Anal. Chem. 1981, 53, 1847-1851
direct contrast to other reports (5, 7 , 1 3 ) ,is currently being investigated. CONCLUSION This paper demonstrates that unmodified electrodes f a bricated from carbon fibers have several ideal properties for electroanalysis in a complex environment such as mammalian brain of an anesthetized animal. At constant potential, dilute solutions of DA can be detected. Backstep correction provides residual current correction for scanning pulsed wave forms. The pulsed wave form prolongs the usable lifetime of the electrode. Their small size, coupled with their rapid response, virtually eliminates artifacts caused by the oxidation of in vivo substances. Since chemical reactions following heterogeneous charge transfer are not apparent with these electrodes, digital subtraction or deconvolution techniques can be employed. While the electrode response certainly deteriorates in vivo, it deteriorates rapidly and then remains constant. Whiile the results presented here do not clearly indicate what compounds are changing in concentration with pharmacological manipulations, they do suggest that the interpretation of in vivo amperometric results is extremely complex. Experiments to clarify these results are in progress. ACKNOWLEDGMENT Helpful discussions with R. Ensman, El. Williamson, and R. Withnell are gratefully acknowledged. LITERATURE CITE11 Dayton, M. A.; Brown, J. C.; Stutts, K. J.; \Nightman, R. M. Anal. Chem. 1980, 52, 946-9!50. Dayton, M. A.; Ewing, A. (2.; Wightman, R. M. Anal. Chern. 1980, 52, 2392-2396. Adams. R. N. Anal. Chern. 1978, 48, 1126A-1138A. Klssinger, P. T. Anal. Chem. 1977, 49, 447A-456A. Lane, R. F.; Hubbard, A. T.; Fukunaga, K.; Blanchard, R. J. Braln Res. 1976, 114, 346-352. Wightman, R. M.; Strope, E.; Plotsky, P.; Adams, R. N. Braln Res. 1978, 159, 55-68.
1847
Conti, J. C.; Strope, E.; Adarns, R. N.; Marsden, C. A. Llfe Sci. 1978, 23, 2705-2716. Lane, R. F.; Hubbard, A. T.; Blaha, C. D. J. Nectroanal. Chem. 1971), 95. 117-122. Lane, R. F.; Hubbarcl, A. T.; Blaha, C. D. Bioelectrochem. Bioenerg. 1978, 5, 504-525. Huff, R.; Adarns, R. N.; Rutledge, C. 0. Braln Res. 1979, 1721, 369-372. Marsden, C. A.; Conti, J.; Strope, E.; Curzon, G.; Adams, R. N. Brah Res. 1979, 171, 85-99. Lindsay, W. S.; Kiuort, 8. L.; Justice, J. 8.; Salamone, J. D.; Neill, C). 8.J. Neuroscl. Meth. 1980, 2 , 373-388. Gonon, F.; Buda, M.; Cespugiio, R.; Jouvet, M.; Pujol, J.-F. Nature (London) 1980, 286, 902-904. Huff, R. M.; Adams, R. N. Neuropharmacology 1980, 19, 587-590. Ossendorfavl, N.; Prad%, J.; Pradleova, J.; Koryta, J. J. Electroanal. Chem. 1975, 58, 285-261. Mattson, J. S.; Jones, T. T. Anal. Chem. 1976, 48, 2164-2167. McCreery, R, L.; Drelilng. R.; Adarns, R. N. Brain Res. 1974, 721, 23-33. Barker, G. C. I n "Prlogress in Polarography"; Zuman, P., Koithoff, I., Eds.; Interscience: lUew York, 1962; Vol. 2, pp 41 1-427. Lane, R. F.; Hubbard, A. T. Anal. Chem. 1978, 48, 1287-1292. Anderson, J. E.; Bond, A. M. Anal. Chem. 1981, 53, 504-508. Ewing, A. G.; Withneil, R.; Wightman, R. M. Rev. Sci. Instrum. 198.1, 52, 454-458. Pellegrino, L. J.; Cushrnan, A. J. "A Stereotaxic Atlas of the Rat Brain"; Appleton Century Crofts: New York, 1967. Dietz, R.; Peover, M. E. J. Mater. Sci. 1971, 6 , 1441-1446. Kambara, T. Bull. Chem. SOC.Jpn. 1954, 27, 523-526. Bard, A. J.; Faulkner, L. R. "Electrochemical Methods: Fundament& and Applications"; Wiley: New York, 1980. Cheng, H. Y.; Schenk, J.; Huff, R.; Adams, R. N. J . Nectroanal. Chem. 1979, 100, 23-31, Lindsay, W. S.; Justice, J. B. Comput. Chem. 1980, 4 , 19-26. Von Voightlander, P. F.; Moore, K. E. J. Pharmacol. Exp. Ther. 1978, 184, 542-552.
RECEIVED for review ,4pril8, 1981. Accepted July 20, 1981. This research was supported by NSF Grant BNS 81-000441. M.A.D. is a combined Medical-Ph.D. candidate, Indiana University. R.M.W. is the recipient of a Research Career Development Award :from the National Institutes of Health (Grant No. 1 KO4 NS 00356).
Analog Instrument for Oxidative and Reductive Potentiometric Stripping Analysis Joan K. Christensen, Kristlan Keiding, Lars Kryger," Jean Rasmussen, and Hans J. Skov Department of Chemistry, Aarhus University, Langelandsgado 140, DK-8000 Aarhus C, Denmark
An Instrument for potenllometric strlpplng analysis is described. With a three-state potentlostat module, both loxldative and reductive potentlometric stripping analysis can be carrled out. I n thls manner, elements such as selenium, sulfur, the halldes, and manganese are added to the llst of substances whlch can be determined by potentiometric: strlpplng analysls. The Instrument can be extended by 8 dlfferentlator module. This module transforms the basic POtentlal vs. time curve to a dtfferential potentlogram, where the analytlcal Information Is represented as peaks. Examples of stripping potentlograms obtalned with the device are given, and those aspects of the analytical performance of the PO.. tentlometrlc stripping technique which depend on instrurnental factors are discussed.
Potentiometric stripping analysis is a relatively new technique for the determinatimon of trace elements in solution (I,
2). In general, the elements which can be determined b y potentiometric stripping analysis are those which can also be determined by voltammetric stripping analysis, and the results obtained by the two types of stripping techniques compare well. However, there me differences which may appear when, for example, samples contain organic traces: with voltammetric techniques, and in particular with pulsed voltammetric techniques, dissolved reversible couples produce interfering signals. This is not the case with potentiometric stripping analysis ( 3 , 4 ) . Potentiometric stripping analysis for elements which can be reduced at a mercury electrode to form dilute amalgams and which exhibit diffusion-controlledredissolution can be determined with very simple instrumentation comprising (apart from the electrochemical cell) a potentiostat, a high-impedance amplifier, a timer switch, and an x-t recorder (I). Such instrumentation is now commercially available (5). This paper describes an instrument for the potentiometriic stripping determination of a larger selection of elements than those determined with oxidative potentiometric stripping
0003-2700/81/0353-1847$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
working electrode is observed while the redissolution process is in progress,. WHg) t
B
~ ' I
I
/---
t
Mn
--m
E mV t Flgure 1. Time sequences of potentials generated during electrolysis by the three-state patentiostat module: (A) oxidative potentiometric stripping analysis, (B) reductive potentiometric stripping analysis with
water-soluble reducing agent, (C) reductive potentiometric stripping analysis with electrolytically generated amalgamated metal as the reducing agent.
analyses (i.e., where the redissolution process is an oxidation). By a simple modificaton of the generator of the plating potential, elements like the halides, preconcentrated as mercurous salts, selenium, preconcentrated as mercuric selenide, and manganese, preconcentrated as manganese dioxide, can now be determined by using reductive potentiometric stripping analysis (i.e., the analyte redissolution is caused by a reducing agent). With voltammeQic techniques, such elements are usually anodically plated and subsequently stripped off the working electrode by a cathodic potential sweep. A similar preconcentration-reductive stripping scheme can be implemented in potentiometric stripping analysis but requires that a simple three-step potential vs. time function, rather than a constant potential is generated during electrolysis. This function can be generated by the device proposed here. Potentiometric stripping curves obtained with analog instrumentation have so far been represented as potential vs. time curves. The analytical signals are here taken as the lengths of the plateaus which indicate the redissolution of preconcentrated analytes. The instrument presented here is provided with an electronic module, which modifies the output to a differential potentiogram (output to an x-y recorder). On the differential output, the analytical information is represented as peaks rather than plateaus. The area of these peaks constitute the analytical signal. The normal potentiogram can be obtained from the differential potentiogram by integration along the potential axis.
PRECONCENTRATION/STRIPPINGSCHEMES I N POTENTIOMETRIC STRIPPING ANALYSIS Oxidative Stripping, Oxidative potentiometric stripping analysis can be carried out by using a single preconcentration potential. This is illustrated in Figure 1A for cadmium. A solution of cadmium(I1) spiked with some oxidizing agent, e.g., mercury(II), is electrolyzed at a mercury film electrode at the potential El e.g., -1300 mV vs. SCE for a time TI + Tz + TS. During plating, cadmium is reduced to form dilute amalgam with the working electrode Cd(II)(aq) + 2e-
Cd(Hg)
(1)
After the electrolysis is completed, potentiostatic control is abandoned and the potential vs. time behavior of the
+ Hg(II)(ad -+ Cd(II)(aq) + Hg(U
(2)
The redissolution process is observed as a plateau on the stripping curve. The duration of this plateau is taken as the analytical signal. Reductive Stripping, Reducing Agent Dissolved i n Aqueous Phase. In reductive potentiometric stripping analysis, the application of a single electrolysis potential is generally not satisfactory: If a single electrolysis potential is used for those elements, which are preconcentrated on solid electrodes by oxidation and determined by reductive potentiometric stripping analysis, irreproducibility mostly results (6). This is because, during the anodic preconcentration of the analyte, the electrode--e.g., platinum-is likely to react with the sample/electrolyte and layers of, for example, oxides and halides are codeposited with the analyte. For a complete regeneration of the electrode surface, polarization at rather cathodic values for one or more minutes is necessary. This may be accomplished as indicated schematically on Figure 1B. Manganese(I1) is preconcentrated on platinum at the potential E2 for T2+ T3seconds (typically +650 mV vs. SCE at pH 6.7) as manganese dioxide Mn(II)(aq) + 2H2Q
-
Mn02(s)+ 4H+ + 2e-
(3)
The redissolution is normally caused by some dissolved mild reducing agent such as hydroquinone
-+
MnOz(s) + 2H+ + C6H4(OH)2 Mn(II)(aq)
C6H402+ 2 H 2 0 (4)
The reaction of the hydroquinone with the coprecipitated oxides/halides is, however, very slow and only by keeping the working electrode at the cathodic potential El for Tl seconds (typically 60 s) prior to preconcentration of the analyte in each plating/stripping cycle, the deposits are redissolved and reproducible results are obtained. Reductive Stripping, Reducing Agent Dissolved in Mercury Phase. The real advantage of the three-step potential generator is, however, its capability of generating, by electrolysis, the reducing agent for reductive potentiometric stripping analysis. The advantage is best understood when reductive potentiometric stripping analysis of selenium or sulfur preconcentrated as insoluble deposits of mercuric selenide and sulfide is attempted. For such experiments, very few water-soluble reducing agents are sufficiently strong and the few possible candidates--e.g., sodium tetrahydroborate-are not sufficiently stable in solution to ensure the necessary constant concentration during several plating/ stripping cycles (7). Moreover, the redissolution of the mercuric selenide deposit by means of sodium tetrahydroborate, appears to proceed to slowly to keep up with diffusion, hence stripping plateaus are poorly developed. To ensure that a constant amount of reducing agent is available for the redissolution, it is advantageous, for each stripping experiment, to generate the reducing agent electrolytically in well-defined amounts, In sodium hydroxide medium, one of the strongest reducing agents known, amalgamated sodium, can be generated electrolytically and stored inside the working electrode until it is needed for the redissolution process (7). This technique is shown in Figure 1C. Firstly, the electrode-a mercury pool--is kept at potential El (e.g., -1500 mV vs. SCE) for TIseconds. At this potential, surface deposits of mercuric selenide from previous runs are stripped off the electrode by reduction, while any amalgamated sodium remaining inside the mercury pool is oxidized simultaneously. Therfore, a clean electrode results at the end of the period TI. During the next period T2at the potential Ea(e.g., -1970
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981 __I
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-___
__-__I
Table I. Controls, Displays, and Input and Output Terminals Available on Front Panel of the Instrument function specifications no. type switch presetlrun potentiometers knobs
1 3 3
selectors (3- or 5-position)
3 pairs
digital voltmeter and amperemeter
2
cell terminal
3.
recorder terminal
1
relay terminal
1.
preset instrument/run experiment adjust E,, E,, E, (preset mode) in preset mode: display of E,, E,, E , on digital voltmeter adjustment of TI, T,, T, 1-5000 s
range -4.500 to +4.500 V selector 1: 1, 2, 5 selector 2: 1. 10. 100. 1000. Time in seconds given by products of selector settings
voltmeter: In preset mode: display of E,, E , according to state of pushbuttons. In run mode: display of actual cell potentials. amperemeter: Display of cell current during electrolysis terminal for cable to working, reference, and counterelec trodes output terminal for E ( t ) to x-t recorder or differentiator module and x-y recorder paper control on x-t recorder. Pen upldown contr ol
mV vs. SCE) a well-defined amount of sodium amalgam is created and stored inside the electrode
I
During the following periold T3,a t the potential E3 (e.g., -500 mV vs. SCE), selenide, which in dilute sodium hydroxide exists as the species HSe-, may be preconcentrated by precipitation on the electrode surface HSe-(aq)
-
+ Hg(l11
HgSe(s)
+ 2e- + H+
+ 2Na(Wg) + €3’
-
2Na+(aq) + Hg(1)
+ MSe-(aq)
J - I
(6)
During T,,some of the recently generated sodium amalgam is lost by reoxidation. The amount can, however, be controlled by selection of T,,T3,Ez, and E3 such that a suitable amount remains a t the end of T3. The redissolution is subsequently caused by amalgamated sodium diffusing toward the electrode surface from the interior of the electrode, and experience shows that the reaction proceeds sufficiently fast for plateaus to develop HgSe(s)
I
I W
(7)
A similar scheme holds for sulfide. The halides may be determined in a similar fashion using a less powerful reducing agent such as zinc amalgam (7).
module.
EXPERIMENTAL SECTION I n s t r u m e n t a t i o n . (A) The Three-State Potentiostat Module. Figure 2, upper part, shows the three-state potentiostat module comprising a simple potentiostat with three electrode terminals (C, counterelectrode; R, reference electrode; W, working electrode) and a timer circuit which controls the electronic switches S1, S2, and Saand hence supplies the selected three-state potential vs. time sequence as input to the potentiostat. The timer circuit also controls the paper-feed function of the strip-chart recorder and, through the state of the electronic switch “PSA”, the selection of the potentiostatic or the potentiometric mode. The operation of the module is activated by the two-state switch shown in the upper left corner. In the “reset” position, this switch causes the two flip-flops FPI and FF2 to be reset, hence the paper-feed function of the recorder “Rec” is disabled and the potentiostatic mode activatled by closing the switch “PSA”. To start an experiment, the switch is moved to the run position. This triggers a series of events, where the three-timer switches “TI”,“Tz”, and *‘Ta’’(all Universal Timer(555)) are sequentially activated for TI,T2, and T3s. After this sequence, the timer “Delay” (also 555) i s activated for a fixed time of D s (2 s in the present version). The switclh S1 is thus closed for T I s. During this period, a potential P1is input to the potentiostat. Similarly, during T 2s, the potential P2is input. After a further T3s, at a potential P3 (switch S3closed), the recorder flip-flop, FF1, is set
simultaneously with the activation of the delay timer, and the paper feed is activated. Two seconds later, the “Delay” timer runs out. This sets the flip-flop FF2, the switch PSA is opened, the device is in potentiometric mode, and the stripping potentiogram is recorded on the recorder which takes input at “V”. Simultaneously, the switch S3is reopened. The potential Psis therefore input to the potentiostat for a total of T4= T3+ D s. The lower part of Figure 2, shows the timing of the device. When any of the timers runs out and activates the next timer, the state of the electronic switches is momentarily undefined, and a transient change of the potential, fed to the potentiostat, may occur. Such transients are, however, of no significance: because of the limited bandwidth of the potentiostat,they are never carried through to the electrodes. Both the potential, V , which, during electrolysis,is currently applied to the electrochemical cell, and the resulting current, I , are monitored on digital displays on the front panel of the module. Although this facility is not strictly necessary, experience shows that it is very convenient for a rapid check of cell connections and potentiostat operation. Table I lists the controls, displays, and inputfoutput terminals available for operation of the module. ( B ) The Differentiator Module. A differential stripping potentiogram can be obtained by differentiating the analytical signal t (time) with respect to tlhe independent variable, E, and plotting d t f d E as a function of E. It is important to observe that in
Flgure 2. Diagram and timing sequence of the three-state potentiostat
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
' 1 I
X
E,
Y
REC
0 N
hl
0 hl
8
m
hl
0
0
0
Figure 3. The differentiator module,
-
_____---
Lo
t
-200
1
mV vs
SCE
/E
time
4
,
_i
i
0
0 u3
0
c.l
l-i
+
m
0 0
0
0
0
rl
rl
+
pi
0 0 0
-e
m
Hi
m
m
+
10s
l-i
Ti
c2
0
1 I
0
I
c.l I
0 0
m
0
rl
rl
I
Figure 4. Oxidative and reductive potentiometric stripping determination
of cadmium(II),lead(II), manganese(II), chloride, and selenide. Cf. Table 11. potentiometric stripping analysis, it is the total times, spent by the working electrode at the redissolution potentials, which constitute the analytical signals ( 4 ) . Therefore, to obtain a differential potentiogram, the function dE/dt vs. t , which is physically readily established by a simple electronic differentiator, is not adequate; moreover, from dE/dt vs. t , it is not possible to recover qualitative analytical information of the stripping potentials of the analytes. The physical implementation of the adequate signal dt/dE vs. E is straightforward in computerized potentiometric stripping analysis (8, 9) and is in fact the most direct way to acquire the stripping signal. With analog instrumentation, the differential potentiogram can be implemented by the circuit shown in Figure 3: Rather than displaying the direct potentiogram, Le., the potential vs. time function E(t) on an x-t recorder, E ( t ) is passed to a differentiating amplifier with a suitable time constant (of the order of 1 8 ) . The time derivative dE/dt is then input to an operational amplifier with variable gain. The resulting output is subsequently passed to a multiplier-divider circuit (Teledyne, Philbrick 4452) along with a constant voltage E,. The resulting output, which is proportional to dtldE, acts as input on the y terminal of an XY recorder, while the E ( t ) function itself acts as input on the x terminal. Electrochemical Cells and Electrodes. The design of electrochemical cell, the choice of electrodes, and further experimental precautions have previously been discussed for oxidative potentiometric stripping analysis (8) and for the two modes of reductive potentiometric stripping analysis (6, 7). In this study, as an extra precaution, the glass surface under the mercury pool electrode was siliconized to prevent any water film from reaching the platinum contact. Chemicals. The trace element solutions used for testing the instrument were prepared as reported previously (6-8).
RESULTS AND DISCUSSION The three operational modes of the instrument were tested on solutions of (a) cadmium(TI)/lead(II) (oxidative stripping),
c L.
c
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
1851
is lost by oxidation during T3despite the value of Tz. Hence, concentrations below sibout lo-' M cannot be determined even with prolonged electrolysis T3. For concentrations much below this level, methods other than stripping analysis should probably be used anyway, sincle theory predicts that for precipitation of the insoluble mercury compounds, a minimum concentration of 5 X M for S2and 5 X lo4 M for C1- is necessary (10, 11). In either of the two modes of reductive stripping analysis,, electrode regeneration between experiments is crucial for the precision. We find that with the values of El and T I given in Table 11,the stripping signals shown in Figures 4D,F,H can be reproduced with a relative standard deviation of 0.03 to 0.06 when eight consecutive stripping experiments are run. E El and TIare, however, sample/electrode dependent and must p be determined by trial runs for a given sample. -700 -400 E mV --vs.SCE t t Differential Potentiometric Stripping Curves. The Figure 5. Signal modification by the differentlator module: (A, B) basic performance of the dlifferentiator module is illustrated in potentiograms E vs. t , preconcentratlon for 2 s and 5 s; (C, D) Figure 5, which shows the results obtained from a lo4 NI derivatives dEldtvs. t ; (E, F) reciprocal derivatives dtldEvs. t; (G, cadmium(II), lo* M lead(I1) in 0.5 M acetate buffer, pH 4.'7 H) differential potentiograms dtldEvs. E . spiked with 10-4 M meircury(I1). The E vs. t plots after plating periods of 2 and 5 s are shown on Figure 5A,B, respectively. (b) manganese(I1) (reductive stripping with reducing agent Figure 5C,D shows the time derivatives output from the dissolved in the aqueous phase), and (c) chloride and selenide differentiating amplifier. The reciprocal derivatives, still a11 (reductive stripping with1 reducing agent dissolved in the a function of time, are shown on Figure 5E,F, while the difmercury phase). The experimental conditions and the inferential potentiograms, in Figure 5G,H, are obtained on an strument settings are listed in Table 11, while Figure 4 Bhows x-y recorder by letting the original E vs. t function control the resulting potentiograims. the x amplitude and d t / d E vs. t the y amplitude. Analytical Performance. The analytical performance It is important to observe that the original analytical (accuracy, precision, resollution, sensitivity, and limit of deinformation-i.e., the plateaus on the E vs. t plots-are re.tection) of potentiometric stripping analysis depends partly flected in the curve portions on dE/dt indicated by arrows. on chemical factors (sample composition and pretreatment), Obviously, the relevant information in dE/dt is that of lowesit partly on the electrode characteristics and quality, and partly amplitude. This low amplitude information must be a d e on factors which are purely instrumental. A discussion of the quately amplified before passing it on to the divider, which noninstrumental factors is beyond the scope of this commuwill otherwise be saturated and not represent the important nication and can be found elsewhere (1,3-9). information correctly. This amplification may, in turn, result At the low noise levels., often found with potentiometric in amplifier saturation during the steep decents on the E vs. stripping analysis, the limit of detection is determined by the t curve, reflected as ininima with high absolute amplitude ability of the instrument to resolve time signals. The senon dE/dt. Hence, for N given sample and a given plating time, sitivity too is dependent on this factor. A discussion of such the amplification of dE/dt must be adjusted to produce instrumental aspects can ]be found in recent papers (4,8,9). maximum sensitivity and simultaneously to prevent overWe shall, however, emphmize that whenever the instrument loading the amplifier between the analytical signals. To assist proposed in this paper is used for reductive stripping with an in this adjustment, the plating time can be varied simultaelectrolytically generated reducing agent, the sensitivity, deneously to ensure a suitable slope of the plateaus on the E: f i e d as the ratio of signal over analyte concentration, depends vs. t curve. These adjustments may be time-consuming, but on those electrolysis parameters used during the analyte once the proper setting is found, several samples of similar precipitation period as well as on those used for generation composition can be run with no further manipulation. of the reducing amalgam: while a long analyte precipitation period improves the sensitivity, long periods of amalgam ACKNOWLEDGMENT formation have the opposite effect (7).The total amount of The technical assiritance of Eigil Hansen and Kaspar reducing amalgam created should, of course, be sufficient to Vorbeck is gratefully acknowledged. redissolve the total amount of analyte precipitated and to LITERATURE CITED cover the losses of amalgam during the period T3.Conversely, Jagner, D. Anal. Chem. 1978, 50, 1924-1929. too large quantities of amalgam result in vanishing stripping Jagner, D.; Graneli, A. Anal. Chim. Acta 1976, 83, 19-26. plateaus (7). To find the proper instrumental settings, the Jagner, D.; Westerlurid, S. Anal. Chlm. Acta 1980, 717, 159-164. Kryger, L. Anal. Chim. Acta 1980, 720, 19-30. following procedure is recommended: Initially, E2 is tiet at . , Jagner, D. Anal. Chirn. Acta 1979, 105, 33-41. the half-wave potential for the formation of the reducing (6) Christensen, J. K.; Kryger, L. Anal. Chim. Acta 1980, 178, 53-64. amalgam. E3 is set a t a potential 200-400 mV anodic alf the (7) Christensen, J. K.; K8ryger,L.; Mortensen, J.; Rasmussen, J. Anal. Chlm. Acta 1980, 721, 71-83. stripping potential of the precipitate. For expected concen(8) Mortensen, J.; Ouziel, E.; Skov, H. J.; Kryger, L. Anal. Chim. Acta trations in the micromolar range, a value of 20 s is a suitable 1979. 112. 297-312. (9) Skov; H. J; Kpger.1. Anal. Chim. Acta 1980, 122, 179-191. choice for T3. Trial stripping experiments are subsequently (10) Vydra, F.; Stuiik, K.; J u l k o J , E. "Electrochemical Stripplng Analysis"; run, while T2is varied until a well-defined stripping plateau Ellis Horwood: Chichester, 1978; Chapter 2. is observed. Further optimization can then be carried out by (11) Brainina, Kh. Z.;Roizenbiat, E. M. Zavod. Lab. 1962, 28, 21-23. small adjustments on E 2 and Ea. If, with a 0.15 mL mercury pool electrode, the timle E3 RECEIVED for review February 24, 1981. Accepted June 1, exceeds about 3 min, all tlhe accumulated sodium amalgam 1981. ~
