The noise level will be defined as T.v. The figure of merit can therefore be expressed as follows:
SYMBOLS
Dimensions O O O O
M’ is invariant nith respect to scanning speed. for the reasons already outlined. I t i i also ~ n v a i ~ a with n t respect to AH, .mce this t j [)e of instrument a h a ) s has a large source resistance. I t ai11 be realized that, in the absence of filtering effects, TI. snd AT should be identical TF the temperature differential across ROa t the conclusion of the transition, is exact13 the amount by m hich the sample ten perature lags the program temperature as a result of the transition Theieforc,, 31’ cannot exceed the reciprocal of the temperature noise
O
C.
c. c. c. c.
O
C. see.-]
O
C.
cal. cal. cal. see.-’ cal. see.-’ cal. sec.-l cal. sec.-l cal. O cal. O
~
C. see. cal.-l O
C. sec cal.-l
O
C. sec. cal.-l
cal. sec.-l sec .
In practice, transitions are always limited by the thermal bandwidth of the system, characterized by the time constant Rocs, and A 7 is considerably larger than Ty. Temperature resolution and sensitivity are incompatible in this type of instrument, since the former requires a small value of Ro,while the latter requires a large value of Ro.
O
C.-l
see.
O C. cm. sec. cal.-] cal. g.-l g. c m . + caL2 c m . O~C.-l ~ sec.-l
O O O O
c.-1 C. c.
c.-1
Definition Sample holder temperature Platform temperature Transition temperature Temperature source .lmbient temperature Scanning speed Temperature resolution of sharp transition Transition energy Energy Sample heat flow rate Measured heat f l rate ~ (ordinate) ~ Ordinatc noise level Peak value of iV Thermal capacity of sample holder Thermal capacity of sample and container Thermal resistance between sample holder and environmerit Output resistance of temperature control system Thermal resistance between sample and sample holder Amplifier transfer function Temperatiire control system time constant Ordinatp readout time constant S a m ~ ~resistivity le Specific transition energy Sample density Convenient sainple parameter Platform area Figure of merit D T h ordinate DTh ordinate noise level DTX figure of merit
ACKNOWLEDGMENT
The author is grateful to E. S. \Tatson, who participated in many helpful discussions relating to the subject matter of this paprr. LITERATURE CITED
(1) Arndt, R. A , , Fujita, F. E., Rev. Sci. Inst. 34, 868 (1963).
Co nti nuo u s
( 2 ) Dauphinee, T. M,, MacDonald, D.
K.C., Preston-Thomas, O., Proc. Roy. Soc. 221, 267 (1954).
( 3 ) Eyraud, C., Compt. Rend. 238, 1511 (1954). ( 4 ) Smith, C. S., Trans. A.I.M.E. 137, 236 (1940). ( 5 ) “Differential Thermal Analysis,”
Smothers, W. J., Chiang, Y., Chemical Publishing Co., New York, 1958.
Qua nti ta tive
(6) Sykes, C., Proc. Roy. Soc. 148, 422 (1935).
(7) Vold, >I. J., Ax.4~. CHEY. 21, 683 (1949). ( 8 ) Westrum, E. F., J r . , Hatcher, J. B., Osborne, D. W., J . Chem. Phys. 21, 419 (1953).
RECEIVEDfor review January 8, 1964. Accepted March 30, 1964.
EIec t rolysis
W. J. BLAEDEL and J. H. STROHL’ Chemistry Department, University o f Wisconsin, Madison, Wis.
b The construction arid operdion of a continuous electrolytic cell, designed for the quantitative electrolysis of material in a flowing stream, are described. Some applications to analysis are presented: quantitative oxidation of Ce(lll), selective prereduction of Fe(1ll)-(UVI) mixtures, and separation of Cu-Pb-Cd-Zn mixtures. At low concentrations, the selectivity of the cell approaches that of controlled potential devices, but selectivity decreases as the concentration of electroactive material increases.
T
HIS article presents the design and operation of a simple apparatus that can be used to electrolyze substances quantitatively in a flowing solution, and some applications to analysis and separations. The continuous electrolytic cell is essentially a short column packed with a material that can act as a n electrode. The cell has several advantages when compared to a metal reductor column, which it resembles in many respects. The cell is packed with a conducting material whose potential can be varied.
Only one packing material i b necessary for a wide variety of tasks. Since the potential is applied from an external source, it is independent of the supporting electrolyte, and there is a aide choice of supporting electrolytes for use in a particular situation. Contamination of samples by the introduction of metal ions is less than in metal reductor columns. The cell can be used for tahks which cannot be performed by Present address, Chemistry Depart-
ment, West Virginia University, Morgantown, W Va. VOL. 36, N O . 7, JUNE 1964
1245
metal re !uctor columns. Oxidations can be performed, and plating processe? followed by stripping procewes can be performed by changing the applied potential. So far, only one device has been reported which has been used in this manner (IO). The cathode consisted of a glass column packed with platinum wire rings. It was claimed to be suitable for the purification of aqueous solutions contaminated by heavy metal ions, and for the separation of carrier-free radioisotopes. C O L U M N CONSTRUCTION
The cell consists of a short column packed with crushed graphite, which serves as a n electrode for effecting various electrocheniical processes. The graphite is in electrical contact with a counter electrode via a potential source and a low resistance solution pat,h. The apparatus is shown in Figure 1. The column proper, I , is a piece of 7-mm. i.d. porous glass ( S o . 7930, Corning Glass Works, Corning, E, Y , ) about 5 cm. long. About 6 mm. of the glass protrudes from the rubber stoppers, E , a t either end. There are two electrodes outside of the porous glass tube: a reference and a counter elect'rode. h single le'ngth of 16-gauge silver wire, D, isolated from the rest of the compartment by a piece of 4-mm. i.d. porous glass, H , is used as a reference electrode. The counter elect'rode, L , is a helix of 16-gauge silver wire in contact with granular silver, G (G. Frederick Smith Chemical Co., Columbus, Ohio), which fills the rest of the reference conipartment,. The reference electrolyte enters and leaves t,he com1)artinent through '/*-inch 0.d. Tygon tubing, &A', which fits into undersized holes in the rubber st3ppers. So that it cannot be pulled out, a short segment of a larger Tygon tubing is sealed over the end nit,h cyclohexanone. The bottom plug, S,is made of Graphitar (Grade 84, L7. 8. Graphite Co., Saginaw, X r h . ) , machined so t,hat about 5 mm. fit,s snugly inside tmhe porous glass tuhc and the rest (about 1 cm.) has the same diarnet,er as the outside of the p n m s glass. '/*-inch hole is drilled axially through the plug and a 3/16-inch depression about 5 mm. deep is drilled into the inside end. The upper plug, B , is made of sections of Tygon from to i?16 inch in dianieter, telescoped and joined together with cyclohexanone; the outer piece of tubing, C, joins the plug to the porous glass, and the inntlr piece is used as the inlet to the cell. The apparatus is asseinbled by inserting the bottom stopper (with porous glass, Tygon tubing, and silver wire already in place) into the bottom of the 24-nim. glass jacket, plwing the piece of 4-mm. i d . porous glass in the compartment, filling the rest of the conipartnient with granulated silver, and then putting the top \toi)per in place. To lire\-ent cracking, the porous glass is wet uniformly before assembly and kept wet. The Tygon sleeves, C, that cover the ends of the purous glass tube should 1246
ANALYTICAL CHEMISTRY
Figure 1. Cross section of assembled cell A,A'.
Tygon tubing, '/*-inch 0.d. Plug m a d e of Tygon tubing C. Tygon tubing, '/lr,-inch 0.d. D. Silver wire reference electrode, 16 gauge E. Rubber stopper F . Gloss cylinder 2.5 cm. long, 24-mm. diameter G. Granulated silver H. Porous glass tubing, 4-mm i.d. I . Porous glass tubing, 7-mm. id. 1. Graphite packing, 1 0 0 - 2 0 0 mesh K . Graphite rad, 2.5 cm. long, 2-mm. diameter 1. Silver wire counter electrode, coiled, 1 6 gauge M. Glass wool plug N. Graphitor plug 0. Copper wire, 2 0 gauge
B.
cover the exposed porous glass completely, t,o prevent it from drying out. The 16-inch Tygon sleeve used to connect the bottom plug to the porous glass can be expanded temporarily by heating it in boiling water to make assembly easier. -After assembly, the Tygon sleeves are wired onto the porous glass and t'he Graphitar plug. .Z loop of 20-gauge copper wire, 0, tightened directly onto the bottom of the Graphitar plug makes electrical contact to the column. The column is packed by first tamping a small plug of glass wool, .If, into the depression in the bottom plug, and then adding enough of a graphite-water slurry (100- t o 200-m~sh graphite) to form a bed about 1 em. deep. h
graphite rod, K (2-mm. diameter and 2.5 em. long), is centered in the column and pressed into the bed. The column is then packed up to the top of the porous glass by alternately adding small portions of slurry and tamping with a piece of glass t,ubing. .Ifter packing, the upper Tygon sleeve, C, is wired onto the porous glass. The graphite packing is prepared by grinding spectrographic graphite elect'rodes (Kational Carbon Co., New York, K. Y.), collecting the 100- to 200mesh portion, and rinsing with acetone, detergent solution, and distilled water. The annular space containing the silver electrodes is filled with 6M XaC1, which is flushed a t a rate of 0.2 to 0.5 nil. per minute during operation. The elect,rical circuit is shown in Figure 2 . The potential of the graphite bed is adjusted manually with the 10-ohm potentiometer. The resistance between the graphite bed and t,he counter electrode is about 5 ohms (measured with a Serfass conductivity bridge, Model RCM15) with 1.5.M NaCIOa in the column. The potential is measured with a Simpson Model 260 vohmeter (20,000 ohms per volt). Current is measured with a S'arian Model G-11 recorder (10 niv. full scale). The use of a separate high impedance circuit to measure the graphite electrode potential compensates for potential drops through the porous glass, but not for potential gradient,s t,hrough the graphite bed or in the solution within the graphite bed. These potential drops can be appreciable under some conditions. The use of a separate reference electrode also compensates for possible polarization of the counter electrode. The graphit,e rod is placed in the graphite bed in an attempt to minimize its longit'udinal resistance. Without, t,he graphite rod, t,he end-to-end resistance of the bed is 6 to 10 ohms; with the rod, it is less than 2 ohms. h relatively high concentration of supporting electrolyte is used to keep the solution resistance within the graphite bed low. GENERAL PROPERTIES
OF
THE C O L U M N
Contamination. Exchange between the sample and the counter electrode solutions can occur through the porous glass tube, causing loss and contamination of the sample solution. T o reduce these effects, the sample is pumped a t as high a rate as convenient. Flow rates of about 6 ml. per minute are easily obtained with a peristaltic pump (Model PA23, New Brunswick Scientific Co., Yew Brunswick, K. J.). At this flow rate, the average residence time of the solution in the column is about 5 seconds. Higher flow rates are difficult to obtain because of the high flow resistance of the graphite bed. Under these conditions, two contaminants reach the sample stream from the reference compartment. Chloride appears in the effluent sample Qtream a t about 0.001X. Traces of silver can enter the iample Ytream if the
U
Figure 2. Electrical circuit C.
P. R.
Counter electrocle (L, Figure 1 I Recorder Reference electrode (D, Figure
1) V. W.
Voltmeter Working electrode (graphite packing, J, Figure 1 )
graphite potential is rnore anodic than f0.4 volt. At more cathodic potentials, the silver is plated onto the graphite bed, but it can be removed periodically by stripping at a n anodic notential. Useful Potential Range. The background current of the cell would be expected t o be similar to t h a t of a graphite electrode of similar area. The useful potential range of the column lies between -1.50 volts (cathodic, with respect to the -4gAgCl reference electrode in 6 M KaC1) and about +2.0 volts (anodic) with 1.5M S a C 1 0 4 (adjusted to p H 3 with HC104) flowing through the cell. The background current is less than 10 ma. throughout the range -1.20 to +1.5 volts. This range is larger than the useful range of graphite as an indicating electrode; the cell can operate effectively with a high background current, since the cell current ib not used for coulometric measurements. Gassing is perceptilile outside of the range -1.0 to +1.4 ~ o l t but s does not appear to interfere with cell operation. Because of the flow of solution through the column, the gac; is constantly removed and there is always sufficient electrode area expclsed to permit quantitative electroly&. Effect of Current on Operation. As the current increacies, the potential gradients in the cell also increase, and the effective electrode potential becomes less than the indicated potential. T o show this effect, samples containing various concentrations of Cu(I1) were prepared in a supporting electrolyte consisting of 1.5.44 NaC104 adjusted to p H 3.0 with HC104. Samples of 0.1-ml. siae were then injected into a stream of the same supporting electrolyte flowing through the column a t 6.5 ml. per minute, a t a particular applied potilntial. The fraction of the injected Cu(I1) that emerged in the effluent stream mas measured with the polarographic flow cell; Figure 3 shows the percentage of the Cu plated
out for various indicated potentials and influent Cu(I1) concentrations. A theoretical curve, A , is calculated from the Nernst equation and the following set of ideal conditions. The sample solution is presumed to travel through the column as a slug, without dilution. Deposition is presumed to occur onto a solid electrode of unit activity which is in equilibrium with the solution. Lastly, since only the shape and slope of the ideal curve are important in the following considerations, a deposition potential of 0.050 volt was assumed, a t which plating just begins to occur. Comparison of the experimental curves of Figure 3 with the theoretical one shows that the slope decreases with increase in Cu(I1) concentration. aUso, the experimental curves shift in a cathodic direction with increased Cu(1I) concentration, rather than in the anodic direction that would be expected theoretically. Both effects can be explained by the previously described potential drops, which increase as the concentration and current increase. Figure 3 shows that for 0.1-ml. samples injected into the column under the previously described conditions, 0.01.k' Cu(I1) could be separated quantitatively from material which is not plated a t potentials more anodic than -0.2 volt, but 0.03.V Cu(I1) could be separated quantitatively only in the presence of materials which are not plated at potentials rnore anodic than -0.45 volt. .It low concentrations and low currents, the selectivity of the cell approaches that of controlled potential systems, but as the current increases the selectivity of the cell decreases. Change in Acidity of EfHuent. At high cathodic potentials there is a change in the acidity of the effluent due t o the reduction of hydrogen ion. Thus a n influent solution of 1.5M XaC104-HC104, p H 3, is changed t o p H 4 when passed a t 6 ml. per minute through the cell a t -1.20 volts and t o p H 6 a t - 1.30 volts. For any particular application, conditions must be chosen so that the p H of the solution remains in a range acceptable for that application. The conditions must be chosen rather empirically, because the p H change also depends upon the eluent composition, column dimensions, flow rate, and any changes in, the electrode due to plating. Any metal plated onto the column may change the hydrogen overvoltage with a resultant change in acidity. For example, the deposition of 0.01111 Cd(I1) from 1.5M NaC104HC104, p H 2, at -1.0 volt results in a n effluent p H of about 10. Also, the reduction of any oxygen in the solution may increase pH, if the solution is poorly buffered.
I
0
Figure 3. copper
,
,
,
,
,
,
-02 -0.4 -0.6 -0.0 INDICATED POTENTIAL, V.
Deposition efficiency for
0.1-ml. samples of Cu(ll1 in N a C I 0 4 - H C I 0 4 injected into NaCI04-HC104 siream flowing a t
6.5 ml./min. through column
A. 6. C.
D.
Theoretical curve 0.01M Cu(ll). Maximum current, 10 ma. 0.03M Cu(ll). Maximum current, 30 ma 0.1M Cu(ll). Maximum current, 60 ma
At high cathodic potentials, increasing the acidity (or buffer capacity) by the addition of strong acid (or buffer) increases the acidity of the effluent. Unfortunately, the background current is also increased because of the increased amount of hydrogen ion available for reduction. Since high currents have a n adverse effect on the selectivity of the cell, these methods of maintaining the acidity of the solution may not be suitable for some applications. Observance of Electrolysis. Electrolysis in the cell may be followed by observing either the cell current or the concentration of various ions in the effluent. At low concentrations electrolysis is more difficult to observe by cell current because it may be obscured by the background current. A continuous polarographic vel1 (6) is convenient for observing the effluent solution. A polarogram of the effluent may be taken, or the polarographic cell may be operated a t a constant potential such that the polarographic current is a measure of the concentration of one or more of the electroactive species in the effluent from the electrolytic cell. Other Packing Materials. While graphite has been used as the electrode material for all of the studies in this paper, other available materials might be more suitable for some apphcations. A platinum packing has been prepared manually by cutting platinum wire (0.015-inch diameter) into short lengths. The useful range of this packing extends almost as far in the anodic direction as that of graphite, hut platinum does not have a very large cathodic range because of its low hydrogen overvoltage. Mercury-coated platinum packing has been prepared from the above platinum packing material by the method of Ramaley, Erubaker, and Enke (11). The platinum bits are VOL. 36, NO. 7, JUNE 1964
e
1247
at all of these applied potentials, showTable 1.
Oxidation of Ce(lll) in 1M
HzS04 Sample solutions: Ce(II1) in 1M H2SOa Flow rate: 6 ml. per minute Applied Influent Ce(II1) potential, molarity volts 0,001 0.002 0.01 yo of influent Ce(II1) oxidized to Ce( I V ) on -passage through column +1.50 +1.75 +2.0
101 101 101
99
100 100
68 80 100
cleaned in boiling 60% HC1O4, rinsed, then placed in a platinum gauze basket and cathodized (to the point of gassing) in an acidic NaC104 solution. The whole basket is then lowered into a mercury pool at the bottom of the container. Most of the excess mercury can be removed from the resultant mixture by placing it in a column and letting the mercury flow off; the rest can be removed by centrifugation. Mercury coatings prepared in this manner remain useful for months, and have the cathodic potential range of mercury. The mercury film can be stripped electrochemically a t potentials anodic of +0.20 volt (us. the Ag-hgC1 electrode in 6.W NaC1) in NaC1O4 media, so the anodic range is limited. Preliminary work indicates that platinum and mercury-coated platinum are acceptable packing materials when restricted to their useful potential ranges. APPLICATIONS
Oxidation of Cerous Ion. Ce(1F') is a strong oxidant which has been prepared by the batchwise electrochemical oxidation of Ce(II1) ( I d ) . It has also been generated coulometrically as a titrant for uranium ( 8 ) . It was possible to oxidize Ce(II1) quantitatively in the continuous electrolytic cell. Ce(II1) solutions of known concentration were prepared by the reduction of reagent grade ceric ammonium nitrate in 1M &So4 with H202, then reoxidized continuously in the cell. The Ce(1V) concentration in the effluent solution was measured a t 410 mp in a Coleman Junior spectrophotometer. Comparison of the effluent Ce(IV) concentration with the known influent Ce(II1) concentration permitted calculation of the extent of oxidation (Table I). Applied potentials sufficiently anodic to oxidize a low concentration of cerium quantitatively may not give quantitative oxidation of a higher Concentration. Howveer, by making the applied potential more anodic, higher concentrations can be quantitatively oxidized. Some gassing was obqerved 1248
ANALYTICAL CHEMISTRY
ing that the current efficiency was less than 100%. I n this manner, a standard solution can be prepared without controlling the current or the current efficiency of the generating device. However, a standard influent solution is necessary. Selective Reduction of Fe(II1)U(V1) Mixtures. M a n y metal reductors have been used for the prereduction of mixtures of Fe(II1) and U(V1). With a zinc reductor, a mixture of Fe(II), U(II1). and UfIV) is obtained t h a t may be titrated with either KMnOd in H2S04or K2Cr207 in HCl. Three breaks have been reported in the potentiometric titration of the mixture ( 7 ) . The U(II1) produced may be oxidized to U(1V) by bubbling air through the solution for a few minutes, after which the titration shows only two end points. Reduction by a 90% cadmium amalgam produces only Fe(I1) and U(1V). The resultant mixture has been titrated spectrophotomet'rically with ceric sulfate ( 6 ) . Other metal reductors which produce only Fe(1I) and U(1V) include silver ( d ) , lead and bismuth amalgams ( I S ) , and lead in HzS04-HC1 medium (1).
These chemical reduction methods have the disadvantage that the reduction is not selective; both components of the mixture are reduced. As a result, a selective method of end point detection is necessary for the titrations. The continuous electrochemical cell is capable of selectively reducing Fe(II1) in the presence of U(VI), thereby making selective end point detection unnecessary. The Fe (111) in a mixture may be determined by selectively prereducing only the Fe(II1) and t,hen titrating the resultant Fe(I1). The sum of Fe(II1) and U(V1) may be determined by prereducing both. As a titrant, KhhO4 posesses some advantages over Ce( IV) or Cr(V1) ; it reacts more rapidly and can act as its own indicator. The precise use of K M n 0 4 as a titrant is precluded for chemical reductors that require chloride media. There is no chloride requirement for the successful operation of the continuous electrolytic cell. EXPERIMENTAL. X standard Fe(II1) solution was prepared by dissolving a weighed amount of primary standard iron wire in H S O a and HzS04, boiling to drive off nitric acid, and diluting to volume. h standard solution of UOZSO4 was prepared from NBS Us08 as described by Amos and Brown ( I ) . -4liquots of these solutions were used for the preparation of samples. Standard O.1N K&'hO4 was prepared and standardized against XazCz04 by a conventional procedure ( 4 ) . For the titrations of Fe(III)-r(VI) mixtures, the 0.1S K > h O a was diluted fivefold.
Conditions for the quantitative reduction of Fe(II1) and U(VI), both singly and in the presence of each other, were established by the titration with standard KMn04 of samples which were passed through the cell under various conditions. The relative standard deviation of the KXln04 titrations was around 0.3%. Samples were prepared by the dilution of a known amount (0.1 meq. each) of Fe(II1) and/or U(V1) to about 50 ml. with 1X NazS04adjusted to p H 2 with HzS04. The samples were deaerated with a stream of nitrogen and then passed through the cell a t about 6 ml. per minute. To ensure quantitative recovery of the samples, the column was rinsed with about 50 ml. of the Sa2S04-H2S04solution. The extent of reduction was determined approximately by passing the effluent through the polarographic flow cell, or precisely by titrating the eluted batch of solution with standard 0.025 KMn04. RESULTSAND COKCLUSIONS.The extent of reduction of Fe(II1) and U(VI), singly and in mixtures, is shown in Table I1 for various applied potentials. The reduction of Fe(II1) alone and U(V1) alone is quantitative a t concentrations around 0.002'V, for potentials more cathodic than f0.20 and -0.60 volt, respectively. .It higher concentrations, more cathodic potentials are required for quantitative reduction. To prereduce quantitatively only the Fe(II1) in a mixture the cell potential must lie between 0 and +0.2 volt. Both Fe(II1) and the U(V1) are quantitatively reduced a t potentials more cathodic than -0.70 volt. If the pH of the influent solution is greater than about 2.5 to 3.0, less than the expected amount of Iln04 is required for the titration. This could be due to precipitation and slow redissolution of iron or uranium, either in the column or during titration. Separation of Mixtures by Deposition and Stripping. The separation of metals bv electrodeposition has the advantage that little contamination is introduced in the process of separation. However, most existing procedures require relatively long times for quantitative deposition and are not used on a continuous basis (9). The plating and stripping of Cu, Pb, Cd, and Zn, separately and in mixtures, were studied under various conditions with the purpose of defining a working procedure that would permit the separation and measurement of the individual components. REAGFNTS. Standard 0.0055434 EDTA was prepared from the disodium salt dihydrate as a primary standard ( 3 ) . Standard 0.00lM metal ion solutions were prepared from the reagent
Figure 4. Deposition efficiency for various metals Influent solution contains m e t a l ion a t 0.001M in 1.5M NaClO4 adjusted to pH 3:O with HC104. Flow rate, 6.5 ml./min.
grade nitrates and standardized by titration with EDTA. SaClOd 1.5M was prepared from reagent grade NaC104.Hz0 (G. Frederick Smith Chemical Co., Columbus, Ohio), made to pH 10 with NaOH, passed through Chelcx 100 chelating resin (Bio-Rad Laboratories, Richmond, Calif.), to remove mzlterials causing a high EDTA blank, and then acidified to t,he desired p H vrith HClO4. All quantitative studies were carried out in this medium. Zinc and cadmium were determined by titration at p H 10 (ammoniaammonium chloride buffer) using Eriochrome Black T indicator. Lead was titrated at pH 10 (ammonia-ammonium chloride buffer and a small amount of sodium tartrate) also using Eriochrome Black T indicator. Copper was titrated at pH 10 (ammonia-ammonium chloride buffer) using Murexide indicat'or.
DEPOSITION AXD STI~IPPIKG OF SINGLE METALS. The extent of deposition of the various metals as a function of cell potential was studied by passing 0.001M solutions of each m e t d ion through the cell. At steady state for each applied potential, the metal io:? concentration in the effluent stream \vvzs measured with the continuous polarographic cell and compared with the known concent,ration in the influent, stream. The results are shown in Figure 4, which indicates the feasibility of separation of a mixture by stepwise selective deposition of each netal in a concent,rat,ionrange around 0.00lM (total). At higher concentrations, the slopes of the curves decrease ( x e Figure 3), and the separations becone less selective. The stripping characteristics were also invest,igated. .ibout 0.05 mmole of each metal was plated on the graphite and st,ripped a t various potentials. The potentials at which the first detectable concentrations (about 10-6M) of metal appeared in the effluent were: Cu, 0.00 volt; Pb, -0.50 volt Cd, -0.75 volt; Zn, - 1.1 volts. At these potentials the deposits take a relatively long time to strip; a t more anodic potentials the concentration in the effluent is higher and the deposits take less time to strip. .it the more anodic potentials, the con-
centration of metal ion in the effluent is dependent upon the amount of metal in the deposit. The foregoing data indicate that separation may be obtained by either plating or stripping. For separations which require a high degree of selectivity, selective stripping is more desirable because the column current may be maintained at relatively low values by a suitable choice of stripping potentials, keeping the selectivity high. The recovery of the various metals plated and stripped singly is summarized in Table 111. Each sample was plated from 50 ml. of solution and then stripped into 100 ml.. which was finally titrated with 0.00554M EDTA. The relative standard deviation of the titrations was around 0.3'%',. -1relatively high anodic potential was required to strip Zn completely into 100 ml.
SEPARATIOK OF MIXTURES BY DEPOSITION us. SEPARATION BY STRIPPING. When mixtures are plated and stripped, complications may arise that determine whether a particular separation is more feasible by stripping or plating. I n the above study, t u o complications were encountered. N h e n Cu was deposited with Cd or Zn and then stripped, the Cu was very difficult to recover quantitatively. It was postulated that, under the highly cathodic deposition conditions, the p H
Table Ill.
Metal cu Pb Cd Zn
Table
(I.
Extent of Reduction of Fe(ll1) and U(VI)
Sample composition: 0.1 meq. Fe(II1) and/or 0.1 meq. U(V1) in about 50 ml. of 1M ?r'anSO4-H2SO4, pH 2 Flow rate: 6 ml./minute Extent of reduction, % U( VI ) Fe(II1) U(V1) and only only Fe(II1)
Applied potential, volt +0.50 +O. 40 $0.30 +0.20 to 0 . 0 0 -0.10
-0.20
-0.30
-0.40 -0.50 -0.60 -0.65 -0.70 -0.75 -0.80
-0.90
12'3 80a
1ooa
100 4 100 4 100 4 100 4 100.4 100.4 100 4 100 4 100.5 100.5 100.5 100.5
Oa
18a
25a 50a
8P loon
99.0
98.1 99 7 99.9 100.2 100.4 100.2 100.4 100.0
a Values obtained by polarographic measurement; other values obtained by KMn04 titrations.
of the effluent became sufficiently high to cause precipitation of Cu as a basic salt, which was slow to redissolve under the stripping conditions. Zn showed the same behavior, but to a lesser extent
Recovery of Metals through a Deposition-Stripping Cycle"
Applied potential, volt Deposition Stripping -0.35 +o, 75 -0.65 -0.10 -1.00 -0.55 -0.50 -1.40
pmoles added 58.56 52.19 50.69 56.23
pmoles found 58.51 52.19 50.47 56.06
Recovery,
70
99.9 100.0 99.6 99.7
a About 50 pmoles of each metal deposited from about 50 ml. of 1.5M NaC104 adjusted to pH 2 with HC104, a t flow rate of 6 ml./min. Deposit stripped into about 100 ml. of
NaC104-HC104 a t 6 ml./min.
Table IV.
Recovery of Metals Separated from Three-Component Mixtures by Various Procedures"
Components plated
cu
Recovery, 70 Pb Zn
Cd
Cu, Pb, Zn Mixture c 1 1
99 .. 3
and Pb Cu, Pb, and Zn
97 lOOb
Cu and Pb Cu, Pb, and Cd
100.2 90*
101.0 100.3
100 3 (sum) ~100.5 100.5
Cu, Pb, Cd Mixture 100.5 100.8
99.2 99.7
a ,Yonplated components determined in effluent from plating process. Plated components then stripped selectively. Plating and stripping conditions same as for Table 111, except that Zn was stripped into 200 ml. of solution instead of 100 ml. b Cu could not be quantitatively stripped unless flow was stopped for 1 hour at open circuit before stripping.
VOL. 36, NO. 7, JUNE 1964
1249
-
I MIN. OR 3.25ml.
Cd
c .-,,xi*+>;
I 1
0
,
:.
I
1
I
0.3 0.6 0.9
POLAROGRAPHIC CELL KXTAGE Figure 6. Continuous removal of Pb from a Pb-Cd mixture Figure 5.
Separation of four-component mixture by stripping Stripping potentials A. -0.80 volt 6. -0.55 volt c . -0.1 0 volt D. +0.50 volt
than Cu. To avoid this complication, it is better to plate out only the Cu (or Cu and Pb) a t less cathodic potentials, so that the pH does not become so high and thc basic salts do not precipitate. The experiments on which these conclusions are based are summarized in Table IV. A second complication arose in the behavior of Zn, which plated onto a Cd deposit at a less cathodic potential than onto graphite alone. Cd could not be quantitatively separated from Zn by plating only the Cd, but the two could be separated by plating both and then selectively stripping each. Binary mixtures may be separated in two ways (unless ruled out by complications similar to those above): by selectively plating the nobler substance and then stripping it, or by plating both and then selectively stripping each. Mixtures of more than two components can be handled similarly. Table IV shows the recoveries obtained when two three-component mixtures were separated in various ways. At lower concentrations, the interactions between components become less, the possibility of precipitation becomes less, and the selectivity is better bccause the current is low. The result is that separations are faster and easier to effect a t lower concentrations. Figure 5 shows the separation obtained for a four-component mixture a t low
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concentration. Here, a stream of 1.5M NaC104-HC104, p H 2.7, was pumped a t 3.25 ml. per minute through the column. Then, with the column set a t -1.45 volts for deposition, 0.1 ml. of a sample solution containing 1 pmole of each component was injected into the stream with a hypodermic needle. For stripping, the column potential was changed in steps, as shown in Figure 5 . The trace shows the elution of each component as it was stripped from the column. The trace was obtained by passing the column effluent through the continuous polarographic cell. The DME was operated a t -1.30 volts us. the Ag-AgC1 electrode in 6 M NaC1, sufficiently cathodic to be in the limiting current region for all four components. The entire operation of plating the sample and selectively stripping four components took about 10 minutes. At higher flow rates, even less time is required. Continuous Separation of TwoComponent Mixture. The Cd content of a solution containing 0.0001M Cd(I1) and 0.001M Pb(I1) cannot be found precisely by a polarographic measurement because the Pb(I1) wave obscures the Cd(1I) wave. However, if the Pb(I1) is removed in some way, the Cd(I1) wave can be observed more precisely. The continuous electrolytic cell can be used to remove the Pb rapidly and continuously.
Sample solution contains 0.031M Pb(ll1 and 0.0001M Cd(ll1 in 1.5M NaC104-HCIO4, p H 2, pumped a t 6 ml./min. Applied electrolytic cell potentials A. -0.30 volt 6. -0.60 volt C. -0.70 to -0.80 Volt
The solution, containing 0.0001M Cd(I1) and 0.001M Pb(II), was passed through the electrolytic cell and then through the continuous polarographic cell. Polarograms of the effluent of the electrolytic cell were made for various potentials applied to the electrolytic cell. Three such polarograms are shown in Figure 6. Curve A , made at a cell potential such that all of the Pb(I1) and Cd(I1) passed through, shows that the Cd(I1) cannot be observed in the presence of the Pb(I1). Curve B was made at a cell potential such that most of the Pb(I1) was plated. Curve C was made a t a cell potential such that all of the Pb(I1) but none of the Cd(I1) was plated. The limiting current due to Cd(I1) was constant within 2% a t applied electrolytic cell potentials from -0.60 to -0.80 volt. This method of removing one component of a mixture can be used on a sample stream whose composition is continuously varying. I t is also very rapid. However, a continuous method of detection is necessary to utilize these advantages. LITERATURE CITED
(1) Amos, W. R., Brown, 1%'. B., ANAL. CHEM.35,309 (1963). ( 2 ) Birnbaum, N., Edmonds, S. M., rim. ENG.CHEM.,AZIAL.ED. 12, 155 (1940). (3) Blaedel, W. J., Knight, H. T., ANAL. &EM. 26,741 (1954)(4) Blaedel, W. J., hleloche, V. W.
“Elementary Quanti1ative Analysis,” 2nd ed., p. 484, Harpw and Row, New York, 1963. (5) Blaedel, R J., Strohl, J. H., ANAL. CHEM36,445 (1964). ( 6 ) Bricker, C E., Sweetser, P. B., Ibad., 2 5 , 764 (1953). ( 7 ) Ewing, D. T., Eldridge, E . F., J . Am. P h ~ mSac 44, 1484 (1922).
(8) Furman, X. H., Bricker, C. E . , Dilts, R. v., ANAL. CHEM.25,482 (1953). ( 9 ) Lingane, J. J., “Electroanalytical Chemistry,” 2nd ed., pp. 316-22, Interscience, New York, 1958. (10) Molnar, J., M a g y . Kem. Folyozrat 68(11), 504 (1962). (11) Ramaley, L., Brubaker, R. L., Enke, C. G., ANAL.CHEM.35, 1088 (1963). (12) Smith, G. F., “Cerate Oxidimetry,”
F. Smith Chemical Co., Columbus. 1942. Someva, K.. Z . Anora. dllaem. Chem.
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RECEIVED for review January 6, 1964. Accepted March 3, 1964. Financial support through Grant No. hT(11-1)-1082 from the U. S. Atomic Energy Commission is gratefully acknowledged.
Resistance Cornpensatio n in Polarography Application to High-Resistance Nonaqueous Systems and to High Current-Density Aqueous Systems WARD B. SCHAAP and PETER S. McKINNEY’ Department o f Chemistry, lndiana University, Bloomington, Ind.
b The technique of derivative polarography was used to detect resistance TWO losses in polarographic cells. limiting cases were investigated-nonaqueous solutions with high specific resistances and low cell currents and aqueous solutions with high cell currents. In the nonaqueous solutions, the solution resistances evaluated at various distances from the D.M.E. were found to agree well with values calculated for a model assuming concentric spherical electrodes. This iR drop, present on all sides of the D.M.E., is not compensated for b y potentiostat action using typical three-electrode cells and can b e appreciable if either the specific resistance or the cell current is abnormally high. Resistance-free halfwave potentials can still b e obtained by extrapolation procedures, however. With concentrated aqueous solutions, an appreciable iR drop was detected when the concentration of the electroactive species exceeded 1 O-* M, though values of id/(: remained constant from to 10-’M.
T
HE DEVELOPMEKT of threeelectrode, resistance-compensating polarographic potentiostats ( 1 , 2 ,13-15) makes it possible to attempt precise polarographic studies in nonaqueous solutions utilizing tolvents of low dielectric constant and high specific resistance. The exper,iments of Arthur (1, 2) and of Kelley, Jones, and Fisher ( 1 4 , 15) show that such polarographic studies are feasible uqing their potentiostats, hut thorough studies in nonaqueous systems were not made. *%ttempts to use these techniques in the nonaqueous electrochemical studies in
Present address, Department of ChemCambridge,
istry, Harvard University, ;\lass.
progress in this laboratory met with only partial success. These preliminary studies indicated t,hat, complete resist’ance compensation was not always attained in such systems, and this sit’uation prompted a more careful investigation of the factors affecting resistance compensat,ion. It was mentioned in the previous paper of this series (26) that it is necessary to distinguish t,wo types of highresistance syst’ems when considering resistance compensation. The first of t.hese includes systems in which the specific resistance of the solution and the cell current are both small or moderate, and the high resistance is primarily a result of cell or electrode design--.Le., a long or restricted current path exists between the D . M . E . and the auxiliary counter electrode. In this first case the ohmic pot’ential difference (iR drop) in the immediate vicinity of the D.M.E. is relatively small, though measurable (19). If care is taken to position the reference electrode on the side of the D.M.E. opposite the counter electrode, the potentiostat will compensate for essentially all the cell resistance effect and undistorted polarograms can be recorded. The potential difference between the D.M.E. and the reference electrode is not influenced by the large iR drop caused by the flow of cell current through the high resistance located between the D.M.E. and the counter electrode. The second type of system includes those in which the specific resistance of solution is very high, so that even a nominal current density in the vicinity of tjhe D.M.E. gives rise to an appreciable ohmic potential drop in this region. In this case a large iR drop can exist even though the cell is designed to minimize overall cell resistanceLe., with a short current path of large
cross-sectional area. Because the D.M.E. is a very small diameter electrode, all points on its surface are essentially equidistant from t,he counter electrode. Thus, current will flow equally from its entire equipotential surface and t’he current path will a t first proceed radially from the D.1I.E. before turning in the direction of the counter electrode. The ohmic potential gradient will be significant on all sides of the D.M.E. if the specific resistance of the solution is very high and will approach spherical symmetry near the drop surface. T i t h this situation, the usual placement of the three electrodesLe., with the D.M.E. betvieen the reference and counter electrode.; and at, appreciable distances ( 2 0.5 em.) from both- will not accomplish complete compensation for resistance losses in the cell and the recorded polarograms will show a distortion, due to iR losses, of a magnit’ude proportional to the cell current and the specific resistance. I t is the iR drop existing in the solution between the D.M.E. surface and the tip of the reference electrode salt bridge that is not compensated for by the action of the potentiostat. I t is then obvious that the effectiveness of resistance compensation depends not only on the specific resistancp of the solution used, but also on the current density and the placement of the controlling reference electrode with respect to the D.1I.E. and counter electrode. In the experiments reported below, the actual magnitude of the uncompensated resistance is measured as a function of D.M.E. to reference-electrode probe distance using anhydrous nbutylamine (dielectric constant = 5.3) ( 2 7 ) as solvent. % . special threeelectrode cell was designed which allowed the position of the reference electrode to be varied and its distance VOL. 36, NO. 7 , JUNE 1964
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