO4)2
1.13M NH4ClOr
0 . lOOM \Ig( CIO4)z
0.99Jf T’U”4C1
0 . lOOM MgC12
1.O O M KCl
0.12-M HC1
8
4 2 16
Molarity
changed, the concentration remaining the same. Flow Rate. Eluate flow rate is one of the factors which must be considered in elution chromatography. T h e effect of flow rate variations on t h e elution of magnesium under t h e
9.3 9.2 8.1 3.6 2.8 3.7 4.0 1.9 IO. 1 12.1 5.4 6.2 3.2 2.6
EHTP, Rlm. 0.9 0.7 0.9 1.2 1.7 1.3 0.6 1.2 1.3 0.8 2.3 0.8 0.6 0.5
Flow Rate.
Ml./Sq.
Cm./Riin. 0.5 0.4 0.4 0.5 1.0 0.4 0.4 0.5 0.4 0.4 0.5 0.5 0.4 0.5
than the (crystallographically) larger ions, i t vias felt that it n-ould be relatively more strongly affected by dehydrating conditions. An experiment in n-hich potassium chloride is used to elute hydrogen ion from resin in the potassium form shows (Table IV) that VOL. 32, NO. 1, JANUARY 1960
69
under the conditions of the present work, the effective radius of hydrogen ion is apparently affected more than that of potassium, as should be the case to agree with the results of the work cited above. Accordingly, values of D for 16% DVB smaller than those for 12% DVB resin indicate that both hydrogen and ammonium show an increased affinity relative to magnesium. This would be caused by a larger relative decrease in effective ionic radius, due to stripping Of the hydration Of the ion as compared with the divalent ion.
(4) Iirau>, I
Ion Exchanger Cellulose citrate CMC-30 Cellulose powder Phosphorylated cellulose CMS-cellulose Oxidized cellulose
-I,
47
45
114
112 48
50
220
200
220
220 200
220
commercial cation eschangers were used. These experiments showed that QAC cations \yere exchanged so strongly that they could not be removed by eluting acids such as hydrochloric, nitric, or sulfuric acid in n-ater, ethyl alcohol, or acetone. The interacting forces of the ion exchanger n-ith the QXC long carbon chain and with the highly basic ammonium group apparently n’ere too strong to be disrupted by an ordinary ion exchange technique. KOreports on this observation, hon-ever, could be found in the literature. Cellulosic materials have been described by several authors as neak cation exchangers (1-3). Several cellulosic ion exchangers were available in this laboratory and u ere investigated. Cellulose powder, cellulose citrate, carboxymethylcellulose, phosphorylated cellulose, oxidized cellulose, and partially carboxymethylated cellulose (CPI IS-cellulose) will exchange QAC cations from aqueous and nonaqueous solutions, either neutral or alkaline. The QAC was quantitatively recovered from the cellulose columns with alcoholic hydrochloric acid. The partially carboxymethylated cellulose (CIlScellulose) was the most effective ion exchanger. An analytical procedure was developed using the ChIS-cellulose colunin for removing and concentrating QXC from solvent estracts and other solutions. The extract or solution is passcd through the column, removed lyith alcoholic acid, and measured colorimetrically by a method similar to that of Wlson (8). EXPERIMENTAL
Apparatus and Reagents. SPECTRO-
