3 r d Ann&
Summer SgmposirCm
- Separations
Separation of Ultramicro Quantities of Elements by Electrodeposition L. B. ROGERS Laboratory for Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge 39, Moss.
E
LECTRODEPOSITIOS is it promising The practicalcmnsiderations present in laboratory air can be a serious method for sep:rratioti and concentrainvolved in devising a source of contamination (10, 17, 18). Conquantitative electroseparatamination is particularly difficult to avoid tion which,, unlike coprecipitatiori, estraction, tion of submicrogram if macro amounts of the element in question and ion exchange, has been the subject of comparatively few studies a t the submicroamounts of elements are. have in the past been handled in the same gram level. In most studies the investigator discussed. room. Frequent mopping of the laboratory has been concerned with the separation of floor and wiping of the desks and shelvegare trace amounts of radioactive elements either necessary, and it is helpful to provide protecfor studying fundamental electrochemical behavior of a radioactive tive covering for the entire electrolytic setup because, for the sake element existing only in trace amounts or for obtaining a carrierof simplicity, electrolyses are usually carried out in open vessels. Sorption. Unfortunately, the sources of error which lead to free preparation of a radioactive nuclide for use as a tracer. However, the radiochemist has also found electrodeposition to be low results arc even more numerous and distinctive than those a useful method for preparing films of an element so thin that described for positive errors. The most serious and general self-absorption of alpha-particlcs (24) or of weak beta-radiaerror arises from sorption, particularly by glass surfaces, because all of a trace element can easily and quickly be removed from a tion ( 2 ) is negligible. .4lthough persons working outside the field solution by this means. This introduces a serious problem in of radioactivity have shown little interest in the electrodeposition storing and handling standard solutions of trace concentrations. of traces, the techniques for liandling su1)microgram quantities Furthermore, the sorption process may be sufficiently strong to of elements are useful for concentrating a trace element prior to its compete successfully a i t h electrodeposition and thereby to analysis or for freeing a solution from traces of impurities. prevent complete deposition. Therefore, it is doubly desirable In any diwussion of analysis a t the submicrograin level, to doat all containers such as reagent bottles, electrolysis vessels, one must consider both the situation in which the sample constirrers, and electrolytic bridges in order to minimize both losses sists of a large volume of extremely dilute solution and that in from sorption and positive errors due to contamination. .4s which the sample is a very small volume of moderately concenwaxes and lacquers differ in their effectiveness (3, Bb), depending trated solution. For esample, in 100 ml. of water, 1 microgram upon the ions that are involved, the best coating for a particuof silver produces a IO-’ i1.I solution, whereas in 0.01 ml. the lar situation has to be determined by trial and error. Ceresine solution is lov3 M . In view of the fact that a modern test wax has been found the most generally applicable coating. ( 1 ) and a recent review ( 1 4 ) on microchemistry failed to mention -4ppreciable sorption. may occur a t the ends of salt bridges any technique for carrying out an electrodeposition using less which are plugged with agar, glass wool, or filter paper, but the than 0.5 nil. of sample, and a new book on ultramicroanalysis losses are usually of the order of 5 to 10% and can be corrected (11)failed to mention electrodeposition as a possible separation for satisfactorily by running appropriate blanks. Fortunately, method, most of the following discussion is nedessarily devoted losses from sorption can be decreased or eliminated if the element to studies involving large volumes of dilute solutipns. Haissinsky is deposited from a solution of a stable complex ion. Thus, (9),in his excellent review on the electrochemidtry of extremely in one study with silver the most serious losses were encountered dilute solutions, devoted most of his efforts to the fundamental in working with nitrate and perchlorate; small losses, with aspects of the problem; the present paper emphasizes the practiainmonia; and undetectable losses, with cyanide (8). cal asperts of interest to the analytical chemist. Radiocolloid Formation. Like sorption, radiocolloid formation has long plagued the radiochemist. Such formation of SOURCES OF ERROR insoluble compounds which remain suspended in solution may be considered simply as another type of compating reaction. Contamination. Anyone who has worked on a microgram To date, positive evidence in electrochemical studies for deviascale using dithizone or a similar reagent is well aware of the difficulty of avoiding serious contamination ‘pf a sample with tions from expected behavior which can be traced to radiocolloid the same or an interfering element which may, be present as an formation is scanty, so one can only point to it aa a possible source of difficulty in future studies. impurity, either in the reagents or on the walls of the equipment Solubility. If one is interested in the deposition of a comwith which the solution comes in contact. Obviously, these pound, such as an oxide, rather than an element, quantitative difficulties become increasingly serious aa the amount of elemeht in the sample is decressed. AB in work with dithisone, it is deposition from extremely dilute solutions may be impossible bwause of the solubility of the compound. Such a limitation has necessary to clean every piece of equipment scrupulously, prefbcen noted in studies dealing with the dioxides of manganese (3) erably just before it is used. Although doubly or triply distilled water is usually sufficiently pure for rinsing equipment, its use in and technetium ( 19). Fortunately, rccovery by electrolytic preparing a solution does not ensure freedom from serious concodeposition appears to be feasible. Even if an element or a compound can be deposited quantitatamination. Almost without exception it has been found necestively on the electrode, one is faced with the problem of minisary to subject solutions of reagents to electrolysis prior to the mizing dissol,ution of the deposit while it is being washed. In introduction of the sample. Such a procedure allows one to use handling submicrogram amounts of material, it is not a t all ordinary distilled water and to eliminate contamination simulunusual for the average depth of a deposit to be less than a monotaneously from both the reagent and the solvent. More spectacular, perhaps, is the fact that dust normally layer, so that a fraction of a second is enough for most of the
1386
VOLUME
22, N O . 11, N O V E M B E R 1 9 5 0
deposit to redissolve unless the required potential is maintained across the electrodes. In the deposition of silver, a relatively noble element, upon platinum electrodes, losses up to 20% were sustained if distilled water (8) or an organic solvent (‘7) were used as the wwh liquid, even if the electrical circuit was not broken during the process. Losses can be decreased somewhat by making the potential of the cathode much more negative imrnediately before starting to wash (28). I n the light of these facts an electrolyte was added to the wash liquid to decrease the losses by facilitating maintenance of a suitable potent,ial. It is important to note, however, that silver, after being deposited quantitatively from a cyanide solution, dissolved to the extent. of 5 t o 10% when washed with fresh portions of the same cyanide solution, even though control of the potential appeared t o be satisfactory. Substit’ution of sodium nitrate or sodium perchlorate for the sodium cyanide in the wash solution reduced the losses to less t.han 0.5% (8). These experiments emphmise the desirability of using in t,he wmh solut>ionan electrolyte which has little or no tendency to dissolve the deposit hp complex formation. Assuming that a deposition and washing have been carried out quantitatively, one is t,hen faccd with the problem of removing the deposit quantitativt4y from the electrode prior to the determination of the element by chemical methods. The worst situation is represented by the classical case of polonium deposited on platinum, where complete dissolution of the deposit can be effected only by dissolving the entire electrode. On the other hand, it is not a t all unusual to find that 5 to 10% of an element cannot be removed from : ~ n“inert” electrode such as plat,inum even by using solutions of strong complexes combined with reversal of the electrode potentid. However, the amount retained by the electrode, in the case of silver deposited upon platinum, is usually decreased l),v continued use of the electrode (3). This behavior m:iy he the result of removing “loose” electrode material during the process of repeated cleaning, I\ hich might, at the same time, decre:tse the number of pores and cracks in the surface. Alternatively, one may consider that the pores and cracks becomc saturated with the particular element after a number of electrolyses, so that further deposition (and eschange) is slowed down or prevented. ,Neither idea by it,self appears t o be ent,irely sntisf:actory as an explanation. CONTROL OF ELECTRODE POTENTIAL
In recent years an:ilvtic:tl chemists have begun to appreciate the fact that electrosrparations can be improved by careful selection and control of the electrode potential. Because of .the deart,h of information on the subject, it was necessary to determine the optimum potential by trial and error until Lingane (15) recently demonstrated the feasibility of using polarographic data obtained with a dropping mercury electrode,to obtain data for electrolytic separations to be performed with a mercury cathode. The idea has Iwen estended by others t,o separations using solid electrodes (8). The f w t that polarograms indicatci only relative rates of deposition of two or more elements is usually not a serious handicap, but if equilibrium conditions are approached, it appears necessary for best results t o consider the relative volumes of the aqueous solution and of the mercury as well as the overvoltage and reversihility of the reactions ( 2 0 ) . unfortunately, if the electrolysis is to be .carried out with a solid electrode and if the amount of deposit is insufficient to form a monolayer, there is no satisfactory method for predicting the potential at which to carry out the electrolysis. A difference of nearly 0.5 volt may be found between the deposition behavior of fractional monolayer of a deposit and a multilayer “macro” deposit (28). However, a smooth transition in behavior has been observed in one study by examining a series of solutions differing only in the initial concentration of silver ion (3). Likewise, the duration of the electrolysis may he a factor to consider (5, 22). As Haissinsky pointed out ( 9 ) , there is no real reason for ex-
1387 pecting that predictions based upon the Nernst equation should describe the deposition behavior of a trace upon an i ert electrode. Haissinsky and his co-workers (4-6) and others 6 3 , 88) have demonstrated conclusively that the electrode material profoundly affects the extent of deposition at a particular potential. The usefulness of the Nernst equation appears to be limited to providing a reference point from which deviations can be measured in describing the behavior of traces deposited upon solid electrodes (23). Finally, the deposition behavior of a trace element may also be profoundly altered by codeposition of a second element, particularly by “macro” amounts of the latter (20, 21). Thus, the selection of a potential for a large number of possible separations must continue t o be determined by trial and error. Although it is usually possihle t o maintain the desired potential automatically nithin a few millivolts by means of specially designed instruments, in exceptional cases regulators may respond too slowly to prevent serious broadening of the limits of control (18). The loss of control appears to occur in a region where gas evolution is just beginning to take plare at an appreciable rate. The region of potentkt1 at which this phenomenon takes place appears to vary with the previous treatment of the electrode as well as with the potential and the electrode material. STUDIES USING ULTRAMICRO VOLUMES
During the. past 8 months, work at the Massachusetts Institute of Technology has been.direrted toward the development of a procedure which will allow analyses to be made using a sample of 5pl. made up to a tot’al volume of 10~1.(16). By combining electrolysis at a cont,rolled potential with coulometric measurement of the deposit, it is possible to minimize interfering elements and to dct.ermine within about, * 10% aniount8sof silver of the order of 5 X 10-8 gr:im. The technique can detect amounts slightly less than 10-10 gr:tm. LITERATURE CITED
(1) Benedetti-Pichler, A. A,. “Introduction to the Microtechiiique of Inorganic Analysis.” New York, John Wiley & Sons, 1342. (2) Rrosi. A. R., and Griess. .J. C.. .Jr.. private communication.
(3) Byrne. J. T., unpublished work., (4) Coche. A.. Comnt. & d , 225. 936 (1947). Coche, A , , Faraggi. H., Avignon, P..and Htissinsky, XI,, J . chim. phya.. 46, 312 (1949). Coche. -4.. and Haissinsky. M., Cumpl. rcnrl., 222, 1284 (1946~: Griess. ,J. C.. .fr.. unpublished work. Griess, J . C., , J i , . , and Rogers, L. B., J . Electrochem. Soc., 95, 129 (1949). Haissinsky, M., “Electrochimie des substances radioactives et des solutions extrement dilukes,” Paris, Hermann & Cie, 1946; J . chim. phys., 43,21 (1946). Hollingsworth, M.. private communication. Kirk, P. L., “Quantitative Cltramicroanalysis,” New York, John Wiley & Sons, 1950. Lamphere. R. R., and Rogers, L. B., A h h i . . CHEM., 22, 463 (1950). Lietske, M.. and Griess, .J. C., J r . . unpublished work. Lindsey, A. J.. Analyst, 73,67 (1948). Lingane, .J. *J., IND.ENG.CHEM..Ax.\r.. ED.,16, 147 (1944). Lord, 8. S., Jr., O’Neill, R. C., and Rogers, L. R., unpublished work. ,Metz, 0. F., private communication. Rodden, C. J., private communication. Rogers, L. B., J . Am. Chem. Soc., 71, 1507 j1949). Rogers, L. B., paper presented a t the 97th meeting of Electrochemical Society, Cleveland, Ohio, -4pril 21, 1950. Rogers, L. B., and Byrne, J. T.. paper presented a t 97th meeting of Electrochemical Society, April 21, 1950. Rogers, L. €3.. Krause, D. P., Griess, J. C., Jr., and Ehrlinger. D. B.. J. Electrochem. SOC..95, 33 (1949). , Rogers, L. B., and Stehney. A. F., Ihid.. 95, 25 (1949). Seaborg, G. T.. Kats, J. J., and Manning, W. M . , “Transuranium Elements,” Vol. I, p. 147, Vol. 11, p. 1140, New York, McQraw-Hill Book Po.. 1949. RECEIVED July 8 , 1950. Work supported in p a r t by the Office of Saval Research and t h e Atomic Energy Coin~niasioii.
