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(13) H. Craig, Science, 133, 1833 (1961). (14) S. Epstein and T. Mayeda, Geochim. Cosmochim. Acta, 4, 213 (1953). (15) Y . Matsuhisa, 0. Matsubaya, and H. Sakai, Mass Spectrosc., 19, 124 (1971). (16) J. R. O'Neii, L. H. Adami, and S . Epstein, J . Res. U . S . Geol. surv., 3, 623 (1975). (17) H. Craig, Geochim. Cosmochim. Acta, 12, 133 (1957). (18) Y . Horibe, Mass Spectrosc., 14, 113 (1966). (19) S. Szapiro and F. Steckel, Trans. Faraday Soc., 63,883 (1967).
(20) 0. Matsubaya, this
Institute, personal communication, 1976.
for review July 9, 1979. Accepted September 20, 1979. One of us (N.K) is grateful to the Ministry of Education for its financial support (General Research D, Grant No. 064127, 1975). RECEIVED
Electrochemical Determination of Trace Mercury in Aqueous Solution Danton D. Nygaard Chemistry Department, Bates College, Le wiston, Maine 04240
Mercury in aqueous solution is routinely determined by one of the many variations on the procedure of Hatch and Ott ( I ) , which involves reduction of aqueous mercury to mercury metal with acidic stannous ion and spectroscopic detection of the purged vapor. Atomic absorption spectroscopy was proposed by Hatch and Ott, and is probably the most widely used method, although plasma emission spectroscopy has also been proposed (2). Both spectroscopic detectors provide a degree of specificity which is probably unnecessary, since phase separation of volatile mercury metal from the solution matrix eliminates any potential interference a t the detector. Therefore, a less specific detector may be adequate. This paper investigates the possibility of using a n electrochemical detector, at which mercury metal vapor is determined through oxidation t o mercuric ion a t a n anodically polarized electrode. T h e membrane covered electrode of Clark e t al. ( 3 ) ,in which a thin, gas permeable polymer membrane separates the gas phase from the electrode and electrolyte, is used t o accomplish the necessary three-phase interface.
EXPERIMENTAL Apparatus. The sample reduction and purge cell is shown in Figure la. It consists of a 30-mL medium porosity fritted glass Gooch crucible, which is cemented to a 3.5-cm diameter funnel and is covered with a neoprene rubber stopper. Purge gas enters through the funnel stem, passes through the glass frit and the sample, and exits through a glass tube inserted through the stopper. Reagents are added to the sample by syringe through the serum cap. Tygon tubing is used to connect the sample cell with the purge gas supply and detector cell. The flow rate of the purge gas is controlled with a two-stage regulator valve and a needle valve, and is monitored with a float type flowmeter. The membrane covered electrode detector is shown in Figure lb. The gas compartment is a 1.5-cm diameter by 2-cm polystyrene vial through which holes have been drilled to accommodate the gas flow ports. The open end of the vial is pressed against the Neoprene rubber O-ring, which holds the membrane in place, forming a gas tight seal. The working electrode is a 0.5-cm diameter inlaid platinum electrode (Beckman Instruments). It forms one arm of a two electrode H-cell, with a saturated KCl in agar salt bridge connecting it to the saturated calomel reference electrode. Purge gas from the sample cell flows through the gas compartment and over the polymer membrane. The working electrode is polarized, and the resulting current amplified, with a PAR Model 174A Polarographic Analyzer. The current is displayed as a function of time on a Bausch and Lomb VOM5 strip chart recorder. Reagents. Reagent grade chemicals are used throughout. The reductant is 10% stannous chloride in 5% concentrated sulfuric acid, as suggested by Velghe et al. ( 4 ) . Mercury solutions (10 and 1 ppm) in 3% concentrated nitric acid and 0.01% potassium dichromate are prepared daily by dilution from a 1000-ppm standard solution. The electrolyte for the detector cell is 0.1 M 0003-2700/80/0352-0358$01 .OO/O
Table I. Sensitivity and Detection Limit for Membrane Covered Electrode sensitivity detection membrane pa/ng Hg limit ng Hg 0.5-mil Teflon 0.9-mil Polyethylene 1.0-mil MEM-213
0.01 2 20
2000 10 1
KNOBin 0.1% concentrated nitric acid, as suggested by Barikov and Songina ( 5 ) . Both nitrogen and air are used interchangeably as the purge gas. Procedure. Ten milliliters of sample are placed in the sample cell and acidified to three molar with concentrated sulfuric acid. The sample cell is sealed with the neoprene stopper, and the purge gas flow rate is adjusted to the desired value. When a base-line current is established for the detector, 1 mL of stannous chloride reductant is injected into the sample cell. After mercury metal is purged from the cell and the detector current has returned to the base-line value, mercury standard solution may be added to the cell for the purpose of standard addition analysis, or the sample may be rinsed from the cell and the next sample added.
RESULTS AND DISCUSSION Reducing Medium. Hydrochloric, sulfuric, nitric, and various mixtures of these acids have been used in different laboratories to acidify the sample before reduction with stannous ion. It has been reported (6) that a sulfuric acid medium yields a more rapid reduction reaction. Three molar hydrochloric and three molar sulfuric acid were compared as reducing media; it was found t h a t the sulfuric acid medium yields a detector response peak which is 40% higher than that which results from the hydrochloric acid medium. Nitric acid is not satisfactory as a reducing medium, because the gaseous nitrogen oxides present in the acid are electroactive, and cause excessively large background currents. Polarizing Potential. T h e detector response to mercury is independent of polarizing potential a t potentials of +0.60 V vs. SCE and above. However, the background current increases with increasingly anodic polarizing potentials. Therefore, +0.70 V vs. SCE was chosen as the operating potential. Purge Gas Flow Rate. A plot of detector response to mercury as a function of purge gas flow rate yields a broad maximum plateau a t flow rates between 120 and 240 mL/min. T h e electrode noise is independent of flow rate. Therefore, 180 mL/min was chosen as the optimum flow rate. Membrane Covered Electrode Detector. T h e peak current response and detection limit of the membrane covered electrode detector depend on the membrane material; they are shown for Teflon, polyethylene, and MEM-213 (General C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 bl
l l !X I
/ I I1
'
I
Figure 1. (a) Sample reduction and purge cell. A, Neoprene stopper; B, glass tubing; C, serum cap; D, 30-mL medium porosity fritted glass Gooch crucible; E, 3.5cm diameter funnel cemented to crucible. Arrows indicate the direction of purge as Row. (b) Membrane covered electrode detector. A, 1.5-cm diameter by 2-cm polystyrene vial; B, membrane; C, Neoprene O-ring; D, 0.5-cm diameter inlaid platinum working electrode; E, 0.1 M K N 0 3 electrolyte level; F, salt bridge, G, SCE reference electrode. Arrows indicate the direction of purge gas flow
Electric Corporation) in Table I. Detector response and detection limit also depend, to some extent, on the degree to which the membrane is stretched during formulation of the electrode. The response to the electrode close to the detection limit is shown in Figure 2 for polyethylene and MEM-213 membranes. The linearity of response and reproducibility of the detector are excellent over the range of concentrations studied (10-1000 ng of Hg), with calibration curves showing a relative standard deviation in the slope of 3% (45 data points) over the lifetime of the electrode. The lifetime of the electrode is limited by the rate of evaporation of the electrolyte through the polymer membrane. Therefore, use of the less permeable polyethylene membrane yields lifetimes as long as a month, whereas use of the more permeable MEM-213
359
membrane usually results in lifetimes of about 2 weeks. For either membrane, the analysis time is about 5 min per sample when a calibration curve is used. Interferences. Interferences can be viewed as being of two types-those that interfere with the reduction of mercury in the sample cell, and those that interfere with the detection of mercury at the detector. Organomercury compounds are the most serious interference of the first kind, since they are incompletely reduced by stannous ion. Therefore, most procedures (7) suggest a preoxidation of the sample with acidic permanganate and reduction of the excess permanganate with hydroxylamine hydrochloride before the stannous ion reduction step. This procedure is compatible with t h e electrochemical detector described in this paper, although the addition of extra reagents tends t o increase the blank. The use of a more powerful reducing agent, such as sodium borohydride, is precluded by the electrochemical detector. Hydrogen, which is slowly produced by attack of borohydride on water, is oxidized a t the platinum detector electrode, resulting in spurious currents. Matrix anions potentially present. the most serious interference problems of the second type. If the sample contains sulfide or sulfite ions, acidification releases hydrogen sulfide gas or sulfur dioxide gas, both of which are oxidized a t the detector. In a similar fashion, acidification of nitrite ion causes slow disproportionation to nitrate ion and nitric oxide gas, which is also oxidized a t the detector. These ions do not, strictly speaking, interfere with the mercury determination, since the electroactive gases are released during the acidification step, and, once they are purged from the system, the detector current returns to base line, and mercury can be reduced and determined as usual. However, the extra time required t o purge these interfering gases from the system increases the analysis time by anywhere from 5 min for sulfide ion to several hours for nitrite ion. I t was found that the previously described acidic permanganate preoxidation step oxidizes sulfide and sulfite to sulfate ion, and nitrite to nitrate ion, thereby eliminating the interference. The addition of nitrate ion to the sample a t the 1 mg/mL level produced no observable interference.
a:
( b)
A
0
L TIME, sec
Figure 2. Membrane covered electrode detector response vs. time. Electrode polarized at f 0 . 7 0 V and purge gas flow rate of 180 mL/min. (a) 0.9-mil Polyethylene membrane; A, blank sample, followed by 50-ng Hg sample and standard additions of 100, 50, and 20 ng Hg. (b) 1.0-mil MEM-213 membrane; B, blank sample, followed by 30 ng Hg sample and standard additions of 20, 20, and 10 ng Hg
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Anal. Chem. 1980, 52, 360-364
CONCLUSIONS In comparison with the usual spectroscopic detectors, the membrane covered electrode is competitive in terms of linearity and reproducibility, but the detection limit is approximately five times larger (with MEM-213) than that reported for plasma emission spectroscopy ( 8 ) ,and the analysis time is about five times longer. However, the instrumentation required for the electrochemical detector is much less expensive. ACKNOWLEDGMENT The author thanks the General Electric Corporation for its contribution of the MEM-213 membranes, and Joan Lathrop, Steven Gellen, and Leslie Anderson for their technical assistance.
LITERATURE CITED (1) Hatch, W. R.; Ott,W. H. Anal. Chem. 1968, 40, 2085. (2) April, R. W.; Hume, D. N. Science 1970, 170, 849. (3) Clark, L. C.: Wolf, R.; Granger, D.; Taylor, 2. J . Appl. Physiol. 1953, R - , 189 .--. (4) Veighe, N.; Campe, A.; Claeys, A. Perkin €/mer At. Absorpt. News/. 1978, 17(2), 37. (5) Barikov, V. G.; Songina, 0. A. Zavodsk. Lab. 1984, 30, 1184. (6) Tong. S. Anal. Chem. 1978, 50, 412. (7) "Atomic Absorption Methods Manual", Fisher Scientific Co., 1977. (8) Gilbert, T. R. Anal. Left. 1977, 10, 599.
RECEIVED for review March 30, 1979. Accepted October 18, 1979. Research supported by a Bates College Faculty Research Grant. Presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 8, 1979.
Digestion of Organic Matrices with a Single Acid for Trace Element Determination Myron M. Schachter and Kenneth W. Boyer" Division of Chemistry and Physics, Food and Drug Administration, Washington, D.C. 20204
Most analytical procedures for determining trace elements in biological and other organic materials require that the organic matrix be completely destroyed before the analysis. Both wet- and dry-ashing procedures are commonly used for matrix destruction ( I ) . Dry-ashing or furnace ashing is more often the method of choice based on convenience. However, problems commonly associated with dry-ashing include loss of elements by volatilization or bonding with the ashing container, formation of difficult to dissolve residue ash, and foaming of the sample (2). Wet-ashing has been the method of choice for metals that are volatile, such as Hg, or which form volatile salts when ignited in the presence of chlorides, such as As, Se, Cr, Fe, Sb, and Pb. Furnace ignition or dry-ashing usually requires ashing aids such as Mg(NO& or H2S04( I , 3) which may introduce unwanted metal contaminants. Wet-ashing methods sometimes require catalysts. Examples include V205for Hg determination (4,and various metal salts, including those of Cu, Ag, Au, Co, V, Ni, P d , and Fe, for Kjeldahl wet-ashing catalysts when determining nitrogen in organic compounds ( 5 ) . These catalysts would also be a source of contamination or interference if the corresponding elements were being determined. Some wet-ashing procedures are permitted to be incomplete. For example, in a digestion procedure prior to Hg determination (6),the fatty acids from waxes, oils, and fats are not broken down by the digestion process. Instead, they are later removed by filtering the digest leaving the Hg salts dissolved in the aqueous filtrate. Another procedure for Hg determination (7) dissolves fatty acids by hydrolyzing the organic matrix with concentrated alkali. Yet another procedure uses either 50% or 30% hydrogen peroxide to produce colorless, clear digest solutions (8). However, these apparently completely digested solutions contain soluble organic peroxides, which darken in color and/or precipitate organic colloid micelles as soon as the peroxides decompose or are reduced. Incomplete destruction of the matrix may result in formation of colloidal substances and colored products of decomposition, which cause problems in the determinative step of most analyses. Such problems include clogging of atomic emission and absorption instrumentation nebulizers, formation of explosive gas mixtures during neutron irradiation for neutron
activation analyses (NAA), and masking of metal chelate absorbance bands during spectrophotometric determinations. A fusion technique using N a N 0 3 and KNOBas a eutectic mixture (9) oxidizes all organics including polyethylene at 390 f 10 O C within a few minutes. Although nitrate fusion is a rapid and effective means of matrix destruction, explosion hazards associated with the use of nitrate salts to destroy organic matrices have been reported (9, IO). A widely used wet-ashing process that completely destroys organic matter ( I I ) uses a combination of nitric, sulfuric, and potentially explosive perchloric acids. That method is effective and safe as long as: (a) the digest is not permitted to boil dry, leading to volatilization losses and possible formation of spontaneously explosive perchlorate esters; (b) digestion of samples having a fat and oil content greater than 50% is not attempted; and (c) the digestion is closely watched to prevent charring which can also lead to volatilization losses and/or possible explosion. Another wet-ashing procedure (12) uses nitric acid, heat, and pressure in conjunction with the Uniseal decomposition vessel to destroy most of the sample matrix. An extensive liberature survey describing applications of this device is available from the manufacturer (Uniseal Decomposition Vessels, Ltd., P.O. Box 9463, Haifa, Israel 31094). The primary disadvantage of this device is the limitation on sample size (approximately 1g of dry weight organic matter). If digestion of too large a sample is attempted, leakage and even explosions may occur owing to pressure buildup in the decomposition vessel. The single acid procedure described here minimizes some of the undesirable features of the procedures described above. It is safer, faster for most matrices, results in acceptable recoveries of most elements, is amenable to multielement methods, and minimizes possibilities of interferences since no catalyst metals are required. EXPERIMENTAL Procedure. Set up apparatus as shown in Figure 1 (see Figure 2 for detailed dimensions). Although we have never experienced an explosion with this procedure, a safety shield is strongly recommended. Weigh sample of up to 5 g (dry weight) into the quartz flask. The material to be digested should be essentially dry (10% or less water) if the shortest possible digestion time is
This article not subject to US. Copyright. Published 1980 by the American Chemical Society
