Rapid Direct- Read ing Spec t roc hemica I Method for the Determination of Trace Metals in the Ash from Catalytic Feed Stocks DEAN HOGGAN, C. E. MARQUART, and W. R. BATTLES Richfield Oil Corp., Wilmington, Calif.
A direct-reading spectrochemical procedure has been developed for determining nickel, copper, vanadium, und iron in catalytic feed stocks present in the nanoigram range. A mixture of 20% sulfuric acid and 8OY0 n-butanol i s used as an ashing additive which reduces losses cind introduces no objectionable impurities. Results in triplicate can be o'btained in less than one hour, with good precision and accuracy. The coefficient of variation was less than 10%.
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catalytic cracking feed stocks have cataly,jt poisons present in the parts-per-billion range. Therefore, the determination of these poisons requires a concentration step. This usually involves a dry or wet ashing procedure which can be very time-consuming. The objectives in the development of thiz method were t,o reduce the time requirement of the determinations, to ohtain a coefficient of variation of less than 10yo,and to assure reasonable accuracy of the results. The developmental work for this method, which extended over a period of eleven years, revealcd that nanogram quantities of nickel, copper, iron, and vanadium could be detected on a l,',inch electrode in a lithium chloridepotassium chloride matrix when excited by a high voltage spark discharge; that organic cobalt added as an internal standard before %.shin$.the feed stocks largely compensated for losses and reduced variations; that potassium iodide aided in dissolving ashes in hydrochloric acid; anti that a mixture of 20% sulfuric acid and 80% n-butanol wa.s an ashing additix.e which helped in reducing losses and did not introduce objectionable impuritit??. YPICAL
EXPERIMENTAL
Equipment. The polychromator used for this method is a n Applied Research Laboratorie!; Quantograph. A 30-micron entrance slit is used in conjunction with 76-micron recei\-er slits. T h e original 8-inch focal length condensing leny was replaced with one having a &inch focal length. This a 1) pro xi mat e 1y do u h le d t I le speed of
the spectrograph. All analytical lines are used in the second order. An Applied Research Laboratories Multisource Model MSC-4 is used as a source unit. For this method the 240pulse unidirectional high voltage components are used. The lower electrode is concave n i t h a 170" included angle. The upper electrode has a 160" conical tip. This 10" difference in the included angles of the tips was found to prevent the discharge from preponderantly striking the outside edge of the lower electrode. The electrodes are now available as Fat,ional Carbon AGKSP 7503 (top) and 7504 (bottom). The tip of the lower electrode is dipped in a 50% acetone solution of Desicot,e (Beckman Instruments, Inc., Fullerton, Calif.) and dried before use. This prevents the ash solution from penetrat'ing the graphite. and thus gives a consequent increase in line-to-background ratio. aluniinum block is used for evaporating sample solutions from electrodes. Holes are drilled in the block for accommodating a thermometer and the l,'&ich electrodes. The block is also used to heat the acid solution in the platinum crurible when dissolving the ash. A ' dropper for transferring ash solutions is made by pulling 3-mm. capillary tubing out t,o a fine tip. Reagents. .\ll reagents must' be of the highest spectrographic purity obtainable, and blanks must be run to correct for a n y metal impurit'ies. -111 water used ie initially deionized or distilled, and is redistilled in a quartz still before using. .I11 acids are initially ACY reagent grade and are likewise quartz distilled. T h e lithium carbonate is a special spectrographic grade obtained from Pacific Spectrochemical Laboratory, 2851 Roberthon 131vd., Culver City, Calif. T h e potassium iodide is initially .ICs reagent grade a n d is recrystallized twice from quartz di.;tilletl water before using. 1 3 U T Y L SULFURIC 1 ~ E . I G I by volume of concentrated ~ulfurivacid, 80% n-butanol. .ISH SOLV-EST.0.46 gram of lithium carbonate antl 1.O grain of potarsium iodide discolved in 200 nil. of 9 S hydrochloric acid. STOCKSOLUTIOKS O . I . 4.6 grams of lithium carljonate and 10.0 grams of potassirmi iodide a r r dissl\-ed in 50 inl. of gL\- h>-drochloric a d . The correct volume of an analyze(! cobalt nitrate
solution (about 1.4% cobalt) is added to give exactly 80.0 mg. of cobalt, and the solution made up to 100 ml. wit,h 6'V hydrochloric acid. STOCKSOLUTION KO.2. Prepare the following four solutions: 0.200 gram of nickel powder in hot 5iV nitric acid, 0.231 gram of ammonium vanadate in 9 X hydrochloric acid, 0.0200 gram of copper powder in concentrated nitric acid, and 2.00 grams of iron wire in aqua regia. Dilute each to 100 ml. with 9N hydrochloric acid. Ten milliliters of each of the diluted solutions are combined and diluted to 100 ml. with 6AVhydrochloric acid. Standards. LOW S T A N D A R D . Ililute 1.00 ml. of stock solution S o . 2 plus 5.00 ml. of stock solution S o . 1 to 10.00 ml. with 6.V hydrochloric acid. HIGH STANDARD.Add 5.00 ml. of stock solution S o . 2 t.o 5.00 mi. of stock solution KO. 1. ISTERKAL STANDARII SOLUTIOX. hpare a solution of cobalt ethyl hexanoate in a Cs-arornatic solvent to contain 40 pg. of cobalt per milliliter by dilution of an analyzed concent.rated solution containing about 8 mg. of cobalt per milliliter. Procedure. Sample the gas oil in chemically cleaned bottles and mix thoroughly. .Ish a portion of the sample sufficient to provide absolute quantities of the elements which fall on the scale of the instrument. S o r mally this will require asliing about 5 grams of the oil. Each sample should be run in tri1)licate. Weigh the oil to the nearest 0.01 grain into 20-ni1. 1)latinuin dishes scrupulously cleaned by iiiiinersiori for a t least 2 hours in hot chromic acid. rinsed with quartz distilled water, and oven dried before use. Pipet 0.10 ml. of the internal standard solution, and for each grain of sample, 0.1 mi. of butyl-sulfuric reagent, into each sample. Stir the mixture.: on a hot [)late at 180" F. for one minute using a small gl propeller-stirrer. I'lac-e the dishes on clal- triangles and heat gently with a Fisher burner until thp oils are i g n i t d . I3urn the mixtures slowly, heating gently with the burner when required to keel] them h r n i n g . Some buinl)ing often occurh early in the burning. I t can be minimized by heating as little as ~~(~ssiihle to niaint:tin I)urning. IVhen the sarniiles are Ijurnt~tlt o a small coke rcasidue, moi5ten csch with 1 nil. of conwntr:ttetI c i i l f w i c zc.i(l antl heat to VOL. 36, NO. 10, SEPTEMBER 1964
1955
dryness over a low flame. Place samples in a muffle furnace a t about llOOo F. for 15 minutes or until all carbon particles have been consumed. I n the dish there will usually be no ash which is visible to the unaided eye. Transfer each ash to an electrode as follows. Add by pipet 0.1 ml. of ash solvent into a cooled dish. Place the dish on the aluminum block which is at 350°F. and scrub the interior thoroughly using a small, curved, glass stirring rod. Rotate the dish with its side in contact with the hot block while scrubbing so that all areas are thoroughly cleansed with the hot solvent. This operation should be completed in 10 to 20 seconds to avoid excessive evaporation of the solvent. Insert a lower electrode into a hole in the aluminum block and transfer a medium-sized drop of the solution from the dish with a fine-tipped dropper to the center of the electrode surface while
Table I. Efficiency of the Micro Ash Method for Nickel, Vanadium, Iron, and Copper
Metal Nickel Vanadium Iron Copper
Table 11.
P.P.B. range present 50 200-300 10-50 50-300 250-300 500 1200 10 50 110 210
apparent recovery of metal present by micro ash analysis,
7a
70-80
80-90 130-160 100-130 60-70 100-110 105-125 75-85 65-85 55-65 45-55
the electrode is still cool. Evaporate the solvent from the surface of the electrode with the aid of an infrared lamp. When the electrode surface is dry, add the remainder of the liquid, drying a drop a t a time until the transfer is complete. Store the dried electrode in a covered electrode holder and keep it warm and dry with the infrared lamp until ready to analyze with the spectrograph. SPECTROCHEMICAL P R O C E D U R E . Prepare low and high standard electrodes in triplicate by evaporating 5 to 10 p!. of the aqueous standard solution on each electrode in the same manner as was done for the sample solutions above. Aidjust the volumes of the standard solutions used so that the integration times for the standards are close to those of the samples. ;i6second ( i1 second) integration time is ideal for the A.R.L. equipment specified. Standard instrument settings are used: primary input voltage 250 volts, capacitance 0.007 microfarads, current 7.0 amperes, and inductance 410 microhenrys. The spark gap is 3 mm. S o prespark is used and the integration time is 4 to 8 seconds. Analytical lines used are: 3274.0 A. for copper, 3021.1 for iron, 3101.6 for nickel, 3184.0 for vanadium, and 3044.0 for the cobalt internal standard. A11 lines were used in the second order. Make adjustments on the readout console so that the integration time for full scale on the cobalt channel is from 4 to 8 seconds for an average gas oil sample carried through the procedure above. Longer integration periods decrease the line-to-background ratios and reduce the sensitivity of the method. Spark the low and high standards in the Quantograph. Read each metal on the direct reader by manually stepping to each channel and noting the absolute amount of each element indicated on a special potentiometer scale. Make the proper instrument adjustments to give the correct a b
Comparison of Minimum Detectable Quantities of Elements by the Micro Ash and Copper Spark Methods ( I )
Copper spark method ( 1 ) Micro ash method Sensitivity Sensitivity in in Spectrum nanograms Spectrum nanograms Element Wavelength order ( l o w 9gram) Wavelength order ( l o w 8gram) 3944 0 1 10 3082 2 2 10 Aluminum 2790 4 1 500 2598 1 2 1,000 Antimony 2860 5 1 500 2780 2 2 1,000 Arsenic 2891 8 1 100 2335 3 2 1,000 Barium 48.34 1 1 100 6142 8 1 10 _.._ 2748 6 1 200 2980 6 2 io0 Cadmium 4012 4 1 100 4186 6 2 10,000 Ceriuma 4186 6 2 50 Cesiuma 8521 1 1 1,000 8521 1 1 50 Chromium 2835 6 2 1 2843 2 1 5 Cobalt 3405 1 2 10 2663 5 1 50 Copper 3247 5 2 0 01 Gold 2676 0 2 1 000 2676 0 1 20 2 1 4383 5 2 50 Iron 3020 64 Y1rke.l 3414 2 1 2116 1 1 10 . . .~~- 8 1 3093 1 1 5 Vanadium 3184 0 2 0 1 200 3345 100 Zinc 3345 0 2 a Infrared film-no filter; spectrograph range: 6150-8650 .4. All other micro ash analyses run on ultraviolet film with a UT- filter. 4336-6835 A. spectrograph range. ~
1956
ANALYTICAL CHEMISTRY
solute value readings for the copper, iron, nickel, and vanadium in the low and high standards. Concentrations of the standards are: Co, 4 pg.; K, 118 pg.; Li, 43 p g . ; Cu, 20 to 100 ng.; Fe, 2000 to 10,000 ng.; S i , 200 to 1000 ng.: and V, 100 to 500 ng. Spark the sample electrodes in the Quantograph and read their metal contents on the potentiometer scale. DISCUSSION
Ultraclean room techniques have been used by one laboratory ( 1 ) which was determining nanogram quantities of elements spectrochemically. Such techniques are not possible in Richfield's Watson Laboratory. Various other activities, such as the determination of ash contents, take place in the sample preparation room. As a result, occasional wild values are obtained. To ensure valid results under these conditions, analyses in triplicate are usually performed. Any wild result, is discarded and the remaining two results are averaged. These precision figures in parts per billion were obtained from triplicate analyses of the same sample on five different days; nickel 152, sigma 4 ; vanadium 68, sigma 4; copper 14, sigma 6; iron 1550, sigma 90. The first step in testing the accuracy of the method mas to analyze a large number of synthetic blends of metnlorganic compounds in oil. The materials used were prepared as follows. GAS OIL DILUENT.The 0 to 90% overhead cut of a laboratory vacuum distillation of a heavy gas oil low in metals content was used. The diluent was analyzed by this method for traces of remaining metals and recorded on film. The metals contents were: nickel, less than 3 p.p.b.; vanadium less than 2 ; copper 5; iron, less than, 100. SICKEL STANDARDS.Xickel 2-ethyl hexanoate was made by reacting an excess of nickel (ic) nitrate with potassium 2-ethyl hexanoate and extracting the potassium nitrate and excess nickel nitrate with water. The purified product was analyzed for nickel by sulfate ashing and converting to the oxide in a niuffle a t 1000" C. Standards of 0 , 1, 2, 5, 10, and 20 p.p.m. of nickel and 273 p.p.b. of nickel were made by dilut,ion of the analyzed hexanoate with a solvent composed of 30y0 xylene (metalfree) and 70% of the gas oil diluent. The 0- to 20-p.p.m. standards were used as calibration standards for x-ray analysis of the 10% bottoms cut from the vacuum distillation of a high nickel content gas oil. The 10% bottoms cut had a nickel content of 18.0 0.3 p,p.m,, using the K a line for nickel a t 1.660 A. on an Applied Research LaboratorieP Production X-ray Quantometer. Later analysis of this sample by neutron activation analysis was made
and gave a value of 19.1 i 0.2 p.p.m. This sample and the sylene-gas oil diluent (30170) were used to make u p known mistures in the 50- to 300-11.p.b. range of nickel content. These samples and the 273-11.p.b. nickel sample prepared from the nickel hexanoate were used for testing the accuracy of the complete method for ni1:kel. VANAI)IULI S T A K D A I I I ) ~ . Vanadium oleate dissolved in xylene was analyzed by sulfate ashing and found to contain 0.847 i .OOj7, vanadium. This material was diluted with the same xylene-gas oil diluent used for nickel to give a 41-p.p.b. vanadium sample. Other vanadium standards were prepared by dilution of a 32-p.p.m. (neutron activation analysis) gas oil 10% bottoms fraction. IROKSTASIIARIIS.X large volume of a light gas oil cut, high in iron content, was passed through an Atapulgas clay column. Aimethyl ethyl ketone extract of the clay was filtered and concentrated by evaporation. The concentrate was wet ashed and analyzed for iron by the ortho-phenanthroline method of Milner et al. (3) and was found to contain 4.30 and 4.45 p.p.ni. of iron (duplicate analyses), A later combustion analysis of the concentrate was made with a modified Beckman oxy-hydrogen burner (2). The combustion gases were absorbed in dilute hydrochloric acid and the iron as determined by the orthophenanthroline method was 4.0 p.p.m. Dilutions of this concentrate were made to the 300- to 1200-p.p.b. level, using the value of 4.4 p.p.m. for the iron content of the concentrate. COPPI:RSTANDARDS. '-A solution of copper 2-ethyl hesanoate, prepared by the same method as the nickel salt, was analyzed by sulfate ashing and found
to contain 2.037, copper. Dilutions were made as above to the 10- to 20 p.p.b. copper range. RESULTS
All of the blends described above were analyzed in quadruplicate a t least. Some of them were subjected to several series of analyses. h summary of the findings is presented in Table I . The amount of each metal present that is apparently recovered by analysis, of course, depends upon the ratio of metal lost to that of the cobalt internal standard lost through the entire process and would be expected to vary for different metals. Table I shows the efficiency of this method for each individual metal when other metals are absent. However, when mixtures of metals are present, minor synergistic effects are often noted. This is especially true of the vanadium recovery a t different concentration levels with varying nickel contents. These effects are of low magnitude and unimportant for routine plant control analyses. However, for the highest accuracy of results, known blends of all four metals approximating the analysis figures of the samples should be made. Analyses of these blends will provide recovery factors for each metal and permit an accurate calculation of the true metals contents of the sample. .4series of analyses of standard blends covering the desired range of metals should be run weekly by each chemist using the method. Thus, correction can be made for any errors caused by variations of individual techniques, contamination of any materials, changes in the inorganic standards. or performance of the equipment. I3y such rigid
control, errors in the true values for each met,al will usually be less than 5 to 10%. This method is intended for use on medium and heavy gas oils. However, it' has been used successfully on light materials with slightly different recovery of metals. A kerosene blend, for example, gave about 10% higher recoveries of nickel and vanadium, with little trend noticed in variation of copper or iron results. The method has been used on occasion for the detection of nanogram quantities of various elements from sources other than gas oils. I n cases where no ashing was involved, the internal standard and buffers were added as stock solution N o . 1 to a solution of the analytes. Standards were prepared in a manner similar to the preparation of stock solution N o 2. Spectra were recorded on film. The approximate minimum detectable quantities of a number of elements by this procedure are given in Table 11. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of E. A. Trunk and G. R. Meador in the development of this method and the preparation of this paper. LITERATURE CITED
( 1 ) Fred, M., Sachtrieb, N . H., Tomkins, F. S., J. Opt. SOC.Am. 37, 279 (1947). ( 2 ) Hoggan, I)., Battles, W. R., ANAL. CHEM.34. 1019 11963). ( 3 ) Milner, h.I., Glass,'J. R., Kirchnew, J. P., Yurick, A. XI., Ibid., 24, 1728 (1952).
RECEIVEDfor review March 6, 1064. Accepted June 19, 1964. Permission to publish this paper was granted by the Richfield Oil Corp.
Spectrochemical Standards for Aluminum Alloys G. P.
KOCH, NICHOLAS CHRIST, and J. L. WEBER, Jr.
heynolds Metals
Co.,Metallurgical Research Division, Richmond, Vu.
b A large number and a considerable variety of emission and x-ray fluorescence standards for aluminum alloys have been cast using small-scale melting equipment and three simple molds. One of these, described as a radial water-spray cooled mold, applies a high cooling rate uniformly in a radial direction, and is the most generally useful of the three. 'The standards obtained are small cylinders 1 '/2 or 21/2 inches in diameter, 3 to 4 inches in length, and have a high degree of homogeneity.
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H E EXTENT to which spectrochemical methods of analysis may be applied is limited frequently by the availability of suitable standards. This is true especially in the analysis of metals, for here it is not always an easy matter to obtain a standard with an appropriate composition, a suitable form, an acceptable homogeneity, and a metallurgical structure equivalent to that of the samples to be analyzed. .kiditionally the standard should be of a size to last the average laboratory a t least one to two years. The situation
may be complicated further, as it is for aluminum, by the esistence of a large number of different alloys containing a large variety of elements, and concentrations ranging from parts per million to 10 or 20YG',. We have attempted to contribute to the solution of the standards problem for aluminum alloys by developing a system for casting standards which requires a minimum of equipment, sl)ace, and skill> offers an excellent o1)I)ortunity in any given rase of being successful on the first attempt, and will produce any number of good VOL. 36, NO. 10, SEPTEMBER 1964
1957
