706
Anal. Chem. 1981,
(normalized Ai, is 75% maximum) and minimum peak width (W,p is 124/n mV compared to its minimum value of 90/n mV). A value of IAEl = 50/n mV (at T = 25 "C) is thus recommended for analytical purposes. Of course in the solution of analytical problems where n may not be known, or where a multicomponent mixture of species with different n values or similar values of Ellz is present, the pulse height should be optimized experimentally. Analytical figures of merit such as detection limits have not been determined in this study. However, the theoretical analysis of both faradaic and capacitative currents and the excellent agreement of experiment with theory at parts-permillion concentration levels without background subtraction suggest that this technique will be competitive with DP voltammetry for analytical purposes. It should be especially useful in nonideal cases where interferences due to electrode passivation, adsorption phenomena, or electrode "conditioning" may be reduced or eliminated by judicious choice of the time and potential parameters of the experiment. At the same time, the technique should be useful for quantitative mechanistic studies of these same phenomena. With the experimental verification of the theory for the reversible case above, the foundation is laid for application of the technique to such cases.
LITERATURE CITED Parry, E. P.; Osteryoung, R. A. Anal. Chem. 1965, 37, 1634-1637, Christie, J. H.; Osteryoung, R. A. J. Electroanal. Chem. 1974, 49,
301-311. Dillard, J. W.;Hanck, K. W. Anal. Chem. 1976, 48, 218-222. Dillard, J. W.;Turner, J. A.; Osteryoung, R. A. Anal. Chem. 1977, 49,
1246-1255. RuziE, J. J. Elecfroanal. Chem. 1977, 75, 25-44. Birke, R. L. Anal. Chem. 1976, 50, 1489-1496. Oldham, K. B. Anal. Chem. 1966, 40, 1024-1031
53,706-709 (8) Keller, H. E.; Osteryoung, R. A. Anal. Chem. 1971, 43, 342-348. (9) Aoki, K.; Osteryoung, Janet J. Elsctroanal. Chem. 1980, 170, 18-36. (IO) Oidham, K. B.; Parry, E. P. Anal. Chem. 1966, 40, 65-69. (11) Christle. J. H.; Parry, E. P. Electrochim. Acta 1966, 1 1 , 1525-1529. (12) Brinkman, A. A. A. M.; Los, J. M. J. Electroanal. Chem. 1964, 7, 171-163. (13) Fonds, A. W.; Brlnkman, A. A. A. M.; Los, J. M. J . Electroanal. Chem. 1967, 14,43-56. (14)Brlnkman, A. A. A. M.; Los, J. M. J. Electroanal. Chem. 1967, 14, 269-284,285-296. (15) O'Deen, W.; Osteryoung, R. A. Anal. Chem. 1971, 43, 1879-1882. (16) Lane, R. F.; Hubbard, A. T. Anal. Chem. 1970, 48, 1287-1293. (17) Aoki, K.; Osteryoung, Janet; Osteryoung, R. A. J. Electroanal. Chem. 1960, 110, 1-18. (18) Barker, G. C. Congress on Analytical Chemistry in Industry, St. Andrews, June 1957. (19) Ramaley, L.; KrauSe, M. S.,Jr. Anal. Chem. 1969, 41, 1362-1365. (20) Krause, M. S.. Jr.; Ramaley, L. Anal. Chem. 1969, 41, 1365-1369. (21) Christie, J. H.; Turner, J. A., Osteryoung, R. A. Anal. Chem. 1977, 49, 1899-1903. (22)Turner, J. A.; Christie, J. H.; Vukovlc, M.; Osteryoung, R. A. Anal. Chem. 1977, 49, 1904-1908. (23) Osteryoung, R. A.; Osteryoung, Janet; O'Dea, J. J. Anal. Chem., in
press.
(24) O'Dea, J. J. Ph.D. Dissettatlon, Colorado State University, Ft. Collins, CO, 1979. (25) Christie, J. H.; Jackson, L. L., Osteryoung, R. A. Anal. Chem. 1976, 48,242-247. (26) Savltzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627-1639. (27) Christie, Joseph H.;Jackson, Lany L.; Osteryoung, R. A. Anal. Chem. 1976, 48, 242-252, (28) Galus, Zbigniew "Fundamentals of Electrochemlcal Analysis"; Wiley: New York, 1976 p 422. (29)Jackson, L. L.; Yarnitzky, Ch., Osteryoung, R. A., Osteryoung, Janet. Chem. Biom6d. Envlron. Instrum. 1960, 10, 175-188. (30) "Instruction Manual, Polarographic Analyzer, Model 174";Princeton Applied Research Corp.: Prlnceton, NJ, 1971. (31) Smith, D. E. I n "Eiectronanalytlcal Chemistry", Bard, A. J., Ed.: Marcel Dekker: New York, 1966;Voi. 1, pp 1-155.
RECEIVED for review August 25, 1980. Accepted January 6, 1981. This work was supported by the National Science Foundation under Grant No. CHE 7917543.
Simultaneous Determination of Iron(I1) and Iron(II1) at Micromolar Concentrations by Differential Pulse Polarography LUIS E. Leon and Donald T. Sawyer* Depattment of Chemistry, University of California, Riverside, California 92521
Trace levels of iron(I1) and Iron(II1) can be assayed by slmulataneous extractlon into chloroform of trls( 8-quinol1nato)iron(II1) and trls(4,7-diphenyl-l,lO-phenanthroline)lron(II) prior to thelr electrochemlcal reductlon in propylene carbonate by differential pulse polarography at -0.55 and -1.25 V vs. SCE, respectlvely. Llnear calibratlon plots are obtalned for both complexes for the concentratlon range from 2 to 200 pM. Because the presence of excess 4,7-dlphenyl-l,lOphenanthroline affects the slope of the callbratlon curve for tris(8-qulnollnato)lron( III),the standard additlon method should be used to obtaln atlalylical accuracy. Copper(I1) and molybdenum(V1) coextract with the Iron specks; when present In large excess they obscure the Iron reductlon peaks.
Many methods for the determination of either Fe(I1) or Fe(II1) (or their ratio) and total iron have been proposed (1). Procedures for the simultaneous determination of the two species have been reported for a number of materials (2-7)
but have not been developed for trace concentrations. The difficulties with electrochemical techniques for the simultaneous determination of iron(I1) and iron(III), in particular the application of normal pulse voltammetry, have been outlined recently (8). Normal pulse polarography also has been employed to study iron speciation in process streams (9). Other workers have used direct current polarographic methods for the simultaneous determination of iron(I1) and iron(II1) in various media (10) and voltammetry at a rotating platinum electrode to determine the two iron species simultaneously in standard rock samples at relatively high concentratiohs (>0.1mM) (3). Although extraction procedures for iron determinations and separations have been utilized for many years (11-13), there are few reliable methods for the eimultaneous extraction of both valence states. Iron(I1) and iron(II1) have been extracted directly into nitrobenzene and tributyl phosphate from aqueous solutions of HBr, HzS04, and HC1 (14). Another approach is the use of dithizone to complex Fe(I1) (15);the complex is extracted together with iron(II1) from an aqueous
0003-2700/61/0353-0706$01.25/00 198I American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
HC1 medium by means of a water-immiscible acetone system a t pH 6-7. Benzyl xanthate complexes Fe(I1) and Fe(II1) in aqueous media a t pH 5-7 and pH 3-4, respectively; both oxidation states can be extracted into chloroform (16). Hydroxamic acid derivatives complex both oxidation states of iron at pH 2-4, which can be extracted into chloroform ( 1 7 ) . Diethyl phosphodithioate (18) and mixtures of hexafluoroacetylacetone and trioctylphosphine oxide (19) also have been proposed for the simultaneous extraction of ferrous and ferric ions. The use of mixed-ligand complexes with triphenylmethane dyes and quaternary ammonium salts for the simultaneous extraction of the two valence states of iron recently has been reviewed (20). 8-Quinolinol (oxine) is a versatile organic ligand (21) and is known to complex the ions of at least 49 metals (13). The partition of several metal-oxinate complexes between water and chloroform, and the effects of such variables as pH and masking agents have been investigated (23). The polarography of metal oxinates in organic solvents has been studied (24,W). Bathophenanthroline (bath; 4,7-diphenyl-l,10phenanthroline) is a specific and highly sensitive reagent for iron(I1) (26). There are few interferences when iron(I1) is complexed with bath a t pH 4 and extracted into chloroform. Copper(1) apparently forms a 1:l complex with bath at or below this pH (27). In the presence of tartrate and perchlorate, the Fe(I1)-bath complex is extracted into chloroform in the range from pH 2 to 9 (28). The polarography of the tris(bath)iron(II) complex has been studied in acetonitrile (29) and dimethylformamide (30). The present paper summarizes the results of an investigation to develop a rapid analytical procedure for the simultaneous determination of iron(I1) and iron(II1) at trace levels in biological samples by differential pulse polarography (DPP). The method consists of a simultaneous extraction and D P P determination of the two iron species in a 0.10 M tetraethylammonium perchlorate-propylene carbonate solvent system. Iron(I1) is extracted as the Fe(I1)-bathophenanthroline complex [Ferr(bath)3](C104)2 and iron(II1) as the Fe(II1)-oxine complex (Ferr1Q3)into chloroform from a sample solution at pH 2.5. Addition of tartrate will eliminate potential interferences from molybdenum, copper, and manganese ions (31). EXPERIMENTAL SECTION Equipment. Differential pulse polarography (DPP) measurements were made with a three-electrode system and a Princeton Applied Research Model 174A polarographic analyzer with a Hewlett-Packard Model 7040A X-Y recorder. The electrochemical cells consisted of Brinkman Model EA 875-20 and PAR Model G57 cell bottoms with corresponding plastic cell tops. The cell tops had provision for inserting a dropping mercury electrode, a reference electrode, and an auxiliary electrode. The top also had access holes for a bubbler to deaerate the solution with argon and to flow argon over the solution while the polarograms were recorded. A Sargent copper-heater kit with a Vycor tube filled with sulfur-free copper wire (Model S-36517) was used to remove traces of oxygen from the argon gas. A Princeton Applied Research Model 170170 drop knocker was used to obtain reproducible drop times. The reference electrode consisted of a silver wire coated with silver chloride in a Pyrex tube with a small soft-glass cracked bead tip. The electrode was filled with a solution of aqueous tetramethylammonium chloride (Matheson Coleman and Bell) with the concentration adjusted such that the electrode potential was 0.000 V vs. SCE. The reference electrode and the auxiliary electrode, a platinum flag sealed in soft glass, were placed directly into the solution. A Leeds and Northrup pH meter equipped with a Broadly James Corp. combination pH electrode was used for all pH measurements. Infrared spectra of the complexes (as KBr disks) were recorded with a Perkin-Elmer Model 283 infrared spectrophotometer. The W-visible spectra for solutions of the complexes were recorded with a Cary Model 219 spectrophotometer.
707
Preparation of Complexes. Tris(8-quinolinato)iron(III) (Fe"'Q3) was prepared by standard methods (32). Tris(4,7-diphenyl-1,lO-phenantholine)iron(II)perchlorate (Fen-bath) was prepared by the method of Sutin and Gordon (33). Both complexes were assayed by their respective absorption spectra (34, 35). Reagents. Dimethyl sulfoxide (Me2SO),Nfl-dimethylformamide (DMF), propylene carbonate (PC), pyridine (pyr), and chloroform (without preservative)were purchased from Burdick and Jackson ("distilled in glass" quality). Hexane (Mallinkrodt, AR) and benzene (Mallinkrodt, AR) were used as purchased. Hexane was added as a preservative to the chloroform (1% v/v) (36). The resulting solution was kept in small, amber bottles (under argon) in a refrigerator. Tetraethylammonium perchlorate (TEAP) (G. Frederick Smith Chemical Co.) was used as the supporting electrolyte. (Other tetraalkylammonium salts such as Et4NBF4would be satisfactory substitutes for TEAP.) Stock solutions of 6.96 X lo-' M FemQ, and 5.21 X M FeII-bath were prepared in 0.10 M TEAP in PC. Standard aqueous solutions were prepared in high-purity water obtained by passing distilled water through a Milli-Q system (Millipore). Fresh stock solutions of Fe(I1) were prepared by dissolving Feu(NH4)2(S04)2 (Mallinkrcdt,AR) in 1L of high-purity water that contained 1 mL of concentrated sulfuric acid (37). Standardized solutions of Fe(II1) were prepared from a small, accurately weighed piece of pure iron wire (99.9%) that was dissolved in concentrated hydrochloric acid (38). The resulting solution was diluted with 3% hydrogen peroxide and boiled for 5-10 min. Next, the solution was transferred to a volumetric flask and made up to volume to yield a 1.805 mM solution of Fe(II1) (100.8 ppm). 8-Quinolinol (Mallinkrodt) was recrystallized twice from aqueous ethanol solution. A 1% solution of 8-HQ in chloroform was freshly prepared immediately before every experiment. 4,7-Diphenyl-l,lO-phenanthroline (bath) (G. Frederick Smith Chemical Co.) was recrystallized from benzene (39). The bathpyridine solution was prepared by dissolving 20 mg of bath in 5 mL of pyridine. Stock solutions of 1 M sodium perchlorate (G. Frederick Smith Chemical Co.) and 0.034 M potassium hydrogen tartrate (Mallinkrcdt, AR) were treated with bath and 8-HQ and extracted several times with chloroform (40).Dissolved chloroform was removed by bubbling argon through the solutions. All glassware was soaked in 7 M hydrochloric acid solution for at least 24 h and rinsed thoroughly with high-purity water. Solutions were stored in polyethylene bottles. The electrochemical cells were soaked overnight in 7 M hydrochloric acid, rinsed, and oven-dried (40). Procedure. The differentialpulse polarograms were recorded between -0.30 and -1.50 V vs. SCE at a scan rate of 2 mV/s with a modulation amplitude of 25 mV. A drop time of 1s was obtained by use of the mechanical drop knocker. The low-pass filter and the noise filter on the mechanical drop knocker were not used. Calibration curves were prepared by the addition of the respective volume of standard solutions of Fe11'Q3 and/or Feu-bath to a 10.0-mL volumetric flask and made up to the volume by addition of 0.10 M TEAP in PC. The solution was transferred to the electrochemical cell (PAR G57 cell bottom) and its DPP recorded. Prior to each polarographic analysis, the cell solution was deaerated for 15 min with argon. RESULTS AND DISCUSSION Iron(III)-8-quinolinato (Fe111Q3) and iron(I1)-bathophenanthroline (Fe'I-bath) complexes can be determined effectively by differential pulse polarography in a propylene carbonate (0.1M tetraethylammonium perchlorate) solution. The reduction of Fen'Q3 occurs at -0).55V vs. SCE and that of Fen-bath at -1.25 V (Figure 1). The reduction of FeLbath is not affected by the reduction of excess 8-quinolinol at -1.9 V. Calibration curves for the two iron complexes are linear for the concentration range from 2 to 200 pM and confirm that they can be accurately determined within this range, either separately or simultaneously. Extraction Procedure. Several modifications had to be made to the original procedure of direct extraction into chloroform in order to obtain quantitative, reproducible re-
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
Table I. Analytical and Extraction Efficiency for Iron(II1) and Iron(I1) as FeInQ3 and Fe"-bath Complexes pmol added pmol found % recovery sample no. Fe(II1) Fe(11) Fe(II1) Fe(1I) Fe(II1) Fe(I1) 1 2.0 x lo-' 2.0 x 2.5 X lo-' 1.8 X 125 90 2 8.0 x 10-2 8.0 x 9.2 X lo-' 7.8 x 115 97 3 3.0 X lo-' 3.0 X lo-' 2.7 X l o - ' 2.6 X lo-' 90 87 4 5.0 X lo-' 5.0 X 10" 4.7 x 10-1 4.1 X 10-1 94 82 1' 2.0 x 5.0 X 10" 2.4 X 4.6 X lo-' 120 92 2' 8.0 X lo-' 3.0 X lo-' 9.8 X lo-' 2.8 X l o - ' 123 93 3.0 X 10" 3' 8.0 x 3.5 x l o - ' 7.0 X 117 87 4' 5.0 X 10" 2.0 x 10-2 4.9 x 10" 1.7 X l o - * 98 85
I
Fen-bolh
)I
I
14
12
O6 4
I
t
h
o
50
I00
150
200
250
[bath], p!!
-0.3
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
E ( V ) v s SCE
Differential pulse polarogram for propylene carbonate solutions (0.10 M TEAP) of 10 p,M Fe'llO, and 10 pM FeII-bath. Flgure 1.
sults. The addition of bath in a 50% ethanol solution (30) causes high results for Fe(II1) and low results for Fe(I1). Apparently, a redox process occurs during the extraction procedure. The substitution of acetone or acetonitrile as the solvent for bath does not improve the results and makes it difficult to measure the corresponding peak current for FeII-bath. The polarographic reduction of 8-quinolinol (8-HQ) in dimethylformamide has been studied previously (41). In acidic medium, the 8-quinolinolinium cation, HzQ+,is formed and is much more easily reduced than the neutral species. On the other hand, the extraction of H2Q+as an ion-pair species in the presence of perchlorate salts has been reported (26,42). Because the extraction of ionic forms of oxine is not expected to occur in oxygen-free solvents (421, the presence of ethanol from the bath reagent solution appears to be the cause for the enhanced extraction of HZQ'. This interferes by enhancing the current for the polarographic measurement of Fe"-bath. The introduction of both reagents, 8-quinolinol and bath, into pure chloroform results in a long and incomplete extraction process. Although pyridine is an effective solvent for bath (43),the combination of the pyridine-bath solution and the sample solution causes the pH of the latter to shift to higher values. Therefore, the reagent solution is added with adjustment of the pH so that it never exceeds pH 2.5. The chloroform for the extraction must not contain ethanol as a stabilizer; instead 1% v/v n-hexane in chloroform is used (36). The presence of sodium perchlorate in the sample solution improves the extraction of both iron species; a concentration of 0.05 M NaC104 appears to be optimal. Selection of Solvent for DPP Determination. Both iron complexes are soluble in MezSO and DMF and yield DPP peaks that are proportional to their concentrations, but the presence of excess oxine in these solvents causes the results to be high for Fe(II1) and low for Fe(I1). Although the solubility of FeQ3in propylene carbonate (PC) is lower than in MezSO or DMF, the analytical accuracy for both iron species is much better in this solvent. The major
Flgure 2. The DPP peak current for 10 pM FeII'Q, in 0.1 M TEAP-PC as a function of bathophenanthroline (bath) concentration.
problem with the use of PC is the presence of a contaminant that is reduced between -1.00 V and -1.15 V (44);vacuum distillation does not improve the problem. The presence of excess bath in the analytical solution affects the determination of FeQ3. Figure 2 illustrates the change in peak current as a function of excess bath for a 10.0 pM solution of Fe"'Q3 in 0.10 M TEAP-PC. The peak current for Fe"-bath is not affected by excess bath. However, for Fe(I1) and Fe(II1) concentrations in excess of 200 pM the base line for the DPP peak of FeII-bath is diffucult to define. Because of the effect of excess bath on the peak current for Fe'nQ,, the calibration curves for this species change with the ratio of Fe(I1)-to-bath. The standard addition method provides a superior means to analytical accuracy and is recommended for the analysis of Fe(I1)-Fe(II1) mixtures. The presence of excess bath also limits the concentration of Fe(I1) that can be determined. Although calibration curves are linear from 2 to 200 hM, the maximum amount that can be determined in the analytical solution by the standard addition method is about 50 pM. Quality of the Extraction. The extraction procedure has been tested by the use of synthetic sample mixtures of iron(I1) and iron(1II). Known amounts of standardized stock solutions have been added to 50 mL of a 0.5 M NaC104 and 4% KH(tartrate) solution in a 250-mL separatory funnel. The acidity of the solution is adjusted to pH 1.8 by the addition of 3 M HC104 or 3 M NaOH. The pyridine solution of bath (0.133 mL) is added dropwise while the pH is kept below pH 2.5 by use of the HC104 solution. After the addition of bath, the solution is adjusted to pH 1.98 and set aside for a t least 20 min with gentle shaking before the solution is extracted once with a 10-mL portion of 1% 8-quinolinol in chloroform. After the mixture is shaken for 3 min, the chloroform layer is transferred to the electrochemical cell. The remaining solution is shaken again with a 5-mL portion of chloroform for 2 min more and the latter layer is collected in the same electrochemical cell. The cell is placed in a water bath a t room temperature in the hood, and the chloroform is removed by evaporation. Then, 10 mL of a 0.10 M TEAP-PC solution is added, the solution is deareated with argon, and the DPP is recorded. The standard addition method is used to calculate
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
709
(10) Beyer, M. E.; Bond, A. M.; McLaughlin, R. Anal. Chem. 1075, 47, 479. (11) Meinick, L. M. “Treatise on Analytical Chemistry”; Kolthoff, I. M., Eivlng, P. J., Eds.; Interscience: New York, 1962;Vol. 2,Part 11, p. 247. (12) Morrison, G. H.; Freiser, H. “Solvent Extraction In Analytical Chemistry”; Wliey: New York, 1957. (13) De, A. K.; Khopkar, S. M.; Chalmers, R. A. “Solvent Extraction of Metals”; Van Nostrand-Reinold: London, 1970. (14) Haggag, A.; Sanad, W.; Alian, A,; Tadroes, N. J. Radioanal. Chem. 1077, 35, 253; Anal. Abstr. 1077, 33, 28140. (15) Matkovich, C. E.; Christian, G. D. Anal. Chem. 1074, 46, 102. (16) Hayashi, K.; Sasakl, Y.; Tagashira, S.;Taneka, T.; Imada, K. Bunsekl Kagaku, 1979, 28, 106;Anal. Abstr. 1979, 37, 58145. (17) Pawde, R.; Tandon, S. 0.Indien Chem. SOC.1977, 54, 990. (18) Budnikov, G. K.; Shakurova, N. K.; Ulakhoulch, N. A,; Cherkasov, R.
-0.3
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
E N ) v s SCE
Flgure 3. Differenthi pulse polarogram for the 0.1 M TEAP-PC Soiution that results from extraction by CHCI, (with 8-HQ and bath) of a n aqueous solution that contains 10 pM each of Fe(III),Fe(II), Mn(II), Cu(II), Zn(II), and Mo(V1).
the concentration of the iron species. The results are summarized in Table I and indicate that there is good reproducibility in the measurements. However, there is a tendency for the assays to be high for iron(II1) and low for iron(I1). This probably is due to some autooxidation of Fe(I1) by residual air in the solvent systems. The use of an initial CHC13 extraction eliminates most ionic interferences. For the extraction conditions copper(I1) and molybdenum(V1) appear to be the only metal ions that are extracted with the iron species (13,23,26-28). However, the concentration of these metals in biological samples is low relative to iron (45). Figure 3 illustrates the DPP that is obtained for the extract from a sample solution which contains approximately 10 pM each of Cu(II), Mn(II), Zn(II), Mo(VI), Fe(II), and Fe(II1). The standard addition method confirms that the respective reduction peaks are due solely to Fe(II1) and Fe(I1).
LITERATURE CITED Kolthoff, I. M., Eiving, P. J., Eds. “Treatise on Analytical Chemistry”; Interscience: New York, 1962;Part 11. Vol. 11. Kawashina, T.; Kozuna, Y.; Nakano, S. Anal. Chlm. Acta 1979, 106,
355.
Moore, W. M. Anal. Chim. Acta 1079, 105, 99. Blen, G. S.; Goldberg, E. D. Anal. Chem. 1056, 28, 97. Ciemencv. C. V.: Haaner. A. F. Anal. Chem. 1061.. 33.. 888. Aked, U..Anal. Chlm: Acta 1960, 47, 495. Schafer, H. N. S. Analyst (London) 1966, 91, 763. Morris, J. L.; Faulkner, L. R. Anal. Chem. 1077 49, 489. Parry, E. P.;Anderson, D. P. Anal. Chem. 1065, 37, 1634.
A.; Ovchinnikov, N. V.; Kutyrev, G. A.; Toropova, V. F. Zh. Anal. Khim. 1977, 32, 1326. (19) Mltchell, J. W.; Ganges, R. Anal. Chem. 1074, 46, 503. (20) Tlkhonov, V. N. Zh. Anal. Khlm. 1977, 32, 1435. (21) Perrln, D. D. “Organic Compiexing Reagents”; Interscience: New York, 1964;p 184. (22) Moeiler. T. Ind. Eng. Chem., Anal. Ed. 1043, 15, 346. (23) Stark, J. Anal. Chlm. Acta 1963, 28, 132, (24) Dagnall, R. M.; Hasamiddin, S. K. Talenfa 1966, 15, 1025. (25) Fyatnltskll, 1. V.; Ruzhanskaya, R. P. Zh. Anal. Khim. 1070, 25, 1083. (26) Dlehl, H.; Smith, 0. F. “The Iron Reagents: Bathophenanthrollne, 2,4,6-Trlpydyt-s-Triazine, and Phenyl-2-Fyridyl Ketoxime”; G. F. Smlth Chemical Co.: Columbus, OH, 1960. (27) Schllt, A. A. “Analytical Applications of 1,lO-Phenanthrollne and Related Compounds”; Pergamon Press: Oxford, 1969. (28) Hill, A. G. Analyst(London) 1976, 103, 521. Sammartano, S.;Bonomo. R. (29) Musumecl, S.;Rizzarelll, E.; Fragaia, I.; P. Inorg. Chim. Acta 1073. 7, 660. (30) Saji, T.; Fukal, T.; Aoyagui, S. J. Electroanal. Chem. 1075, 66, 81. (31) Gentry, C. H. R.; Shenington, L. G. J. Chem. SOC. 1950, 75, 17. (32) Ohkaku. N.; Nakamoto, K. Inorg. Chem. 1971, 10, 798. (33) Sutin, N.; Gordon, B. M. J. Am. Chem. SOC. 1061, 83, 70. (34) thomklnson, J. C.; Williams, R. J. P. J. Chem. SOC. 1076, 1153. (35) COlllnS, P.; Diehl, H. Anal. Chem. 1950, 31, 1693. (36) Kawai, S. Yakugaku Zasshi 1066, 86, 1125. (37) Skoog, D. A.; West, D. M. ”Fundamentals of Analyticai Chemistry”; Holt, Rinehart, and Winston: New York, 1969;p 690. (38) Moore, W. M. Anal. Chim. Acta 1079, 105, 99. (39) Levillaln, P.; Bourdon, R. Bull. Soc. Chim. Fr. 1072, 371. (40) Zlef, M.; MRchell, J. W. I n “Chemical Analysis”; Elving, P. J., Kokhoff, I. M., Eds.; Wlley: New York, 1976;pp 98,69. (41) Fujlnaga, T.; Izutsu, K.; Takaoka, K. J. Electroanal. Chem. 1066, 76,
69. (42) Kuz’mln, N. M.; Khorklna, L. S.;Lebedev, A. I.; Zoiotov, Yu. A. Zh. Anal. Khim. 1070, 25, 1257. (43) Zharovskii, F. G.;Ryzhenko, V. L. Zh. Anal. Khim. 1967, 22, 1142. (44) B?hserman, P.; Sawyer, D. T.; Page, A. L. Anal. Chem. 1976, 50, 1JUU.
(45) Wllshire, J. P.; Leon, L.; Bosserman, P.; Sawyer, D. T.; Buchanan, R. M.; Pierpont, C. G. Chem. Uses Molybdenum, Proc. Int. Conf. 3rd 1970, 264-269.
RECEVIED for review November 17,1980. Accepted January 13,1981. This work was supported by the National Science Foundation under Grant No. CHE-7922040. We thank CONICIT, Caracas, Venezuela, and Simon Bolivar University, Miranda, Venezuela, for Fellowship support to Luis E. Leon during the courese of this investigation.
