B. Further proof that dimer is not extracted by the vacuum-extraction method was obtained by placing a known amount of purified dimer in a sample tube and vacuum-extracting it for 2 hours at 200' C. and a t 1 mm. of mercury pressure. This procedure did not remove any detectable amount of dimer from the sample tube, as determined by weight loss of the sample tube, and no condensed material was found in the cold trap. Because vacuum extraction will not extract dimer from polymer samples, n-hereas water extraction does, the difference in results obtained by the
two procedures on a given sample indicates the amount of dimer, plus any mater-soluble, linear molecules present in the sample.
and 0.4iyc moisture was found with standard deviations of 0.03 and 0.06Ycj respectively. LITERATURE CITED
PRECISION
A statistical study was carried out on 16 analyses on each of two flakes of different monomer and moisture contents. I n the higher range, a n average of i.99% monomer and 1.70% moisture was obtained with standard deviations of 0.26 and 0.25%, respectively. For a polymer of lorn monomer and moisture content, an average of 0.53% monomer
(1) Hanford,
W. E., Joyce, R . RI., J . Polymer Sci.3, 167 119.18). (2) Hymans, P. H., Rer. trav chini. 72, /98-812 (1953).
(3) Velden, P. F. van, Want, G. 11. van der. Heikens. D.. KI&sink. Ch. .I.. Hermans, P. H., Staverman, A . J.; Ibid., 74, 1376-94 (1955).
RECEIVED for revie% September 4, 1956. Accepted January 8, 1957.
Applications of Anion Exchange Resins to Determination of Boron JOHN D. WOLSZON' and JOHN
R.
HAYES
The Pennsylvania State University, Universify Park, Pa. WILLIAM H. HILL Graduate School o f Public Health, University of Pittsburgh, Pittsburgh, Pa.
,The behavior of boric acid and of several known interferences has been investigated using both weak and strong base anion exchange resins. A simple, rapid chromatographic technique employs chloride ion as an elution indicator, applicable to the concentration and separation of boron from dilute solutions of low total salt content. Boron is separated from most of the known titrimetric interferences by a mixed bed ion exchange column. The procedure is rapid, requires no special treatment of the sample prior to the exchange process, and compares favorably with existing techniques. However, it cannot be recommended for separation of boron prior to colorimetric determination.
S
rapid and accurate procedures are available for the deterniination of boron. but there is no single mcthod of suitable accuracy which can 11c applicd to a variety of sample types without the prior wparation of boron from its determinative interferences. Scyarativc techniques which have been employed include ion exchange, previpitation, extraction, distillation, paper chromatography, and electrolytic proEVERAL
Present address, Marshall College, Huntington, W. Va.
cedures. Of these, only the distillatioii of boric acid as the methyl ester and the ion exchange procedures have shown general applicability. However, the methylborate distillation technique is time-consuming and tedious, and gives slightly low boron recovery even when multiple distillations are carried out. The utilization of ion exchange resins for the separation of boron has been confined, with one exception (8),to the separation of boron from cationic interference by retaining the cations on a strong acid cation exchange resin while the boric acid and other anions pass into the effluent. Such procedures are not applicable to samples for titrimetric determination which contain reasonable concentrations of anionic interferences whether these are of the buffering or complexing nature. $1~0, cations forming anionic complexes might not be removed by the cation exchange resin. A strong acid efluent is obtained 1%hich, on neutralization, produces a high ionic strength medium and thereby decreases the accuracy of a titrimetric process involving the use of color indicators. STRONG BASE RESINS
I o n Exchange Chromatography. Two general methods use strong base resins t o separate boron from
its inteifeiences. The first of these renioyes all anions fiom t h e sample and quantitatively elutes them in ordei of their increasing affinity foi the iesin. The second uses t h e resin in a n anionic form such t h a t the boric acid from the sample solution is not held by the resin, n hik the, interferences are retained. Schutz ( 8 ) reports the use of strong base anion exchange r e m s for the quantitative separation of boron. He uses the resin Anibcrlite IRA400 in the formate form to separate boric and phosphoric acids in the determination of boron in fertilizers. The phosphoric acid is retained by the resin while the boric acid is mashed into the effluent by distilled water. Samples are of lo^ total salt content and relatively large resin bed capacity. Positive errors of 0.1 to 0.4 nig. of boron are obtained on 1-gram samples containing 25 to 40 mg. of boron. Prior to the above puhlication, similar separations had been attempted in this laboratory using the chloride and the nitrate forms of the same resin. The results obtained are accurate to about 1 0 . 2 nig. of boron, but serious contamination of the boron-containing effluent is noted. Less accurate results were obtained as the total foreign ion concentration increased. Preliminary studies of the behavior of boric acid and the common anionic VOL. 2 9 , NO. 5 , M A Y 1957
829
interferences, phosphate, arsenite, and arsenate, produce the following order of elution from strong base resins in the hydroxide form: borate, chloride, nitrate, arsenite, phosphate, and arsenate. Borate appears to be weakly held by the resin, chloride ion somewhat more strongly, and the succeeding ions were retained strongly. The presence of chloride ion, silver chloride, silver nitrate, or mixtures of these, does not interfere with the titrimetric determination of boron. It thus appears feasible to use chloride ion to indicate the complete elution of borate from the column. By placing acidified silver nitrate in the receiving flask, and adding a small amount of chloride ion to the sample, it would be possible to elute with a relatively concentrated reagent until a very definite chloride test appeared in the receiver. At this point all of the borate and none of the more strongly retained ions would have been eluted. Such a procedure would allow the collection of a single eluent fraction as the sample, and would keep the total volume of eluent reasonably low. WEAK BASE RESINS
Mixed Resin Bed Separations. As the name implies, weak base exchange resins contain ionizable groups having weakly basic properties. The extent of absorption of a weak acid by such a resin is largely dependent upon the ionization constant of the weak acid (4). Because of the great affinity of resins of this type for the hydroxyl ion, i t is possible to separate very weak acids from stronger acids by the hydrolysis of the resin salts formed by the anions of the former. Keak base resins will not retain such acids as boric, silicic, or carbonic ( 7 ) . If boric acid can be quantitatively recovered from a bed of a weak base resin, a simple and rapid method of separation of boric acid from all other anions except those of extremely weak acids is available. Further, this resin could be mixed with a strong acid cation exchange resin to give a mixed resin bed which mould simultaneously remove cations and most anions without the necessity for an elution procedure. By using the cation exchanger in the free acid form and the anion exchanger in the free base form, the sample could be deionized. The effluent from such an ideal ion exchange column would contain only the boric acid and such other weak acids as would not be retained by the resin. A single mixed resin bed has three distinct advantages (6). It increases the efficiency of both of the component resins over that achieved with either alone; a mixed resin bed has a sharp break-through point for ionic solids which allows a more complete utiliza-
830
ANALYTICAL CHEMISTRY
tion of the resin bed capacity; and the quality of the effluent is, within reasonable limits, independent of the concentration of ionic solids in the sample. The technique has been used widely for the treatment of water, and for the deionization of analytical samples, but apparently no analytical separations of weak acids by this means have been reported. EXPERIMENTAL
Reagents. Standard boric acid solutions were prepared by dissolving weighed amounts of anhydrous boric oxide in water t o give a concentration of approximately 1.0 mg. of boron per milliliter. The boric oxide was prepared by fusion of reagent grade boric acid in a platinum dish. All other reagents were prepared by solution of the appropriate reagent grade chemical. Carbonate-free sodium hydroxide solutions were prepared from saturated sodium hydroxide solution and were standardized by potentiometric titration of aliquots of the standard boric acid solution using the procedure described by Martin and Hayes (3). This procedure was also used for the determination of the boron in the samples eluted from the ion exchange columns. The method involves the adjustment of sample and complexing agent (invert sugar) to a pH of 6.90, mixing of the two, and a titration with 0.02N carbonate-free sodium hydroxide of the complex acid formed to restore the p H of 6.90. As shown by Hollander and Rieman ( I ) , there is a small error in this type of titration. The accuracy which may be expected is indicated in Table I. A degree of familiarity with the procedure is required before highly precise results are obtained. Plus or minus 0.03 mg. of boron was selected as a safe limit of accuracy in the titration.
Table
I.
Standardization of Sodium Hydroxide Solutions
Solu- NO. of tion Replicates No. 1 2 3 4
5
6
Titer, M g . Std. Dev., Mg. B/Ml. B/ML 0,2520 0.014
5
0.2838
5
0,2332 0.2681
6 6
0.2431
0.011
0.004 0.008
0,010
Resins. STROXG BASERESIN,Amberlite IRA-400. WEAK BASE RESINS, Amberlite IR-4B, Amberlite IR-45, DeAcidite, Nalcite WBR. STRONGACID RESIN, Nalcite H C R (Dowex 50), 8% cross linkage, 50 t o 100 mesh. The resins were obtained from the manufacturer in the fully regenerated free acid or base forms. The resins were air-dried, crushed in a mortar, screened to the appropriate mesh size,
and washed free of fines. Used resins were regenerated or converted into other ionic forms by using 1N solutions of the appropriate acid or base and rinsing with distilled water until the rinse water was approximately neutral to methyl orange or phenolphthalein indicator. Ion Exchange Column. The ion exchange columns consisted of borosilicate glass tubing of varied diameter and length equipped with three-way stopcocks a t the bottom. One outlet mms used to collect the effluent and the other was permanently connected by rubber tubing to an elevated distilled water reservoir. A l-inchthick, glass-wool plug was used to support the resin. The columns were filled by adding approximately 40 ml. of resin as a slurry to the column containing about 1 inch of water above the plug. The column was drained to about the resin level, and the bed was then raised by adding water from the reservoir. This settled the resin and removed trapped air. The stopcock controlled flow rates. Samples, rinse, and eluent solutions were admitted from a 250-ml. separatory funnel connected to the top of the column by a rubber stopper. The funnel created an air lock in the column by which the liquid level was maintained a t least 1 inch above the resin bed. In this manner the sample was admitted a t the flow rate determined by the column stopcock, and the stopcock of the funnel was left open. Columns about 60 cm. in length and 11 mm. in inner diameter were used for determinations. Columns of 22-mm. inner diameter were generally used for regeneration. I O N EXCHANGE CHROMATOGRAPHY
Borate was separated from several anionic interferences such as phosphate, arsenite, and arsenate by addition of chloride ion to the sample solution before admitting it to an ion exchange column containing a strong base resin in the free base form. Elution from the column was continued until a positive test for chloride ion was noted in the receiving flask. While it was possible to effect successful separations, such a method appears to have limited application. MIXED RESIN BED SEPARATIONS
Evaluation of Resins. Some weak base resins such as Amberlite IR-4B and DeAcidite retained small amounts of boric acid and quantitative recovery of the borate either by washing or by elution with sodium chloride solution was impossible. The resin Amberlite IR-45 did not exhibit this behavior and was used in subsequent work. The cation exchange resin chosen was Nalcite HCR (Dowex 50). This type of resin was shown by Martin and Hayes (3) and by Kramer (2) to provide a
good separation of borate from reasonably large quantities of 15 different cations. Procedure. The resins in their fully regenerated form were crushed, screened to 40 to 60 mesh, washed free of fines, and combined on the basis of equivalent exchange capacity using the capacity ratings of the manufacturer. Successive portions of boric acid were passed through 50 ml. of the mixed resin a t a constant flow rate of 2.8 ml. per minute. The resin was rinsed with distilled water until 250 ml. of effluent had been obtained, and 100-ml. aliquots were titrated bv the above method. The more sigkficant results are shown in Table 11.
Table II. Evaluation of Resins ME. of Boron Recovered. IR-45Nalcite WBRNalcite HCR Nalcite HCR I
3.94 3.95
3.93 3.93 3.94 3.94
(3.93 mg. of B as H3B03, 10 mg. of P a8 KH*POI, and 10 mg. of As as .4sO~--present) 3 . 93 3.93
3.96 3.94
Separations from Knovin Interferences Mg. B No. Sample0 Found 1 120 mg. of Fe as 3.92 Fer(S04)iin 10 ml. 1N HC1 2 As above plus 10 mg. of 3 . 9 4 P as PO,--- and 10 mg. 3
of AS as ASOi--118 mg. of Sn as SnC4 plus HC1 to prevent
3.95
40.2 rng.-of i f 0 as
3.92
hydrolysis 10 ml. of 0 . l M KblnOa 3.92 in 1 X H2S04 5 10 ml. of 0.1M K2Cr207 3.91 in lill H&O, 4
6
7
Mool-1 meq. of NaHCOz 10 mea. of KaHC02 5 meq.=ofNaC2H30g 10 mea. of NaGHIOI 0.5 mei. of ~a2CsCj45 meq. of Il-a&s04
8 9 10 11 12 13 10 ml. of 0.lM KF
3.94 3 93 3.94 3 89
3.93 3.93 3.65* 2 989 3 .9 I d
3.93 mg. of B as &Bo3 present. Sample titrated directly without separation. Sample passed through ion exchange bed before titration. d Boron added after KF soln. passed through ion exchange column.
*
For further evaluation of IR-45Nalcite HCR, it vias assumed that ions removed by the cation exchanger alone would also be removed by the resin when used in a mixed bed. Hence, only those interferences noted by workers who used cation exchange resins to separate boron and anionic in-
terferences were studied. The results shown in Table I1 were obtained with fresh resin for each sample. I n utilizing a mixed resin bed there was some question regarding the behavior of cations which form insoluble hydroxides as the p H of the solution rises, due to the neutralization of excess acidity by the exchange process. Samples 1 and 2 (Table 11), containing iron, gave no difficulties. Spot tests on the effluent indicated the absence of iron, sulfate, and phosphate. Only a trace of chloride was present, and the pH was found to be 5.6. The stannic chloride sample produced a thin gelatinous layer on the upper surface of the resin bed, with marked reduction of the flow rate. Agitation of the upper few milliliters of the resin bed dispersed the precipitate and normal flow was restored. No turbidity or interference in the titration was noted in the effluent sample. Shaking small amounts of stannic chloride in a test tube with a portion of the mixed resin resulted in the appearance and then disappearance of a turbidity, indicating that ion exchange rather than a physical retention by the resin bed had taken place. Kramer (2) suggested that tin be removed prior to a cation separation by plating it out on metallic zinc. It seems probable that samples known to contain large amounts of tin could be satisfactorily handled by a combined batch and column operation. Permanganate, dichromate, and molybdate attacked some cation exchange resins in the free acid form (6). Rfolybdate was reduced in part to molybdenum blues, which passed into the effluent and prohibited the use of color indicators for the boric acid titration. However, molybdate was reported to be retained by weak base resins in the chloride form (9). Permanganate and dichromate attacked the resin. but this had no evident effect on the separation or boric acid recovery. The molybdate did not seem to attack the resin, and no molybdenum blues appeared in the effluent. Anions of weak organic acids. such as formate, acetate, and oxalate, interfered in the boric acid titration because of their buffering effect. Samples such as 8, 10, and 12 (Table 11) could not be titrated with reasonable accuracy without the separation of boric acid from these anions. The potentiometric titration will tolerate small amounts of these anions. KO buffering was noted with samples 7 to 12 after passage through the ion exchange column. Boric acid could not be separated from fluoride ion using this technique. The fluoride interfered by forming fluoboric acids of sufficiently acidic character to be retained by the anion exchange resin. The resin bed retained
free fluoride ion, but apparently did not decompose the fluoboric acids, as indicated by the data for sample 13 in Table 11. The passage of an acidic sample through the mixed resin bed liberated small bubbles of carbon dioxide from the anion exchange resin. (The latter was always found to contain some carbonate.) These carbon dioxide pockets are small and disappear as the resin is rinsed with water, and have no apparent adverse effects on the separation or recovery of boric acid. The small gas pockets did not alter the flow rates appreciably, but indicated the amount of resin that had been exhausted, because the bubbles appeared as far down the column as the free acids had reached. N o leakage of ions was noted unless the bubbles reached the bottom of the column; in such cases insufficient resin had been used and the sample was discarded. The resin pairs differed in color and in density and could be separated readily for regeneration. One batch of resin showed an appreciable blank after several regenerations; therefore, it is recommended that they be discarded after use, or else rigidly checked after each regeneration. There was sufficient surface attraction between the two resins in the fully regenerated forms to keep the mivture homogeneous if a 60- to 100-mesh size wab used. When larger particle sizes n-ere used, the resins tended t o separate. Smaller sizes decreased the flow rates markedly. Practical Applications. To compare the proposed mixed bed ion exchange separation technique with existing separation methods, the analysis of a glass and of a synthetic steel !vas undertaken. These represent fairly difficult types of samples, for which comparable data have been published using other methods. PROCEDURE A. DETERMINATION OF BORONIN GLASS. Il-ational Bureau of Standards glass No. 93 (12.76% boron oxide) and silica samples were ignited a t 500' C. for 1 hour. Samples of 0.2 to 0.3 gram were fused in a platinum crucible with approximately six times the sample weight of potassium carbonate; the melt was taken up in a minimum of hot water and transferred to a 250-ml. separatory funnel containing 30 to 40 ml. of mixed, 60 to 100 mesh, IR-45 and Nalcite HCR resins in the free base and acid forms, respectively. The funnel was swirled and contents allowed to settle until no further sign of carbon dioxide evolution was apparent. The resin solution mixture was transferred to a glass column 11 mm. in inner diameter, which already contained about 20 ml. of the mixed resin. The separatory funnel was connected to the top of the column, rinsed with several small portions of VOL. 2 9 , NO. 5 , M A Y 1957
831
distilled water, and then filled with distilled water. Rinsing was continued a t a rate of 2.5 ml. per minute until a total effluent of 250 ml. was collected in a volumetric flask. Aliquots of 100 ml. were then taken for the determination of the boric acid. For a blank determination, samples of RIallinckrodt analytical grade SiO2. zH20, 100-mesh, were given the same treatment as the glass samples. A known amount of boric acid was added just after the melt was transferred to the separatory funnel. The average error caused by loss of boric acid was added to the amount of boron found in the glass analyses. The results are tabulated in Table 111.
Table 111. Determination of Blank in Analysis of Glass Samples Rt. of Mg. of Boron
SiO,, Gram 0.2695 0.1697 0.2367
‘
Added
Found
Not recovered
10.47 10.47 10.47 10.47 10.47 10.47
10.39 10.32 10.41 10.40 10.33 10.35
0.08 0.15 0 06 0.07 0.14 0.12
Average 0 . 1 0
Table IV.
Analysis of Glass Sample
Mg. of Boron
Not % BzO3
w t . of Glass, Gram
re-
(Cor-
Prescov- rected for ent Found ered Blank) 0.2022 8.02 7.90 0.12 12.74 7.91 0 . 1 1 5.94 0.13 5.91 0.16 0.2029 8.02 7 . 8 9 0.13 0.3137 12.44 12.20 0 . 2 4 Average 0.1529
Table V.
6.07
12.70 12.68 12.69 12.70
Comparison of Separation Procedures
NBS certificate of analysis Precipitation of SiOs (1)” Precipitation of Si02 by cation exchange (2)“ Extraction (10)“ Mixed bed ion exchange Denotes literature reference.
% ’ &Os
12.76 12.51 12.44 12.72 12.70
The average blank correction applied is not exactly correct, in that the ratio of silica to boron varies from blank t o sample, and the same blank was applied to all weights of sample. However the magnitude of this variation is about the same as the probable error in the determination of the blank. The precision obtained with varied weights of sample indicated that this assumption is reasonabIe. The addition of the carbonate melt
832
ANALYTICAL CHEMISTRY
Wt. of Steel,
Gram
0.2538 0.2538 0.2538 0.2538 0.2538 0.2538 0
Table VI. Analysis of Synthetic Steel Samples hlg. As Added Mg. P Added Mg. B Added as As03--as KHIPOl as H3B03
5 10
5 10
I l g . B Found
0.983 9.83
1.00 9.83O
Results obtained on 100- to 250-ml. aliquots of effluent.
to the resin in the separatory funnel was necessary so that the solution could be neutralized, and carbon dioxide evolved, before the sample was admitted to the column. This prevented complete disruption of the resin bed by the formation of large amounts of carbon dioxide in the column. Such action results in a relatively slow neutralization of the carbonate by the resin and closely approximates a homogeneous precipitation of the silica, thereby reducing possible loss of boron by coprecipitation. Some boron is lost, as is indicated by the silica blank data. By correcting for this loss, good agreement vas reached with the value of 12.76% boron oxide given by the National Bureau of Standards (Table IV). Colloidal silica appeared in the effluent, but caused no interference in the potentiometric titration. Some difficulty was encountered due to the accumulation of silica on the top of the resin bed, but was easily overcome by occasional stirring of the upper few milliliters of the resin. Results obtained with the procedure are reproducible and should provide accuracy to a t least &0.10 mg. of boron for glass samples. The other methods of separation which have been applied to the analysis of National Bureau of Standards glass KO.93 include methylborate distillation, precipitation of interferences, cation exchange, and liquid-liquid extraction. Comparative data are given in Table V. The mixed bed ion exchange procedure appears to give satisfactory results with less manipulative skill and time consumption than many of the other published methods. A single analysis can be performed in approximately 3 hours after the solution of the sample, and the analyst’s attention is only required for approximately half of this time. PROCEDURE B. ANALYSISOF SYKTHETIC STEELS.Synthetic standards were prepared using National Bureau of Standards steel 55a; 6.3444 grams were dissolved in 80 ml. of 1 to 1 hydrochloric acid with gentle heating. Two milliliters of 30% hydrogen peroxide were added after the steel had dissolved, to destroy carbonaceous material. The solution was boiled for 10 minutes, cooled, and diluted t o 250 ml. Known amounts of boric acid and interferences were added to 10-ml. aliquots (which contained 0.2538 gram of steel). These
samples n ere passed through approximately 50 ml. of the mixed resin at a flow rate of 2.5 ml. per minute. The column was rinsed with distilled water a t the same rate until 250 ml. of eflluent mere collected. Either an aliquot or the entire sample was analyzed for boron by the titrimetric procedure mentioned above. The results are given in Table VI. All results obtained were within the experimental error of the titration step. The same results might be expected with the cation exchange method, if the samples do not contain phosphorus or arsenic. Colorimetric Determination of Boron. Attempts were made to employ various colorimetric methods t o the determination of the boron content of the effluent from the mixed bed ion exchange columns. The results were somewhat high, and the method cannot be recommended at the present time for the determination of microgram quantities of boron. The magnitude of the error, averaging about plus 3 to 5 y of boron on samples containing 21 y, indicates that the recovery of boron is quantitative to well within the accuracy which might be expected of a titrimetric procedure. LITERATURE CITED
Hollander, M., Rieman, K.,111, IND. EXG.CHEU., A N A L ED. 18, i 8 8 (1946).
Kramer, H., A 4 ~ . 4CHEW ~. 27, 144 (1955).
Martin, J. R., Hayes, J. R., Ibid., 24, 182 (1952).
Nachod, F., “Ion Exchange,” p. 71, Academic Press, Kew York, 1949. (5) Rohm & Haas Co., “Amberlite
Monobed Deionization,” Bull. M-
15-50, p. 5, 1950. (6) Runeberg,,G.,Samuelson, 0 Srensk Kem. Tadskr. 57, 250 (1945). ( 7 ) Samuelson, O., “Ion Exchangers in Analvtical Chemistry,” p. 169, Wilei, New York, 1953. (8) Schutz, E., Mitt. Gebiete Lebensm. u. Hyg. 44, 213 (1953). (9) Sussman, S., Nachod, F. C., Wood, W..Ind. Enq. Chem. 37, 618 (1945). (10) Webster, P. A., Lyle, A. K., J . Am. Ceram. SOC. 23,235 (1940). RECEIVEDfor review May 24, 1956. Accepted January 18, 1957. Pittsburgh
Conference on Analytical Chemistry and Applied Spectroscopy, February 1956.
