18)
Substituting Equation 22 into 21, we get R, = 2/(w2C R C,) (23)
Solving this with respect to R,,
R,
=
ACKNOWLEDGMENT
(21)
R
=
.4’RoZ/(+” - i’) - Ro
(20)
Yumerical valum of R and C are obtained from Equations 19 and 20 b j ~ using the values of A’ and R,. Quantities of the sample, R, and C,, are then obtained by multiplying Equation 4 by Equation 5 to give CR
c, + dR,2 =
C,C,(C, w2CC2 R,
+ c,
+ C,) R z 2 w2C, C R, R
w4
CcCZR2- 4wz/C= (Cz
+ C,)
0
c,
=
-
The authors wish to express their gratitude for the support by the Grant in Aid of the Ministry of Education in Japan. LITERATURE CITED
Solving this,
I
c, * dC2 + W ~ C C 2 C ~ X Z 2
With the condition that C, must be positive,
lr-hichcan be rewritten [ts w’C,(C,
As Equation 21 refers to two positive real roots for R,, it is necessary to have
(1) Blaedel, W. J., Malmstadt, N. V.,
Petitjean, D. L., Anderson, W. K., ANAL.CHEM.24, 1240 (1952). (2) Fujiwara, S., Hayashi, S., Zbid., 26, 239 (1954).
(3) Reilley, C. N., McCurdy, W. H., Jr., Ibid., 25, 86 (1953).
RECEIVED for review September 10, 1958 hccepted hpril 17, 1959.
+1=0
Arsenic in Naphthas GEORGE W. POWERS, Jr., RONALD L. MARTIN, and FRANK J. PIEHL Research and Development Depwtment, Standard Oil Co. (Indiana), Whiting, Ind. J. MARCUS GRIFFIN Research Department, Utah Oil Refining Co., Salt Lake City, Utah
,Arsenic in naphthas is determined a t the parts per billion level b y a combination of chromatography and colorimetry. It is first quantitatively adsorbed from naphtha onto silica gel impregnated with sulfuric acid. The adsorbent is then’digested with sulfuric, nitric, and perchloric acids to remove organic matter and to dissolve the arsenic, which is subsequently converted to arsine and determined colorimetrically with silver diethyldithiocarbamate in pyridine. Precision and accuracy ore about 3% relative or 0.3 p.p.b., whichever is greater. Four to six samples are analyzed in 8 hours. Periodic analyses of naphthas during storage show that an inherent source of error in any method is adsorption of arsenic on the sample container. Although as much as of the arsenic present in a typical naphtha can b e lost in 4 hours, simple techniques eliminate the loss. The arsenic compounds in virgin naphtha seem to b e alkyl arsines.
T
constituents in petroleum profoundly affect catalytic processes. I n catalytic reforming over platinum, a few parts per billion of arsenic in the naphtha feed gradually deactivate the catalyst (8) and eventually destroy its activity. Because naphthas contain varying small amounts of RACE
arsenic (7, l a ) , they must be monitored for arsenic content. The problem is twofold; the arsenic must be concentrated and then determined. Three ways of concentrating the arsenic in naphtha have been described. The usual one is digestion with sulfuric acid, nitric arid, and hydrogen peroxide (10, 12, IS) to oxidize organic matter and dissolve the arsenic. Combustion of the naphtha in a Beckman oxyhydrogen burner and absorption of the combustion products in sodium hydroxide solution have been described recently (1). Chromatographic adsorption on alumina (9, 22) or silica gel (4) has been suggested but has not been adequately tested. The simplicity and speed of the chromatographic approach make it potentially the most useful. Four methods of determining arsenic have been used. Neutron activation is the most sensitive (16, 18), but few laboratories have the needed equipment; moreover, errors as large as 20 p.p.b. a t the level of 50 p.p.b. have been encountered in the method as used a t Oak Ridge (11). Emission spectrosropy (22) lacks precision and sensitivity. The Gutzeit method (15), in which arsine reacts with mercuric bromide to form a yellow spot on paper, is the most sensitive chemical method; although most procedures (9, 10, 13) use it, obtaining reproducible spots
and measuring the intensity of them quantitatively is difficult. The molybdenum blue method (6, 19) avoids these difficulties (12) because the color is formed in solution; however, it is only one tenth as sensitive as the Gutzeit method. A new method combines the advantages of chromatographic concentration and colorimetric determination. The arsenic is concentrated on a new adsorbent-silica gel impregnated with sulfuric acid-and determined with silver diethyldithiocarbamate in pyridine, which gives a soluble red complex with arsine (5, 20, 21). This reagent is somewhat simpler to use than molybdenum blue and is equally sensitive. DEVELOPMENT
OF METHOD
I n developing the new method, four problems were: how to collect samples for analysis, to concentrate the arsenic, to convert it to arsine, and to determine the arsine. Two special sampling techniques were devised when it was discovered that as much as 30% of the arsenic can be adsorbed on the sample container in 4 hours. I n one, the naphtha is sampled into a bottle that has been prewashed with acid; after the naphtha has been withdrawn, the arsenic is desorbed from the glass by rinsing x i t h concentrated sulfuric acid, and the acid and naphtha VOL. 31, NO. 9, SEPTEMBER 1959
1589
are analyzed together. I n the other, the naphtha is passed directly from a sampling point into a special sampling column containing the chromatographic adsorbent. Sone of the adsorbents described in the literature was entirely satisfactory for concentrating the arsenic from naphthas. -4lthough clay (14) and peroxide plus acid on such supports as silica gel (5) are used commercially, they do not remove arsenic quantitatively. Silica gel removes some arsenic compounds qiiantitatively, but it is too selective. Basic alumina removes arsenic trioxide and arsanilic acid quantitatively (Q),but an adsorbent with a greater capacity for natural arsenic compounds was desired. Although numerous metal salts on a variety of supports remove arsenic quantitatively ( 2 ) , they could cause difficulty in the subsequent generation of arsine. Concentrated sulfuric acid on silica gel removed arsenic from naphthas quantitatively and had a much larger capacity than other adsorbents. When 500 ml. of virgin naphtha containing 0 to 100 p.p.b. of arsenic as trimethylarsine, tributylarsine, triphenylarsine, or naturally occurring arsenic compounds are passed through a column containing 5 grams of this adsorbent, the effluent contains less than 0.2 p.p.b. of arsenic as determined by this method and by the method of Maranowski, Snyder, and Clark ( I S ) . This adsorbent does not introduce interferences or contribute significantly to the method blank. For recovering the arsenic from the adsorbent, digestion with a mixture of sulfuric, nitric, and perchloric acids proved most satisfactory. Perchloric acid speeds up the oxidation of organic matter and lessens the possible loss of arsenic from carbonization. Although zinc and acid conveniently convert the recovered arsenic to arsine, the technique (13) previously used with the Gutzeit method by the authors was modified in three ways. Glass wool moistened with lead acetate solution could not be used for removing hydrogen sulfide because enough water sometimes remained on it to dissolve arsine and cause low recoveries. Cotton impregnated nith lead acetate and dried to the touch eliminated the loss. Because arsine is absorbed in solution, the rate of hydrogen generation proved less critical than in the Gutzeit method. If the acid solution is chilled a t the start of the generation and the rate of agitation is controlled, all the arsine is liberated in a reasonable time but not so rapidly that any escapes from the absorber. Because the presence of the silica gel tends to slow down the evolution of arsine, hydrogen is evolved for 75 minutes to ensure quantitative recovery of arsenic. 1590
ANALYTICAL CHEMISTRY
+
f
F I - C M
''
I.D.
COARSE-POROSITY FRITTED GLASS 2-MM
CAPILLARY
IOCM
i
IMPREGNATED COTTON
T
3-ML
RESERVOIR
1 I/
I4 1
19/38
JOINT
h
100-ML KJELDAHL FLASK
7/8-INCH MOLDED T E F L O N STIRRING BAR
MAGNETIC STIRRER \
Figure 1 . absorber
MANTLE
Digestion flask and arsine
Although determining the arsine colorimetrically with silver diethyldithiocarbamate (5) was generally adequate, a new arsine absorber had to be devised to prevent loss. EQUIPMENT A N D REAGENTS
All new glassware must be cleaned with hot concentrated sulfuric acid and rinsed with water and then with acetone. If the glassware is reserved for trace-arsenic work, the sulfuric acid can be omitted in subsequent washings. The chromatographic column is 10 cm. long and has an inside diameter of 1 em.; it has a male 14/35 T joint a t the bottom and a conical-shaped 500ml. reservoir with a 28/14 socket joint a t the top. The column has a detachable tip made from a female 14/35 T joint and a No. 2 Teflon stopcock. Details of the digestion flask and arsine absorber are shown in Figure 1. A 50-ml. heating mantle rated a t 50 volts must be used to deliver enough heat to reflux sulfuric acid. Optical measurements are made with a Beckman Model B spectrophotometer, or equivalent, equipped with matched 1-cm. cells. All reagents are analytical reagent grade unless otherwise specified, and all references to water mean ion exchanged water. Sulfuric acid manufactured a t the East Chicago, Ind., plant of E. I. du Pont de Nemours & Co. is recommended because it is distilled in glass and contains less arsenic than acid from other suppliers. Silica gel from the Davison Chemical Co., Grade 62, 60 to 200-mesh, is used because it adsorbs more than its own weight of sulfuric acid and remains free flowing. The adsorbent is prepared by mixing 20 ml. of sulfuric acid nith 50 grams of silica gel until the mixture is free of lumps. It should be stored in a tightly stoppered bottle and prepared fresh each week. Impregnated cotton is prepared by
soaking cotton in a filtered, saturated, aqueous solution of lead acetate. The cotton is compressed to remove as much liquid as possible and then dried under vacuum until just dry to the touch. It can be stored indefinitely in a tightly covered jar. Silver diethyldithiocarbamate is prepared by slowly adding a 0.1M aqueous solution of silver nitrate to a n equal volume of a vigorously stirred 0.1M aqueous solution of sodium diethyldithiocarbamate trihydrate (Eastman White Label). Water is removed with a beaker filter, and the precipitate is washed 3 times with water. The solutions and rinses are chilled to below 8" C. before mixing, and the precipitate is kept below 8" C. until the last rinse has been completed. The precipitate is finally dried under vacuum at room temperature. The silver diethyldithiocarbamate solution is prepared by dissolving 1 gram of the dry salt in 200 ml. of pyridine, which has been percolated over activated alumina and then over silica gel, PROCEDURE
Sampling and Concentration. When sampling in bottles, flush out t h e sampling point, fill a clean bottle, and close it with a cap lined with Teflon or polyethylene. Place a wad of cotton in t h e stopcock section of the chromatographic column, and fill the barrel of the column with n-hexane (Phillips Petroleum Co. technical grade). As slurries in n-hexane, add enough silica gel t o fill the stopcock section and enough adsorbent to form a column about 10 cm. high. Drain excess n-hexane from the column and tap it occasionally t o remove air bubbles. Measure the volume of naphtha, transfer all of it to the column, and allow it to flow through the column under gravity or with applied pressure, as desired. Rinse the bottle and the walls of the reservoir with two IO-ml. portions of n-hexane and allow them to flow through the column. Rinse the sample bottle with 12 ml. of concentrated sulfuric acid and transfer it to the column. Rotate the column at different angles to wet the entire inner surface of the reservoir with acid. Quickly remove the stopcock section (discard the cotton and silica gel in the stopcock section) and insert the end of the column into the neck of a 100-ml. Kjeldahl flask. \Then all of the adsorbent and acid have drained into the flask, rinse the bottle and the column with an additional 10 ml. of concentrated sulfuric acid in the same way. Finally rinse the column with 3 ml. of 70y0 perchloric acid and with 5 ml. of concentrated nitric acid, and add the rinses to the flask. Khen sampling directly from a sampling point, close the bottom of a sampling column (a 16-cm. length of 1-em. glass tubing with a constriction near one end) with neoprene tubing and a pinch clamp. Fill it about one fourth with n-hexane, and push a wad of glasp wool to the constricted portion of the tube. As a slurry in k-hexane, add
enough adsorbent to form a column about 10 cm. high. Drain excess nhexane from the column, and tap it occasionally. Flush out the sampling point and attach the sampling column. Allow 500 ml. of naphtha to flow through the column over a period of about 1 hour, and disconnect the column. Pass two 10-ml. portions of n-hexane through it. Transfer the adsorbent and glass wool t o a 100-ml. Kjeldahl flask by applying gentle air pressure to the bottom of the column. Rinse the column successively with two 10-ml. portions of concentrated sulfuric acid, with 3 ml. of 70% perchloric acid, and with 5 ml. of concentrated nitric acid, and add the rinses to the flask. Conversion to Arsine. Stir the contents of the flask rapidly for about 15 minutes without applying heat. Reduce the stirring speed as much as possible while maintaining uniform rotation of the stirring bar. Gradually apply heat to boil off the hydrocarbon. When no hydrocarbon remains, increase the stirring speed and apply more heat. If the oxidation becomes too vigorous, remove the flask from the mantle, and cool. When vigorous oxidation subsides, apply more heat. If the mixture darkens, add more nitric acid. Whcn reaction ceases, cover the flask with asbestos or glass cloth and apply enough heat to reflux the sulfuric acid to about 2 inches from the top for 1.5 minutes. If a yellow or orange color persists, add a few drops of perchloric acid and continue the oxidation. Cool the flask in cold water and cautiously add 10 nil. of water and then 10 ml. of saturated aqueous ammonium oxalate, Replace the flask on the mantle, stir, and heat. When most of the water has boiled off, again reflux the sulfuric acid for 15 minutes. Cool the flask and cautiously add 50 ml. of water. Cool again, add 20 ml. of isopropyl alcohol, and chill the flask in an ice bath. Stir and add 2 ml. of 15Oj, aqueous potassium iodide. Return the flask to the ice bath for 5 minutes. Remove, stir the contents, and add 12 drops of stannous chloride solution (40 grams dissolved in 100 ml. of concentrated hydrochloric acid). Allow the flask to stand in the ice bath for 15 minutes. Pack a small wad of the impregnated cotton firmly inside the joint of the arsine absorber. Grcase the joint with a water-soluble stopcock lubricant (Fisher Scientific Co. Nonaq or equivalent), Pipet 3.00 ml. of silver diethyldithiocarbamate solution into the absorber, and draw most of it through the fritted-glass disks into the 3-ml. reservoir. Remove the flask from the ice bath and stir the contents moderately. Add 6.0 & 0.1 grams of zinc (Mallinckrodt Chemical Korks analytical reagent, 20-mesh) and quickly connect the arsine absorber to the flask. Stir slowly for 30 minutes, and then rapidly for 45 minutes. Disconnect the absorber and inspect the impregnated cotton. If most of it is blackened, discard the analysis because hydrogen sulfide may hare reached the absorber.
Determination of Arsine. Draw the liquid in the absorber back and forth through the fritted-glass disks until it is uniform in color, and then force i t out of the absorber into -an absorption cell. Determine the absorbance of the solution a t 540 mp with silver diethyldithiocarbamate solution in the reference cell. From a calibration curve, read the number of micrograms of arsenic corresponding to the absorbance. Determine a blank by preparing an extra column and carrying it through the entire procedure. If the blank exceeds 0.2 y of arsenic, one of the reagents is contaminated. Calculate the arsenic content of the naphtha from the equation: P.p.b. As
=
(-4- B)(1000)
(WD) where A is micrograms of arsenic in the sample, B is micrograms of arsenic in the blank, C is milliliters of naphtha, and D is the density of the naphtha. Calibrate with about six solutions that span the range of 0 to 15 y of arsenic. To each flask add freshly diluted standard sodium arsenite solution (IS), 5 grams of adsorbent, 5 ml. of n-hexane, 20 ml. of concentrated sulfuric acid, 3 ml. of 70% perchloric acid, and 5 ml. of concentrated nitric acid. Oxidize the contents and develop the color. From the absorbance of each solution a t 540 mfi, subtract the absorbance of the solution containing no added arsenic. Plot the differences against micrograms of arsenic. PRECISION AND ACCURACY
Precision of the method was established by analyzing virgin naphthas obtained by both sampling techniques. Six duplicate samples were taken with the sampling column and analyzed. The duplicate analyses, in parts per billion of arsenic, were: 0.7, 0.8; 2.5, 2.6; 6.5, 6.6; 6.6, 6.8; 9.6, 9.6; and 19.7, 23.3. With one exception, the difference is 0.2 p.p.b. or less. Five replicate samples of virgin naphthas were taken in bottles and analyzed. The replicate analyses, in parts per billion of arsenic, were: 1.4, 1.5, 1.6; 2.9, 3.0; 5.3, 5.8; 5.6, 5.7, 5.7, 6.1; and 6.9, 7.1. None differed by more than 0.5 p.p.b. Duplicate samples were also collected by each sampling method and analyzed. The average values, in parts per billion of arsenic, were: Sampling Columns 8 6 5 4
9 3 4 2
Rinsed Bottles 9 6 4 4
1 1 8 0
The methods agree nithin the demonstrated precision. Accuracy was determined by analyzing synthetic mixtures of commercial triphenylarsine and tributylarsine in virgin naphtha (Table I). The virgin
I.
Table
Accuracy with Mixtures
Synthetic
Arsenic, P.P.B. Added Found Error Tributylarslne
0.9 5 1 10.3 25.7 45.7 91.4 Triphenylarsine 0 . 9 5.5 13.7 43.0 86.0
1.1 5.0 9.8 24.5 45 0 86.6 0.3 5.5 13.7 41.0 82.4
+O. 2 -0.1 -0.5 -1.2 -0.7 -4.8 -0 . 6 0.0 0.0 -2.0 -3.6
Table II. Recovery of Natural Arsenic Compounds and Trimethylarsine from Glass Containers
Days
Arsenic, P.P.B. Naphtha Glass Total Natural Arsenic Compounds
0 18 0
7 14
4.3 2.2" 9 1" 8.1
5.8"
0.0 2.3" 0.0" 1.3 2.9"
4.3 4 5" 9.1" 9.3 8 7"
Trimethylarsine 0 2 7 21 70 Av.
6 . 9 , 6 . 9 0.0, 0 . 0 6 . 9 , 6 . 9 5 . 1 , 6 . 2 1.7, 1 . 2 6 . 8 , 7 . 4 4.1,4.1 2.9,2.4 7.0,6.5 4.0 6.8 2'. 8 4.7 7.2 2.5 of two determinations
naphtha contained 0.4 p.p.b. of arsenic, which has been subtracted from the reported values. Below 10 p.p.b., the average error is 0.3 p.p.b.; a t higher concentrations, it is about 3% relativetypical for a colorimetric method. To find out whether naturally occurring arsenic compounds are completely recovered, a virgin naphtha that contained 9.6 p.p.b. mas diluted with 20, 40, 60, and 80% of another naphtha that contained only 0.3 p.p.b. The amount of arsenic recovered in each case differed from the calculated value by no more than 0.3 p.p.b. BEHAVIOR OF NATURAL ARSENIC COMPOUNDS
The arsenic content of naphtha drops significantly in a few hours after sampling because arsenic is adsorbed on the sample container. A 500-ml. sample of reforming feed was taken with a sampling column, and a 3-gallon sample was collected simultaneously in a bottle. Within 1 hour, the 3-gallon sample was divided among four glass, four steel, and four polyethylene containers. One sample from each type of container was analyzed after 4 hours and after 7, 14, and 28 days; the containers xere not rinsed with acid. Analysis with the sampling column established that the naphtha originally contained 12.1 p.p.b. of arsenic. As shown in Figure 2, the arsenic content falls rapidly. After 4 VOL. 31, NO. 9, SEPTEMBER 1959
1591
hours in the glass container, 27% was lost. Although the lws was somewhat more rapid in glass, the arsenic content in all three types of containers leveled out in 2 weeks at about 20% of that originally present. Even though the containers remaining a t 28 days were shaken vigorously before the naphtha was removed, no additional arsenic was recovered. I n two similar experiments, glass containers were rinsed viith acid after the naphtha was removed, and the naphtha and acid were analyzed separately. The natural arsenic compounds lost from the naphtha were recovered quantitatively from the glass container (Table 11). The loss of arsenic from these naphthas was less rapid than from the naphtha depicted in Figure 2. Stability of synthetic solutions was also studied to find out what kinds of arsenic compounds behave in this manner. I n solutions. containing 20 p.p.b. of arsenic as tributylarsine and as triphenylarsine, no loss of arsenic could be detected after 1 week. Hoivever, as shown in Table 11, the arsenic content of a naphtha with added trimethylarsine declined, but not as rapidly as that of the naphtha M ith natural arsenic. compounds. As before, d l the lost arsenic was recovered by rinsing the bottles with sulfuric acid and by analyzing the acid separately. The average total amount of arsenic recovered was 6.9 p.p.b., with a standard deviation of 0.3 p.p.b. A clue to the structure of naturally occurring arsenic compounds was also found. When adsorbent that had been used to purify a large volume of naphtha was dumped into hot n-ater, the characteristic odor of low molecular weight alkyl arsines was noted. Although this odor was distinguishable from that of arsine, it was not possible to distinguish among mono-, di-, and trialkyl arsines. The loss of arsenic can be explained as a combination of oxidation and adsorption. With the exception of triaryl arsines, the alkyl and aryl arsines are readily oxidized by air (17). The products, which contain oxygen. should be less soluble in naphtha and readily adsorbed on the walls of the container. Thus, the loss in arsenic depicted in Figure 2 could result from oxidation and adsorption of arsines, whereas the leveling out could represent arsines that are either not oxidized or are oxidized to soluble products. DISCUSSION
The chromatographic concentration of arsenic has several advantages. Contamination by arsenic in reagents and 011 glassware is less a problem because ten times more arsenic is being handled. Consequently, the selection and purification of reagents and the 1592
0
ANALYTICAL CHEMISTRY
1
“I IN S T E E L A IN GLASS
m IN POLYETHYLENE
0
O
I
1
5
IO
I
I
15
20
25
i 30
DAYS
Figure 2. naphthas
Loss of arsenic from virgin
naphthas and analyzed. Recoveries, in weight %, were: Hexane-benzene (1 :1) Catalytically reformed Thermally rracked Cokestill
99 94, 105 91, 88 85, 96
Results for the thermally cracked and cokestill naphthas have been corrected for 2 p.p.b. of arsenic originally present. With t,he thermally cracked and cokestill naphthas, the adsorbent became black, and some arsenic was not adsorbed. I n such cases, accuracy can be improved a t the expense of sensitivity by the use of a smaller sample. CONCLUSION
preparation of standard solutions of arsenic arc easier. Furthermore, the oxidation of less organic matter is simpler, and arsenic is less likely to be lost. The time required per analysis is about the same as for other methods. Four to six samples can be analyzed in 8 hours. Although it is convenient to set up the chromatographic columns a t the end of one day and to allow the naphtha to percolate overnight under gravity, this step can be completed in 30 minutes by applying pressure to the column. Three hours usually suffice to complete the digestion. but some samples require more. For maximum accuracy, the sample should contain betaeen 3 and 11 y of arsenic. Although 500 ml. of naphtha may not alnays contain 3 y of arsenic, this amount of sample provides adequate sensitivity. The calibration is usually linear from 0 to 18 y ; occasionally it is slightly curved from 0 to 5 y and linear a t higher levels. For the linear calibration, a value of 0.070 absorbance unit per microgram was found. The blank is usually about 0.1 y or 0.2 p.p.b.; although larger in micrograms than that for some other published procedures, it is actually smaller in parts per billion. Of the two sampling procedures, direct sampling into sulfuric acid on silica gel is preferred, because the naphtha is transferred one less time. The columns can be stored for a t least 2 weeks without loss of arsenic : Days Stored
.4rsenic, P.P.B.
0 7 14
8.6, 9 . 2 8.7, 9 . 1 8.8. 8 . 8
To determine w-hether naphthas other than virgin naphtha deactivate the adsorbent and cause loss of arsenic, synthetic samples containing about 30 p.p.b. of arsenic as tributylarsine were prepared in hexane-benzene and three
Two extensions seem possible. Presumably arsenic in virgin naphthas could be determined a t lolt-er levels by using a larger sample, and more adsorbent if necessary. The chroniatographic adsorbent should be useful in concentrating other metals; lead has been removed quantitatively a t the parts per million level. By concentrating some of the naturally occurring arsenic conipounds on the adsorbent, and xith the aid of mass spectronietry and gas chromatography, it should be possible to obtain conclusive evidence for the presence of alkyl and aryl arsines in naphtha?. ACKNOWLEDGMENT
The authors thank A. L. Hensley for synthesizing the trimethylarsine. LITERATURE CITED (1) Albert, D. K., Granatelli, Lawence, ANAL.CHEM.31,1593 (1959).
( 2 ) Appell, H. R. (to Universal Oil
Products Co.), U. S. Patent 2,781,297 (Feb. 12, 1957). (3) Bicek, E. J. (to Universal Oil Products Co.), Ibid., 2,782,143 (Feb. 19, lnK7\ rvu‘,.
(4) Bond, G. Et., Jr., Harriz, C. G., ANAL.CHEM.29, 177 (1957). (5) Committee on
