unit. Powers el al. (6) recently found that naturally occurring arsenic compounds in naphthas are unstable and undergo a change in concentration with aging. The method has been satisfactory in the analyses of refinery naphthas. Most of these naphthas were from the Gulf Coast region and contained arsenic on the order of 2 to 8 p.p.b. The maximum concentration range of arsenic for which the combustion technique
can be applied has not been determined, but the method should also be satisfactory for parts per million ranges of arsenic. LITERATURE CITED
(1) Cali, L. J., Loveland, J. W., Partikian, D. G., AXAL. CHEM.30, 74 (1958). (2) Granatelli, Lawrence, Zbid., 27, 266 (195.51. (3j Ibyd., 29, 238 (1957). (4) Hinsvark, 0. N., O'Hara, F. J., Zbid., 29, 1315.(1957).
(5) Houghton, N. W., Zbid., 29, 1513 (1957). (6) Powers, G. W., Jr., Martin, R. L., Piehl, F. J., Griffin, J. M., Zbid., 31, 1589 (1959). (7) Sandell, E. B., "Colorimetric Determination of Traces of Metals," 2nd ed., p. 178, Interscience, New York, 1950. (8) Stickler, W. C., Zbid., 24, 1219 (1952). RECEIVEDfor review March 9, 1959. Accepted May 25, 1959. Division of Petroleum Chemistry, 135th Meeting, .4CS, Boston, Mass., April 1959.
Separation and Determination of Microgram Amounts of Sulfur R. P. LARSEN, L. E. ROSS, and N. M. INGBER' Chemical Engineering Division, Argonne Nafional laboratory, lemoni, 111.
b A method has been devised for the determination of parts per million of sulfur in uranium trioxide, sodium zirconium fluoride, and hydrofluoric acid. Sulfur trioxide evolved by fusion with vanadium pentoxide is reduced over copper a t 950" C. to the dioxide, which is absorbed in sodium tetrachloromercurate and determined spectrophotometrically with pararosaniline and formaldehyde. The coefficient of variation for samples containing 5 to 10 y of sulfur is 5.
H
and Faust (2) demonstrated that sulfur can be volatilized from a wide variety of refractory substances by fusion with vanadium pentoxide a t 900" C. in an air atmosphere. The evolved sulfur trioxide was determined alkimetrically or gravimetrically. To extend the range of the method to the determination of 10 p.p.m. or less of sulfur in uranium trioxide, and subsequently both hydrofluoric acid and sodium zirconium fluoride, several more sensitive reactions for the detection of sulfate were considered. Colorimetric methods have been described which are based on the metathesis of barium chloranilate ( I ) and barium chromate (4) with sulfuric acid. The chloranilate method is not sufficiently sensitive. The chromate method, although very sensitive, is adversely affected by the solubility of barium chromate when appreciable concentrations of such anions as chloride and nitrate are present. AGERMAN
* Present address, Borden Cliemical Co., Philadelphia, Pa. 1596
0
ANALYTICAL CHEMISTRY
Attention was directed, therefore, t o ways of reducing the evolved sulfur trioxide to the dioxide so that the more selective and sensitive reactions of sulfite might be used. Copper was found t o be very satisfactory for this purpose. The quartz packing used in the furnace section of the combustion tube by Hagerman and Faust (8) was replaced with copper turnings and nitrogen was used instead of air as the sweep gas. The method of West and Gaeke (5) was satisfactory for the microgram detection of sulfur dioxide. The dioxide was absorbed in a sodium tetrachloromercurate solution and determined with pararosaniline and formaldehyde. After this work was completed, Helwig and Gordon (3) reported that sulfur dioxide could be absorbed directly in the pararosaniline-formaldehyde mixture and that the color developed more rapidly and was more intense. The sensitivity of the method reported here might be increased somewhat by using their color development procedure.
REAGENTS
Use reagent grade chemicals and distilled water, which has been passed through a mixed-bed ion exchange column (Illinois Water Treatment Co., Rockford, Ill.). COPPERTURNINGS. Wash with acetone to remove oil film. KITROGEN.Pass through a gas washing bottle containing 0.1M sodium tetrachloromercurate. VAKADIUM PENTOXIDE.Heat in a quartz dish overnight a t 1000" C. Grind. Prepare melts by fusing about 125 mg. in a quartz boat a t 1000" C. Remove from the boat by gentle tapping. STANDARD SULFATE SOLUTION, 18 y of sulfur per ml. Prepare from anhydrous potassium sulfate. STANDARD SULFITE SOLUTION, 32 y of sulfur per ml. Dissolve 126 mg. of anhydrous sodium sulfite and dilute to 1 liter with 0.1M sodium tetrachloromercurate. SODIUM TETRACHLOROMERCURATE (0.lM), PARAROSANIIJNE (0.04%), and FORMALDEHYDE (02%). Prepare each solution in the manner described by West and Gaeke (5).
APPARATUS
PROCEDURE
FURNACE ASSEMBLY. The combustion tube is a 19 X 530 mm. quartz tube fitted with a 24/40 joint a t the inlet and an 18/7 spherical joint a t the outlet. Copper turnings are snugly packed into a 15-cm. section of the tube near the outlet. The packing is heated by a 20cm. tube furnace (Hevi-Duty, Milwaukee, Wis.). QUARTZ BOATS. 76 X 11 X 10 mm. (Microchemical Specialties Co., Berkeley, Calif.). Wash with concentrated hydrochloric acid before use. S P E C T R O P H O T O M EBeckman TER, Model B, with matched 1-cm. cells.
Calibration Curve. A small negative deviation from Beer's law exists above 4 y of sulfur per 10 ml. of solution; therefore, a calibration curve must be used. Prepare the curve by pipetting aliquots of the standard sulfite solution containing from 0.5 to 8 y of sulfur into 10-ml. volumetric flasks. Add 1 ml. of the pararosaniline solution and 1 ml. of 0.2% formaldehyde, and make up to volume with 0.1111 sodium tetrachloromercurate. Mix well and let stand 25 minutes for full color development. Measure the absorbance in 1-cm. cells vs. a reagent blank a t 560
mfi. Run a curve for each set of color development reagents. General Procedure. With the furnace heated t o a temperature of 950' C., weigh a ground sample containing 2 to 8 y of sulfur into a quartz boat lined with a vanadium pentoxide melt. Cover the sample with a similar melt and position the boat in %he combustion tube, 30 mm. from the furnace. Adjust the nitrogen flow rate to about 80 ml. per minute, and sweep out the apparatus for 10 minutes. Place the absorber, a 10-ml. volumetric flask containing 5 ml. of 0.1M sodium tetrachloromercurate, in position. Place a platinum-gauze heat reflector on the combustion tube over the sample. With the Meker blast burner adjusted to provide a temperature of 900' to 950" C., heat the combustion tube. Start near the inlet end and move the burner slowly toward the sample, allowing 10 minutes for this operation. Ignite the sample for 20 minutes. Lower the collection flask and wash the outlet tube with tetrachloromercurate solution. Develop and measure the color as described above. Determine a reagent blank by igniting a comparable amount of vanadium pentoxide. Procedure for Hydrofluoric Acid
Samples. Pipet an aliquot containing 2 to 8 y of sulfur with a plastic pipet into the quartz boat to be used for the fusion. Add 0.5 ml. of bromine water to oxidize sulfide and/or sulfite to sulfate, and evaporate to dryness over a phosphoric acid bath a t 120' C. Place a vanadium pentoxide melt in the boat and carry out the fusion as described. Procedure for Sodium-Zirconium
Samples. Weigh a finely ground sample containing 2 to 8 7 of sulfur into a quartz boat lined with a vanadium pentoxide melt. Cover with a similar melt. Fuse in the furnace, using air for a sweep gas according to the procedure of Hagerman and Faust ( 8 ) . Evaporate the hydrogen peroxide catch solution to incipient dryness in a platinum dish over a phosphoric acid bath a t 120" C. Transfer the residue to a quartz boat with a small amount of water and treat as a hydrofluoric acid sample. EXPERIMENTAL
The volatilization of sulfur from pure compounds and the reduction of the evolved sulfur trioxide to the dioxide over coppei was initially demonstrated at the milligram level. Potassium sulfate, cuprous sulfide, and sodium sulfite were used as standards. The dioxide was collected in dilute sodium hydroxide, and the sulfite was determined iodiometrically. For these compounds, 99, 97, and loo%, respectively, of the sulfur taken w a recovered. ~ When attemptg were made to extend the range of the method to microgram
amounts of sulfur, using the method of West and Gaeke (6) to assay the sulfur dioxide in the sweep gas, the blanks were found to be prohibitively high, 15 to 90 y. Each of the components in the fusion-the porcelain boat, the silica used to line the boat, and the vanadium pentoxidewas found to have an appreciable sulfur content. By use of quartz boats which had been preheated to 900" C., by elimination of the silica, and by fusion of the vanadium pentoxide overnight a t 1OOO' C. in a quartz dish and then regrinding before use, the blank was reduced to 0.5 y. At this level, it was found that scratches made in the quartz boat during the removal of previous melts could increase the blank as much as 0.3 y. Gentle tapping, followed by a hydrochloric acid wa$, proved the most satisfactory way of removing the melts. As a further precaution, only new glassware which had been rinsed with dilute hydrochloric acid and distilled water was used. When all these precautions were taken, blanks of 0.3 to 0.7 y were obtained. The results shown in Table 1 were obtained by pipetting aliquots of standard potassium sulfate solution containing 2 to 7 y of sulfur into a quartz boat, drying a t 110' C., and then treating as described above. To convert liquid samples-e.g., hydrofluoric acid-to a form satisfactory for fusion, it was necessary to evaporate them t o dryness. A phosphoric acid bath a t 120' C. was used, as it had been previously demonstrated that no losses of sulfuric acid occurred when the evaporation was carried out in this manner. Low results were obtained when samples were pipetted directly onto the vanadium pentoxide melts and dried. In the determination of sulfur in sodium zirconium fluoride, the volatility of vanadium fluoride in the nitrogen atmosphere necessitated a preliminary separation of the anionic constituents. By fusion of the sample in an air atmosphere first, according to the procedure described by Hagerman and Faust (a), only sulfur trioxide and hydrogen fluoride were volatilized. The peroxide catch solution was then amenable to the procedure developed for hydrofluoric acid samples. Interferences. Although less than 10 p.p.m. of nitrogen dioxide will prevent color formation in the sodium chloromercurate catch solution (6), nitrogen dioxide formed by the breakdown of nitrate impurities in the sample during the fusion step is effectively removed by the hot copper. No interference was noted when 15 mg. of nitrate was added t o a uranium
Table 1. Recovery of Solfur from Potassium Sulfate at Microgram Level
Taken 2.8 4.6 7.4
Sulfur, y
-
Found
2.7, 2.9 4 . 6 , 4 . 5 ,4.6, 4.5, 4.5 7.6, 7.4
Table 11. Determination of Sulfur in Uranium Trioxide, Hydrofluoric Acid, and Sodium Zirconium Fluoride
Material UOa HF
Sulfur Found, P.P.M. B y By authors others 129,134 76, 73 9.4, 8 . 4 49, 51 36, 38 21.20 2.4,1.9 0.11, 0.17 4.5, 5 . 3
13'
60' 35b 13.5'
NaF-ZrF, 8 Turbidimetric analysis by the New Brunswick Laboratory of the Atomic Energy Commission. b Activation analysis by G. W. Leddicotte of Oak Ridge National Laboratory. trioxide sample known to contain 10 p.p.m. of sulfur, and the fusion was carried out in the apparatus described above. When 150 mg. of nitrate was added to a similar sample, no color could be developed. However, there is no reason to believe that increasing amounts of nitrogen dioxide could not be handled by increasing the size of the copper reductor. (Product specifications for uranium trioxide limit the nitrate content to 0.8%.) RESULTS
Resulb typical of those obtained in a large number of determinations of the sulfur in uranium trioxide, hydrofluoric acid, and sodium zirconium fluoride are given in Table 11. LITERATURE CITED
(1) Bertolacini, R. J., Barney, J. E., 11, ANAL.CHEW29, 281 (1957). (2) Hagerman, D. B., Faust, R. A., Ibid., 27, 1970 (1955). (3) Helwig, H. C., Gordon, C. L., Zbid., 30, 1810 (1958). (4) Iwasaki, I., Utsumi, S., Tarutani, T., J . Chem. Soe. Japan, Pure Chcm. Sec. 74,400 (1953). (5) West, P. W., Gaeke, G. C., ANAL. CJ~EM. 28,1816 (1956).
RECEIVEDfor review March 1, 1959. Accepted May 20, 1959. Work erformed Atomic under the auspices of the U.
8.
Energy Commission. Pittsburgh Confer ence on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1959.
VOL 31,
NO. 9, SEPTEMBER 1959
1597
