Determination of Trace Quantities of Selenium in Petroleum and Petroleum Products by Atomic Absorption Spectrometry H. H. Walker* Mobil Research and Development Corporation, Paulsboro, N.J. 08066
J. H. Runnels and Ruth Merryfield Phillips Petroleum Co., Bartlesville. Okla. 74004
A method is described for determinlngselenium in petroleum and petroleum products at or above the 10 ng/g level. The sample matrix is destroyed by acid digestion under reflux, and the selenium is measured by hydrogen selenide generationatomic absorption using either a flame-heated Vycor furnace or a hydrogen-argon-air flame. The method was evaluated in two cooperatlng laboratories, and the results were compared to those obtained by a third laboratory using neutron activation.
Selenium in trace quantities is an essential dietary requirement for rats, livestock, and other mammals (1-4). In larger concentrations selenium is toxic, and its harmful effect on cattle and other livestock that graze on plants grown in seleniferous soil has long been recognized. In nature selenium is frequently associated with sulfur, a common component of petroleum. Since large quantities of petroleum are consumed annually to meet the energy requirements of the nation, and since small amounts of selenium may have environmental effects, the selenium content of petroleum has attracted growing attention. Analytical methods for the determination of selenium in a variety of materials have been reported (3, 5-10). Redox titrations have been used for milligram quantities, while photometric and fluorometric methods have been applied to lesser amounts (1,5, 7,8,11). In recent years atomic absorption has been widely used (3, 4, 6, 10, 12-14), primarily because of the high sensitivity that can be obtained by the hydride generation technique and by the general availability of atomic absorption facilities. Macro amounts of selenium in crude oil have been determined gravimetrically as the element by precipitation with various reducing agents (9,15). Smaller amounts have been determined by neutron activation (16-18); however, this technique is not readily available in most laboratories and, as a result, only limited data are available. This paper describes a method for measuring selenium down to 10 ng/g in petroleum and petroleum products. The method is reasonably straightforward and can be utilized in any laboratory with atomic absorption capability. The sample matrix is destroyed by acid digestion under reflux and the selenium is measured by hydride generation-atomic absorption. Either a hydrogen-argon-air flame or a flame-heated Vycor furnace may be used for the measurement.
EXPERIMENTAL Instrumentation. A Jarrell-Ash Model 82-546 atomic absorption spectrometer and a Perkin-Elmer Model 403 spectrophotometer were used in the development of the method. The Jarrell-Ash spectrometer was equipped with a Varian-Techtron optical rail and burner assembly, and the monochromator was elevated to provide straightthrough optics; a Vycor furnace was supported in the optical path of the spectrometer with clamps and lens mounts (Figures 1and 2). The furnace was heated with a hydrogen-argon-air flame, and the hy2056
drogen selenide from the generating flask was introduced directly into the furnace. The Perkin-Elmer spectrophotometer was equipped with a deuterium background corrector and a 3-slot premix burner. The hydrogen selenide generator was connected to the auxiliary oxidant port of the burner, and the selenium was measured with a hydrogen-argon-air flame. In both cases, the resultant signal was recorded on a strip chart recorder, The parameters established to provide maximum selenium response for the two instruments are presented in Table I. Hydrogen Selenide Generators. Two different generating systems were developed for the different instruments. The generator developed for use with the Vycor furnance measurement system is shown in Figure 3. The generator is connected to the Vycor furnance with a Tygon tube. The 90' stopcock allows the generating flask to be purged with argon and when rotated provides a means of introducing zinc dust into the generating solution. The extra coarse frit serves as a spray trap and prevents large droplets of spray from being introduced into the Vycor furnace. The hydrogen selenide generator that was developed for use with the Perkin-Elmer hydrogen-argon system is shown in Figure 4. The generator is connected to the burner with Tygon tubing. A plastic tube partly filled with loosely packed glass wool serves as a spray trap that prevents spray droplets from being introduced into the burner. Zinc dust is introduced by means of a pinched-off Tygon tube. Sample Preparation Apparatus. The apparatus used to decompose the samples is shown in Figure 5 . Basically it is a 300-ml Kjeldahl flask with a ground-glass joint and fitted with a water-cooled reflux condenser. A heating mantle and 140-volt auto-transformer are provided to regulate the heat, and the contents of the flask are stirred by means of a magnetic stirrer. At several points during the decomposition step, the condenser is removed from the flask and the heating mantle and flask are separated. Consequently, sufficient flexibility should be provided to allow for these operations. Reagents a n d Standards. Reagent-grade mineral acids were generally purified by sub-boiling distillation prior to use. Purification was necessary because some lots of acids gave a high selenium blank. Other lots of the same acids gave lower blanks, whiIe still other lots could be used without further purification. Purification was accomplished by the general sub-boiling distillation procedure described by Kuehner (19).Heat for the distillation was supplied by a heating mantle and was regulated by an auto-transformer. The receiving flask was protected from airborne contamination throughout the distillation. Water was redistilled from potassium permanganate solution acidified with sulfuric acid. Stannous chloride and sulfamic acid (Matheson, Coleman and Bell) were used without further purification. Zinc dust was used as received (Matheson, Coleman and Bell) or prepared from Z-15,20-mesh zinc (J. T. Baker) by grinding until it passed through a 400-mesh sieve. Stannous chloride solutions that contained 20% wt/vol of tin(I1) chloride dihydrate in concentrated hydrochloric acid were prepared fresh daily. Aqueous calibration standards were prepared by diluting stock aqueous selenium dioxide to the desired concentration with water. Nonaqueous standards were prepared by dissolving dilauryl selenide (available from Chevron Chemical Co. as Oronite 250) in toluene. Hydrogen selenide generating solutions used in the Vycor furnace measurement method were 5.5 N in sulfuric acid and 2.8 N in hydrochloric acid. The generating solution for the flame measurement was 7.7 N in sulfuric acid and 5.2 N in hydrochloric acid. Samples f o r Interlaboratory Study. Five samples that represented a variety of petroleum products were selected and prepared for interlaboratory evaluation of the described method. Three of the samples contained native selenium and no additional selenium was
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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VYCOR FURNACE
1 ;
LENS
OPTICAL
e BURNER
ARGON
~
RAIL
Figure 1. Modified Jarrell-Ash spectrophotometer
1 _ -
195 mm
1
1
I
MAGNETIC STIRRER
Flgure 4. Selenide generator for flame measurement
1 1 1 1-
I /~l/
1.7 mrn C A P I L L A R Y
Figure 2. Vycor glass furnace
KJELDAHL
FLASK
E X T R A - COARSE HEATING MANTLE
#/
Z I N C DOSING BURET
Figure 5. Wet oxidation apparatus
Table I. Instrument Settings
i 1
"
Jarrell-Ash 82-546
125 H L FLASK
MAGNETIC
STIRRER
I
Figure 3. Selenide generator for Vycor furnace measurement
added; the other two were spiked with a known amount of selenium as dilauryl selenide. The samples were analyzed in one laboratory by the flame measurement method, in another laboratory by the Vycor furnace measurement method, and in a third laboratory by neutron activation. The results are discussed below. Procedure. Sample Preparation. Add 1-2 g of sample and 10 ml of concentrated sulfuric acid to a 300-ml Kjeldahl flask (Figure 5). Add 10 ml of 90% (fuming) nitric acid through the condenser and apply heat for 30 min at fi4 scale of the auto-transformer. Increase heat to % scale for an additional 30 min. Raise the flask several inches above the heating mantle, and add 5-10 ml of 70% perchloric acid through the condenser. Return the flask to the heating mantle and heat a t 3h scale until the mixture becomes a light amber color (normally 30-90 min). Turn off the heat, add 20 ml of water through the condenser and allow it to drain completely. Disconnect the condenser and heat again at the same auto-transformer setting until fumes of perchloric acid appear. Increase the auto-transformer setting to full scale and heat until fuming ceases. Cool, and add 10 ml of water and 0.1 g of sulfamic
Anal. wavelength L a m p current Background correction Burner height Damping Slit Hydrogen Argon Air Recorder Mode Scale e x a
Perkin-Elmer 403
1960 A 10 mA
...
1960 A 13 mA On
...
19
2 100 p (ent.), 150 p (exit) 32 l./min. 1.7 l./min. 7.0 l./min. 2-5 mV Conc. 5x
2 4 14.5 l./min. 13 l./min. Entrained 2 mV Absorbance
..
acid. Adjust the acid concentration to the values given previously. If the selenium is to be measured with the Vycor furnace method, add 5 ml of the 20% stannous chloride solution and let the solution stand a t room temperature for 30 min. Stannous chloride is not required in the flame measurement method. Transfer the solution to a generating flask and generate the hydrogen selenide as described below. Generation of Hydrogen Selenide and Measurement of Selenium. Attach the generating flask to the spectrometer, stir the solution and purge the flask with argon until a steady baseline is obtained. Continue to stir and add the zinc dust. Record the selenium response until
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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Table 11. Recovery of Added Selenium by Vycor Furnace Measurement Sample Selenium form Added" Foundn Crude oil Di-lauryl selenide 200 182 Mineral oil Di-lauryl selenide 54 47 Mineral oil 108 109 Di-lauryl selenide Mineral oil Di-lauryl selenide 162 175 Gasoline Di-lauryl selenide 105 110 Gasoline Di-lauryl selenide 200 198 Gasoline Di-lauryl selenide 110 106 No. 2 Fuel oil Di-lauryl selenide 100 94 No. 2 Fuel oil Di-lauryl selenide 105 122 Mineral oil Selenium dioxide 50 60 Mineral oil Selenium dioxide 100 96 Mineral oil Selenium dioxide 150 160 a
Selenium added,
Selenium found by analysis, ng/g Laboratory Laboratory Laboratory
Sample nglg 14 2b 3c Gasoline 0.0
