1970
ANALYTICAL CHEMISTRY
Results. The results obtained over the entire range of molybdenum tested are in good agreement with the values sought, not only for the simulated tungsten-bearing steels, but also for the other combinations. The presence or absence of tungsten apparently does not influence the results. Cobalt up to 5y0 is also without effect. Corrections made for chromium appear to be satisfactory. The largest correction for chromium amounted to 6% of the observed per cent transmittance. One of the samples included in Table V, A-C 19-9 DL, is not a certified standard sample, but it bears a molybdenum value which is the average of results obtained by the Research Laboratories and cooperating laboratories. The proposed method gave good results against the accepted value whenever this sample was used. Statistical Analysis. A statistical analysis was made of 55 results for 46 different samples, not including those which required a multiplication factor of 10. The analysis of these results, made as outlined by Youden ( l o ) , showed the method to be free of both constant and relative errors. The standard deviation of a single determination of molybdenum is 0.0149. Therefore, 95% of the values determined in the range of 0.01 to 1.34% will be within ~k0.03%from the true value. Aliquot Samples. Samples 132, 134, and 160 in Table V required a multiplication factor of 10, because a one-tenth aliquot of the original sample was taken. Slightly low results were obtained. However, they were comparable to the lowest values reported on the certificates for each of the samples. The results obtained for Kational Bureau of Standards standards 132, 134, and 160 were 6.98, 8.59, and 2.90%, respectively. The cor-
responding lowest values reported on the respective certificates were 7.01, 8.58, and 2.91%. Elapsed Time. This method produces results i n about 1 hour as compared with 2 hours for the peroxide-sulfuric acid method and 3 hours for the titrimetric method. Neither of these slower methods can be expected to give results which are more accurate than those obtained by the proposed method. Limits of Application. Maximum values encountered in the testing of the method were 8.68% molybdenum, 18.05% tungsten, 19.13% chromium, and 5.00% cobalt. Although it is unlikely that higher values will be encountered in routine work, it is possible that the method can be applied satisfactorily in such cases. It may be necessary, hon-ever, to extend correction curves and/or to.make other adjustments of the method. LITERATURE CITED (1) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffman, J. I., “8pplied Inorgaric Analysis,” 2nd ed., p. 315, Wiley, New York, 1953. (2) Zhid., p. 316. (3) Zhid., p. 689. and Allen, H. O., IXD.ENG.CHEM.,ANAL.ED.,7, (4) Hurd, L. 396-8 (1935). ( 5 ) James, L. H., Ihid., 4, 89-90 (1932). (6) Kapron, M., and Hehman, P., Ibid., 17, 573-6 (1945).
c.,
(7) Killeffer, D. H., and Lina, d.,“Molybdenum Compounds,” p. 154, Interscience, New York, 1952. (8) Methods of Analysis Committee, J . I r o n Steel Znat. ( L o n d o n ) , 172, 413-15 (1952). (9) Poole, G. M., Zron Age, 148, 62 (Oct. 9, 1941). (10) Youden, TV. J., AXAL.CHEY.,19, 946-50 (1947). RECEIVEDfor review April 15, 1955. Accepted August 31, 1955. Division of Analytical Chemistry, 127th Xeeting, ACS, Cincinnati, Ohio, March 1955.
Use of Vanadium Pentoxide in Combustion Method for Sulfur in Refractory Materials D. B. HAGERMAN and R. A. FAUST Research and Development Department, Socony
Mobil Laboratories, Paulsboro,
A method is proposed for the determination of sulfur in inorganic materials that are highly resistant to decomposition by pyrolysis. The method is based on the reaction of sulfur with vanadium pentoxide, possibly by replacement of the sulfur through the formation of metal pyrovanadates. The oxide is mixed intimately with the powdered sample and heated in a quartz tube at 900” to 950” C . The liberated gases are absorbed in a solution of hydrogen peroxide and the resulting sulfuric acid is determined allralimetrically. Such materials as inorganic sulfates, silica-alumina catalysts, and suspensions of barium sulfate in oil have been successfully analyzed by this method. Inorganic sulfides require the addition of chromium to the ignition mixture, to obtain complete sulfur recovery. An analysis may be completed in 35 to 45 minutes, thus offering an appreciable time-saving factor as compared to any of the conventional wet chemical methods.
N. J.
There are, however, two methods which do not use combustion tube technique. One involves fusion of the refractory sample with sodium peroxide in a metal bomb (2). The other requiree treatment of the material with hydrofluoric and perchloric acids (0), reduction of sulfates to sulfides, and a final iodometric titration. Both methods are lengthy and the latter procedure is of uncertain value in estimating sulfur in forms other than sulfate. To circumvent the disadvantages of the afore-mentioned methods, attention was directed to some rather meager information revealed by Kirsten (6-8), who volatilized residual sulfur from the ash of certain biological materials by heating in a combustion tube with vanadium pentoxide. It was also used by Zinneke ( 1 2 ) in his micro combustion method for sulfur in organic compounds containing phosphorus and fluorine. On the basis of this information, a rapid and simplified analytical method was developed for the total sulfur assay of inorganic and certain organic materials by combustion with vanadium pentoxide APPARATUS
C
OMBUSTION methods for determining sulfur have generally proved inadequate when applied to refractory materials. Well established procedures such as the Carius ( S ) , the Parr oxygen bomb (I), heating in a horizontal (4,11) or vertical tube (6) in the presence of oxygen or air have failed to provide a satisfactory means for determining aulfur in such substances as cracking or reforming catalysts, certain inorganic sulfates, and inorganic sulfides.
Furnace Assembly (Figure 1). The apparatus employed in this investigation consists of a single quartz combustion tube and furnace unit similar to that described by Peters, Rounds, and Agazzi (IO),except that a single rather than a dual unit was used. REAGENTS
BARIUM SULFATE, rea ent grade. CALCIUM SULFATE,( 8aS04.2H20),reagent grade.
1971
V O L U M E 27, NO. 12, D E C E M B E R 1 9 5 5 HYDROGEN PEROXIDE, 6% solution. Dilute 200 ml. of reagent grade 30% hydrogen peroxide to l liter with water. METHYLPURPLE INDICATOR, prepared solution available from E. Machlett & Co., New York 10, N. Y. SILICA,powdered reagent grade. SODIUM HYDROXIDE SOLUTIONS, carbonate-free, 0.10 and 0.01N, prepared from reagent grade sodium hydroxide. SODIUM SULFATE,anhydrous, reagent grade. VANADIUM PENTOXIDE, reagent grade, available from Eimer & Amend, New York, S. Y. PROCEDURE
With the combustion tube in position and the furnace adjusted to 900' to 950' C., 30- and 10-ml. volumes of 6% hydrogen peroxide solution are added to the primary and secondary absorbers, respectively. Air flow through the combustion assembly is regulated to a rate of 2 liters per minute by applying vacuum at the absorber exit.
SPRAY TRAP SECONDARY ABSORBER COMBUST ION TUBE
FURNACE
H PRI M A R Y ABSORBER
AIR-I
RHEOSTAT'
PYROMETER
Figure 1. Combustion apparatus
Preparatory to analysis of the sample, a thin layer of silica is distributed evenly on the bottom of the combustion boat, followed by a similar layer of vanadium pentoxide (approximately 1 gram). A 0.3- to 0.6-gram portion (depending on sulfur content) of the finely-ground sample is distributed on a thin uniform layer over a prepared bed of the weighed combustion boat which is subsequently reweighed to obtain sample weight. Another layer of vanadium pentoxide, followed by a final covering of silica similar in amount to the first two layers, is added preparatory to inserting the boat in the tube. The boat is placed on a concave metal shield made of thin gage nickel and approximately as long as the boat, to protect the quartz tube against splattering. With the aid of a metal rod, the shield and boat are pushed into the tube to a point within 50 mm. of the heated zone of the furnace. A wire gauze heat-reflector is placed on the outside of the combustion tube above the sample. Heat from a Fisher blast burner is applied approximately 30 mm. in front of the boat. The burner is adjusted to provide a temperature of 900" to 950" C., similar to that obtained in the furnace. After 3 minutes, the flame is advanced slowly along the tube until the furnace is reached, allowing 25 to 30 minutes for the completion of this operation. Following this, the short exit end of the tube is also heated with the gas flame for 1 to 2 minutes. The open end of the combustion train is stoppered and the vacuum line at the absorber outlet is disconnected. The partial vacuum remaining in the system serves to draw the contents of the secondary absorber into the primary absorber. After collecting the water washings of the secondary in the primary absorber and addition of 3 drops of methyl purple indicator, the sulfuric acid is titrated directly in the absorber using 0.01 or 0.1N sodium hydroxide, depending on the expected sulfur content. Acidity due to compounds other than sulfuric acid must be determined in order to apply proper correction. DISCUSSION OF METHOD
Vanadium pentoxide reacted SO vigorously a t elevated temperatures that it fused with the glazed surfaces of the porcelain boats rendering them unfit for more than a single determination. This difficulty was overcome, however, by lining the boat with a protective layer of silica before introducing the sample and vanadium pentoxide. Also an additional layer of silica was
placed on the combustion mixture to aid in retarding the vigorous reaction when fusion of vanadium pentoxide with the sample occurred. Extending the heating period t o 55 minutes was found necessary in the case of those samples containing approximately 15% or more of sulfur. An air purification assembly is commonly used with the a p paratus prescribed for this method and is highly recommended if the presence of atmospheric sulfur is a t all likely. However, the possibility of error from this source in the procedure as described was considered to be negligible. Replicate determinations of as little as 0.01% sulfur (Table 11) appear t o justify this conclusion. RELIABILITY OF METHOD
Synthetic samples containing known amounts of calcium sulfate, sodium sulfate, and inorganic sulfides were mixed with finely ground silica or chromia-alumina compounds, and were subjected to the above analytical scheme Sulfur recoveries obtained on these materials are presented in Table I. Molybdenum sulfide required the presence of chromia in the combustion mixture, in order to obtain acceptable results. This was accomplished by including a powdered chromia-alumina matrix, containing 20 to 30% chromium, in the combustion mixture (Table I), in the proportion of about five parts of chromium to one of sulfur.
Table I.
Analyses of Inorganic Sulfates and Sulfidea in Synthetic Catalysts
Sulfur, Wt. Yo Added Found 0.50 0 . 5 1 , O .4 8 . 0 . 4 9 0.36,0.36 0.36 0.22 0.22,0.22 1.20 1 . 1 8 , 1 . 2 3 , l .1 6 , 1 . 2 0 2.40 2.36,2.42 5.11,5.10 5.27 Sulfide values obtained b y adding t o combustion mixture a chromiaalumina catalyst containing 27% chromium. Inorganic Compound Used in Mixture Barium sulfate Calcium sulfate Sodium sulfate Molybdenum sulfide
Table 11.
Solid Catalysts and Related Substances
Material Tested T C R bead cata.lysta A B TCR bead catalyst plus molybdenum sulfide
C
D NazSOi-catalyst mixture Petroleum coke deposit 30% chromia, 70% alumina.
Total Sulfur, Wt. % Theory or determined VnOa method 0.01,0.00,0.01 0.42
1.20 2.39 0.66 7.44,7.50
0.01,0.01,0.01 0 . 4 1 , O .41,O. 43 1.11,1.16,1.20 2.35,2,42 0.66,O. 66 7.39, 7.47. 7.35, 7.43
The agreement between the amount of sulfur added and that found by the proposed method is acceptable. These results attest to the effectiveness of the vanadium pentoxide combustion technique in quantitatively liberating sulfur from materials of high thermal stability. -4pplication of the proposed method to the determination of total sulfur in solid catalysts and related substances was next investigated. Several experimental catalysts were obtained and the sulfur content was determined by the vanadium pentoxide method. These results, compared to either calculated values or to values obtained by the iodometric procedure ( 7 ) are reported in Table 11. I n order to extend the application of the method to the sulfur assay of organic systems of diverse compositions, such as may be encountered in petroleum research laboratories, a series of blended samples was analyzed. The first two samples repre-
1972
ANALYTICAL CHEMISTRY
sented lubricating oils containing a barium sulfonate additive in moderately high concentrations. The third sample contained a suspension of barium sulfate in mineral oil. Other tests were run on blends consisting of barium sulfonate mixed with an oil of high disulfide content. These blends ranged in concentration from approximately 4 t o 20% sulfur. Analytical data on these materials are given in Table 111. On the basis of the above data, the standard deviation for per cent of sulfur is as follows: 70s 0 to 1 1to 5
Over 5
Std. Dev. 0.006 0.06 0.10
INTERFERENCES
Other acid-forming constituents interfere, in which case a gravimetric determination of the evolved sulfur is required. CONCLUSION
Total d f u r may be readily determined in materials of high thermal stability by combustion with vanadium pentoxide at 900” t o 950’ C. Elements such as calcium, barium, sodium, and aluminum do not interfere, although acid-forming elements do cause interference. The method is applicable t o organic as well as inorganic substances. I n materials containing molybdenum sulfide, the presence of chromium in the combustion mixture is necessary. Application of the method to the sulfur assay of paints, ceramics, shale, and minerals is suggested. ACKNOWLEDGMENT
The authors wish t o express their appreciation t o E. T. Scafe and Perry Swanson for their cooperation and assistance in the
Table 111. Application to Organic Systems Containing Barium Sulfur Sample Added Barium sulfonate in lubricating oil 1 1.51 2 1.82 Barium sulfate suspended in mineral oil 0.14 Barium sulfonate and disulfide oil 1 2 3 4
Wt. % of Found
Rt. % of Barium Present
1 44.1 51 1 8 0 , l 85
4 84
0 13,0.14
0 59
7 48
preparation of this paper, and t o A . G. Herzog for obtaining part of the analytical data. LITERATURE CITED (1) Am. Sac. Testing Materials, Philadelphia, Pa., “Standards on Petroleum Products and Lubricants,” Method 129-52, 1954. (2) Chalmers, A,, and Rigby, G. W., IXD.ENG.CHEM., ASAL.ED , 4, 162 (1932). (2) Gatterman, L., “Laboratory Methods of Organic Chemistry,” pp. 65-8, blacmillan, Xew York, 1932. (4) Grate, W., and Krekler, H., Angew. Chem., 46, 106 (1938). (5) Hagerman, D. B., ANAL.CHEX.,19, 381 (1947). (6) Kirsten, W., Ibid., 2 5 , 7 4 (1953). (7) Kirsten, W., Mikrochim. Acta, 35, 175 (1950). (8) Kirsten, W., unpublished paper, Institute of lledical Chemistry, University of Uppsala, Uppsala, Sweden. (9) Luke, C . L., IND. ENG.CHEM.,A N ~ LED., . 5 , 298 (1945). (10) Peters, E. D., Rounds, G. C., and .Igasai, E. J., A s ~ L .CHEM., 4, 710 (1952). (11) Rice-Jones, TV. G., Ibid., 25, 1383 (1953). (12) Zinneke, F., Z.anal. Chem., 132, lis (1951).
RECEIVED for review M a y 12, 1955. Accepted September 1
1955
Determination of lead in lead Sulfide Ores and Concentrates CHARLES A. G O E T Z and FREDERICK 1. DEBBRECHT lowa State Co//ege, Ames, b w a
The present work was begun to develop a more accurate and rapid method for the determination of lead in lead sulfide ores and concentrates. .-i method employing the perchloric acid dissolution of the sample, electrodeposition of lead as lead dioxide, and complexometric titration of the lead with HexaVer is proposed. The entire procedure for the determination of lead can be completed in less than one hour with a precision of about one part in a thousand. A longer procedure is given for the elimination of arsenic, antimony, and tin if these elements are present in sufficient amounts to interfere in the deposition of lead. The method avoids the time-consuming and inaccurate acetate extraction from separation of lead from barium. Not only is the complexometric titration faster than drying and weighing the lead dioxide deposit, but it also avoids the use of an empirical factor for the deposit.
T
H E soparation of lead from barium in the presence of sulfate ions based upon the acetate extraction of the lead has been shown to be incomplete (f, 5 ) . The Separation of lead as lead dioxide by electrodeposition with subsequent weighing is objectionable, because of the empirical conversion factor that varies with the conditions. This factor varies from 0.845 ( 3 )up to the theoretical factor of 0.8662 ( 7 ) . Loomis ( 4 ) shoJTed that lead and copper couldbesimultaneously
deposited on the anode and cathode, and that the lead could be determined volumetrically using HexaVer (disodium dihydrogen 1,2-diaminocyclohexane iV,,V,‘,S’,S’-tetraacetate), by adding an excess of the HexaVer and back-titrating tT-ith standard magnesium chloride solution. Flaschka ( 2 ) titrated lead directly using Versenate [disodium dihydrogen (ethylenedinitrilo) tetraacetate] in the presence of tartrate to keep the lend in solution at a p H of 10. The authors found that the lead ore samples used for student analysis (prepared and sold by Hach Chemical Co., .hies, Iowa) could be dissolved rapidly and completely in about 5 minutes using boiling 72% perchloric acid, and that the lead could be quantitatively separated from the barium, iron, arsenic, bismuth, and silica present in the ores even when appreciable amounts of sulfate ion were present. The lead dioxide was viashed and dissolved using nitric acid and hydroxylanimoniuni chloride, and the lead determined by direct titration using Hexaver. MATERIALS AYD APPARATUS
Solutions. STANDARD HEXAVER. rlnalytical reagent grade HexaVer (obtained from Hach Chemical Co., h i e s , Iowa) was used in preparing the standard solution, by dissolvihg about 7.8 grams per liter of distilled water and then standardizing against the primary standard lead nitrate, standard calcium chloride solution, and the gravimetriaally standardized lead perchlorate solution. LEADPERCHLORATE. Twenty-one grams of test lead riere dissolved by boiling in 72% perchloric acid for a few minutes. The
