A,, Grunwald, E. A., Ind. Eng. Chem. 40, 1500 (1948). (10) Hiller, L. A., J . Polymer Sci. 14, 555 (1954). (11) McKenzie, A. W., Higgins, H. G., Holzforschung 10, 179 (1957). (12) Malm, C. J., Barkey, K. T., May, D. C., Abell, P. I., Znd. Eng. Chem. 46, 557 (1954). ( 1 3 ) &falm, ’C. J., Barkey, K. T., May, D. C., Lefferts, E. B., Zbid., 44, 2904 (1952).
Tech. Bull. “Dimethyl Sulfoxide,” Deo
(14) Malm, C. J., Tanghe, L. J., Zbid., 47, 995 (1955). (15) Malm, C. J., Tanghe, L. J., Laird, B. C., Zbid., 38, 77 (1946). (16) Proffit, J. R., Graham, H. &I., Purchase, E. R., Blume, R. C., T a p p i 37, 28 (1954). (17) Rosenbaum, C. K., Walton, J. H., J . Am. Chem. SOC.52, 3366 (1930). (18) Schierz, E. R., Zbid., 45, 455 (1923). (19) Stepan Chemical Co., Chicago, Ill.,
29, 1954. (20) Whitford, E. L., J . Am. Chem. SOC. 47, 2934, 2939 (1925).
RECEIVED for review December 23, 1957.
Accepted March 29, 1958. Divison of Cellulose Chemistry, 132nd Meeting, ACS, New York, N. Y., September 1957. Contribution No. 33 from the Olympic Research Division, Rayonier Inc., Shelton, Wash.
Determination of Arsenic in Petroleum Fractions and Reforming Catalysts DAVID LIEDERMAN, J.
E. BOWEN,
and 0. 1. MILNER
Research and Development laboratory, Socony Mobil Oil Co., Inc., Paulsboro, N. J. ,Small amounts of arsenic found in typical petroleum charge stocks are harmful to present-day reforming catalysts. To provide the required control, methods were developed for determining this arsenic in both naphthas and catalysts. The arsenic is extracted from the naphtha or, in the case of catalysts, solubilized by fusion. It is then separated by distillation as arsenic trichloride and determined colorimetrically as the arsenic-molybdenum blue complex. With these methods, arsenic can be determined in concentrations of 1 to 50 p.p.b. in naphtha and 1 to 1000 p.p.m. in catalysts. The method has been applied successfully to a wide variety of materials, both in commercial refinery operation and in laboratory experimentation.
T
petroleum industry has been concerned with the poisoning of platinum-bearing reforming catalysts by small amounts of arsenic that are present in the charge stocks. To determine the extent of this poisoning and to be able to study the course of remedial action, it is necessary to have analytical methods capable of determining extremely low concentrations of arsenic in naphtha (1 to 50 p.p.b.), as well as in pretreating and reforming catalysts (1 to 1000 p.p.m.). Many methods have been published for the determination of small amounts of arsenic in mrious materials. Most are based on the separation of the arsenic by arsine evolution (5, 6, 9, 12) or by distillation as arsenic trichloride (11) or pentabromide ( I , I S ) . The final determination is usually made b y modifications of the Gutzeit method-Le., absorption of arsine or string, paper, or powder impregnated with silver or mercury salts (6, 9)-or by the formation of the arsenic-molybdenum blue complex HE
THERMOMETER WELL
0.01% (weight by volume) hydrazine sulfate. B y selecting an appropriate volume for the h a 1 solution and varying the length of the light path used for the measurement, quantities of arsenic ranging from 0.02 to 40 y can be determined. APPARATUS
Digestion apparatus, a 200-ml. roundbottomed flask with a 24/40 groundglass joint, fitted with a Vigreux column. Distillation apparatus (Figure 1). Spectrophotometer, Beckman Model B, with matched 1-cm. cells. Spectrophotometer cells, special 7-em. (Figure 2). Cylinders, 10-ml., glass-stoppered.
40
200ML
-
90
f
Figure 1. Arsenic trichloride distillation apparatus All dimensions in mm.
(1-5, 7 , 8, 10, 14-16). Recently, a method using neutron-activation analysis for arsenic in platinum-bearing catalysts was presented (15). I n the present method as applied to liquid stocks, the arsenic is extracted and simultaneously oxidized by a mixture of sulfuric acid and hydrogen peroxide. The arsenic is then separated and concentrated by distillation as the trichloride, and it is finally determined spectrophotometrically as the molybdenum blue complex. Catalysts are solubilized by fusing with sodium peroxide and the arsenic is then distilled and determined as above. The method map be used for pretreating catalysts as well as for reforming catalysts. The final color is developed in a solution that is 0.27N in hydrochloric acid and contains 0 . 0 5 ~ o(m-eight by volume) ammonium molybdate and
SOLUTIONS
The distilled water used throughout this determination should be redistilled from all-glass apparatus or ion exchanged. All sulfuric acid is redistilled. Standard Arsenic Solutions, 1, 10, and 100 y per ml. Dissolve arsenious oxide in dilute sodium hydroxide, acidify with sulfuric acid, and dilute to volume. Acid Ammonium Molybdate SoluDissolve 1.67 grams of tion. 0.0841,. ,ammonium molybdate tetGhydrate [ (NH4)6M07014.4H20] in water containing 75 ml. of concentrated hydrochloric acid, and dilute to 2 liters. Acid Ammonium Molybdate Solution, 0.05%. Dissolve 0.555 gram of ammonium molybdate tetrahydrate in water containing 25 nil. of concentrated hydrochloric acid, and dilute to 1 liter. Hvdrazine Sulfate Solution. Dissolve 0.1 gram of hydrazine sulfate in 100 ml. of water. Prepare fresh weekly. PROCEDURES
Arsenic in Liquid Stocks. Place 25 ml. each of 30% hydrogen perVOL. 30,
NO. 9, SEPTEMBER 1958
* 1543
oxide a n d concentrated sulfuric acid in a 200-nil. round-bottomed flask. Mix, cool t o 0" t o 5" C., a n d weigh about 40 grams of sample into the flask. (Some samples, especially those high in olefins, react vigorously with the acid-peroxide mixture. Therefore, depending upon the reactivity of the sample, either of the procedures described below is followed.) EXTRACTIOK FROhl SAMPLES LOW I N OLEFINS. Attach the Vigreux column and shake the mixture vigorously for 2 minutes. Heat under reflux. After 15 minutes, shake the mixture carefully with the column still attached. Then, while the solution is hot, shake it as vigorously as possible for an additional 2 minutes. Heat under reflux for 15 minutes more. Remove the column, boil off the unreacted sample, and heat t o light fumes of sulfuric acid. If the mixture starts to char, add more hydrogen peroxide in small increments to oxidize the carbonaceous material. Cool the solution and add 20 ml. of water. EXTRACTION FROM SAMPLES HIGHIN OLEFINS. Attach the Vigreux column and shake the flask until the aqueous layer becomes brownish yellow. Stop the shaking until the color lightens. (Caution: If the reaction proceeds too rapidly, cool the flask in ice water until the reaction subsides.) Shake again until the aqueous layer darkens. Then stop shaking until the color lightens. Repeat this until the aqueous layer no longer clears, but assumes a reddish color. Cool, remove the condenser, and add 15 ml. of peroxide while swirling the solution. Heat strongly to drive off the unreacted sample. If severe coking occurs, add concentrated nitric acid dropwise until the solution clears. Heat to light fumes of sulfuric acid and, if necessary, add 30% hydrogen peroxide dropwise to keep the solution from darkening. Cool the solution and add 20 ml. of r a t e r .
DISTILLATIOX AND MEASUREMEKT OF ARSENIC. T o the cool solution add 1.5
grams of ferrous sulfate and 10 grams of sodium chloride. Without swirling, add 50 ml. of concentrated hydrochloric acid. Quickly attach the distilling head and the absorber containing 15 ml. of concentrated nitric acid. Transfer the apparatus to a hood and place the absorber in an ice bath. Heat the flask gently a t first, then gradually increase the heat so that the distillation proceeds s l o d y and the temperature rises to 50" C. in about 1 hour. Continue to increase the heat until the temperature rises above 100" C. and several drops of liquid distill over. Stop the heating and immediately remove the absorber from the distillation head. Transfer the contents to a 50-ml. beaker, rinsing the absorber with 10 ml. of water. Force the water through the frit with a rubber bulb. Evaporate the contents of the beaker to dryness on a sand bath maintained a t 135" =k 10' C. in a hood. Place the beaker in an oven, held at 135" Z!Z 10" C., for 5 minutes. Cool, and then pipet 4.50 ml. of 0.05% ammonium molybdate solution and 0.50 ml. of hydrazine sulfate solution into the bottom of the beaker. Swirl to mix 1544
ANALYTICAL CHEMISTRY
CEMENTED DISK
'OD
OPTICALLY FLAT PLATE IN END OF TUBE
OPTICALLY FLAT PLATE IN END OF TUBE
yPPoRT
Y
IO aD
1-
I
I c- 20
b 70 Figure 2.
48
CEMENTED DISK
Spectrophotometer cell
All dimensions in mm. Paint all outside surfaces, except 1 0-mm. tube ends, with optically black paint
...
evar,orated. Develor, the color as described above, but use 15 ml. of 0.08% acid-molybdate ~ solution ~ and 2.5 l ml. of hvdrazine sulfate. and dilute to 25 ml. i i a volumetric 'flask before heating. Use a, 1-em. cell for the color measurement.
92 96
DISCUSSION
Table 1. Recovery of Arsenic after Distillation and color ~ ~ Arsenic, y Added Found
Recovery, %
0,000
0.000
0.50 1.00
0.97
0.100
300
500 700
0,092 0.48
97
286
95
692 970
99
470
1000
94
Average
97 95.0
Table II. Reagent Blanks Arsenic Content Y P.P.B .a 0.15 3.8 0.15 3.8 0 . iS
4.0
0.13
3.3
0.17 0.13 0.15
a
4.2 3.3 3.8
0.15 3.8 Av. 0.149 3.75 Based on 40-gram sample.
the solutions. Pour the beaker contents into a 10-ml. glass-stoppered cylinder without rinsing, and heat in a boiling water bath for 10 minutes. Cool, transfer the solution to the 7-em. measuring cell, and measure the absorbance a t 840 mp us. water in the reference cell. Arsenic in Catalysts. Weigh 0.6 gram of finely ground catalyst (for arsenic contents estimated t o be less t h a n 0.05%) or 0.3 gram (for more t h a n 0.05'%) into a platinum crucible. Add 3 grams of sodium peroxide and mix well. Cover the mixture with an additional 1 gram of sodium peroxide and fuse b y heating in a muffle a t 500" f 25' C. for 1 hour. Leach in about 25 ml. of water, cool, and carefully acidify with 40 ml. of 1 to 1 sulfuric acid. Transfer the solution to a round-bottomed flask, evaporate t o light fumes of sulfuric acid, and proceed with the distillation as described for naphthas. If the arsenic is estimated to exceed 40 y, adjust the absorber solution to 25 ml. and remove a suitable aliquot to be
Standardization. T o prepare calibration curves, known amounts of arsenic are carried through the distillation step and subsequent operations. Table I shows t h a t t h e recovery is uniform, regardless of the quantity distilled. Therefore, occasional checks of standard curves may be made b y heating known amounts of arsenic with a few drops of aqua regia, drying a t 135" C., and developing the color. It may be assumed that this intensity is equivalent to 105% of the intensity that would be given b y the same amount of arsenic carried through the operations. Color System. When quantities of arsenic ranging from 5 t o 40 y were treated with molybdate and hydrazine and measured in a 1-cm. cell, the relationship was linear n ith a coefficient of variation of 1.1%. Smaller quantities of arsenic, measured in the special long-path cell (Figure 2), gave about a 5% variation from linearity over the range of several tenths to 2 y. This was attributed to stray light effects along the transparent walls of the 10mm. tube. Reagent Blank. It is important t o choose reagents \\-hose arsenic content is as low as possible, particularly in the analysis of naphtha, where t h e total amount of arsenic dealt with is usually 0.1 to 1 y. A new blank should, of course, be determined each time a nelv lot of a reagent is used or when the quantity is altered-for example, in handling olefinic stocks. The redistilled water should be collected in a large container and considered as a reagent, Typical successive blank values obtainable b y observing these precautions are shown in Table 11. Avoidance of Contamination. Reagents should be reserved for this method only. All glassware must be made scrupulously clean b y boiling in concentrated nitric acid before use.
~
Kew distillation apparatus should be used for reagent blank determinations before being put into general use. Determinations should be performed in a n atmosphere free of excessire dust articles that might contaminate the solutions. If naphthas (at lo^ arsenic concentration levels) as well as catalysts (at relatively high levels) are to be analyzed, separate sets of apparatus should be maintained.
results were compared with those that had previously been obtained b y neutron activation. Table VI1 shows the excellent agreement b e h e e n the tTvo methods. INTERFERENCES
On the basis of literature information on interfering elements and knowledge of the composition of the materials
studied, no interferences were expected or investigated. Antimony, tin, tellurium, germanium, and selenium may distill over with the arsenic. Of these, only germanium will interfere b y producing a color under the prescribed conditions. It is believed that germanium, if present, mould be volatilized during the evaporation step. Phosphorus, which gives a molybdenum blue color under the same conditione
RESULTS
Table 111.
Extraction of Arsenic from Naphthas. T o test t h e efficiency of t h e ex-
traction and oxidation procedure in bringing t h e arsenic compounds in naphtha into aqueous solution, determinations were made on a base midcontinent naphtha with known amounts of added arsenic. The compounds used for this purpose were triphenylarsine and benzenearsonic acid. Results are shown in Table 111. T o check further the applicability of the proposed extraction procedure to the types of arsenic compounds present in naphtha, a 100-nil. portion of a plant sample of naphtha which had been analyzed b y this method mas percolated through a column containing 1 gram of activated alumina. The alumina (presumed to have adsorbed the arsenic) was analyzed before and after percolation, b y the proposed method and also b y a completely independent method involving neutron activation ( I S ) (Table IV). The extremely good agreement between the arsenic content of the naphtha as obtained b y direct extraction and as calculated from the adsorption data is further evidence of the validity of the extraction technique. Arsenic Determination in Plant Naphtha Samples. T h e precision of t h e method as applied t o typical naphthas of unknown arsenic content is illustrated in Table V. Segative values indicate recoveries less than the reagent blank-actually within the precision of the method, which is represented b y a standard deviation of about 1.5 p.p.b. Arsenic Determination in Catalysts.
T h e procedure described above was applied t o nevi platinum-bearing reforming catalysts and chromia-alumina-molybdena pretreating catalysts, t o n hich known amounts of arsenious acid w r e added. Similar tests were made on typical spent commercial catalysts to establish possible interference by other elements a p t to be present. The data show good recoveries for both new and used catalysts of both types (Table VI). As a further check of the method, a number of commercial samples of platinum reforming catalysts were analyzed by the proposed method, and
Recovery of Arsenic from Naphtha
Recovered, P.P.B., ildded as Triphenylarsine Benzenearsonic acid 2.8 3.4.3.9.3 2 . 3 . 8 ... 8.0 9 8; 9.0; 7.0; 8 . 6 , 6 7 13.4 14.1,14.1 14.7,’i 4 . 4 26.8 25.0,25.0 28.4,27.8,24.9,27.4 53.9,50.8,49.5 ... 53.5 Based on 50 ml. of naphtha base (about 37 grams). Added, P.P.B.a
a
Table IV. Determination of Arsenic in Naphtha b y Adsorption on Alumina
Found by Xeutron Extracactitionvation color -4s in A1203, ?/gram
Before percolation 0.33 After percolation 1.88 Calculated As in naphtha, ?/lo0 m1.a 1 55 a Found to contain 1.54 y/100 direct extraction method. Table V.
0.37
A
B Thermal gasoline Pretreated naphtha Pretreated thermal gasoline -4 B California naphtha Visbreaker gasoline Straight-run heavy naphtha A B
Table VI.
ACKNOWLEDGMENT
1.85 1.48
ml. by
Analytical Precision on Typical Naphtha Samples
Identification Mid-continent naphtha
as arsenic, is not distilled. It may, horn-ever, be carried over mechanically b y too vigorous distillation. Small amounts of silica t h a t might be introduced from the glassware will not interfere with the color development a t the acid concentration used.
Arsenic Found, P.P.B. 23, 19, 20 7.8. 8.6 5.4; 3.8 -1.6, -1 .3, 0.0 3.5, 2.4 1.3, 0.0 51,48 32,28 20,21 4.0, 5.2
The authors wish to thank J. B. Evaul for providing data on the repeatability of reagent blanks and G. F. Shipman who performed the neutron activation analyses. LITERATURE CITED
(1) Bartlet, J. C., Wood, M,, Chapman, R. A,, .49.4~. CHEY.24, 1821 (1952).
Table VII. Determination of Arsenic in Used Platinum Catalysts
Seutron Bctivation
(Arsenic, 7c) Colorimetric Method 0 085,O. 087,O.090 0 037,O. 034,0,035
0.096 0.040
0,0037 0,0002,0,0001 0.0013,0.0012,0.0011 0.109
0.0039 0.0003,0,0002 0.0011,0.0012 0.100
0.097
0.098
Recovery of Arsenic from New and Used Catalysts
Arsenic, 70 Found on base
ooooi
Kew reforming
0
Kew pretreating
0 0001,o 0001
Used reforming
0 0037 0 053 0 063,O 064
Used pretreating
-4dded 0 0031 0 051 0 104 0 0097 0.058 0 0022 0 056 0 103
Total 0031 051 104
0 0 0 0 0 0 0 0
0098
058 0059 109 167
Found 0 0030
0 0 0 0 0 0 0
VOL. 30, NO. 9, SEPTEMBER 1958
051 104
0105
055
0059
110 168
1545
‘(2) Boltz, D. F., Mellon, M. G. IND. ENG.CHEV.,ANAL.ED. 19, 873 (1947). (3) Chaney, A. L., Magnuson, H. J., Zbid., 12,69! (1940). (4) Denigks, G., Compt. rend. 171, 802 (1920). (5) Jacobs, M. B., Sagler, J., IND. ENG. CHEM.,ANAL.ED.14, 442 (1942). (6) Jay, R. R., Dickson, L. R., Petroleum Processing 9, 374 (1954). ( 7 ) Kingsley, G. R., Schaffert, R. R., ANAL. CHEM.23,914 (1951).
(8) Magnuson, H. J., Watson, E. B., IND.ENG. CHEM.,ANALED. 16, 339 (1944). (9) Maranowski, N. C., Snyder, R. E., Clark, R. O., ANAL. CHEM. 29, 353 (1957). (10) Morris, H. J., Calvery, H. O., IND. EKG.CHEM.,ANAL. ED. 9 , 447 (1937). (11) Moser, L., Ehrlick, J., Ber. 55, 437 (1922). (12) Sandell, E. B., “Colorimetric Deter-
mination of Traces of Metals,” 2nd ed. Interscience, New York, 1950. (13) Shipman, G. F., Milner, 0. I., ANAL. CHEM.30, 210 (1958). (14) Truo E., Meyer, A. H., IND. ENG. CHEM.,~ N A L ED. . 1, 136 (1929) (15) Woods, J. T., Mellon, M. G., Zbid., 13,760 (1941). (16) Zinzadze, C., Ibid., 7, 230 (1935).
RECEIVED for review November 8, 1957. Accepted April 25, 1958.
Separation of Fluoride from Inorganic Compounds by Pyrolysis R. H. POWELL and OSCAR MENIS Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn. An effective pyrolytic method has been developed for the separation of micro and macro quantities of fluoride from inorganic materials. The pyrolysis is carried out under a stream of moist oxygen in a fused silica reactor tube. The fluoride, which is volatilized, i s absorbed in a small volume of dilute sodium hydroxide, and then determined either by an acidimetric or spectrophotometric titration. Excessive dilution of the released fluoride by condensate, which occurs if pyrohydrolytic or Willard and Winter distillation techniques are used, is avoided; this is advantageous when microgram quantities of fluoride are to be determined. For the more difficultly decomposed fluorides,-e.g., alkali and alkaline earth fluorides-a reactive oxide such as uranium oxide or tungstic oxide is added to the sample to accelerate release of the fluoride. Optimum operating conditions are established whereby milligram quantities of fluoride can b e separated in 20 minutes or less and subsequently determined with a coefficient of variation of 0.1 For trace amounts of fluoride (-10 y) the coefficient of variation is of the order of 5%.
70.
N
for a rapid, simple, effective method for the separation of microgram quantities of fluoride from compounds of thorium and uranium, and their aqueous slurries, prior to the determination of the fluoride, led to the development of a pyrolytic method which has distinct advantages over earlier methods. Fluoride was usually separated from interfering substances as silicon tetrafluoride, prior to the development of the Willard and Winter distillation method (9) in 1933, in which fluoride was volatilized as hydrofluosilicic acid. EED
1546
ANALYTICAL CHEMISTRY
The next significant advance was the introduction of the pyrohydrolytic method of Warf, Cline, and Tevebaugh (8), whereby fluoride salts were hydrolyzed in a stream of superheated steam, the volatile fluoride mas recovered in the condensate, and then determined b y one of several methods (4, 6). Modifications of Warf’s apparatus have since appeared in the literature. Susano, White, and Lee, (7) substituted a nickel apparatus for the platinum tube. Lee, Edgerton, and Kelley (3) constructed a special apparatus for the pyrohydrolysis of semimicro and micro quantities of fluoride compounds in radioactive materials. Hibbits ( 2 ) applied pyrohydrolysis to the removal of fluoride from uranyl fluoride. Recently, Gillies, Keen, Lister, and Rees (1) described a silica apparatus for the pyrohydrolytic determination of fluoride and uranium in uranium tetrafluoride. All earlier work involved the hydrolysis of fluoride salts a t a high temperature by superheated steam, for lvhich Karf adopted the term pyrohydrolysis. The proposed method uses moist oxygen rather than steam in the reactor to purge the system during the high temperature pyrolysis of either readily or difficultly decomposed fluoride compounds. I n the pyrolysis of the latter, the release of fluoride is accelerated b y the addition of a reactive oxide to the sample; uranium oxide, which is widely used in the pyrohydrolytic method, is a satisfactory accelerator. Of other oxides evaluated, tungstic oxide was found to be more effective than uranium oxide a t a lower temperature. I n addition, a simplified apparatus was designed in which the reactor tube, fabricated of fused silica with a n air-cooled delivery tube, replaced the water-cooled condenser. The effect of variables, such as reactor tem-
perature, f l o ~rate, moisture content of the purge gas, and materials used as reactants, on the separation of micro and macro quantities of fluoride was studied, and optimum operating conditions were established. I n general, the pyrolytic method is more rapid than either the pyrohydrolytic or Willard and TJ7inter distillation procedures. The fluoride is recovered in a small volume of solution rather than in a large volume of condensate. This is advantageous if microgram quantities of fluoride are to be determined, because high sensitivity can be attained without concentrating the solution prior to determination of the fluoride. Also, the pyrolytic method is applicable to a wider variety of materials than the hydrofluosilicic acid distillation method of Willard and Winter, the limitations of ivhich are summarized b y Simons (6). Aluminum, zirconium, or gelatinous silica, which retard or prevent volatilization of the fluoride in the distillation procedure, offer no difficulty in the pyrolytic separation. APPARATUS
The apparatus (Figure I) consists of the following components. Gas cylinder n-ith flowmeter and regulator. Oxygen is supplied from the cylinder, while the flow of the gas is measured b y means of a calibrated flowmeter. Water tower. Oxygen is passed through the scrubbing water tower, containing water at room temperature, to saturate the gas with moisture. Reactor tube, fused silica, 24 inches in length and 11/4 inches in outer diameter. The reactor tube is connected to the gas supply by a 29/42 standard taper joint, lubricated with powdered graphite. Delivery tube, fused silica, 10 inches in length and 3/s inch in outer diameter. This tube is fused a t right angles t o the exit end of the reactor tube.
