(8) Hiskey, C. F., Ibid., 33,927 (1961). (9) Hoskins, A. L., Sherman, A. I., Allen, W. M., J. Biol. Chem. 182,429 (1950). (10) Hotchkiss, R. D., Zbid., 175, 315 (1958). (11) Jones, J. H., Clark, G. R., Harrow, L. S., J . Assoc. Ofic. Agr. Chemists 34, 135 (1951). (12) Ibid., p. 149. (13) Morris, W. W., Haenni, E. O., Zbid., 45, 92 (1962) (14) Morris, W. W., Wilkie, J. B., Jones, S. W., Friedman, Leo, ANAL.CHEM. 34.381 (1962). (15) Morton, R. A., Stubbs, A. L., Anal@
71,348 (1946). (16) Morton, R. A., Stubbs, A. L., Biochem. J . 41, 525 (1947). (17) Morton, R. A., Stubbs, A. L., Zbid., 42, 195 (1948). (18) Rotondaro, F. A., J . Assoc. Ofic. Agr. Chemists 40,824 (1957). (19) Schiaffino, S. S., Ph.D. thesis,
(22) Tunnicliff, D. D., Rasmussen, R. S., Morse, M. L., ANAL. CHEM.21, 895 (1949). (23) Wilkie, J. B., J . Assoc. Ofic. Agr. Chemists 32,455 (1949). (24) Wilkie, J. B., Ibid., 46, 920 (1963). (25) Wright, Norman, IND.ENG.CHEM., ANAL.ED. 13, 1 (1941).
1956. (20) Schiaffino, S.S., Loy, H. W., Kline, 0. L., Harrow, L. S., J . Assoc. Ofic. B g r . Chemists 5 9 , 180 (1956). (21) Tardif, Real, J . Pharm. Sci. 5 0 , 693 (1961).
RECEIVED for review September 30, 1963. Accepted January 21, 1964. Presented in art a t Pittsburgh Conference on Anayytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1961.
Georgetown Univ., Washington, D. C.,
Anion Exchange Separation of Calcium and Strontium JAMES S. FRITZ, HlROHlKO WAKI, and BARBARA B. GARRALDA Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
b A new anion exchange method for separation of calcium and strontium was developed using the mixed solution of 0.25M nitric acid and 95% (v./v.) methyl alcohol as the eluent. Magnesium, calcium, and strontium (total volume 1 mmote) were separated quantitatively from each other with a 22-1111. column of the Amberlyst XN-1002 nitrate form. Even the separation of the sample of Ca/Sr = 500 showed fairly satisfactory results for the strontium content.
A
a number of methods for the ion exchange separation of strontium(I1) from other alkaline earths have been reported (1, 2, 6-8), there is no paper on the use of a simple mineral acid as the eluent for an anion exchange method. It is advantageous to use a mineral acid that can be easily volatilized and does not interfere in the subsequent quantitative analysis of the fractions containing the separated elements. The separation depends on the selective formation of complexes in the resin phase between the metal ions and the anion of the eluting acid. For the alkaline earths, the eluent must contain a high proportion of an organic solvent in order to attain the necessary complex formation. I n a previous paper (4), the separation of magnesium and calcium has been accomplished by anion exchange using isopropyl alcohol-nitric acid. The separation of calcium and strontium seems to be much more difficult, because the separation factor in the ion exchange separation is not so large as in the case of magnesium and calcium. The possibility of alkaline earths separation has been outlined before (S), but a more precise and quantitative method was required. In this work, the separation of calcium and strontium is examined under various conditions and is accomplished using the mixture of LTHOUGH
900
ANALYTICAL CHEMISTRY
0.25M nitric acid and !%yo methyl alcohol. The mutual separation of magnesium, calcium, and strontium is also done by using the eluents, 0.25M nitric acid 9570 ethyl alcohol and 0.2544 nitric acid %yo methyl alcohol. The separation of barium is important. By preliminary tests, it should be possible to separate barium from other alkaline earth elements by a similar technique. This was not studied here, however, because of the extremely low solubility of barium(I1) in alcoholnitric acid media.
+
+
EXPERIMENTAL
Column. The anion exchange resin Amberlyst XN-1002 (Rohm & Haas Co.) is ground t o 60 t o 100 mesh and converted t o the nitrate form with nitric acid. This resin is covered with 95y0 methyl alcohol or 95% ethyl alcohol and then put in a glass column t o make a uniformly packed resin bed of 1 sq. cm. X 22 cm. It is convenient t o add about one to two ml. of excess resin because the resin
Figure
3,.
usually packs down a little during the early stages of elution. Reagents and Solutions. Nitric acid, alcohols, (ethylenedinitrilo) tetraacetic acid (EDTA), and other reagents are all reagent grade and are used without purification except when otherwise stated. Stock solutions of magnesium, calcium, and strontium are prepared by dissolving their nitrates in 5M nitric acid. Sample solutions are prepared by adding ethyl or methyl alcohol to these stock solutions, so that the 0.25-11 HN03 95% (v./v.) alcohol solutions are produced. The eluent is prepared by mixing one volume of 536 nitric acid with 19 volumes of methyl or ethyl alcohol and then adding water equal to the volume contraction resulting from mixing. In some cases magnesium and calcium are purified by the ion-exchange technique developed in this work, because impurities in these reagents may become significant when the sample mixture consisting of large amounts of magnesium and calcium and very small amounts of strontium is used in the separation experiments. Procedure for Ca-Sr. The column is pretreated by passing 40 to 50 ml.
+
EFFLUENT VOLUME, ML. Elution curve for the Ca-Sr separation in ethyl alcohol medium Column: 14 ml. (4 1 rq. cm.) Flow rate: 0.5 to 0.6 ml./min. Sample solution volume: 10 ml. Ca, Sr: each 0.1 mmole
+
of the 0.25M nitric acid 9501, methyl alcohol. T h m , 10 ml. of a sample solution (con ,aining less than 0.1 mmole of strontium) is added to the column and passed through at the flow rate 0.5 to O X ml./min. The volume of effluent if measured from this point. When no liquid is left above the resin bed, the elution is carried out continuoL sly by dropwise addition of the 0.25M nitric acid 95% methyl alcohol solution from a separatory funnel. The flow rate is always maintained 0.5 to 0.6 ml./min. The first 10 ml. of effluent is discarded because it contains no metal ions. The next 80 to 100 ml. (depending on Ca/Sr ratio) is collected for the determination of calcium. Then the eluent is changed and stron:ium is stripped with 50 ml. of 9574 methyl alcohol or water a t the same flow rate. Procedure for Mg-Ca-Sr. The column is pretreated b y passing 40 t o 50 ml. of the 0.25M nitric acid 95oJ, ethyl alcohol and then 20 t o 30 ml. (depending on the amount of dissolwd 9olutc.s) of a sample solution through t h r column, followed bv a continuous elution with the 0 25M nitric acid 95% ethyl alcohol solution at a flow rate 0.5 to O.(? ml./min. The first 10 ml. of the effluent is discarded, and the next 70 ml. is collected for the determination of magnesium. Then, the eluent is changed and calcium is eluted with 70 to 100 ml. (depending on the Ca/Sr ratio) of t i e 0.25M nitric acid 95% methyl alcohol. Finally strontium is stripped with 50 ml. of 95% methyl alcohol or water. After stripping strontium, a thorough washing of the column with dilute nitric acid and water is sometimes desired because impurities possibly remaining on the coluinn might cause error in the determination of small
L Q 2 ,
I
I
MeOH -95%
H&+95%
1
I
I
MeOH
+
+
+
+
Table
Ratio 1 1
1
5
5 1
100 100 1 300 300 1
300 1
200 400
Mg
Ca Sr
Mg
Ca Sr
Mg
Ca
Sr Mg
Ca
Sr
Ca
Sr
Mg
Ca
Taken, mmole 0.0997 0.0990 0 I0991 0.25 0.2.5 0.0504 0.499 0.498 0.0056 0.495 0.4826 0.0016 0,231.5 0.00075 0.1710 0.3658
1
Sr
Ca
0.2311 0,00050
Ca
0.5 0 . .i
Sr Mg
Sr
E t : 0.25M HNOI
Figure 2.
Elution curves for the Ca-Sr separation in methyl alcohol medium Column: 2 2 ml. (+ 1 sq. cm.) Sample volume: 10 ml. Flow rate: 0.5 to 0.6 ml./min. Ca, Sr: each 0.1 mmole
Table 1.
Comparison of Various Eluents
- Solution composition
"03, M
Alcohol, %
1
80 iso-
0.5
80 iso-
1
1
1 0.5
1 0 .5
0.25
D,.S,
Vm,,. Cab volume column
De.ca'
V r n . Y * Rr
column volume
sc
PrOH
3.5
11.5
3.3
PrOH 70 isoPrOH 80 EtOH 85 EtOH 90 EtOH 85 MeOH 90 MeOH 95 MeOH
2.1
4.3
2.1
1.6
2.9
1.25 2.41 4.85 0.48 0.64
4.44 9.77 13.6 2.28 2.54
1.8
3.0 3.7 2.6 3.3 3.0 4.0
1.57 6.29 metal amt./meq. of resin. De = -Distribution ratio D,is defined as: metal amt./ml. of soh. V,,, indicates the effluent volume a t maximum concentration of metal. Separation factor is calculated from the following equation: s = De, ___sr 0 32 V D L L D e .a, 0.32 Or = V mnx. . ra' __ See the previous paper ( 4 ) .
+ +
Quantitative Separation of Strontium from Calcium and Magnesium
500 1 500 500 1 a
Element
II.
EFFLUENT VOLUME, ML.
0.00094
0.0010
+ 9594 ethyl alcohol.
Found, mmole 0.0992 0.0988 0.0995 0.0502 0.498 0.496 0.0055 0.496 0.481 0.0015 0.2315 0.00083 0.1702 0.3662 0.00098 0.2316 0.00058 0.0012 Me: 0.25M HN03
Error, mmole -0.0005
-0.0002 +O ,0004
Sample solution vol . 30 ml.
-0.002 -0.0001 +O. 001 -0.0015 -0.0001 0.0000 +O ,00008 - 0.0008
+0.0004 +O ,00004 +O. 0003
+o. 00008
+o ,0002
Vol. a t which the eluent is changed, ml. E t 80 Me 70
20 ml.
E t 10-70 Me 0-70
E t 80 Me 100
25 ml.
E t 10-60 Me 0-70
E t 80 Me 100
25 ml.
E t 10-80 Me 0-70
Et 90 Me 80
15 ml.
Me 10-85
Me 105
25 ml.
Et 10-70 Me 0-80
E t 90 Me 100
15 ml.
Me 10-90
Me 100
25 ml.
E t 10-80 Me 0-70
E t 90 Me 80
- 0.0002 -0.001
Fraction taken for detn., ml. Et" 12-70 Me 10-60
+ 95% methyl alcohol. VOL. 36, NO. 4, APRIL 1 9 6 4
901
quantities of strontium later when the same column is employed. All column operations are carried out at the room temperature 24' =k 1'C. Determination. Standard E D T A titration methods using Eriochrome Black T indicator are employed. Magnesium is titrated directly with 0.01M E D T A , and calcium and strontium are titrated in the presence of Mg-EDTA. The effluent for each metal is evaporated t o remove alcohol and excess nitric acid, or an aliquot (10 to 20 ml.) of the effluent is taken for the titration without evaporation. For comparison, the standard solution containing the same amount of each metal as the sample is titrated under the same conditions as the eluted sample solution. In making elution curves, each 10 ml. fraction of the effluent is analyzed individually. For small amounts of strontium, a spectrophotometric method using ocresolphthalein complexone is used. The total fraction for strontium or its aliquot is evaporated and dissolved in water. After adding the ammonium chloride-ammonia buffer solution (pH 10.6) and the indicator solution, the color is measured a t 575 nip using Beckman Spectrophotometer Model B. Although the calibration curve deviates from a straight line, it is reproducible. RESULTS AND DISCUSSION
Calcium-Strontium Separation. Amberlyst XN-1002 resin has been recommended for separations from nonaqueous solutions because of its rapid sorptive rate. Use of this resin 3s advantageous when aqueous-alcohol mixtures containing a high proportion of alcohol are employed. Generally the sorbability of metal ions by a given resin depends mainly on the acid (electrolyte) concentration, and on the concentration and kind of organic
I
I
---0.25M HN03+95% EtOH-
--
I
I
I
- --0.25MHN03t95% MeOH - - -
EFFLENT VOUIME,ML. Figure 4.
Elution behavior of M g and Ca in heavy loading Cdumn: 2 2 ml. (6 1 rq. cm.) Sample volume: 25 ml. Flow rate: 0.5 to 0.6 ml./min. Mg, Ca: each 0.5 mmole
solvent. Changing the nitric acid concentration does not seem to change appreciably the separation factor of similar metal ions; instead it results in a parallel change of sorbabilities of these ions. In the previous magnesiumcalcium separation (4), higher alcohols did not give better separations than did the lower alcohols. Furthermore the sorptive rates are slower with higher alcohols and the sorbability of calcium is too high for rapid elution. For these reasons, either methyl or ethyl alcohol seemed best to use in the separation of calcium and strontium. All preliminary experiments using isopropyl alcohol were unsuccessful. Among the eluents containing ethyl alcohol, the best separation was obtained with I A l nitric
+
acid 85% ethyl alcohol. Although calcium and strontium could be separated quantitatively here, the elution curve (Figure 1) still shows some tailing and a fairly long time is taken for the elution of calcium. The combination of a higher alcohol content and a lower concentration of nitric acid might result in a better separation. However, if 95% ethyl alcohol is used, the acid concentration should be lowered below 0.1M to elute calcium within a reasonable time. Such a low acid concentration might lead to unreproducible elution behavior because a small change in the amount of solute will affect the elution appreciably. Better results were obtained using methyl alcohol. It is less viscous than ethyl alcohol and gives a larger solubility for metal ions. Especially 0.25M nitric 95% methyl alcohol gave good acid results (Figure 2, Table I). Calcium can be eluted in 2 or 3 hours, and there is a reasonable blank range. If magnesium is present in the sample solution, it should accompany calcium. In a practical case, when the separation must be done under the different conditions such as increased sample volume, different flow ' rate, different column dimensions, etc., a check on the completeness of elution is recommended. Magnesium Calcium - Strontium. One may come to the conclusion t h a t the same conditions employed in the previous Mg-Ca separation should be used for the first step of Mg-Ca-Sr separation. Although a 10-ml. column is adequate for Mg-Ca separation, i t is too short for the next step (Ca-Sr separation). As mentioned in the former section, a 22-ml. column is required for its complete
+
-
Figure 3.
EFFWENr voum,m. Elution curve for Mg-Ca-Sr separation Column: 22 ml. (6 1 rq. cm.) Flow rate: 0.5 to 0.6 ml./min. Sample solution volume: 30 ml. Mg, Ca, Sr: about 0.1 mmole each
902
0
ANALYTICAL CHEMISTRY
4
separation. Use of such a large column is advantageous bedause i t may permit use of a large amount of metals and large volume of a sample solution. Sometimes it is necessary to increase the volume of the sample solution because strontium has a compitrably low solubility in ethyl alcohol-nitric acid media. Here the 0.25iW nitric acid 95% ethyl alcohol eluent is used to elute magnesium instead of isopropyl alcoholnitric acid. It is also a satisfactory eluent for the separai,ion of only magnesium and calcium. As shown in Figure 3, there is no tailing of magnesium and there i3 a considerable blank range before breakthrough of calcium. Even if a total of 1 mmole of magnesium and cdcium is treated,’ the separation is quit: safe (Figure 4). For the following Ca-Sr separation, the eluent, 0.25M nitric acid 95y0 methyl alcohol, is employed. The ethyl alcohol remaining in the column after changing the eluent does not give any trouble to Ca-Sr separation. Strontium is stripped
+
+
with 50 ml. of 950/, methyl alcohol or water. Results of calcium-strontium and magnesium - calcium - strontium separation are given in Table 11. The results were quantitative when each element was present in a comparable order of magnitude in the sample. They were also fairly satisfactory for the sample in which the amount of strontium is extremely small compared with those of magqesium and calcium. For the strontium determination in such cases, high precision may not usually be required. However, in treating small amounts of strontium, a thorough washing of the used column may be necessary prior to the separation, and the alteration of eluent should be made as late as possible (closest to the breakthrough of strontium) to minimize the interference by calcium tailing. Wh‘en extremely high precision is required for strontium or when a much larger ratio of Ca to Sr must be handled, the use of a longer column than 22 ml. is recommended.
On the other hand, when a much larger amount of the solute and therefore much larger volume than 30 ml. of sample solution is treated, a bigger column of the same length can be employed. By increasing the flow rate, the separation can be accomplished in the same time, provided the number of plates in the column is kept constant. LITERATURE CITED
(1) Davis, P. S., Nature 183,674 (1959). ( 2 ) Eulitz, G., Nukleonik 2, 85 (1960). ( 3 ) Fritz, J. S., Garralda, B. B., un-
published data.
(4) Fritz, J. S., Waki, H., ANAL CHEM.
35, 1079 (1963). ( 5 ) Honda, M., Bunseki Kagaku 3, 132 (1954). ( 6 ) Nelson, F., Kraus, K. A., J . Am. Chem. Soc. 77,801 (1955). (7) Tsubota, H., Kitano, Y., Bull. Chem. SOC.J a p a n 33,770 (1960). (8)Wunsch,. L.., Chem. Listv 51, 376 (1957).
RECEIVED for review November 20, 1963. Accepted January 8, 1964.
Monitoring Countercurrent Distribution with a Recording Refractometer R. 0.BUTTERFIELD
and
H. J.
DUTTON
Northern Regional Research laboratory, Peoria, 111.
b Deterrents to greater use of countercurrent distribution are the labor and time required for determining weight distribution. These usually can be eliminated by the US(? of a sensitive differential recording refractometer to monitor effluents either (a) as they are discharged to the fraction collector or (b) as they are reifitroduced to the first tube of the irain during the recycle operation. Under (a), the single withdrawal operation, one has a concentration versus volume curve immediately available to integrate for analysis or to guide the combination of tubes for preparative purposes. Under (b), the “recycle“ operation, one can observe the progress of fractionation of solutes, determine the maximum length of recycle, and decide when conversion to single withdrawal operation is desirable. Necessary servo-, pumping, and control mechanisms, which automatically change the refractometer range, transport solutions, and program operations are described.
c
distribution (CCD) is frequently the analytical and preparative method of choice for a OUNTERCURRENT
variety of substances difficult to separate, particularly those that are nondistillable, heat labile, or otherwise not amenable to gas liquid chromatography. One disadvantage of the technique, however, has been the lack of an automatic detection device comparable to the catherometer (or the gas density balance) for gas chromatography. The universal method of detection for CCD has been a laborious process: solutions from individual extraction tubes are evaporated in tared flasks, the residue of each is weighed, and a weight distribution curve is established. Refractive index measurements determine the concentration of solutions, particularly those dilute solutions for which refractive index varies linearly with concentration. Within a related group of compounds, such as the higher fatty acid methyl esters whose individual refractive indices vary within small limits, the refractive index of their solutions may be used as a measure of the concentration or the “total solids content.” By observing changes in this colligative property of matter (the refractive index), one has the elements of a universal linear detector for
monitoring eluates from CCD trains. Differential, recording, continuous flow refractometers of high sensitivity (10-6 to IO-’) have recently become available commercially and are adaptable to this function. This presentation is divided into a description of the necessary modifications in instrumentation and a discussion of selected applications in CCD. EXPERIMENTAL
Modifications of Refractometer. The continuous-flow, differential recording-type refractometer used in this work has a sensitivity of four units in the sixth decimal place. The Phoenix Instrument Co., Philadelphia, Pa., makes such a refractometer, although mention of this company and its products does not constitute a n endorsement by the Department of Agriculture over other firms making similar products. The instrument works on the principle of following a refracted light beam with an octagonal beam-splitting prism whose apex is held on the light beam by a servomechanism. A strip-chart recorder indicates the position of the octagonal prism. Since the change in refractive VOL 36, NO. 4, APRIL 1 9 6 4
903
