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Anal. Chem. 1981, 53, 861-864
dation of the TBTO and some loss to the glass walls of the container. While this determination of TBTO is not necessarily a trace technique, it serves to illustrate the ease and convenience of analysis on the HAFID. Environmental analysis is an important area in which organotin analytical methodology can be applied. For example, one of the most widely ueied miticides in the fruit industry is Plictran, a product of the Dow Chemical CQ.,which contains tricyclohexyltin hydroxidle as the active ingredient. Figure 6 shows the results of an analysis of apple orchard leaves for tricyclohexyltin hydroxidle in which the organotin was converted to the bromide derivative and determined as tricyclohexyltin bromide. Figure 7 shows a similar analysis of a fungicide, Duter (Haywood Chem. Co., Kansas City, KS), which contains triphenyltiri hydroxide, in which the organotin was converted to the metlhyltriphenyltin derivative using a Grignard reagent prior to injection into the GC. Uses of organotins are expanding an industry and agriculture as they replace more toxic chemicals as pesticides, preservatives, stabilizers, etc. But with increased production
comes increased danger of environmental contamination. Whether for process control or environmental monitoring, the need for quantification and identification of specific organotin compounds is growing in importance. Modification of a FID to a HAFID offers the analyst an inexpensive option for the sensitive and selective detection of organotin compounds.
LITERATURE CITED (1) Aue, W. A.; Flin, C. G. J . Chromatogr. 1977, 50, 1435. (2) Gauer, W. 0.; Seiber, J. N.; Crosby, D. G. J. Agric. Food Chem. 1979, 22, 252. (3) Wagner, J. H.; Lillle, D. H.; DuPuis, M. D.; Hill, H. H., Jr. Anal. Chem. 1980, 52, 1614. (4) DuPuls, M. D.; Hill, H. H., Jr. Anal. Chem. 1979, 52, 292. (5) Wrlght, B. W.; Lee, M. L.; Booth, G. M. HRC CC, J . Hlgh Resolut. Chromatogr. Chromatogr. Commun. 1979, 189. (6) Hill, H. H., Jr.; Aue, W. A. J . Chromatogr. 1878, 122, 515. (7) Sternberg, J. D.; Gallaway, W. S.; Jones T. L. Gas Chromatogr. (U.S.), 1962, 231-267.
RECEIVED for review October 22, 1980. Accepted February 3,1981. This work was supported by the National Science Foundation under Grant CHE77-25743.
Spectrophotometric Determination of Iron(I1) Ferrozine Complex for the Indirect Determination of Phosphate Fredrick Bet-Pera, Amit K. Srlvastava, and Bruno Jaselskis" Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60626
Phosphate in parts-per-billion amounts is determined indirectly by use of the lron(I1) ferrozine complex and measuring the absorbance at 562 nm after the extraction of 12-molybdophosphoric acld into isobutyl acetate and reduction of moiybdate by amalgamated zinc In 1.44 M hydrochloric acid and reoxidation of molybdenum(111) to -(VI) with iron(II1). Phosphate in Environmental Protection Agency and natural water samples is determined in amounts as low as 4 ppb in the final solution, with a relative precision of 6% at la value. The apparent molar absorptivity for this method is 9.66 X IO5 compared to the theoretical value of 10.8 X lo5.
Analytical methods for the determination of microamounts of orthophosphate are usually based on the formation of 12molybdophosphoric acid from phosphate and excess molybdate in acid solution, and subsequent extraction of the 12molybdophosphoric acid from excess molybdate by oxygencontaining organic solvents. The extraction is followed by an absorbance measurement of' the yellow 12-molybdophosphoric acid or the reduced blue heteropoly compound. Different reducing agents have been reported in literature such as stannous chloride (1,2),ferrous ammonium sulfate (3,4), ascorbic acid ( 5 , 6 ) ,hydroquinone (7), hydrazone sulfate (8), 2-amino-4-chlorobenzenethiol hydrochloride salt (9), and l-amino-2-naphthol-4-sulfonic acid (10, 11). Different methods for the phosphate determination have been used to overcome interferences due to arsenate, silicate, and germanate. With these methods interferences have been eliminated by using a suitable organic solvent which selectively extracts 12-molybdophosphoric acid into the organic phase 0003-2700/8 110353-0861$01.25/0
and leaves interfering species in the aqueous phase (12-16). Other methods for removal of interferences include controlling hydrogen ion concentration (17,18), preliminary volatilization of the interfering species as bromides (19),and differences in the rate of formation of heteropoly blue (20). Small amounts of phosphorus have also been determined indirectly by polarography (21)and by atomic absorption (15,22). In the method described in this paper, phosphate is converted to 12-molybdophosphoricacid which is then extracted into isobutyl acetate, as recommended by Paul (23). After evaporation of the organic solvent the complex is dissolved in alkaline solution, and after acidification molybdenum(V1) is reduced to molybdenum(II1). The resulting molybdenum(111)is reoxidized with iron(II1) to molybdate and the resulting iron(I1) is determined as iron(I1) ferrozine complex. In this manner submicroamountsof phosphate have been determined by measuring the absorbance of the iron(I1) ferrozine complex.
EXPERIMENTAL SECTION Apparatus. A Cary 14 (Varian Instrument Group, Palo Alto, CA) spectrophotometer was used for all spectrophotometric measurements. The pH of solutions was determined by using an Orion (Orion Research Inc., Cambridge MA) Model 12 pH meter. Small amounts of reagents were weighted with a semimicrobalance. Reagents. All chemicals were either analytical or primary standard grade and were used without further purification. Deionized water was used to make all solutions, Glassware. All glassware was washed with 6 M hydrochloric acid and rinsed with deionized water. Preparation of Standard Solutions. Stock standard phosphate solution was prepared by dissolving 0.1000 g of oven-dried analytical reagent grade disodium hydrogen phosphate, Na2HP04,in 100 mL of deionized water and was stored in a 0 1981 Amerlcan Chemical Society
862
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6 , MAY 1981
polyethylene volumetric flask. Working solutions of lower phosphate concentrations were prepared daily by diluting the stock solutions. Stock standard sodium phosphate (tripoly) was prepared by dissolving 0.0390 g of oven-dried sodium phosphate (tripoly), NaP3Ol0,in 100 mL of deionized water. A 50-mL portion of this solution was transferred to a 100-mL beaker followed by addition of 1mL of 1:3 sulfuric acid, and the mixture was heated on a hot plate with gentle heat until the volume was reduced to about 10 mL. Several drops of phenolphthalein indicator were added, and the solution was titrated with 1M sodium hydroxide. One drop of 1:3 sulfuric acid was added to decolorize the solution. The colorless solution was cooled to room temperature and diluted to 50 mL with deionized water and was used as the stock solution for hydrolyzable phosphate. The solution was stored in a polyethylene container. Working solutions of lower concentration were prepared daily by diluting the stock solution. The phosphate in tripoly phosphate stock solution was determined gravimetrically and was found to be 96% pure. A 0.003 M iron(II1)solution was prepared by dissolving 0.1446 g of ferric ammonium sulfate (Mallinckrodt-analyzed (Mallinckrodt, Inc., St. Louis, MO)) in about 50 mL of deionized water with 0.5 mL of concentrated perchloric acid and 0.2 mL of concentrated nitric acid. The solution was heated until perchloric acid fumes appeared. Then the contents were diluted to 100 mL with deionized water. Ferrozine, 3-(2-pyridyl)-5,6-bis(4phenylsulfonic acid)-l,2,4-triazinemonosodium monohydrate salt, was purchased from Hach Chemical Co. (Ames, IA). A 0.012 M stock solution of ferrozine was prepared by dissolving 0.6125 g in 100 mL of deionized water. The Jones reductor was prepared by taking enough 20-30 mesh zinc to occupy 7-8 cm of a 50-mL buret. Zinc was then amalgamated by placing it in a solution containing 15 mL of 3% mercuric chloride and 15 mL of 1.44 M sulfuric acid and stirring for about 10 min. After the amalgamated zinc was washed, the Jones reductor was made in a conventional manner as described in most of the beginner's quantitative analysis texts and was activated by running 10-20 mL of 1.44 M HC1 through the column prior to use. The reductor was stored in deionized water while it was not in use. Procedure. Preparation of the Standard Curve Using Phosphate Solutions. Various volumes (0-5 mL) of the stock 7.04 X lo9 M disodium hydrogen phosphate were transferred to 100-mL volumetric flasks and diluted to the mark with deionized water. One milliliter of dilute phosphate solution and 1.5 mL of 5% ammonium molybdate (w/v) were placed in a 25-mL separatory funnel, and 1.25mL of 2 M hydrochloric acid was added to bring the hydrogen ion concentration to 0.66 M. This solution was then shaken and left standing for 15 min to ensure complete formation of the heteropoly acid. Heteropoly acid was then extracted with 5.0 mL of isobutyl acetate by shaking the contents of the separatory funnel vigorously for 1 min. The two phases were allowed to separate, and then the lower aqueous phase was discarded. One milliliter of the organic phase was pipetted into a 25-mL beaker and was air-dried in a hood. After complete evaporation of the organic phase 2.0 mL of 0.125 M NaOH was added to break up to heteropoly acid, and to this 5.6 mL of 2 M HCl was added to bring the solution to a final hydrogen ion concentration of 1.44 M. This solution was passed through a Jones reductor at a rate of 1 drop every 5 s, and the effluent which contained Mo(II1) was collected in a 25-mL beaker containing M ferric ammonium sulfate. The collection 5.0 mL of 3 X beaker was covered with a paraffin film and with nitrogen throughout the entire operation. The Jones reductor was washed with three 4-mL portions of 1.44 M HCl to recover all of the molybdenum. Four milliliters of 0.012 M ferrozine was added immediately to the beaker containing effluent from the Jones reductor. The pH of this solution was adjusted to be in the range of 3.0-3.5 (by pH meter) by slowly adding small amounts of solid sodium bicarbonate. The adjusted solution was transferred into a 100-mL volumetric flask and diluted to the mark. The absorbance of the complex was measured immediately at 562 nm by using a 1-cm cell. Preparation of the Standard Curve Using Hydrolyzed Sodium Phosphate (Tripoly). Various volumes (0-5 mL) of the 1.06 X M sodium phosphate (tripoly)stock solution were transferred
to 100-mLvolumetric flasks and diluted to volume with deionized water. The rest of the procedure for the determination of the hydrolyzed phosphate was the same as in the previous section. Determination of Phosphate i n Natural Waters or at Low Level. Surface water was placed in an acid-washed Nalgene container and immediately cooled on ice. Then the sample was filtered through a 0.45-rm membrane filter, and the filtrate was analyzed by the procedure used in the determination of pure phosphate solutions. If the concentration of phosphate was found to be low, the amount of organic phase taken for analysis was increased to 80% of the organic phase rather than taking 20% as in the previously described procedure. Calculations. The amount of phosphorus in parts per billion of the final solution having volume, V, of 100 mL is obtained by the equation (3.1 X 107)A/(9.66 X
lo5)
(1)
where A is the corrected absorbance for the blank. The amount of phosphorus in the original solution is calculated by taking into account the following: (1)volume of the sample, V,, (2) the volume of the isobutyl acetate, V,, used for the extraction, (3) the volume of organic phase taken for analysis, V,, and (4) the final volume, V, of solution being measured. The amount of phosphorus in parts per billion in the original aqueous sample is X
1O7)A/(9.66 X 105)l~[(VoV~)/(Vo,V,)1 (2)
Standard addition method is recommended for the sample having absorbance values less than 0.1. In this case the amount of phosphorus in the original sample is calculated as In this equation A,a corresponds to the uncorrected absorbance of the standard and A s a + d corresponds to the absorbance of the standard containing the unknown. In all of these experiments the volumes of aqueous and organic phases before extraction are 3.75 and 5.0 mL, respectively, or the ratio between aqueous and organic phases is kept to 0.75.
RESULTS AND DISCUSSION The determination of phosphate is achieved indirectly. First, phosphate is converted to 12-molybdophosphoric acid in acidic solution according to eq 4 and then extracted from
the excess molybdate by isobutyl acetate. The method of molybdophosphoric acid formation and extraction described in this paper is a modification of the procedure developed by Paul (23). The aqueous phase hydrogen ion concentration selected by Paul to eliminate silicate interference (0.66 N) is used here. The optimum aqueous phase molybdate concentration for molybdophosphoric acid is achieved by varying the concentration of molybdenum(V1) and hydrogen ion concentration. Quantitative formation of molybdophosphoric acid is obtained when the aqueous phase molybdate concentration is 0.11 M molybdenum. The efficiency of various organic solvents in extracting heteropoly acids was investigated by Wadelin and Mellon (16). Their work shows that oxygen-containing solvents are good extractants. Paul (23)has reported that isobutyl acetate not only selectively extrads molybdophosphoric acid away from molybdoarsenic acid but also, when the aqueous phase hydrogen ion concentration is carefully adjusted, selectively extracts molybdophosphoric acid away from both molybdoarsenic and silicomolybdic acids. Isobutyl acetate is therefore used in this study because of its ability to preferentially extract phosphate away from both silicate and arsenate interferences which may be present in natural waters. The extraction efficiency in this method is almost 100%. A small amount of molybdenum as molybdic acid is carried over; therefore, it is necessary to run a blank deter-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
863
Table I. Absorbance as a Function of Phosphorus Concentration" amt of P in final solution, ppb taken found
mmol of P taken X lo4 0.704 1.408 2.11 2.82 3.52
4.50 8.83 13.32 17.33 21.73
4.37 8.73 13.10 17.46 21.83
calcd molar absorptivity X lo5
absorbanceC 0.140 f 0.275 f 0.415 f 0.540 f 0.670 f
0.002 0.006 0.007 0.005 0.006
A,, a
9.84 9.69 9.74 9.50 9.53 = 9.66d
Initial concentration of phosphate is varied by taking varying aliquots of the standard solution and after dilution to 100
mL taking 1.00 mL for extraction. The following are kept constant: sample volume, V,, 1.00 mL; volume of isobutyl Amount of acetate, V,, 5.00 mL; volume of organic phase taken for analysis, V,,, 1.00 mL; final volume, Vf, 1.00 mL. phosphorus in ppb is calculated by using the average of three determinations by the equation: (3.1 X 10') A / (9.66 X 10');
Average absorbance value based on three determinations where 9.66 X l o 5 is the average apparent molar absorptivity. The average apparent molar absorptivity is calculated by using all data points and leastwith the standard deviation. squares method: A,, = 9.66 X lo', correlation coefficient, 0.999.
-the absence of oxygen is reoxidized by iron(II1) to molybdenum(V1).
Table 11. Absorbance as a Function of Hydrolyzed Polyphosphate in Terms of Phosphorus" mmol of amt cof P in triphosphate final solution, taken in ppb aqueous phase X l o 5 taken foundb
Mo(II1)
1.44 M HC1
+ 4H20 + 3Fe(III)
moderately fast
MoC?~~+ 3Fe(II)
+ 8H+ (6)
~
1.06 2.1 2 3.18 4.24 5.30
0.64 1.25 1.89 2.51 3.10
0.63 1.26 1.89 2.52 3.16
This step should be performed in a medium completely free of oxygen, in order to prevent oxidation of some of the molybdenum(II1) and iron(I1) which is produced in reaction 6. Finally, the liberated iron(I1) is chelated with ferrozine in the pH range of 3.0-3.5.
absorbanceC 0.060 f 0.117 f 0.177 f 0.235 f 0.290 f
0.005 0.004 0.007 0.005 0.006
Fe(I1)
Heteropoly acid formed from the hydrolyzed polyphosphate is extracted into 5.00 mL of isobutyl acetate, 1 mL of which is used for analysis, and final volume is 100 mL. Calculated from the average absorbance (3.1 X 107)A/(9.66X lo5). Average absorbance and standard deviation based on three determinations. a
mination along with the standard phosphate solutions and unknown samples. Reduction of Mo(V1) to Mo(II1) is carried out in acidic solutions by using Jones reductor as follows: (5) For quantitative reduction of Mo(V1) to Mo(III), not only must the Jones reductoir be completely amalgamated and activated with 1.44 M hydrochloric acid prior to use but the acidity of the aqueous solution must be adjusted to 1.4-1.5 M with hydrochloric acid. Furthermore, the solution must be passed through the Jones reductor a t a rate of 1 drop per 4 or 5 s, otherwise the redluction of molybdenum(V1) to -(III) will be incomplete. Molybdenum(II1) in acid media and in
pH 3.0-3.6 + 3Ferz2- fast Fe"(Fer~)~~-
(7)
The formation of iron(I1) ferrozine complex requires pHs higher than 3.0. However, in our case the pH is kept in the range of 3.0-3.5 in order to prevent the formation of an insoluble iron(II1) hydrous oxide, which can be a problem with high iron(II1) concentrations. The interference of iron(II1) can be overcome either by keeping the pH below 3.5 or by the addition of ammonium fluoride. The color of iron(I1) ferrozine is stable for over 1 h and requires no special timing before the measurement. Since the molar absorptivity for Fe"(Ferz)?- at 562 nm is 28000 and since each phosphate is associated with 12 molybdenum atoms in this procedure, 36 iron(I1) ions are produced. Thus, on the basis of these reactions, the expected apparent molar absorptivity for phosphate is 1080000. Results which have been obtained for the determination of phosphate are reported in Table I. The determined apparent molar absorptivity, as calculated in Table I, corresponds to 9.66 x lo5L mol-l cm-l indicating that this value is approximately 4.1% lower than the theoretical. This can be attributed to the overall efficiency of the
Table 111. Analysis of Synthetic and EPA Samples" amt of P in final solution, ppb taken found
amt of Na,P,O,, and H,PO, as P, pg/mL A
NaSP3010
H,PO, a s p
B NaSP30,0
H,PO, a s p C HJO, as P std
22.50 2.18 22.50 10.91 2.18
4.37
4.59
21.83 17.46
21.73 17.48
absorbance 0.143
0.677 0.545 (0.605) D EPA no. 2 .t H,PO, (std) 2.37 18.98 19.00 (0.653) EPAno. 2 0.19 1.52 1.54c 0.028 a Analysis of unhydrolyzed polyphosphate mixture: 1-mL sample; 5 mL of isobutyl acetate; final volume 100 mL for samples A and B. Sample C and D contain 2 mL of isobutyl acetate, and final volume 50 mL. Average absorbance for
duplicate samples. Absorbance values in parentheses are uncorrected for blank. method from the uncorrected absorbance values in samnles C and D.
Calculated value using standard addition
a64
Anal. Chem. 1981, 53, 864-867
operations. At the same time the measurement of absorbancy for the replicate samples can be reproduced with a relative standard deviation of 1-3%. Polyphosphate does not interfere in the determination of phosphate and can be determined in a similar manner as orthophosphate after the hydrolysis in acid solution (24). Results are summarized in Table 11. The values of phosphorus in polyphosphate are calculated by using the apparent molar absorptivity of 9.66 x lo6 as determined from pure orthophosphate. Results for determination of phosphate in synthetic and the EPA samples are summarized in Table 111. Synthetic samples containing polyphosphate as much as 10 times that of orthophosphate can be analyzed without difficulty; however, at higher concentrations of polyphosphate the results are higher than expected because of the slow hydrolysis of polyphosphate during the formation and extraction of the heteropoly acid. Common cations as well as iron(II1) in the amounts 1000 times that of phosphate do not interfere. Interferences by arsenic(V) and silica are prevented by using isobutyl acetate as the organic phase and careful control of the hydrogen ion of the aqueous solution.
LITERATURE CITED (1) Schaffer, Fredrick; Fong, Jean; Kirk, Paul Anal. Chem. 1953, 25,
343-346. (2) Vander, G. J. Analyst(London)1948, 73, 411.
Rockstein, Morris; Herron, Paul W. Anal. Chem. 1951, 2 3 , 1500-1 50 1. Chalmers, Robert A.; Thomson, Adele D. Anal. Chlm. Acta 1958, 78,
575-577. Chen, P.
S.;Torlbara, T.
Y.; Warner, Huber Anal. Chem. 1956, 28,
1756-1756. Fogg, D. N.; Wiikinson, N. T. Analyst (London) 1958, 83, 406-414. Ging, Nelson S. Anal. Chem. 1956, 28. 1330-1333. Teiep, George; Ehriich, Robert Anal. Chem. 1956, 30, 1146-1148. Djurkin, V.; Kirkbright, G. F.; West, T. S. Ana/yst (London) 1988, 97,
89-93. Bauminger, B. B.; Waiter, G. Analyst (London) 1966, 97, 205-206. Bottcher, C. J. F.; VanGent, C. M.; Pries, C. Anal. Chlm. Acta 1961, 24, 203-204. DeSesa, Michael; Rogers, L. B. Anal. Chem. 1954, 26, 1381-1383. Jintakanon, S.;Kerven, G. L.; Edwards, D. G.; Asher, C. J. Analyst (London) 1975, 700, 406-414. Paul, J. Anal. Chlm. Acta 1980, 23, 178-182. Kirkbright, G. F.; Smith, A. M.; West, T. S.Analyst(London)1987, 92,
411-416. Wadeiin, C.; Meiion, M. 0. Anal. Chem. 1953, 25, 1668-1673. Baghurst, H. C.; Norman, V. J. Anal. Chem. 1957, 29, 778-782. Simon, S.J.; B o k , D. F. Anal. Chem. 1975, 47, 1758-1783. Lueck, Charles H.; Boitz, D. F. Anal. Chem. 1958, 28, 1168-1171. Ingle, J. D.; Crouch, S.R. Anal. Chem. 1971, 43, 7-10. Boltz, D. F.; DeVries, T.; Meiion, M. G. Anal. Chem. 1949, 27,
563-565. Ramakrishna, T. V.; Robinson, J. W.; West, Philip W. Anal. Chlm. Acta 1969: 43: 43-48. Paul, J. Mikrochim. Acta 1965, 5-6, 630-835. Environmental Protection Agency, “Methods for Chemical Analysis of Water and Wastes”; U.S. Government Printing Office: Washington, DC, 1971;pp 235-268.
RECEIVED for review September 8, 1980. Accepted January 30, 1981.
Electroanalytical Chemistry of (Carbon monoxy)heme R. C. Gurira‘ and Joseph Jordan” Department of Chemistty, The Pennsylvania State University, 152 Da vey Laboratory, University Park, Pennsylvania 16802
Dlssolutlon of ferrlheme In water-ethanol mlxtures buffered at pH 9-13 yields a hydroxo(ethano1ato)hemlchrome. When the heme-Iron was electroreduced from the ferric to the ferrous state in the presence of carbon monoxlde, one axial ligand, vlz., hyydroxyl, was concomitantly replaced by CO while the trans axlal ligand (ethanol) remalned unaffected. The electrogeneration of the relevant (carbonyl monoxy)(ethano1ato)hemochrome has been Investigated by potentlometry, chronoamperometry, polarography, and cycllc voltammetry. The electrochemical rate constant (prevailing at the formal potentlal where k,, = kred)was 0.001 cm s-’ whlch Is remarkably slow compared to other hemlchromehemochrome redox couples. The slugglsh electron transfer reactivity is accounted for by a change In spin rnultipliclty from the high-spin state of the hydroxy(ethano1ato)hemlchrome to be low-spin state of the (carbon monoxy)(ethanoiato)hemochrome.
Electrode kinetics involving numerous hemochromes have been reported in the literature ( I , 2). Surprisingly, however, any such investigation of the (carbon monoxy)heme complex (which-incidentally-accounts for the toxicity of CO) is ‘Present address: Department of Chemistry, Grinnell College, Grinnell, IA.
conspicuous by its absence. The work described below is devoted to fiiing this void. The electroreduction of an ethanol ferriheme moiety to the corresponding (carbon monoxy)ferroheme has been investigated by potentiometry, chronoamperometry, polarography, and cyclic voltammetry. Results are reported which revealed that remarkably large activation energies were involved. This is accounted for by the fact that the relevant ferrous-ferric electron transfer required a concomitant change from a low-spin to a high-spin state.
EXPERIMENTAL SECTION Current-voltage and current-time curves were obtained with the aid of the “Model 170 Electrochemical System” supplied by Princeton Applied Research Corp., princeton, NJ. Special precautions were taken to compensate for iR drops. Uncompensated iR drops never exceeded 1 mV. For cyclic voltammetry and chronoamperometry,a three-electrode system was used consisting of a hanging mercury drop electrode (HMDE) of Kemula’s design (Model E-140, Metrohm Limited, Herisau, Switzerland) as the indicator electrode, a platinum wire counterelectrode, and a commercial (Beckman)saturated calomel reference electrode. For classical steady-state polarographic measurements the HMDE was replaced by a conventional dropping mercury indicator electrode. Two types of readout devices were used, depending on the applied potential sweep rates. An X-Y recorder supplied by Houston Instruments, Bellaire, TX) was employed for sweep rates up to 200 mV/s and an appropriate oscilloscope (Tektronix Model 503) at faster sweep rates. A 100-mL electrolysis cell, jacketed for the purpose of circulating water at a temperature of 25 f 0.1 “C, was used throughout.
0003-2700/81/0353-0864$01.25/00 I981 American Chemical Society
