Determination of Sub-Microgram Quantities of Mercury by Atomic Absorption Spectrophotometry W. Ronald Hatch and Welland L. Ott Falconbridge Nickel Mines Limited, Metallurgical Laboratories, Thornhill, Ontario, Canada The procedure outlined describes an extremely sensitive and accurate method for the determination of mercury down to 1.0 ppb in solution. This procedure has been applied to nickel and cobalt metal as well as rock samples and soil samples containing organic materials. The sample is taken into solution by an oxidizing acid attack. Mercury i s then reduced to the elemental state and aerated from solution in a closed system. The mercury vapor passes through a quartz absorption cell of an atomic absorption spectrophotometer where its concentration is measured. The procedure is free from interferences due to organic matter or other volatile constituents of the sample. large amounts of easily reducible elements must be absent from the solution.
THE following method for the determination of mercury in solution employs a simple reduction-ae-atio2 procedure to produce and introduce mercury vagor into a closed system where the absorption of the 2537A line is mesisured in a quartz windowed cell. Several spectrophotometric reagents have been employed for the determination of traces of mercury ( I ) . Of these, dithizone is the most populer because it exhibits a high sensitivity. Many elements interfere and various separations and complexing agents are required in the analysis of materials containing heavy metals. Conventional atomic absorption techniques using a flame lack the sensitivity that is required for trace analytical work (15 ppm for 1% Abs). Addition of a reducing agent to the solution improves the sensitivity slightly. Several instruments are available for the atomic absorptiometric determination of mercury vapor (2, 3). These range from small portable units to the elaborate instruments used for very low concentrations of mercury. The mercury vapor is produced by direct heating of the sample, thus limiting their application to solid samples. The following procedure requires a commercial atomic absorption spectrophotometer with an otherwise simple apparatus to obtain extremely sensitive and accurate results for mercury down to 1.0 ppb in solution. The procedure is described for nickel and cobalt analysis, and has also been applied to rock analysis and soils containing organic matter. The method should find applications in biological analysis, where traces of mercury are present in aqueous solutions. EXPERIMENTAL
Excellent results have been obtained in the determination of mercury by the procedure of Kimura and Miller ( I ) who employ an aeration step in the separation of mercury prior to its determination with dithizone. Attempts were made to measure the mercury aerated from various solutions by passing the vapor through a quartzwindowed cell. The effects of different acids and several (1) Y. Kimura and V. L. Miller, Anal. Chim. Acta, 27, 325 (1962). (2) W. W. Vaughn and J. H. McCarthy Jr., U. S. Geol. Surcey Prof. Paper 501-D, p p D 123-127 (1964). (3) A. R. Barringer, Institution of Mining and Metallurgy, 75, Bull No. 714, May (1966).
Figure 1. Apparatus used (I) Absorption cell (2) Reaction flask (3) Flask containing drying agent (4) Disconnect for bleeding mercury from system (5) Neptune Dyna-Pump
reducing agents were investigated. The presence of hydrochloric acid caused high absorbance values which may be due to its high mercury content (4). Aeration was satisfactory from dilute perchloric acid as well as dilute nitric and sulfuric acid solutions. Hypophosphorous acid is suitable for reduction of the mercury but contributes a small blank value. For nickel and cobalt samples, nitric and sulfuric acid were found most satisfactory with stannous sulfate as reducing agent. Reduction was carried out in a sodium chloridehydroxylamine sulfate medium. Very low blank values were observed using these reagents. Apparatus. ATOMIC ABSORPTIONSPECTROPHOTOMETER. Perkin-Elmer Model 303 equipped with Automatic Null Recorder Readout. Other commercial units having an open sample presentation area to mount the absorption cell could be used. MERCURY HOLLOWCATHODELAMP. Westinghouse WL22847, argon filled. RECORDER.Sargent Model SRL used in the logarithmic mode for direct absorbance measurement. ABSORPTION CELL. Constructed from borosilicate glass tubing, 25-mm 0.d. X 15 cm. The ends were ground perpendicular to the longitudinal axis and quartz windows (25mm diameter X 2-mm thickness) were cemented in place. Gas inlet and outlet ports were attached approximately 2 cm from each end. VARIAC,0 to 130 volts, for controlling pump speed. NEPTUNEDYNA-PUMP,Model 3. The pump was disassembled, valves and diaphragm were removed, and all metallic surfaces were sprayed with a clear plastic to resist corrosion. When dry, the unit was reassembled. AERATION TUBING. Borosilicate glass of 5-mm i.d. FLASK containing 10 grams of magSUCTIONFILTRATION nesium perchlorate. The apparatus is assembled as shown in the accompanying (4) E. B. Sandell, 3rd Ed., “Colorimetric Determination of Trace Metals,” Interscience, New York, 1959, p 625. VOL. 40, NO. 14, DECEMBER 1968
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Table I. Variation in Mercury Vapor Absorbance with Time Time, seconds Absorbance 0.000 0.108
0
24 48 72 96
0.198 0.245 0.268 0.282 0.290 0.292 0.292 0.292 0.292
120
144 168 192 216 240
Table 11. Calibration and Analysis of Nickel and Cobalt Samples Sample Blank Blank Blank Blank Blank Nickel (1.0 g) Nickel (1.0 g) Cobalt (1.0 g) Cobalt (1.0 g)
Hg
added, pg 0.00 0.30 0.60 1.00
2.00 0.00 1.00 0.00 1.00
AbHg sorbance found, pg 0.002 0.090 0.175 0.268 0.440 0.040 0.12 0.305 1.16 0.020 0.07 0,270 1.03
Parts per billion
120.0 70.0
photograph. Tygon tubing is used for passage of the mercury vapor with a disconnect in the line for bleeding mercury from the closed system. The cell is aligned in the light beam by the use of two 2-inch by 2-inch cards. Twenty-five-millimeter holes are cut in the middle of each card and placed over each end of the cell. The cell is then positioned and adjusted vertically and horizontally to give the maximum transmittance. Reagents. STANNOUS SULFATE, 10 w/v solution in O.5N sulfuric acid. SODIUM CHLORIDE, 30% wiv solution. HYDROXYLAMINE SULFATE, 25 % w/v solution. SODIUMCHLORIDE-HYDROXYLAMINE SULFATESOLUTION. Transfer 60 ml of the 25% hydroxylamine sulfate and 50 ml of the 30% sodium chloride solutions to a 500-ml volumetric flask and dilute to the mark with water. POTASSIUM PERMANGANATE, 5 % w/v solution. STANDARD MERCURY SOLUTION, 0.100%. Dissolve 0.1354 gram of mercuric chloride in 100 ml of 1Nsulfuric acid. From this solution prepare by dilution a 0.001% solution in 1N sulfuric acid. Dilute solutions should be prepared fresh daily.
Instrument Settings. Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer with Automatic Null Recorder Readout Lamp Current 10 mA Wavelength 2537 A (UV) Slit Setting 3 x1 . Scale Noise Suppression 2 Recorder Settings Logarithmic Mode Scale Selector-mV Range-10 mV Chart Speed 0.5 inch per minute Calibration. Transfer aliquots of the standard mercury solution containing 0.2 to 2.0 pg of mercury to 250-ml roundbottom flasks. Add 25 ml of 18N sulfuric acid and 10 ml of 7N nitric acid to each and dilute to 100 ml. Treating each sample individually, add 20 ml of the sodium chloride-hydroxylamine sulfate solution, followed by 10 ml of the stannous sulfate solution. Immediately attach the flask to the aeration apparatus forming a closed system. Turn on the circulating pump and adjust the aeration rate using the Variac. The absorbance value will increase and reach a constant value within a period of 3 minutes. Open the system at the disconnect and continue the aeration until the absorbance returns to its minimum value. The absorbance readings thus obtained are plotted against micrograms of mercury to establish the calibration curve. Absorbance values obtained on aeration of 1.1 pg Hg from solution are given in Table I. A maximum value is reached after 3 minutes at a circulation rate of approximately 2 liters per minute. Procedure. Transfer 1.0 gram of nickel or cobalt metal to a 250-ml round-bottom flask. Add 10 ml of 7 N nitric acid and dissolve without the application of heat. Add 25 ml of 18N sulfuric acid and dilute to 100 ml. Cool to 20 “C and add 20 ml of mixed sodium chloride-hydroxylamine sulfate solution. Add 10 ml of stannous sulfate and immediately attach to the aeration apparatus. Aerate the solution according to the directions given under “Calibration curve” obtaining the absorbance of the mercury vapor. A variation of this procedure is used for the analysis of rock samples. Treat 1 to 4 grams of finely-ground material with 25 ml of concentrated sulfuric acid in a 250-ml round-bottom flask. Carefully make three, 1-ml additions of 50% hydrogen peroxide solution to the flask, allowing sufficient time for the decomposition of peroxide between additions. Heat the flask gently to decompose any remaining peroxide. Cool to 20 “C. (5) M. Fleischer, Geochim. Cosmochim. Acta, 29, 1271 (1965).
Table 111. Results Obtained on U.S.G.S. Rocks. and Falconbridge Secondary Rock Standard “A” Mercury-Parts per Million Neutron Atomic Sample Proposed method activation (5) absorption ( 2 ) Dithizone spectrophotometryb 0.13 0.090;0.070; 0.095 0.11; 0.08 0.34 61 0.34 0.17; 0.16 0.17 0.18; 0.18 w-1 0.36;0.33; Falconbridge Secondary Rock 0.36;0.36; Standard “A” 0.39; 0.33; 0.32~
Standard Rock Samples-Granite (G-1) and Diabase (W-1) issued by the United States Geological Survey. b Analysis performed at Falconbridge, Thornhill. c Mean 0.35 Standard deviation 0.024 a
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Carefully add 100 ml of water and 5Z potassium permanganate solution until a permanent pink color is obtained. Cool the solution to 20 “C and continue as in the above procedure adding the hydroxylamine sulfate-sodium chloride solution and the stannous sulfate solution. RESULTS AND DISCUSSION
The values obtained in establishing the calibration curve and the results obtained from the analysis of nickel and cobalt samples are given in Table 11. Table I11 lists the values obtained on U. S. Geological Survey rocks G-1 and W-1 using the modified procedure. Results obtained in this laboratory using the dithizone procedure of Kimura and Miller ( I ) and other available data are also shown.
Also shown in Table I11 are the results of seven determinations performed on a finely-ground rock sample used as a secondary standard. From these data the precision of the method was calculated. There are few interferences under the conditions described. The procedure cannot be applied to metal samples such as copper, which is easily reduced, thus preventing the complete aeration of mercury. Large amounts of elements such as tellurium, which are easily reduced to their elemental state, cause low results by coprecipitating some of the mercury. Usually this interference can be detected by visual inspection of the solution after the addition of stannous sulfate. Normally these elements are not present in sufficient quantity to cause interference. RECEIVED for review May 27, 1968. Accepted July 18, 1968.
Enhancement of Zirconium Atomic Absorption by Nitrogen-Containing Compounds and its Use in the Determination of Ammonia A. M. Bond Department of Inorganic Chemistry, Unicersity of Melbourne, Parkville, Victoria 3052, Australia J. B. Willis Diaision of Chemical Physics, CSIRO Chemical Research Laboratories, P. 0. Box 160, Clayton, Victoria 3168, Australia The absorption by zirconium atoms in the nitrous oxide-acetylene flame is enhanced in the presence of many nitrogen-containing compounds which can act as Lewis bases. For ammonia the magnitude of this enhancement is proportional to the concentration of base over the range 1 X l F 4to 5 X 10-aMand can be used to determine ammonia in the absence of phosphate and of certain nitrogen-containing compounds. When interfering species are present the method can be used, after prior distillation of the ammonia, to replace spectrophotometric methods which require considerable time for color development. The analytical usefulness of the enhancement technique is demonstrated by the determination of ammonia in biological systems.
AMOSAND WILLIS(1) observed that fluoride ion enhanced the absorption by zirconium atoms in the nitrous oxide-acetylene flame, and Bond and O’Donnell (2), while investigating this effect as a method for determining fluoride, found that ammonium ion interfered by causing an enhancement similar to that produced by fluoride. This suggested that the atomic ab sorption technique might be applied to the determination of ammonia and be a useful supplement to existing methods, most of which require prior distillation of the ammonia and time-consuming color development before making spectrophotometric measurements (3). In order to investigate further the principles behind the enhancement of zirconium absorption by ammonia, a range of other nitrogen-containing compounds was also investigated. (1) M. D. Amos and J. B. Willis, Spectrochim. Acta, 22, 1325, 2128 (1966). (2) A. M. Bond and T. A. O’Donnell, ANAL. CHEM.,40, 560 (1968). (3) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Vol5, Interscience, New York, 1961, pp 279-85.
EXPERIMENTAL
Ammonium chloride (B.D.H. “AnalaR” quality) and zirconium oxychloride octahydrate (B.D.H. “L.R.” quality, 98.5z ex. C1 analysis) were assumed to be stoichiometric. Other nitrogen-containing compounds were of analytical reagent grade wherever available, and if any doubts existed about purity the materials were purified by distillation or recrystallization, as appropriate. Atomic absorption measurements at the 3601.2A line of zirconium were made with an AA-4 or AA-100 atomic absorption spectrophotometer (Varian Techtron Pty. Ltd., North Springvale, Victoria 3170, Australia) fitted with a 50- X 0.5-mm slot burner for use with nitrous oxide-acetylene mixtures. Absorption was measured in a fuel-rich, slightly luminous flame having an interconal zone about 30 mm high. ESH.%\CE\IENT OF ZIRCONIU\I ABSORPTION BY SITROGEN-COSTz\ISISG COMPOUSDS
Figure 1 shows the effect of a number of nitrogenous compounds on zirconium absorption. The absorbance at first increases linearlj, with the concentration of compound, and then flattens out to a plateau when the molar ratio of nitrogenous compound to zirconium exceeds a value of about four. The solutions measured contained 0.8M hydrochloric acid to keep the nitrogenous compounds in solution and 0.006M potassium chloride to suppress the ionization of zirconium, which is appreciable in the nitrous oxide-acetylene flame (4). Table I shows the results of semi-quantitative survey of a wider range of compounds; in general, the most basic compounds show the strongest enhancement of zirconium absorption, which seems to depend principally on the availability of an unshared electron-pair at the nitrogen atom. (4) D. C. Manning, Atomic Absorption Newsletter, 5, 127 (1966). VOL. 40, NO. 14, DECEMBER 1968
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