Table IV. Effect of Gamma Radiation on 1,5-DiphenyIcarbohydrazide
Composition of test solutions: [DPC], 8 X 10-4M [HzSO(],0.1M Procedure, see Procedure section Radiation damage factor, Radiation, Radiation mmoles/ml./ r. X 10-5 damage,a 70 r. X 1Olo 0.35 0.70 3.5 7.0 11
6.0 8.95 22.4 30.0 38.4
Radiation damage, % =
13.7 10.2 5.7 3.4 3.8
(V) 100
where C = mmoles of KzCr207required for the clontrol aliquot, and Z = mmoles of KzCrz07 required for the irradiated aliquot
to do so would be a major research problem in itself. The results of this experiment did not show specifically whether the unreacted D P C or the Cr-DPC color intermediate was being attacked. Because such a large excess of D P C was
present, the disappearance of the color following the irradiation appeared due to an attack on the intermediate. However, to clarify this point and also to measure the relative susceptibility to radiation of the unreacted D P C itself as compared with that of the colored intermediate, a study was made on D P C solutions at various radiation levels. The composition of the solution was the same as that used for the CrD P C reaction except that no chromium was present. The degradation of the D P C on being irradiated was measured by titration with a standard solution of K2Cr207,as described above. This titration procedure is very tedious and requires extreme care in determining the end point. The erratic results of the gamma irradiation of D P C are shown in Table IV. Apparently, the effect of radiation is fairly fast initially but slows down as more degradation products grow into the solution. Several attempts were made to obtain less erratic data, but each series of test solutions gave, in general, the same results. The following conclusions can be drawn from these data: the radiation damager per unit of radiation seems to be less severe a t the higher radiation levels; the Cr-DPC color intermediate
is no more sensitive to radiation than is the D P C itself and may be slightly less sensitive a t the lower radiation levels; and gamma radiation levels of less than lo3 r. will introduce no appreciable error after the Cr(V1)-DPC reaction has taken place. LITERATURE CITED
( I ) Allen, A. O., J . Phys. Colloid Chem. 52,479 (1948). (2) Allen, A. O., “The Radiation Chemistry of Water and Aqueous Solutions,” Van Nostrand, New York, 1961. (3) Bakko, A. K., Palii, L. A., Zh. Analit. Khim. 5, 272 (1950). (4) Lefort, M., J . Chim. Phys. 54, 782 (1957). (5) Lichtenstein, E., Allen, T. L., J . Am. Chem. SOC.81, 1040 (1959). ( 6 ) Miller, F. J., Zittel, H. E., ANAL. CHEW35,299 (1963). (7) Miller, N., Rev. Pure A p p l . Chem. 7.123 (1957). (8) ‘Pflaum, R. T., Howick, L. C., J . Am. Chem. SOC.78,4862 (1956). (9) Proc. 2nd Int. Conf. Peaceful Uses At. Energy, Geneva, 29, (1958). (10) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 392, Interscience, New York, 1959. (11) Weiss, J., Allen, A. O., Schwarz, H. A., Proc. 1st Int. Conf. Peaceful Uses At. Energy, Geneva, 14, I79 (1955). RECEIVEDfor review October 8, 1962. Accepted January 7, 1963. Oak Ridge National Laboratory is operated by Union Carbide Corp. for the U. S. Atomic
Energy Commission.
Determination of Trace Metals in Oils by Sulfur Incineration and Spectrophotometric Measurements E. J. AGAZZI, D. C. BURTNER,’ D. J. CRITTENDEN, and D. R. PATTERSON* Shell Development Co ., Emeryville, Calif.
b Sensitive reliable procedures for the determination of iron, nickel, and vanadium in petroleum oils have been developed. By adding sulfur to oils they may be burned to a residual ash with no loss of these metals. The procedures are based upon this simple means of reducing the oil to an ash followed by the measurement of the metals by sensitive spectrophotometric methods developed in these laboratories. The metals were determined, using 25 grams of oil, with a relative standard deviation equal to 1 to 5% of the mean or 0.02 to 0.05 p.p.m., whichever is larger.
T
HE NEED for the determination of trace metals in oils has been well established (1, 2, 12, 14). Of the many metals occurring in oils, sodium, iron, nickel, vanadium, chromium, and copper are of the most interest. The
332
ANALYTICAL CHEMISTRY
direct meawrement of one or more of these metals is possible by emission spectroscopy (17), x-ray fluore:-cence (4, I I ) , electron paramagnetic resonance (19), or neutron activation (10). When direct measurement is not feasible, concentration of metals before analysis is necessary. The simplest procedure is to burn off the oil leaving the metals as residual ash, but appreciable loss of some metals occurs when some oils are burned (12, 14). To overcome this loss, rarious investigators have modified the ashing procedure ( I d ) , extracted metals from oils (1, 2 ) ) or acid-oxidized oils to various degrees ( I , 7 ) . Shott, Garland, and Clark (20) recently reported that loss of vanadium and nickel during incineration can be prevented by adding benezenesulfonic acid to the oil before burning. Forrester and Jones (6) have reported on sensitive spectrophotometric methods for the determination of iron, nickel, and vana-
dium in oils. In their case, the oil was oxidized with perchloric acid; a maximum of 2 grams of sample was taken. In this laboratory it was found some years ago that addition of sulfur to the oil prior to burning prevented loss of metals. Sulfur has the advantages that it can be obtained practically free of metals and is more effective in lower concentrations than is benzenesulfonic acid. Incineration with added sulfur is simple but still time-consuming. Use of the least possible quantity of sample is therefore desirable and this demands employment of sensitive methods for measurement of metals. We have developed a number of sensitive spectrophotometric methods which are generally applicable and allow the use of Present address, Fresno State College, Department of Chemistry, Fresno, Calif. 2 Present address, Shell Chemical Co., Union, S. J.
small samples for the determination of trace metals. This report describes the application of these methods t o oils after sulfur incineration. Of the metals mentioned above, iron, nickel, and vanadium were determined. INCINERATION WITH SULFUR
The effectiveness of sulfur in preventing loss of metals during burning is shown by the data in Table I. Oils were selected which had decided loss of nickel and vanadium during ordinary incineration. The good agreement between results given by acid oxidation (where no loss of metal should occur) and incineration with sulfur demonstrates sulfur prevents loss of these metals. Although elemental sulfur dissolves in petroleum distillates, little is known concerning the mechanism whereby metals are retained by sulfur incineration. We found t h a t considerable quantities of hydrogen sulfide are formed during sulfur incineration, however, treatment of oils with hydrogen sulfide both before and during' incineration failed to improve recovery of metals. I n addition to formation of hydrogen sulfide, sulfurization of the oil occurs with the formation of appreciable amounts of an asphaltic and cokelike mixture. The residue from an oil originally containing 2.5% sulfur was found to contain 5.4% sulfur even after completion of burning to a cokelike residue. The ash from sulfur-addition incineration is obtained as metal sulfates. These observations suggest that the improved retention of metals is caused by sulfurization of volatile metal-organic compounds to form less volatile sulfur linked compounds which can be decomposed without appreciable loss of metal during subsequent ashing. The acid oxidation procedure used in this work involves a preliminary charring of the sample with concentrated sulfuric acid. Nitric acid is then added and the solution heated until the bulk of the organic matter is oxidized and the solution is light straw colored. The acid solution is transferred to a platinum dish, evaporated to dryness, and ignited to 500' C. to remove traces of organic matter. The residue is then dissolved in sulfuric acid and treated with hydrofluoric acid to remove silica. Incineration with sulfur is carried out as follows: Transfer 25 grams or less of sample to a weighed 50-ml. platinum crucible. Reweigh, and add sulfur equal to 10% of the weight of oil. Heat gently with a Bunsen burner until the contents ignite and burn readily. Remove the burner and allow the oil to burn. When the flame extinguishes, reignite with the burner and set the crucible in the air bath (see Apparatus). Some oils form a crust of carbon over the surface during the burning process
Table 1.
Sample Flash Dist. PR-1 Straight run residue 105-2
Effect of Sulfur on Metal Loss During Incineration
Ordin. incin. 6.8 56
Ordin. incin.
Incin. with S
8.7
0.59
1.00
1.05
57 3.43
56 3.44
8.6 75
69
Flash Dist. 105-1 11.3 16.0 16.1 a Average of three to four determinations.
which may entrap enough gas to cause the partially burned material to expand over the top and the sides of the crucible. To prevent this, heat the sides of the crucible projecting above the transite top of the air bath (Figure 1) with the burner until the carbon shrinks enough to allow the combustible gases to escape. Continue the burning process until only carbon remains. Place the crucible in a muffle furnace a t 550 =t 50' C. Leave the muffle door slightly ajar until no carbon remains in the crucible. Cool the crucible to room temperature. Dissolve the ash as described for acid oxidation using 0.5 ml. of 1:l sulfuric acid and 2 to 3 drops of hydrofluoric acid and transfer the solution to a 5-ml. volumetric flask. A study was made to determine the minimum amount of sulfur that could be added to the oil to prevent loss of nickel and vanadium. Results from the tests showed that the minimum ratio of sulfur to oil lay between 5 and 10 parts by weight of sulfur to 100 parts by weight of oil. -41 to 10 weight ratio was selected. Since the metal content of the sulfur added to oils should be low, a number of available grades of sulfur were analyzed for ash content. Ash content of various lots of sublimed sulfur ranged from 20 to 63 p.p.m. Commercially available "precipitated" sulfur and "washed" sulfur were found to contain 220 and 280 p.p.m., respectively. Chief constituents of the ash were calcium, iron, and silicon. S o appreciable amounts of nickel and vanadium were detected by either spectrophotometric or spectrographic analysis of the ash. Sulfur obtained from a petroleum source (from modified Claus process) mas found t o be very low in
Figure 1 . Hot plate burning oils
v. D.U.rn."
Ni.,. u.D.m.0 Incin. Acid oxid. with S
modified for
I
49 2.9
Acid oxid.
ash (0.5 to 5 p.p.m.). Chief contaminant in the ash was iron with 0.1 p.p.m. or less of nickel and vanadium. This sulfur was therefore used. SPECTROPHOTOMETRIC METHODS FOR ANALYSIS OF ASH
Apparatus and Reagents. .4bsorbance readings were taken on a Beckman Model B spectrophotometer equipped with 1 and special small volume, about 4.5 ml. (American Instrument Co., San Francisco, Calif.) 5-cm. cells. The Beckman instrument was modified t o increase the range over which i t gives a linear response to absorbance. The loss of linearity (at high absorbance) was found to be the result of stray light. This was eliminated by masking the exit beam a t the sample side of the shutter opening. The mask was made by taping the opening with electrician's tape to allow passage only of the well defined rectangular image of the exit slit. All reagents were of the highest purity available. Most oils will not continue burning in platinum crucibles after ignition unless external heat is applied. The crucible may be heated with a burner, but this requires considerable operator attention. Setting the crucible on a hot plate proved only moderately successful because insufficient heat is conducted up the crucible to maintain burning. To provide more uniform heating, the top of a hot plate was removed and covered with a shallow box with holes in which the crucibles were inserted and held just above the heating coils. Such an arrangement, utilizing a long, narrow, high temperature hot plate, is shown in Figure 1. Determination of Iron. Iron was determined with 1,lO-phenanthroline. The procedure embodies many improvements described in the literature over a period of many years. Iron is reduced with hydroxylamine hydrochloride, and ammonium acetateammonium citrate buffer solution is added. Citrate alone is often used b u t color development is slow. Acetate has little or no effect on the rate of color development, therefore it was substituted for citrate as the buffer and just sufficient citrate was added to hold any aluminum in solution. The absorptivity obeys Beer's law over the range of 0 to 60 pg. of iron when color is developed in a volume of 5 ml. The relative standard deviation of the absorptivity obtained with aqueous VOL. 35, NO. 3, MARCH 1963
333
iron solution free of interferences has been determined to be 1% when the corrected absorbance readings lie between 0.200 and 2.5. COLORDEVELOPMENT.Transfer to a 5 m l . volumetric flask a n aliquot of the dissolved oil ash containing 0.02 to 60 pg. of iron. Add 2 drops of 1: 1 hydrochloric acid, 0.2 ml. of 10% hydroxylamine hydrochloride, 0.2 ml. of the acetate-citrate buffer (5 grams diammonium citrate and 10 grams ammonium acetate in 85 ml. of water), and 1.0 ml. 0.2% o-phenanthroline, mixing thoroughly between the addition of each reagent. Drop in a small piece of Congo red indicator paper and add 1:4 ammonium hydroxide dropwise until the Congo red indicator paper changes from blue to red, then add one drop excess. Dilute to volume and mix thoroughly. After 10 minutes, measure the absorbance relative to distilled water a t 510 mp. CALIRRATION. With micropipets introduce exactly 0, 0.02, 0.06, 0.20, and 0.30 ml. of the standard iron solution (0.200 mg. iron per ml.) into separahe 5-ml. volumetric flasks. Dilute to about 2 ml. and develop color as described above. Read the absorbance and correct each absorbance by subtracting the absorbance of the 0-ml. standard and calculate each absorptivity in liters per gram cm. Average the absorptivities and use the average in calculating iron content. The absorptivity should average about 196 liters per gram cm. Determination of Nickel. The present method was developed from a restudy of the spectrophotometric determination of nickel with dimethylglyoxime. There have been many refinements to increase the color stability, extend the useful concentration range and eliminate interferences (3, 13, 15, 16, 18). With all of these changes, the method has become more complex and less desirable for routine use. The stability of the color formed depends upon the pH of the solution and a t high pH the color remains stable for days. A pH range of 10.5 to 12.5 was found optimum for nickel color development; however, a pH range of 12 to 12.5 was selected to assure the formation of the colorless iron tartrate complex. I t was observed that ammonium ion must be present to obtain soluble nickel dimethylglyoxime a t any pH. This information has been combined with the use of tartrate, citrate, and (ethylenedismine)tetraacetic acid (EDTA) to eliminate the interferences from iron, aluminum, and copper to yield a direct color development method which is rapid, requires no heating, and has high stability. The absorptivity obeys Beer's law from 0 to 60 pg. nickel per 5 ml. The relative standard deviation of the absorptivity has been determined to be 1%. COLORDEVELOPMENT. To the solution in a 5-ml. volumetric flask add 0.4 ml. of 10% diammonium citrate, 0.4 ml. of 20% sodium potassium tartrate,
334 *
ANALYTICAL CHEMISTRY
and a piece of Congo red paper. Neutralize with 2N sodium hydroxide until the paper turns from blue to full red. Add 0.5 ml. of 2N sodium hydroxide and 0.5 ml. of saturated bromine water with swirling. Without delay, add 0.2 ml. of 1% dimethylglyoxime in ethanol and again swirl the contents of the flask to ensure complete mixing. After about 30 seconds add 0.2 ml. of 10% EDTA solution, dilute to the mark with distilled water, and mix thoroughly. Measure the absorbance of the solution relative to distilled water a t 470 mfi. Establish the average absorptivity as with iron using a known nickel solution. The average absorptivity should be about 228 liters per gram cm. Determination of Vanadium. One of the most sensitive means for the determination of vanadium is its reaction with diphenylbenzidine (DPB) to form a yellow (5, 9) or violet compound (21). The yellow compound provides more sensitivity than the violet, and Eeckhout and Weynants (5) developed a method for vanadium based upon the yellow color. But their system has the disadvantage that the reaction is carried out in acetic acid which is not a good solvent for inorganic materials. The violet compound forms in aqueous phosphoric acid which has better solvent properties. The color change of DPB from colorless to violet is striking and DPB is widely employed as an indicator in the titration of dichromate or vanadic acid with ferrous iron. However, little has been done in using DPB-violet for the spectrophotometric determination of vanadium. Wrightson (21) described a method successfully applied to oil ashes, but made no study of the method variables. We found that his method gave good results when the concentration of vanadium exceeded about 0.4 pg. per ml. At lower concentrations, color often failed to develop. The violet compound is very sensitive to excess DPB. Apparently it reacts with DPB to form the yellow compound and prevents the determination of small amounts of vanadium. The reaction is reversible and the yellow compound can be reconverted to the violet by extracting excess DPB with carbon tetrachloride. To maintain a low excess of DPB, advantage was taken of the fact that DPB is much more soluble in organic solvents than water (0.06 mg. per liter of water). The reagent was dissolved in carbon tetrachloride and the reaction carried out by shaking this solution with the aqueous acid vanadium solution. The violet color formed readily and was insoluble in carbon tetrachloride. However, phase separation was poor and results were erratic. Addition of acetone led to good phase separation, but acetone was not entirely satisfactory, for some batches contained material which reduced either vanadium or DPB-violet. Acetonitrile proved as effective as acetone and was free of reducing material. The volume of acetonitrile was not critical. Very little DPB was extracted from the carbon tetrachloride by water. I n
blank determinations, only 3 to 6 pg. of DPB were found in 25 ml. of aqueous phase. The solvent system was thus very effective in maintaining a low excess of DPB in the aqueous phase. When a solution of vanadium is allowed to stand in a glass vessel in the presence of sulfuric acid, the recovery of vanadium by the DPB-violet methods decreased with time. It appears that some of the vanadium(V) is converted to forms of vanadium incapable of oxidizing DPB. By adding base, vanadium is converted to the soluble vanadate ion, transformed to the desired pervanadyl ion upon acidification, and recovery is complete. I n the method, adjustment of pH is made using aluminum ion as an indicator ion. Organic indicators were not used because of the danger of reducing some vanadium. DPB in carbon tetrachloride can be oxidized by ultraviolet light to yield the same oxidation product as that produced by vanadium(V). Therefore, the reagent is made and stored in low actinic glass and the color reaction is carried out in low actinic glassware. The high acidity required is obtained by use of phosphoric acid. Phosphoric acid contains traces of materials capable of reducing vanadium(V). The effect of these trace contaminants is eliminated by treating the acid with potassium permanganate. To do this, mix two volumes of the concentrated acid and one of water. Add 0.1N potassium permanganate dropwise to the acid solution until the solution is a definite pink. Heat to 115' C., cool, and make a blank determination as described under Color Development. The blank should have an absorbance of less than 0.100 in a 5-cm. cell. If the absorbance is greater than this, heat a t 115' C. until the absorbance drops below this value. The absorptivity obeys Beer's law from 0.5 to 100 pg. of vanadium per 10 ml. The relative standard deviation of the absorptivity was 4y0. COLORDEVELOPMENT. Obtain in a 10-ml. volumetric flask, 1 t o 3 ml. of solution containing 0.5 to 100 Fg. of vanadium and about 0.1 ml. of concentrated sulfuric acid. To each flask add 0.3 ml. of aluminum solution (10 mg. per ml.) and 0.2 ml. of saturated bromine water; allow to stand 5 minutes and remove the bromine by boiling. Cool. Add 6N sodium hydrovide dropwise until precipitation of aluminum hydroxide begins; then, add 0.2 ml. of 2N sodium hydroxide. Heat the solution on a hot plate until boiling begins. Remove from the hot plate and cool to room temperature. To aroid formation of unreactive forms of vanadium by prolonged standing in acid solution, from this point on, do one sample a t a time. Add 4.8 ml. of 2 : l phosphoric acid; mix thoroughly. Add 2 ml. of acetonitrile, mix, and make the solution to volume with water. Pour the solution into a 125-ml. glass-stoppered Erlenmeyer flask made of low actinic glass. Rinse the volumetric flask with two, 5-ml. portions of the carbon tetra-
Table
Iron, p.p.m. Incin. Material with S Flash Dist. No. PR-1 3.87 3.99 3.99 Av. 3.95 S.D: 5 0 . 0 7 Flash Dist. KO.105-1 7.73 8.04 8.07 Av . i . 9 5 S.D. f O .19 Flash Dist. No. 0.14 WRC-12 0.11 0.11 Av. 0.12 S.D. h 0 . 0 2 Combination feed 0.22 0.25 0.26 0.24 AV. S.D. f 0 . 0 2 Charge gas oil BRL1.00 55-1705 1.03 0.95 Av. 0.99 [S.D. f 0 . 0 4 SRR No. 18703 8.23 S.40 8.44 8.3,5 AV. S.D. AO.11 Catalytic cracker feed 41.6 40.4 40.7 Av. 40.9 S.D. f 0 . 6 Catalytic cracker 76.8 pitch 76.0 79.3 77.3 AV. S.D. 4 ~ 1 . 7 Flasher pitch 12.0 11,s 13.4 12.4 AV. S.D. 1 0 . 9 a
II. Trace Metals in Oil Nickel, p.p.m. Incin. Acid with S oxid. 8.5 8.8 8.3 8.6 8.2 8.9 8.3 8.8 10.2 15.9 16.1 16.0 16.1 16.0 16.5 16.0 16.2 ~~
~
f O .3 1.60 1.61 1.61 1.61
11.0
78 l i
77 77 50.7
143 141 142 142
11.7 11.3 11.8
11.6 78 82 81 81
1.68 1.65 1.69 1.67 f0.02 26.0 26.1 26.3 26.1 f0.2 149 148 152 150
154 148 150 151 10.8
io. 5
1.63 24.1 24.2 24.0 24.1 14s 142 143 144
10.7 10.7
29 1
-2s _1-
287 281 286 f 5
277 270 276
23.1 23.5 23.1 23.2 f0.2
22.5 22.4 22.0 22.3
S.D. = standard deviation.
chloride-diphenylbenzidine solution (0.1 mg. DPB per ml. CC14) and add each rinse t o the Erlenmeyer flask. Stopper the flask and shake for 2 minutes. Allow 2 to 3 minutes for the phases to separate and transfer the upper, aqueous phase to a centrifuge tube. Centrifuge for 2 to 3 minutes. Measure the absorbance of the centrifuged solution relative to distilled water at 580 mp. Determine the average absorptivity a s with iron using a standard solution of ammonium vanadate. The average absorptivity should be about 500 liters per gram cm.
LITERATURE CITED
1.61 1.65
h2
h1
10.4 10.6 10.3 10.5 10.3
The authors are indebted t o E. D. Peters and G. W. Bond for their interest and suggestions during the course of this investigation.
Less than 0.05
f0.2 r)-
ACKNOWLEDGMENT
f0.07
fO.O1
0.016 0.022 0.030 0.023 f0.007 0.16 0.15 0.14 0.14 fO.01 11.2 11.2 10.9
Vanadium, p.p.m. Incin. Acid oxid. with S 0.99 1.07 1.00 0.99 1.00 1.09 1.00 1.05 f0.01 3.46 3.02 3.32 3.52 3.51 3.79 3.43 3.44 10.12 2.20 2.08 2.20 2.16
of metals by reduced-scale sulfur incineration, Eomparison was made in many cases with values obtained by acid oxidation. Iron was not determined by acid oxidation because of the large blanks encountered. The data show iron, nickel, and vanadium can be measured on a 25-gram portion of oil with a relative standard deviation equal to 1 to 5% of the mean 01 0.02 t o 0.05 p.p.m., whichever is larger. About 8 hours are required t o complete a single sample. However, the time per sample is considerably less when a series of samples are run. Because many of the operations require no attention, the method is well suited t o serial analysis.
TRACE METALS IN OILS
Reduced-scale sulfur incineration with spectrophotometric finish was applied t o a number of oils. The data are summarized in Table 11. The maximum amount of oil burned for the determination of iron, nickel, and vanadium mas about 25 grams. Smaller amounts mere taken for samples high in these metals. I n all this work, blank determinations were made and corrections applied to the data. To demonstrate the complete recovery
(1) Barney, J. E., (1990).
ANAL.
CHEM.27, 1283
(2) Barney, J. E., Haight, G. P., Ibid., 27,1285 (1955). (3) Blackwell, -4. T., Danel, A. M., Miller, J. D., Ibid., 28, 1209 (1956). (4) Dwiggins, C. W., Jr., Dunning, H., Ibid., 32, 1137 (1960). (5) Eeckhout, J., W-eynants, A., Anal. Chiin. Acta 15, 145 (1956). (6) Forrester, J. S., Jones, J. L., ANAL. CHEM.32,1443 (1960). ( 7 ) Gamble, L. W., Jones, W. H., Ibid., 27, 1456 (1955). (8) Hale, C. C., King, W. H., Jr., Ibid., 33, 74 (1961). (9) Hoste, J., .4nal. Chim. Acta 3 , 36 (1949). (10) Johnson, R . .4.,Shell Development Co., Emeryville, Calif., unpublished data. (11) Kang, Chia-Chen Chu, Keel, E. W., Solomon, E., . ~ N A L . CHEM. 32, 221 (1960). (12) Kirchmer, J. H., Gunn, E. L., Ibid., 24, 1133 (1952). (13) Liberman, rl., Analyst 80, 595 (1955). (14) Milner, 0.O., Glass, J. R ., Kirchner, .J. P.. Turick. il. N.. A N A L . CHEM.24. i728 ’( 1952). ’ (15) Mitchell, 4. hl., Mellon, M. G., I N D . ESG. CHEY., A N A L . ED. 17, 380 (1946). (16) Oelschlager, W., Z. A n d . Chem. 146, 339, 349 (1955). (17) Pagliassotti, J. P., Porsche, F. W., A S A L . CHEM. 23, 1820 (1951). (18) Sandell, E. H . , “Colorimetric Determination of Trace Metals,” Vol. 111, p. 669, third ed., Interscience, New York, 1959. (19) Saraceno, A. J., Fanale, D. T., Coggeshall, N. D., AXAL. CHEM. 33, ,500 11961). (20) Shott, J . E., Garland, T. J., Clark, R. O., Zhid., p. 506. (21) Wrightson, F. M., Ibid., 21, 1543 (1949). RECEIVEDfor review October 11, 1962. Accepted January 7, 1963. VOL 35,
NO. 3, MARCH 1963
0
335
