(14) Minerals and Chemicals Corp. of America, 210 West Washington Square, Philadelphia 5, Pa., “Preparation of Petroleum Feeds for Platinum Cata-
lysts.”
(15) Satterlee, H. S., Blodgett, Gertrude, IND.ENG.CHEM.,ANAL.ED. 16, 400 (1944). (16) Shipman, G . F., Milner, 0. I., ANAL.CHEM.30,210 (1958).
(17) Sidgwick, N . Y., “The Chejmical Elements and Their Compounds, Vol. I, pp. 761-7, Clarendon Press, Oxford, 1950. (18) Smales, A.. iz., Pate, B. D., ANAL. CHEY.24,717 (1952). (19) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. I, pp. 1YO-8, Van Nostrand, New York, 1949.
(20) Vasak, V., Sedivec, V., Chem. lisdy 46, 341 (1952). (21) Vecera, Z., Bieber, B., Giessereilechnik 3, 61-4 (1957). (22) Wagner, F., Vitt, R., Erdol u. Kohle 11,460 (1958). RECEIVEI) for review February 20, 1959. Accepted May 22, 1959. Division of Petroleum Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Determination of Microgram Quantities of Arsenic in Naphthas with Oxyhydrogen Burner D. KENDALL ALBERT and LAWRENCE GRANATELLI Research ond Development Department, American Oil Co., Texas City, Tex. Arsenic in naphthas is determined at the parts per billion concentration level by combustion of the sample in the flame of a Beckman oxyhydrogen burner. Products of combustion are absorbed in sodium hydroxide solution. Arsenic in the sodium hydroxide solution is subsequently converted to arsine and determined spectrophotometrically with silver diethyldithiocarbamate in pyridine. Combustion of large samples avoids possible contamination from reagents which are used in wet oxidation methods. The method of combustion is rapid as indicated by a burning rate of about 100 ml. of sample per hour. The standard deviation varies from l p.p.b. at the 8 p.p.b. level to 4 p.p.b. a t the 78 p.p.b. level. The relative error a t the corresponding levels varies from 12 to 6%, respectively. The lower limit of the method is approximately 2 p.p.b.
A
and reliable analytical methods are needed for the determination of arsenic in reformer charge stock naphthas in the parts per billion concentration range. Several methods for the determination of arsenic ‘in naphthas (6) are accurate and reliable but require meticulous techniques and strictly controlled operation conditions. The method described uses the Beckman oxyhydrogen burner for sample decomposition. The burner has been used advantageously for the determination of sulfur (6, 4, 6) and chlorine (1, 3 ) in petroleum products. This technique permits the combustion of large samples and avoids possible contamination from reagents which are used in wet oxidation methods. The ability to burn large samples allows a spectrophotometric, rather than the Gutzeit, method to be used because CCURATE
relatively larger quantities of arsenic are available for determination. Spectrophotometric procedures based on the formation of molybdenum blue (7) are generally preferred, but in recent years the use of silver diethyldithiocarbamate has been shown to be an excellent reagent for the spectrophotometric determination of arsenic (6). The method described here is based on a procedure described by Powers et al. (6). It is reproducible and simple t o perform. REAGENTS
All chemicals used were of analytical reagent grade. All aqueous solutions were prepared in distilled water demineralized by ion exchange. hlany of the reagents used were the same as those described by Powers et al. (6) and were of the same quality. Glassware was also treated in this manner. Hydrochloric acid solution, 1 to 1. Sodium hydroxide solution, I yo. Sulfuric acid solution, 1%. Standard arsenic solution, prepared by dissolving 0.1320 gram of oven-dried arsenic trioxide in 10 ml. of 40% aqueous sodium hydroxide and diluting to 1 liter with water. For calibration purposes, the stock solution was diluted as needed in a ratio of 1 to 100 to obtain a titer of 1 y of arsenic per milliliter. Silver diethyldithiocarbamate, prepared a t about 10” C. by precipitation of a soliltion of sodium diethyldithiocarbamate (2.25 grams in 100 ml. of water) with a solution of silver nitrate (1.7 grams in 100 ml. of water). The yellow precipitate was collected on a suction filter, washed with about 100 ml. of cold water, and dried under vacuum a t room temperature. Silver diethyldithiocarbamate pyridine solution, prepared by dissolving 1 gram of the salt in 200 ml. of pyridine from a freshly opened bottle. The solution was filtered and stored in a brown glass bottle.
Hydrogen, Air Reduction Co., Inc. Oxygen, Linde Co. APPARATUS
Assembly of the apparatus used for burning naphthas is shown schematically in Figure 1. It is essentially a modification of the apparatus described by Cali, Loveland, and Partikian ( I ) . There is a high noise level of combustion which may be a point to consider in choosing a location for the apparatus. BURNER,CHIMNEYASSEMBLY.Hydrogen and oxygen cylinders were fitted with double-stage pressurereducer gages, 18,19. The flow of gases to the burner was controlled by 1/4-inch Hoke needle valves, 16, 17, and instant control of the hydrogen and oxygen streams was obtained by 1/4-inch Hoke toggle valvm, 14, 15. The Beckman oxyhydrogen burner capillary, 12, was extended to approximately 31/4 inches with 16-gage hypcdermic tubing. The tubing was sealed to the burner capillary tube with Sealit Cement (Fisher Scientific Co.). (Alternatively, the tubing may be silver soldered.) The chimney, 10, approximately I2 inches long and with a 3/4-inch diameter inlet, was fabricated from 51-mm. outside diameter Vycor brand glass tubing. The outlet was fused to a Vycor 35/20 ball joint, 9. ABSORBER.A 350-ml. gas washing bottle, 7 , with a coarse fritted-glass disk, contained the absorbent. The side inlet was made from 22-mm. outside diameter borosilicate glass tubing. A Kjeldahl-type spray trap, 6, a 300-mm. condenser, 5, and a 500-ml. Erlenmeyer flask, 4, were modified as shown in Figure 1. The components were assembled using 24/40 T joints and standard taper Teflon sleeves (Quorn, Arthur F. Smith Co., Rochester, N. Y.). VACUUMPUMP. This had a minimum capacity of about 18 liters of air per minute and was connected to a mercury manometer and to the outlet of the water receiver, 4, through a cold tr8p. 3. Connections were made with rubber vacuum tubing. A screw clamp, VOL. 3 1 , NO. 9, SEPTEMBER 1959
1593
1, was located immediately before the Pump. SAMPLEHOLDER.The holder, 13, was 3 inches in height and 23/* inches in outside diameter,-with a capacity of approximately 300 ml. A capillary hole at the top of the holder allowed entry of the burner capillary into the sample. A 10/30 T taper, with stopper, was fused to the side of the sample holder to allow sample entry, and to facilitate the relief of any vapor pressure that might occur during burning. ARSINEGENERATOR.This apparatus is illustrated in Figure 2. The flask, 9, and the connecting unit, 7, were components of the arsine generator unit manufactured by the Corning Glass Works, Corning, N. Y. The capillaryabsorber unit, 1, 3, was essentially that of Powers et al. (6) with minor modifications. The 24/40 -$ joints were lubricated with Fisher’s Nonaq stopcock lubricant. Arsine was generated directly into 3 ml. of the silver diethyldithiocarbamate reagent contained in the holder, 1. Fritted disks, 2, allowed uniform mixing of the gas in the reagent. The reservoir, 4, facilitated the washing of the fritted disks, 2, when the tube contents were drained. Fritted disk 6 supported a piece of cotton which was moistened with lead acetate solution to remove hydrogen sulfide evolved during arsine generation. A Beckman Model DU spectrophotometer, equipped with matched 1-cm. cells, was used.
Figure 1. 1. 2.
3. 4.
5. 6. 7. 8.
9.
Assembly for burning naphthas
Screw clamp Dewar f l a s h c o l d bath Vapor trap Erlenmeyer flask Condenser Spray trap Absorber Beaker Vycor 3 5 / 2 0 ball joint
PROCEDURE
Sample Combustion. The absorber, charged with 50 ml. of 1% sodium hydroxide solution, was placed in the modified 3-liter beaker (Figure 1) and the beaker, condenser, and vapor trap were charged with coolants. A centrifugal pump circulated ice water through the condenser. Tap water was sufficient for cooling the absorber. With screw clamp, 1, closed, the vacuum pump was started. The screw clamp was then opened until the manometer indicated a pressure difference of about 7 em. of mercury. Oxygen and hydrogen pressures a t the gas cylinders werc regulated to about 15 and 5 p.s.i.g., respectively. The oxygen flow to the burner was turned fully on by opening the needle and toggle valves, then hydrogen was allowed to flow to the burner, and the gas mixture was ignited. The hydrogen needle valve was regulated to yield a flame about a/8 inch high. The burner was clamped into position with the burner tip located a t the center of the chimney inlet and about inch below the opening. A warmup period of about 2 minutes was allowed before sample combustion was started. The sample to be burned was weighed in the sample holder, and the holder positioned beneath the burner. Combustion was started by raising the holder quickly and allowing the capillary to extend into the sample. Whenever the flame went out during operation, the 1594
0
ANALYTICAL CHEMISTRY
Figure 2. Arsine generator 1.
2.
3. 4. 5. 6. 7. 8.
9.
Arsine absorber, 1 2 cm. X 1 cm. inside diameter Coarse fritted disks, 2cm. spacing Capillary tubing, 2 mm. inside diameter Reservoir, 3-ml. volume 24/40 Coarse fritted disk Connecting unit T 24/40 Erlenmeyer flask, 1 2 5 ml.
hvdrogen sumlv. and then the oxygen
10. 1 1. 12.
13. 14, 16,
18. 19.
Chimney, Vycor Oxyhydrogen burner Capillary extension Sample holder 15. Toggle valves 17. Needle valves Hydrogen pressure regulator Oxygen pressure regulator
tinued, and the burner was removed from the chimney. The sample holder was reweighed to determine the amount of sample burned. The solution in the absorber was quantitatively transferred to a 400-ml. beaker, and the absorber, chimney, connecting tube, and spray trap were rinsed with boiling 1% sulfuric acid solution. Nitrogen pressure was used to force the rinsing solution back and forth through the fritted disk in the absorber and to completely drain the liquid from the disk. The rinsinge aere added to the beaker, and the solution was evaporated to a volume of about 10 ml. The concentrated absorber solution was quantitatively transferred to an arsine generator flask using 20 ml. of 1 to 1 hydrochloric acid solution and a few milliliters of hot distilled water. The flask was cooled to room temperature before proceeding with arsine generation. Arsenic Determination. The method used here was essentially the same as that used by Powers et al. ( 6 ) . Arsine was generated from a solution having a total volume of 50 ml.
Calibration Curve. The arsenic content was determined from a calibration curve covering the range of 0 to 5 y of arsenic using the same arsine generation procedure as used for the sample. The calibration curve was linear in the 0- to 5-7 arsenic range with a slope of about 0.07 absorbance unit per microgram of arsenic. EXPERIMENTAL RESULTS
of Stickler ( 8 ) , was used as the arsenic source. K'aphtha desulfurized over cobalt molybdate catalyst was used as the base stock. Analyses of the naphtha indicated an arsenic content between 0 and 1p.p.b. For experimental purposes an arsenic content of 1 p.p.b. was assumed. Determinations were made on freshly prepared solutions. Table I shows the results of replicate determinations a t the various concentration leveis of arsenic. Samples were prepared to cover the concentration range of 2 to 78 p.p.b. arsenic. The quantity of sample taken for combustion was selected to yield between 1 and 5 y of arsenic. The standard deviation varies from about 1 p.p.b. a t the 8-p.p.b. level to about 4 p.p.b. a t the 78-p.p.b. level. Average recoveries within the 8- to 78-p.p.b. range varied from 88 to 94%. Limited analyses a t the 2-p.p.b. level indicated a relative error of about 50%, or greater, a t that level. The lower limit of the method is approximately 2 p.p.b. Samples were burned a t an average rate of 80 grams per hour with most sample sizes varying within the range of 50 to 200 grams. Samples larger than 200 grams were burned by recharging the sample holder.
DISCUSSION
The main advantage of the combustion apparatus is that a relatively small amount of water of combustion is accumulated in the absorber, thus reducing the evaporation time in preparing the absorber solution for analysis. Combustion of large samples produces large volumes of water from both the burner and from the sample. Most of the water of combustion is removed from the absorber by the flow of hot gases and is condensed into the water receiver. During long periods of burning, it was sometimes necessary to stop the combustion briefly to allow the burner t o cool. Prolonged exposure to heat will prevent the burner from operating properly. Burning characteristics were improved by directing jets of air near the top and the bottom of the chimney. The rate of evaporation of water from the absorber was controlled by periodically adjusting the amount of water flowing through the cooling bath. Excessive accumulation of mater in the absorber must be preGented; otherwise, mechanical carry-over of the absorbent into the condenser may occur. In such cases, the contents of the mater receiver must be analyzed, or else the sample must be rejected. During normal operation, the spray trap is sufficient to prevent mechanical carry-over of the absorbent into the condenser. Excessive heat flowing to the absorber wdl,
of course, evaporate the absorber contents to dryness. Dilute sodium hydroxide solution was satisfactory as an absorbent. Relatively low arsenic recoveries were obtained when distilled water or dilute sulfuric acid sdlutions were used as absorbents. I n burning large volumes of samples which were high in sulfur and/or nitrogen content, it was necessary to adjust the basicity of the absorbent after an hour or more of burning. Presumably, neutralization of the basic absorbent is due to the formation of sulfuric and/or nitric acids during combustion. Solutions of sodium hydroxide stronger than 1% could not be used because of excessive foaming. Boiling sulfuric acid solution was necessary to remove arsenic adsorbed to the glassware. Extreme care must be taken in rinsing the absorber to ensure complete removal of arsenic from the fritted disk. Low arsenic recoveries usually result from the incomplete recovery of arsenic which has adsorbed to the chimney walls during combustion. It was necessary t o carry out the arsine generation with considerable precaution. Arsine was generated directly into the silver diethyldithiocarbamate reagent at a moderate rate of evolution. Incomplete absorption of arsine in the reagent, or loss of pyridine solvent by mechanical action or volatilization may result if vigorous evolution occurs. Chilling the reaction mixture was necessary to slow the zinc-acid reaction. Insufficient cooling before zinc n-as added sometimes resulted in a dark, frothy reaction mixture. In such cases, the sample was discarded. This reaction behavior is attributed t o the formation of excessive hydrogen sulfide, because in these instances unusual amounts of lead sulfide were observed on the cotton plug. ru'ormally, only trace amounts of hydrogen sulfide were detected during arsine generation. Spectrophotometric measurements were made directly on 3 ml. of reagent. This mas the minimum volume of reagent that could be used for maximum sensitivity and yet be sufficient to fill the regular 1-em. Beckman cells. Absorbance measurements were made a t a wave length of 540 nip using a 0.02mm. slit width and normal sensitivity. The wave length of maximum absorbance was determined from absorption spectra of the arsenic(II1)-diethyldithiocarbamate complex a t concentrations of 1, 3, and 5 y of arsenic. Procedure blanks were run to compensate for any arsine which might have been present in the hydrogen source. The data shorn in Table I were obtained using one fritted disk, 2, in the arsine absorber, Figure 2 . dlthough results were satisfactory with this
design, a second fritted disk was added later to reduce any error that might result from incomplete absorption of arsine in the reagent (6). In recent work, the sample combustion procedure has been slightly modified. After combustion of the entire sample, the sample holder and sampling container are rinsed thoroughly with isopropyl alcohol and the rinsings burned. The rinsing step was added to reduce any error caused by adsorption of naphtha-insoluble organoarsenic compounds on the sampling containers. To reduce error, naphtha samples should be analyzed immediately after they have been withdrawn from the
Table
I.
Determination of Arsenic" in Naphtha*
Deviation
61 1 56 3 54 9
78.0
55 5 54 1
53 5 53 9
75 4 51 1
54 6
84 0 81 8 83 0 70 0 79 5 84 8 82 9
84 4
82 5 79 8 92 91 90 92 95
2 9 5 4
6 94 9 90 2 94 6 89 1 92 8
180 178
182 180
181
156 163
255 451
66.0 72.5 77.4 72.2 72.6 69.0 78.5 75.7 68.1
77.7
Mean 73.0 Std. dev. 39.0 37.0 34.8 33.1 34.0 37.2 38 0 39 5 40 8
36 5 30 2 Mean 36 2 Std dev 19 5 17 4 19 0 16 6 16 2 15 3
19 6 16 8 19 1 15 0 17 2 Mean 17 2 Std dev 7 8 6 3 7 0 7 7 6 6 5 7 8 0 6 8 Mean 6 9 Std. dev. 2.0
0
-7 0 -0 +4 -0 -0 -4 +5
5
4
8
4 0 5
+2 7 -4 9 +4 '7 -f4 3
+O 8 -1 4
-3 1 -2 2 +I 0
+2
i
+o
3
+o
2
+3 3 -146 -6 0 1 3 2 +1 -0 -1 -1 +2
8
$1
0
6 0
9 4 -0 4 -2 2
0 f l6 -0 6
+o
+O -0 -1 +1 -0 &O
1
8
3
2
1 1 8
-0 9
1.7 +O 8 1 0 466 -0 1 Mean 0.9 Added as triphenylarsine. Hydrodesulfurized naphtha. Corrected for 1 p.p.b. of arsenic in base stock.
VOL. 31,
NO. 9,
SEPTEMBER 1959
1595
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
