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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduckion, negatives! niay be obtained from Business Operations, Books and Journals Division, American Chemical Society, 1155 16th St., N.W., Washington. D.C. 20036. Full bibliographic citation (journal, title of article, author) and prepayment, check or money order for $5.50 for photocopy ($7.00 foreign) or $3.00 for microfiche ($4.00 foreign) are required.
LITERATURE CITED (1) "Manual of Symbols and Terminology for Physicochemical Quantities and Units", IUPAC, Butterworths, London, 1970, p 33.
(2) R. A. Durst and B. R. Staples. Clin. Chem. ( Winston-Salem, N . C . ) .18, 206 (1972). (3) R. G. Bates and R. A. Robinson, Anal. Chem., 45, 420 (1973). (4) R. G. Bates, R. N. Roy, and R. A. Robinson, Anal. Cbem., 45. 1663 (1973). (5) S. J. G. Semple. G. Mattock. and R. LJncles, J . Biol. Chem., 237, 963 (1962). (6) J. H. Ladenson, C. H. Smith, 0. N. Dietzler, and J. E. Davis, Ciin. Chem (Winston-Salem, N . C . ) , 20, 1337 (1974).
(7) R. A. Durst and R. G. Bates, in "Blood pH, Gases, and Electrolytes", R. A . Durst, Ed., NBS Spec. Pub/., 450, Washington, D.C., 1977, p 247. (8) C. C. Westcott and T. Johns, Appl. Res. Tech Rep , 542, Beckman Instruments. Inc., Fullerton, Calif. (9) R G. Bates, "Determination of pH, Theory and Practice". 2nd ed..Wiley, New York. N.Y.. 1973. (10) C. A. Vega and R. G. Rates, Anal. Chem, 48, 1293 (1976). (1 1) R G. Rates. E. A. Guggenheim. H. S. Harned. D. J. G. Jves. G. J. Janz, C B. Monk. J, E. Prue, R. A . Robinson, R . H. Stokes, and W. F K. Wynne-Jones, J . Chem. Phys.. 25, 361 (1956): 26, 222 (1957). (12) R. G. Bates and E. A. Guggenheim, Pure Appl. Chem., 1, 163 (1960). (13) A. H. J. Maas. ref. 7, p 195. (14) A H . J. Maas. private communication (15) R. G. Bates, 8 . R. Staples, and R. A. Robiwon. Anal Chem., 42, 867 (1970). (16) H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions", 3rd ed., Reinhold, New York. N.Y., 1958, Chap. 14.
RECEIVED for review March 16, 1978. Accepted April 12, 1978. This work wa5 supported in part by the National Science Foundation under Grant OC"E76 24384.
Differential Pulse Polarographic Determination of Molybdenum at Parts-per-Billion Levels Paula Bosserman and Donald T. Sawyer" Department of Chemistry, University of California, Riverside. California 9252 1
Albert L. Page Department of Soil and Environmental Sciences, University of Cdifornia. Riverside, California 9252 1
A new method for the determination of molybdenum at trace levels has been developed, which is based on the electrochemical reduction of dioxobis(8-quinolinolato)molybdenum(V1). The complex is formed and extracted from the sample matrix into chloroform prior to its reduction at -1.08 V vs. SCE in dimethyl formamide. Only tungsten co-extracts and it does not interfere with the analysis. A linear calibration curve is obtained for the concentration range from 0.1 pM (9.6 ppb) to 100 pM (9.6 ppm). The method has been applied to plant samples that have been grown on fly-ash amended soils.
Numerous analytical methods have been developed for the determination of molybdenum ( 1 ) . Among the most common are t h e spectrophotometric analysis of M O ( S C N )(~2 ) and flameless atomic absorption spectrometrk (3). Althoiigh these methods are reasonably sensitive, they are subject to numerous interferences. Other sensitive methods for the determination of molybdenum include neutron activation analysip and x-ray fluorescence spectrometry. The detection limit for neutron activation analysis is several nanograms for a 10-h irradiation with a flux of l0l3 neutrons cm-2 s ( 4 ) . A detection limit of 7 2 ng of molybdenum has been reported for x-ray fluorescence spectrometry ( 4 ) . Both of these methods require the use of elahorate and often unavailable instrumentation Furthermore, sensitive neutron activation methods for molybdenum require long irradiation times. A method based on the EPR spectrum of Mo(W (.5) is a variant of the Mo(SCN); spectrophotometric procedure. Although it is sensitive, the response is nonlinear and provides poor precision. Inductively 0003-2700/7810350-1300$01.00/0
coiipled plasma emission spectroscopy also offers a sensitive method of molybdenum analysis with a detection limit of 5 ng/mI, ( 4 ) . This method also is subject to matrix interferences. Several polarographic methods for the determination of niolybdenim have been developed. These include Mo(V1) by anodic stripping voltammetry at mercury (6) and graphit,e 17) electrodes. In both cases, the limit of detection is 5 pM. T h e sensitivity of molybdenum determinations can be enhanced by the use of solvent extraction to both isolate and concentrate molybdenum complexes. Molybdennni(V1) forms an especially useful complex wit,h 8-quinolinol (8-hydroxyquinoline. oxine, HQ) in acidic solution which can he quantitatively extracted into chloroform 18). This complex also can be extracted into isobutyl methyl ketone and ethyl acetate 19). Polarographic studies have established that the bis(8-quincilinolato)molybdenum(VI)complex has a reduction half-wave potential at 4 . 3 0 V vs. a Hg pool in isobutyl methyl ketone and ethyl acetate, and a t -0.46 V vs. a Hg pool in chloroform. The voltammetric hehavior of the Mo(V1)-8quinolinol complex in dimethyl sulfoxide a t a platinum working electrode has been described ( 1 0 ) . A quasi-reversible peak at 1.15V vs. SCE is observed for the complex, arid free ligand is not reduced in this system at potentials less negative than -1.95 V vs. SCE. Differential pulse polarography has heen demonstrated to be a sensitive voltammetric method for the analysis of many trace metals. Although most differential pulse polarographic methods have been developed for aqueous media, the selectivity and precnncentration of solvent extraction has led to the present stitdy. This has resulted in a method for the ~
(c 1978 American Chemical Socrety
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9. AUGUST 1978 determination of the Mo(VI) oxine complex in aprotic solvents t h a t is rapid, selective, and sensitive to trace levels of molybdenum in biological matrices.
EXPERIMENTAL Apparatus. Differential pulse polarograms were ohtained by use of a Princeton Applied Research Model 174A Polarographic Analyzer using a three-electrode system, and a Hewlett-Packard Model S040A X-Y recorder. The electrochemical cell consisted of a 100-mL electrolytic beaker and a Leeds and Northrup polyethylene electrochemical cell top. The cell top had provision for inserting a dropping mercury electrode, which was used as the working electrode, a reference electrode, an auxiliary electrode, a bubbler used to deaerate the solutions with argon, and a short piece of glass tubing used to flow argon over the cell while the polarograms were recorded. A tapered dropping mercury electrode capillary tube was connected to a mercury reservoir by means of Tygon tubing. Inserted in the tubing was a platinum wire contact which had been sealed in soft glass. A Princeton Applied Research Model 174170 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 closed with an unfired Vycor tip. The electrode was filled with a solution of tetramethylammonium chloride with the concentration adjusted such that the electrode potential was 0.00 V vs. SCE. The auxiliary electrode was a platinum flag sealed in soft glass. Neither an auxiliary compartment nor a Luggin capillary was used. A Leeds and Northrup pH meter equipped with a Broadly James Corp. combination pH electrode was used for all pH measurements. Reagents. Dioxohis(8-quinolinolato)molyhdenum(VI) (MoTv02Q,)was prepared by the method of Isbell ( I O ) The product was filtered and washed three times with methanol. It, was then dried under vacuum for four days and stored under vacuum. Stock solutions of 1.00 X 10-3M MoV'02Q2in dimethyl sulfoxide (DMSO), propylene carbonate (PC), and dimethyl formamide (DMF) were prepared by use of Burdick and Jackson "distilled in glass" solvents, All of the electrochemical studies made use of these solvents. Tetra-n-butylammonium perchlorate was obtained from Frederick Smith Chemical Company and was used as the supporting electrolyte. Stock solutions of Mo(V1) were prepared by dissolving in distilled water Mallinckrodt ammonium molybdate ((NH4)6M~)7024.4H20) which had been dried for 2 h at 110 "C. A 170solution of 8-quinolinol in chloroform was made by use of Matheson, Coleman. and Bell 8-quinolinol and Mallinckrodt reagent grade chloroform. Stock solutions of EDTA were prepared by dissolving J. T. Baker Na2H2EDTAin distilled water. Bethlehem triple distilled instrument grade mercury was used for the dropping mercury electrode. Procedure. The differential pulse polarogram was recorded between an initial potential of -0.80 V and a final potential of - 1.40 V. A scan rate of 2 mV/s was used with a modulation amplitude of 25 mV. A drop time of 1 s was obtained by use of the mechanical drop knocker. The low pass filter and the noise filter on the mechanical drop knocker were left off. Calibration curves were prepared hy the addition of MoV'02Q, to a cell that contained 50 mL of solvent with 0.1 M TBAP as supporting electrolyte. For some of the differential pulse polarograms 20 pL of concentrated acetic acid was added to the cell solution. Prior to the polarographic analysis the cell solutions were degassed for 5 min with argon.
RESULTS AND DISCUSSION Molybdenum in the form of the dioxobis(8-quinolinolacan be determined to)molybdenum(VI) complex (MoV1O2Q2) effectively by differential pulse polarography in a dimethyl formamide (DMF) solvent systpm. Figure 1 illustrates the differential pulse polarograms for 0.1 pM and 1.0 pM MoV'02Q2in DMF. T h e peak current a t -1.08 V vs. SCE is directly proportional t o the concentration of molybdenum, as illustrated by the calibration curves of Figure 2 . T h e response is linear from 0.1 pM to 100 pM molybdenum if the sample solution contains 7 ptM acetic acid. Selection of Solvent. Because the Mo(V1) oxine complex is reasonably soluble in dimethyl sulfoxide !DMSO) and this
1301
I
--6
16
r
b,
5c
Figure 1. Differential pulse pohrograms for dimethylformamide solutions that contain (a) 0.1 pM and (b) 1.0 pM Mo(V1)-8-quinolinol, 0.1 M TBAP, and 7 ILM acetic acid
a C
a
203
:
160
t
PO
O ,
5'7
:4c
'M$C2C;!.
c 6C
:K
I
cc
'LN
Figure 2. Calibration curves for the peak current of Mo(VIt8-quinolinol as a function of its concentration in DMF that contains 7 FM acetic acid and 0.1 M TBAP as supporting electrolyte
represented a solvent with known electrochemistry for t h e complex ( I O ) , DMSO received first consideration for the determination of molybdenum. For a DMSO solvent with 0.1 M tetra-n-butylammonium perchlorate (TRAP) as t h e supporting electrolyte the detection limit for MoV1O2Q2is 1 pM. However, the calibration curve is not linear because of the appearance of a doublet peak at concentrations of molybdenum above 18 pM. Below t,his concentration, Mom0,Q2 is reduced a t a peak potential of -1.04 V vs. SCE, and for
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
Table I. Analytical and Extraction Efficiency for Molybdenum as M O ~ I O ~from Q ~ an Aqueous Solution of (NH,)&fo,O, 4H,O pmol added
pmol found 0
0 1.0 x 1 0 5.0r 10 1.0 Y 10-2 5.0 x 10
1 . o u IO-' 5.0 x 10
1.2 x 5.1 X 3.1 x 4.9 x 1.1 x 4.6 X
10-3
%
recovery 120
102
10.10-2 10-2
110
10
110
10
'
fi3
98
"
."~
.
92
concentrations above 80 pM t h e complex is reduced a t -1.10 V. Propylene carbonate also received consideration as a solvent system. With it and 0.1 M TRAP as the supporting electrolyte, linear calibration curves are obtained for the concentration range from 0.5 pM t o 100 pM MoV*O2Q2.Peak splitting does not occur within t h e concentration range. However, the quality of the Burdick and Jackson propylene carbonate was highly variable. A contaminant often appeared with a reduction peak a t -1.09 V. Vacuum distillation did not result in a significant improvement. T h e deficiencies of DMSO and propylene carbonate prompted us t o try dimethylformamide (DMF) as a solvent for t h e determination of MoVIOzQz. In D M F with 0.1 M T B A P as t h e supporting electrolyte, t h e detection limit for molybdenum is 0.2 pM. For these conditions, the calibration curves are not linear because of the same peak splitting phenomenon that is observed in DMSO. The addition of a small amount of concentrated acetic acid to the cell solution (about 7 pM) eliminates this problem a t all concentrations and results in linear calibration curves. Addition of acetic acid increases the peak current for Mov10zQ2at all concentrations and causes the reduction peak potential to he stable a t -1.08 V vs. SCE. Figure 3 illustrates the effect of acetic acid concentration on the peak current for the Mo(V1)-oxine complex in DMF. Quality of Extraction. T o test the extraction procedure, known amounts of ammonium molybdate have been extracted from distilled water according to the method of Morrison and Freiser (8). T o a beaker that contains known amounts of ammonium molybdate in approximately 50 mI, of distilled water is added 5 mL of 0.02 M EDTA solution. The solution is diluted to about I00 mL and the p H adjusted to p H 1 5 5 by use of concentrated NaOH and 3 M HC'1. The solution is then transferred to a 250-mI, separatory funnel and extracted twice by shaking for 2 min with lO-mI, portions of a 1% solution of 8-quinolinnl in chloroform. The chloroform layer is then transferred to a n electrochemical cell, and t h e chloroform removed by evaporation at rooni temperature. Next, 10 mL of DMF that contains 1 mmol of TRAP and 0.07 mmol of acetic acid is added to the cell. The differential pulse polarogram is recorded after the solution is deaerated with argon for 5 min, and the peak current compared to the calibration curves to determine the percent recovery. Table
~
~~
3: 5
2'3
! t G k ] , m,?J
Figure 3. Dependence of peak current for the reduction of MO~'O,Q, in DMF (0.1 M TBAP) as a function of the concentration of acetic acid
I summarizes the results. Theoretically, 99% of t h e molybdenum should be extracted. Applications. The differential pulse polarographic method has been applied to the determination of molybdenum in plants that have been grown on fly-ash amended soils. Two types of plant materials, alfalfa and fescue, have been analyzed by use of a combustion procedure. Approximately 2.5 g of air-dried plant material are placed in Vycor crucibles and dry-ashed in a muffle furnace at 550 "C for 8 h. (Attempts to webash plant samples failed because of incomplete oxidation of the organic matter.) After ashing, the samples are allowed to cool to room temperature. The ash is then dissolved in 3 M hydrochloric acid and diluted to exactly 50 m L with 3 M hydrochloric acid. Occasionally, a small amount of residue does not dissolve in the acid. In these cases, the sample is centrifuged until all of the particulate matter is separated from the bulk of the solution. For the purposes of evaluating the procedure, each dry-ashed and dissolved sample has been divided into two portions to test for interferences and percent recovery. Ten-mL aliquots of each portion have been extracted by the previously-described procedure. One of these is analyzed by the differential pulse polarographic procedure. Then, by means of the standard addition method, the sample is spiked with a known amount of MoV'O2Q2and reanalyzed for molybdenum. T h e results confirm that the reduction current only is due to the presence of molybdenum and that interferences are not extracted. These results are in accord with the expectation that only the molybdenum and tungsten oxine complexes are extracted into chloroform a t p H 1.55. The second portion of sample solution has been extracted and analyzed in the usual manner except that, before extraction, a known amount of ammonium molybdate has been added to the sample solution to determine t h e efficiency of the extraction process. T h e results are summarized in Table I1 and are compared to the results t.hat are obtained by the M O ( S C N )spectro~ photometric method ( 2 1 ) . The differential pulse polarographic method provides a sensitive, selective means for the determination of trace levels of molybdenum in environmental samples. By use of a solvent extraction cleanup, interferences are excluded from the electrochemical solution. The latter has a detection limit of 0.1 pM Mo(V1) oxine; a linear peak
Table 11. Determination of Molybdenum in Plant Samples
sample fescue 1 fescue 2 alfalfa 1 alfalfa 2
alfalfa 3
4.-
molvbdenum content standard addition, differential pulse differential pulse polarography, polarography, PPm PPm 6.6 6.6 1.7 1.7 4.8 6.6 2.4 2.1 4.3 5.0
spectr ophotometry, ppm 5.2 2.4 5.1 1.5 2.7
ex traction efficienry, %
94 100 112 95
97
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
current response is obtained up to concentrations of 100 pM.
ACKNOWLEDGMENT We are grateful to Tony Gange of the Department of Soil and Environmental Sciences, University of California, Riverside, for supplying the plant samples and the comparison analyses. LITERATURE CITED (1) A. I. Busev, "Analytical Chemistry of Molybdenum", Academy of Sciences of the U.S.S.R., Moscow, 1962 (Israel Program for Scientific Translators, Jerusalem, 1964). (2) I. M. Kdtoff, E. B. Sandell, E. J. Meeham, and S. &udtensteln, "Ouantftative Chemical Analysis", 4th ed., Macmillan, New York, N.Y., 1969, p 1136. (3) D. A. Segar and A. Y. Cantillo, in "Analytical Methods in Oceanography", T. R. P. Gibb, Jr., Ed., Chap. 7, Advances in Chernisv, No. 173, American
(4) (5) (6) (7) (8) (9) (10) (11)
1303
Chemical Society, Washington, D.C., 1975. J. J. Dulka and T. H. Risby, Anal. Chem., 48, 640A (1976). G. Hanson, A. Szabo, and N. D. Chasteen, Anal. Chem., 49, 461 (1977). P. Lagrange and J.-P. Schwing, Anal. Chem., 42, 1844 (1970). V. V. Pnev, G. N. Popov, and V. G. Nagarev, Zh. Anal. Khlm., 28, 2050 (1973). G.H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry". John Wiley and Sons, New York, N.Y., 1957. R . M. Dagnaii and S. K. Hasanuddin, Talanta, 15, 1025 (1968). A. F. Isbell and D. T. Sawyer, Inorg. Chem., 10, 2449 (1971). C. M. Johnson and T. H. Arkley, Anal. Chem., 26, 572 (1954).
RECEIVED for review March 23, 1978. Accepted May 19,1978. The study was supported by the National Science Foundation under Grant No. CHE-76-24555, and by the Kearriey Foundation of Soil Science, University of California.
Differential Pulse Polarography of Phenylarsine Oxide J. H. Lowry,' R. 6. Smart, and K. H. Mancy Department of Environmental & Industrial Health, The Environmental Chemistry Laboratory, 2530 School of Public Health I, The University of Michigan, Ann Arbor, Michigan 48 109
Differential pulse polarography was applied to analyze for phenylarsine oxide (PAO) in aqueous solutions at dillerent pH values. Optimization of the instrumental artifacts resulted In a detection limit of lo-' M PA0 at pH 7.3, a relative standard deviation of 1.7 YO,and a maximum sensitivity of 450 pA/mM PAO. The polarographic reduction of phenylarslne oxide was found to be pH dependent. Cyclic voltammetry and coulometry were used to characterize the electrode process.
Several studies have been reported on the polarographic reduction of organic arsenic species (1-5). This interest is the result of the use of these compounds as herbicides (6) as well as the fact that organic arsenic compounds may be produced from inorganic arsenic by natural biological processes ( 7 ) . The determination of inorganic arsenic by differential pulse polarography (DPP) (8) and differential pulse anodic stripping voltammetry (9) has been reported previously. The polarographic reduction of dimethylarsinic acid and methylarsonic acid has also been reported (3). Bess et al. (4) studied the D P P of a series of alkylarsonic and dialkylarsinic acids below p H 2. They found the peak potentials were p H dependent, shifting to more anodic values a t lower p H values. Recently, Bess e t al. ( 5 ) reported on the D P P of aromatic arsonic and arsinic acids. They found peak potentials as well as peak currents were p H dependent below p H 2. Phenyl arsonic acid and phenyl arsonous acid were studied extensively by Watson and Svehla ( 1 , Z ) using dc polarography a t a DME. They suggested that phenylarsine oxide (PAO) existed in aqueous solution as phenyl arsonous acid and the diffusion current was p H independent. Unfortunately, above p H 2 their waves were poorly formed and exhibited broad maxima. No data were presented above this pH, and most of the work was carried out in 0.1 M HC1 a t concentrations below 1 X M, where P A 0 showed two main reduction processes. The half-wave potentials were shown to be p H dependent as the reduction processes became increasingly irreversible with increasing pH. A reaction scheme for PA0 0003-2700/78/0350-1303$01 .OO/O
in 0.1 M HC1 was proposed, where the reduction product reacts with the electroactive species with the formation of an insoluble polymeric product. The first wave was attributed to the reduction of P A 0 t o phenylarsine, where each mole of phenylarsine combines with an additional two moles of P A 0 to form the insoluble polymeric product. The second wave was due to an increase in the fraction of PA0 molecules that undergoes reduction to phenylarsine and a decrease in the fraction of P A 0 molecules that reacts with the phenylarsine. A t this wave there is a net increase in the average number of electrons consumed per molecule of PAO. Further information on the reaction schemes can be found in their original article. PA0 is used as a titrant for the direct and indirect determination of residual chlorine and ozone in water and wastewater (10). Preliminary investigations on the direct measurement of PA0 by D P P indicate that this technique is a promising method for lowering the detection limits in the indirect measurement of these oxidants (11). Since the control of p H is a necessary consideration in free and combined chlorine analysis with P A 0 ( 1 0 , I Z ) as well as the stability and measurement of ozone (10, 13),an investigation of the effect of pH on the D P P of P A 0 was mandatory. In the following discussion we describe the D P P behavior of PA0 and suitable conditions for analysis. For the purpose of analytical method development, the effect of p H on the reduction of P A 0 was investigated, and the reduction was shown to have strong pH dependence. This is in contrast to earlier findings by Watson and Svehla (I,Z ) , emphasizing the need for a better understanding of the reduction processes. In addition to investigating the reduction processes, we have optimized instrumental artifacts (14) for the D P P technique, resulting in a highly reproducible analysis of P A 0 whose sensitivity is p H dependent.
EXPERIMENTAL Apparatus. A Princeton Applied Research, Inc. (PAR) Model 174 Polarographic Analyzer and a Model 174/50 drop timer were used. A PAR Model 175 Universal Voltage Programmer was used for cyclic voltammetry. Temperature studies were conducted using the PAR Model 9350 water jacketed cell with a Haake TP-42 0 1978 American Chemical Society
