ACKNOWLEDGMENT
The authors express their hearty thanks to G. Van Dyke Tiers who kindly permitted them to quote 7-values from his table. They are also indebted to Shueo Hattori of the Nagoya University and ToAimasa Seki of the Japan Electron Optics Laboratory, Ltd., for their valuable discussions, to Yoichiro Mashiko and Shinnosuke Saeki of the Government Chemical Industrial Research Institute, Tokyo, for their
kmd encouragement throughout this work, and to many institutes and laboratories which provided the valuable samples. LITERATURE CITED
(1) Bothner-hy, A. A., Glick,
R. E., “NMR Spectra and Structure Correlations, Vol. 1, Mellon Institute, Pittsburgh, Pa., unpublished notes, 1956; Bothner-by, A. A,, Naar-Colin, (2)C., chamberlam, Shapiro,B. L., N, Ibid., F., Vol. 2,CHEM, 1958. 31,56 (1959).
(3) Heathcock, C., Can. J . Chem. 40, 1865 (1962).
(4) Meyer, L. H., Saika, A,, Gutowsky,
H. S., J . Am. Chem. SOC.75,4567 (1953). (5) Shoolery, J. N., Technical Information Bulletin 2, No. 3, Varian Assooiates, Pa10 Alto, Calif., 1959. ( 6 ) Tiers, G. V. D., “Characteristic
Nuclear Magnetic Resonance Shielding Values for Hydrogen m Organic Struetures,” Part I: Tables of T-Values for a Variety of Organic Compounds, Minnesota Minipg and Manufacturing CO.,St. Paul, Mmn., 1958. RECEIVED for review February 18, 1963. Accepted June 24, 1963.
Precipitation Chromatography Separation of Metals as Ferrocyanides on Filter Paper ADELE M. LllMATTA and JAMES D. SPAIN Department of Chemistry and Chemical Engineering, Michigan College of Mining and Technology, Houghton, Mich.
b
When a mixture of certain metal ions is allowed to flow from a capillary tube onto a piece of filter paper impregnated with a precipitating agent such as manganese ferrocyanide, concentric rings af metal precipitate develop. These rings form in the order of increasing solubility, since more soluble precipitates will form only after the metal ions of a less soluble one are exhausted by the precipitation process. A procedure for separation and identification of 1 1 ions which farm insoluble ferrocyanides is described. If the precipitant is dispersed uniformly on the paper, then the area of the spot produced is proportional ta the total number of equivalents of metal ion applied. A quantitative procedure which is based on this principle is described and certain variables in the procedure are discussed.
Figure 1.
impregnated with precipitant and dried. The impregnation technique has been used by Velculescu and Cornea (6) for the separation of halide ions using silver nitrate. Nagai (4, 6) has puhlished several papers on the separation of metal ions using 8-quinolinol. Vyakhirev and Kulaev (7) carried out an extensive study of metd ion separation using papers impregnated with a variety of soluble precipitants. Using this technique, Kulaev in a later paper (3) related the size of the spot to the solubility product and the concentration. The diameter of the spot varied inversely with the solubility product constant and directly with the concentration. These relationships, which were derived with the use of soluble precipitants, did not bold true for the insoluble precipitants employed in the present investigations. The importance of using an insoluble precipitant was first appreciated by Clarke and Hermance ( 1 , $). These investigators separated various metalion pairs using papers impregnated with insoluble cadmium sulfide, zinc ferrocyanide, and barium carbonate. Another contribution of these investigators was the use of an elaborate capillary applicator so that the sample flowed from essentially a point source. The present investigation is intended
PRECIPITATION
CHROFTOQRAPHY is a differential migration procedure in which a distribution takes place between a nonmobde precipitate and mobile ions in solution. Separations result from differences in the well known solubility product equilibrium. Either gels or filter paper can be used as the supporting medium. Precipitation chromatography on paper can take place by simply placing a drop of precipitant on paper followed by a drop of solution containing the metal ions to be separated. Such a technique haa been employed by Zolotavin and associates (8, 9 ) for the separation of anions using silver nitrate, and for the separation of metallic cations using potassium iodide. A more elegant procedure is to employ a paper which has been previously
1898
ANALYTICAL CHEMISTRY
Apparatus
to provide the basis for both qualitative and quantitative application of the type of precipitation chromatography which employs papers impregnated with an insoluble precipitant. A type of capillary apparatus is introduced which greatly simplifies the technique, and makes it possible to run several samples simultaneously.
Method of Preparation of Manganese Ferrocyanide Papers. Strips of Whatman No. 1 filter paper, approximately 7 em. by 30 em., were dipped into a solution of 0.1M potassium ferrocyanide and blotted between two strips of the same paper to remove excess solution. The papers were then rolled loosely and placed on edge on a hot plate or in an oven until dry. Then they were submerged rapidly in a solution of manganese nitrate containing Mnt2 in a concentration of 0.2lM and allowed to stand in this solution for approximately 5 minutes. Next, the papers were blotted and‘placed in a water bath for a few minutes to remove excess reagents, blotted again, and dried in the same manner as before. The order in which the reagents were applied to the paper bad a distinct effect on the type of chromatograms obtained. If the manganese solution were applied first, followed by the potassium ferrocyanide solution, the spots obtained when the paper was used had quite irregnlar boundaries and a light ring always formed around the spot during the washing process. When the potassium ferrocyanide solution was applied first, followed by the manganese nitrate solution, the spots obtained bad more even boundaries and the halo effect previously produced by washing was completely absent.
Preparation of Solutions. The test solutions used were prepared from the nitrates of the metals, if possible. Stock solutions conhining the ion in a concentration of 50 mg. per ml. were first prepared, and aliquots of these were diluted to give the desired concentrations for the various tests. Most of the qualitative work was done using solutions having a concentration of 5 mg. per nil. Application of the Sample. The apparatus used for supporting the paper consisted of a glass tube bent so t h a t the paper could be stretched out with the ends rclled around the tube and secured with wooden splints and rubber bands. Tiiis holder served to keep the paper elevated and thus prevent it from resting on any surface while the test solutions were being applied. The holder was placed on a wooden baseboard which had a strip of plumber's strap iron attached in such a way that the tubes containing the samples could be passed through the holes in the strip and down onto the paper. This strip served to support the tubes upright and made it possible to apply samples all along the length of the paper simultaneously. This greatly facilitated the procecs when a series of tests was being run. The apparatus is shown in Figure 1. If there is no need to run a large number of tests, a simple temporary apparatus for single runs can be readily assembled from a few pieces of common laboratory glassware. A circle of filter paper is used instead of a strip and this is placed on a concave watch glass which serves to support the paper and keep it from resting on any surface. The support for the samplt, tube consists of a buret funnel placed wide end down on the paper. The sainde tube can be readily passed down through the stem of the funnel on to the surface of the paper. For strictly qualit ttive work, the solutions were applied from capillary melting point tubes. The best spots were obtained if the ends of the tubes were drawn to a fine opening-approximating a point source as closely as was practical. For quantitative work the sample solutions were applied with micropipets. Washing. For blith qualitative and auantitativc worlr. the mots were washid with a manganese nitrate solution containing 0 2 M Mnt2 until stable dimensions wer? attained. The washing was done by drawing up the wash solution in the same tube which had been used for applying the sample, and then replacing the tube at approsimately the same point at which the sample was applied. Determination of Areas. All areas used in the quantitative work were obtained by measuring the spots with a planimeter. Because of the relative inefficiency of the planimeter for measurement of smalI areas, an average of five to 10 readings of a single spot are necessary to provide a valid measurement of area. Rapid estimalions of spot area may be performed uaing a 13~11'sEye template
Table I.
Figure 2.
Typical chromatograms
Smallest amount detectable,
Ion Ag+
on photographic film. This was prepared by drawing the Bull's Eye with relative areas indirated and then reducing its size photographically using a copy camera.
Cuf2 Fe+3 Hgf2 Co + 2
QUALITATIVE RESULTS
Eleven ions were found which gave characteristically colored precipitates on manganese ferrocyanide paper. These included Fe+3, C U + ~ ,Ag+, Hg+z, Cd+2, C O + ~ ,Ni+2, Zn+2, Bi+3, Sn+4, and UOz+2. Cd+2 and Zn+2 could only be detected when in combination with some other ion having a highly-colored ferrocyanide. Ni+2gave its characteristic color only after standing for some time. The characteristic colors of the ferrocyanides for all the ions tested, and the limits of detection on the paper preparcd as described are presented in Table I, in order of increasing solubility. Limits of detection will depend upon the Concentration of precipitant on the paper. The technique for paper preparation described resulted in papers impregnated with manganese ferrocyanide in the approximate concentration of 0.4 to 0.5 mg. per sq. em. Papers prepared at the same time showed good agreement as to the amount of precipitate contained on them. The colors and relative positions of the precipitates are somewhat dependent on pH. The data given in Table I apply for acidic solutions of pH less than 5 . In general, good separations were achieved with various combinations of from two to five ions. However, if more than one of the ions in the group gave a light colored band, and these bands happened to fall adjacent to one another, it was sometimes difficult to distinguish the separate zones. Figure 2 shows some typical chromatograms. Some tailing was noted with ironcopper solutions in that the iron could not be washed out from the copper region entirely. The chromatogram therefore consisted of a purple central area, containing both ferric and cupric ferrocyanides, surrounded by a deep blue band due to ferric ferrocyanide alone. Presumably this results from the similarity of solubility product constants. Iron itself gave a spot having the characteristic deep blue center surrounded by an irregular ring of lighter blue which appeared during washing. However, if the impregnated
Qualitative Data on Metal Ferrocyanides
Ni f 2 Znf2 Cd+2 Bi + 3 Sn + 4 UO2 + 2
Characteristic color Pg. White (thin blue 45 band develops around white center upon standing) Deep wine red 0.045 Deep blue 0.045 Bluish-green 0.27 Mint green ini0.20 tially; gradually turns purple at outer edge Very pale greenish- Difficult t o white detect alone rVhite Cannot be detected alone Creamy white Cannot be detected alone Light yellow-green 0.5 Greenish-white 0.5 Light orange-red 0.15
paper were placed in an oven at, about 100" C. for a short time before application of the iron solution, the secondary banding did not occur. Some additions can be made to the solubility series of ferrocyanides worked out by Kulaev. He gave the following series based on the use of potassium ferrocyanide as the precipitant. Ag-Fe, Cu-Zn, Co, Xi-Cd-Bi-Mn-Pb-A1 Using manganese ferrocyanide paper, some fine distinctions in solubilities can be made and this results in the following series arranged according to increasing solubility. Ag-Cu-Fe-Hg-Co-Zn,
Ni-Cd-Bi-( Mn )
The uranyl ion was not studied in combinations, but its solubility seemed to be very close to that of manganese ferrocyanide. The position of manganese is derived from the fact that the other ions are able to replace it on the paper. QUANTITATIVE RESULTS
Relationship of Spot Area to Concentration. When an insoluble precipitant such as manganese ferrocyanide is precipitated uniformly over a piece of paper, the area of the spot obtained by application of a solution of a n ion was found to be proportional to the concentration in a linear fashion. Thus, a plot of spot areas vs. concentration for an ion will result in a straight line which can be expressed by the equation A = Kc, where A is the area a t concentration c, and K is a constant which can be determined from the slope of the line. VOL. 35, NO. 12, NOVEMBER 1963
1899
.
100PS.
IZO
Figure 3. Area of spot vs. quantity of metal ions present
P5 PB.
50 W.
'.
Spots obtained fron
Other Factors A study was made of the effects of elution, sample delivery time, and the time interval between washings on the final spot size. The spot size increased with the number of washes up to a point a t which the spot attaiued a stable size, and further washing had no significant effect. The number of washings r e quired will vary depending on the concentration and on the particular ion or ions involved, hut five washings were usually sufficient. The time requii?ed for all of the sample solution to dirain from the sample inal yInn+ *"IT, :F tuhe effects the LALyyu the tube delivers so rapidly that the solution pours out faster than the precipitate can form on the paper. In the present work, tubes which delivered in approximately 40 seconds were used, and this length of time was sufficient to eliminate the effect of sample delivery time on the spot area. Varying the time lapse between applications of wash solution from 10 seconds to 10 minutes had no effect on the final spot size.
Figure 3 represents spot area us. concentration cnrves for Cu+l and F'e+J, respectively, which illustrate this straight line relationship. Figure 4 shows a series of spots ohtained from varying quantities of Cuf' which illustrates the dependence of spot size on concentration. I t is noteworthy that these results are not in agreement with those obtained by Kulaev (3) in his work using soluble precipitants. Il[e found a linear relatioriship hetween spot diameter and concentration rather than between spot area and concentration. The difference is probably due to the fact that when a soluble precipitant is used, it is continually washed away as the sample solution advances, and this results in an entirely different situation than exists with a precipitant that is held immobile in the form of an insoluble material. Relationship of Spot Area to EqnivDISCUSSION d e n t Weight. Theoretically, when The technique described has a number an insoluble precipitant is used, the of interesting advantages which could sizes of the spots ohtained from singleion solutions of the same weight make it the procedure of choice for many specific applications, particularly concentration should vary inversely as the equivalent weights of the ions in field analysis where precision is not involved. However, this did not the most important factor. hold true except for ions such as It is fast. Several sample solutions C U + and ~ Fe+8 that give ferrocyanides may he analyzed simultaneously in a which do not differ appreciably in solu10-minute period. The apparatus is bility. Ions forming ferrocyanides havinexpensive, and can be built in less ing very low solubilities gave spots than an hour if materials are availahle. many times smaller than would be exCapillary tubes and paper are dispospected if equivalent weight were the able so there is no clean up or contamonly determining factor, and ions formination problem. ing ferrocyanides of fairly high solubiliThe method is specific for a given ties gave spots which were considerahly metal ion, but at the same timeis cap* larger than expected. When the ions ble of qualitatively analyzing for several metal ions simultaneously. This instudied were arranged in series according to increasing solubility and spot volved as many as 11 in the specific system described. The results may area, and decreasing equivalent weight, be retaiued in a permanent form by the spot area series corresponded most simply stapling the paper chromatoclosely with the equivalent weight series, but was not identicd to it. graph in the note book, and quantitative This inrlirnt.ea II rnmhinatinn nf _._I _ _l.__t.ha.t. . ".._" l _"...l_ll"-"I ". analysis may he performed a t any time factors determines the final spot size, after the analysis with a simple measwith equivalent weight and solubility uring tool such as a planimeter or a tembeing the most important. plate, assuming that a known volume &"O ylyI
I
1900
ANALYTICAL CHEMISTRY
Quantitative analyses may also be performed simultaneously on more than one metal ion when k,, is sufficiently ditlerent that overlap of spots does not occur. The apparatus is portable. It may he carried almost anywhere in a briefcase, and set up for operation in a few minutes. The paper impregnated with precipitant is stable, and may he prepared in large lots prior to use, and stored in a clean dry container. In addition, a glass tank for maintaining an atmosphere saturated with solvent vapor is not necessary, as with other types of paper chromatography. The general theory of separation, and hasis for quantitative analysis are easily understood by personnel with a minimal training in chemistry. These advantages suggest that the method could very well find a whole area of application in field testing of such things as of geochemical samples, soils, ground water, etc. It is clear that a great deal more work needs to he done to develop this technique completely. LITERATURE CITED
RECEIVEDfor review January 21, 1963. Accepted July 31, 1963. Presented at Division of Analyticd Chemistry, 140th meeting, ACS Chicago, Ill., September 1961. Taken 'in part from a thesis presented to the Graduate School of the Michigan College of Miniig and Technology by Adele M. Liimatta in pmtial ful6Ument of the requirementa for the master of science degree.
