Amine Cobalt( II) Hexacyanoferrate(II) Compounds as Inorganic Ion Exchangers Richard B. Hahn and Henry C. Klein Chemistry Department, Wayne State Uniuersity, Detroit, Mich. 48202
THESYNTHESIS of an inorganic ion exchanger, potassium cobalt(I1) hexacyanoferrate(II), K2C03[Fe(CN)&, (designated as KCFC) has been described by Prout, Russell, and Groh ( I ) . The use of this material for the determination of cesium-137 in various environmental samples has been described by Boni (2). Petrow and Levine (3) improved upon this exchange material by substituting the ammonium ion for the potassium ion in KCFC, thus forming ACFC (A = NH4+)also having excellent exchange properties for cesium-1 37. The advantage of the ammonium compound over the potassium compound lies in the fact that all potassium compounds contain some naturally occurring radioactive potassium-40 which may introduce errors in counting low levels of cesium-137 (3). If the ammonium ion is substituted by another ion of higher molecular weight and larger ionic radius, there should be even less tendency to exchange with other ions and the column then should become more selective for cesium ions. This may be accomplished by substituting an amine group for the ammonium ion in the compound. In the following investigation, various amines were used to prepare columns of amine cobalt(I1) hexacyanoferrate(I1) which were then tested for exchange properties. EXPERIMENTAL
The following procedure was used to prepare the various amine compounds: 0.0375 mole of potassium hexacyanoferrate(I1) was dissolved in 65 ml of distilled water and this solution was run through a 4-cm X 75-cm column containing 200 ml of Dowex 5OW-X10, 20-50 mesh in the acid form. It was found that washing the column with 185 ml of distilled water eluted essentially all of the hydroferrocyanic acid. The final eluate was about 250 ml in total volume and was brilliant green in color. To this eluate 0.075 mole of the selected amine was added to form the amine hydrogen salt A2H2Fe(CN)6. The amine hydrogen salt solution was poured, with stirring, into 250 ml of a solution containing 0.065 mole cobalt(I1) nitrate. A thick precipitate of the amine-cobalt hexacyanoferrate(I1) was formed immediately and varied in shade from yellow-brown to bright green depending upon the amine used. The precipitate was filtered by suction and washed with water until the filtrate was free of color from the cobalt ion. The precipitate was dried at 110 "C, then ground and sieved to 30-60 mesh. These granules were washed free of fines with water and dried at 110 "C. The dried column material was usually black, but blue-green or green granules were sometimes obtained depending on the type and amount of amine. In addition to the amine columns, one was prepared (1) W. E. Prout, E. R. Russell, and H. J. Groh, J. Inorg. Nucl. Chem., 27, 243 (1956). (2) A. L. Boni, ANAL.CHEM., 38, 89 (1966). (3) H. G. Petrow and H. Levine, ibid., 39, 360 (1967).
using ammonia as the base, and one was prepared using no base. The exchange efficiencies of the various compounds were determined as follows: A known activity of 36,000 cpm of carrier free cesium-137 was diluted with 50 rnl of water and this solution was run through a 15-mm x 4 0 m m column of the compound at the rate of approximately 5 ml per minute. Counts were made both on the eluate and on the columns themselves, by placing them in the 2- X 2-inch sodium iodide well type scintillation detector and scaler. The per cent cesium-137 retained by the various columns is given in Table I. DISCUSSION
Experiments were carried out to find the effect of varying the mole ratio of A2H~Fe(CN)6 to cobalt on the properties and exchange efficiencies of the columns. Prout, Russell, and Groh ( I ) recommend a molar ratio of 0.0375 mole amine salt to 0.065 mole of cobalt(I1) ion. These ratios were used in preparing the above columns. When the amount of amine salt was varied between 0.01 mole to 0.1 mole, or the amount of cobalt ion was varied from 0.01 mole to 0.2 mole, there were no noticeable changes in the physical form or ion-exchange properties of the final product. When 0.375 mole of amine salt and 0.65 mole cobalt ion were mixed to make large amounts of column material at one time, the granules obtained broke down into mud upon washing. When the material was prepared in the smaller batches, however, the particles were clean, hard, and easily washed. Table I. Retention of Cesium-137 by Various Amine Cobalt(I1) Hexacyanoferrate(I1) Compounds Basea Cesium-137 retained, % Ammonia 100 2-Aminoethanol 100 Aniline 100 Benzylamine 100 Butylamine 100 Isobutylamine 99 Ethylamine 98 N-Ethylaniline 100 Ethylenediamine 86 1-Glutamic acid 100 100 Hexanediamine Hydrazine 92 Methylamine 100 Morpholine 100 dl-p-Phenylalanine 46 Phenylhydrazine 100 Pyridine 100 Quinoline 100 No added base 100 a Cyclohexylamine, N-N diethylhexylamine, and m-diethylaminophenol would not form granular column material.
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Table 11. Elution of the 14CLabeled Columns Methylamine Aniline column eluate column eluate Eluate activity, dpm activity, dpm 1st 500 ml water 5600 1850 2nd 500 ml water 980 720 2170 1330 O.OOOlM cs+ 2730 1260 0.001M Csf 2170 1330 0.01M Cs+ 50 mlO.1M CsCl 4760 1620 50 mlO.1 M NH4C1 1260 1400 Table 111. Amine Content of Column Material Using 0.035 Mole Acid Moles methylamine Amine in to prepare column compound, 0.014 5.1 0,028 5.0 0.07 11.0 0.14 (excess) 12.1 0.35 (excess) 13.0
The effect of varying the amount of amine from the usual 0.075 moles was also studied. Several columns were prepared using methyl amine in amounts from 0.01 mole to 0.375 mole with usual acid salt to cobalt ratios and also with various other acid salt t o cobalt ratios. Although these variations produced columns of many shades of green, and many different hardnesses, they all formed columns which exchanged with the cesium-137. Because a column prepared with no amine also exchanged with cesium, the mechanism of the exchange reaction was questioned. In order to determine whether the exchange occurred between cesium and the amine, columns were prepared containing 2.2 X 106 dpm of lC labeled methyl amine or 2.2 x 106 dpm of IClabeled aniline. These columns were eluted with solutions of nonradioactive cesium chloride, and the eluate was beta counted for carbon-14 using a Packard Tricarb liquid scintillation spectrometer, Series 314E. These labeled columns were prepared as described previously using methyl amine and aniline of known specific activity. The columns were first washed with 1 liter of distilled-deionized water. The various concentrations of cesium ion were put on the column in 10-ml samples and then washed with 25 ml of water. 500-pl aliquots of the total eluate were counted in the liquid scintillation spectrometer using 15 ml of aqueous scintillation liquid. The results are given in Table 11. After the elution with the 0.01M Cs+ solution, 36,000 cpm of Cs13’ in 10 ml of water was eluted through the column. The column retained all of this activity. Because the column did not appear to be saturated with cesium a t this point, 50 ml of a 0.1M cesium chloride solution was eluted through the column in an attempt to saturate the sites. The eluate from this elution produced very little added activity, (See Table 11) but the eluate contained about 0.01M cobalt(I1) ion. T o test the effect of other more concentrated salts on the column, 50 ml of a 0.1M ammonium chloride solution was eluted through the column. The eluant contained n o cobalt ion, and very low activity (See Table 11). To determine whether the cobalt(I1) ion was involved in the exchange reaction, a methylamine column was prepared and labeled with 2.2 X lo6 dpm of cobalt-60 in the cobalt nitrate solution. The column was eluted with various solutions and aliquots of the eluate were gamma counted using a well type sodium iodide crystal detector and scaler. 1 136
ANALYTICAL CHEMISTRY
No activity was observed in the eluate when the column was eluted with 50 ml of lO-6M cesium chloride solution or with 50 ml of lO-3M cesium chloride. Approximately 1000 cpm of 6OCo activity appeared in the eluate when the column was eluted with 50 ml of 0.1M cesium chloride. An equal activity was observed when the column was eluted with 50 ml of 0.1M hydrochloric acid. This slight loss of cobalt from the column is attributed to the dissolution of small amounts of the column and does not arise from exchange of cesium with cobalt. To determine if one amine might exchange another, a new column containing 2.2 X lo6 dpm of IC labeled methylamine was prepared. This labeled column was eluted with 7 X moles of nonradioactive aniline hydrogen sulfate diluted with 10 ml of water. The column was then washed and the total eluate (35 ml) contained 4000 dpm of ‘C. Similarly, a labeled column with aniline was prepared and eluted with 7 X 10-4 mole of nonradioactive methylamine hydrochloride in 10 ml of water. The column was washed and the total eluate contained 1700 dpm of l4C, which is about the amount eluted with a similar amount of water. The following conclusions were drawn from the above 14C and BOC experiments: (1) The cobalt ion apparently is not involved in the exchange with cesium ions. (2) Although exchange occurs between cesium ions in solution and the amine in the column, there is no quantitative relationship between the concentration of cesium in the eluant and the amount of amine eluted. The data in Table I1 could be explained by assuming that rapid exchange takes place at the surface of the ACFC particles along with a much slower reaction involving the exchange of cesium with amines in the interior of the particle. Thus an exchanger with its surface saturated with cesium ions would slowly release a more or less constant amount of amine when eluted with cesium ions regardless of the concentration of cesium in the elutant. (3) There is a slight tendency for aniline t o displace methyl amine from the column. There is also some question concerning the exact composition of these complex compounds. Prout, Russell, and Groh ( I ) suggest the formula of the compound would be A ~ C O R(A , = amine, R = [Fe(CN)6]4-) while Tananaev and Korol’kov ( 4 ) suggest the formula AzCo3Rz. With methyl amine as A, the calculated per cent amine in AzCoR is 19.1 %, while in A2C03R2the calculated per cent amine is 9.6% and in AHCo3R2it is 5.2 %. In order t o determine the composition of the methyl amine compound, several samples were synthesized using 0.014, 0.028, 0.07, 0.14, and 0.35 mole methyl amine with 0.035 mole H4R. The compounds were prepared in the usual manner, and samples from 1 t o 2 grams of the material were analyzed for amine by a modified Kjeldahl procedure. The material was mixed with 200 ml of 10M sodium hydroxide and the amine distilled into 100 ml of a nearly saturated boric acid solution. The amount of base thus distilled was determined by titrating the boric acid solution with standard 0.1M HC1 to a bromcresol green end point. The results of these analyses are given in Table 111. These data indicate that the composition of the compounds formed is probably AHCo3R2when a limited amount of amine is used and AZCo3R2 with excess amounts of amine. RECEIVED for review January 26, 1968. Accepted March 20, 1968. Work supported by a grant from the National Science Foundation. (4) I. V. Tananaev and A. P. Korol’kov, Izv. Akad. Nauk. SSSR, Neorgan. Materialy 1 (9), 1577-81 (1965); Chem. Abstr. 64,
4557c (1966).
