lonophoresis Much More Than a Low-Cost Analytical Technique in the Qualitative Analysis Laboratory Daniel W a l e and Fernando Labandera Cdtedra de Q.A. Cualltatlva. Facultad de Quimlca, CC1157, Montevideo. Uruguay
In the past several years, a number of new techniques have been more used more and more freauentlv in the aualitative analysis laboratory. In some cases, these techniques have replaced the classical methods of precipitation and dissolution, while in other cases they have complemented them. Amone these is electro~horesis, . lone - and widelv used to separate complex organic mixtures, mainly of proteins,' and recently introduced to the inorganic qualitative analysis laboratory.2 In this application electrophoresis is known as ion-
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Ionophoresis is based on the action exerted by an electric field on the ions present in an aqueous solution. This action brings about the differential migration of the ions through a porous support in the direction of the corresponding electrode. deoendine on the size and sien of the charee - and of the size of thk ion. In order that experiments can be reproduced in the laboratory, mobility (u)is defined according to the following
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Figure 1. Apparatus used for the experiment. A, paper; C, elecholyte: 0 , carbon electrode.
power
supply: 6, d r i p of
where d is the distance traveled by the ion, t the time during which the potential difference V is applied, and 1 the distance between the electrodes. In a basic chemistry course, laboratory exercises must obviously be easy to carry out, as students are just starting to e t&e get acquainted &th laboratory techniques. ~ t t h same they must stimulate the students' interest and adequately integrate previously learned material and skills. In our laboratory, we have included paper ionophoresis in the regular laboratorv sessions. as we consider that this techniaue meets the above-mentioned requirements. The theoreticil bases of ionoohoresis have been recentlv added to the curriculum of which already included other procedures such as our ion exchange and chromatography.
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Procedures Several strips (approximately 1.5 X 25 cm)of No. 1Chromatogrsohv Whstman naner are immersed in a vessel containine a solution i f ,f aauitehle eieckdvte. ~,The strins are then allowed to drain to remove excess liquid. We have found that leaving the strips in the solution for 5 min i sufficient time to ohmin ionophorograms wirh an adequately homogenous development. The strip of paper is then held by either end snd placed on a flat glass plate (5 X 12 em) at the top of two 250-mL beakers, as shown in Figure 1. Both ends of the strip must be immersed in the beakers, checking that the electrolyte level in the beakers is at 2.5 em of either end of the strin. Carbon electrodes are olaced in the vessels and connected to the teminak of a POWPI JUPPIY. Using egraphite pencil, r h point ~ or points where the reagenrsare applied hy means of a capillnry tube are marked. ~
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Gaal, D.; Medgyesi. G. A,; Vereczkey. L. Electrophwesls In the Separation of Biological Macromolecules; Wlley: New Yoh, 1980. Lederer. M.: Mosini. V . J. Chromatax 1973, 77. 464-466. ~~edreer; ~ . c h e mlnd. . 1954, 48, 1481. Grassini, G.: Lederer, M. J. Chrornatogr. WS9, 2, 326. 178
Journal of Chemical Education
Figure 2. Resolution of a midurs containing hsxacyanoferrate(lt).hexacyano ferrate(lil),and thiooyanate. A, point where me midure to be analyzed is applled;8. blue spot due to hexacyanolwate(ll):C, blue spot due to hexacyanoferrate(lil):D,red spot due to thkyanate: E, point where the midure of Fe(ll) and Fe(iil)Is applied. Once reagents have been applied, the glass and paper strip are covered with another glass with the same dimensions as the first one. In order to carry out an ionophoretic run, the potential of the power supply is adjusted to the desired value, and we allow for the necessary time to elapse. Although this is a safe technique, all necessary precautions when handling these voltages must be taken. Speolllc Examples Ledere13 and Grassini4 studied the se~arationof different ions, including hexacyanoferrate(l1) ind hexacyanoferrate(1ll). It is of analytical interest tostudy the resolution of a m&re of these two, ions with thiocy&ate. For that purpose, we have developed in our laboratory the procedure shown in Figure 2.
The color development is carried out using H2S under a hood. and the ionoohoroeram shown in Fieure 3 is thus obtahed. A black PbS spot can be observed a t C, which indicates that the Pb(I1) cation reacted with HC1 to give insoluble PbC12, thereby remaining a t the point where the mixture was applied. A brown CUSspot can be observed at A, whereas a yellow CdS spot is present in R.Taking into account the fact that the distance between A and the p&t where the mixture is applied is twice the distance between said point and B, i t can he inferred that Cu(I1) migrates in the form of an ion with acharge twice the charge of Cd(I1); i.e., Cu(I1) migrates as Cu2+,while Cd(I1) migrates as CdClf. The black HgS spot a t D indicates that, contrary to that might be expected, the Hg(I1) cation migrates toward the anode; as the distance from D to C issimilar to that from C toB, i t can be concluded that the Hg(I1) complex has a negative charge, i.e., i t is HeClr-. . . --" Using this technique, we can separate manv other ions whose similar chemical properties make traditional procedures slower or difficult. This is the case of several groups of anions such as phosphate, arseniate and arsenite, &d Ehloride, bromide, and iodide, which can be separated using KN03 0.05 M as electrolyte, a 300-Vvoltage and AgNO, as color developer.
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Figure 3. Resolution of mlxture containing Cu(ll), Cd(ll), Hg(ll), and Pb(ll). A. bnlwn spot due toCuS: 0, yellow spot due to CdS C. black spot due to PbS; D. black spot due to HgS.
Themixture to beanalyzed isapplied at A, while adrop of the solution containing ions Fe(I1) and Fe(l1Ij is applied at E. As a potential difference results, cations migrate in an opposite direction to anions, thus producing the color development simultaneous to the ionophoretir run. Both hexacyanoferrate(1l)and hexacyanoferrate(lI1j form nonionized compounds that remain at the point where they meet ions F e W and Fe(III), which move toward hexacyanoferrate(1I) and hexacyanoferrate(II1) IH and C). After reacting with ions Fe(1ll). thiocvanate continues to move forward. although more s l o w l ~in the form of a complex ion (D). This enables us to separate, as much as desired, the thiocyanate spot from the remaining spots. In order to do so, we only need t o allow the ionophorogram to develop enough time. In this specific example we used a voltage of 250 V and 0.01 M HCl as electrolyte. The separation of ions Cu(II), Cd(II), Ph(II), and Hg(I1) is of analytical interest, as they are included in the same group in the classical analytical scheme. I t also provides an excellent opportunity to visualize the formation of complex ions. The e a u i ~ m e nused t is the same as in the orevious example, but ihe ration mixture to he analyzed k placed at the center of the filter paver strip. HCI 0.1 M is used as electrolyte, and a 300-Vv;lGge is applied during 15min. After this time has elapsed, the current is cut off, the strip of paper removed and allowed to dry by exposure to a draft.
Conclusions The advantages of applying ionophoresis for the separation of the above mentioned ions are mainly economic. Most laboratories possess apower supply, and the cost of purchasing one, if necessary, is not high. Carbon rods contained inside dry cells can be used as electrodes. Filter paper is certainly the most inexpensive we can use as support. Ion migration phenomena occur slowly enough so that the formation of complex ions and their movement can be easily visualized. Hence students acquire a concrete image of a process they have only learned through diagrams, figures, and equations. Finally, i t should be pointed out that even students with little training can obtain satisfactory results, which makes them feel confident in their own skills and in the method itself.
Volume 68 Number 2 Februaly 1991
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