W. Joe Frierson, Nina, Marable, ond Becky Bruce
Agnes Scott College
Decotur, Georgia
A Quantitative Experiment with Paper Chromatography
Experiments in the qualitative separation of metallic ions by paper chromatography have been rather widely used in general chemistry or qualitative analysis; hut few, if any, paper chromatography experiments have been introduced into quantitative work. A procedure developed in this laboratory for the separation and determination of cobalt, copper, nickel, and zinc' has been combined with more recent methods for the spectrophotometric determiuation of cobalt and nickel2a and adapted to student use. The amount of time (approximately six three-hour laboratory periods) required for the experiment is not considered excessive since the student receives trainiug in several techniques and principles. A glass pipet cleaning jar, 46 X 15 cm, covered with a desiccator top serves as a convenient chamber. A slide staining dish atop a 38- X 3.8-cm hydrometer cylinder may he used as the solvent reservoir for preparing descending chromatograms on strips of Whatman No. 3 mm filter paper. The Experiment Solvent I . 60 mi n-butyl alcohol, 11ml water, 5 ml12 M HCI. Mix just prior to use. Solvent 11. 50 ml acetone, 42 ml 3-methyl-2-butanone, 6 ml 6 M HCI. Mix just prior to use.
408 / Journal of Chemical Education
Indicators. 0.01% solution of dithizone in CC4. Keep refrigerated. 0.01% solution of rubeanic acid in 1 M NaOH solution. Prepare fresh daily. Solution of Metallic Ions. The following weights of the anhydrous salts me used in preparing 250 mlof solution: 0.77% g ZnO dissolved in 10 ml 6 M HCI, 1.5695 g CuSO*, 0.5508 g CoC12, and 3.3125 g NiCb. This solution contains 2500 ppm of capper, 2500oom of sine. 1000 oom of cobalt. and 6000oom of nickel. ~ u & r ~olution. Mix 1 part of 0:2 M boric k i d with 9 parts of 0.05 M borax to obtain a pH of 9.0. ( ' 0 1 ~ 1T. F U Q P P ~ ~ T ~ .k r u g e n r f r v,qpcr a d zh.r is 1 rcpard by o i l 1 g f i n L d l u t h ! Chw.iril I'r.,dtl:ts C. , I , 1 . i r l 2 1111 uf 1 _I/ S n ~ l n1.d l dlluring 1%.100 1111 with water. This solution should be freshly preparedeach week. The reagent for nickel and cobalt is prepared by dissolving 0.291 e of 1.2.3-evclahexanetrione trionime (Eastman No. 76601 in 100 mi of 95% ethyl alcohol.
For each chromatographic jar to be used, cut six paper strips 43 cm long, and fold each strip 3 and 3.5 cm from one end. Apply 25 lambda of the standard solution of the ions across 5 of the 6 strips per jar a t a point 3 cm below the second fold. Five chromatograms are run to insure against trouble in subsequent steps and still to have two results for reporting. The volume applied will contain 62.5 pg of each zinc and copper, 25.0 L F ~W. J.,~ REARICK, ~ ~D. A,,~ YOE, ~ J. H., ~ Anal. , Chem., 30,468 (1958). W. J., PATTERSON,N., HLRRILL,H., MARABLE, *FRIERSON, N . , Anal. Chem., 33,1096(1961). 8 FRIERSON, W. J., MARABLE, N.,A d . Chem., 34, 210 (1962).
rg of cobalt, and 150.0 pg of nickel. The sixth strip, without any ions, is carried through the same chromatographic procedures and the section corresponding t o a given ion is cut, eluted, and used as the reagent blank in the spectrophotometric determinations. Mix the components of solvent I and pour it into the slide staining dish. The dish is placed on top of the hydrometer cylinder which is standing in the pipet jar. Cover the jar and allow it to become saturated with the vapor of the solvent (20 to 30 min). Hang the strips over the edge of the dish so that all the folded edges dip into the solvent to the same extent. The strips should be held in place with a glass slide and should not be allowed to touch each other or any of the glassware below the edge of the dish. Allow the solvent to migrate down the strip for 17-24 hr or until the zinc is 4 4 em from the end of the strip. I n most cases a faint brown line on the strip indicates the position of zinc; however, if this line does not show up, one strip must be removed and developed with the dithizone in order to determine when all strips are to be removed. During this time the solvent will begin to drip off the end of the strip and the cobalt, nickel, and copper will remain near the point of application. The relative positions of the ions are shown in Figure 1. After migration is complete, remove the strips and allow them to dry. At the second laboratory period, while the jar is becoming saturated with solvent 11, swab the lower 8 cm of one strip from each jar with the dithizone reagent to locate the zinc band. Using this developed strip a s a guide, clip off the lower section of all the strips cutting 1-2 cm above the indicated position of the zinc. Hang the remaining upper portion of the original strips in solvent 11. Allow this separation to proceed until
Figure 1. Left, position of the ions after using solvent I. 'Right, pwition of the remaining ions after using rolvent 11.
Figure 2.
the yellow copper band has moved to 6 cm from the end of the strip, (4-5 hr). I n most runs a blue band near the center of the strip shows the position of cobalt; however, the rubeanic acid test as described will definitely show the position of cobalt if the blue band does not appear. Develop one strip immediately while it is still damp by hanging it in ammonia vapor until the copper band turns blue and then by swabbing the entire strip with the rnbeanic acid solution. The ,copper will now appear as a green band near the bottom
of the strip, the cobalt as a yellow band near the center, and the nickel as a blue band near the top. Figure 2 shows the relative positions of the three ions after using solvent 11. While this separation is in progress, elute the zinc by cutting the strip to a slight point a t the end nearest the zinc band, creasing the other end and hanging in 0.1M HC1 contained in a Petri dish. The Petri dish is mounted on an inverted beaker such that the eluting liquid from the short strips can be conveniently caught in small beakers. This arrangement is placed inside a desiccator or other container which can be kept closed during the elution. The elution should be complete in two hours when about 2 ml of the eluate has collected in the beaker. Before the strips are removed it is advisable to place about 10 drops of the eluting acid a t the top of the strip so that it may flow down the strip and elute any remaining ions. Do not allow the eluates to evaporate before the spectrophotometric determinations are made. A Beckman DU spectrophotometer with 1.0 cm cells was used in making the measurements. I n subsequent laboratory periods cut from the strip separate sections for each nickel, cobalt, and copper, and elute each individually: the nickel with 0.1 M HC1, and the cobalt and the copper with 3 M HC1. The following methods are recommended; the references will supply complete directions. Copper and Zinc. Transfer the eluted ions to 50 ml volumetric flasks, add 20 ml of borax-boric acid buffer and 3 ml of the Zincon solution. If the pH does not check between 8.5 and 9.5 use ammonia to adjust it. Dilute to 50 rnl with water, mix well, and make absorbance measurements at 620 mp against a reagent blank.' Cobalt. To the eluate add 0.8 ml of the solution of 1,2,3-cyclohexanetrione trioxime, adjust the pH between 3 and 4, transfer to a 10 rnl volumetric flask and dilute with water. After 30 min make absorbance readings at 400 mp against a reagent blank.% Nickel. To the eluate add 3 ml of 1.0% gelatin solution and 0.75 ml of the solution of 1,2,3-cyclohexanetrione trioxime. The pH normally falls in the correct range of 3 4 , but any necessary adjustments may be made with ammonia or hydrochloric acid. Transfer to a 10 rnl volumetric flask and dilute with water. Absorbance measurements are made at 560 mr against a reagent and gelatin blank3 All water used should be redistilled since very small amounts of ions present in it will interfere. The amount of each ion recovered from the chromatograms is read from Beer's law absorbancy graphs for concentrations in the ranges of 1.&25 ppm of nickel, 1.0-5.0 ppm for cobalt, and 0.5-2.0 ppm for copper and zinc. These graphs may be determined by using aliquots of stock solutions of the various ions to prepare solutions of known concentrations in the appropriate ranges and then treating these solutions according to the given procedure. Once these curves have been prepared, they may be used from year to year with fairly good results. Students with no previous training in any of the special techniques involved can readily achieve an accuracy of 2 to 10%. Volume
40, Number 8, August 1963
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