Quantitative Spectrographic Determination of Vanadium in Petroleum Products by logarithmic Sector Method J. A. KANEHANN Technical Service Department, Socony M o b i l Laboratories, Brooklyn 22,
Vanadium present in minute quantities in certain oils poses a serious problem to the petroleum industry, as it causes catalyst poisoning and boiler corrosion. To keep equipment and catalyst replacement at a minimum, it is necessary to know the vanadium concentration in crude oil, charge stocks, and residual fuels. This paper presents a rapid logarithmic-sector spectrographic method where a sample is ashed by a controlled procedure and diluted with a buffer of graphite and silica to which 1% titanium has been added as an internal standard. A portion of this mixture is packed into a cratered graphite electrode and arced under prescribed conditions. The titanium and vanadium line lengths are determined with a modified Bausch & Lomb optical magnifier. The vanadium concentration is determined by referring to calibration curves, plotting line length difference versus concentration.
R
ECENTLY the petroleum industry has become increasingly an-are of the deleterious nature of vanadium in petroleum processes. Generally. the vanadium is present as a porphyrin complex ( 5 , 6 ) and is oxidized during cracking processes, yielding a lo^ melting pentoxide or vanadate. These products deposit readily and contribute to catalrst poisoning, corrosion, and combustion deposits. Although the vanadium concentration in the sample is relatively lowr,the large throughput of crude and fuel oils causes the deposits to grow rapidly. Because of the corrosive effect on "crackers" and the large expense involved in replacing or regenerating poisoned catalyst, it is worth while to linon the vanadium content of the crude oils and charge stocks, so that adequate precautions may be taken. Prior to the use of the spectrograph, colorimetric and polarographic procedures, nhich Ivere extremely time-consuming and subject to various interferences, were used. The phosphotungstate colorimetric method especially is hindered by chromium, and requires prior separation of this element. Thus, because of the inherent difficulties of polarographic extraction or fusion, coupled with colorimetric interferences, the uqe of the spectrograph is a logical alternative. Spectrographic determination of vanadium is not new, as Murray and Plagge (Q), Carlson and Gunn ( 3 ) )and Anderson and Hughes ( 2 )have all used the spectrograph to attack this problem. Carlson and Gunn's technique of quenched electrodes gives only fair agreement with chemical results. Murray and Plagge's step sector method compares samples with known standards to estimate the vanadium concentration. ,inderson and Hughes introduced an internal standard and used a densitometer to determine vanadium quantitatively. This method was used in this laboratory for some time, but because it required 2 to 3 man-hours to perform, a method using essentially the Anderson procedure with the exception of substituting a logarithmic sector disk for an expensive densitometer was adopted. The logarithmic sector, which can be considered as a step sector with an infinite number of steps, is a disk whose periphery is cut to a logarithmic curve. As the sector rotates in front of the entrance slit, the image produced on the photographic plate is a tapered line whose length is proportional to the concentration. Any variation of the vanadium line length caused by differences in emulsions or by changes in the arc during the exposure
N. Y.
period is compensated for by the addition of an internal standard, titanium. No plate calibration is required with the logarithmic sector method. The chief advantages of this method are that a vanadium determination can be performed in about l man-hour, which is half the time required by the densitometric procedure, and that no expensive densitometer is necessary. EQUIPAlENT
A Bausch & Lomb large Littrow spectrograph with the camera positioned to photograph the 2500 to 3500 A. range is used. The exposure is controlled automatically with a timer switch. All electrodes are of high purity spectrographic grade graphite with the anode 2 inches long and '/a inch in diameter, flat cut, and the cathode 1.5 inches long and 0.25 inch in diameter. The end of the cathode is a 30" cone containing an axial crater '/le inch in diameter, '/I6 inch deep. A standard 5000 r.p.m. motor with a 5a/,,-inch logarithmic sector disk obtained from the Jarrell ilsh Co. is placed in the optical path. A direct current motor generator, 115-volt, 60-ampere) is used as a source. The arc is initiated by a radio-frequency spark. The fived slit used is 20 microns wide, 13 mm. high, and modified with a human hair about 3 mm. below the pomt of maximum radius of the logarithmic sector disk. Photographic equipment consists of Eastman spectrum analysis plates No. 1, Eastman D-19 developer, a 3% acetic acid Ehort stop, and Eastman rapid liquid fixer with hardener. Circulating cold n ater for wash and a drying oven with circulating warm air are also required The plates are placed on a special viewing box and read with a Bausch & Lomb optical magnifier, which is modified as follows. Use a razor blade to cut out a typical line ( 7 )from a plate exposed under test conditions (after softening emulsion in water). Float this bit of emulsion containing the line on top of water in a large evaporating dish. Slqnly bring the removed bottom glass from the magnifier up under the floating piece of emulsion. Take care that the glass surface is clean and no bubbles are trapped under the bit of emulsion. Dry in a dust-free location. When thoroughly dry, brush a thin coating of transparent lacquer or collodion over the emulsion and glass, and when dry, reassemble the magnifier. METHOD FOR INTERNAL AYD BUFFER STAR-DkRDS
Prepare synthetic standards of various vanadium concentrations by using C.P. vanadium pentoxide and spectrographic graphite as diIuent. Vanadium concentrations of I, 3, 9, 30, 56% have been found adequate. Mix these samples with the buffer internal standard mixture in a ratio of 1 part of sample to 9 parts of buffer-internal standard mixture. Prepare the buffer-internal standard mixture as follows. Prepare a mixture of 1% titanium (as the oxide) in C.P. silicon dioxide and mix the titanium-silicon dioxide powder with spectrographic graphite in a ratio of 1 part of titanium-silicon dioxide with 2 parts of graphite. All the above chemicals are ground so as to go through 200mesh screens. Set the spectrograph as indicated in Table I and arc the samples. Table I.
Excitation Conditions
Type of excitation Circuit constants Voltage Current Capacitance Resistance Arc gap Spectral region Exposure time Log sector disk and motor
1873
D.c. arc withr.f. initiatingspark 60 volts across the electrodes 7.5 amperes 0.4 pfd. 20 ohms 5 mm. 2500 t o 3500 A. 75 seconds I n place
ANALYTICAL CHEMISTRY
1874
No condensing lenses, diaphragms, or filters were used. Develop the photographic plate for 2.5 minutes a t 68' F., short stop for 10 seconds, and fix for 5 minutes. Wash the plate in cold running water for 10 minutes and dry in a drying oven maintained a t 95' F. Draw calibration curve^ b y plotting difference in the line length of the vanadium line a t 2977.5 A. and titanium line a t 2956.1 ,A,, against t,he log of the vanadium concentration. A typical calibration C U N ~ with 95% confidence limits is shown in Figure 1.
t /
0
iM ANALYSIS OF SAMPLES
Thoroughly mix the sample in its original container by vigorous shaking. In addition t o shaking viscous samples require heating and dry samples must be ground in a mortar. If samples do not require ash treatment they are run directly after mixing.
P TI
b
N1r. S%
C 0 N
V
560 WM.
Figure 2.
relationship Speetr!w rams . showing . between line length and Eoneentratlon
C N
I
Subtract t h e length of the titanium 2956.1 A. line from the length of the vanadium 2977.1 A line.
N
CALCULATION
If reporting composition of ash or inorganic sample, report F i g u r e 1. Vanadium 2977.5 A. calibration curve
Ignite 50 =! 0.1 grams of sample in a porcelain crucible (Coors No. 3) tared to the nearest 0.1 ma. Allow sample to burn gently,
elemental concentrations directly in per cent, but if reporting on original sample basis, cdcnlate as per cent or parts per million b y the formulas:
% = % element in ash X % ash X 0.01 P.P.M. = %element in ash X % ash X 100 DISCUSSION AND RESULTS
into 8 muffle maintained a t 1000" f 50" F. until d1:ll'thecar6on is consumed. Cool in a desiccator and weigh crucible against the same tare to 0.1 mg. Thoroughly mix the ash, weigh 10 mg. into a screw cap vial, and add nine times the sample weight of buffer-internal standard mixture. After thoroughly mixing the semple-buffer-internal standard in a mortar, storein origical vial. Pack the mixture into a cratered electrode by pushing the hollowed end of the electrode into some of the sitmole contained in the CaD of the vial. thus Dackine - the samole leveiwith the toD of the electrode. Next arc the samples and develop the plate as previously described. Determine the length of lines by moving the modified eyepiece dong the line being measured until the rate of extinction of the
The complete method, including ashing the sample, requires 1 man-hour t o perform and has been successfully applied to crude oil, charge stocks, residual fuel, and deposit andyses. Correlation between experimental data and known elemental concentrations is very high. The average correlation coefficient for all concentrations equals 0.99. Perfect correlation would equal 1.00. Based on the data. obtained from repeated tests, the repeatability in terms of 95% confidence limits averages 35%. The 95% coniidence limits are a stringent measurement of precisian in that they predict only one result in 20 to exceed these limits. That means, if a sample of 10 p.p.m. vanadium was run 20 times only one result would be expected t o exceed 14 p.p.m. or he less than 6 p,p.m. A comparison of logarithmic sector data compared t o densitometrio answers is shown in Table 11. I n all cases the agreement is excellent and well within the precision of the method.
of the slit hairline a c t h e base from which to measure all 1engtG. A typical plate is shown in Figure 2
ACKNOWLEDGMENT
The author wishes t o thank Owen J. Black for aid in the statistical analyses of the data. Table 11. Comparison of Log Sector and Densitometric Results
90Vanadium
Gemple NO.
1 2 3 4 5
6 7 8
D
10
Type Italian orude No. 6 fuel oil Shedgum residuum Bunker C Krasnoor residuum Oean Residuum No. 6 fuel oil Unidentified residuum Unidentified residuum Unidentified residuum
in Original Sample8 Log sector Densitometer 0.0020 0.0024 0.0152 0.0140 0.0056 0.0043
0.006P 0.0012 0.0175 0.0132 0.0177 0.0125 0.0074
0.0063
0.0007 0.0207 0.0172 0.0251 0.0106 0.0054
LITERATURE CITED
(1) Am. Soc. Testing Materials. Philadelphia, Pa., "ASTM Standards on Petroleum Products and Lubricants," D 482-46 (Note 3)
(19501.
(2) Anderson. J. W., and Hughes, H. K.. ANAL.C ~ E M 23, . , 1358
(1951).
(3) Carlson, M. T., and Gunn, E. L., Ibid.. 22, 1118 (1950). (4) Murray. M.J.. and PI~gge,H. A.. Proc. Am. Petroleum Inst. 111, 29 M,84 (1949). (5) Sacks, W..Can. J . Technd.. 29,492-5 (1951). (6) Skinner. D.A,, I d . Ene. Chm., 44, 1159-65 (1952). (7) Wilhelm, H.A.. IND. EN^. CHEM.,ANAL.ED.,9,170 (1937). Rmcirvno for review April ' I . 1955. Aceebted August 8, 1955
