0; zo/nF
-
(dr/dt) (-422) K i t h the aid of conventional Laplace transform methods the condition a t the electrode surface can be restated in the form of the integral equation ~zotl12/nFnl/2 C*Dll2 - CDll2 1:
=
=
D(dC/dx)
or alternatively
&t/nF
=
r* -
After T ~ ,the time at which C reaches zero, Equation A6 can be written in the form
(A301 The integral assumes zero value after T~ because its argument vanishes at that point. The argument of the integral can be given explicit form by substitution of Equation 410. Making that substitution and performing the indicated integration leads to iot/nF
To solve either of these equations for the transition time, it 16 necessary to assunie some explicit relation betm-een C at the electrode and I’. CASE I. Assume r and C not to be related b j an equilibrium expression and 1’ to react electrochemically before C. ZIathematically this means that Then
r > 0, then
C
=
C*
(A25) IYhile r has a finite value, Equatioii A24 siniphfies to znt/nF = I * - r Ixcause the argument of the integral is Aero. hfter T ~ ,the time at which r hcconies zero, Equation A25 can be put in the form 2C(t
- n)/nF
=
D’/2
L1-r‘ x
( C * - C ) / + l 2 ( t - 71 - B ) W B (A27) nhich is cxactly the same form as the expression n-ould have if there nere no adsorption and elt.ctrolysis began at time 71rather than time zero. CASE 11. Assume l7 and C not to be related by an equilibrium expression rind C to be reduced to zero before reaction of r begins. IhIathematically, when C > 0, then ( d r / d t ) = 0 (A28)
\Yhilc tlierc is still a finite concentration, C, a t the electrode, Equation A23 can be bimplified to 2 ~ o t ” ~ / r ~ F ( r r D= ) ” C* ~ - C (A29) lmausc the argument of the integral is zero.
%n
=
1’*
-
I?
+
[arcsin(2+t) 41 -
(? -
1)’
+ 71/21
(A431)
The expression for 7 2 can be obtained from Equation A31 by noting that a t t = T p J r = 0.
+
CASE 111. Assume r and C to be related by an equilibrium of the form
r = K,C at z = 0 (A32) The integral equation resulting from the substitution of Equation A32 into Equation A23 or A25 can be solved readily by Laplace transform methods and the result has been given by Lorenz (6). NOMENCLATURE
C*
=
C, C*,
= =
D
= = =
erf erfc exp E
= =
E”’
=
F H
= =
20
= = =
ti
kj
concentration of species 0 prior to electrolysis instantaneous concentration of 0 hypothetical Concentration of 0 prior to electrolysis assuming complete conversion of Y to 0 diffusion coefficient error function error function complement natural exponential function instantaneous cathode potential us. unpolarized reference formal standard potential of the 0-R couple Faraday’s constant height of mercury head in polarographic experiment currentdensity polarographic limiting current formal rate constant for chemical reaction in forn-ard direction
formal rate constant for chemical reaction in reverse direction R, = equilibrium constant for chemical reaction ( k , / k f ) K O = equilibrium constant for adsorption obeying linear isotherm n = number of electrons per molecule in reduction of 0 to R 0 = oxidized form of electrochemically reactive couple = stoichiometric constant for chemp ical reaction R = reduced form of electrochemically reactive couple t = time from start of electrolysis z = linear distance from electrode surface X, Y = electrochemically inactive participants in chemical reaction A0 = defined by Equation A10 r = instantaneous amount of adsorbed species per unit area r* = initial amount of adsorbed species per unit area 7 = transition time (numerical subscript indicates chronological order) T~ = hypothetical transition time n-hich mould be observed if all chemical kinetic steps were infinitely rapid k,
=
LITERATURE CITED
(1) Davis, D. G., Ganchoff, J., J . Electroa m l . Chem. 1, 248 (1960). ( 2 ) Delahay, P., Berzins, T., J. Am. Chem. SOC.75, 2486 (1953).
(3) Fischer, O., Dracka, O., Fischerova, E., Collection Czechoslav. Chem. Communs. 25, 323 (1960). (4) Gierst, L., Juliard, A,, J . Phys. Cliem. 57, 701 (1953). (5) Koutecky, J., Cizik, J., Collectzon Czechoslov. Chem. Communs. 22, 914 (1959). (6) Lorenz, W.,2. Elektrochern. 59, 730 (1955). ( 7 ) Moorhead, E. D., Furman, N. H., ASAL. CHEW32, 1506 (1960). (8) Reinmuth, W. H., Ibid., 32, 1514 (1960).
(9)’ Rosebrugh, T. R., Miller, W. L.. J . Phys. Chem. 14, 816 (1910). (10) Testa, A. C., Reinmuth, SIr. H., A N A L . CHEM. 32, 1512 (1960). (11) Ibid., p. 1518. RECEIVEDfor review October 21, 1960. lccepted December 22, 1960.
Application of Stripping Analysis to the Determination of Iodide with Silver Microelectrodes IRVING SHAlN and S. P, PERONE Deparfment o f Chemistry, University of Wisconsin, Madison, Wis. The extension of stripping analysis to the determination of halides with a silver microelectrode has been investigated. During the pre-electrolysis step, a portion of the halide was deposited b y a controlled potential oxidation of the silver electrode. Two methods of stripping the silver halide deposit from the electrode
were investigated: electrolysis with constant potential, and electrolysis with linearly varying potential. The quantity of electricity measured in the stripping step was a direct function of the pre-electrolysis time and the bulk concentration of halide. The method was applied to iodide solutions as dilute a s 4 X 10-8M.
R
RESEARCH on stripping analysis has shoum t h a t the method is very sensitive for the determination of electroactive materials. The technique consists of a pre-electrolysis step, during which the sample is concentrated by electrodeposition on a n electrode. The actual analysis takes place during a subsequent elecECEST
VOL. 33,
NO. 3,
MARCH 1961
325
trodissolution (stripping) step, in which a n y of several electroanalytical methods can be used. Most of the important applications of stripping analysis have been applied to the determination of metal ions in solution. Hanging mercury drop electrodes (3, 4) or mercury pool electrodes (14) were used, and during the pre-electrolysis step amalgams were formed. Solid inert electrodes also have been used for stripping analysis. They must be used for the analysis of materials which are more noble than mercury (9) or which cannot be stripped from a mercury electrode because of irreversibility ( 1 2 ) . Another important type of stripping analysis involves the use of oxidizable electrodes for the determination of substances which can be precipitated on an electrode surface as the electrode is anodized. The methods are similar to the conventional methods of stripping analysis, except that the actual determination is made during a cathodic stripping process. This type of stripping analysis, using mercury electrodes, has been applied to the determination of chloride (1). This paper describes the application of silver electrodes to the stripping analysis of halides, primarily iodide. The method involved the controlled potential oxidation of a silver Plectrode immersed in a halide solution. A potential was selected anodic enough so that the silver electrode could be oxidized readily to the silver halide, but not so anodic as to permit significant amounts of free silver to form. During this pre-electrolysis period, t h e experimental conditions were maintained as constant as possible so that a reproducible portion of the sample was deposited each time. The deposited film of silver halide \vas cathodically stripped by using either of two methods: electrolysis with constant potential or voltammetry \$ ith linearly varying potential. I n either case the correlation between the quantity of silver halide deposited and the original bulk concentration vias made by measuring the quantity of electricity involved in the stripping process. It was possible to analyze iodide solutions as dilute as 4 X 1 O - * X . The method was less sensitive for bromide and chloride, m-hich forin more soluble silver salts.
EXPERIMENTAL
Apparatus. TKO types of instruments were used in this work. One was a general purpose voltammetric instrument based on t h e operational features of the analog computer amplifiers manufactured b y G. A. Philbrick Researches, Inc., Boston, Mass., and used some of the ideas suggested by DeFord ( 2 ) . The circuit 326
ANALYTICAL CHEMISTRY
was arranged so that the potential between the working electrode and a reference electrode was controlled to within 1 mv., while the current passed between the working electrode and a counter electrode. For the potentiostatic stripping method, this control potential could be changed suddenly (within 1 msec.) t o cathodic potentials, and a n analog computer integrator circuit was used to measure the quantity of electricity. On the other hand, when voltammetry with linearly varying potential was used for the cathodic stripping process, the potential was scanned toward cathodic potentials using rates of voltage change in the range of 15 to 50 mv. per second. The current-voltage curves were recorded on a Leeds &. Sorthrup 10-mv. (0.4second) recorder. The quantity of electricity was obtained by measuring the area under the recorded curve n-ith a planimeter. The second instrument used was a modified version of the Sargent Model XV Polarograph (E. H. Sargent Co., Chicago, Ill.). This instrument was suitable for stripping analysis when using voltammetry with linearly varying potential. Three rates of voltage scan were available: 16.7, 33.3, and 50.0 mv. per second. I n all experiments, the working electrode was a Beckman S o . 1281-5 silver electrode, surface area approximately 0.06 sq. em. I n those experiments performed with the working electrode a t controlled potential, the counter clectrode was a platinum wire. The reference electrode was a Beckman Yo. 39270 calomel electrode placed in a compartment containing 2-11 potassium nitrate, and connected to the cell by a Luggin capillary salt bridge. I n those eyperiments performed with a tn-o-electrode system, the combined counter-reference electrode was a large saturated calomel electrode, connected to the cell by a double junction salt bridge. The salt bridge was arranged so that a d o n stream of 2-11 potassium nitrate flowed through the center compartment, thus effectively preventing cross contamination of the reference electrode and the sample. The cell was a polystyrcne tumbler, 300-nil. capacity, painted black to prevent photoreduction of the silver halide deposit. Reproducible stirring was provided by a glass stirrer rotated a t 600 r.p.m. by a Sargent synchronous rotator (E. H. Sargent Co., Chicago). N o attempt was made to control the cell temperpture in these euperiments. Materials. A+lllmaterials were reagent grade and were used nithout further purification. The indifferent electrolyte in each evperinient was 0.1N acetic acid-sodium acetate buffer, pH 4.7. All solutions nere prepared with triply distilled a ater. K h e n preparing and using solutions more dilute than 10-6M i t was necessary to equilibrate all containers and the cell assembly. A less stringent version of the procedure described previously (3) was used. High purity nitrogen was passed through a gas washing bottle containing
the indifferent electrolyte, and then was used t o remove oxygen from the cell. RESULTS AND DISCUSSION
The oxidation of silver electrodes in halide solutions has been investigated extensively, particularly in connection with coulometry (8) and coulometric titrations ('7). Electrodeposited halide films are adherent (8, 10) and the electrode reactions are relatively uncomplicated. Thus the experiments n ere designed to test ri-hether a reproducihle portion of silver halide could be electrodeposited during the pre-electrolysis step, and whether such electrodepmited silver halide films could be reduced quantitatively during the subsequent cathodic stripping step. Pre-electrolysis Step. SELECTION OF PRE-ELECTROLYSIS POTENTIAL. During t h e pre-electrolysis step, t h e potential of t h e working electrode must be controlled carefully. Lingane and Small (8) selected the proper working potential by considering the standard electrode potentials of the various silver halides and constructing a potential-pX diagram. Those calculations were confirmed in this n ork by obtaining current-voltage curves n-ith a stationary silver electrode in stirred solution (Figure 1). Exactly the same conditions-stirring, electrode and cell geometry, indifferent electrolyte, d e . were used in these experiments as n-ere used in the actual stripping analyses with more dilute solutions. I n each case a limiting current region 11-as obtained. A rather broad plateau was obtained for iodide solutions, for example, while a narron one was obtaincd for chloride solutions, reflecting the differences in solubility of silver halides. In order to use such voltammetric curves to help select the proper preelectrolysis potential. shifts in the halfwave potential on dilution of the halide must be considered. Since. a microelectrode was used in this n-ork, the silver halide deposit could be assumed to be relatively thick. e ~ c nfor very dilute solutions. On this basis, it was assumed that the activity of the silver halide deposit did not varx during the electrolysis. Thus, an equation developed from a consideration of the Nernst diffusion layer theory (5) could be used, For the anodic reaction, Ag
+ X-
+
AgX
4-e
(1)
one obtains El,* = const
C,- RT n F In 2
(2)
where C,- is the bulk concentration of the halide, Ellz is the half-wave poten-
tial and €2, T, n, and F have the usual significance. From Equation 2, the half-wave potential should shift to more anodic values b y approximately 60 mv. for each 10-fold decrease in the halide concentration, until the silver halide wave merges TT ith the limiting wave corresponding to the oxidation of the electrode to free silver ion. Thus, for the determination of iodide, the limit of sensitivity (ignoring residual current and other interferences) cannot be lower than about 10-9-V. Using these data, a pre-electrolysis potential of +0.18 volt us. S.C.E. was selected for the stripping analysis of iodide. The solubility of silver chloride is so high that the two lvaves merge n-hen the chloride concentration is reduced below about 10-4AlZ. The sensitivity of stripping analyses for chloride can be improved, however, by using a solvent in which the solubility of silver chloride is decreased. Coulometric titrations of chloride have been improved by using a solvent containing 80% ethanol ( 7 ) . This decreased solubility of silver chloride in 80% ethanol also is shonn in Figure 1, nhere a broader, but lower, limiting current region indicates that in this solvent, the limit of sensitivity should approach 10-6JI. From these data, a pre-electrolysis potential of +0.30 volt vs. S.C.E. was selected for stripping analysis of chloride in 80% ethanol. The u e of mercury electrodes for stripping analysis of halides should lead to even more sensitive determinations, since the solubility products of the mercurous halides are much lower than the corresponding silver halides. However, mercury electrodes were not investigated because of the apparent passivation of stationary mercury electrodes on anodization in halide solution (6). ELECTRODE PRETREATMENT. As is frequently the case when solid electrodes are used, the condition of the electrode surface was a critical experinicntal factor. It was necessary to maintain the condition of the electrode surface as constant as possible to obtain maximum precision and accuracy. I n gcncral, the pretreatment consisted of a prc-electrolysis, followed by stripping to leave a freshly precipitated layer of silver. on the electrode surface. The conditions used were essentially the samc as in an actual determination, except that care was taken to ensure that the quantity of frt.shly precipitated silver formed was greater than that required for the subsequent determinations. This \vas accomplished by performing the pretreatment in a halide solution more concentrated than that expected in the analysis, and/or continuing the pre-electrolysis for a longer time than required for the analysis. I n a series of analyses, each preceding determination could serve as a pre-
I
I'
I
04
02
1
1
1
0 -02 VOLTS, v s S C E
-04
-06
Figure 1. Current voltage curves for oxidation of silver electrode in stirred halide solutions Each solution, 1 O-3M in halide, 0.1 M acetate buffer A. KI B . KBr C. KCI D. KCI in 80% ethanol
treatment when working above 10-6M, except when the concentration level increased markedly between determinations. K i t h the more dilute solutions, the best reproducibility was obtained by pretreating the electrode in a more concentrated solution before each determination. I n spite of the Pretreatment, a gradual increase in the residual current was noted LT hen the electrode was used for a series of analyses. K h e n this interference became significant, the electrode could be restored to its original condition by polishing i t gently using a slurry of 600X Alundum and n-ater. The main consideration in the development of a pretreatment procedure was to handle the electrode in exactly the same fashion prior to each analysis
Table 1.
Concn., C, Moles/Liter 4 00 x 10-4 4 00
in a series. Consistent results could be obtained only when the surface of the electrode \J as restored to some reproducible condition before each use. PRE-ELECTROLYSIS PROCEDURE. After the pre-treatment, the electrode was equilibrated with an aliquot of the sample solution, and then immersed in the sample. To remove oxygen, nitrogen was passed through the solution for 10 to 20 minutes. During the remainder of the analysis the nitrogen continued to flow over the surface of the solution. The elertrode potential then mas set a t -0.40 volt (us. S.C.E.) to ensure that the surface was entirely reduced. This was followed by the carefully timed pre-electrolysis step a t the appropriate anodization potential. Pre-electrolysis times varied rn ith the concentration level, and normally ranged from 5 minutes a t l O - 5 J f to 30 minutes a t 10-8,U. Reproducible stirring conditions could be obtained in a series of analyses by exercising normal care in placement of the electrode and stirrer in the cell assembly. Changes in cell geometry are reflected in the different electrode calibrations of Tables I and 11. Cathodic Stripping. After t h e preelectrolysis step, t h e quantity of material n hich has been concentrated on t h e electrode surface must be determined. Since t h e activity of t h e deposited film does not vary in a n y reproducible manner during t h e stripping procedure, t h e total a m o u n t of oxidized silver can be determined only through some type of coulometric meawrcment (13). The three most
Constant Potential Stripping Analysis of Iodide
Ket Preelectrolysis Time, t, Minutesa
x 10-5
4.00 x 10-6 4.00 x 10-7 4 . 0 0 X lo-*
Average Quantity of Electricity, Q, pcoulombea,b
3
2540
7
421
&/Ct X
Av. Dev.,6 % 1 1
2 12
10
15 30
Ket quantity of electricity for a net pre-electrolysis time was obtained by subtracting the total coulombs for a short pre-electrolysis from that for a longer pre-electrolysis. Average and average deviation of 5 replicate determinations.
Table It.
Concn., C, hloles/Liter 4.00 x 10" 4.00 X 4 . 0 0 x 10-7 4.00 X a
Linearly Varying Potential Stripping Analysis of Iodide
Pre-electrolysis Time, t , Minutes 5.27 10 15
30
Average Quantity of Electricity, &, pcoulombs402 76.2 11.3 2.27
Q/Ct
x
Av. Dev.,O
10-6 1.91 1.90 1.88
1.89
70
0.6 0.9
2.0 2.5
Average and average deviation of 3 to 5 replicate determinations.
VOL. 33, NO. 3, MARCH 1961
327
straightforward methods of reducing the silver halide film are constant current, constant potential, or linearly varying potential methods. The last two were investigated in this M ork.
POTENTIOSTATIC STRIPPIKGMETHOne of the simplest IT-ays of removing an oxidized film from a solid electrode is to change the potential suddenly to some cathodic value. Current time curves for the potentiostatic stripping of silver iodide films are shown in Figure 2. After the carefully timed pre-electrolysis step, the potential was changed to -0.40 volt us. S.C.E. for the reduction. For analytical purposes, the current-time curves were integrated automatically by an analog computer integrator circuit. Since this method is essentially coulometry a t constant potential, it is subject to the various errors discussed by RSeites and Moros (11). However, the time required to reduce the silver iodide film is so short that only two of these sources of error are important. The first interference was the residual current resulting from the reduction of impurities in the solution, primarily the remaining traces of dissolved oxygen. The effect of this source of error was minimized by using the same time for the cathodic stripping in all cases. It was found that even for the most concentrated solution analyzed, the silver iodide could be reduced quantitatively in less than 10 seconds. A standard stripping time of 15 seconds was selected, and all current-time curves were integrated for this length of time. Thus the blank could have been used to account for this source of error. The second major source of interference was the charging current resulting from the sudden change in the electrode potential from the anodization potential (+0.18 volt) to the stripping potential (-0.40 volt). This is the major part of the current indicated in Figure 2, curve A . Unfortunately, a correction for this charging current could not be made simply by subtracting from the total coulombs the quantity of electricity associated with the blank. The presence of the silver iodide deposit causes the capacity of the electrical double layer of the electrode to change markedly. This was shown by performing a series of stripping analyses on a 4 X lO-’JI solution of iodide using various pre-electrolysis times. Plotting the total measured stripping coulombs for each pre-electrolysis time, a n excellent straight line n-as formed which extrapolated to a value of 17.1 pcoulombs for zero pre-electrolysis time. This represents the value of the blank (mostly charging current) for an electrode with a deposited silver iodide film. HOWever, when actually measured in the absence of iodide, the blank n a s of the ODS.
328
ANALYTICAL CHEMISTRY
0
1
2
3 4 5 TIME, SECONDS
6
7
0
Figure 2. Cathodic stripping of silver iodide films using potentiostatic method Stripping potential, -0.40 volt and pre-electrolysis times were A. Blank B. 4 X 10-6M, 10 min. C. 4 X 10-6M, 5 min. D. 4 X lo-“, 2 min.
order of 20.7 pcoulonibs, and essentially independent of the pre-electrolysis time. These results indicate that the presence of the oxidized film reduces the capacity of the electrical double layer markedly. These data, however, indicate a n alternate method of correcting for the blank. Two stripping analyses are run on the same solution, one with a relatively long pre-electrolysis time, the other for a short pre-electrolysis time. Then the difference in the stripping coulombs which is obtained corresponds to the difference in pre-electrolysis times. This procedure effectively corrects for both the charging current and the rebidual current. Typical results are given in Table I The quantity of electricity measured for the stripping process was proportional to the product of the bulk iodide concentration and the pre-electrolysis time over a wide range of concentrations. The major source of error is probably in the dilution and handling of the solutions a t these concentration levels. LINEARLY VARYING POTEXTIAL STRIPPING METHODS.A second way to reduce the silver iodide film is to scan the potential toward cathodic values. A typical peaked current voltage curve is obtained (Figure 3), and the quantity of electricity involved in the reduction can be obtained by measuring the area under the curve with a planimeter. Although the potentiostatic method has the advantage that the entire measurement and read-out of the quantity of electricity can be performed autoniatically, rather complex equipment is required. The equipment needed for the linearly varying potential method, on the other hand, is much simpler and is commercially available. Thus, the scanning technique also was investigated thoroughly.
VI.
S.C.E.;
iodide concentrations
The selection of the rate of voltage scan was important. I n general, the more rapid rates of voltage scan produced sharper peaks without markedly increasing the charging current. Thus, the area under the curve was somewhat easier to measure than when the curves were obtained with the slower voltage scans. On the other hand, high rates of voltage scan could not be used indiscriminately at the higher concentration levels because the potential of the hydrogen wave was reached before all the silver iodide had been reduced. Thus, the proper rate of voltage scan was a compromise between these two factors, and depended on the sample concentration. The approximate suitable ranges were 10 to 15 mv. per second a t to 10-8111 iodide; 15 to 35 mv. per second a t to lo-7x; and 40 to 50 niv, per second belorv 10-731. Another factor n-hich affected the sharpness of the peak obtained was the rate of removal of free iodide as it was released from the electrode surface during the reduction. At the higher concentration levels (above 10-6JI) the large amounts of iodide released a t the electrode surface change the equilibrium potential and result in rather drawn out curves. Thus, a t these higher concentrations i t was necessary to continue the stirring during the stripping process. The improved shape of the current-voltage curve more than made up for the increased noise on the trace. For solutions more dilute than lO-’-lI, stirring had no effect on the shape of the current-voltage curves obtained and, therefore, was not used. The typical current-voltage curves shown in Figure 3 indicate that a straightforward correction for residual current could be made by inspection (dashed line). Attempts to use a blank
bOLTS, 0
,
02
"5
SCE -02
-0:
8h 6a 2
a
7 4._
2-
I
O t J + 0,2 0
-02 LOL'S,
vi
-04 SCE
Figure 3. Cathodic stripping of silver iodide films using voltammetry with linearly varying potential Concentrations and pre-electrolysis times were A. 4 X 1 0-8M, 30 min. 6. Blank C. 4 X 1 O - W , 1 0 min.
I
I
-0.2
I
-04 VOLTS, v s S C E 0
_ . I -0.6
Figure 4. Cathodic stripping of silver chloride film using voltammetry with linearly varying potential Solution contained 80% ethanol, 1 5-min. pre-electrolysis time A. 4 X 10-6M KCI 6. Blank
electrolysis tiincs of 10 minutes or of C. 0. Huber during the early stages longer, the correction was negligible. of this n-ork. SOLUTIONS. AKALYSISOF CHLORIDE as a guide in subtracting the residual LITERATURE CITED The linearly varying potential method current were not successful. As in the (1) Ball, R. G., Manning, D. L., RIenis, of stripping was applied to the analysis potentiostatic method, the charging O., ANAL.CHEX 32,621 (1960). of chloride in 80% ethanol. The pro(2) DeFord, D. D., Division of Analytical current for a n unoxidized electrode was cedure was essentially the same as used Chemistry, 133rd Meeting, ACS, San markedly different than the obvious for the analysis of iodide solutions. Francisco, Calif., April 1958. base line of the analytical current(3) DeNars, R. D., Shain, I., ANAL Analyses were made in solutions as voltage curve (Figure 3, curve B ) . CHEW29, 1826 (1957). dilute as 4 x 10-631 but the results (4) Kemula, W.,Kublik, Z., And. Chim. The analytical results are summarized (Table 111) were not as precise as were Acta 18, 104 (1958). in Table 11, :tnd the correlation bethose obtained with iodide. The cur( 5 ) Kolthoff, I. M., Lingane, J. J , tween the stripping coulombs and the "Polarography," Vol. 2, p. 578, Interrent-voltage curves were rather drawn product of the pre-electrolysis time and science, Sew York, 1952. out, and tended to split into two peaks (6) Kuwana, T., Adams, R. T.,-4nal. bulk concentration is excellent. The (Figure 4,.4), possibly indicating that Chim.Acta 20, 51, 60 (1959). odd pre-electrolysis time for the -1 X the silver chloride is deposited in two (7) Lingane, J. J., AUL. CHEX. 26, 622 10-5JI solution reflects the fact that a n (1954). forms. anodic current continues to flon during (8) Lingane, J. J., Small, L. A., Ibid., 21,1119 (1949). the early part of the cathodic scan, and ACKNOWLEDGMENT (9) Lord, S. S., Jr., O'Seill, R. C., Rogers, the nominal 5-minute pre-electrolysis L. B., Ibzd., 24, 209 (1952). iws correctcd accordingly. For preThe authors are grateful for the help (10) RIcXevin, W. M, Baker, B. B., SlcIver, R. D., Ibid., 25, 274 (1953). (11) Meites,- L., Moros, S. A,, Ibid., 31, 23 (1999). (12) Kicholson, M. M., Ibid., 32, 1058 (1960). Table 111. Linearly Varying Potential Stripping Analysis of Chloride in 80% Ethanol (13) Kicholson, M. M., J . Am. Chem. Sac. 79, 7 (1957). Average (14) Nikelly, J. G., Cooke, W.D., ANAL. Pre-electrolysis Quantity of CHEM.29,933 (1957). Concn., C, Time, t, Electricity, &, Av. Dev.," RIoles/Liter Minutes ficoulombsa &/Ct x 10-6 % RECEIVEDfor review October 20, 1960. Accepted December 19, 1960. Work sup4.00 x 10-4 5 1996 1.00 2.4 ported in part by funds received from the 4.00 X 10 412 1.03 2.6 U. S. Atomic Energy Commission under 4.00 X 15 59.8 1.02 2.6 Contract x o . AT(11-1)-64, Project KO. 17. Part of the work was performed Average and average deviation of 5 replicate determinations, n-hile S. P. Perone held a Woodrow Wilson Fellowship. 0
VOL. 33, NO. 3, MARCH 1961
329
