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V O L U M E 21, NO. 1 2 , D E C E M B E R 1 9 4 9 16) Iirubets, M. C . , Duel, H. J., a n d Hanley, B. J., J . Nutrition, 29, 245 (1945). (6) Koehn and Sherman, J . Biol. C‘hem., 132, 527 (1940). ’7) Lesher, M., Brody, J. K., Williams, H. H., and Macy, I. B., Am. J . Diseases Children, 70, 182 (1945). ‘8) Macy, I. G., Outhouse, J., Graham, A . , and Long, M. L., J . B b l . Chem., 73, 175 (1927). 19) Oser, B. L., Melnick, D., arid Pader, M.,IND.ESG. CHEX.. ANAL.E D . , 15, 724 (1943). ‘10) Parrish, D. B., and Caldwell, $1. J., J . Lab. Clin. X e d . , 29, 992 (1944). (11) Sobel, A . E., and Snow, S.D., J Biol. Chem., 171, 617 (1947).
(12) Sobel, 8 . E., and Werbin, H., ANAL.CHEM.,19, 107 (1947). (13) Sobel, A . E., and Werbin, H., IND.EXQ.CHEM.,ANAL.ED., 18, 570 (1946). (14) Sobel, A. E., and Werbin, H., J . B i d . Chem., 159, 681 (1945). (15) Squibb, R., Cannon, C . Y., and Allen, B. S., J . Dairy Sci., 6, 421 (1948). (16) Swain, L. A , , ISD. ENG.CHEM.,ANAL.ED.,16, 241 (1944). (17) Wall, M. E., and Kelley, E. D., ANAL.CHEM.,20, 757 (1948). RECEIVED February 17, 1949. Presented before the Dit ision of Biological Chemistry at the 114th Meeting of the AxEnxrAN C H E w c A L SOCIETY, Washington, D. C.
Determination of Traces of Iron, Nickel, and Vanadium in Petroleum Oils FRANCES M. WRIGHTSON The M . W . Kellogp Company, Jersey City, A‘. J . Methods have been developed for the determination of iron, nickel, and vanadium in petroleum oils. The sample is ashed and taken up with potassium bisulfate and the aqueous solution of the bisulfate cake is analyzed by spectrophotometric procedures. The measurements are based on the development of colored solutions by reagents specific for one element under the analytical conditions. Iron is determined as the iron-2,2’-bipyridine complex, nickel as col-
T
HE present trend in ratalytic cracking is in the direction of utilizing heavier oils as feed stocks. It has been found,
however, that as deeper cuts are taken into the crude the concentration of metal contaminants increases rapidly. These metals, which are in the main deposited on the cracking catalyst, may adversely affect both the ielectivity and activity of the catalyst. This is understandable when it is realized that a catalyst replacement rate of 0 5 pound (0.23 kg ) of catalyst per barrel of oil containing only 1 p p.m. of inetal could result in an equilibrium catalyst containing about 0 06 weight % of the metal The major metal cont:tniiriarits in petroleum oils are usually aluminum, sodium, iron, nickel, and vanadium with, frequently. lesser amounts of silica, magnesium, calcium, copper, manganese. lead, tin, barium, zinc, molybdenum, chromium, and titanium The form in which the metals occur in the crude is not known; in all probability they are present both in suspension as inorganic salts and in solution as metallo-organic compounds. illthough the metals concentrate in the residual fraction when the crude 1s processed by either distillation or propane extraction, small quantities of the metals appear i n the lighter fractions from either method of processing The concentrations of iron, nickel, and vanadium have beeu determined on a number of cracking feed stocks in order to obtain an index as to the probable degree of contamination of the cracking catalyst I t has been found necessary to devise methods for determining as little as 0.5 p p m of each of these metals irith a prohable accuracy of 10% In order to provide an analytical procedure that would be applicable to crudes from a variety of fields, it was necessary to develop methods that would tie subject to a minimum of interference from a large number of possible elements. I n the interests of speed and economy it was also desirable to avoid the use of separation techniques in eliminating the interfrrences
loidal nickel dimethylglyoxime, and vanadium by reaction with diphenylbenzidine. The analyses can be made with an accuracy of *lo%, or better, in the presence of the many elements likely to occur in petroleum oils. Interferences are minimized or eliminated without recourse to separation techniques. The detection limits are approximately 0.005 mg. of vanadium pentoxide of nickel oxide, 0.007 mg. of nickel oxide, and 0.01 mg. of ferric oxide.
The spectrophotometric procedure reported here fills these requirements when applied to the residue left from the complete ashing of the oil. SAMPLE PREPARATIOh
The oil to be analyzed is completely ashed by the follon-ing technique. The sample is thoroughly agitated and a weighed amount is poured into a 200-ml. platinum evaporating dish. It is then warmed to the flash point and allowed to burn quietly without further heating. The residual coke is ignited by heating n i t h a Meker burner. Fresh charges of oil are added until an observable amount of ash, which may be evidenced as a stain on the dish, is obtained. It is not necessary to ash completely between successive additions of oil. For concentrations of the order of 1p.p.m., 100 to 200 grams of oil are usually sufficient. When sufficient oil has been burned the residue is ignited to constant weight. The silica is then removed in the usual manner by treatment with hydrofluoric acid. Following this the residue is fused with 5 grams of analytical grade potassium bisulfate and, after cooling, the bisulfate cake is dissolved in water containing not more than 1 ml. of 12 X hydrochloric acid. If much acid is used, too great a dilution occurs in subsequent neutralizations. The solution is diluted to 100 ml. in a volumetric flask. A bisulfate fusion is used, rather than a carbonate fusion or an acid, to ensure the complete dissolution of aluminum oxide which may occlude the other elements. It has been found desirable to use 5 grams of potassium bisulfate in order to enwre adequate wetting of the platinum dish. DEVELOPMENT OF METHODS
All colors are developed in 25-ml. volumetric fl&sksto minimize the volumes of the solutions t o be tested. Color densities are measured with a Beckman spectrophotometer (Model DU) using 1-em. Corex cuvettes with distilled water in the reference cell. Greater sensitivity could be obtainea bv the use of longer cuvettes; in this case a reagent blank should he employed in the reference cell, whereas in the procedures used here the blank correction was negligible. All reagents emplored are of analytical grade, except as noted. Reference solutions of the
ANALYTICAL CHEMISTRY
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metal ions are prepared from ammonium metavanadate, nickel ammonium sulfate hexahydrate, and electrolytic iron.
Iron. Of the many methods available for the colorimetric determination of iron, the method of Moss and Mellon (3) has been selected as being relatively simple and free of interferences from the other metals likely to be present. This procedure is based on the red complex formed between 2,2’-bipyridine and ferrous ion. Tests have shown that comparable concentrations of nickel, vanadium, and zinc, present simultaneously, lead to an interference of about 374, which is within tolerance. Of the interferences listed by Moss and Mellon, these three metals are the only ones that have been found to occur in petroleum in high enough concentrations to be troublesome. Aluminum will interfere if present in 100-fold excess, owing to the formation of the hydroxide. Table I.
Duplicate Analysefi
Venezuelan distillate KO.1 Venezuelan distillate No. 2
Fez08 P.p.m. 1.1 1.0 0.4 1.1
VzOs
NiO P.p.m.
P.p.m.
0.30 0.30 0.9 1.1
3.0 2.9 16 14
Table 11. Determination of Iron, Nickel, and Vanadium in Known Solutions (All quantities are expressed as mg. in 100 ml.) Fez03 NiO VzOs hIoOa MnOz CrzOs MgO 74.3 10.2 14.3 4.8 0.0 0.0 15.1 0.0 Synthesis 74.2 10.1 13.7 .. .. .. ... Analysis 17.2 25.5 85.7 2.4 0.0 0.0 15.1 0.0 Synthesis 17.2 24.3 80.3 .. .. .. ... ... Analysis 10.0 9 . 9 3 200 0.0 1.58 0.0 200 8.3 Synthesis ,. .. .. ... ... 10.1 9 . 5 1 195 Analysis 3.8 4.07 0.0 0.0 0.0 7.6 0.0 Synthesisa 2 1 . 4 .. .. 3.98 .. . , . ... Analysis 0.0 0.0 3.62 0 . 0 31.7 29.2 0.0 0.0 Synthe?isa .. .. 3.77 . . .. ... ... Analysis 5 Samples ignited and subjected t o bisulfate fusion t o simulate sample preparation step.
...
.
. .
...
Only two revisions in the procedure of Moss and Mellon are required. The pH of the solution is held between 5 and 8, and the color development solution is allowed to stand overnight before readings are taken. Moss and Mellon indicate that the color development reaches a maximum almost immediately. In this application the color forms slowly, probably because of the high salt concentration arising from the bisulfate fusion. The absorption due to the 2,2’-dipyridine-iron complex is measured a t 5200 A. The range is from 1.0 to 7.0 micrograms of ferric oxide (FenOa) per ml., corresponding to transmittances of between 80 and 20%. Beer’s law is obeyed over the indicated range. Nickel. Most colorimetric methods for nickel employ the development of the wine-red color of colloidal nickelic dimethylglyoxime. This method, although the most reliable, suffers from interferences and it has been recommended (d, 4 ) that a separation be carried out prior to color development. The procedure developed here is satisfactory without prior separation. Iron interference, for concentrations less than 100 micrograms per ml., is suppressed by the addition of tartrate ion. Aluminum, vanadium, molybdenum, and manganese cause less than 2% error when present in 50-fold excess. The major interferences, copper and cobalt, are not important in this case, as it has been shown by qualitative spectrographic analyses of a number of oils that they occur in negligible amounts. Alcohol is not added to stabilize the colloid, as in the procedure of Mitchell and blellon ( d ) , because this would precipitate potassium sulfate. The solution to be tested is neutralized with 0.5 N ammonia, and tartaric acid is added, then bromine water, followed by con-
centrated ammonia; dimethylglyoxime is added before the solution is made up to volume. The color develops gradually, reaching maximum intensity in about 1 hour, and then fades slowly. Because the time to attain maximum intensity depends on the other constituents present, several readings are taken to determineothe maximum value. The transmittance is measured a t 4660 A. The range is from 0.5 to 5 micrograms of nickel oxide per ml. Beer’s law is obeyed over the indicated range.
Vanadium. The two most thoroughly investigated methods for vanadium are the hydrogen peroxide ( 7 ) and the phosphotungstic acid (6) procedures. These methods are not applicable, because the peroxide method is not sufficiently sensitive and the formation of vanadophosphotungstate is prevented by the precipitation of potassium tungstate. An attempt to use the vanadophosphomolybdate complex, as in phosphorus analysis, has not been successful. A method based on diphenylbenzidene has been developed. Oxidation indicators such as aniline, diphenylamine, phenylenediamine, benzidine (1,6),and diphenylbenzidine ( 4 ) have frequently been suggested as colorimetric agents. Their chief fault is that they are not specific for any particular reducible ion. It is possible to consider them in the present application, inasmuch as the coking, ashing, and bisulfate fusion used in sample preparation preclude the presence of most metals in their highest oxidation state. Diphenylbenzidine has been chosen as giving the most stable color and apparently having the highest oxidation potential of the indicators tested. Ferric iron is the element most likely to have an oxidation potential high enough to be considered as an interference. In acid solution ferric ion concentration of 4 micrograms per ml. causes no color formation. The blue color due to the partially oxidized form of diphenylbenzidine is most stable in acid solution, phosphoric acid being particularly favorable; this is a fortunate circumstance, for phosphoric acid removes ferric ion by complex formation. Other active oxidizing agents, including nitric acid, must be absent. In the determination of vanadium a slightly acid ali uot of sample is treated with a few drops of bromine water, wqich is then expelled by gentle heating; this step is necessary to ensure that all the vanadium is in the +5 state of oxidation. The acidic bromine solution does not oxidize other metallic ions to valence states capable of forming the blue oxidation product of diphenylbenzidine. Concentrated phosphoric acid and water are then added and after the solution has been cooled the diphenylbenfidine reagent is added. The transmittance is read a t 5750 A., 15 minutes after color development. The colored solutions obey Beer’s law over the working range of 0.4 to 3 micrograms of vanadium pentoxide per mi. RECOMMENDED COLORIMETRIC PROCEDURES
Iron. REdGENTS. Hydroxylamine hydrochloride, 5% in water. Ammonium hydroxide, 0.5 N and 2 N . 2,P‘-Bipyridine, 0.05% in water. PROCEDURE. Titrate an aliquot of the sample with 0.5 N or 2 N ammonium hydroxide solution to the methyl orange and phenolphthalein end points; the choice of normality depends on the acidity of the sample. Transfer another aliquot of the sample containing 0.02 to 0.18 mg. of iron (as FesOs) to a 25-ml. volumetric flask, and add 1 ml. of hydroxylamine hydrochloride solution. Adjust the pH to betmeen 5 and 8 with a volume of ammonium hydroxide equivalent to the mean of the values determined by the aforementioned titration. Add 5 ml. of 2,2’bipyridine solution, dilute to 25 ml. with water, mix, and allow the solution to stand overnight. Read the transmittance at 5200 A. (0.05-mm. slit width) using distilled water as a blank. Determine the ferric oxide content from a calibration curve and calculate the total iron in the original sample from the dilutions used. An optical density of 0.1 in a 1-cm. cell is equivalent to 0.89 microgram of ferric oxide per ml. Nickel. REAGENTS.Tartaric acid, 20% in water. Saturated bromine water. Ammonium hydroxide solution, concentrated, 0.5 N and 2 N . Dimethylglyoxime, 1%in ethanol. PROCEDURE. Transfer an aliquot of the sample containing 0.01 to 0.12 mg. of nickel oxide to a 25-ml. volumetric flask. Adjust the pH, using the same relative amount of ammonium hydroxide as in the procedure for iron. 4 d d 1.25 ml. of tartaric
V O L U M E 2 1 , N O . 12, D E C E M B E R 1 9 4 9
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Table 111. Analyses of Processed Crude Oils Oil
oA.P.I
Arabian residuum Arabian vacuum distillate California residuum California vacuum distillate Colorado residuum Colorado propane decarbonized oil Hsstings propane decarbonized oil Juesepin vacuum distillate Lighthlercedesvacuumdistillate
Mid-continent vacuuni distillate
16.3 21.4 16.7 26.1 17.8 24.5 22.4 20.2 23.8 33.5
FenOs
Xi0
VzOs
P.p.m.
P.p.m.
P.p.m.
139 7.7 25 0.1 12.0 0.81 0.68 4.6
2.0 0.0 46 0.5 1.3 0.14 0.86 0.70 0.30 0.60
6.5 1.8 59 0.8 4.3 0.11 0.20 0.60 3.0 2.4
1.1
1.3
acid solution, followed by 1.25 ml. of saturated bromine water and 2.5 ml. of concentrated ammonium hydroxide. Mix the solutions and then add 1.25 ml. of dimethylglyoxime solution. Dilute to 25 ml. with water and mix. Read the transmittance a t 4660 -1.(0.05-mm. slit width) a t the time of maximum color development (approximately 1 hour after mixing), using distilled water as the blank. Determine the nickel oxide content from a calibration curve and, from this the nickel content of the original sample. An optical density of 0.1 in a 1-cm. cell is equivalent to 0.57 microgram of nickel oxide per ml. Vanadium. REAGENTS. Diphenylbenzidine. Prepare a saturated solution of diphenylbenzidine in 957Cethanol. Filter and dilute 1 to 1 with phosphoric acid, specific gravity 1.71. This procedure was desirable in order to provide a clear, stable solution that does not produce cloudiness during color development. Saturated bromine water. Phosphoric acid, specific gravity 1.71. PROCEDURE. Transfer an aliquot of the sample containing 0.01 to 0.08 mg. of vanadium pentoxide to a 25-ml. volumetric flask. Add 1 drop of saturated bromine water and then heat the flask gently over a microburner until all the color due to the bromine is removed; this takes about 5 minutes. After cooling the flask, add 10 ml. of phosphoric acid and cool again. Dilute with water to almost 20 ml. and cool thoroughly after mixing. Add 5 nil. of the diphenylbenzidine solution andodilute with water to the mark. Read the transmittance a t 5750 A. (0.05-mm. slit width) 15 minutes after color development, using distilled water as the blank. Determine the vanadium pentoxide content from a calibration curve. An optical density of 0.1 in a 1-cm. cell is equivalent t o 0.38 microgram of vanadium pentoxide per ml.
Table IY. Jfetal Content Yersus Extent of Distillation Yield of distillate, vol. R FetOa, ~ i . p . i n . KiO, p . p . n i . VnOr, p.p.m.
l?! (feed) JJ
38 435
66.2 0.40 0.85 13.8
54.2 0.30 0.35 6.2
The reagent solution is very dilute and its strength should be determined by following the above procedure, but using an excess of standard vanadate solution and 5 ml. of reagent. From this the maximum optical density that can be relied upon may be determined. If, in an analysis, this maximum optical density is approached, the unknown solution should be diluted. PRECISION AND ACCURACY
The precision of the method can be considered as dependent on three separate stages of the analysis: photometry, color development, and sample preparation. The accuracy of measurement with a Beckman spectrophotometer is considered to be within about 1%. The color development on standard samples of nickel, vanadium, and iron is accurate to within 275, as deviations from the best straight line through the calibration points (optical density us. metal content) were all within this value. The slope of the curve was the same on two independent calibrations more than a year apart for all three determinations. The ashing procedure may have inherent errors, principally due to volatilization and incomplete fusion. However, check analyses indicate that this is not large. Table I shows two examples of the reproducibility to be ex-
pected. The two analyses of distillate S o . 2 were run on samples taken several months apart; the discrepancy in iron content is possibly due to contamination from the container. The accuracy depends on the size of the sample prepared from a reasonable amount of oil and on interferences. The colorimetric procedures can determine 0.01 mg., or less, of each of ferric oxide, nickel oxide, and vanadium pentoxide. Hence, the original 100-ml. solution should contain a t least 0.1 mg. of the metal oxides. Smaller quantities of the three elements can be detected and determined 11hen it is possible to prepare the test solution in a smaller volume; however, in some instances the unused part of the 100-mI. solution has been used for other tests or determinations. The interferences are the most important and least known cause of error. Interferences due to a likely amount of any one of the metals known to be in the oils should be less than 2%. When several of these metals are present together, the error may be greater than this. It is advisable to precede any series of analyses of oils from a given region by a semiquantitative spectrographic analysis, in order to asses3 the possibility of major interferences. The results obtained on a number of synthetics are shown i n Table 11. In addition to the interference tests on individual elements, the synthetic solutions were prepared to determine the effect of widely varying compositions with respect to both the nature of the metals present and the relative concentration of the specific elements. The representative mixtures listed in the table shon- good agreement between synthesis and analysis in all cases. APPLICATION
The procedures descrihed above have been applied to a variety of oils. Analyses of several oils from various fields and a t various depths of cut are given in Table 111. These data indicate the wide variations in composition that may be expected. The principal application has been to feed stocks for catalytic cracking obtained from reduced crudes. It has been observed that as the percentage overhead obtained by vacuum distillation of a reduced crude is increased, the amount of metal contaminants in the overhead oil is also increased. A typical set of analyses illustrating this effect is given in Table IV for a Venezuelan reduced crude feed and for 54 and 66% fractions from this feed Iiy vacuum distillation. The concentrations vary in a regular manner with the yield of distillate. From such data it is possible to predict the effect of depth of cut into a crude on the characteristics of the catalyst employed in the cracking operation. The analytical procedures have also been applied, with minor modifications, to the analysis of cracking catalyst.; contaminated by the feed stocks. ACKSOW LEDGBIEST
The author wishes to acknon ledge the advice and encouragenicnt given by Ernest Solomon and P. -4.LeFranqois and the aidstance of G. A. Tirpak in carrying out much of the experimental work. LITERATURE CITED dlimarin, J . Applied Phys. (C.S.S.R.),17, 8 3 (1944). ASAL.ED.,17, 380 (1945). Mitchelland Mellon, IND.Esc,. CHEM., Moss and Mellon, Ibid.,1 4 , 8 6 2 (1942). Sandell, E. B., “Colorimetric Metal Aknalysis.” N e w T o r k , Interscience Publishers, 1944. ( 5 ) Trepka-Bloch, 2.anal. Chcm., 125, 276 (1943). (6) Weissler, IND.ENG.CHEM.,ANAL.E D . , 17, 695 (1945). (7) Wright and hlellon, Ibid.,9, 261 (1937). (1) (2) (3) (4)
RECEIVEDApril 22, 1949. Presented before the Divisions of Petroleum and Analytical and Micro Chemistry. Symposium on Microchemistry in the Petroleum Industry, at the 1 1 5 t h 11eeting of the AMERICANCHEMICAL SOCIETY, San Francisco, Calif.
