Anal. Chem. 1980, 52, 105-108
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Determination of Mercury in Water by Furnace Atomic Absorption Spectrometry after Reduction and Aeration Darryl D. Siemer” Exxon Nuclear Idaho Company, Idaho Falls, Idaho 8340 1
Lynn Hageman Montana State University, Bozeman, Montana 597 15
Mercury present in water samples Is reduced to the metal In a bubbler and sparged from the solution with a stream of air. The gas stream is filtered through porous, gold-plated graphke furnace atomizer tubes which retain the mercury. The tubes are then placed into either a carbon rod atomizer or a Woodriff furnace for the actual atomic absorption (AA) mercury determination. Each deterrnlnation takes about 3 minutes. The detection limit Is about 10 parts-per-trillion.
Mercury determination in aqueous solutions a t low concentrations is usually done by either some variation of the Hatch and Ott (1)process or by direct graphite furnace atomic absorption spectrometry. Both methods have serious limitations. In the Hatch and Ott technique, a reducing agent (usually stannous ion) is added to the water sample and the elemental mercury vapor released is flushed into a quartz-windowed cuvette mounted on an AA spectrometer. Two variations of the procedure are commonly used. In the first, the mercury vapor is recirculated through the bubbler-cuvette system and a steady-state signal is observed. In the second, the vapor passes only once through the cuvette, giving a peak response. Typically only a small fraction of the total mercury in the original sample is ever in the cuvette a t any one time and therefore the potential maximum analytical response is not closely approached. The reasons for the relatively poor sensitivity include: large reaction vessel and solution volumes with respect to the volume of the cuvette, cuvettes with large cross sections with respect to their volume, and a partition coefficient of mercury between aqueous solutions and the gas phase favoring the solution phase. Typically, a microgram of mercury will give an atomic absorbance of about 0.3. However, since sample solution volumes of 50 to 250 cm3 can be used, the concentration-based sensitivity is often adequate for real water samples. Direct furnace atomic absorption analysis of solutions of mercury stabilized by the addition of dichromate gives much better absolute sensitivities (typically an absorbance signal of 0.3 for from 10 to 30 ng); however, somewhat poorer concentration sensitivities than those of the Hatch and Ott approach are attained because sample volumes are limited to 10 to 100 pL. With either approach, the determination of mercury at concentrations at or below the recommended EPA maximum level is somewhat difficult. For example, 100 mL of a 0.5 ppb mercury solution will give a signal on the order of only 0.05 absorbance unit in the original version of the Hatch and Ott apparatus; with direct injection of 100 pL of the same solution into a typical graphite furnace atomizer, the expected signal would be on the order of only 0.001 absorbance unit. Base-line “noise” in most AA spectrometers is usually larger than this figure. 0003-2700/80/0352-0105$01.00/0
A number of authors (2-5) stress the importance of increasing the efficiency of sparging the mercury released from sample solutions by the Hatch and Ott reduction process into a minimum volume of carrier gas and then transferring the “plug” of mercury-containing vapor to the absorption cell without further dilution. Hawley and Ingle ( 5 )optimized the one-pass system by minimizing dead volumes and using an efficient fritted glass sparging system with a long cuvette having a very small cross section. With a sample solution volume of 1mL and an outstandingly noise-free, dc electronic system capable of detecting an absorbance of they g absolute. achieved detection limits of 1 ppt, or However, there are practical limitations to the wide application of Hawley and Ingle’s approach. First, the atomic absorption spectrometer used for the work would not be useful for the general run of atomic absorption measurements because there is no provision for either atomizer emission light suppression or for the correction of nonatomic absorption. The instrument is useful for only a single application. I t is the experience of these authors that commercially available atomic absorption spectrometers with background correctors cannot generally detect transient signals of less than absorbance unit with any degree of reliability and that the really analytically useful signals should be considerably greater than lo-’ absorbance unit. Second, the analytical response (peak absorbance signal) observed for a given mass of mercury is dependent upon factors influencing the volume of carrier gas containing the bulk of the mercury “slug”. These factors include: gas flow rate, solution volume, solution viscosity, and any changes in solution composition affecting the partition coefficient of the mercury between the vapor and liquid phases. Koirtyohann and Khalil (6) in one study and Tong (7) in another determined these partition coefficients in a number of solutions containing different common acids; their results range from 0.4 to about 0.7 a t 25 O C . Hwang et al. ( 8 ) ,reporting upon the performance of a commercial cold vapor mercury determination accessory, describe the effects of changing a number of experimental variables upon the peak response of a typical modern Hatch and Ott system. T o combine the excellent absolute sensitivity of graphite furnace atomic absorption with the large sample volume capability of the Hatch and Ott process, we undertook to adapt the gold-plated, porous graphite atomizer tube mercury filtration system previously applied to air and solids analysis to water analysis (9-11). This technique should also free the overall analytical response (peak height absorbance signal) from any sensitivity to variations in experimental conditions which affect only the rate a t which a given mass of mercury is sparged from the sample solution. In order to have a reasonably fair basis of comparison between the system and a modern version of a cold vapor sweep system (Le., all experiments done on the same spectrometer), ‘C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980 /
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A’ Figure 2. Conventional cold vapor sweep system. (A) Frit. (B) Drain. (C) Sample port with removable septum. (D) Open ended quartz tube cuvette
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Flgure 1. Reduction-sparging vessel and filter adapter. (A) Zinc-silver mercury absorber. (B) Rubber stopper. (C) Rubber septum. (D) Glass “fingers” to break bubbles. (E) Stopcock. (F) Plastic filter adaptor. (G) To vacuum source. (H) Detail of filter adaptor showing air flow. (I) Glass wool
a homemade Hatch and Ott apparatus utilizing a n efficient sparging cell and a quartz tube cuvette was constructed and tested too.
EXPERIMENTAL The bubbler (Figure 1)used with the gold plated graphite filters is constructed of 30-mm Pyrex tubing and is about 25 cm long. A Teflon stopcock is sealed to the bottom of the tube to serve as a drain. The filter adaptor is connected to a 15-cm long, I-cm o.d., “L” shaped glass tube inserted through a rubber stopper in the top of the bubbler. A short length of 1.0-cm tubing sealed to the top of the bubbler and capped with a rubber vacuum line septum serves as the sample and stannous ion reductant introduction port. The sample solution is sparged of the elemental mercury released by the stannous ion by drawing air through a thistle tube filled with a mercury vapor absorbant. Either a “house” vacuum line or a water aspirator was used for drawing the sparging air through the system. The filter adaptor is constructed of Teflon rod stock and silicone rubber gasket materials. It can hold either a Woodriff furnace cup or a carbon rod atomizer tube from 13 to 14 mm long and 6 mm in 0.d. The air is drawn in through the end of the adaptor to the inside of the cup or tube and then out through the walls of the cup or tube. These tubes are electroplated on the inside with about 2 or 3 mg of gold. (Construction details for the filter adaptor and plated graphite tubes are available in ref. 9--11.) The mercury absorber in the thistle tube is made by swirling 150 g of granulated zinc along with about 100 mL of water in a beaker while slowly adding a solution of 10 g of silver nitrate dissolved in 50 mL of water. The mixture of remaining zinc and precipitated silver is filtered and washed first with water and then with acetone through a Buchner funnel. The highly divided silver is extremely effective in removing the mercury from air drawn through it. The remaining granulated zinc prevents the silver powder from packing so closely together so as to seriously restrict airflow.
The bubbler used for the conventional type Hatch and Ott system is constructed of 34-mm borosilicate glass tubing and is about 14 cm long. To enhance the sparging efficiency, the entire bottom of the bubbler consists of a coarse glass frit (about 8 cm2 area) through which the gas was bubbled up through the solution. The mercury vapor released from the sample solution upon reduction was swept through a 10-cm long, 1-cm i.d. open ended quartz tube mounted in the optical path of the spectrometer. Figure 2 depicts the system. The Varian CRA 63 carbon rod atomizer and power supply are used without modification with the gold-plated tubes; the rods are, however, adjusted to accommodate the somewhat larger than standard tube diameters. An atomization power supply program of 30 s at 1.5 for “dry” cycle (on the “dry” control potentiometer), 5 s at “0” (on the “ash” control potentiometer) for the ash, and 2.5 s a t “2” (on the atomization control potentiometer) for atomization serves first to dry out the tube and remove any possibly interfering absorbed organic vapors and then to atomize the mercury without damaging the gold in the tube. It is important that the atomization tube’s maximum temperature be kept below gold’s melting point (1063 OC) if the tubes are to be reused. In the majority of cases the drying step is unnecessary. The Woodriff furnace was run at 900 K. Varian hollow cathodes and hydrogen lamps were used. The usual 253.6-nm resonance line was used instead of the much more sensitive 184.9-nm line because of practical instrumental light throughput limitations. The atomic absorption spectrometer used with the carbon rod atomizer consists of a Varian AA6 monochromator optical rail, beam splitter, and photomultiplier tube assembly. A homemade dual frequency modulation hollow cathode and hydrogen lamp power supply was used with a laboratory-constructed system utilizing two lock-in amplifiers and dual H P 3447 system voltmeters with an HPIB interface to permit data acquisition with a Hewlett-Packard 9825 mini-computer. The Woodriff furnace was mounted on a homemade background-corrected spectrometer controlled by a Hewlett-Packard 9830 computer. The use of computer controlled spectrometers increased the ease of data collection and permitted more flexibility in standardization and in the calculation and graphic display of results, but did not in any way influence the basic detection capabilities of either instrument. In that respect, both instruments are comparable to typical, modern “off the shelf’ single beam AA instrumentation. Solutions. Mercury stock solution: 0.0343 g of mercury metal was weighed in a 50-mL volumetric flask, dissolved in 1 mL of “OB, and the solution diluted to volume with water. Ten percent SnClz in 3 M HC1: 20 g of SnC12was weighed in a 2W-cm3plastic bottle and 50 mL of concentrated HC1 and 150 cm3of water were added. Ten percent SnC12in 3 M HzSO4: 20 g of SnC12was weighed into a 200 cm3 plastic bottle and 32 mL of concentrated H2S04and 160 mL of water were added. A n a l y t i c a l Procedure. A porous CRA tube (or Woodriff furnace cup) precleaned by heating in the atomizer is inserted
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
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Figure 3. Sparging curve for mercury vapor
into the filter adaptor. Then the water sample is pipetted into the sample port and 1mL of the stannous reagent is added and rinsed into the bubbler with a stream of water from a wash bottle. The port is quickly stopped up with the septum cap and the vacuum source applied. Air is drawn through the system for 2 or 3 min. Then the adaptor is opened and the tube (cup) either placed in the atomizer for an immediate determination, or stored in a mercury-vapor-absorber-filleddesiccator for later attention. The stopcock is then opened and the contents of the bubbler are rinsed into a waste beaker with a steam of distilled water introduced through the entrance port. The pressure drop across the walls of the carbon fdter is about one-half an atmosphere, resulting in air flows of approximately 1100 cm3/minute through the walls of the filters. Standards were usually introduced into the bubbler by pipetting 5 to 100 WL aliquots of relatively concentrated solutions into the sample port of the bubbler and then washing them into the bubbler with enough water to bring the total volume up to the desired level (1 to 40 mL) for the experiment. The handling and storage of sub part-per-billion mercury solutions was avoided whenever possible to avoid adsorption errors.
RESULTS AND DISCUSSION Figure 3 is a plot of flushing time vs. peak atomic absorbance signal seen for 13.7 ng of mercury in 25 mL of sample solution sparged for various lengths of time. The fraction of the total mercury removed is an exponential function of the flushing time with a one-half time of about 8 s. The volume of air flushed through the solution in 8 s is about 150 cm3, or about six times the volume of the aqueous phase. The partition coefficient of elemental mercury between the aqueous phase and air was determined using the syringe cold vapor absorption cell method described by Koirtyohann and Khalil(6). I t turned out to be 0.42, in good agreement with their work. Assuming that equilibrium occurs between the sparging air and the liquid, an estimate for the relative volumes of air flushed through the liquid to that of the liquid necessary to remove one half of the mercury (n)can be obtained with the following equations: ratio of volumes (1 - partition coefficient) = (1- 0.42)” = 0.5 solving for n
n = 1.27 =
vol air vol aqueous phase
In practice about six volumes of air must be passed through a given volume of liquid to flush out one half of the mercury; this shows that the bubbler system does not equilibrate the two phases. Using lower gas sparging rates increases the one-half time proportionately indicating that the reduction kinetics are fast relative to the sparging rate. Incorporating a frit on the end of the thistle tube would probably increase the volume sparging efficiency but would
107
also increase the resistance to the gas stream and lower the rate of gas flow. A 2-min bubbling time (about 15 “1/2 times”) suffices for greater than 99% “stripping efficiency”; this was deemed short enough to make further efforts as speeding up the process of little real value. The absolute sensitivity of the method depends on the atomizer and its mode of operation. Typical sensitivities (in the units traditionally used for furnace AA, g/0.0043 absorg for the CKA and 0.9 X bance unit) are about 1.3 X g for the particular Woodriff furnace used. The height of the peak signal is strongly dependent upon the atomizer heating rate in the CRA but the peak area is much less affected. These sensitivities are identical to those obtained when microliter sized aliquots of standard solutions are directly deposited inside the tubes (cups), carefu2ly dried, and then atomized. The reproducibility achieved using the bubbler is somewhat better than that obtained by drymg standard solutions directly in the tubes. The improvement in reproducibility probably results from the fact that the usually critical solution drying stage is eliminated when the bubbler is used. The mean peak area obtained from a series of 5 dcterminat,ions of 13.6 ng of mercury in 40 mI, water samples using the CRA was 0.297 absorbance second with a relative standard deviation of 2.2%. The standard deviation of a series of blanks was 0.014 absorbance second giving a “2s” detection liniit of 32 parts per trillion (ppt). The mean and RSI) of the peak height signals observed for the same set of samples was 0.452 and 3.2‘%. The standard deviation of the series of blanks was 0.009 absorbance, giving a detection limit of 14 ppt. Since sample volume is limited only by the size of the bubbler used, detection limits may be lowered at will by constructing a larger bubbler. It is not generally necessary (or desirable) to use a flow of nitrogen or argon around the atomizer tube during the atomization heating stage as is usually the practice in atomizing other metals. The tube never gets hot enough to oxidize appreciably in the air and the gas serves no useful function. A series of mercury determinations in 40 cm3 total sample volumes were performed using the simple one pass Hatch and Ott system. These were done by pipeting the sample into the bubbler (Figure 2) and then inserting the septum into the sample port. Then two cm3 of the stannous iori reductant was injected into the solution with a syringe to release the mercury. A sparging gas flow of 1.5 L/min of air was used to carry the mercury to the cuvette mounted in the spectrometer. Lock-in amplifier time constants of 1 s were used instead of the 25 ms required to accurately follow the much more rapid signal transients gotten with the carbon atomizer system. The 1-s time constants significantly reduced base.line “noise” (but not drift) and were short enough to not seriously perturb the shape of the much broader signal transients gotten with the cold vapor sweep method. Sample solutions were prepared in 0.005 M HzSO, and 0.005% NazCrzO7to stablize the mercury in them. The bubbler was rinsed out between runs with ti “blank’ solution of the same reagents to ensure the oxidation of any unreacted stannous ion left in the system before the new sample was introduced. A series of eight determinations of 0.5 pcg of mercury in 40 cm3 of solution (12.5 ppb) gave a mean peak response of 0.382 absorbance unit with a relative standard deviation (RSD) of 5.6%. The mean integrated signal was 3.491 absorbance seconds with an RSD of 1.2%. A series of eight blanks gave a mean peak response of 0.0138 and a standard deviation of 0.0055. The mean integrated signal for the blanks was 0.145 with a standard deviation of 0.064. An analysis of these data indicates several features of interest. First of all, the homemade cold vapor sweep system
108 * ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
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Table I. Atomizer ‘rube Storage Time vs. Peak Absorbance time of storage
signal
10 min 1 0 niin l h 5 h 24 h
0.472 0.450 0.441 0.410 0.466
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time of storage 30 48 48 72 78
h h h h h 97 h
Table 11. Volume of Sample vs. Peak Response
signal 0.458 0.440 0.440 0.445 0.468 0.453
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volume of sample, mL
signal, absi17 ng Hg
1 2 5 10 20 40
0.555 0.543 0.548 0.561 0.540 0.557
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has a sensitivity comparable (slightly better) to typical state-of-the-art commercially available systems (see ref. 12). However, its sensitivity is about 30-fold worse than that obtained with the gold plated filter system. More important is the fact that the detection limit capability of the system (utilizing peak height data, 2 s criterion) is 0.37 ppb or about 30 times worse than that obtained with the filters. If the calculation is based on peak area data, it suffers even more in comparison with the filter system. With the cold vapor sweep apparatus, the main limitation to detection capability lies in the base-line drift inherent in the single beam AA spectrometer. The response of the cold sweep system was of course subject to all of the perturbations (caused by changes in gas sparging rates, solution viscosities, solution volumes, etc.) usually seen in such systems (8). The detection capability of the method incorporating the gold plated filters was initially determined by the reagent blank contributed by the reducing solution. Preliminary work done using a reducing solution composed of 10% by weight stannous chloride in 3 M HCl indicated these blanks were both very high and difficult to reduce by bubbling nitrogen through the reagent prior to its use in the bubbler. However, 10% by weight SnC12reagent prepared in 3 M HLS04instead of HC1 was very readily purged of its initial mercury content by bubbling the inert gas through it. The probable reason for this is that as the chloride concentration is raised beyond the unit molar level, the overall reaction potential for the reduction of mercurous ion by stannous ion in hydrochloric acid ceases to overwhelmingly favor the formation of elemental mercury. Reduction potentials (13) indicate the net driving force for the reaction is only 129 mV in 1 M chloride and that the driving force becomes even less at higher concentrations. The use of reagents prepared from the sulfate salts and sulfuric acid is strongly recommended. When presparged stannous reductant prepared in sulfuric acid is used, the limiting factor in the overall analysis became “electronic” noise and base-line drift. The actual quantity of the reducing solution used for each determination is not critical in water samples free of oxidants or complexing species. One milliliter of the 10% SnCILin 3 M H2S04reagent proved sufficient to reduce the mercury quantitatively in from 5 to 40 mL of normal water samples with a bubbling time of 2 min. Of course, if large amounts of other reducible substances are present in the sample owing to a prior sample pretreatment with oxidants in order to break up “bound” forms of mercury, more reagent may be required. In this case, the detection limit will usually be determined by an overall reagent blank and not by “electronic” noise. Table I lists the results of an experiment done to determine how long the mercury sorbed onto the gold-plated tubes would stay associated with the tube. Ten different tubes were sequentially placed into the adaptor and 13.7 ng of mercury (originally in sample volumes of 10 mL) were transferred to each. These tubes were placed in the wells of a porcelain spot plate stored in a small aluminum desiccator; the desiccator was charged with 100 g of granulated copper chips mixed with
100 g of the same zincsilver mixture used in the thistle tube. The first four tubes were sequentially inserted into the CRA atomizer and the mercury absorbance signal was measured a t various times during the first day; the others were run on succeeding days. There was no tendency for the tubes to lose the mercury amalgamated into the gold layer when stored a t room temperature. In practice this means that it is not necessary to tie up the AA spectrometer during the entire analytical procedure. The reduction/filtration steps can be done separately and the tubes analyzed a t a later time if it is more convenient to do so. Gold plated tubes will slowly scavenge mercury vapor from the air in the open laboratory and should be stored in a desiccator filled with an effective mercury vapor absorber if an appreciable delay is anticipated between the separationconcentration step and the actual determination. We did not observe any advantage in handling the graphite with tweezers as opposed to handling the graphite with clean, dry fingers. Most of the mercury contamination in our laboratories appeared to be present as volatile mercury metal in the air and not as particulate dust on equipment or hands. Variations in the volume of water sample containing a given mass of mercury have no effect on the analytical response of the system. Twenty-five microliter aliquots of a 0.686 ppm mercuric nitrate standard solution were washed into the bubbler with volumes of water ranging from 1 to 40 mL. One milliliter of the stannous ion reductant was added in each case and the mercury transferred to a CRA tube. Table I1 gives the peak height responses obtained. The same graphite tube was used for all of these samples. The advantages of the carbon filter system as opposed to the cold vapor sweep approach include better sensitivity and an independence of the analytical response on factors changing only the kinetics of the reduction/sparging process. The ability to separate the separation/filtration process from the actual AA finish may save considerable instrument time as well as reduce the seriousness of the long term drift in spectrometer sensitivity often encountered in mercury AA determinations. The main disadvantages are the additional manipulations required to perform an analysis and the fact that the analyst will have to make his own tubes and filter adaptor as they are not commercially available accessories.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
W. R. Hatch and W. L. Otl, Anal. Chem., 40, 2085 (1968). T. R. Gilbert and D. N. Hume, Anal. Chlm. Acta, 65, 461 (1973). M. P. Stainton, Anal. Chem., 43, 625 (1971). J, F. Uthe, F. A. J. Armstrong and M. P. Stainton, J . Fish. Res. Board Can., 27, 805 (1978). J. E. Hawley and J. D. Ingle, Jr., Anal. Chem., 47, 719 (1975). S.R. Koirtyohann and M. Khalil, Anal. Chem., 48, 136 (1976). S. L. Tong, Anal. Chem.. 50, 411 (1978). J. H. Hwang, P. A. Ullucci, and A. L. Malenfant, Can. Spectfosc., 4, 2 (1971). D. Siemer and R. Woodriff, Anal. Chem., 46, 597 (1974). D. Siemer, J. Lech, and R. Woodriff, Appl. Spectrosc., 28, 68 (1974). J. Lech, D. Siemer. and R. Woodriff. Appl. Spectfosc., 28, 78 (1974). K. G. Brodie, Am. Lab., 11, 58-66 (1979). “CRC Handbook of Chemistry and Physics”, 54th ed, R. C. Weast, Ed., CRC Press, Cleveland, Ohio, 1974, p D122.
RECEIVED for review May 7, 1979. Accepted October 22, 1979.
