the resulting signal decreases accordingly. Coating the inner surface of the furnace with a thermally-stable carbide precludes subsequent carbide formation by the sample and enhances the atomization efficiency and resulting signal accordingly. The similarity in results obtained for beryllium, chromium, manganese, and aluminum indicates that the competitive processes of atomization and carbide formation may occur for a large number of other elements and that the detection limit for these elements should be improved by treating the furnaces of the HGA-70 type with an element that forms a non-volatile carbide.
At sufficiently high temperatures the carbides formed by the sample-furnace interaction should be thermally decomposed and the element atomized. However, the process of carbide formation, followed by thermal dissociation, reduces the peak height and detection limit accordingly. This suggests that both maximum atomization temperature and the rate a t which maximum temperature is attained affect the response and, consequently, the detection limit.
CONCLUSIONS
T. S. West, PureAppl. Chem., 26, 27 (1971). J. W. Robinson and P. J. Slevin, Amer. Lab., 4, 8 (1972). G. F. Kirkbright, Analyst(London), 96, 609 (1971). C. W. Fuller, Proc. Soc. Anal. Chem.. 9, 279 (1972). (5) H. L. Kahn and J. D. Kerber, J. Am. OilChem. SOC.,48, 434 (1972). (6) J. Kleinberg, W. J. Argersinger, Jr., and E. Griswaid, "inorganic Chemistry", D. C.Heath and Company, Boston, 1960, Chap 13. (7) T. Moeller, "Inorganic Chemistry", John Wiley and Sons, Inc., New York and London, 1952 Chap. 16. (8) B. V. L'vov, "Atomic Absorption Spectrochemical Analysis", Adam Hilger. London, 1970, Chap. V. (9) G. D. Renshaw, At. Absorption News/., 12, 6 (1973). (IO) D. C. Manning and F. J. Fernandes, At. AbsorptionNewsl., 9, 65 (1970). (11) F. J. Fernandes, At. Absorption News/., 11, 123 (1973). (12) R. Muzzarelii and R. Rocchetti, Anal. Chlm. Acta, 64, 271 (1973). (13) S.Henning and T. L. Jackson, At. Absorption News/., 12, 101 (1973).
The detection limit for some elements can be substantially improved by treating the furnace with an element that forms a thermally stable carbide when heated to elevated temperatures in the presence of carbon. The net result of the furnace treatment procedure is that perturbation of the atomization process by sample-furnace interaction is eliminated or reduced to an insignificant level. Therefore, atomization efficiency increases the peak height and improves the detection limit accordingly. The improvement in detection limit is most apparent for those elements that are volatile a t the furnace atomization temperature and which react with the furnace to form carbides that are thermally stable a t that temperature.
LITERATURE CITED (1) (2) (3) (4)
RECEIVEDfor review December 13, 1974. Accepted March 26, 1975.
Analysis of Petroleum for Trace Metals1-Determination of Mercury in Petroleum and Petroleum Products Henry E. Knauer Analytical Section, Mobil Research a n d Development Corporation, Paulsboro, NJ
George E. Milliman Analytical & Information Division, Exxon Research and Engineering Company, Linden, NJ
Two methods based on decomposition and cold-vapor atomic absorption have been developed for the determination of mercury in petroleum and petroleum products down to a concentration level of 5-10 ng/g. One method involves acid decomposition of an oil sample in a closed system, and the other uses Wickbold oxy-hydrogen combustlon to decompose the oil. The standard deviation of the acid decomposition procedure is 3 ng/g at the 42 ng/g level. That of the Wickbold combustion procedure is 4 ng/g over the range 20-100 ng/g. An interlaboratory cross-check program showed that a good deal of experience with either method was required to obtain reliable results. In each case, the initiating laboratory obtained accurate results.
C o n t r i b u t i o n o f Trace M e t a l s P r o j e c t P a r t i c i p a t i n g L a b o r a t o ries: A t l a n t i c R i c h f i e l d Company, Harvey, Illinois; Chevron Research Company, Richmond, California; E x x o n Research a n d Engineering Company, L i n d e n , New Jersey; M o b i l Research a n d Development Corporation, Paulsboro, New Jersey; a n d P h i l l i p s P e t r o l e u m Company, Bartlesville, Oklahoma.
The accurate determination of mercury a t the parts-perbillion (ng/g) level in organic materials has become increasingly important as its potential hazard to health has become more broadly recognized. Although much has been published on the occurrence of mercury in coal, blood serum, fish and grain (1-91, and traces of mercury have been found in natural gas from certain wells in The Netherlands and Germany (10, I I ) , little is known about the concentration of mercury in petroleum. Mercury has been reported to be present in certain crude oils in amounts ranging from 20 parts-per-billion to, in isolated cases, the parts-per-million level (12, 13).These data are based on neutron activation studies, but a method that is more readily implemented with generally available laboratory equipment has been needed to provide more comprehensive information. Wet digestion and various combustion techniques have been used to decompose organic materials prior to obtaining the mercury in aqueous solution for final measurement (3, 6, 8, 14-17), but the materials have generally been relatively nonrefractory andlor contained much higher concentrations of mercury than are likely to be found in petroANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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leum. This paper describes two methods that can be carried out with equipment available in most laboratories and which can be used to determine mercury a t concentration levels down to 5-10 ng/g. One method involves digesting the sample with sulfuric and nitric acids in a closed system which prevents the loss of mercury during sample decomposition (18). The other method is based on burning the sample in a Wickbold oxy-hydrogen combustion apparatus and collecting the volatilized mercury in acidic permanganate solution. In both cases, the mercury in the resultant aqueous solution is determined by an adaptation of the Hatch and Ott cold vapor atomic absorption procedure (19).
EXPERIMENTAL Apparatus. All glassware is cleaned with 1:l aqueous nitric acid. W e t Oxidation Decomposition (Figure 1). The digestion apparatus consists of a 500-ml, 2-neck flask; a 75-ml distillation receiver equipped with a 12.5 mm-o.d., medium-wall side arm and a 2-way Teflon stopcock; a 60-ml addition funnel; and a 300-mm Friedrichs condenser. All ground-glass joints are lubricated with concentrated sulfuric acid. Wickbold Combustion A p p a r a t u s fFigure 2). This consists of a 250-ml sample reservoir, a stainless steel burner, quartz combustion chamber, gas pressure regulators for oxygen, nitrogen, and hydrogen, and an absorber fitted with an extra-coarse frit. The unit is manufactured by the Koehler Instrument Co., Bohemia, NY. The apparatus should be used behind a protective shield. Mercury A t o m i c Ahsorption Cell a n d Aspiration S y s t e m . The apparatus used in conjunction with the wet oxidation procedure (Figure 3) consisted of a 200- by 22-mm diameter borosilicate absorption cell with quartz windows, an oscillating plastic-lined pump, a variable transformer, bubbler, a 300-ml pear-shaped flask, and a water knock-out trap constructed from a 16- by 100-mm test tube that is fitted with an impinger and filled with glass wool. Connections were made with Tygon tubing. The apparatus used with the Wickbold combustion procedure was similar except that the absorption cell was 100 mm long and 25 mm in diameter. The circulating pump was omitted. One end of the cell was open to the atmosphere while the other was connected to the aspiration system used to sweep mercury directly out of the Wickbold absorber. Air was regulated to flow through the aspiration system a t a rate of 1 l./minute. Air was turned off by means of a two-way stopcock at the inlet side of the absorber. A t o m i c Absorption Spectrometer. A Perkin-Elmer Model No. 403 AA spectrometer, Perkin-Elmer Corporation, Norwalk, CT, was used in the development of the methods. The operating parameters for the spectrometer are given in Table I. Reagents. All water was deionized before use. Concentrated sulfuric, nitric, and hydrochloric acids designated as “suitable for mercury determinations” were obtained from J. T. Baker, Phillipsburg, NJ. Potassium permanganate, sodium chloride, mercury(I1) chloride. and hydroxylamine hydrochloride were ACS reagent grade. Hydroxylamine sulfate was 99+% from Matheson, Coleman and Bell, Norwood, OH; a 12% solution in water was prepared. Tin(I1) sulfate was purified grade obtained from Fisher Scientific, Pittsburgh, P A , and was used as a 10% solution in 0.6N HC1 or 0.2N HzS04. n-Hexane was obtained from Burdick and Jackson, Muskegon, MI. Diphenyl mercury was obtained from Eastman Kodak, Rochester, NY. Benzene and isopropanol were ACS re1264
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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Figure 2.
Wickbold combustion apparatus
Table I. Instrumental Settings for the Determination of Mercury by Cold-Vapor Atomic Absorption Spectrometry Using a Perkin-Elmer No. 403 Spectrometer Slit: Range : Wavelength: Lamp:
5 (3 n i m )
uv 253.7 n m , peaked Hollow c a t h o d e m e r c u r y vapor, P e r k i n E l m e r No. 303-6044, o p e r a t e d at 10
mA . Mode : Concentration Concentration dial: 110: set s o t h a t 1 n g of m e r c u r y g i v e s 0001 on t h e d i g i t a l r e a d o u t . T h i s c o r r e s p o n d s to a n a b s o r b a n c e of a p p r o x i m a t e l y 0.0003 Function: 10 a v e r a g e . r e p e a t agent grade. The absorbing solution used in the Wickbold combustion method was prepared by dissolving 2 g of potassium permanganate in 90 ml of water and then adding 5 ml each of concentrated nitric and sulfuric acids. This solution was prepared directly in the Wickbold absorber, just prior to a combustion. Stock aqueous mercury standard, 1.0 mg/ml, was prepared by dissolving 0.1354 g of dried mercuric chloride (HgC12) in 100 ml of 3:97 sulfuric acid. A 0.1 kg/ml solution was prepared by diluting the stock mercury solution with water. This dilute standard was freshly prepared immediately before use. Nonaqueous mercury standard, 1.0 mg/ml, was prepared by dissolving 0.1750 g of diphenyl mercury in 90 ml of isooctane and 10 ml of toluene. For the Wickbold combustion method, a blank solution consisting of 50 ml of 4:l benzene-isopropanol was prepared. S a m p l e s for Interlaboratory Cross-Check Program. Diesel fuel, No. 2 heating oil, and South Louisiana crude, each containing 42 ng/g of added mercury, and Nigerian, Light Arabian, and South Louisiana Crude oils containing 20, 50, and 100 ng/g of added mercury, respectively, were prepared by mixing the appropriate volume of the diphenyl mercury standard with 1000-1200 g of sample. Procedure. Acid Digestion Procedure. This procedure should be performed behind a shield. Add 25 ml of concentrated sulfuric acid to 5 g of sample in the digestion apparatus (Figure 1).Stir the solution and add 50 ml of concentrated nitric acid dropwise through the addition funnel. After addition of nitric acid is complete, gradually raise the temperature of the solution and reflux for one hour until fumes of nitrogen oxides cease to evolve. Evaporate the mixture to fumes of sulfuric acid and collect the volatile components in the distillation receiver. Fume for 5 minutes and then add, dropwise, 25 ml of concentrated nitric acid. Continue to collect distillate until all the nitric acid has been added and fumes of sulfuric acid are generated in the flask. Allow the solution to fume for 5 minutes. Return the distillate dropwise to the flask. After all the distillate has been returned, reflux for 15 minutes. Repeat the distillation once more and then allow the solution to cool. Wash
the interior of the condenser and distillation receiver with approximately 100 ml of water. Collect the washings in the flask. Extract the solution with three 100-ml portions of n-hexane to remove water-insoluble, undigested organic matter. Bubble nitrogen through the extracted solution until no bubblets of n- hexane are visually observed. Avoid prolonged sweeping of the mixture with nitrogen to minimize mercury loss. Add dropwise, and with stirring, 20 ml of a solution containing 1 2 g of sodium chloride and 12 g of hydroxylamine sulfate in 100 ml of water. Stir the solution until evolution of gas is complete, and then sweep the air space above the solution with nitrogen. Transfer the solution to the pearshaped flask of the cold-vapor apparatus and adjust the volume to 250 ml with water. Measure the mercury content of this solution as described below. Reagent Blank Solutions. Prepare three reagent blanks. BLANK1. Add 5 ml of 5% potassium permanganate solution, 25 ml of concentrated sulfuric acid, and 75 ml of concentrated nitric acid to 150 ml of water. Heat the solution on a hot plate until most of the water and nitric acid have evaporated. Cool the solution, dilute to 100 ml with water, and shake with three 100-ml portions of n- hexane. Process the solution as described above. BLANK2. Add 5 ml of 5% potassium permanganate and 20 ml of 12% hydroxylamine sulfate solutions to 230 ml of water. BLANK3. Add 20 ml of 12% hydroxylamine sulfate to 230 ml of water. Calibration Standards. To a series of pear-shaped flasks containing 250 ml of 1:9 sulfuric acid, add separate aliquots of the 0.1 Kg/ml mercury standard to cover a range of 40 to 1000 ng. Wickbold Combustion Procedure. The designated sample size is for samples estimated to contain less than 100 ng of mercury per gram. Smaller samples should be taken if higher concentrations of mercury are expected. For crude oils and heavy fractions, weigh 20 g of oil into a 250ml beaker and add 40 ml of benzene and 10 ml of isopropanol. Dissolve the sample completely and transfer the solution to the sample reservoir of the Wickbold apparatus. Light distillate fractions may be analyzed directly. Adjust the oxygen regulator to 9 psi and the hydrogen regulator to 3 psi. Immediately light the burner with the induction coil. Attach the combustion chamber to the burner. Prepare the absorbing solution in the absorber, attach the spray trap, and connect the absorber to the outlet of the combustion chamber with a clamp. Carefully increase the nitrogen pressure on the sample reservoir until the flame is several inches long (nitrogen pressure will be about 4 psi) and burns cleanly without soot formation. Burn the entire sample. Just before the last droplets of sample are discharged into the flame, momentarily discontinue the nitrogen flow and rinse the reservoir with 10 ml of benzene. (Otherwise, sputtering of the last drop of sample causes troublesome soot formation.) Turn the nitrogen flow back on, and continue to burn until just before the last droplets of wash solution enter the flame. Then turn off, in order, the nitrogen, hydrogen, and oxygen. Remove the absorber, and wash the spray trap and the cooled combustion chamber with water, adding the washings to the solution in the absorber. Burn the blank solution (50 ml of 4:l benzene-isopropanol) in the same manner. Measure the mercury concentration in the blank solution and the sample solutions by the procedure described below. T o ensure complete recovery of the mercury, all of the color due to permanganate and manganese dioxide must be discharged by the hydroxylamine hydrochloride before the divalent tin is added. If preferred, sample solutions may be accumulated and the atomic absorption measurements made in a group. However, a separate calibration is required for each group of measurements. Calibration. Add aliquots of the 0.1 wg/ml mercury standard to a series of Wickhold absorber vessels, each containing 100 ml of the absorbing solution, so that the mercury content ranges from 100 to 2000 nanograms. Add sufficient hydroxylamine hydrochloride solution to reduce the potassium permanganate (usually 10-20 ml). Measure the absorbance due to the mercury in each solution by the procedure described below. Plot the best straight line relating absorbance to ng of mercury standards, and extrapolate to 0 ng of mercury added (blank). Draw a parallel line passing through the origin to serve as a calibration curve. The response is linear up t o 2000 ng. Cold-Vapor Measurement of Mercury. Place the absorption cell in the beam of the hollow-cathode mercury-vapor lamp. Optimize the spectrometer to measure mercury using the operating parameters shown in Table I as a guide. Assemble the remaining apparatus shown in Figure 3 , except the pear-shaped flask. Add 10
Figure 3. Apparatus for cold-vapor mercury absorption measure-
ment ml of 10% tin(I1) sulfate in 0.2N sulfuric acid to the solution in the pear-shaped flask and rapidly connect the flask to the assembled apparatus. Turn the pump on to circulate the mercury vapor through the cell. The absorbance will stabilize after approximately 5 min. Record the absorbance. Switch the pump off and disconnect the tubing connected to the inlet side of the pump. Empty the pear-shaped flask, wash with 1:1 nitric acid, and fill with 250 ml of 1:9 sulfuric acid and 10 ml of tin(I1) sulfate solution. Reconnect the flask to the apparatus. Switch on the pump and vent the elemental mercury in the system to a hood. After all the mercury vapor is exhausted, approximately 10 min, reconnect the tubing to the pump. After the absorbance stabilizes (approximately 6 min.) record the zero reading. Subtract the zero reading from the sample solution absorbance. Measure the absorbance of blank solutions 1, 2, and 3 and the calibration standards. Response is linear over the range of 40 to 1000 nanograms. In most of the measurements of mercury in solutions from the Wickbold combustion, the above circulating procedure was modified as follows: Place each absorber in turn in the aspiration train. Add 10 ml of tin(I1) sulfate solution and immediately connect the air supply to the inlet of the absorber. Note the gradual increase in absorbance as the mercury is aspirated out of the solution, and record the maximum. Continue aspirating until the absorbance is again zero. Calculation. Obtain the amount of mercury in each of the solutions from the calibration graph, subtracting an appropriate blank. In the acid digestion procedure B = B1 - (B2 - B 3 ) , where B is the blank to be applied to the sample, and B1, Bp, and B3 are the nanograms of mercury in blank solutions 1, 2, and 3, respectively.
RESULTS AND DISCUSSION Acid Digestion Procedure. All glassware had to be thoroughly cleaned to remove traces of mercury before analyses were begun. Nitric acid washing of glassware not previously used for mercury determinations was often not sufficient to remove all traces of mercury. However, after using the glassware for one or two digestions of a mercuryfree oil, e.g., white oil, all traces of mercury were removed. After this initial treatment, washing with 1:1 nitric acid was usually sufficient t o remove traces of mercury. Several authors report that mercury and mercury compounds are easily lost from aqueous and acidic solutions (20-22). The closed-system digestion apparatus of Gorsuch ( 1 8 ) ,modified as shown in Figure 1, was adopted to minimize mercury losses. An addition funnel was necessary because two separate additions of nitric acid were required in the decomposition of oils. To avoid flooding of the distillation receiver side arm if light or narrow boiling-range petroleum distillates were being digested, the side arm was enlarged to the size indicated in Figure 1. When low-boiling hydrocarbons were digested, some organic material remained. Since this material could contain compounds absorbing a t 253.7 nm, and interfere with mercury measurements, the digestion mixture was extracted with hexane prior to measurement. After extraction, no broad band absorption was observed. Measurement of Mercury. Many systems for the coldvapor atomic absorption determination of mercury in aqueous solutions have been described (16, 17, 23-31). The adaptation of the Hatch and Ott apparatus shown in Figure 3 performed satisfactorily and was described in the acid digestion procedure. The speed of the recirculating pump can ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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Table 11. Recovery of Mercury Added to Various Oils" Using Various Mercury Compounds (Acid Digestion Procedure) 139 ny 4 011
Added
Found
Hg a d d e d as
Crude oil A
50 25 15 40 25
46, 55 32, 32, 1 4 12 32 17, 21, 22
13
9, 11, 15
Diphenyl mercury Diphenyl mercury Diphenyl mercury Mercury octanoate NBS mercury in coal standard 1630 NBS,mercury in coal standard 1630
Fuel oil White oil
Unspiked Oils were found to contain less than 10 ng Hg/g oil.
he varied within wide limits with little effect on the absorbance of a given amount of mercury. A simple bubbler was used to sweep the mercury vapor from the flask because the pressure drop across it was much less than across a sintered glass bubbler. The pump could then be operated under relatively mild conditions prolonging its life. Other adaptations of the cold-vapor measuring technique, such as the noncirculating measurement used in the development of the Wickbold combustion method, served equally well. Mercury Content of the Reagents. The mercury content of the reagents had to be measured indirectly because it was impossible to generate elemental mercury from a solution of 1:3 nitric acid using tin(I1) sulfate. Furthermore, Gorsuch (18) showed that mercury could not be separated from nitric acid by distillation. However, mercury was not lost from a mixture of concentrated nitric and sulfuric acids when potassium permanganate was added and the solution heated to remove water and nitric acid. This was confirmed by adding 200 ng of mercury to the acid mixture and recovering 205, 193, and 198 ng of mercury in three separate experiments. Since potassium permanganate is not used in the digestion of oils, its mercury content must be measured separately. The total mercury content of the reagents was found to be about 60 ng. Recovery of Mercury from Oils. Evaluation of the accuracy of the method was difficult because no certified oils of low-level mercury content were available, the type of mercury compounds which might occur in petroleum was unknown, and recovery of mercury may be dependent upon sample composition. The data presented in Table I1 indicate that recovery of mercury from oils seems unrelated to the type of hydrocarbon or mercury compound. Some of the results in Table I1 were obtained from samples to which coal, certified to contain 127 ng Hg/g by the U.S. National Bureau of Standards, was added. While mercury may not be present in petroleum in the same form as it is in coal, the results indicate that mercury associated with a fossil fuel is recovered using this procedure. Wickbold Combustion Procedure. Blank. In this procedure, the amount of mercury in the reagents making up the absorbing solution could be measured directly. The sulfuric-nitric acid solution containing hydroxylamine hydrochloride and stannous chloride, but not the permanganate, was found to contain 40 ng of mercury. The acids plus 2 g of potassium permanganate was found to contain 160 ng of mercury, indicating that the potassium permanganate was the main source of mercury in the blank. I t is therefore essential that each new bottle of permanganate be thoroughly checked for its mercury content. The blank absorbance of the absorbing solution could be subtracted from the absorbance of the standard solutions 1266
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Table 111.Mercury in Petroleum Samples by Wickbold Combustion Method Sample
Isooctane Lt. Arabian Lt. Arabian Lt. Arabian Lt. Arabian
crude crude crude crude
A d d e d , ng!g
oil oil oil oil
None None 23 1020 1030
Found,
"414
